Strength Development of Concrete

Strength Development of Concrete

8

Main Headings

• Compressive strength • Tensile and flexural strength • Impact loading • Resistance to high temperatures

Synopsis The effects of several factors relating to the incorporation of recycled aggregates on the strength development of concrete subjected to compressive, tensile and flexural loading are assessed in this chapter. The factors inherent to recycled aggregate that have been studied include replacement level, size, type and quality. The influence of the interactions of recycled aggregate with mineral additions, moisture condition and chemical admixtures are also addressed in this chapter. Emphasis is also placed on the effects of recycled aggregate incorporation on the resistance to impact loading and to high temperature exposure conditions. Keywords: Construction and demolition waste, Recycled aggregate concrete, Compressive strength, Tensile strength, Flexural strength, Impact loading, Resistance to high temperature.

Sustainable Construction Materials: Recycled Aggregates. https://doi.org/10.1016/B978-0-08-100985-7.00008-X Copyright © 2019 Elsevier Ltd. All rights reserved.

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8.1  Introduction The use of recycled aggregate (RA) as an alternative to natural aggregate (NA) has been widely looked upon as the most sustainable solution for the use of construction and demolition waste (CDW). Even though it is mostly used in some geotechnical or road pavement applications, an insufficient volume of CDW is assimilated in them and they are often considered as downcycling of RA. Concrete, on the other hand, as the second most consumed material in the world, can certainly serve as a cost-effective outlet for the greater reduction of CDW volume. Indeed, applying adequate recycling and beneficiation procedures to CDW gives rise to RA that may present characteristics close to those of conventionally used NA and thus can be integrated into the concrete production industry. However, despite the several specifications concerning RA for concrete production (BCSJ, 1977; BRE, 1998; BS-8500, 2006; DAfStb, 1998; DIN-4226, 2002; EHE-08, 2010; JIS-5021, 2011; JIS-5022, 2012; JIS-5023, 2012; LNEC-E471, 2006; NBR-15116, 2005; OT-70085, 2006; PTV-406, 2003; WBTC-No. 12, 2002; RILEM, 1994; TFSCCS, 2004), there are still some discrepancies concerning the classification of RA, in terms of composition and physical properties. As a result, structural codes for the design of concrete have yet to present comprehensive clauses that can provide a greater insight into the effects of RA on the strength development of concrete. The literature review on this subject revealed the possibility of using RA in a broad spectrum of concrete applications, such as non-structural low-cement concrete (Lopez-Uceda et al., 2016a, 2016b), pervious concrete (Zhang et al., 2015, 2017; Guneyisi et al., 2016; Barnhouse and Srubar, 2016), self-compacting concrete (Milosevic et al., 2016; Kapoor et al., 2016; Gesoglu et al., 2015), breakwater concrete blocks (Etxeberria et al., 2016a,b), fibre-reinforced concrete (Carneiro et al., 2014; Mohammad et al., 2016; Islam and Siddique, 2017) and fibre-reinforced polymer confined concrete (Gao et al., 2016), among several others. Despite the interest of each of these individual applications, since the effects of RA incorporation are fundamentally the same in the cementitious composite, the evaluation performed throughout this chapter is based on the effects of RA on the main properties of concrete, rather than focusing on specific applications. Nevertheless, there is some divergence in the findings concerning the effects of RA on the mechanical performance of RA concrete (RAC). Despite the fact that some studies have highlighted the distinct features of RA coming from different sources, which must be accounted for in the production of concrete (De Brito and Robles, 2010, Dhir and Paine, 2007, Kikuchi et al., 1998, Dhir et al., 2004), it is clear that most studies on the subject have shown no regard for the quality of RA or how this may affect the strength development of concrete. Thus, this chapter presents a compilation, analysis and evaluation of published information on the influence of RA on the main mechanical properties of concrete, which can be further considered in the design of structural concrete.

