materials
Review A Review on Alkali-Silica Reaction Evolution in Recycled Aggregate Concrete
Miguel Barreto Santos 1, Jorge De Brito 2,* and António Santos Silva 3
1 School of Technology and Management, Polytechnic of Leiria, Campus 2—Morro do Lena—Alto do Vieiro, 2411-901 Leiria, Portugal; [email protected] 2 CERIS, Instituto Superior Técnico, University of Lisbon, Av. Rovisco Pais, 1-1049-001 Lisbon, Portugal 3 National Laboratory for Civil Engineering, Av. do Brasil 101, 1700-066 Lisbon, Portugal; [email protected] * Correspondence: [email protected]
Received: 7 May 2020; Accepted: 4 June 2020; Published: 9 June 2020
Abstract: Alkali-silica reaction (ASR) is one of the major degradation causes of concrete. This highly deleterious reaction has aroused the attention of researchers, in order to develop methodologies for its prevention and mitigation, but despite the efforts made, there is still no efficient cure to control its expansive consequences. The incorporation of recycled aggregates in concrete raises several ASR issues, mainly due to the difficult control of the source concrete reactivity level and the lack of knowledge on ASR’s evolution in new recycled aggregate concrete. This paper reviews several research works on ASR in concrete with recycled aggregates, and the main findings are presented in order to contribute to the knowledge and discussion of ASR in recycled aggregate concrete. It has been observed that age, exposure conditions, crushing and the heterogeneity source can influence the alkalis and reactive silica contents in the recycled aggregates. The use of low contents of highly reactive recycled aggregates as a replacement for natural aggregates can be done without an increase in expansion of concrete. ASR expansion tests and ASR mitigation measures need to be further researched to incorporate a higher content of recycled aggregates.
Keywords: alkali-silica reaction; recycled aggregate; concrete degradation; test-methods; characterization
1. Introduction Concrete is produced with materials extracted from nature and, therefore, the use of non-renewable natural resources for its manufacture has, over the years, become a sustainability problem. The reduction of natural resources with the minimum characteristics required to not compromise the durability and performance of concrete structures is a current concern of the technical and scientific community. On the other hand, the construction and rehabilitation of structures produces waste as well as the demolition of some older structures. This cycle will continue over the years due to the finite useful life of the structures; therefore, it is necessary to ensure an effective and sustainable destination for the construction and demolition waste. Reuse of construction and demolition waste (CDW) began at a large scale after the end of World War II, with large quantities of concrete rubble becoming available and an increased need for new aggregates for reconstruction of damaged or destroyed structures [1]. CDW recycling is one of the guiding strategies of today’s sustainable culture. The use of recycled aggregates (RA) from concrete structures on a new concrete mix is pointed out as a contribution to sustainable construction criteria. However, RA constitution and behaviour on concrete is different from natural aggregates (NA). In this article, more emphasis will be given to coarse RA. Coarse RA consists of coarse NA and adhered mortar or cement paste. The adhered mortar consists of the cement matrix (composed by the interfacial transition zone (ITZ) between the aggregates
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Materials 2020, 13, x FOR PEER REVIEW 2 of 21 and the cement paste, and by the cement paste itself) and fine NA [2,3]. Figure1 exemplifies coarse aggregates and the cement paste, and by the cement paste itself) and fine NA [2,3]. Figure 1 RA’s constitution. exemplifies coarse RA’s constitution.
Figure 1. RA constitution. (A) Coarse NA; (B) Adhered mortar; (C) Interface between coarse NA and Figure 1. RA constitution. (A) Coarse NA; (B) Adhered mortar; (C) Interface between coarse NA and adhered mortar. adhered mortar. The replacement of NA with RA in concrete causes several changes in the fresh and hardened The replacement of NA with RA in concrete causes several changes in the fresh and hardened properties of concrete due to RA’s characteristics. Besides, depending on their properties and properties of concrete due to RA’s characteristics. Besides, depending on their properties and replacement ratios, the use of RA in concrete requires some additional attention to achieve acceptable replacement ratios, the use of RA in concrete requires some additional attention to achieve acceptable properties. Concrete performance is frequently inferior in recycled aggregate concrete (RAC) than properties. Concrete performance is frequently inferior in recycled aggregate concrete (RAC) than in in natural aggregate concrete (NAC), but the variation of its properties is similar to that of NAC [4]. natural aggregate concrete (NAC), but the variation of its properties is similar to that of NAC [4]. Therefore, it is feasible to control and recommend its use with normative regulations. Therefore, it is feasible to control and recommend its use with normative regulations. The incorporation of RA in concrete leads to questions about its durability, including its behaviour The incorporation of RA in concrete leads to questions about its durability, including its related to alkali-silica reaction (ASR), which is one of the major causes of concrete degradation. behaviour related to alkali-silica reaction (ASR), which is one of the major causes of concrete ASR occurs in concrete as a result of reaction between alkaline pore solution and reactive silica forms degradation. ASR occurs in concrete as a result of reaction between alkaline pore solution and within the aggregates [5]. The reaction develops a gel that expands by water uptake and causes reactive silica forms within the aggregates [5]. The reaction develops a gel that expands by water concrete cracking [6–8]. ASR gel expansion increases the internal stresses of concrete and contributes to uptake and causes concrete cracking [6–8]. ASR gel expansion increases the internal stresses of decrease its modulus of elasticity, flexural strength, and compressive strength, and activates/intensifies concrete and contributes to decrease its modulus of elasticity, flexural strength, and compressive other concrete degradation processes [6,9–13]. strength, and activates/intensifies other concrete degradation processes [6,9–13]. Assuming that ASR can occur in RAC and its event may be a consequence of the natural aggregates’ Assuming that ASR can occur in RAC and its event may be a consequence of the natural reactivity also present in the RAC, it is important to understand when and why the replacement (partial aggregates’ reactivity also present in the RAC, it is important to understand when and why the or total) of NA (reactive or non-reactive) with RA becomes harmful to the new concrete. replacement (partial or total) of NA (reactive or non-reactive) with RA becomes harmful to the new This paper reviews the main findings on ASR development in RAC. concrete. 2. ResearchThis paper Methodology reviews the main findings on ASR development in RAC.
2. ResearchThe main Methodology objective of this research is to create a study basis on ASR in RAC, so that the research that currently exists on NAC can be similarly pursued on RAC. Considering this intention, a research methodologyThe main was objective outlined. of this research is to create a study basis on ASR in RAC, so that the research that currentlyFirst, a list exists of the on principal NAC can publications be similarly about pursued RA on and RAC. RAC Considering was collected this in intention, order to summarize a research recentmethodology knowledge was outlined. on this material. Research results were organized, and the main findings were organizedFirst, a and list brieflyof the principal described. publications Summarized about comments RA and on RAC RA was and collected RAC presented in order on to this summarize item are mainlyrecent knowledge qualitative, on comparing this material. the main Research properties results with were conventional organized, and and well-known the main findings properties were of NAorganized and NAC. and briefly described. Summarized comments on RA and RAC presented on this item are mainlyASR qualitative, problem in comparing conventional the concretemain properties is presented with in conventional Section4, with and focus well-known on aggregate properties reactivity of evaluation,NA and NAC. ASR diagnosis and prognosis, the prevention and mitigation, as well as the impact on NAC.ASR The problem research attemptsin conventional to indicate concrete the most is presented recent knowledge in Section on the4, with expansive focus reactionon aggregate issue, indicatingreactivity evaluation, the current referenceASR diagnosis manuals, and methodologies, prognosis, the tests,prevention and threshold and mitigation, limits on as ASR well study. as the impact on NAC. The research attempts to indicate the most recent knowledge on the expansive reaction issue, indicating the current reference manuals, methodologies, tests, and threshold limits on ASR study.
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Afterwards, considering the properties of the RA and ASR problem in conventional concrete, the main doubts and questions about the possible development and impact of ASR in RAC were outlined and answers were sought, as summarized in Section5. In this review, the lack of information about RA’s reactivity is evident and articles about ASR in RAC are also reduced. Taking this into consideration, the review strategy had to be more comprehensive and considered the following types of publication to be reliable: articles in the Scientific Citation Index; articles in conference proceedings; Doctoral and Master thesis; published technical reports. For each publication, an analysis was made in order to establish a conductive line of research. The use of different aggregate types, cement, mix design, aging or test performed hindered the collective analysis. The option was that each publication was individually assessed, highlighting the commonalities on main results and conclusions. Several doubts are still without answer and compromise the development of specifications for the use of RAC, promoting a sustainable future in construction.
