Effect of early carbonation curing on concrete resistance to weathering carbonation
by Tianlu Liu
Department of Civil Engineering and Applied Mechanics McGill University, Montréal, Québec, Canada June 2016
A thesis submitted to McGill University in partial fulfillment of the requirements of the degree of Master of Civil Engineering
©Tianlu Liu, 2016
Abstract
Carbonation curing of precast concrete has shown enhanced durability performance and carbon storage capacity. However, carbonation curing may reduce the pH of concrete.
Furthermore, the progressive weathering carbonation in service may aggravate the problem, leading to the corrosion of steel in concrete. This thesis is to investigate the effect of early carbonation curing on concrete resistance to weathering carbonation. The concrete cubes were prepared with two water-to-cement (w/c) ratio: 0.65 and 0.40. The samples were initially in-mold cured, off-mold fan dried, then carbonated with pure CO2 gas at 5 bars for 2 and 12 hours respectively, and finally subsequent by hydrated. To evaluate the degree of weathering carbonation, carbonation depth, carbonation coefficient, pH distribution, carbon content, and X-ray diffraction analysis were performed. It was found that the volume of permeable voids was lower in carbonated samples than in hydration reference, which was indirectly proved to be beneficial to decrease the permeability and porosity of concrete by carbonation curing. For concretes with w/c = 0.65, the carbonation depth, pH value and carbon content analysis were found to be more affected by weathering carbonation in 12-hour carbonated concrete than in 2-hour carbonated and hydrated concretes. Early carbonation curing even helped to reduce the carbonation coefficient in weathering carbonation. For concretes with w/c = 0.40, the carbonation depth, carbonation coefficient, pH value and carbon content of different carbonated and hydrated concretes were comparable during weathering carbonation. Therefore, the early carbonation curing was not more detrimental to precast reinforced concrete in this mix design over weathering carbonation.
i
Résumé
Le béton préfabriqué mûri par carbonatation accélérée a démontré une meilleure performance de la durabilité et de la capacité d’emmagasinage de carbone. Cependant, le mûrissement par carbonatation accélérée réduit le pH du béton. En outre, la carbonatation atmosphérique du service peut aggraver le problème, menant à la corrosion de l'acier dans le béton. Cette thèse est d'étudier l'effet initial de la carbonatation accélérée lors du mûrissement sur la résistance du béton à la carbonatation atmosphérique. Les cubes de béton ont été préparés avec deux rapports d’eau-ciment (E/C) : 0,65 et 0,40. Les
échantillons ont initialement mûri dans le moule, séchés hors-moule par ventilateur, puis carbonatés avec du gaz CO2 pur à 5 bars pendant 2 heures et 12 heures respectivement, et enfin ultérieurement hydratés. Pour évaluer le degré de carbonatation atmosphérique, la profondeur de la carbonatation, le coefficient de carbonatation, la distribution du pH, la teneur en carbone et l'analyse par diffraction de rayons X ont été effectués. Il fut constaté que le volume des vides perméables était plus faible dans les échantillons soumis à la carbonatation accélérée par rapport aux échantillons de référence hydratés, ce qui s’est indirectement prouvé être bénéfique pour la réduction de la perméabilité et de la porosité du béton suite au mûrissement par carbonatation accélérée. Pour les bétons avec E/C =
0.65, la profondeur de carbonatation, la valeur du pH et la teneur en carbone se sont révélées être plus affectées par la carbonatation accélérée dans les bétons carbonatés pendant 12 heures que dans le béton carbonaté 2 heures et hydraté, même un mûrissement initial par carbonatation accélérée a contribué à réduire le coefficient de carbonatation dans la carbonatation atmosphérique. Pour les bétons avec ratio E/C = 0.40, la profondeur de la carbonatation, le coefficient de carbonatation, la valeur du pH et la teneur en carbone de
ii différents bétons carbonatés et hydratées sont comparables pendant la carbonatation atmosphérique. Par conséquent, le mûrissement initial par carbonatation accélérée n'a pas
été plus préjudiciable au béton armé préfabriqué avec ce mélange par rapport à la carbonatation atmosphérique.
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Acknowledgement
I would like to express my sincere gratitude to my supervisor, Professor Yixin Shao, for his encouragement, support and priceless guidance throughout my research and writing of this thesis.
My special thanks go to my colleague Duo Zhang for his helpful suggestions and valuable contributions on either the writing or the procedure to carry along the research. I am deeply grateful to him for carefully reading and commenting on this manuscript. Thanks are extended to the laboratory technicians John Bartczak, Bill Cook and the administrative staffs in the Department of Civil Engineering and Applied Mechanics.
I would like to thank my parents for their everlasting love, constant encouragements and understanding through my entire life. Thanks to my friends, colleagues and relatives for their support, advices and always being on my side during my time at McGill.
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Table of Contents
Abstract ...... i
Résumé ...... ii
Acknowledgement ...... iv
List of tables ...... viii
List of figures ...... ix
Chapter 1 Introduction ...... 1
1.1 General Overview ...... 1
1.2 Research Objectives ...... 2
Chapter 2 Literature Review ...... 4
2.1 Carbonation Curing ...... 4
2.1.1 Reaction Mechanism ...... 4
2.1.2 Factors Affecting Carbonation Curing of Fresh Concrete ...... 5
2.1.3 Performance of carbonation-cured products ...... 8
2.2 Weathering Carbonation ...... 11
2.2.1 Reaction Mechanism and Carbonation Behavior ...... 11
2.2.2 Factors affecting weathering carbonation ...... 12
2.2.3 Threshold pH Value to Initiate Corrosion of Steel in Reinforced Concrete .... 14
2.2.4 Durability ...... 15
v
Chapter 3 Experimental Program ...... 19
3.1 Materials ...... 19
3.1.1 Cementitious Binder ...... 19
3.1.2 Coarse and Fine aggregate ...... 20
3.2 Mix design and sample preparation ...... 20
3.3 Sample curing ...... 21
3.3.1 Conventional hydration curing ...... 21
3.3.2 Early carbonation curing ...... 22
3.4 Accelerated weathering carbonation ...... 25
3.5 Carbonation regimes ...... 28
3.6 Evaluation of carbonation curing ...... 29
3.6.1 Water loss ...... 29
3.6.2 Carbon uptake ...... 30
3.6.3 Compressive strength test ...... 32
3.6.4 Permeable voids test ...... 32
3.6.5 Carbonation depth ...... 34
3.6.6 pH value of pore solution ...... 35
3.6.7 X-ray diffraction (XRD) ...... 35
3.7 Evaluation of weathering carbonation ...... 36
Chapter 4 Results and discussion ...... 39
4.1 Early carbonation curing behaviour of concrete ...... 39
4.1.1 Carbonation Behavior ...... 39
4.1.2 Compressive strength test ...... 43
vi
4.1.3 Permeable voids test ...... 45
4.1.4 Carbonation depth ...... 47
4.1.5 pH value of pore solution ...... 53
4.1.6 X-ray diffraction (XRD) ...... 57
4.2 Effect of early carbonation curing on weathering carbonation ...... 60
4.2.1 Compressive strength test ...... 60
4.2.2 Carbonation depth due to weathering carbonation ...... 62
4.2.3 pH value of pore solution ...... 71
4.2.4 Carbon content analysis ...... 76
4.2.5 X-ray diffraction (XRD) ...... 78
Chapter 5. Conclusions and recommendations ...... 81
5.1 Conclusions ...... 81
5.2 Recommendations for future work ...... 83
Reference: ...... 85
vii
List of tables
Table 3.1: Chemical composition of Type GU Portland cement ...... 19
Table 3.2: Main compounds of Type GU Portland cement ...... 20
Table 3.3: Mix proportions of concrete ...... 21
Table 3.4: Carbonation regimes ...... 28
Table 4.1: Water content (%) of concrete based on initial mixing water ...... 40
Table 4.2: Carbon uptake of concrete ...... 42
Table 4.3: Water absorption and volume of permeable voids (%) ...... 46
Table 4.4: Qualitative depth of carbonation of concrete after carbonation curing and
subsequent hydration ...... 48
Table 4.5: Carbon content at layer of 10 – 25 mm depth of Mix I concrete in weathering
carbonation (%) ...... 77
Table 4.6: Carbon content at layer of 10 – 25 mm depth of Mix II concrete in weathering
carbonation (%) ...... 78
viii
List of figures
Figure 3.1: Subsequent hydration in moisture room ...... 23
Figure 3.2: Carbonation curing setup ...... 24
Figure 3.3: Weathering carbonation apparatus ...... 26
Figure 3.4: Construction weather tracker for testing humidity and temperature ...... 27
Figure 3.5: Cross-section of split concrete cube samples after weathering carbonation .. 37
Figure 4.1: Mass curves of concrete during 12-hour carbonation ...... 42
Figure 4.2: Compressive strength of concretes subject to early carbonation curing ...... 44
Figure 4.3: Qualitative depth of carbonation of Mix I concrete after carbonation curing 49
Figure 4.4: Qualitative depth of carbonation of Mix II concrete after carbonation curing50
Figure 4.5: Qualitative depth of carbonation of Mix I concrete after carbonation curing and
subsequent hydration up to 28 days ...... 51
Figure 4.6: Qualitative depth of carbonation of Mix II concrete after carbonation curing
and subsequent hydration up to 28 days ...... 52
Figure 4.7: Qualitative depth of carbonation of concrete after 28-day hydration ...... 53
Figure 4.8: Distribution of pH values of Mix I concrete at 1 day ...... 55
Figure 4.9: Distribution of pH values of Mix II concrete at 1 day ...... 55
Figure 4.10: Distribution of pH values of Mix I concrete after 27-day subsequent hydration
...... 56
Figure 4.11: Distribution of pH values of Mix II concrete after 27-day subsequent hydration
...... 56
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Figure 4.12: XRD patterns of Mix I concrete after initial curing and subsequent hydration
...... 58
Figure 4.13: XRD patterns of Mix II concrete after initial curing and subsequent hydration
...... 59
Figure 4.15: Compressive strength of Mix II concrete subject to weathering carbonation
...... 62
Figure 4.16: Weathering carbonation depth of Mix I concrete ...... 64
Figure 4.17: Weathering carbonation depth of Mix II concrete ...... 64
Figure 4.19: Qualitative depth of carbonation of Mix I concrete after 4-week weathering
carbonation ...... 65
Figure 4.18: Qualitative depth of carbonation of Mix I concrete after carbonation curing
and subsequent hydration up to 28 days ...... 65
Figure 4.21: Qualitative depth of carbonation of Mix I concrete after 12-week weathering
carbonation ...... 66
Figure 4.20: Qualitative depth of carbonation of Mix I concrete after 8-week weathering
carbonation ...... 66
Figure 4.23: Qualitative depth of carbonation of Mix II concrete after 4-week weathering
carbonation ...... 67
Figure 4.22: Qualitative depth of carbonation of Mix II concrete after carbonation curing
and subsequent hydration up to 28 days ...... 67
Figure 4.25: Qualitative depth of carbonation of Mix I concrete after 12-week weathering
carbonation ...... 68
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Figure 4.24: Qualitative depth of carbonation of Mix I concrete after 8-week weathering
carbonation ...... 68
Figure 4.26: Weathering carbonation coefficient of Mix I concrete ...... 70
Figure 4.27: Weathering carbonation coefficient of Mix II concrete ...... 70
Figure 4.28: pH at surface of Mix I concrete in weathering carbonation ...... 72
Figure 4.29: pH at 10 mm depth of Mix I concrete in weathering carbonation ...... 72
Figure 4.30: pH at 25 mm depth of Mix I concrete in weathering carbonation ...... 73
Figure 4.31: pH at core of Mix I concrete in weathering carbonation ...... 73
Figure 4.32: pH at surface of Mix II concrete in weathering carbonation ...... 74
Figure 4.33: pH at 10 mm depth of Mix II concrete in weathering carbonation ...... 75
Figure 4.34: pH at 25 mm depth of Mix II concrete in weathering carbonation ...... 75
Figure 4.35: pH at core of Mix II concrete in weathering carbonation ...... 76
Figure 4.36: XRD patterns of Mix I concrete in weathering carbonation ...... 79
Figure 4.37: XRD patterns of Mix II concrete in weathering carbonation ...... 80
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Chapter 1 Introduction
1.1 General Overview
It is widely accepted by the scientists that the emission of carbon dioxide (CO2) plays a crucial role on the problem of global warming and climate change, which is the result of human activities. To limit or reduce the emission of CO2 to atmosphere, carbon capture and storage is necessary.