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8.2  Compressive Strength The compressive strength of concrete is the most commonly used property to evaluate its overall performance. It usually presents a good correlation with other mechanical properties and also the durability of concrete, thereby being widely looked upon as a quality indicator of concrete in general. In the specific case of RAC, there are several factors related to the incorporation of RA that influence the compressive strength of concrete. The replacement level is often considered to be the factor with the highest impact on the property, yet other more specific variables must also be considered in such an analysis, namely, the size, type and physical properties of RA. Given the inherently higher porosity of RA, its moisture state must always be taken into consideration in the production of RAC. It not only has an influence on the consistence (workability), but also has a noteworthy impact on the strength development of the concrete. Furthermore, depending on the size and nature of the RA, its use in concrete may have varying interactions with mineral additions and chemical admixtures such as water-reducing admixtures (WRAs).

8.2.1  Influence of Recycled Aggregate Content The literature concerning the effects of incorporating RA on the mechanical properties of concrete has shown that there is a general consensus in that the performance generally declines with increasing RA content. Figure 8.1 shows the 28-day compressive strength of concrete obtained from various studies plotted against increasing replacement of coarse or fine RA content. The data were obtained from 977 concrete mixes, sourced from 79 publications (Adams et al., 2016; Akca et al., 2015; Anderson et al., 2016; Montero and Laserna, 2017; Pedro et al., 2017; Pickel et al., 2017; Lotfi et al., 2015; Wardeh et al., 2015; Brand et al., 2015; Huda and Alam, 2014; Garcia-Gonzalez et al., 2014, 2017; Medina et al., 2014; Carro-Lopez et al., 2015; Akbarnezhad et al., 2011; Amorim et al., 2012; Barra and Vázquez, 1998; Butler et al., 2011; Buyle-Bodin and Hadjieva-Zaharieva, 2002; Cachim, 2009; Casuccio et al., 2008; Chen et al., 2003; Choi and Yun, 2012; Corinaldesi, 2010; Corinaldesi and Moriconi, 2007; Correia et al., 2006; Dapena et al., 2011; Debieb and Kenai, 2008; Dhir et al., 1999, 2003; Dhir and Paine, 2007; Domingo-Cabo et al., 2009; Dosho, 2007; Etxeberria et al., 2007; Evangelista and de Brito, 2007, 2010; Ferreira et al., 2011; Gomez-Soberon, 2002; Gonçalves et al., 2004; González-Fonteboa and Martínez-Abella, 2004; Jau et al., 2004; Juan and Gutiérrez, 2004; Kenai et al., 2002; Khalaf and DeVenny, 2004a, 2005; Khatib, 2005; Kim et al., 2013b; Kim and Yun, 2013; Knights, 1998; Kou and Poon, 2009a; Kou et al., 2007; Koulouris et al., 2004; Limbachiya et al., 2012; Manzi et al., 2013; Matias et al., 2013; Nagataki et al., 2004; Olorunsogo, 1999; Otsuki et al., 2003; Park, 1999; Pereira et al., 2012b; Poon and Kou, 2010; Poon et al., 2004a,b; Rahal, 2007; Rao et al., 2010; Ravindrarajah et al., 1987; Razaqpur et al., 2010; Ridzuan et al., 2005; Salem et al., 2003; Sarhat, 2007; Shayan and Xu, 2003; Tang et al., 2007; Teranishi et al., 1998; Thomas

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Sustainable Construction Materials: Recycled Aggregates