3. Properties of Concrete with Recycled Aggregates This section summarizes the main concepts on RA and RAC. The most important consequences of the total or partial incorporation of coarse RA in concrete are highlighted. The RA properties are mainly conditioned by the source concrete’s properties, which include the type of natural aggregate, the ITZ and the adhered mortar quality. The adhered mortar is considered the main cause of the RA properties’ decrease. Coarse RA have higher porosity and water absorption than coarse NA of similar origin, caused by the adhered mortar. RA can absorb 3–12% of water, depending on whether it is coarse or fine and its old concrete characteristics [3,14,15]. RA also show a lower density, bulk density, crushing, and abrasion resistance, usually depending on the content and quality of the adhered mortar [16–18]. Juan & Gutiérrez [19] state that only coarse RA obtained from a source concrete with a minimum of 25 MPa of compressive strength and a maximum of 44% of adhered mortar should be used. Concrete’s physical characteristics and the presence of anhydrous cement particles in RA are two opposing mechanisms that, according to Katz [15], can affect the properties of RAC, together with the characteristics of the new cement matrix. The properties of RA have an impact on the properties of RAC that are fairly discussed in the literature [3,15–17,20–43], leading to some summary conclusions:
The porosity of coarse RA negatively influences the compressive strength and modulus of • elasticity, tensile strength, abrasion resistance, shrinkage, water absorption, carbonation resistance, and chloride ion penetration. The effect of RA depends on the property under analysis and on the mechanical strength of the RAC to be produced; There are no marked differences between RAC and NAC, if the quality of RA is good, especially • in the properties that are more dependent on the new ITZ, such as compressive strength, splitting tensile strength and abrasion; The microstructure characteristics of the ITZ of coarse RA are different from those of the ITZ of • coarse NA and are influenced by the original concrete (OC)’s properties; RAC fracture can occur through RA and not necessarily through the new ITZ. Hence, the adhered • paste may be the weak point when its strength is less than that of the new ITZ; There is a strong bond between the coarse RA and the surrounding paste in RAC probably due to • the angularity of coarse RA and a residual cohesive effect on its surface; The modulus of elasticity, shrinkage, water absorption, and properties related with durability are • more dependent on the adhered mortar’s characteristics in RA. So, it is more difficult to maintain these properties in RAC equal to those of a reference NAC, although there are interesting results of concrete properties with RA of good quality. Materials 2020, 13, 2625 4 of 20
4. ASR Problem on Concrete ASR is an issue still gaining importance in NAC [44–46], although there are already preventive recommendations to avoid ASR occurrence in new concrete constructions [47]. These recommendations include limiting the alkalinity of the concrete’s pore solution, ensuring the use of non-reactive aggregate combinations, reducing moisture access or maintaining the concrete sufficiently dry, and also modifying the properties of the gel to be non-expansive [47]. However, despite advances in prevention, new cases of affected structures continue to be discovered around the world. ASR occurs between unstable reactive silica constituents that exist in some aggregates and the hydroxyl ions (OH-) and alkali ions (Na+,K+) present in the interstitial binder solution. Hydroxyl ions initially attack the silanol group (Si–OH) releasing water and attracting the alkali cations in the middle. The siloxane group (Si–O–Si) is then attacked, in a second stage, by hydroxyl ions, fragmenting the bonds of the group and replacing them with silicates. Solution silicates reacts with calcium to form calcium and aluminium silicate hydrates (C–S–H and C-A-S-H) and ASR gel [48–50]. Several discussions concerning the fundamentals of ASR mechanism were analysed and summarized in a Figueira et al. [46] review. Methodologies for ASR diagnosis [12,51–55] are essential to identify and confirm its presence in concrete. However, it is important that the aggregates’ reactivity is properly detected prior to its use, since ASR development is generally slow and is observed only several years after the structure is in service. RILEM published in 2016 a compilation reviewing and updating the methodologies and recommendations for ASR prevention and mitigation in NAC. The test-methods to evaluate the potential alkali reactivity of aggregates for use in mortar or concrete are: the petrographic examination method according to RILEM AAR-1 [47], similar to the ASTM C295 standard [56]; the accelerated mortar-bar test method (AMBT) according to RILEM AAR-2 [47], similar to ASTM C1260 standard [57]; the concrete prism test (CPT) at 38 ◦C according to RILEM AAR-3 [47], similar to ASTM C1293 standard [58]; and the concrete prism test (CPT) at 60 ◦C according to RILEM AAR-4 [47], similar to AFNOR P 18-454 standard [59]. In the AMBT, the aggregate is considered potentially deleterious if the average expansion of the mortar bar after 14 days of immersion in NaOH at 80 ◦C is higher than 0.20%. For expansion values within 0.10–0.20% at 14 days, it is recommended that the test proceeds until 28 days, the aggregate’s reactivity being considered doubtful when the expansion at 28 days is lower than 0.20%. Expansions of less than 0.10 % at 14 days after immersion are indicative of innocuous behaviour in most cases. In the CPT methods, the composition in evaluation is considered non-expansive if the average length of the prisms conserved in a moist environment at 38 ◦C is below 0.05% at 1 year or 0.02% at 12–15 weeks when conserved at 60 ◦C. The long duration of the tests and the aggressive environment used to accelerate the ASR development can compromise the prompt assessment of aggregate reactivity. Several suggestions for tests modifications or new analysis procedures are present in literature, such as the use of a CPT test at 38 ◦C with prims that immerged in an alkaline solution [60] or the application of a stiffness damage test for assessing the depreciation of concrete mechanical properties [61]. Cement is one of the main sources of alkalis supply, although any other source of alkalis can be mobilized for ASR development. The equivalent sodium oxide content (Na2Oeq) is conventionally used to indicate the alkali content of Portland cement and is usually limited to values less than 0.60% to mitigate ASR. Nowadays, it is more recommended to compute all alkalis available in concrete constituents instead of only the alkalis content of the cement, and the concrete Na2Oeq is generally limited to a maximum of 2.5 kg/m3. In terms of ASR prognosis, the knowledge about the residual expansion and the soluble alkalis content is of the uttermost importance [62]. It is known that the ASR expansion increases with the alkali enrichment of cement paste with ageing, which is mainly attributed to the alkalis released by the aggregates [63]. The evaluation of the alkalis released by the aggregates is usually determined by Materials 2020, 13, 2625 5 of 20 laboratory extraction tests, where the aggregates are immersed in alkaline solutions [47,62]. Recently, Berra [64] suggested an autoclave method to minimize leaching and increase the alkalis extraction in a short testing period.
5. Research Issues and Concerns about ASR Development on Concrete with RA In general, standards regard RA as a potentially reactive aggregate. When RA of concrete are intended to be used, EN 12620 [65] states that it will be necessary to confirm that the OC does not contain potentially reactive aggregates, and when the alkalis content of the new concrete (or cement) is limited, the alkalis content of recycled concrete aggregates should be determined and taken into account. The use of the same prevention and evaluation methodology on RA reactivity is very restrictive since it is quite difficult to control, analyse and limit the origin of RA, and thus, the reactive silica present in the fine and coarse original NA. The amount of alkalis content of the adhered mortar present in a RA structure is also laborious to control. Age, exposure conditions of the old concrete before it is recycled, and the heterogeneity of RA have influence on its reactivity. However, in addition, other differences in ASR development can occur either due to the properties of the RA or of the RAC itself. It is not certain that all RA have a negative influence on the ASR development, in particular when their reactivity has been extinguished or is reduced. Also, the characteristics of RAC are distinct from those of NAC, which increases doubts about its behaviour in relation to ASR. Research about ASR in RAC exists but is still very preliminary, and there are still several doubts about the influence of RA from concrete on ASR development. Considering previous reasoning, the main research issues and concerns about ASR development on RAC are:
Are the existing reactivity tests sufficient and adequate to study RA? • Can an increase in alkalis content of a new concrete mix or an increase of fresh reactive aggregate • surfaces originated during the RA’s preparation make a RA reactive? Can a concrete with natural reactive aggregate that has not been exposed to the optimal humidity • conditions for ASR development become reactive in a new RAC? Since the porosity is higher in RAC than in NAC, can this characteristic benefit the ASR gel • accommodation and counteract its expansion? Can a long exposure of RA to alkalis decrease or extinguish its potential reactivity or, on the • contrary, can it be reactivated in contact with new cement? Has the ASR gel formed in RAC different properties than in NAC? • Are the properties of RAC and NAC different when ASR occurs? • To answer these questions, a critical literature review and analysis was made. Section6 separates the articles by themes and focuses on the main objectives of each research, the common parameters and the main conclusions obtained, as well as providing a discussion.