Concrete is the most widely used man-made material throughout the world. The amount of concrete used worldwide is twice by mass that of steel, wood, plastics, and aluminum combined. Amongst them, the precast concrete has been developing since the industrial era and currently constitutes 20-30% of overall concrete industry production [1]. At the same time, it is estimated that the cement industry contributes to a comparatively high proportion of CO2 emission in the world, which accounts for approximately 5% of the global anthropogenic CO2 emission [2]. Through the cement production, the carbon dioxide is released by fossil fuel combustion and limestone decomposition. It will be beneficial if emitted carbon dioxide from cement production can be utilized in concrete production.
Precast concrete industry often use steam to accelerate curing since most precast products require fast curing using steam. The steam curing is a high energy consumption process. In order to reduce the energy consumption and the emission of carbon dioxide, carbonation
1 curing, a cost-effective method, has been studied recently [3, 4, 5]. Moreover, with the improvement of carbon-capture technologies, highly pure carbon dioxide gas can be mass produced at comparatively low cost [6]. In the precast concrete production, early carbonation curing can be used to replace steam curing to achieve high early strength and enhance the concrete durability performance by permanently storing carbon dioxide gas into concrete products in the form of stable calcium carbonates [7]. Therefore, the economical and technical benefits of using early carbonation curing can motivate concrete producers to take part in global CO2 utilization activities [8, 9].
However, despite the early strength gain [4, 5], reduced atmospheric carbonation shrinkage in service [10, 11] and the improvement of durability performance [12], the process of early carbonation curing will reduce the pH value of concrete and consume calcium hydroxide
(Ca(OH)2), which could generate corrosion of steel reinforcement. Moreover, progressive weathering carbonation occurs when matured concrete reacts with atmospheric carbon dioxide. The weathering carbonation will further reduce the pH value of concrete and is detrimental to the durability against carbonation corrosion. Therefore, the effect of early carbonation curing on concrete resistance to weathering carbonation should be examined for concrete with steel reinforcement.
1.2 Research Objectives
The purpose of this research is to understand the effect of early age carbonation curing on weathering carbonation behavior for precast reinforced concrete. The experiments focused on both high strength (70 MPa) concrete and normal strength concrete (30 MPa) with
2
different cement contents (452 and 335 kg/m3) and water to cement ratios (0.40 and 0.65).
The research also attempted to study the effect of carbonation reaction degree on related
properties by using different carbonation durations (2-hour and 12-hour). To evaluate the
degree of weathering carbonation after the early carbonation curing and subsequent
hydration, carbonation depth, carbonation coefficient, pH distribution and carbon pyrolysis
by layer were also investigated among normally hydrated and carbonation cured concretes.
The main objectives of this research were:
1) To investigate the carbonation behaviour and mechanical performance of reinforced
concretes subjected to early age carbonation curing based on their carbon dioxide uptake,
strength gain, absorption permeability, pH values, carbonation depth and XRD analysis.
The effect of early carbonation on subsequent hydration is also studied for the compressive
strength, pH values, carbonation depth and phase analysis.
2) To study the weathering carbonation degree of carbonated reinforced concrete after the
early carbonation curing and subsequent 27-day hydration period by the pH distributions,
weathering carbonation depth, carbonation rate (diffusion coefficient), carbon content by
layer and intensity (XRD) of calcium hydroxide and calcium carbonate against time.
3) To compare the carbonation degree in terms of different carbonation durations and water-
to-cement ratios and to determine the optimal mix proportion of precast reinforced concrete
and its early carbonation curing process to maximize its strength and durability.
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Chapter 2 Literature Review
2.1 Carbonation Curing
2.1.1 Reaction Mechanism
Accelerated carbonation curing (using carbon dioxide) of fresh Portland cement relies on the rapid hydration of calcium silicates to a combination of calcium-silicate-hydrates
(CSH) and calcium carbonates (CaCO ). Young et al. [4] observed that, during the first 10 min, the carbonation reaction of tricalcium silicate (C3S) and dicalcium silicate (C2S) was extremely rapid. Accelerated carbonation curing is initiated through the reaction between calcium silicate and dissolved carbon dioxide that produces calcium carbonate and CSH which reacts with carbon dioxide in the subsequent reaction. For C3S compacts, it takes only 3 min to reach the degree of reaction achieved by normal hydration. These two types of reactions have comparable stoichiometry but normal hydration produces calcium hydroxide instead of calcium carbonate. Subsequent reactions primarily involve CSH, which rapidly carbonate depleting lime and water. Overall these reactions produce calcium carbonate and silica gel. The reaction of calcium silicate with carbon dioxide is exothermic.
The shift of primary reactant from calcium silicate to CSH is due to the evaporation of water caused by the high heat developed initially. Depletion of water due to high heat also renders the core material of the sample largely unreacted.
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The preceding mechanism depicts the primary reactions if carbonation is performed immediately after casting. If carbonation is applied after a short period of hydration, the reaction concerns both anhydrous (C3S and C2S) and hydrous (Ca(OH)2, CSH) phases as described in Equations. 2.1 – 2.4. In both approaches, the reaction produces hybrid of hydrates and carbonates.
C S + 3 − x CO + yH O → C SH + 3 − x CaCO (2.1)
C S + 2 − x CO + yH O → C SH + 2 − x CaCO (2.2)
Ca(OH) + CO → CaCO + H O (2.3)
C − S − H + 2CO → SiO + 2CaCO + H O (2.4)
As is indicated in Equations. 2.1 – 2.4, carbonation curing at early stage is a carbon uptake process where gaseous CO2 is converted to solid carbonates and is permanently stored thereafter for emission reduction. Kashef-Haghighi et al [13] presented a mathematical framework that predicts CO2 uptake and distribution during carbonation curing by combining equations describing CO2 gas transport, dissolution in concrete pore water and reaction with cement compounds.
2.1.2 Factors Affecting Carbonation Curing of Fresh Concrete
In spite of the abundance of available literatures pertaining to carbonation curing of pure
C3S, C2S and low w/s ratio cement paste compacts [3, 4], few investigations were dedicated to the elucidation of carbonation curing of fresh concrete with higher w/s ratio required to
5 facilitate mixing and maintain workability over a substantially long period of time for placement and molding.
It’s already been revealed that CO2 concentration, relative humidity during carbon dioxide exposure and specimen size are important factors affecting carbonation curing [14]. In another research activity conducted by Goodbrake et al [5], a positive correlation between the carbonation reactivity of anhydrous C3S, C2S powders and parameters such as reaction temperature, particle-surface area, reaction time, relative humidity and partial pressure of carbon dioxide was observed. However, for fresh concrete with higher w/c ratio, there are a few other factors that influence the carbonation curing process.
2.1.2.1 Water Content
Water plays a substantial role in carbonation reactions dissolving calcium ions from the anhydrous cement phases which then react to form CaCO3. The amount of CaCO3 generated and the consequent strength development is thus affected by water content [15].
However, the mechanism of early carbonation is not straightforward in terms of water demand. On the one hand, low w/c ratio is appreciated in carbon dioxide curing since low pore saturation facilitates carbon dioxide diffusion into the material. On the other hand, complete hydration of the cement demands lower gas permeability and higher w/c ratios are therefore more desirable. Optimal w/c ratio has been reported to range from 0.09 to
0.15 [3,16].
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2.1.2.2 Temperature
Both ambient temperature and carbon dioxide temperature affects the efficiency of carbonation curing. On contrary to carbon dioxide solubility that increases with decreasing temperature, degree of carbonation was reported to increase with increasing temperature
[17]. To maintain the same level of carbon dioxide concentration at higher temperatures, almost twice the amount of carbon dioxide is required. A temperature of 15 ℃ or higher was suggested by Maries [18] to avoid delays in the induction period.
2.1.2.3 Preconditioning prior to carbonation curing
When vibration compaction is utilized to produce the precast concrete products, water of the mixture tends to move toward the surface of the concrete and saturate the capillaries close to the surface. The gas permeability is thus reduced and the CO2 uptake declines accordingly [19]. Several preconditioning techniques have been utilized prior to carbonation curing to improve CO2 uptake. Air drying is frequently used for concrete block production as a standard curing practice [20]. About 7% CO2 uptake in 2h carbonation curing following 18h air drying has been reported by Rostami [12]. Another means of preconditioning is fan drying [21]. In laboratory experiments, climatic chamber has been heavily used for preconditioning thanks to the improved controllability of ambient temperature and humidity. Shi et al. [22] reported doubled CO2 uptake when climatic chamber preconditioning was used prior to carbonation curing.
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2.1.3 Performance of carbonation-cured products
2.1.3.1 Compressive Strength
Strength gain from carbonation is rapid when compared with conventional hydration.
During a carbonation process, the amount of C3S reacted within 3 (utilizing a pure C3S sample) minutes is comparable to that of normal hydration after 12 hours [4]. C2S demonstrates a similar strength development profile compared to that of C3S in spite of the somewhat slow initial stages of carbonation. The increase in strength continues after 27 minutes of carbonation whereas negligible consumption of C3S is observed, implying that reaction between CSH and carbon dioxide is responsible for the strength gain during later stages of carbonation. Similar results were proposed by Bukowski and Berger [23] by utilizing C2S and Portland cement mortars. In their research, strength gain after 5 minutes of carbonation is comparable to that obtained after 1 day of normal hydration.
During the post-carbonation stage, the core materials of cement specimens are largely chemically inert when no remarkable strength gain is observed. The outer carbonated edge is found responsible for early strength development. Additional strength can be obtained by applying moist curing to the carbonated samples, analogous to the normal hydration [4].
Klemm and Berger studied the influence of post-carbonation hydration on the strength gain of carbonated cement samples using Type II mortars (sand:cement:water = 1:1:0.12) [24].
They recorded the strength of carbonated and non-carbonated samples after1, 3, 7 and 14
8 days of initial curing. Immediately following carbonation, the sample demonstrated higher compressive strength than that of normally hydrated sample after 1 day. After 1 day the strength of carbonated sample increased by 30%, comparable to that of hydrated sample after 7 days. After its initial rapid development of strength, the carbonated samples reached a strength plateau after 3 days to 2 weeks. One interesting observation is that, although the strength of carbonated sample proceeded to grow during the subsequent hydration, its strength gain did not catch up with that of normal hydration, the explanation of which requires further investigation.
Hannawayya et al. studied the compressive strength development in carbonation-hydration process [25]. It was observed that immediate strength gain after 38 minutes of carbonation exceeded that of 28 days’ hydration but the initial strength gain decreased after weeks’ of curing in water or air, whereas samples under normal hydration with relatively low initial strength continued to increase its strength and eventually beat carbonated samples after 28 days. They then concluded that the cement-based products under carbonation hydration process were inherently weaker than that under normal hydration.
Sodium carbonate (Na2CO3) falls in the category of accelerating admixture which mainly impact the acceleration of tricalcium silicate (C3S) hydration. Janotka et al. reported the decrease of long-term compressive strength of carbonated sodium carbonate modified cement [26]. One possible explanation for this phenomenon was the loss of binding capacity resulting from the preferential built-up of calcium carbonate produced during early-age cement hydration.
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Increase in surface hardness and decrease in permeability through intentional carbonation treatments were also observed which were believed to be the results of reduction in porosity thanks to the formation of calcium carbonate in previously empty pore sites. As a consequence, carbonation treatment has then been implemented to increase frost and surface resistance, as well as resistance to atmospheric carbonation and alkali-aggregate reaction [27]. The surface permeability decrease can be used to build a protective barrier to reduce the ingress of water which is harmful for the concrete structure. Carbonation also reduced creep prior to loading but the opposite was observed when the concrete was under loading [28].
2.1.3.2 Durability
The reaction between carbon dioxide and hydration products causes the slow shrinkage of concrete products in service. Intentional carbonation curing following a period of hydration can substantially reduce the shrinkage as was reported by Shideler [10] and Verbeck [14].