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et al., 2013b; Vieira et al., 2011; Waleed and Canisius, 2007; Wang et al., 2011; Yang et al., 2008, 2011). The maximum and minimum 28-day compressive strength values of RAC with 100% RA content were, respectively, 1.38 and 0.50 times lower than those of the corresponding control mixes. This considerable scatter indicates that attempting to explain the compressive strength variation on the basis of merely the replacement level is not feasible. Furthermore, it should be noted that the scatter observed here is not an indication of the lower reliability of the properties of RAC, which tend to be as consistent as those of a corresponding NA concrete (NAC) (Huda and Alam, 2015), but rather a result of the several variables involved in the test programmes undertaken. The lower and upper limits (dashed lines in Figure 8.1) of the 95% confidence interval were analysed in an attempt to further understand the boundaries of this behaviour. There is a probability of 95% that RAC with 100% coarse or fine RA content will exhibit a compressive strength value between 1.29 and 0.39 times that of the control NAC mix. This variation, as explained further on, is primarily dependent on the quality and moisture state of the RA. However, some of the greater-than-expected variation in the compressive strength values of specimens with lower replacement levels propagated to higher levels, which led to somewhat unrepresentative values. For example, the dashed line of the lower limit cannot represent the compressive strength loss of mixes with replacement levels of 100%, as it is considerably below the minimum value of the sample. Therefore, new upper and lower limits (solid lines in Figure 8.1) of the 95% confidence interval were made by taking into account specimens made solely with 100% RA content. This analysis was made since a greater sample size was collected for mixes with 100% RA content, which can more accurately represent the behaviour. Also, because the compressive strength loss or gain progresses in a linear fashion, it can be said that it may vary in line with the extreme values, and thus strength variation of intermediate replacement levels can be interpolated without considering the scatter normally associated with concrete testing. The upper and lower limits of the 95% confidence interval of specimens with 100% RA content suggest that the compressive strength of RAC is likely to vary between 1.13 and 0.66 times, respectively, that of the corresponding control specimens. As stated, the results plotted in Figure 8.1 show a considerable scatter, which can be attributed to the combined effects of several variables related to the use of RA. A specific combination of some of those variables leads to compressive strength gain, which is very much possible, though unlikely to be observed with the use of conventional RA from beneficiated CDW. This strength gain usually occurs as a result of RA coming from highstrength materials and/or the use of higher contents of WRAs in combination with partly dry RA. Naturally, the latter approach produces specimens with notably lower effective water/cement (w/c) ratios with increasing replacement level, which are not comparable to the physicochemical and hydration mechanisms of mixes containing water-compensated RA, nor from a microstructure viewpoint (Leite and Monteiro, 2016; Jeong et al., 2016).

8.2.2  Size of Recycled Aggregate It has been stated that replacing NA with up to 30% coarse or 20% fine recycled concrete aggregate (RCA) will have little influence on the mechanical performance of concrete (Dhir and Paine, 2004) and thus could be used as a rule of thumb when producing RAC,

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Figure 8.2  Relative compressive strength of recycled aggregate concrete (RAC) mixes with increasing (a) fine or (b) coarse recycled aggregate (RA) content (dashed lines represent the upper and lower limits of the 95% confidence interval of the whole sample, whilst solid lines concern those of specimens with replacement levels of 100%).

since the compressive strength would only gradually decrease for higher replacement levels (Teranishi et al., 1998; Dhir et al., 1999; Limbachiya, 2004; Etxeberria et al., 2007; Yang et al., 2008; Akbarnezhad et al., 2011; Fan et al., 2016). Despite some of the empirical evidence pointing towards this, it should be noted that this does not occur for all RAs, which present extremely changing properties depending on the strength and type of their main composing material, the beneficiation process to which they were subjected and also the moisture state upon their inclusion in the mixing process. In light of the aforementioned variation, the data in Figure 8.1 were further broken down in terms of one of the most known factors influencing the strength development of RAC (i.e., the size of the RA). Figure 8.2 separately plots the relative compressive strengths of RAC made with fine and coarse RA. It is immediately perceivable that coarse RA is capable of producing concrete with higher compressive strength than that

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of corresponding control specimens, whereas fine RA almost always results in lower strength, though an improvement may also be observed (Puthussery et al., 2017). Findings on the use of large RCA, with a particle size up to 80 mm, also showed negligible differences compared with NAC for replacement levels up to 40% (Li et al., 2016c). The limits of the 95% confidence interval in Figure 8.2a suggest that the compressive strength of specimens with 100% fine RA is likely to vary between about 1.10 and 0.35 times that of the control mixes. In parallel, Figure 8.2b shows that for 100% coarse RA, these numbers are around 1.30 and 0.46 times those of the corresponding control specimens, thereby indicating a greater probability of producing concrete with lower strength loss if it is made with coarse RA. As previously observed, however, in the attempt to include 95% of the sample inside the interval (dashed lines in Figure 8.2), the error observed in lower levels propagated to higher replacement levels, which led to somewhat unrepresentative limits. Therefore, limits for specimens with 100% RA content are also presented in Figure 8.2 (solid lines). These results indicate that RACs with 100% fine RA are likely to be between 1.02 and 0.54 times that of the control, which is more in agreement with the literature (Khatib, 2005; Yang et al., 2008; Evangelista and de Brito, 2007, 2010; Pereira et al., 2012a,b; Fan et al., 2016). For mixes with 100% coarse RA, this interval was between 1.12 and 0.60. Nevertheless, it can be clearly observed that the variation in the compressive strength of RAC cannot also be explained in terms of the RA’s size, as the intervals fail to represent much of the strength development.