6. ASR in Concrete with Potential Reactive RA
6.1. Suitability of ASR Accelerated Tests to Evaluate RA Reactivity The adhered mortar in RA increases the porosity of the aggregate and can accommodate some of the ASR gel formed, being able to “hide” or “delay” the expansion. Crushing and washing the mortar during sample preparation for the accelerated tests may also change the properties of the RA or the sample under study. The heterogeneity that may exist in RA with different origins can make this assessment difficult. Also, the high duration of tests is still a concern when RA are used. Several proposals of modified tests are presented in the literature to overcome these difficulties, and some are summarized next. Gress & Kozikowski [66] propose the acceleration of AMBT and CPT ASTM test-methods through an increase in temperature, microwave energy applications, ultrasonic energy utilization, and alkali boosting. The authors also proposed to increase the size of AMBT test specimens to 76.2 76.2 × × Materials 2020, 13, 2625 6 of 20
279 mm3 when RA are used. They also used different test specimens: larger prisms; prisms with lateral holes to facilitate the NaOH access; solid cubes; and cubes with lateral holes. The changes introduced in the test-methods procedures were effective in ASR acceleration. Scott IV [67] states that ASTM C 1260 test for RA crushes and destroys the aggregate matrix, which causes separation of the aggregate from the mortar, thus not testing RA as a whole. The author considered that ASR tests should be focused on RA’s absorption capacity, mortar fraction and free alkalis content. His proposal is to replace the test specimens used in the mortar-bar test with others with the size of the concrete prisms used in the ASTM C 1293 test, but applying ASTM C 1260 test conditions. Furthermore, the limit expansion value at 28 days was decreased from 0.10 to 0.04%. Scott IV [67] used altered samples, based on the work of Gress and Kozikowski [66], but vacuum sealed using a plastic bag. Considering that cathodic protection of reinforced concrete promotes ASR, Scott IV [67] also studied a method to accelerate the reaction through an electric current. Adams et al. [68] considered that aggregate washing, used in NA AMBT and CPT tests, can reduce adhered cement paste in the RA, promote the hydration of anhydrous cement particles in RA, and/or leach calcium or alkalis from aggregates and adhered paste. The authors proposed limited washing at time intervals for each of the size fractions. The authors analysed four types of RA from NAC blocks that were under natural exposure over a long period in a CANMET study [69]. The blocks were selected by age, extent of ASR damage and mineralogy of reactive NA. Adams et al. [68] also modified the mixing process because of the water absorption of RA. First they immersed the RA in water for 30 min and reached 85% of RA saturation. After this period, extended for another 30 s in mixing, they inserted cement and continued mixing. The results showed that AMBT can be used to evaluate ASR in RAC as long as precautions are taken in the mix. Johnson & Shehata [70] proposed the use of concrete micro-bar test (CMBT) based on the RILEM AAR-5 test method [47], to minimize RA characteristics modification and compare results with those of the AMBT. They used RA from the same source used by Adams et al. [68]. The required fractions for AMBT were produced using large and small jaw crushers as well as a disk pulveriser. After crushing, RA were washed based on the Adams et al. [68] method. In CMBT, the batch water was corrected for aggregate absorption during mixing and the alkalis were boosted until 1.5% Na2Oeq in some mixes. After 24 h curing, the samples were soaked in water at 80 ◦C for another 24 h and then immersed in a 1 N NaOH solution at 80 ◦C. The authors believe that AMBT and CMBT can be used to evaluate the reactivity of RA if high absorption of the RA is considered and a representative sample of the crushed materials is used (maintaining the relative proportion of residual mortar-to-stone). Avoiding several crushing steps to achieve sand size is indicated as the main advantage of CMBT, because the adhered mortar and NA are represented in the same proportion and condition as they are in concrete. CMBT was effective in evaluating the reactivity of RA and NA when an expansion limit of 0.04% at 28 days or 0.10% at 56 days was used. For AMBT, the authors propose to process the RA with a jaw crusher followed by a disk pulveriser, which was found to produce a fine aggregate size distribution that better represents the reactivity of the coarse RA. Johnson & Shehata [70] observed that washing RA slightly reduces the expansion in AMBT, while changes in the crushing method were more significant in the expansion. To study the influence of the water absorption of RA on ASR tests, Delobel et al. [71] studied two standard experimental procedures using mortars and microbars, which were evaluated according to the NF P18-594 test-method. The authors used two industrial coarse RA incorporating a siliceous limestone aggregate and a mortar rich in alkalis. They tested three protocols to consider the water absorption of RA during mortar manufacture: using dried RA without taking into account the water absorption; using dried RA and the water corresponding to absorption added to the bowl during mixing; pre-saturation of aggregates 96 h before mixing with the water absorption value +5%. Although all results were positive for ASR, the authors observed a higher expansion on AMBT when they did not take into account the water absorption because, according to them, as soon as the mixing starts, part of the mixing water is absorbed by the aggregates, reducing the quantity of effective water, which Materials 2020, 13, 2625 7 of 20
reduces the workability of the mortar and the effective water–cement ratio of the in-place material, Materialsleading 2020 to, a13 reduction, x FOR PEER in REVIEW the matrix porosity and probably a larger expansion of material caused7 of 21 by expansive products. BeaucheminBeauchemin et et al. al. [72] [72 ]evaluated evaluated CPT CPT to to assess assess the the potential potential ASR ASR reactivity reactivity of of RA. RA. The The authors authors usedused reactive reactive coarse coarse RA RA from from concrete concrete blocks blocks made made with with a variety a variety of reactive of reactive coarse coarse aggregates aggregates from CANMETfrom CANMET [69] and [69 ]coarse and coarse RA from RA from three three different different structural structural elements elements of ofa ademolished demolished concrete concrete overpassoverpass (foundations; (foundations; bridge deck;deck; columns)columns) aaffectedffected by by ASR. ASR. The The residual residual mortar mortar content content of theof the RA RAwas was also also analysed. analysed. CPT CPT was was made made according according to CSA to CSA A23.2-14A. A23.2-14A. The authorsThe authors produced produced several several series seriesof concrete of concrete with replacementwith replacement ratios ratios of 25, of 50 25, and 50 100% and of100% coarse of coarse RA with RA changes with changes in adhered in adhered mortar mortarcontent content and RA’s and moisture RA’s moisture conditions. conditions. BeaucheminBeauchemin et et al. al. [72] [72] recommended recommended using using aggr aggregatesegates under under a a saturated saturated state. state. The The authors authors consideredconsidered that usingusing in in the the mix mix process process an additional an additional water contentwater content will increase will theincrease risk of the segregation. risk of segregation.Because of the Because variability of the of adheredvariability mortar of adhered content withmortar RA content size, the with authors RA also size, suggested the authors following also suggestedASTM C1293 following by using ASTM the dry-rodded C1293 by bulk using density the todry-rodded determine thebulk aggregate density/sand to determine ratio, instead the of aggregate/sandthe fixed 60:40 ratioratio, of instead CSA A23.2-14A. of the fixed 60:40 ratio of CSA A23.2-14A.
6.2.6.2. Potential Potential Influence Influence of of RA RA Recycling Recycling Process Process on on ASR ASR Development Development ExposureExposure of of fresh fresh faces faces of of the the original original reactive reactive NA NA resulting resulting from from the the RA RA crushing crushing seems seems to to increaseincrease RA’s RA’s reactivity, reactivity, getting getting worse worse with with multiple multiple cycles. cycles. Still, Still, adhered adhered mortar mortar and and aggregate aggregate only only withwith mortar mortar can can also also underestimate underestimate or or attenuate attenuate the the ability ability to to expansion. expansion. There There are are certainties certainties on on researchresearch about about the the recycling recycling effect effect on on RA RA cracking cracking and and NA NA exposure, exposure, but but doubts doubts exist exist on on its its effect effect on on expansionexpansion due due to to the the aggregate aggregate reactivity reactivity level level and and to to the the adhered adhered mortar mortar damping. damping. ShehataShehata et et al. al. [73] [73 ]found found that that in in AMBT AMBT the the fine fine fraction fraction produced produced by by primary primary crushing crushing caused caused lowerlower expansions expansions thanthan thatthat resulting resulting from from secondary secondar crushingy crushing of coarse of coarse RA (Figure RA (Figure2). Still, 2). the Still, authors the authorsalso noted also that noted it is that possible it is thatpossible processing that processing of coarse RAof coarse to reduce RA theto reduce particles the size particles for AMBT size mayfor AMBTproduce may samples produce with samples less reactive with less aggregate reactive andaggregate more mortar and more than mortar in the than original in the coarse original RA, coarse which RA,will which underestimate will underestimate the coarse RA’s the reactivity.coarse RA’s The reac authorstivity. used The concreteauthors withused ASRconcrete that waswith for ASR 12 yearsthat wasunder for natural12 years exposure under natural according exposure to ASTM according C 1260 to ASTM and CSA C 1260 A23.2-14A and CSA procedures. A23.2-14A procedures. The authors Thevaried authors the alkali varied content the alkali of the content compositions of the compos and theitions content and of the mineral content additions of mineral and additions lithium nitrate. and lithiumIn addition nitrate. to freshIn addition new faces, to fresh Shehata new etfaces, al. [ 73Shehata] stated et that al. [73] high stated RAC that expansions high RAC can expansions be attributed can to bea significantattributed to increase a significant in alkali increase contribution in alkali by contribution residual mortar by residual in RA andmortar/or expansionin RA and/or of theexpansion ASR gel ofin the RA ASR when gel exposed in RA when to a high exposed level to of a moisture high level in of the moisture new concrete. in the new concrete.