Improved hardness and strength, reduced porosity as well as better volume-stability have also been observed for concrete subject to carbonation curing [10]. Toennies [11] reported an average reduction of 43% in early-age drying shrinkage by pure CO2 and of up to 53% by flue gas carbonation. Some recent investigations of Rostami et al. [7, 12] revealed that early carbonation curing showed the effect of eliminating the calcium hydroxide phases from the treated concrete surface which consequently decreased surface air permeability as well as reduced chloride ion migration and enhanced initial sorptivity. It was also found
10 that the carbonation-curing technique increased surface electrical resistivity, sulfate resistance, and freeze-thaw resistance because of the chemically and physically modified microstructure [12].
It was stated that reduction in pH value in pore solution could be induced by early age carbon dioxide treatment, therefore, early age carbonation has been avoided for use in steel reinforced products. Nevertheless, quantitative analysis pertaining to the pH reduction of pore solution by early-age carbonation was absent.
2.2 Weathering Carbonation
2.2.1 Reaction Mechanism and Carbonation Behavior
In the field, concrete is exposed to atmospheric air with a carbon dioxide concentration of approximately 0.03% which reacts with the hydration products of hardened cement paste and forms calcium carbonate. This carbonation after full hydration and under a low CO2 pressure is referred to as weathering carbonation. Carbonation primarily occurs between
Ca(OH)2 and CSH which is given in Equations. 2.3 and 2.4 [23].
Ca(OH) + CO → CaCO + H O (2.3)
C − S − H + 2CO → SiO + 2CaCO + H O (2.4)
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It has been observed that concrete strength could be improved through weathering carbonation. However, the decrease in pH inside the pore solution results in depassivation and then corrosion of rebars and the overall influence of weathering carbonation is detrimental. Also, the enhancement in strength is observed only for a w/c ratio under 0.5 and for a higher w/c ratio, the overall strength decreases.
2.2.2 Factors affecting weathering carbonation
2.2.2.1 Relative Humidity
Due to lack of reported research activities pertaining to the influence of relative humidity
(RH) in early carbonation curing of fresh concrete, the following review focuses exclusively on the optimal RH range (to achieve maximum degree of carbonation) for accelerated carbonation testing of hardened concrete rather than early carbonation curing of fresh concrete.
For a relatively high carbonation rate, the relationship between RH and carbonation is influenced by the specimen size while for slow carbonation or small specimen, it is relatively easy for internal RH to achieve equilibrium with the external controlled humidity
[14]. It is therefore, necessary to dry and stabilize the specimen prior to accelerated carbonation testing to obtain equilibrated internal and external humidity by placing the hardened concrete sample over a specific salt solution inside a desiccator.
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The clarification of the influences of RH on carbonation reactions is an ongoing effort and different ranges of optimum RH in hardened concrete for maximum degree of carbonation have been reported. For example, Mmusi et al. [30] reported an optimal RH level of 80% in a study testing four RH levels between 80% and 95% after weight stabilization at 60% relative humidity. In another investigation conducted by Roy et al. [31], an inconsistent correlation between RH and carbonation depth was observed.
2.2.2.2 Curing Period
The curing period of concrete affects significantly the hydration process. Longer early curing period leads to more adequate hydration, higher paste denseness, and improved carbonation resistance. It has been observed that elongated curing period is beneficial for the reduction of carbonation depth [46, 47, 48]. Concrete subject to air curing shows higher porosity as well as increased connectivity resulting in higher carbonation rate.
2.2.2.3 Carbon Dioxide Concentration
Castellote et al. [32] employed respectively Si Magic Angle Spinning-Nuclear Magnetic
Resonance (Si MAS-NMR), TGA, and XRD to analyse effects of carbon dioxide concentration on the chemical and phase changes of cement paste. The result indicated that, compared to 10% and 100% CO2 treatments, the microstructure of cement paste carbonated with 3% CO2 is closest to that of natural carbonation.
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2.2.3 Threshold pH Value to Initiate Corrosion of Steel in Reinforced
Concrete
The pore solution in concrete is typically alkaline with a pH level greater than 12.5. The alkaline aqueous phase facilitate the formation of a thin oxide layer on the surface of reinforcement that protects the reinforcing steel against corrosion. The integrity and protective effect of this layer is largely dependant on the level of alkalinity. Therefore, the pH value of the pore solution surrounding the reinforcing steel determines the effectiveness of this passivating film on the steel surface [33]. The alkaline aqueous solution is favourable for the protective film thus the pH value of the pore solution must be maintained at a certain level. Completely carbonated concrete had a pH value lower than 9.0 [29].
Parrott et al argued that in order to assess the odds of reinforcement corrosion only regions with pH value lower than 9.0 should be detected [34].
Over the last five decades the research community have made enormous efforts towards the determination of the threshold pH value below which the reinforcement corrosion shows a substantial probability. A study of submerged corrosion of steel in alkaline solutions is utilized to evaluate the behaviour of reinforcing steel in concrete pore solution.
It was reported that a passivating film was usually maintained for a pore solution with a pH value higher than 11.5 because of the absence of certain ions. A pH value of 11.5 is therefore considered as the upper bound of the threshold pH to sufficiently inhibit corrosion of reinforcing steel. In an independent study, alkaline solutions with various pH values were assessed to investigate the effects of pH on corrosion [35]. Again, the pH value of
14
11.5 was found to be sufficient to maintain stable passivity. Therefore, this pH value of
11.5 was widely accepted by the research community of carbonation corrosion.
2.2.4 Durability
2.2.4.1 Porosity
The weathering carbonation of concrete leads to reduction in porosity and permeability
[27]. The formation of calcium carbonate resulting from the reaction of calcium hydroxide causes the concrete volume to grow and the newly formed calcium carbonate is deposited in the empty pores and capillaries thus resulting in reduction in porosity. The overall distribution of pore size also witnesses a shift towards a lower value [36]. On the contrary, when pozzolanic materials are utilized, the permeability of concrete may increase and the strength may decrease unless the materials are sufficiently cured. The decreased porosity and permeability through carbonation leads to the formation of a protective outer layer from aggressive chemicals [30]. Nevertheless, for poorly cured or highly porous concrete, the drop in permeability is insufficient to protect concrete from oxygen and chloride infiltration thus providing little resistance to reinforcement corrosion [27].
De Ceuklaire and Nieuwenburg [37] points out that weathering carbonation yields an increase in solid volume of 11% through the conversion of Ca(OH) to CaCO . An increase in molecular weights of 35% has also been observed. The carbonation process produces silicone gel. A common opinion is that silicone gel would occupy the capillaries in cement
15 paste which consequently leads to refinement of pores. However, Manns et al indicated that since the carbonation process consumes ettringite in the paste, the porosity would increase [38].
Bier studied the pore structures of various types of cement paste before and after carbonation [39]. It was observed that cement paste with high slag content experienced a reduction in porosity after carbonation and cement paste containing 50% furnace slag also showed deterioration in pore structures. The observation was attributed to the dissociation of CSH gel. Further investigation [40] revealed that, the carbonation of CSH that generates silicone gel induced condensation effects resulting in shrinkage and cracking, decrease in volume and pore structure deterioration. Pore size without carbonation ranges between
10nm~50nm, where as that after carbonation ranges between 100nm~1000nm because of the carbonation of CSH gel.
Saeki et al. studied the change in micro-structure of mortar due to carbonation considering water-cement ratio and initial curing condition. They reported a reduction in pore volume in the carbonated portion as well as an increase in denseness.
2.2.4.2 pH of Pore Solution
Carbonation of matured concrete has been given extensive attention since the durability of the concrete is of great concern. The research efforts have been focused on the reduction in pH value which is responsible for the corrosion of reinforcing steel. When completely
16 carbonated, the pH value of the pore solution can be reduced from 12.5 to 13.5 to approximately 8.3 through carbonation reaction [33]. This reduction in pH value leads to the destruction of the protective passive film that typically covers the reinforcing steel and prevents corrosion [41]. Therefore, extensive investigations have been conducted to assess the pH values of pore solution in carbonated cement paste and concrete to evaluate the degree of carbonation-induced corrosion.
Jerga et all [42] have investigated the impact of carbonation on physic-mechanical properties of hardened concrete and observed that cement specimens subjected to enhanced carbon dioxide concentration demonstrated higher degree of carbonation with pH value lower than 8 whereas non-carbonated specimens with normal method of curing showed pH values higher than 11.5.
Comprehensive studies have been conducted on pore solution analysis to evaluate the impact of carbonation on corrosion-induced properties of concrete. In a study conducted by Anstice et al [43], matured cement paste specimens with water/cement ratio of 0.6 were first prepared and then cured in saturated air for 2 weeks and subsequently at 38°C for 12 weeks after remolding. The specimens were then cut to 5 mm-thick discs to be carbonated subject to carbon dioxide concentrations of 100%, 5% and 0.03%. The high-pressure extraction technique was then applied to these samples operating at pressure up to 300
MPa. Non-carbonated samples had pH values of 13.49 whereas carbonated samples with same composition subject to different carbon dioxide concentrations all yielded pH values lower than 11.0. Studies also revealed that higher carbon dioxide concentrations yielded
17 more complete neutralization in the pore solution. The lowest pH value, 7.1, was with sample under carbon dioxide concentration of 100 %, as expected.
Three zones of carbonation based on the corresponding carbonation degree and pH values of pore solution were defined when the concrete carbonation depth was being investigated
[44]. Concrete cylinders were first cure for 4 weeks with water/cement ratio of 0.65 and then subjected to accelerated carbonation at 23 C with 70% RH and 20% carbon dioxide concentration for 8 and 16 weeks. Pulverized powder with sizes ranging from 2 to 5 mm was mixed with water to prepare suspension solution with liquid to water ratio of 0.1. The prepared solution was then placed in a sealed container at 15°C for 3 weeks and filtered prior to measurement. pH electrode was then employed to detect the pH value of the pore solution and it was observed that the pH value of the pore solution correlated with the degree of carbonation. The non-carbonated zone showed a pH value of 11.5 while the carbonated zone demonstrated a reduction in pH value to 9.0 for carbonation degree of
50%. When the carbonation was completed, a pH value lower than 7.5 was observed.
18
Chapter 3 Experimental Program
This chapter describes the experimental procedure used in this research. Precast concretes were prepared following concrete mix design of wet-mix concretes for general use. Two mixture proportions were studied: low cement content, normal mixture with water to cement ratio (w/c) of 0.65; and high cement content, high performance mixture for w/c of
0.40.
3.1 Materials
3.1.1 Cementitious Binder
Experiments were conducted using Canadian CSA-A3001 (Canadian Standard Association)
Type GU Ordinary Portland Cement (Lafarge OPC) were used in preparing concrete samples. Its chemical analysis and mineralogical composition, as determined by XRF, are presented in Table 3.1 and Table 3.2, respectively.
Table 3.1: Chemical composition of Type GU Portland cement
Constituent %
CaO SiO2 Al2O3 Fe2O3 MgO Na2O K2O SO3 CO2 LOI
63.1 19.8 4.9 2.0 2.0 0.85* - 3.8 1.89 1.66
* Na2O alkali equivalent including K2O.
19
Table 3.2: Main compounds of Type GU Portland cement
Compounds %
C3S C2S C3A C4AF
59.3 12.1 9.5 6.2
3.1.2 Coarse and Fine aggregate
Granite coarse and fine aggregates were used to avoid the influence of carbon content in aggregate for thermal analysis of concrete. The fine aggregate had a fineness modulus of
3.0 and the coarse aggregate had a maximum aggregate size of 12mm. The water absorption of fine and coarse aggregate is about 4.3% and 1.6%, respectively.
3.2 Mix design and sample preparation
Precast concretes were prepared following concrete mix design of wet-mix concretes for general use. Two mixture proportions were studied, as presented in Table 3.3: low cement content, normal mixture with water to cement ratio (w/c) of 0.65; and high cement content, high performance mixture for w/c of 0.40. Concrete samples of 10 x 10 x 10 cm were cast, following these two mix designs: Mix I. w/c = 0.65, 14.6 wt.% OPC, 9.5 wt.% water, 75.9 wt.% granite aggregates; Mix II. w/c = 0.40, 19.1 wt.% OPC, 7.6 wt.% water, 73.4 wt.% granite aggregates. To maintain the same slump value of approximately 230mm of Mix I, dosage of superplasticizer (sp) is necessary for Mix II to adjust slump of fresh mixture. The
20
W. R. Grace (AVDA Cast 575) superplasticizer was used and concretes were formed by
30 seconds vibration on a vibration table.