8.2.3  Type of Recycled Aggregate As stated in Chapter 5 and according to existing specifications on the matter, there are three main types of RA from processed CDW that are suitable for the production of structural concrete: RCA, recycled masonry aggregate (RMA) and mixed recycled aggregate (MRA). Though considered by many as a type of RA, recycled asphalt pavement is excluded from this chapter’s scope in light of its extremely nefarious effects on the performance of concrete (Abdel-Mohti et al., 2016; Huang et al., 2005, 2006; McGinnis et al., 2017; Seara-Paz et al., 2016). When comparing the three RA types and their influence on the compressive strength of concrete, the findings from the literature point to a greater strength decline when using increasing RMA content, whereas RCA has often produced concrete materials with negligible losses. Figure 8.3 plots the same values as those presented in Figure 8.1, but divided by RA type (RCA, MRA and RMA in Figure 8.3a, b and c, respectively). The limits of the 95% confidence intervals for the whole sample and mixes with 100% RA content are presented in Table 8.1. The results are in line with the literature’s findings in that the use of RCA as an NA replacement is more likely to produce concrete with lower strength loss, followed by MRA and RMA, for which strength loss of up to 50% can be observed (Correia et al., 2006; Dhir and Paine, 2007). The degree of this strength loss is greater when both coarse and fine fractions of RA are used (Kumar et al., 2017; Gesoglu et al., 2015).

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Figure 8.3  Relative compressive strength of recycled aggregate concrete (RAC) mixes with increasing (a) recycled concrete aggregate (RCA), (b) mixed recycled aggregate (MRA) or (c) recycled masonry aggregate (RMA) content (dashed lines represent the upper and lower limits of the 95% confidence interval of the whole sample, whilst solid lines concern those of specimens with replacement levels of 100%).

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Table 8.1  Limits of the 95% confidence interval of the relative compressive strength of specimens made with different type of recycled aggregates RA Type RCA

MRA

RMA

95% Confidence Interval of: Whole Sample

100% RA Content

Whole Sample

100% RA Content

Whole Sample

100% RA Content

Upper limit

1.20

1.15

1.00

0.98

1.04

1.02

Lower limit

0.57

0.72

0.46

0.62

0.36

0.51

MRA, mixed recycled aggregate; RA, recycled aggregate; RCA, recycled concrete aggregate; RMA, recycled masonry aggregate.

Concrete made with MRA is expected to exhibit intermediate strength losses compared with mixes with RCA and RMA (Chen et al., 2003; Dhir and Paine, 2007; Knights, 1998; Yang et al., 2011). Typically, with increasing amounts of ceramic particles or other fragments coming from the demolition of masonry walls in RA (Anastasiou et al., 2014), the RAC containing it is likely to show progressively lower compressive strength (Dhir and Paine, 2007; Gomes et al., 2014). Contrary to general belief, however, RMA will not always lead to a considerable decline in mechanical performance. The use of fine RMA, even though likely to cause a decrease in compressive strength, will not present an overall strength reduction as noticeable as that from using coarse RMA (Debieb and Kenai, 2008; Khatib, 2005; Vieira, 2013; Vieira et al., 2016; Alves et al., 2014). This can be attributed to the pozzolanicity of RMA, the reaction of which is potentiated by the greater surface area of its ground state. Furthermore, using fine RMA as a sand replacement may even lead to increased strength development of the concrete over time, similar to what happens with fly ash-containing concrete (Khatib, 2005; Wild et al., 1996; Alves et al., 2014; Vieira et al., 2016; Ghernouti et al., 2016; Etxeberria and Vegas, 2015).