0.45 Expansion limit
0.40 Secundary Crushing RA
0.35 Primary Crushing RA Origin NA 0.30
0.25
Expansion (%) Expansion 0.20
0.15
0.10
0.05
0.00 02468101214 Time (days)
Figure 2. Expansion of RA from primary and secondary crushing in the ASTM C 1260 test, adapted Figure 2. Expansion of RA from primary and secondary crushing in the ASTM C 1260 test, adapted from [73]. from [73].
Adams et al. [68] found that the increment in RA crushing, as also found by Shehata et al. [73], leads to higher access to reactive NA and, consequently, higher expansions. RA’s reactivity will depend on the amount of reactive NA present in the RA and also on the replacement ratio of NA with RA.
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Adams et al. [68] found that the increment in RA crushing, as also found by Shehata et al. [73], leads to higher access to reactive NA and, consequently, higher expansions. RA’s reactivity will depend on the amount of reactive NA present in the RA and also on the replacement ratio of NA with RA. Beauchemin et al. [72] found that the reactivity/expansion of the OC of RA will influence the expansion of RA. The author’s results show that RA with a high degree of reactivity/expansion/damage can undergo less expansion than concrete with a similar content of unreacted aggregate material of the same origin. The authors attributed this fact to the consumption of reactive phases of RA and to the fact that RA have adhered mortar that limits the tendency of NA to react. However, aggregate crushing also causes the exposure of new fresh aggregate surfaces that may increase reactivity, as Shehata et al. [73] also mentioned.
6.3. Potential Influence of RA Alkalis Content on ASR Development The increase in concrete alkalis’ content due to alkalis present in RA’s adhered mortar and in new cement used in a RAC mix is a concern to ASR development. Stark [74] states that the ASR expansion regulator in an OC of the RA was the alkalis content of the interstitial solution and that the re-introduction of alkalis through the new cement in a RAC restarted ASR. Moreover, Stark [74] suspected that ASR is more dependent on the alkalis content of the new cement of RAC mix than on that of the old concrete. In a Stark’s [74] study, coarse and fine potentially reactive aggregates were tested with Portland cement with 0.50, 0.75 and 1.00% of Na2Oeq, which were afterwards exposed for 48 months to ASTM C 227 test conditions (Figure3). The samples were removed after 2 months of testing, at half and at the maximum expansion level. The author states that no washing of the RA was done to avoid losing characteristics, mainly to preserve alkalis content in the pore solution in the OC and thus also in the RA used. After crushing, concrete expansion tests were made with high alkali RAC of coarse RA from low alkali OC (Figure4) or high alkali OC (Figure5), and high alkali cement. Innocuous fine NA was used. All RAC expansion was higher than OC expansion, boosted by OC alkalis content of the interstitial solution. Stark [74] also observed that the largest expansion occurred in RAC with RA produced with 1.00% of Na2Oeq and crushed at its maximum expansion value in ASTM C 227. Although reactive NA does not react in concrete with low alkali cement, the reactivity of NA remains after crushing. It was also observed that, in some cases, ASR leads to excessive expansions when cement with high alkali Materialscontent 2020 is, 13 used, x FOR mixed PEER REVIEW with RA from an OC with low alkali content. 9 of 21
Expansion limit NAC (0.5% cement alkali content) NAC (0.75% cement alkali content) NAC (1.00% cement alkali content) 0.40
0.35
0.30
0.25
0.20 Expansion (%) Expansion
0.15
0.10
0.05
0.00 0 6 12 18 24 30 36 42 48 Time (months) Figure 3. Expansion of NA with cements of different alkalis content in ASTM C227, adapted from [74]. Figure 3. Expansion of NA with cements of different alkalis content in ASTM C227, adapted from [74].
Expansion limit NAC (0.5% cement alkali content) 0.40 RAC (recycle at 2 months) 0.31%real RAC (recycle at about half maximum expansion) expansion 0.35
0.30
0.25
0.22%real 0.20 expansion 0.15 concrete (%) 0.10
0.05
0.00 Cumulative xpansionrelativeto original 0 6 12 18 24 30 36 42 48 Time (months) Figure 4. Expansion of high alkali mixes with coarse RA, from low alkali OC, and high alkali cement in ASTM C 227, adapted from [74].
Expansion limit NAC (1.0% cement alkali content) RAC (recycle at 2 months) RAC (recycle at about half maximum expansion) 0.24%real expansion 0.60 RAC (recycle at about full maximum expansion) 0.55
0.50 0.23%real 0.45 expansion 0.40 0.35 0.30 0.25 (%) 0.20 0.15 0.10 0.10%real 0.05 expansion 0.00 0 6 12 18 24 30 36 42 48 Time (months) Cumulative xpansionrelative to original concrete concrete original to xpansionrelative Cumulative Figure 5. Expansion of high alkali mixes with coarse RA, from high alkali OC, and high alkali cement in ASTM C 227, adapted from [74].
Materials 2020, 13, x FOR PEER REVIEW 9 of 21 Materials 2020, 13, x FOR PEER REVIEW 9 of 21
Expansion limit NAC (0.5% cement alkali content) NAC (0.75% cement alkali content) NAC (1.00% cement alkali content) Expansion limit NAC (0.5% cement alkali content) 0.40 NAC (0.75% cement alkali content) NAC (1.00% cement alkali content) 0.40 0.35 0.35 0.30 0.30 0.25 0.25 0.20 Expansion (%) Expansion 0.20 Expansion (%) Expansion 0.15 0.15 0.10 0.10 0.05 0.05 0.00 0.00 0 6 12 18 24 30 36 42 48 Time (months) 0 6 12 18 24 30 36 42 48 Time (months) Figure 3. Expansion of NA with cements of different alkalis content in ASTM C227, adapted from [74].Figure 3. Expansion of NA with cements of different alkalis content in ASTM C227, adapted from Materials 2020, 13, 2625 9 of 20 [74].
Expansion limit NACExpansion (0.5% limitcement alkali content) 0.40 RACNAC (recycle (0.5% cement at 2 months) alkali content) 0.31%real 0.40 RAC (recycle at about half maximum expansion) expansion 0.35 RAC (recycle at 2 months) 0.31%real RAC (recycle at about half maximum expansion) expansion 0.35 0.30 0.30 0.25
0.25 0.22%real 0.20 expansion 0.22%real 0.20 0.15 expansion
concrete (%) 0.15 0.10 concrete (%) 0.10 0.05 0.05 0.00 Cumulative xpansionrelativeto original 0.00 0 6 12 18 24 30 36 42 48 Cumulative xpansionrelativeto original Time (months) 0 6 12 18 24 30 36 42 48 Time (months) Figure 4. Expansion of high alkali mixes with coarse RA, from low alkali OC, and high alkali cement Figure 4. Expansion of high alkali mixes with coarse RA, from low alkali OC, and high alkali cement in inFigure ASTM 4. CExpansion 227, adapted of high from alkali [74]. mixes with coarse RA, from low alkali OC, and high alkali cement ASTM C 227, adapted from [74]. in ASTM C 227, adapted from [74]. Expansion limit NAC (1.0% cement alkali content) Expansion limit RAC (recycle at 2 months) NAC (1.0% cement alkali content) RAC (recycle at about half maximum expansion) 0.24%real RAC (recycle at 2 months) RAC (recycle at about full maximum expansion) expansion 0.60 RAC (recycle at about half maximum expansion) 0.24%real expansion 0.550.60 RAC (recycle at about full maximum expansion) 0.500.55 0.23%real 0.45 expansion 0.50 0.23%real 0.400.45 expansion 0.350.40 0.300.35 0.250.30 (%) 0.200.25 (%) 0.150.20 0.100.15 0.10%real expansion 0.050.10 0.10%real 0.000.05 expansion 0.00 0 6 12 18 24 30 36 42 48 Time (months) Cumulative xpansionrelative to original concrete concrete original to xpansionrelative Cumulative 0 6 12 18 24 30 36 42 48 Time (months) Cumulative xpansionrelative to original concrete concrete original to xpansionrelative Cumulative FigureFigure 5. 