Table 3.3: Mix proportions of concrete
unit Mix I Mix II
w/c ratio - 0.65 0.4
Cement (c) kg/m3 335 452
Water (w) kg/m3 218 181
Coarse granite kg/m3 1060 1060
Fine granite kg/m3 680 680
Concrete kg/m3 2293 2373
SP/c % - 0.8
Slump mm 228 225
3.3 Sample curing
3.3.1 Conventional hydration curing
Conventional hydration curing samples were prepared as reference. A constant water content was maintained over time using this curing method. It is important to assess the influence of water loss due to presetting and carbonation as well as the influence of water spray and subsequent hydration curing. Directly after casting, the samples were sealed to
21 prevent water loss. After 24 hours, the samples were demolded and placed in moisture room (25°C and 100% RH) for another 27 days hydration.
3.3.2 Early carbonation curing
The early carbonation curing process developed previously [46] was adopted in this project.
It was involved four steps: (1) in-mold curing, (2) off-mold preconditioning, (3) carbonation curing and (4) subsequent hydration. For the wet-mix concrete, step 1 provided time for initial setting of fresh concrete because of the high slump. Directly after casting, the samples were subject to air curing at ambient condition (25°C and 60% relative humidity). The time required for this step varies among different mixture proportions. For
Mix I and II, it took about 5 hours, after which, the samples were demolded by removing the four side plates and placed on bottom plates. Demolded concretes were fan dried at a wind speed of 1 m/s in lab condition at a temperature of 25°C and relative humidity of 60%.
Step 2 off-mold preconditioning by fan drying allowed part of the free water to evaporate and made more space for carbon dioxide to penetrate, which was critical to improve carbon uptake. For 40% target loss of initial mixed water, it took approximately 7 hours. After the
12 hours of presetting, samples weight was measured. The difference in mass before and after presetting represented the actually removed free water during steps 1 and 2. In step 3, dried samples were moved to a pressure chamber for carbonation durations of 2 and 12 hours with 5 bars pressure using a CO2 gas of 99.8% purity. The samples were weighed separately as a starting point for the carbonation using mass gain method. Carbonation behaviour was assessed by carbon uptake of the concrete. After completing the prescribed
22 processing conditions, all samples were immersed into water for 24 hours and then underwent subsequent hydration up to 28 days in the moisture room.
Figure 3.1: Subsequent hydration in moisture room
23
Figure 3.2: Carbonation curing setup
A scheme of early carbonation curing setup is shown in Figure 3.2. The apparatus is composed of carbon dioxide gas tank, pressure regulator and transducer, carbonation chamber, digital balance and data acquisition system. The CO2 with a purity of 99.8% was used to simulate the recovered CO2 from industrial exhaust gas in cement plant. Two pressure gauges were fitted to the regulator, one to monitor the tank pressure and the second to indicate the pressure inside the chamber. By adjusting the regulator to the desired gas pressure level, one can ensure a constant pressure to be maintained during carbonation curing. The pressure vessel was equipped with a steel plate as a tray for supporting the samples. Clamping bolts were used to secure the vessel and a lid fitted with a rubber O- ring seal ensured the air-tightness. The mass change due to the CO2 uptake throughout the carbonation process was recorded by a digital balance connected directly to its own data acquisition system. The electrical scale has a weighing capacity of 32kg with a precision of 0.1g and the interval of mass recording over carbonation curing was specified to be 30 seconds.
24
After completing the prescribed preconditioning process, the mass of each sample was measured and then, they were moved into the carbonation chamber which was placed on a digital balance to monitor the mass gain through a mass curve measurement, representing carbon uptake by concrete. As soon as all samples were in the chamber, the balance was zeroed. Carbon dioxide gas of 99.8% purity was injected into the chamber until the pressure reached to 0.5MPa. The interior pressure was regulated to maintain constant by ensuring a continuous supply of CO2 to chamber during the whole carbonation period of 2 hours and
12 hours, respectively. The mass change of the system was recorded over the injection and carbonation period. After carbonation, the water of concretes evaporated during carbonation curing was condensed on the wall of chamber and was collected by absorbent paper, which was used to calculate the CO2 uptake. The weight of each sample was measured again to calculate the carbon uptake using mass gain method.
3.4 Accelerated weathering carbonation
Carbonation curing reduces the amount of calcium hydroxide and decreases the porosity of concrete, which was proved to be beneficial to durability improvement [7, 12]. However, there is concern that the reduction of CH and alkalinity may cause the corrosion of reinforcement inside and promote more carbonation depth in weathering carbonation.
Therefore, an accelerated weathering carbonation test (AWCT) after early carbonation curing and subsequent hydration period was performed in the laboratory for a period of 12 weeks. The 28-day properties were used as baseline. Concrete cubes after 28 days were stored in 25 ± 5 °C, 70 ± 5% RH for 24 hours before they were coated with wax on four
25 sides, leaving two sides exposed. The wax coating ensures that carbon dioxide could diffuse only into the specimens in a one-dimensional mode, which can be used to calculate the CO2 diffusion coefficient of concretes against time. The samples were then set into a sealed weathering carbonation chamber and CO2 gas with concentration of 20 ± 5% was injected. Humidity and temperature were measured in chamber and the condition inside should be maintained 25 ± 5 °C (laboratory temperature), 70 ± 5% RH so that setup of weathering carbonation treatment had to be daily inspected. Samples then be carbonated for 3 months in total with testing period of each 4 weeks. Individual readings of compressive strength, carbonation depth (using phenolphthalein), pH distribution, TGA, and XRD were taken on samples after 4, 8 and 12 weeks.
Figure 3.3: Weathering carbonation apparatus
26
Figure 3.4: Construction weather tracker for testing humidity and temperature
Accelerated weathering carbonation was simulated in an air-tight apparatus, the major components of which included a tank with a volume of 273 litres (60 gallons), compressed carbon dioxide gas cylinder, chemical for controlling humidity, temperature and humidity tester and carbon dioxide analyzer. The AWCT system is shown pictorially in Figure 3.3.
The AWCT chamber measured 889mm in length by 584mm in width and depth, and a fitted polyethylene lid. After the concrete cubes were placed on the steel plates which were stored previously in the chamber, the lid was sealed on the tank. An interface was cut in the center of the lid and fitted with a Plexiglas door for inspecting the inside temperature and humidity on daily base. To allow circulation and maintain a constant humidity and carbon dioxide concentration throughout the tank, a 0.052 m3/s fan was installed inside the chamber. Quantek Instruments Model906 Carbon Dioxide Analyzer was used to monitor the concentration of carbon dioxide in the AWCT chamber. The analyzer had a full
27 detection range of 0 to 100% with a precision of 0.1 %. The valve allowing air flow to the analyzer was opened and then, air flow through the analyzer was generated with its own pump. Carbon dioxide gas of 99.8% purity was connected to the chamber to replenish CO2 rich environment daily. Moreover, a construction weather tracker was placed inside the chamber to check the temperature and humidity. Sodium chloride salt was used as chemical to control the humidity and would be supplied after each measuring period.
3.5 Carbonation regimes
Table 3.4: Carbonation regimes
Testing age Hydration 2hr Carbonation 12hr Carbonation
Early carbonation
1 day A-1 B-1 C-1
28 days A-2 B-2 C-2
Weathering carbonation
4 weeks A-3 B-3 C-3
8 weeks A-4 B-4 C-4
12 weeks A-5 B-5 C-5
Table 3.4 summarizes the carbonation regimes for both early carbonation curing and accelerated weathering carbonation. Each batch for early carbonation curing had 5 samples
28
(3 for compressive strength test and 2 for water absorption test) while batch for weathering carbonation had 3 samples, with two different w/c ratio (0.65 and 0.40), for a total of 114 concrete cubes (60 samples for early carbonation test and 54 samples for accelerated weathering carbonation test). The hydration samples (Batch A) were served as references to 2-hour carbonation (Batch B) and 12-hour carbonation (Batch C). They were tested after
1 day and 28 days respectively to examine the effect of early carbonation curing. The 28- day properties were used as baseline for accelerated weathering carbonation test (AWCT).
The accelerated weathering carbonation tests started after 27-day hydration. Total of 54 concrete cubes were placed in weathering chamber and the weathered samples were to be tested every 4 weeks and up to 12 weeks.
3.6 Evaluation of carbonation curing
3.6.1 Water loss
The water loss during the initial curing and carbonation, and water gain due to the immersion into water for 24 hours were monitored for each step and is expressed in terms of initial mixing water. This can be calculated by Equation. 3.1:
Water loss % = × 100% (3.1)
Where “Water loss” (%) is the percentage of water lost during the process, “Wafter” (g) is the weight of sample after the process, “Wbefore” (g) is the sample weight before and “Winitial” is the weight of initial mixing water. The initial water content was calculated from the
29 measured amount of concrete cubes with w/c = 0.65 and 0.40 respectively in the molds and assumed to be consistent over time.
3.6.2 Carbon uptake
The degree of carbonation behavior is quantified by CO2 uptake by the concrete during early carbonation curing process and is measured by two different methods: mass gain method and mass curve method, as the ratio of the bound CO2 to dry cement content.
3.6.2.1 Mass gain method
The first method to quantify the CO2 uptake is described by Monkman et al [10]. The mass gain during carbonation curing is due to the transformation of carbon dioxide into solid calcium carbonates, therefore, the CO2 uptake in concrete was calculated by comparing the mass of samples before and after carbonation. Evaporated water from the exothermic carbonation reaction was collected using absorbing paper on the internal wall and lid of the carbonation chamber and added to the mass after carbonation since it was initially inside the samples prior to carbonation. This calculation is given by Equation. 3.2:
( ) CO uptake % = × 100% (3.2)
Where “CO2 uptake” (%) is the carbon uptake quantified by mass gain method, “Wafter carbonation” and “Wbefore carbonation” (g) are the sample weight after and before carbonation
30 curing respectively, “Wwater lost” (g) the weight of water lost during carbonation and retrieve on the chamber side and lid, and “Wcement” (g) is the weight of dry cement used in sample.
3.6.2.2 Mass curve method
Mass curve can be obtained using digital balance system by recording mass change of the closed system with respect to exposure time in the entire carbonation process. After concrete samples were placed in the chamber and the lid was closed, the balance was zeroed. The gas was then injected at 5 bars pressure and the mass increase was recorded as a function of time. Since the pressure is maintained as constant, the increase in mass of the system is due to the carbon uptake by concrete. At the end of carbonation curing at which time CO2 was released and the residual mass, was measured. To calculate the carbon uptake only caused by reaction, the system was calibrated by repeating the same test using CO2- nonreactive polystyrene foam samples of the same volume to obtain second residual mass.
The difference between the masses of the two test runs represented the CO2 uptake by concrete (Equation. 3.3). Data collected by mass gain and mass curve methods are two simultaneous measurements from the same process and therefore should be comparable.
Because they are also independent from any carbon content existed before carbonation and are the average of entire batch, the values of carbon uptake by the two methods could be used to verify consistency.
CO uptake % = × 100% (3.3)
31
Where "CO2 uptake" (%) is the CO2 uptake quantified by mass curve method at a time "t".
"Wmass curve" (g) is the whole system mass at a time "t", "Wcalibration" (g) is the mass of the residual CO2 inside the chamber at a time "t" obtained by calibration, and "Wcement" (in g) is the weight of the dry cement used in the sample (constant during carbonation).
3.6.3 Compressive strength test
Concrete cubes were tested for their compressive strength by a MTS rock compression machine at 1 day after initial curing and 28 days after subsequent hydration according to
ASTM C140-03. Each sample was loaded at a rate of 0.5mm/min until failure and the peak axial load was recorded. The compressive strength was then calculated by dividing the peak axial load by the cross-sectional area. To estimate the effect of early carbonation curing, three samples were tested for each batch and the average was presented with deviation.
3.6.4 Permeable voids test
To investigate the effect of early carbonation curing on the permeable voids of concretes, samples cured under both carbonation (2 hours and 12 hours) and hydration processes were tested following general guidelines provided by ASTM C 642, the standard test method for density, absorption, and voids for hardened concrete.