8.2.4  Quality of Recycled Aggregate The quality of RA can be considered as a subjective term to portray RA, considering the several variables affecting it, including the nature of the original NA itself (Arcas et al., 2016; Laserna and Montero, 2016). Nonetheless, it can be quantitatively measured by evaluating its main properties and characteristics and subsequently comparing them with those of conventionally used aggregates. Having such a benchmark not only will facilitate the categorisation of RA and eventually its certification for use in concrete for high-end purposes, but also presents a very strong correlation with the performance of concrete containing it (Lotfi et al., 2017; Silva et al., 2014a,b; 2015; Abdulla, 2015; Andal et al., 2016).

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Among other conventionally studied properties, such as particle size distribution or organic content, which must be evaluated in NA before its use in concrete, special attention must also be given to the water absorption, density and resistance to fragmentation of RA. Assessing these properties can provide an indirect assessment of the porosity of aggregates as well as the strength of the source material, which is known to have a considerable effect on the mechanical performance of concrete (Hansen, 1992; Kikuchi et al., 1998; Nagataki et al., 2004; Nagataki and Lida, 2001; Otsuki et al., 2003; Poon et al., 2004a; Wang et al., 2011; Yang et al., 2008; Silva et al., 2014b; Surya et al., 2015). As explained in Chapter 5, these properties can be actively controlled during the RA’s processing phase, wherein the application of additional treatments, such as the heating and grinding method (Voinitchi et al., 2014), acid treatments (Katkhuda and Shatarat, 2016; Güneyisi et al., 2014), submersion in water glass (Güneyisi et al., 2014) or accelerated carbonation curing (Kou et al., 2014b), will decrease the material’s porosity, thereby improving the mechanical performance of concrete containing it. Although this is usually associated with RCA coming from high-strength concrete, equivalent compressive strength values can also be observed with the use of RMA coming from high-strength ceramic brick units (Khalaf, 2006, Khalaf and DeVenny, 2004a,b, 2005). Crushing bricks with enhanced mechanical performance yields aggregates with lower water absorption, higher oven-dry density and better resistance to fragmentation than those obtained from regular ceramic brick units. In the previous section, since the separation by type of RA did not fully explain the strength loss of RAC, the data were further broken down with respect to the several quality classes of RA according to the methodology proposed by the authors in their previous studies (Silva et al., 2014a,b). The reason for this separation lies in the need to further classify RA based on their quality rather than solely according to their composition, which has been the approach followed thus far. Furthermore, applying such categorisation not only will increase the quality of certification of the material, but also will improve the confidence of stakeholders that are likely to use the aggregates (De Brito and Silva, 2016; Silva et al., 2017). Figure 8.4 presents the relative compressive strength of RAC mixes with increasing RA content separated according to their quality class (classes A, B, C and D). In one of the previous authors’ studies (Silva et al., 2014a), a similar analysis was carried out on a smaller sized sample available at the time. In that investigation, some of the data were filtered out, specifically those related to exceedingly different effective w/c ratios. The results’ coefficients of correlation showed strong to very strong correlations between the compressive strength loss and the increasing RA content, divided by quality class (Piaw, 2006). Nevertheless, here, further and more recent data were used in the analysis to increase the representativeness of the proposed models. Table 8.2 presents the upper and lower limits of the 95% confidence interval of the whole sample and of mixes containing only 100% RA content divided by quality class. Despite the considerable spread, which is expected in such type of analysis because of the considerable number of different variables related to the mix design (e.g., cement content, w/c ratio, aggregate-to-cement ratio, target strength, among others), equivalent and even higher compressive strength values could also be observed in

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Figure 8.4  Relative compressive strength of recycled aggregate concrete (RAC) mixes with increasing recycled aggregate (RA) of classes (a) A, (b) B, (c) C and (d) D (dashed lines represent the upper and lower limits of the 95% confidence interval of the whole sample, whilst solid lines concern those of specimens with replacement levels of 100%).