5. ExpansionExpansion of of high high alkali alkali mixes mixes with with coarse coarse RA, RA, from from high high alkali alkali OC, OC, and and high high alkali alkali cement cement inFigurein ASTM ASTM 5. CExpansion C 227, 227, adapted adapted of high from from alkali [74]. [74 ].mixes with coarse RA, from high alkali OC, and high alkali cement in ASTM C 227, adapted from [74]. Gress & Kozikowski [66] results, with modified AMBT, shows that RA exhibits a smaller expansion than NA when using cement with 1.15% of Na2Oeq. The authors justified the results with the lower reactivity of RA per unit volume and its higher porosity to accommodate ASR gel. RAC expand less than NAC, although the opposite occurs when alkalis’ content increases. Scott IV & Gress [75] found initial expansion in RAC. The authors state that the paste fraction of RA is responsible for the expansion. According to the authors, the high absorption allowed water to enter the paste fraction of the RA of concrete. This caused expansion in concrete during early hydration, when the new matrix was soft. Dhir et al. [76] examined the problem of ASR when using RA from several origins and observed that all aggregates studied achieved values in AMBT below the expansion limit of 0.10% at 14 days. Regarding alkalis release, the highest values were obtained from bricks and concrete blocks, with Na2Oeq release values, by mass, between 0.47 and 0.61%. Concrete from demolition and concrete produced in laboratory released about 0.15 and 0.20% of Na2Oeq by mass, respectively. Dhir et al.’s [76] study consists of evaluating aggregates’ reactivity and alkalis release from CDW using ASTM C 1260 procedure and the alkalis CANMET extraction test-method [77]. They tested materials produced recently and from demolitions with a cement Na2Oeq of 0.6%, making different RA groups (lightweight concrete, brick, plasters, floor-regulating aggregates, among others). The authors Materials 2020, 13, 2625 10 of 20 Materials 2020, 13, x FOR PEER REVIEW 10 of 21
alsoDhir produced et al.’s [76] concrete study withconsists a high of evaluating and normal aggregates’ reactivity, reactivity some with and mineral alkalis release additions from (fly CDW ash or usinggranulated-blast ASTM C 1260 furnace procedure slag) and and the compressive alkalis CANMET strength betweenextraction 25 test-method and 50 MPa, [77]. crushed They at tested 28 days. materialsSome of produced the materials recently were and subjected from demolitions to CPT, according with a cement to BS Na 812-123,2Oeq of similar0.6%, making to RILEM different AAR-4. RAConcrete groups (lightweight samples were concrete, manufactured brick, plasters with a high, floor-regulating alkali cement aggregates, and fine and among coarse others). RA, from The RA authorsgroups also mentioned produced above, concrete and with then a combinedhigh and no withrmal a reactivity, low coarse some or fine with reactive mineral natural additions aggregate. (fly ashAfter or granulated-blast curing, the concrete furnace samples slag) were and conditionedcompressive in strength a moist between cabinet at 25 60 and◦C for50 MPa, 12 weeks. crushed With at the 28 exceptiondays. Some of concreteof the materials samples withwere RA subjected from the to road CPT, planning’s, according which to BS reached 812-123, high similar expansion to RILEM values, AAR-4.the remaining Concrete concretesamples sampleswere manufactured showed low with reactivity a high andalkali little cement risk ofand ASR fine development. and coarse RA, from RA groupsLi [78 mentioned] stated that above, the RAand should then combined contribute with with a low alkalis coarse to theor fine cement reactive paste natural system aggregate. since it has Aftersoluble curing, alkalis the concrete in the old samples paste. were However, conditioned the interstitial in a moist solution cabinet does at 60 not °C show for 12 the weeks. effect With of this theincrease; exception so of the concrete author samples argues that with the RA interstitial from the solutionroad planning’s, is fundamentally which reached controlled high byexpansion the alkalis values,content the ofremaining the cement concrete and thesamples alkalis showed present low in reactivity the RA can and be little in balance risk of ASR with development. the alkalis in the poreLi [78] solution. stated RA that used the RA by Lishould [78] was contribute obtained with from alkalis recycling to the acement concrete paste pavement system thatsince had it has been solubleaffected alkalis by ASR. in the ASR old was paste. caused However, by a coarse the inters quartzitetitial /solutionquartz-biotite does naturalnot show aggregate the effect designated of this increase;by “Blue so Rock”,the author usually argues used that as the an ASRinterstitial reference solution aggregate. is fundamentally To compare controlled the behaviour by the of alkalis RA and contentreference of the NA, cement the researcher and the alkalis used hydrochloricpresent in the acid RA tocan dissolve be in balance the cement with pastethe alkalis adhered in the to pore the RA solution.and tested RA used them by with Li ASTM[78] was C obtained 1260 procedure. from re Licycling [78] used a concrete vacuum pavement saturation that of had the RAbeen in affected the AMBT byand ASR. CPT ASR ASTM was caused tests. Cements by a coarse with quartzite/quartz-biotite high alkali content (1.09% natural Na2 Oaggregateeq) and, indesignated some cases, by one“Blue with Rock”,a low usually alkali contentused as (0.26%an ASR Nareference2Oeq), wereaggregate. used inTo the compare mixes. the The behaviour author used of RA RA’s and saturation reference to NA,eliminate the researcher the eff ectused of hydrochloric cement hydration acid to on disso thelve final the expansion. cement paste However, adhered in toAMBT, the RA thereand tested was no themlarge with variation ASTM betweenC 1260 procedure. saturated andLi [78] unsaturated used vacuum RA (Figuresaturation6) probably of the RA due in tothe the AMBT small and size CPT of the ASTMparticles. tests. InCements CPT, saturated with high RAC alkali has largercontent expansions (1.09% Na at2O theeq) sameand, in ages some than cases, NAC one (Figure with7 ),a aslow also alkaliobserved content by (0.26% Scott IVNa &2O Gresseq), were [75]. used Li [78 in] the commented mixes. The that author this can used be RA’s due to saturation swelling ofto theeliminate old ASR thegel effect in the of RA, cement the reactivation hydration ofonASR the orfinal the expansion. increase in alkaliHowever, content in ofAMBT, the new there cement was paste.no large variationScott between IV [67 ]saturated observed and a higher unsaturated content ofRA soluble (Figure alkalis 6) probably in RA, which due to was the attributed small size to theof the paste particles.fraction In of CPT, the RA. saturated Moreover, RAC he has observed larger expansio that RACns had at the a high same initial ages expansion, than NAC which (Figure was 7), attributedas also observedto the waterby Scott absorption IV & Gress of the[75]. mix Li [78] during commented the initial that hydration this can be of concrete.due to swelling Thus, of he the pre-saturated old ASR gelthe in the RA RA, for 48the h reactivation before mixing, of ASR filling or the their incr poresease in and alkali assuming content that of the it wouldnew cement eliminate paste. the initial expansions,Scott IV [67] and observed used pre-saturation a higher content water of insolubl the mix,e alkalis avoiding in RA, alkalis which loss. was attributed to the paste fractionShehata of the etRA. al. Moreover, [73] observed he thatobserved washing that the RAC RA had for 18a hhigh in runninginitial expansion, water did notwhich result was in a attributedsignificant to the reduction water absorption in expansion. of the The mix authors during considered the initial thathydration it was of not concrete. enough Thus, to clean he apre- large saturatedamount the of alkalisRA for from48 h before the residual mixing, adhered filling mortartheir pores of RA and and assuming/or that there that it were would suffi eliminatecient alkalis the in initialthe newexpansions, concrete and toreactivate used pre-saturation the expansion water on in the the fresh mix, faces avoiding of NA alkalis present loss. in RA.
0.35 Expansion limit RA 0.30 RA soaked 0.25 NA (Blue Rock)
0.20
Expansion0.15 (%)
0.10
0.05
0.00 0 2 4 6 8 10121416182022242628 Time (days)
Figure 6. Expansion of saturated and unsaturated NA and RA in the ASTM C 1260 test, adapted Figurefrom 6. [ 78Expansion]. of saturated and unsaturated NA and RA in the ASTM C 1260 test, adapted from [78].