At 1 day (with initial curing) and 28 days (after subsequent hydration), concrete cube samples were dried in an oven at a temperature of 105°C for 24 hours. After removing each specimen from the oven, the samples were allowed to cool down in a laboratory room with
32 a temperature of 24°C and then, the mass of each specimen was determined. Next, all the samples were placed in the oven for a second drying treatment of 24 hours. Room temperature mass of each sample was verified again to ensure the difference of two successive values of mass did not exceed 0.5% of the lesser value. The second measured mass of each specimen was designated as the concrete’s oven-dry mass, “A”.
The oven-dried samples were immersed in distilled water at approximately 21°C for 48 hours and until two successive values of mass of surface-dried sample at intervals of 24 hours show an increase in mass of less than 0.5% of the larger value. Surface-dry each specimen by removing surface moisture with a towel, and the larger values of mass were determined as the concrete’s saturated mass after immersion, “B”.
After immersion, the soaked samples were placed in a water bath, covered with tap water and boiled for 5 hours. They were then allowed to cool by natural loss of heat for not less than 14 hours to room temperature around 24°C. The boiled, cooled, surface-dried mass of the concrete cube was assigned as the saturated mass after boiling, “C”.
Every boiled sample was placed in a steel basket and suspended by a wire in water. The immersed apparent mass “D” was designated by weighing the suspended basket.
By using the values for mass determined in accordance with the procedures described above, the absorption and the volume of permeable voids could be calculated by Equations.
3.4 – 3.6.
33
Absorption after immersion, % = (B − A)/A × 100 % (3.4)
Absorption after immersion and boiling, % = C − A /A] ×100 % (3.5)
Volume of permeable voids, % = [(C − A)/(C − D)] × 100 % (3.6)
Where:
A = mass of oven-dried sample in air, g
B = mass of surface-dry sample in air after immersion, g
C = mass of surface-dry sample in air after immersion and boiling, g
D = apparent mass of sample in water after immersion and boiling, g
3.6.5 Carbonation depth
Carbonation of samples can be detected by using phenolphthalein indicator solution. A solution of 1% phenolphthalein and 70% ethyl alcohol is recommended to measure carbonation depth in hardened concrete by RILEM (1988). Phenolphthalein indicator solution was sprayed on the fresh fracture surface immediately after compressive strength testing. The color pattern was observed and the pictures were taken immediately after indicator spraying. Phenolphthalein solution turns non-carbonated concrete red-purple due to high pH and remains colorless in carbonated concrete where alkalinity is reduced. The average depth of the colorless phenolphthalein region was measured from three points, perpendicular to the two edges of the split face. It was known that such a solution can only
34 indicate whether the pH value is higher than 9.2 or not. The test was used to compare the difference in color patterns between various treatments.
3.6.6 pH value of pore solution
The pH values of the pore solution of the carbonated and hydrated samples were measured to estimate the extent of pH reduction caused by carbonation. The depth of carbonation and pH value at the depth of steel bar in reinforced concretes were important parameters in terms of assessing the risk of carbonation corrosion. Moreover, the pH measurement could make it possible to correlate between the carbonation induced consumption of calcium hydrate and the pH of pore solution in different ages. Therefore, to investigate the effect of carbonation treatment on pH value distribution with respect to depth from surface, a pH meter (Extech PH110) with a flat sensor at its head of 6 mm in diameter was used to measure pH values at different depths: outer surface, 10 mm depth, 25 mm depth and core area. The pH measurement was carried out on the fresh split cross section after the measurement of carbonation depth. An absorptive paper of approximately 10x10mm size was placed on the studied area and then, three drops of distilled water were deposited on the paper. Equilibrium of hydroxyl ions is reached after 15 minutes of extraction by diffusion mechanism. The pH probe measured pH values of the extracted pore solution through absorptive paper. To ensure consistent results, the pH was measured three times at each depth to acquire the average.
3.6.7 X-ray diffraction (XRD)
35
Powder samples were collected from each fractured specimen after the pH measurement and used for both X-Ray Diffraction (XRD). Split face of the concrete specimens was oven- dried over night at 105°C prior to taking samples. The cross-section area of every specimen was separated into three parts: outer surface, 10 – 25 mm depth from surface and core area, the same as pH measurement. The powder of the cement was collected from area of 10 –
25 mm depth using the electric drill with a 0.15mm sieve. With respect to regions of the spilt surface where samples were not taken yet, plastic film was used to avoid carbonation of the fresh surface from CO2 in the ambient air.
Crystalline phases were analyzed using X-ray diffraction (XRD) analysis performed on
Philips PW 1710 Powder X-Ray Diffractometer with Cu Kα radiation to identify the phases such as calcium hydroxide, calcium carbonates, calcium silicates and silica in carbonated and hydrated concretes. Patterns were scanned at a 2 theta angle from 10° to 70° and a
0.02° step size with 0.5 seconds per step.
3.7 Evaluation of weathering carbonation
Compressive strength test was conducted on weathered samples after 4, 8, and 12 weeks according to Section 3.6.3. The pH distribution along fractured section was measured at each weathering carbonation testing age, as outline in Section 3.6.6. X-ray diffraction analysis (XRD) was also conducted at the layer of 10 – 25 mm depth from surface at each weathering carbonation testing age according to Section 3.6.7. The 28-day properties served as baseline.
36
The carbonation depth was measured for weathering carbonation, at each specified test age described in Table 3.4. Since carbon dioxide can only diffuse into the concrete cubes in a one-dimensional mode due to the four-side wax coating (Figure 3.5), the CO2 diffusion coefficient of concretes against time can be calculated to indicate weathering carbonation rate. By applying Fick’s first law, the carbonation coefficient A can be determined by
Equation. 3.7:
x = A t (3.7)
Where “x” is the carbonation depth (mm) at time t, “t” is the exposure time to accelerated weathering carbonation (week), and “A” is the carbonation coefficient (mm/√week). It is a simplified formulation agreed upon by numerous researchers, especially when the carbonation tests were performed under controlled indoor conditions.
One-dimensional diffusion
Figure 3.5: Cross-section of split concrete cube samples after weathering carbonation
37
Over the weathering carbonation period, the thermal decomposition analysis was performed to estimate the carbon content on the area of 10 – 25 mm depth away from the surface and its correlation with carbonation depth and pH value. Due to the use of aggregate in concrete sample, there was only small quantity of cement paste; therefore, it was not feasible to use Thermogravimetric analysis (TGA). Instead, a laboratory furnace was employed for pyrolysis analysis of large concrete samples with a mass of approximately
25 g for each collected from 10 – 25 mm depth from the surface. Concrete chunk samples were cut by concrete saw and heated up to 105, 550 and 1000°C to quantitatively measure the corresponding evaporable water, bound water in hydration products, and carbon dioxide due to the decomposition of calcium carbonates, respectively. The mass at each temperature was recorded. As an indicator of weathering carbonation degree, carbon increase in concrete was compared among hydration, 2-hour pre-carbonation and 12-hour pre-carbonation. The carbon content could be calculated by Equation. 3.8:
℃ ℃ CO2 content (%) = × 100% (3.8)
The cement mass in Equation (3.8) could be estimated by concrete chunk mass multiplied by 0.146 and 0.191 for concretes with w/c = 0.65 and 0.40, respectively, which was the cement content in concrete.
38
Chapter 4 Results and discussion
This chapter presents the results and discussion on carbonation performance of precast reinforced concrete, and its effect on concrete resistance to weathering carbonation. The chapter is divided into two parts. Part 1 presents the results of carbonation behaviour of concrete immediately after carbonation and 28 days after subsequent hydration. Part 2 presents the results on accelerated weathering carbonation. Water loss, carbon uptake and water absorption tests were conducted immediately after early carbonation curing as well as after subsequent hydration. For all batches of concrete cubes subject to both early carbonation curing and accelerated weathering carbonation, compressive strength, qualitative depth of carbonation, pH value distribution, XRD and TGA were recorded.
Especially, the 28-day properties were used as the baseline for the evaluation of accelerated weathering carbonation. Carbonation curing durations of 2 hours and 12 hours were tested in order to assess the effect of exposure time on carbonation. Two mix designs (w/c = 0.65 and 0.40) were studied to determine the optimal mixture proportion for precast reinforced concrete subject to carbonation curing. Summaries of results and detailed experimental data are presented throughout this chapter.
4.1 Early carbonation curing behaviour of concrete
4.1.1 Carbonation Behavior
The water content of the concrete samples at different steps is presented in Table 4.1. The preconditioned samples with initial curing including 5h in mold and 7h off-mold curing
39 lost about 40% of their mixing water. This initial curing was critical for early carbonation curing to remove free water and facilitate carbon dioxide diffusion. Water evaporated during carbonation was in a range of 2-5% which was much less than the water removed by preconditioning. It was obvious that longer carbonation period generated more water loss by the reaction and higher water-to-cement ratio concretes provided more permeable voids for carbon dioxide to penetrate which could also have more evaporated water over carbonation. The water loss was compensated by immersing samples into water for 24 hours after carbonation. The hydrated samples were sealed and therefore retained the initial mixing water. The water content in concrete played a critical role in late strength development.
Table 4.1: Water content (%) of concrete based on initial mixing water
After Curing After After Water w/c initial Final condition casting carbonation added curing Not Hydration 100 100 0 100 applicable
2h 0.65 100 60.3 57.9 42.1 100 carbonation
12h 100 59.7 55.4 44.6 100 carbonation
Not Hydration 100 100 0 100 applicable
2h 0.4 100 59.1 56.9 43.1 100 carbonation
12h 100 59.1 55.4 44.6 100 carbonation
40
To evaluate the carbonation degree, the carbon dioxide uptake was obtained by mass gain and mass curve method. Mass curves of the two concrete cubes during 12-hour carbonation are presented in Figure 4.2. For both mix designs, the rate of carbonation reaction was not constant over time while the reactions were fast in first two hours, during which time more than 50% of the carbon dioxide were reacted with concrete if the uptake at 12 hours was taken as reference. The reactions then continued at a reduced rate up until 12 hours due to the consumption of calcium silicates and the formation of calcium carbonates on surface, therefore, carbonation curing was indicated to follow a diffusion-controlled reduced-rate reaction model. Carbon uptake was summarized in Table 4.1 and the values by mass gain and mass curve method were comparable and self-verified. The values calculated by mass gain method were generally slightly smaller because the evaporated water due to exothermic carbonation reaction could not be completely collected in Equation. 3.2. In average, the CO2 uptake of Mix I (w/c=0.65) reached up to 10.6% and 20.8% after 2-hour and 12-hour carbonation respectively while, for Mix II (w/c=0.4), 4.9% and 9.8%, relative to the dry cement content used in the concrete. The samples prepared with w/c = 0.65 measured a CO2 uptake twice more than the samples prepared using w/c = 0.40. With higher water-to-cement ratio, samples were more carbon dioxide reactive although they had lower cement content. It was due to the more void spaces created by presetting and drying which improved the penetration of carbon dioxide on the surface of samples.
41
25
20
15
10 CO2 (%)Uptake
5 w/c = 0.65
w/c = 0.40 0 0 2 4 6 8 10 12 14 Reaction Time (h)
Figure 4.1: Mass curves of concrete during 12-hour carbonation
Table 4.2: Carbon uptake of concrete
Mass gain Mass curve Average w/c Curing condition (%) (%) (%)
2h carbonation 10.7±0.5 10.5 10.6 0.65 12h carbonation 20.5±1.2 21.1 20.8
2h carbonation 4.8±0.3 5.0 4.9 0.40 12h carbonation 9.4±1.0 10.2 9.8
42
4.1.2 Compressive strength test
The compressive strength of concretes subjected to different curing schemes are summarized in Figure 4.2. Carbonated concretes were in-mold cured for 5 hours and fan- dried for additional 5 hours to achieve target 40% of initial mixed water removal, followed by 2-hour and 12-hour carbonation respectively at 5 bars. The hydration batch was served as reference. They were tested 1 day after casting and 28 days after subsequent hydration.
For both concretes, the 1-day strength was considerably increased by 12-hour carbonation.
