A Performance Based Approach for Durability of Concrete Exposed to Carbonation Emmanuel Rozière, Ahmed Loukili, François Cussigh
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A performance based approach for durability of concrete exposed to carbonation Emmanuel Rozière, Ahmed Loukili, François Cussigh To cite this version: Emmanuel Rozière, Ahmed Loukili, François Cussigh. A performance based approach for durability of concrete exposed to carbonation. Construction and Building Materials, Elsevier, 2009, 23 (1), pp.190-199. 10.1016/j.conbuildmat.2008.01.006. hal-01004926 HAL Id: hal-01004926 https://hal.archives-ouvertes.fr/hal-01004926 Submitted on 14 Oct 2017 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. A performance based approach for durability of concrete exposed to carbonation Emmanuel Rozie`re a, Ahmed Loukili a,*, Francßois Cussigh b a Research Institute of Civil Engineering and Mechanics (GeM), UMR CNRS 6183, Ecole Centrale de Nantes, 1 rue de la Noe¨, BP 92101, 44321 Nantes Cedex 3, France b Direction technique, VINCI Construction France, Nanterre, France Carbonation is a widespread degradation of concrete and may be coupled with more severe degradations. In order to change from prescriptive requirements to performance based specifications for durability, through the equivalent performance concept, it is necessary to find relevant performance tests and indicators. Concrete mixtures of the reported study were designed complying with required binder contents and water–cement ratios to investigate the effects of binder composition, aggregate type and curing conditions. Early drying severely affected performances, as well as cement replacement by low-calcium fly ash. Aggregates had indirect effects on actual water content and curing. Porosity, gas permeability and chloride diffusivity were found as unreliable indicators for carbonation, since they only characterise compactness of concrete. An accelerated carbonation test is proposed as a performance test, as it is sensitive enough and results were consistent with natural carbonation in the studied exposure conditions. Another indicator, based on chloride diffusivity/ initial CaO content ratio, could provide useful data for given aggregate mixes and curing conditions, as it takes into account compactness of concrete and chemical reactivity of binder. Keywords: Durability; Carbonation; Equivalent performance concept; Accelerated test 1. Introduction ation accelerates chloride ingress [1]. Besides, ductility of concrete would decrease with an increase in the degree of Corrosion of steel reinforcement is one of the major carbonation [2]. Therefore carbonation has to be taken into causes of degradation of concrete structures. Initiation of account for designing most of concrete mixtures. corrosion may be due to ingress of chloride ions or carbon- Carbonation is the result of a chemical reaction between ation. Carbonation results in a drop of pH of surface con- carbon dioxide and concrete hydrates, such as portlandite crete, then corrosion of steel bars is likely to occur because Ca(OH)2 and calcium silicate hydrates (CSH), producing they are not passivated anymore. Although carbonation is calcite CaCO3 and water. The mechanism of degradation often considered as less severe than chloride ingress, it is is now well known and models [3–6] describe the propaga- much more widespread, because it involves carbon dioxide tion of the dissolution front, mainly in Portland cement (CO2) from air. Furthermore, carbonation coupled with pastes or concrete. But the recent development of perfor- chloride ingress reduces durability of concrete exposed to mance based specifications for durability of concrete show de-icing salts and marine environments, because carbon- the need for relevant indicators that might be applied to all kinds of binders and would be relatively easy to assess [7– 9]. Most of the experimental studies have dealt with the * Corresponding author. Tel.: +33 240 37 1667; fax: +33 240 37 2535. effect of compactness through variations in water–cement E-mail address: [email protected] (A. Loukili). ratio, as it is a major parameter as far as durability of 1 concrete is concerned. However, in construction practice, Table 1 concrete mixtures are often designed from the maximum Properties of cementitious materials water/binder ratio complying with standards and varying Blaine surface (cm2/g) Cement CEM I 52,5 N Fly ash other parameters such as aggregate and binder types. 