Materials 2020, 13, 2625 11 of 20 Materials 2020, 13, x FOR PEER REVIEW 11 of 21
Expansion limit 0.20 RA unsoaked 0.18 RA Soaked NA (Blue Rock) 0.16 0.14 0.12 0.10
Expansion (%) Expansion 0.08 0.06 0.04 0.02 0.00 0 2 4 6 8 1012141618202224 Time (months)
Figure 7. Expansion of saturated and unsaturated NA and RA in the ASTM C 1293 test, adapted Figurefrom 7. [78 Expansion]. of saturated and unsaturated NA and RA in the ASTM C 1293 test, adapted from [78]. 6.4. Potential Influence of RA Replacement Ratio on ASR Development Shehata et al. [73] observed that washing the RA for 18 h in running water did not result in a The reactivity level of concrete depends on the amount of reactive aggregate used and on its significant reduction in expansion. The authors considered that it was not enough to clean a large combination with other aggregates. Hence, ASR should not be a problem if only RA with non-reactive amount of alkalis from the residual adhered mortar of RA and/or that there were sufficient alkalis in NA are used. However, as already mentioned, it is very difficult to control reactivity level/activity the new concrete to reactivate the expansion on the fresh faces of NA present in RA. of the RA’s source concrete. Studies demonstrate that even using RA with reactive NA, the concrete expansion level depends on the replacement ratio of NA with reactive RA. 6.4. Potential Influence of RA Replacement Ratio on ASR Development The Gottfredsen & Thøgersen’s [79] study showed that the incorporation of 20% coarse RA in newThe concrete reactivity did level not causeof concrete a critical depends expansion. on the The amount authors of observedreactive aggregate that the OC used was and aff ectedon its by combinationASR due to with reactive other constituents aggregates. in Hence, the aggregates’ ASR should fine not fraction. be a problem So, the if RAC only had RA awith non-reactive non-reactive coarse NANA are and used. a potentially However, reactiveas already mortar. mentioned, it is very difficult to control reactivity level/activity of the RA’sEtxeberria source concrete. [2] studied Studies the microstructural demonstrate that and structuraleven using behaviour RA with ofreactive RAC [ 80NA,]. The the author concrete used expansionmixes with level 0, depends 25, 50 and on 100% the replacem of RA froment aratio recycling of NA industry, with reactive and with RA. a CEM I 52.5R with Na2Oeq contentThe Gottfredsen higher than & 0.6%, Thøgersen’s keeping 28-day [79] study compressive showed strengththat the constant.incorporation The coarse of 20% RA’s coarse and RA adhered in newmortar’s concrete reactivity did not werecause studied a critical separately expansion. using The the authors ASTM observed C 1260 test that procedure. the OC was affected by ASR dueEtxeberria to reactive [2] constituents AMBT tests resultsin the aggregates’ showed at 14fine days fraction. an expansion So, the RAC of 0.07% had fora non-reactive the coarse RA coarseand NA 0.10% and for a thepotentially adhered reactive mortar. mortar. The tests were extended up to 56 days and the adhered mortar’s expansionEtxeberria increased [2] studied to 0.19%the microstructural at 28 days and and 0.37% stru atctural 56 days behaviour (Figure of8). RAC Therefore, [80]. The the author original used fine mixesNA with present 0, 25, in 50 adhered and 100% mortar of RA was from suspected a recycling to be industry, potentially and alkali-reactive.with a CEM I 52.5R Etxeberria with Na [22O] alsoeq contentcarried higher out macroscopic than 0.6%, and keeping microscopic 28-day tests compre in concretessive strength specimens constant. in order toThe confirm coarse ASR RA’s evolution. and adheredThe tests mortar’s were madereactivity after were the concrete studied separa was storedtely using in a humidity the ASTM chamber C 1260 test for 8procedure. months at 20 ◦C and 100%Etxeberria RH. Scanning [2] AMBT electron tests microscoperesults showed (SEM) at analysis14 days showsan expansion ASR gel of in 0.07% the interfaces for the coarse between RA RA andand 0.10% cement for the paste, adhered this presence mortar. beingThe tests justified were by extended the increase up to in 56 the days alkalis and content the adhered of the mortar’s mix due to expansionthe new increased cement employed to 0.19% (CEMat 28 days I 52.5 and R), originating0.37% at 56 ITZsdays with(Figure high 8). alkalis Therefore, content. the original The interstitial fine NAsolution present reached in adhered a high mortar pH, promotingwas suspected the dissolution to be potentially of reactive alkali-reactive. silica on the Etxeberria aggregates’ [2] surfacealso carriedand producingout macroscopic ASR gel. and However, microscopic Etxeberria tests in did concrete not find specimens losses in RAC’s in order properties to confirm due toASR ASR, evolution.but it may The have tests beenwere becausemade after the the manufactured concrete was beams stored for in structurala humidity evaluation chamber for were 8 months placed at in a 20 dry°C and environment. 100% RH. Scanning electron microscope (SEM) analysis shows ASR gel in the interfaces betweenIn RA Calder and &cement McKenzie’s paste, [this81] work,presence the RAbeing was ju classifiedstified by asthe non-expansive. increase in the The alkalis authors content claim of the themodification mix due to the of ASR new classification cement employed from high (CEM tonormal I 52.5 R), reactivity. originating The ITZs authors with observed high alkalis the formation content. of TheASR interstitial gel by microscopic solution reached analysis. a Although high pH, ASR prom geloting appeared the dissolution in some concrete of reactive specimens, silica the on authors the aggregates’did not observe surface significant and producing expansions, ASR evengel. However, though petrography Etxeberria indicateddid not find the presencelosses in of RAC’s reactive propertiessilica forms due in to some ASR, RA, but that it weremay classifiedhave been as potentiallybecause the ASR-reactive. manufactured beams for structural evaluation were placed in a dry environment.
MaterialsMaterials 20202020, 13, ,x13 FOR, 2625 PEER REVIEW 12 of12 21 of 20
0.35 Expansion limit
0.30 Recycled aggregate
0.25 Adhered mortar
0.20
Expansion0.15 (%)
0.10
0.05
0.00 14 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 Time (days)
Figure 8. Expansion of recycled aggregate and adhered mortar after 14 days and after 56 days in ASTM FigureC 1260, 8. Expansion adapted from of recycled [80]. aggregate and adhered mortar after 14 days and after 56 days in ASTM C 1260, adapted from [80]. Barreto Santos et al. [82] confirmed that the use of coarse RA with non-reactive NA caused no remarkableIn Calder expansion& McKenzie’s in RAC [81] (Figure work,9 the), and RA observed was classified that, up as to non-expans 20% of coarseive. RA The as authors NA replacement, claim the themodification mix’s reactivity of ASR seems classifica not to betion significantly from high a fftoected normal (Figure reac 10tivity.). The The authors authors also studiedobserved the the eff ect formationof total orof partialASR gel use by of reactivemicroscopic or non-reactive analysis. RAAlthough on RAC’s ASR behaviour. gel appeared For this in purpose,some concrete controlled specimens,reactive (fullthe authors reactive did NA) not and observe non-reactive significan (fullt expansions, non-reactive even NA) though concrete petrography mixes were indicated used, which the werepresence differently of reactive aged silica to simulate forms in old some and RA, recent that concrete.were classified Before as recycling, potentially both ASR-reactive. types of concrete Barreto Santos et al. [82] confirmed that the use of coarse RA with non-reactive NA caused no underwent either accelerated aging (climatic chamber at 38 2 ◦C and relative humidity higher than remarkable expansion in RAC (Figure 9), and observed that,± up to 20% of coarse RA as NA 95%) or natural exposure condition. The RA and the coarse NA were both separated in different di/Di replacement,sizes, and the mixed mix’s using reactivity a method seems based not onto be the si Faury’sgnificantly reference affected curve. (Figure The 10). author’s The authors prepared also four studiedconcrete the familieseffect of withtotal diorff erencespartial use in mix of reactive reactivity, orRA non-reactive replacement RA contents on RAC’s (0%, behaviour. 20%, 50%, For and this 100%), purpose,RA age controlled (according reactive to aging (full suff ered)reactive and NA) cement and typenon-reactive (42.5R or (f 52.5R).ull non-reactive The reactivity NA) of concrete aggregates mixeswas were evaluated used, which using ASTMwere differently C1260, and aged the to reactivity simulate of old concrete and recent mixes concrete. was evaluated Before recycling, according to bothRILEM types AAR-3of concrete CPT. underwent The authors either also accelerated comment that aging the (climatic age and exposurechamber at conditions 38 ± 2 °C ofand concrete relative will Materials 2020, 13, x FOR PEER REVIEW 13 of 21 humidityhave an higher influence than on 95%) the RA’sor na alkali-reactivitytural exposure potential.condition. The RA and the coarse NA were both separated in different di/Di sizes, and mixed using a method based on the Faury’s reference curve. The author’s prepared four concretenon-reactive families NA reference with differences 20%in mixnon-reactive reactivity, RA RA replacement 50% non-reactive RA 100% non-reactive RA contents (0%, 20%, 50%, 0.15and 100%), RA age (according to aging suffered) and cement type (42.5R or 52.5R). The reactivity of aggregates was evaluated using ASTM C1260, and the reactivity of concrete 0.13 mixes was evaluated according to RILEM AAR-3 CPT. The authors also comment that the age and exposure conditions of concrete0.11 will have an influence on the RA’s alkali-reactivity potential. 0.09
0.07
0.05 Expans i on (% ) 0.03
0.01
-0.01 0 28 56 84 112 140 168 196 224 252 280 308 336 364 Time (days)
Figure 9. Evolution of the expansion in non-reactive concrete mixes using the RILEM AAR-3 method [82]. Figure 9. Evolution of the expansion in non-reactive concrete mixes using the RILEM AAR-3 method [82].
reactive NA reference 20% reactive RA 50% reactive RA 100% reactive RA non-reactive NA reference 0.15
0.13
0.11
0.09
0.07
Expansion Expansion (% ) 0.05
0.03
0.01
-0.01 0 28 56 84 112 140 168 196 224 252 280 308 336 364 Time (days)
Figure 10. Evolution of the expansion in reactive concrete mixes using the RILEM AAR-3 method [82].