Compared with the reference of hydration strength, Mix I was able to increase by 43.4% and Mix II by 17.8%. However, the increase of strength was only about 3.5% after 2-hour carbonation for both mixtures. The high strength increase was due to high carbon uptake after longer carbonation period. For longer carbonation time, concretes with higher w/c ratio had more free spaces for carbon dioxide to penetrate and then react, thereafter, the percentage of strength increase was significantly larger in this mixture. Due to a shorter duration of carbonation, although higher water-to-cement ratio generated more CO2 uptake, cement content accounted for a more critical role on strength gain. It was also interesting to notice that the 1-day compressive strength of Mix II could reach up to around 40 MPa while Mix I only stayed in a range of 11- 16 MPa, indicating that high cement content and low w/c ratio were critical for early strength gain.
The 28-day compressive strength of concretes after subsequent hydration in moisture room was also shown in Figure 4.3. For Mix II concrete, carbonated samples still showed higher compressive strength than the hydration reference, nevertheless, the increase rate largely reduced comparing to that of 1-day strength. The strength increase was 3.6% for 12-hour
43 carbonation curing and 0.6% for 2-hour carbonation curing. However, for Mix I concrete, a slightly lower strength of the carbonated samples was observed in comparison to the hydration reference, revealing that the surface part of concrete had been completely carbonated and could not be recovered even after subsequent hydration with 100% water compensation. It could be also observed that the 28-day compressive strength of Mix I was yet lower than the 1-day strength of Mix II, which again confirmed that w/c ratio played a main decisive role on strength. In conclusion, to maximize the degree of carbonation and consider the strength gain at early age, it is better to have a concrete mix design with reasonable lower w/c ratio subject to longer carbonation curing.
80
69.7 70 Hydration 2h carbonation 12h carbonation 67.3 67.7
60
50 41.7 40 35.4 36.6 31.5 28.8 28.5 30 Compressive strength strength Compressive (Mpa) 20 16.2 11.3 11.7 10
0 I-1 day I-28 days II-1 day II-28 days Sample batch
Figure 4.2: Compressive strength of concretes subject to early carbonation curing
44
4.1.3 Permeable voids test
To investigate the effect of early carbonation curing on the total voids volume of concretes, samples cured under both carbonation (2 hours and 12 hours) and hydration processes were tested following general guidelines provided by ASTM C 642. This test intended to find out if the total permeable voids volume could be decreased by carbonation curing. The lower reading of permeable voids volume of concrete indicated a higher resistance of water penetration.
The volume of permeable voids after certain curing conditions is presented in Table 4.3.
Two samples for each batch were tested 1 day after casting and 28 days after subsequent hydration, and the values were averaged. In 24 hours right after carbonation curing, for both mixtures, the hydration reference consistently exhibited higher volume of permeable voids than the carbonated samples. Mix I measured a percentage decrease of voids volume of 5.8% for 2-hour carbonation curing and 19.9% for 12-hour while Mix II with a decrease of 4.8% for 2-hour and 10.5% for 12-hour. It could be observed that, with longer carbonation duration, concrete showed a higher percent reduction of permeable voids volume. It was caused by the fact that more calcium hydroxide was reacted with carbon dioxide and transformed into solid calcium carbonates which was proved to be beneficial to decrease the permeability and porosity of concrete. Even though the percentage drop of voids volume was considerable in concretes with higher w/c ratio, those concretes still had larger volume of voids. Again, it suggested that w/c ratio played a more important role in affecting the water absorption.
45
Similarly, carbonation curing was also found to reduce the permeable voids volume after
27-day subsequent hydration in moisture room according to Table 4.3. With water compensation, hydration reaction continued and all the samples were shown to be denser.
In summary, carbonation curing could slow down the water movement through concrete densification, and the resistance to water penetration of carbonated concretes could be indirectly proved to be increased.
Table 4.3: The volume of permeable voids (%)
w/c Curing condition 1 day 28 days
Hydration 17.1 15.8
0.65 2h carbonation 16.1 15.0
12h carbonation 13.7 12.9
Hydration 12.4 10.8
0.4 2h carbonation 11.8 10.4
12h carbonation 11.1 9.9
46
4.1.4 Carbonation depth
Using phenolphthalein indicator solution, the depths of carbonation of concrete cubes at 1 day after casting and 28 days after subsequent hydration are summarized in Table 4.4.
Moreover, typical pictures of depth of carbonation are shown in Figure 4.3 – Figure 4.6.
The hydration samples were served as reference shown in Figure 4.7. After 12 hours of carbonation, the samples prepared with w/c = 0.65 measured a CO2 uptake of 20.8% by cement mass and a penetration depth of 12.1 ± 1.1 mm and, there was a distinguishable colorless outermost surface surrounding an inner core region of purple. While the samples prepared using w/c = 0.40 recorded a carbon dioxide uptake of only approximate 9.8 wt.% cement and a depth of 2.6 ± 1.7 mm, the border between colorless and purple region was faint. Similarly, 2-hour carbonated samples prepared using w/c = 0.4 displayed more CO2 penetration resistance than ones prepared using w/c = 0.65, where 2 hours of carbonation yielded 1.3 ± 0.8 mm of penetration for the former (with 4.9% carbon uptake) and 5.2 ±
0.9 mm for the latter (with 10.6% carbon uptake). Almost no colorless region was able to be identified for 2-hour carbonation in concretes with w/c = 0.40. It could be conclusive that with lower water-to-cement ratio, concretes had more cement content; even though with the same CO2 uptake, a less carbonation depth was found, indicating a more promising mix design against excessive CO2 permeation. While, for concretes with the same w/c ratio, it revealed that the colorless region was dependent on the carbon dioxide uptake through carbonation curing.
47
After the subsequent 27-day hydration, it was observed that in concretes prepared using w/c = 0.40, this led to considerable reduction of colorless area and the border between colorless and purple region became faint (for both 2-hour and 12-hour carbonation curing).
However, for Mix I, no remarkable difference could be found following further hydration at the same age. Especially for 12-hour carbonation batch, some colorless spots still remained. It demonstrated that the surface part of carbonated concretes using w/c = 0.65 had been completely carbonated and could not be recovered as shown with phenolphthalein solution indicator. Therefore, further tests for the subsequent hydration of concretes in the form of pH value distribution and carbon content were necessary.
Table 4.4: Carbonation depth of concrete due to early carbonation curing
w/c Curing condition 1 day (mm) 28 days (mm)
Hydration 0.0 0.0
0.65 2h carbonation 5.2 ± 0.9 5.0 ± 1.1
12h carbonation 12.1 ± 1.1 11.8 ± 1.2
Hydration 0.0 0.0
0.40 2h carbonation 1.3 ± 0.8 0.0
12h carbonation 2.6 ± 1.7 0.0
48
(a) After 12-hour carbonation curing
(b) After 2-hour carbonation curing
Figure 4.3: Qualitative depth of carbonation of Mix I concrete after carbonation curing
49
(a) After 12-hour carbonation curing
(b) After 2-hour carbonation curing
Figure 4.4: Qualitative depth of carbonation of Mix II concrete after carbonation curing
50
(a) After 12-hour carbonation curing and 27-day subsequent hydration
(b) After 2-hour carbonation curing and 27-day subsequent hydration
Figure 4.5: Qualitative depth of carbonation of Mix I concrete after carbonation curing
and subsequent hydration up to 28 days
51
(a) After 12-hour carbonation curing and 27-day subsequent hydration
(b) After 2-hour carbonation curing and 27-day subsequent hydration
Figure 4.6: Qualitative depth of carbonation of Mix II concrete after carbonation curing
and subsequent hydration up to 28 days
52
Figure 4.7: Qualitative depth of carbonation of concrete after 28-day hydration
4.1.5 pH value of pore solution
The distribution of pH value from surface to core in carbonated concrete after early carbonation curing and after 27-day subsequent hydration together with references of only hydration are presented in Figure 4.8 – Figure 4.11. For Mix I concrete, the pH values of the surface after 2-hour and 12-hour carbonation curing were reduced to 10.5 and 9.8 respectively, which depended upon the quantity of carbon dioxide uptake through carbonation curing. As described in 4.1.4, the depth of carbonation of 12-hour carbonated concrete was 12.1 mm; correspondingly, the pH of this concrete at a distance of 10 mm from the surface was 10.5, much lower than that of hydrated reference (12.1 ± 0.3) and 2- hour carbonated samples (12.0 ± 0.4). The pH values of concrete after either 2-hour or 12- hour early carbonation at more than 25 mm away from the edge were almost equal to those of the reference samples and their pH values were over 12.0, which indicated that these regions were not affected by early-age carbonation. Similarly, for Mix II concrete, the pH values of the surface on 2-hour and 12-hour carbonated concretes were decreased to 11.1 53 and 10.6 respectively due to the transformation of calcium hydroxide to calcium carbonate, which was consistent with carbon uptake. Moreover, the pH of both carbonated concretes could be preserved above 11.5, very close to those of hydrated samples, except for the outer
10 mm surface layer. It could be observed that the distributions of pH value from surface to core in all carbonated concrete cubes after 1 day were somehow corresponding to carbonation depth determined using phenolphthalein indicator solution.
After 27-day subsequent hydration, for Mix I concrete, carbonated concrete samples for both 2 hours and 12 hours did not see any further pH retrieval, consistent with the patterns of carbonation depth. While, for both carbonated Mix II concretes, the carbonation depth disappeared and the pH value within first 10 mm layer was increased to approximately 12.0 which was close to that at core and beneficial to the reinforcement protection. During the subsequent hydration, strength also increased at the same time in both carbonated and hydrated samples (Figure 4.2), revealing that subsequent hydration was not impeded by early carbonation treatment.
As previously assuming that the area of 10 – 25 mm from the surface represented the location of steel reinforcement, the pH value over this region should be achieved to remain over the depassivation threshold (above 11.5) through controlled carbonation curing.
Therefore, the results above suggested that early carbonation curing was not likely detrimental with regard to carbonation induced corrosion in precast concrete with reinforcing steel with certain control of carbonation duration and mix design. Furthermore, the diffusion resistance would be improved by densified surface to prevent weathering
54 carbonation in service.
14.0
Hydration 2h carbonation 12h carbonation 13.0 12.8 12.7 12.412.4 12.5 12.0 12.112.0 12.1 12.0
11.0 10.5 10.5 pH 10.0 9.8
9.0
8.0
7.0 0 mm 10 mm 25 mm 50 mm Depth from concrete surface
Figure 4.8: Distribution of pH values of Mix I concrete at 1 day
14.0
Hydration 2h carbonation 12h carbonation 13.0 13.0 12.612.7 12.312.3 12.412.412.3 12.1 12.0 11.7
11.1 11.0 10.6 pH 10.0
9.0
8.0
7.0 0 mm 10 mm 25mm 50 mm Depth from concrete surface
Figure 4.9: Distribution of pH values of Mix II concrete at 1 day
55
14.0
Hydration 2h carbonation 12h carbonation 12.8 13.0 12.612.6 12.512.412.4 12.2 12.0 12.1 12.0
10.9 11.0 10.6 10.2 pH 10.0
9.0
8.0
7.0 0 mm 10 mm 25 mm 50 mm Depth from concrete surface
Figure 4.10: Distribution of pH values of Mix I concrete after 27-day subsequent
hydration
14.0
Hydration 2h carbonation 12h carbonation
13.0 12.812.812.8 12.6 12.512.4 12.512.4 12.2 12.1 12.0
11.1 11.0 10.8 pH 10.0
9.0
8.0
7.0 0 mm 10 mm 25mm 50 mm Depth from concrete surface
Figure 4.11: Distribution of pH values of Mix II concrete after 27-day subsequent
hydration
56
4.1.6 X-ray diffraction (XRD)
The concrete samples formed after 1-day initial curing and following subsequent hydration were examined through X-ray diffraction analysis. The XRD patterns of Mix I and II concretes at the layer of 10 – 25 mm are displayed in Figure 4.12 and 4.13 separately. Due to the use of aggregate, quartz and feldspar had a very strong intensity compared to other materials. In both Mix I and II concretes, after early carbonation curing, both 2-hour and
12-hour carbonated samples showed the peak at 29°, indicating formation of crystals of calcium carbonate, mostly the calcite. Nevertheless, XRD pattern of Mix II concrete revealed calcium carbonate of less intensity, which was likely related to high cement content and demonstrated that the reinforcing steel layer in Mix II concrete was less carbonated. After following 27-day subsequent hydration, the reduction in combined peaks of C3S and C2S indicated the consumption of the silicate phases by both hydration and carbonation. On the other hand, the calcium carbonate generated by carbonation were found to be stable after subsequent hydration. The absence or less intensity of calcium hydroxide phases in both carbonated concretes after 27-day hydration suggested that although the strength gain in subsequent hydration was evident, it was not accompanied by the growth of calcium hydroxide. It was interesting to notice that even the Ca(OH)2 did not increase through subsequent hydration in carbonated concretes, the pH values were increased slowly with time (Figure 4.8 – 4.11), revealing that the alkali content in cement could also play a part.