Sustainable Construction Materials: Recycled Aggregates

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Table 8.2  Limits of the 95% confidence interval of the relative compressive strength of specimens made with recycled aggregate of different quality classes RA Type Class A

Class B

Class C

Class D

95% Confidence Interval of: Whole Sample

100% 100% 100% RA Whole RA Whole RA Content Sample Content Sample Content

100% Whole RA Sample Content

Upper limit

1.30

1.13

1.26

1.15

1.14

1.01

0.91

0.89

Lower limit

0.76

0.79

0.56

0.71

0.37

0.55

0.32

0.50

RA, recycled aggregate.

RAC. The upper limits suggest that it is more probable to produce RAC exhibiting higher compressive strength than that of its corresponding control NAC when the RA is of better quality. Nevertheless, since this increase is extremely dependent on the mix design, moisture content of the RA and content of the WRA, its corresponding limits cannot be considered as representative of the actual behaviour of RAC. Regardless of the replacement level, the better approach for the manufacture of RAC is to maintain equivalent w/c ratio, by compensating for the absorbed water, in order to have workable mixes over time. RACs produced with this approach are likely to exhibit some loss in mechanical behaviour, such as that observed in previous results. Because of this, the lower limits associated with these mixes’ results can be considered as representative of the actual behaviour. Indeed, the lower limits presented in Table 8.2 show a clear logical decrease in relative compressive strength of RAC with decreasing quality of RA. By adopting a method that is capable of measuring the quality of the RA, its effects on the mechanical behaviour of concrete become more easily predictable and extrapolatable to its structural performance (De Brito et al., 2016). Several other models have been proposed for the variation in the mechanical behaviour of RAC, based on the modification of methods in existing codes (Seara-Paz et al., 2016), non-linear and multilinear regression analysis (Younis and Pilakoutas, 2013; Shin and Kim, 2013; Tam et al., 2008; Abdulla, 2015; Biglarijoo et al., 2017; González-Taboada et al., 2017; Liu et al., 2017; Pepe et al., 2016), fuzzy logic (Topçu and Sarıdemir, 2008), gene expression programming (Abdollahzadeh et al., 2016; Gonzalez-Taboada et al., 2016) and artificial neural networks (Topçu and Sarıdemir, 2008; Duan et al., 2013, 2017; Dantas et al., 2013; Kim et al., 2013a; Duan and Poon, 2014a). However, the authors firmly believe that the use of some of the aforementioned sophisticated and complicated methods may often fail to make physical sense and are unlikely to attract the attention of professionals in the construction industry. Therefore, the purpose of presenting such straightforward values for predicting the compressive strength loss of RAC (Table 8.2) is to deliver a

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model that can provide an accurate representation of the actual physical phenomena and, at the same time, be accessible enough to engineers with little knowledge of concrete science (ACI-209, 2008; Silva et al., 2014a). An economic prediction model based on few input data, simplicity, easy accessibility and representation of reality generally demonstrates better performance over time (Huettmann and Gottschalk, 2011; Burnham and Anderson, 2002). Another factor related to the quality of RA is its strength in relation to that of the new cementitious matrix. The fracture of a failing conventional concrete specimen usually occurs at the interfacial transition zone (ITZ) between the mortar and the NA, whereas in RAC, it is likely to develop through the RA, probably at the old ITZ (Peng et al., 2016), since this may contain several microcracks making it weaker than the adjacent new mortar (Ayob et al., 2017; Li et al., 2017). Thus, it has been observed that, as the RA content increases, the strength reduction of RAC mixes is more pronounced for mixes with lower w/c ratios (Chen et al., 2003; Dhir et al., 1999; Otsuki et al., 2003; Rao et al., 2007; Ray and Venkateswarlu, 1991; Teranishi et al., 1998; Limbachiya, 2004; McGinnis et al., 2017). From a probabilistic point of view, for relatively lower w/c ratio, the compressive strength of an RAC specimen is likely to depend more on the strength of its aggregates, which in turn also depends on the strength of the original material (Le et al., 2017). In the case of RCAs, these can be sourced from concrete products with originally different w/c ratios, thus altering the strength of the resulting aggregates’ adhered mortar. For this reason, RAC mixes produced with lower w/c ratios, but incorporating RCA from concrete products with relatively lower strength, will cause a failure to occur in the relatively weaker old adhered mortar. However, this effect seems to be insignificant for RAC with higher w/c ratios (Amer et al., 2016; Lopez-Uceda et al., 2016a, 2016b; Deng et al., 2016). In these cases, the new cement paste is relatively weak because of the higher water content, which increases porosity and yields poorer ITZ bond strength. Therefore, the ultimate compressive strength of concrete mixes with higher w/c ratio depends more on the strength of the new cement paste than on the strength of the RA (Lopez-Uceda et al., 2016a, Otsuki et al., 2003, Pedro et al., 2014a, 2015, 2017).