6.5. Potential Influence of RA Age and Exposure Condition on ASR Development The exposure time of a concrete to favourable conditions to ASR development seems to limit the reactivity state of the aggregates and their ability to continue to react after recycling process and incorporation in a new concrete. Desmyter & Blockmans [83] observed that the reactive elements of studied RA preserved only a limited reactivity after years of deleterious ASR in old concrete. The authors analysed a potentially alkali-reactive NA (aggregate B), a non-alkali reactive NA (aggregate C) and a RA (aggregate SB) from the demolition of a bridge affected by ASR. The reactivity of the aggregates was studied by CPT according to the French standard NF P 18-587, similar to the RILEM AAR-3 test-method. Desmyter & Blockmans [83] tests results showed that NA “Aggregate B” (reactive) confirmed to be reactive, while NA “aggregate C” (non-reactive) proved to be non-reactive. RA from the demolition of a bridge affected by ASR, denominated “Aggregate SB” showed an expansive behaviour in the first months, but did not exceed the 0.04% threshold expansion limit at 8 months of the NF P 18-587 standard (Figure 11).
Materials 2020, 13, x FOR PEER REVIEW 13 of 21
non-reactive NA reference 20% non-reactive RA 50% non-reactive RA 100% non-reactive RA 0.15
0.13
0.11
0.09
0.07
0.05 Expans i on (% ) 0.03
0.01
-0.01 0 28 56 84 112 140 168 196 224 252 280 308 336 364 Time (days)
Figure 9. Evolution of the expansion in non-reactive concrete mixes using the RILEM AAR-3 method Materials 2020, 13, 2625 13 of 20 [82].
reactive NA reference 20% reactive RA 50% reactive RA 100% reactive RA non-reactive NA reference 0.15
0.13
0.11
0.09
0.07
Expansion Expansion (% ) 0.05
0.03
0.01
-0.01 0 28 56 84 112 140 168 196 224 252 280 308 336 364 Time (days)
Figure 10. Evolution of the expansion in reactive concrete mixes using the RILEM AAR-3 method [82]. Figure 10. Evolution of the expansion in reactive concrete mixes using the RILEM AAR-3 method [82]. 6.5. Potential Influence of RA Age and Exposure Condition on ASR Development 6.5. PotentialThe exposure Influence time of RA of aAge concrete and Exposure to favourable Condition conditions on ASR Development to ASR development seems to limit the reactivityThe exposure state time of the of aggregates a concrete to and favourable their ability conditions to continue to ASR to react development after recycling seems process to limit andthe reactivityincorporation state in of a the new aggregates concrete. Desmyterand their &ability Blockmans to continue [83] observed to react thatafter the recycling reactive process elements and of incorporationstudied RA preserved in a new only concrete. a limited Desmyter reactivity & after Blockman years ofs deleterious[83] observed ASR that in oldthe concrete.reactive elements The authors of studiedanalysed RA a potentially preserved alkali-reactiveonly a limited NA reactivity (aggregate after B), years a non-alkali of deleterious reactive ASR NA in (aggregate old concrete. C) and The a authorsRA (aggregate analysed SB) a from potentially the demolition alkali-reactive of a bridge NA a(aggregateffected by ASR. B), a Thenon-alkali reactivity reactive of the NA aggregates (aggregate was C)studied and a by RA CPT (aggregate according SB) to thefrom French the demolition standard NF of P a 18-587, bridge similar affected to theby RILEMASR. The AAR-3 reactivity test-method. of the aggregatesDesmyter & was Blockmans studied by [83 CPT] tests according results showed to the Fr thatench NA standard “Aggregate NF P B”18-587, (reactive) similar confirmed to the RILEM to be AAR-3reactive, test-method. while NA “aggregate Desmyter C” & (non-reactive)Blockmans [83] proved tests to results be non-reactive. showed that RA fromNA “Aggregate the demolition B” (reactive)of a bridge confirmed affected byto be ASR, reactive, denominated while NA “Aggregate “aggregate SB”C” (non-reactive) showed an expansive proved to behaviour be non-reactive. in the MaterialsRAfirst from months, 2020 the, 13 ,but demolitionx FOR did PEER not REVIEW exceedof a bridge the 0.04% affected threshold by ASR, expansion denominated limit at “Aggregate 8 months of SB” the NFshowed P14 18-587 of an21 expansivestandard (Figure behaviour 11). in the first months, but did not exceed the 0.04% threshold expansion limit at 8 months of the NF P 18-587 standard (Figure 11). 0.15 Expansion limit SB-a SB-b B-a 0.13 B-b C-a 0.11 C-b
0.09
0.07
Expansion (%)0.05
0.03
0.01
-0.01 012345678
Time (months) Figure 11. Evolution of expansion of selected aggregates in NF P 18-587, adapted from [83]. Figure 11. Evolution of expansion of selected aggregates in NF P 18-587, adapted from [83]. Nevertheless, Sugiyama et al. [84] found that the existing coarse NA inside the studied coarse RA maintainedNevertheless, almost Sugiyama all their et al. reactivity, [84] found even that after the 30 existing years in coarse the concrete NA inside of a the bridge studied affected coarse by RAASR. maintained The authors almost analysed all their the reactivity, RA’s reactivity even ofafter the 30 columns years in of the a bridgeconcrete aff ectedof a bridge by ASR, affected in Japan. by ASR.The bridge’s The authors concrete analysed contains the a RA’s non-reactive reactivity river of sandthe columns and an alkali-reactive of a bridge affected andesite by coarse ASR, aggregate.in Japan. TheTwo bridge’s types of RAconcrete were producedcontains a by non-reactive crushing, one river coming sand directly and an from alkali-reactive crushing (RA-L) andesite and anothercoarse aggregate.that after crushing Two types underwent of RA were abrasion produced to minimize by crushing, the amount one coming of adhered directly mortar from (RA-H). crushing RA-L (RA-L) was and another that after crushing underwent abrasion to minimize the amount of adhered mortar (RA- H). RA-L was about 60% of adhered mortar and had 6% water absorption, while RA-H had about 30% adhered mortar and had 3% water absorption. All aggregates were classified as alkali-reactive by AMBT (Figure 12). Using CPT, authors claimed that ASR development in concrete with coarse RA was delayed due to the dampening effect of the adhered mortar and the reactive ring around the coarse RA.
1.00 Expansion limit 0.90 Origine NA 0.80 0.70 RA-H 0.60 RA-L 0.50
Expansion0.40 (%) 0.30 0.20 0.10 0.00 0 7 14 21 28 Time (days)
Figure 12. Expansion behaviour of RA and NA according ASTM C1260, adapted from [84].
6.6. Potential Mitigation Measures There are several procedures, some in use and others still in study, to mitigate ASR in NAC, such as: the use of supplementary cementitious materials (SCM) [85] and lithium-based chemical additions [86–89]. SCM’s influence the alkalis content in the paste pore solution by reducing the amount of alkalis available for ASR since some were fixed in the reaction with SCM. According to Thomas [85], the reaction that occurs with SCMs is similar to ASR. The reactive silica of SCMs reacts with the alkali-
Materials 2020, 13, x FOR PEER REVIEW 14 of 21
0.15 Expansion limit SB-a SB-b B-a 0.13 B-b C-a 0.11 C-b
0.09
0.07
Expansion (%)0.05
0.03
0.01
-0.01 012345678
Time (months)
Figure 11. Evolution of expansion of selected aggregates in NF P 18-587, adapted from [83].
Nevertheless, Sugiyama et al. [84] found that the existing coarse NA inside the studied coarse RA maintained almost all their reactivity, even after 30 years in the concrete of a bridge affected by ASR. The authors analysed the RA’s reactivity of the columns of a bridge affected by ASR, in Japan. The bridge’s concrete contains a non-reactive river sand and an alkali-reactive andesite coarse aggregate. Two types of RA were produced by crushing, one coming directly from crushing (RA-L) Materials 2020, 13, 2625 14 of 20 and another that after crushing underwent abrasion to minimize the amount of adhered mortar (RA- H). RA-L was about 60% of adhered mortar and had 6% water absorption, while RA-H had about 30%about adhered 60% of adheredmortar and mortar had and 3% had water 6% absorption. water absorption, All aggregates while RA-H were had classified about 30% as adheredalkali-reactive mortar byand AMBT had 3% (Figure water 12). absorption. Using CPT, All authors aggregates claimed were th classifiedat ASR development as alkali-reactive in concrete by AMBT with (Figurecoarse RA12). wasUsing delayed CPT, authors due to claimed the dampening that ASR effect development of the adhered in concrete mortar with and coarse the RA reactive was delayed ring around due to the the coarsedampening RA. effect of the adhered mortar and the reactive ring around the coarse RA.