57
3 4 2 1 28d 12C 3 5 3 1 3
3 1 4 2 1 3 5 28d 2C 3 1 3 3 1 1 4 28d H 3 3 1 Intensity 3 3 3 4 1 2 1 3 5 1d 12C 3 1 3 5 3 4 2 5 1 1d 2C 1 3 3 1 3 5 3 4 1 1d H 1 3 5 1 3
10 15 20 25 30 35 40 45 50 55 60 65 70 75 Position ( 2 theta)
Figure 4.12: XRD patterns of Mix I concrete after initial curing and subsequent hydration
1: Ca(OH)2; 2: CaCO3; 3:Quartz; 4: Feldspar; 5: C3S, C2S
58
3
4 2 1 3 5 1 3 3 28d 12C 1 3 5 3 4 1 2 5 1 28d 2C 3 1 3 5 3 4 1 3 1 28d H 3 1 3 3
Intensity 4 2 1 3 5 1 1d 12C 3 1 3 5 3 4 5 1 1 2 3 1d 2C 3 3 1 3 5 3 4 1 5 1 1d H 3 1 3
10 15 20 25 30 35 40 45 50 55 60 65 70 75 Position ( 2 theta)
Figure 4.13: XRD patterns of Mix II concrete after initial curing and subsequent
hydration
59
4.2 Effect of early carbonation curing on weathering carbonation
4.2.1 Compressive strength test
The results of compressive strength of Mix I and Mix II concretes subject to weathering carbonation after initial curing and subsequent hydration in moisture room are presented in
Figure 4.14 and Figure 4.15 respectively. Samples were tested every 4 weeks up to 12 weeks and the 28-day strength was served as baseline. For Mix I concrete in Figure 4.14, it is obvious that the strength of both pre-carbonated and hydrated samples was increased by accelerated weathering carbonation. The rate of strength gain was not constant due to the different diffusion coefficient of carbon dioxide and it exhibited a higher rate in first 4 weeks, during which a percentage increase of strength of around 27.9%, 24.3% and 21.1% in hydrated, 2-hour carbonated and 12-hour carbonated concretes separately. At the age of
12-week weathering carbonation, the strength gain of those concretes was approximately
40% in a comparison to the 28-day strength, indicating that the increase of strength in the first 4 weeks accounted for more than 50% in total. This result was likely related to the carbon dioxide uptake during the accelerated weathering carbonation which would generate deeper carbonation front and furthermore reduce the pH value in concrete.
Therefore, the qualitative depth of carbonation and pH value in the area of steel reinforcement location should be investigated in order to detect if carbonation induced corrosion on reinforcement would happen in pre-carbonated and hydrated concrete prepared using w/c = 0.65 during weathering carbonation. The resistance to weathering carbonation of concrete cured by early-age carbonation was a major concern in this
60 research.
It was shown in Figure 4.15 that the effect of weathering carbonation on compressive strength for the Mix II concrete in differently prepared batches were not apparent. Strength recorded indicated that a high w/c (0.65) consistently yielded lower strength than low w/c of 0.40, for the differently cured samples (2-hour carbonated, 12-hour carbonated, and hydrated). Moreover, it was evident that both early carbonated and hydrated samples showed weathering strengths in the same order of magnitude. In summary, the weathering carbonation did not alter significantly the compressive strength of precast concrete cured by early-age carbonation.
90
80 Hydration 2h carbonation 12h carbonation
70
60
50 42.0 42.9 40.3 40.5 38.938.4 39.6 40 35.834.5 31.5 28.828.5 30 Compressive strength strength Compressive (Mpa)
20
10
0 0 4 8 12 Time (week)
Figure 4.14: Compressive strength of Mix I concrete subject to weathering carbonation
61
90.0
Hydration 2h carbonation 12h carbonation 80.0
71.1 71.3 72.0 71.9 72.0 72.4 69.7 69.0 68.8 69.6 70.0 67.3 67.7
60.0
50.0
40.0
30.0 Compressive strength strength Compressive (Mpa)
20.0
10.0
0.0 0 4 8 12 Time (week)
Figure 4.15: Compressive strength of Mix II concrete subject to weathering carbonation
4.2.2 Carbonation depth due to weathering carbonation
Carbonation depth due to weathering carbonation was measured by phenolphthalein indicator solution on split cross section. The results of Mix I concrete are shown in Figure
4.16. The 28-day properties were served as baseline and the initial carbonation front of 2- hour and 12-hour carbonated concretes in Mix I were 5.0 mm and 12.1 mm respectively.
After 12 weeks of weathering carbonation, the 12-hour pre-carbonated samples experienced a higher rate of penetration (17.6 mm) than the 2-hour early-age carbonated and hydrated reference samples (w/c = 0.65), which both showed a displacement for the carbonation front of approximately 13.3 mm. Even with initial carbonation depth of 5.0 mm, the 2-hour early carbonated samples finally caught up the hydrated references and
62 experienced almost the same carbonation front. It suggested that the rate of weathering carbonation was different for the three cured samples. Using the subtraction of 28-day data, the carbonation depth generated in weathering carbonation was only 8.4 mm and 5.5 mm in 2-hour and 12-hour pre-carbonated concretes separately, which was much smaller than that of reference samples (13.3 mm). The results above indicated that for concretes with high water-to-cement ratio, the early carbonation curing was beneficial to slow down the rate of penetration over accelerated weathering carbonation period. However, considering as a whole, since the early-age carbonation depth remained after 27-day subsequent hydration, the total carbonation depth reached 17.6mm and steel reinforcement should be placed further away from the carbonation zone.
As shown in Figure 4.17, for Mix II concrete, the carbonation depth was much smaller than
Mix I concrete due to low water to cement ratio. In addition, both early-age carbonated and hydrated reference samples showed weathering carbonation depth in almost the same order of magnitude. After 12 weeks weathering carbonation, the carbonation depth was 4.0 mm,
3.8 mm and 3.9 mm in normally hydrated, 2-hour carbonated and 12-hour carbonated concretes respectively, which showed no difference among these three curing conditions.
It revealed that the early carbonation curing on concretes prepared using w/c = 0.40 did not generate more carbon dioxide penetration during weathering carbonation.
63
20.0 Hydration 18.0 2h carbonation 17.6
12h carbonation 16.5 16.0
14.4 13.4 14.0
12.0 12.1 13.3 11.3 10.0 10.0
8.0 7.7 Carbonationdepth(mm) 6.0 6.2 5.0 4.0
2.0
0.0 0.0 0 2 4 6 8 10 12 14 Time (week)
Figure 4.16: Weathering carbonation depth of Mix I concrete
20.0
Hydration 18.0 2h carbonation 16.0 12h carbonation
14.0
12.0
10.0
8.0
Carbonationdepth(mm) 6.0
4.0 3.3 4.0 2.0 3.9 2.9 3.1 3.8 2.0 1.5 1.8
0.0 0.0 0 2 4 6 8 10 12 14 Time (week)
Figure 4.17: Weathering carbonation depth of Mix II concrete
64
Typical pictures of carbonation depth of Mix I concrete due to weathering carbonation are shown in Figure 4.18 – Figure 4.21.
(a) 12-hour carbonation (b) 2-hour carbonation (c) Hydration
Figure 4.18: Qualitative depth of carbonation of Mix I concrete after
carbonation curing and subsequent hydration up to 28 days
(a) 12-hour carbonation (b) 2-hour carbonation (c) Hydration
Figure 4.19: Qualitative depth of carbonation of Mix I concrete after 4-week
weathering carbonation
65
(a) 12-hour carbonation (b) 2-hour carbonation (c) Hydration
Figure 4.20: Qualitative depth of carbonation of Mix I concrete after 8-week
weathering carbonation
(a) 12-hour carbonation (b) 2-hour carbonation (c) Hydration
Figure 4.21: Qualitative depth of carbonation of Mix I concrete after 12-week
weathering carbonation
66
Typical pictures of carbonation depth of Mix II concrete due to weathering carbonation are shown in Figure 4.22 – Figure 4.25.
(a) 12-hour carbonation (b) 2-hour carbonation (c) Hydration
Figure 4.22: Qualitative depth of carbonation of Mix II concrete after
carbonation curing and subsequent hydration up to 28 days
(a) 12-hour carbonation (b) 2-hour carbonation (c) Hydration
Figure 4.23: Qualitative depth of carbonation of Mix II concrete after 4-week
weathering carbonation
67
(a) 12-hour carbonation (b) 2-hour carbonation (c) Hydration
Figure 4.24: Qualitative depth of carbonation of Mix I concrete after 8-week
weathering carbonation
(a) 12-hour carbonation (b) 2-hour carbonation (c) Hydration
Figure 4.25: Qualitative depth of carbonation of Mix I concrete after 12-week
weathering carbonation
68
The carbonation depth of Mix I and Mix II concretes was plotted against square root weathering time in Figure 4.26 and Figure 4.27 separately to indicate weathering carbonation rate from calculating the CO2 diffusion coefficient. The carbonation coefficient A can be determined by Equation (3.4):
x = A t (3.4)
For concrete prepared with w/c = 0.65, the linear regression yielded carbonation coefficient of 3.9 mm/√weeks for hydrated references, 2.4 mm/√weeks for 2-hour carbonated samples, and 1.5 mm/√weeks for 12-hour carbonated samples. Again, this results showed that the early carbonation curing helped to reduce the rate of penetration in this mix design concrete during weathering carbonation time. The longer carbonation curing concrete experienced, the more resistance to carbon dioxide diffusion it had in weathering carbonation. For concrete using w/c = 0.40, the carbonation coefficient determined by the linear regression was of close value of more or less 1.1 mm/√weeks for the differently cured samples
(hydrated, 2-hour carbonated and 12-hour carbonated). No difference among the three curing conditions demonstrated that the early carbonation curing on concrete with low w/c ratio did not produce more carbonation front over weathering carbonation period. In conclusion, it seemed that the effect of early carbonation curing on carbon dioxide diffusion of both two mix design concretes is beneficial.
69
20.0 Hydration 18.0 2h carbonation 12h carbonation 16.0
14.0 A = 1.5 mm/√weeks 12.0
10.0 A = 2.4 mm/√weeks 8.0
Carbonationdepth(mm) 6.0
4.0 A = 3.9 mm/√weeks
2.0
0.0 0 0.5 1 1.5 2 2.5 3 3.5 4 Square root of time (√weeks)
Figure 4.26: Weathering carbonation coefficient of Mix I concrete
20.0 Hydration A = 1.2 mm/√weeks 18.0 2h carbonation A = 1.1 mm/√weeks 16.0 12h carbonation A = 1.1 mm/√weeks
14.0
12.0
10.0
8.0
Carbonationdepth(mm) 6.0
4.0
2.0
0.0 0 0.5 1 1.5 2 2.5 3 3.5 4 Square root of time (√weeks)
Figure 4.27: Weathering carbonation coefficient of Mix II concrete
70
4.2.3 pH value of pore solution
The pH values at different depths from the surface on Mix I concrete were compared against the time during weathering carbonation in Figure 4.28 – Figure 4.31. It is shown that the surface pH in normally hydrated sample was higher at beginning but became close to other two early carbonated samples after 4 weeks, and remained around 9.0 after 12 weeks. The pH value at 10 mm away from edge reduced in all samples and was about 10.0 after 12-week weathering carbonation. However, pH at 25 mm from surface dropped slightly, keeping above 11.0 in all batches. The results again demonstrated that weathering carbonation reaction had faster rate in hydrated concretes than in pre-carbonated concretes.
The pH value at core stayed higher than 12.0, indicating that carbon dioxide had not diffused to this area yet. As assuming the region between 10 – 25 mm from the surface was the location of steel reinforcement, the results also showed that early carbonation cured concrete prepared with w/c = 0.65 might be vulnerable to steel bar corrosion due to weathering carbonation because the pH value could not be recovered after subsequent hydration.