3440 3840 As the assessment of life time of a concrete structure Mean diameter (lm) – 24 from a given concrete mixture is still difficult, standards Clinker (%) 98.0 – on concrete often rely on prescriptive specifications, assum- CaO 63.77 2.2 ing that they will result in appropriate durability. In the SiO2 20.21 52.5 new European Standard EN 206-1 [10] on performance, Al2O3 4.18 27.9 production and conformity of concrete, exposure classes Fe2O3 2.22 5.6 are defined. XC exposure classes deal with carbonation. SO3 2.76 0.6 For each exposure class, minimum binder content and maximum water–binder ratio are specified. Minimum strength is also mentioned, and in French standard maxi- Portland cement (C) and 30% fly ash (FA). Properties of mum mineral admixtures contents are also defined. How- cement and fly ash are given in Table 1. Portland cement ever, the new European standard allows performance was substituted by fly ash (FA) according to the maximum based specifications through the equivalent performance FA/(C + FA) ratio allowed by the standard. Two aggre- concept [10–12]. Equivalent performance has to be shown gates mixtures were chosen to take into account the possi- through a comparison with a reference concrete mixture, ble effects of aggregate type on interfacial transition zone, which complies with prescriptive requirements for a given and effects of grading of aggregates on compactness of con- exposure. The comparison may be done through durability crete. Boulonnais sand and gravel are crushed dense lime- tests, provided that they give reliable ranking of perfor- stone aggregates. They have been used in other mances of concrete exposed to a given degradation, such laboratory studies [13] and the main feature of the sand as carbonation. This study is part of a project aimed at is a high proportion of fine elements (6.7% below 63 lm). designing a methodology to implement the equivalent per- The other type is the sea aggregates which are washed formance concept to every exposure class. This study deals and have a relatively high porosity in terms of Water with reference concrete mixtures, as they are not defined in absorption, as shown in Table 2. The design of the mix- the standard. The purpose is to investigate the variability of tures was performed by the Faury method, adjusting the performances of concrete mixtures which comply with pre- mixture curve (including cement and admixture) with a ref- scriptive requirements, i.e. threshold values of standard, in erence granulometric curve [14]. The second aggregate mix order to provide data to choose reference concrete mix- made of sea aggregates lacks fine elements between 63 and tures, taking into account the effect of mineral admixtures, 315 lm. Fine siliceous sand (0/1 mm) was used in B8 con- aggregate types and curing conditions. The experimental crete mixture to compare the effect of mineral admixture study includes assessment of durability indicators and mer- (fly ash) and grading of aggregates. Two batches of each concrete were made and cast in cury intrusion porosimetry on hardened concrete before 3 degradation, accelerated carbonation test and natural car- 70 Â 70 Â 280 mm moulds for accelerated carbonation bonation, in order to find performance indicators of con- test and Ø 110 Â 220 mm moulds for other tests. After crete exposed to carbonation. 24 h of sealed curing, the first set of specimens was cured under water for 28 days and will be referred to as ‘‘24 h – 2. Experimental program Water”. The second set of specimens was sealed cured until concrete reached 50% of the minimum 28-day characteris- 2.1. Materials, mixture proportions and curing tic strength from standard NF EN 206-1, which is 20 MPa for XC1–XC2 classes and 25 MPa for XC3–XC4 classes. In European standard EN 206-1 [10], XC exposure clas- Then they were cured in a room at a constant temperature ses classifies the environmental conditions from low (XC1) of 20 °C and a constant relative humidity of 50% RH for 28 to high (XC4) risk of carbonation, depending on carbon days. They will be referred to as ‘‘50%”. dioxide concentration and saturation degree of concrete. Only two sets of prescriptive specifications are defined in the French standard NF EN 206-1. The two levels of min- Table 2 imum strength of concrete (C20/25 and C25/30), maximum Properties of aggregates water–binder ratio (0.65 and 0.60) and minimum binder Boulonnais Sea aggregates Fine sand content (260 and 280 kg/m3) respectively correspond to aggregates XC1–XC2 and XC3–XC4 exposure classes. Two series of Sand Gravels Sand Gravels five and three concrete mixtures have been designed, keep- Passing <63 lm (%) 6.7 0.5 1.4 0.1 0.0 ing these values constant, and varying binder and aggre- Passing <125 lm (%) 15.7 0.5 2.6 0.1 7.1 gates types. Two binders were used, namely: Portland Water absorption (%) 0.8 0.7 0.6 2.1 0.2 3 cement CEM I 52.5 N and another binder made of 70% Density (kg/m ) 2650 2670 2580 2600 2650 2 2.2. Experimental procedures tial durability. Each result (Table 4) is a mean value, from free tests on different samples.