1.00 Expansion limit 0.90 Origine NA 0.80 0.70 RA-H 0.60 RA-L 0.50
Expansion0.40 (%) 0.30 0.20 0.10 0.00 0 7 14 21 28 Time (days) Figure 12. Expansion behaviour of RA and NA according ASTM C1260, adapted from [84]. Figure 12. Expansion behaviour of RA and NA according ASTM C1260, adapted from [84]. 6.6. Potential Mitigation Measures 6.6. Potential Mitigation Measures There are several procedures, some in use and others still in study, to mitigate ASR in NAC, suchThere as: the are use several of supplementary procedures, some cementitious in use and materials others (SCM)still in [study,85] and to lithium-basedmitigate ASR chemicalin NAC, suchadditions as: the [86 use–89 ].of supplementary cementitious materials (SCM) [85] and lithium-based chemical additionsSCM’s [86–89]. influence the alkalis content in the paste pore solution by reducing the amount of alkalis availableSCM’s for influence ASR since the some alkalis were content fixed in in the the reaction paste po withre SCM.solution According by reducing to Thomas the amount [85], the of reaction alkalis availablethat occurs for with ASR SCMs since is some similar were to ASR.fixed The in the reactive reaction silica with of SCMs SCM. reacts According with the to alkali-hydroxides,Thomas [85], the reactionforming ASRthat occurs gels with with a relatively SCMs is lowsimilar Ca/ Sito ratioASR. without The reactive expansion silica behaviour. of SCMs reacts This author with the concludes alkali- that higher amounts of SCMs are required when there is an increase in the alkalis content of concrete or the potential alkali-reactivity of the aggregates. Silica fume, metakaolin, low-calcium fly ash, and ground granulated blast furnace slags are the most used SCMs. Lithium-based chemical additions, mainly lithium nitrate, are recognized to be efficient in the prevention and decreasing of the dissolution of reactive silica. However, this mitigation mechanism is not yet fully understood. Tremblay et al. [87] stated that the mechanism that could explain the phenomenon is due to the higher chemical stability of silica in the presence of lithium ions. Kim and Olek [88] stated that Li+ facilitates the emergence of a physical barrier on the surface of reactive aggregate and hampers the consumption of alkalis ions. Despite these works, a recent study of Liu et al. [90] on lithium use for ASR mitigation demonstrated that the use of a high concentration of lithium nitrate can be effective for ASR mitigation at the early stages, but after a long period of time, the formation of lithium silicate (Li2SiO3) occurs and that causes expansion and cracks in concrete. RAC research also presents proposals for ASR mitigation, with similar measures to those of NAC. Stark [74] recommended the use of low alkali cement and fly ash to inhibit ASR expansion. The author observed that the replacement of cement with 20% of fly ash re-established, in most cases, the levels of safety related to expansion. However, Stark [74] considered that, in order to reduce expansions to values below the limit, without exception, it is necessary to use the combination of low alkali cement and fly ash. Desmyter & Blockmans [83] also observed that the use of low alkali cement or incorporation of granulated-blast furnace slag might improve the durability of RAC, when ASR is a concern. Sánchez de Juan [91] introduced the ASR problem in RAC during a state-of-the-art analysis on structural concrete design with RA. The author referred to the possibility that RA could potentially be reactive to alkalis, which was justified, according to BRE Digest 433 [92] and Sagoe-Crentsil & Materials 2020, 13, 2625 15 of 20
Brown [93], by the high alkali content in the adhered mortar. Sánchez de Juan (2004) considered that the use of RA may potentiate ASR in RAC due to the difficulty in monitoring the reactivity of the RA, because of its heterogeneous origin and, as noted by Etxeberria [2], requires separate ASR tests for the characterization of the coarse and fine fractions. Sánchez de Juan [91] found through theoretical calculations that in RAC the use of low alkali cement contents would be necessary, unlike in NAC. Li [78] states that small amounts of mineral additions or lithium salts should be used to mitigate ASR in concrete formulated with the reference Blue Rock aggregate. The author concluded that 25% of fly ash, or 55% of granulated blast furnace slag or 12% of silica fume, all as cement replacement can also mitigate ASR, but the formation of silica fume clusters can reduce this effectiveness. The author also analysed the mitigation capacity of the use of lithium ions, either by incorporation in the mix or by application in the structure itself. Li [78] found the need for larger amounts of lithium to mitigate ASR in RAC compared to what happens in NAC. In the surface application of lithium nitrate in pavement blocks, a penetration of about 25 mm of lithium ion was observed, with reductions in surface expansion, although, according to the researcher, there was no effect on internal expansion. Li [78] also made an analysis of the interstitial cement paste of mortars produced with the natural Blue Rock aggregate or with RA. The analyses, which were complemented by determining the calcium hydroxide content by thermogravimetry, showed that the mineral additions significantly reduced the contents of calcium hydroxide and alkalis and the pH of the interstitial solution. Scott IV [67] found that only mixes with low alkali cement, 55% granulated blast furnace slag, 25% fly ash or 100% lithium nitrate relative to the total alkali content were effective in ASR mitigation. Similar to previous researchers, Shehata et al. [73] also observed that RAC suffered similar expansions to the reference NAC, although they noted a need to introduce larger amounts of mineral additions to mitigate ASR.
7. Conclusions and Challenges for Future Research One of the main sustainability challenges in construction is focused on eliminating waste and preserving natural resources. The reuse of construction and demolition waste has become a reality to improve such a purpose. The incorporation of RA in structural concrete to construction intends to promote the sustainability and reduce environmental issues. However, according to the literature, there are a growing number of structures affected by ASR, which increases the possibility of having potentially reactive RA in the future if recycled. Since RA do not come only from one type of concrete or from one structural element, unlike controlled concrete, it is difficult to measure and control the amount of alkalis present in the adhered mortar of RA or the content of reactive silica present in NA. Literature results are often contradictory about the effects of RA on ASR occurrence. However, the main conclusions of the analysis of literature, in order to contribute to ASR in RAC’s knowledge and discussion, are the following:
ASR can continue in RAC, in particular: by increasing the alkali content of the medium; fresh • faces of NA contained in the RA because of crushing; a more favourable environmental exposure for ASR development; Secondary crushing increases RA reactivity if reactive NA are present. Adhered mortar can limit • the tendency of NA to react; Age and exposure conditions of recycled origin concrete seem to be reflected on RA reactivity, • probably due to the consumption of aggregate reactive phases; RA’s mitigating effect on the AMBT and CPT expansion, caused by the adhered mortar, should be • considered in future tests, reviewing methodologies and expansion limits; The size of test specimens should be increased when RA are used; AMBT and CPT tests should • be reviewed to accommodate RA; RA washing limitation and water absorption of RA should be considered; Materials 2020, 13, 2625 16 of 20
AMBT for ASR evaluation, although faster, can cause misleading results due to the fragmentation • of RA, which causes the detachment of the adhered mortar, creating fine RA with different characteristics from those of the original coarse RA; our suggestion is to test separately the adhered mortar and NA or that only RA from coarse RA (i.e., from secondary crushing); Soluble alkalis content of the RA should not be ignored since they will increase the alkali content • of RAC; High amounts of mineral additions (e.g., fly ash, silica fume or granulated blast furnace slag) are • required to mitigate ASR in RAC to achieve NAC’s levels. RA composed of non-reactive NA and non-reactive adhered mortar do not cause ASR in RAC; • when RA are exclusively composed of high alkali reactive NA, ASR expansion in RAC increases with the increase of replacement ratio of NA with RA. A replacement level of 20% does not suggest a significant increase in mix reactivity.
Several doubts still without answer compromise the development of specifications for the use of RAC, promoting a sustainable future in construction. Conclusions are unanimous in that the study of RAC should always include the assessment of RA’s reactivity to alkalis. However, the heterogeneity that may exist in alkalis and reactive silica contents in RA makes this process more difficult to achieve. ASR combat does not just mean eliminating the risk associated with the use of reactive aggregates, as this will have serious economic, environmental and sustainability implications. The main challenge for future research is to find ways to minimize the occurrence of the reaction even in the presence of reactive aggregates.
Author Contributions: M.B.S. performed the literature review. The analyses of the references and interpretation of the results were developed by M.B.S., J.D.B. and A.S.S. The original draft of this paper was written by M.B.S. and the review and editing was performed by J.D.B. and A.S.S. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors gratefully acknowledge the support of CERIS, from IST-University of Lisbon, the Foundation for Science and Technology, and also National Laboratory for Civil Engineering—LNEC for their support through the project RE-IMPROVE (Expansive reactions in concrete—Prevention and mitigation of their effects). Conflicts of Interest: The authors declare no conflict of interest.
Abbreviation
AMBT accelerated mortar-bar test method ASR alkali-silica reaction CMBT concrete micro-bar test CPT concrete prism test NAC natural aggregate concrete RAC recycled aggregate concrete CDW construction and demolition waste ITZ interfacial transition zone NA natural aggregates OC original concrete RA recycled aggregates
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