71
14.0
On surface 13.0
12.0
11.0 pH 10.0
9.0
Hydration
8.0 2h carbonation 12h carbonation
7.0 0 2 4 6 8 10 12 14 Time (week)
Figure 4.28: pH at surface of Mix I concrete in weathering carbonation
14.0 10 mm from surface 13.0
12.0
11.0 pH 10.0
9.0
Hydration
8.0 2h carbonation
12h carbonation 7.0 0 2 4 6 8 10 12 14 Time (week)
Figure 4.29: pH at 10 mm depth of Mix I concrete in weathering carbonation
72
14.0
25 mm from surface 13.0
12.0
11.0 pH 10.0
9.0
Hydration
8.0 2h carbonation
12h carbonation 7.0 0 2 4 6 8 10 12 14 Time (week)
Figure 4.30: pH at 25 mm depth of Mix I concrete in weathering carbonation
14.0 At the core, 50 mm from surface 13.0
12.0
11.0 pH 10.0
9.0
Hydration
8.0 2h carbonation
12h carbonation 7.0 0 2 4 6 8 10 12 14 Time (week)
Figure 4.31: pH at core of Mix I concrete in weathering carbonation
73
Figure 4.32 – Figure 4.35 presented the distribution of pH value from surface to core in
Mix II concrete over weathering carbonation time. The decrease of surface pH value was found in all batches, where pH was approximately 10.0 after 12 weeks weathering carbonation. The pH values at 10 mm or other deeper depths maintained almost the same over this period in both hydrated and carbonation cured concretes, which kept above 11.5.
The pH results of Mix II concretes were corresponding to the carbonation depth, revealing that the early carbonation cured concretes had the same rate of weathering carbonation reaction as the normally hydrated concretes.
14.0
On surface 13.0
12.0
11.0 pH 10.0
9.0
Hydration
8.0 2h carbonation 12h carbonation
7.0 0 2 4 6 8 10 12 14 Time (week)
Figure 4.32: pH at surface of Mix II concrete in weathering carbonation
74
14.0 10 mm from surface 13.0
12.0
11.0 pH 10.0
9.0
Hydration
8.0 2h carbonation
12h carbonation 7.0 0 2 4 6 8 10 12 14 Time (week)
Figure 4.33: pH at 10 mm depth of Mix II concrete in weathering carbonation
14.0
25 mm from surface 13.0
12.0
11.0 pH 10.0
9.0
Hydration
8.0 2h carbonation
12h carbonation 7.0 0 2 4 6 8 10 12 14 Time (week)
Figure 4.34: pH at 25 mm depth of Mix II concrete in weathering carbonation
75
14.0 At the core, 50 mm from surface 13.0
12.0
11.0 pH 10.0
9.0
Hydration
8.0 2h carbonation
12h carbonation 7.0 0 2 4 6 8 10 12 14 Time (week)
Figure 4.35: pH at core of Mix II concrete in weathering carbonation
4.2.4 Carbon content analysis
As an indicator of weathering carbonation degree, carbon content in concrete layer 10 -
25mm from surface was compared among hydrated, 2-hour pre-carbonated and 12-hour pre-carbonated concretes. Table 4.5 and 4.6 summarize the carbon content in Mix I and II concretes over weathering carbonation period. For Mix I concrete, the carbon content in normally hydrated sample was 1.4% before weathering test. The initial carbon content is due to the limestone addition in Portland cement. The 2-hour carbonation and 12-hour carbonation had a carbon content of 3.3% and 7.1% respectively. After 8 weeks, CO2 content in hydration began to be close to other two early carbonated samples, and it
76 increased up to around 20.0% in all differently cured samples after 12 weeks weathering.
The results above indicated that the layer of 10-25mm from surface was heavily carbonated during weathering carbonation. Therefore, for high w/c concrete (Mix I), the reinforcing steel shall be placed to avoid the carbonation zone. For Mix II concrete, the carbon content in layer of 10 – 25 mm depth was rather small due to high strength mixture and high cement content. However, CO2 content increased at almost the same order of magnitude among all differently cured concretes. After 12 weeks weathering carbonation, carbon content was close to 5.0% in all batches, which demonstrated that the early carbonation curing did not generate more carbon content in weathering carbonation. For both Mix I and Mix II concretes, the carbon content was consistent with the results of carbonation depth and pH value.
Table 4.5: Carbon content at layer of 10 – 25 mm depth of Mix I concrete in weathering carbonation (%)
Testing time Hydration 2h carbonation 12h carbonation
0 1.40 3.30 7.10
4 weeks 4.10 8.90 12.60
8 weeks 12.10 14.60 16.80
12 weeks 18.60 20.00 21.60
77
Table 4.6: Carbon content at layer of 10 – 25 mm depth of Mix II concrete in weathering carbonation (%)
Testing time Hydration 2h carbonation 12h carbonation
0 0.90 1.60 2.10
4 weeks 2.90 3.10 3.50
8 weeks 3.90 3.90 4.10
12 weeks 4.70 4.50 5.00
4.2.5 X-ray diffraction (XRD)
Figure 4.36 and 4.37 compare the XRD patterns of Mix I and II concretes respectively at the layer of 10 – 25 mm depth in weathering carbonation. After 12 weeks weathering carbonation, hydrated samples for Mix I showed a peak at 29° indicating the formation of calcium carbonate, and the calcium hydroxide peaks were much shorter. However, for Mix
II concrete, no CaCO3 peak appeared and the Ca(OH)2 were found to be stable in hydration, which again confirmed that concrete with low w/c ratio had a slower penetration of carbon dioxide over weathering carbonation. For Mix I concrete samples, the CH peak present at
18° for hydration reference disappeared in both 2-hour and 12-hour carbonated samples.
Especially in 12-hour pre-carbonated concrete, the CaCO3 peak at 29° had a very strong intensity and that at 47° appeared while the CH peak at 34° was totally disappeared.
Nevertheless, for Mix II concrete, after 12 weeks weathering carbonation, no remarkable
78 difference was observed for both 2-hour and 12-hour early carbonated samples. It was consistent with the previous results that the early carbonation curing was not more detrimental to concrete with high cement content in weathering carbonation.
3 2 4 12w 12C 3 3 3 1 2 3 3 4 2 1 12w 2C 3 1 3 3 4 2 1 12w H 1 3 3 3 1 3
Intensity 3 4 1 3 2 1 3 28d 12C 5 3
3 4 1 1 3 2 1 28d 2C 5 3 3 3 4 1 3 3 28d H 1 3 1 3
10 15 20 25 30 35 40 45 50 55 60 65 70 75 Position ( 2 theta)
Figure 4.36: XRD patterns of Mix I concrete in weathering carbonation
1: Ca(OH)2; 2: CaCO3; 3:Quartz; 4: Feldspar; 5: C3S, C2S
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3
4 2 1 12w 12C 1 3 1 3 3 4 2 5 1 3 12w 2C 1 3 1 3 3 4 1 3 12w H 1 3 1 3 3 Intensity 4 2 5 1 3 3 3 28d 12C 1 3 1 5 3 4 2 5 1 28d 2C 1 3 1 35 3 4 1 28d H 1 3 3 1 3
10 15 20 25 30 35 40 45 50 55 60 65 70 75 Position ( 2 theta)
Figure 4.37: XRD patterns of Mix II concrete in weathering carbonation
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Chapter 5. Conclusions and recommendations
5.1 Conclusions
The effect of early carbonation curing on weathering carbonation of precast reinforced
concrete was studied. Early carbonation curing could reduce pH value immediately after
curing. This reduction could be aggravated by weathering carbonation in service. To
evaluate the carbonation degree, carbonation depth, pH distribution, carbon content and
intensity of calcium hydroxide and calcium carbonate were investigated. Two mix
proportions were studied representing different precast concrete products. Two durations
of early carbonation curing were considered and compared with hydration reference.
Though experimental studies, the following conclusions were drawn:
From early carbonation curing testing:
1) The precast concretes prepared with w/c = 0.65 measured a CO2 uptake twice as high as
the concretes prepared using w/c = 0.40. With higher water-to-cement ratio, concretes are
more carbon dioxide reactive although they have lower cement content. It was due to the
high porosity created by preconditioning and drying which improved the penetration of
carbon dioxide on the surface of samples.
2) Early age compressive strength of 12-hour carbonated samples remarkably exceeded that
of hydrated and 2-hour carbonated samples, which was due to higher carbon uptake after
prolonged carbonation period. After 27-day subsequent hydration, carbonated concretes
prepared with low w/c ratio still showed higher compressive strength than the hydration
reference, nevertheless, the increase rate was largely reduced compared to the 1-day
81
strength. However, a slightly lower strength of the carbonated samples prepared using high
w/c ratio was observed in comparison to the normal hydration.
3) The volume of permeable voids was lower in carbonated samples than that in hydration
reference. With longer carbonation duration, concrete showed a higher percent reduction
of permeable voids volume because more calcium hydroxide reacted with carbon dioxide
and transformed into solid calcium carbonates, which was beneficial to decrease the
permeability and porosity of concrete.
4) Carbonation depth using phenolphthalein solution was found to be deeper in concretes with
higher w/c ratio through longer carbonation time, due to the higher carbon uptake. After
subsequent hydration, the colorless area in concrete with high w/c ratio still remained while
that in concrete with low w/c ratio disappeared.
5) pH values of both carbonated concretes with low w/c ratio as well as 2-hour carbonated
concrete with high w/c ratio remained above 11.5. Therefore, carbonation curing was not
likely to be detrimental to reinforced concrete with certain control of carbonation period
and mix design.
From weathering carbonation testing:
6) The strengths of different carbonation cured and hydration cured samples were comparable
and increased after weathering carbonation.
7) For concretes with high w/c ratio, the carbonation depth, pH value and carbon content
analysis were found to be more affected by accelerated weathering carbonation tests in 12-
hour carbonated concrete than those in 2-hour carbonated and hydrated concretes. Even
early carbonation curing helped to reduce the carbonation coefficient during weathering
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carbonation. As a result, the carbonated reinforced concretes in this mix design was more
vulnerable to weathering carbonation if reinforcing steel is placed 25mm from the surface.
8) For concretes with low w/c ratio, the carbonation depth, carbonation coefficient, pH value
and carbon content of different early carbonated and normally hydrated concretes were
comparable during weathering carbonation period. Therefore, the early carbonation curing
was not more detrimental to precast reinforced concrete in this mix design over weathering
carbonation.
9) Early carbonation does not have apparent effect on the resistance to weathering
carbonation. It neither enhances, nor reduces, the weathering carbonation reaction. The
final degree of carbonation is dependent on the degree of early carbonation curing.
10) Both concretes can be used for carbonation curing treatment. For low water to cement ratio
concretes, the pH reduction at 25mm depth is not significant. Therefore, rebar can be placed
near the surface (as close as 25 mm from surface). However, for high water to cement ratio
concretes, if the same carbonation curing is used, the location of rebar has to be determined
to avoid the pH reduction and the carbonation corrosion.
5.2 Recommendations for future work
Further investigation and studies are still necessary, and the following list identifies some
of the major items:
1) Carbonated concrete with w/c = 0.65 failed to maintain the pH over 11.5 during weathering
carbonation while carbonated concrete with w/c = 0.40 was not more vulnerable to
weathering carbonation. Additional tests can focus on the effect of early carbonation curing
on weathering carbonation of precast reinforced concrete with various w/c ratio.
83
2) Investigation into the weathering carbonation of carbonated precast reinforced concrete
containing mineral admixtures, such as steel slag, blast-furnace slag, fly ash, silica fume
and so on. With wide use of mineral admixtures in concrete, the weathering carbonation of
carbonation cured concrete containing reinforcement should be fully studied too.
3) In this research, only the carbonation-induced corrosion potential by pH measurement of
the carbonation cured concrete was studied, which might not be convincing enough to
apply the carbonation curing technology at the industry level. A full-scale corrosion
research on carbonated concrete is essential.
4) Pure gas carbonation curing at 5 bars was conducted in this study, in order to get a higher
carbon uptake and a lower permeability. Further investigation should be performed to find
the cost-effective concentration and pressure to improve properties and carbon uptake.
84
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