8.2.5  Influence of Mineral Additions Mineral additions, as partial replacement of or an addition to cement, are used for a number of reasons, including improved consistence (workability), enhanced resistance in some durability-related properties and, in some cases, higher strength. The interaction of coarse RA with additions, due to the former’s higher specific surface area, is likely to have a more inert role in the hydration reactions of concrete. However, because of the specific chemical composition of some of the RAs, new products of hydration may arise over time at the ITZ between the RA and the cementitious matrix. These chemical reactions are potentiated by the decreasing size of the RA itself, being capable of interacting with the mineral additions and the cement.

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Sustainable Construction Materials: Recycled Aggregates

8.2.6  Fly Ash It is widely known that the use of fly ash as a cement replacement will cause a reduction in the 28-day compressive strength of concrete, being inversely proportional to the amount of cement replaced, unless very fine fly ash is used (Shaikh, 2016). The use of fly ash in combination with RA leads to a combined effect typically resulting in an even greater decline in the mechanical performance of the concrete. However, it has been suggested that fly ash may chemically interact with the adhered mortar of RCA, consequently yielding materials with a lower than expected loss in mechanical performance. Kou et al. (2007) observed that the decline in the compressive strength of concrete with increasing coarse RCA content was lower when the fly ash content increased, which suggests some interaction between the two materials. This decline further widens with the combined use of coarse fly ash as the sand replacement and fine RCA (Ravindrarajah and Tam, 1987; Kou and Poon, 2009a,b; Kurda et al., 2017b). The use of such addition with RCA exhibiting greater surface area will have a beneficial outcome on the strength development of concrete, depending on its size and pozzolanic activity with cement. The results shown in Table 8.3 suggest that there may be a positive effect on the 28-day compressive strength from the combination of both fly ash and increasing fine RCA in comparison with the behaviour of the control specimens containing 100% coarse RCA.

8.2.7  Silica Fume Concrete containing a small portion of silica fume, as a cement replacement, usually has improved mechanical performance, albeit largely dependent on the average particle size of the addition (Pedro et al., 2017; Mukharjee and Barai, 2015a,b, 2017; Prusty et al., 2015). Apart from the formation of additional products of hydration as a result of the reaction between the addition and Ca(OH)2, since silica fume presents small particle size, some of its particles may fill the RA’s surface pores and microcracks, thereby enhancing the ITZs and preventing crack propagation through them (Mukharjee and Barai, 2017; Yaragal et al., 2016). The resulting compressive strength of the concrete is a combination of the strength loss caused by the use of RA and the strength increase proportional to the silica fume content (Corinaldesi et al., 2002). Figure 8.5 shows a parallel strength development (i.e., 10% strength increase for 10% silica fume) with increasing RA content, thereby suggesting minimal reaction between the mineral addition and the aggregate (Kou et al., 2011b). The findings of Pedro et al. (2017) also suggest similar consequences, wherein negligible differences were observed with the combined use of RA and silica fume.

8.2.8  Ground Granulated Blast-Furnace Slag The combined incorporation of ground granulated blast-furnace slag and RA is likely to cause a strength reduction in concrete at the initial ages after casting, compensated for by a higher strength gain rate at later ages (Berndt, 2009; Kou et al., 2011b). Nonetheless, the outcome of using this addition as a cement replacement is generally expected not to be influenced by the incorporation of coarse RA.

Mix Coarse RCA, %

Fine RCA, %

Cement, kg/m3

W/Beff Ratio

Coarse Fly Ash, kg/m3

Fine Fly Ash, kg/m3

fcm,cube, MPa

fc,RAC/Fc,NAC

R0

100

0

340

0.53

200

0

44.3

1.00

R25

100

25

340

0.53

200

0

44.5

1.00

R50

100

50

340

0.53

200

0

43.4

0.98

R75

100

75

340

0.53

200

0

41.3

0.93

R100

100

100

340

0.53

200

0

38.7

0.87

R0

100

0

340

0.44

200

70

53.7

1.00

R25

100

25

340

0.44

200

70

64.3

1.20

R50

100

50

340

0.44

200

70

62.3

1.16

R75

100

75

340

0.44

200

70

56.3

1.05

R100

100

100

340

0.44

200

70

53.2

0.99

Strength Development of Concrete

Table 8.3  Results of self-compacting recycled aggregate concrete mixes with increasing fine recycled concrete aggregate, different fly ash contents and different water/binder ratios

NAC, natural aggregate concrete; RAC, recycled aggregate concrete; RCA, recycled concrete aggregate; w/b, water/binder. Data from Kou and Poon (2009b).

235

236

Sustainable Construction Materials: Recycled Aggregates

8.2.9  Other Additions The effects of using other less conventional additions on the properties of RAC mixes have also been evaluated, including metakaolin (Kou et al., 2011b), brewery spent diatomite (Letelier et al., 2016), CDW powder (Kim, 2017; Corinaldesi and Moriconi, 2011, 2004; Corinaldesi et al., 2002; Asensio et al., 2016; Letelier et al., 2017a; Li and Yang, 2017; De Lucas et al., 2016; Ge et al., 2015; Kim et al., 2014), municipal solid waste incinerator bottom ash (Corinaldesi and Moriconi, 2004), waste paper sludge ash (Fauzi et al., 2016) and even temperature-treated hydrated cement powder (Letelier et al., 2017b). The findings of these and other similar studies indicate that the expected strength gain or loss from these additions is not affected by increasing coarse RA content in the mix. For example, in the case of metakaolin (Figure 8.5), the incorporation of this addition is expected to result in a strength increase in comparison with a corresponding mix without the addition. Nevertheless, it is natural that there is a decline in the mechanical behaviour from the addition of RA, leading to a material with intermediate strength. The use of pulverised CDW was found to have positive results depending on the debris constituents (Corinaldesi and Moriconi, 2011; Kim, 2017; Asensio et al., 2016). High-strength self-compacting concrete was produced with the addition of high silica-bearing (85% by weight) CDW powder; using the same content (about 20% by weight of cement), control mixes with CDW powder were capable of presenting equivalent slump flow and compressive strength compared with control mixes with fly ash (Corinaldesi and Moriconi, 2011). However, this positive outcome in strength

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Figure 8.5  Influence of different mineral additions (CC, control concrete; SF, silica fume; MK, metakaolin; FA, fly ash; GGBS, ground granulated blast-furnace slag) as partial cement replacements on the 28-day compressive strength of concrete with increasing coarse recycled concrete aggregate content. Data sourced from Kou et al. (2011b).

Strength Development of Concrete

237

development is likely to be witnessed only for additions wherein the original CDW materials were mostly composed of ceramic bricks and tiles (Asensio et al., 2016; De Lucas et al., 2016; Ge et al., 2015). Indeed, the use of waste concrete powder as a partial cement replacement, without any further treatment, will result in a considerable decline in the mechanical performance of the concrete (Kim et al., 2014). The use of such addition replacing 45% of cement can lead to a decrease of 50% in the 28-day compressive strength of concrete (Kim, 2017), which can be easily explained by the already hydrated state of the cement particles and absence of reactive constituents that can lead to pozzolanic reactions.

8.2.10  Moisture State of Recycled Aggregate It is usual for concrete producers to disregard the moisture content of NAs when producing conventional concrete, since their water absorption is typically low. Nevertheless, considerable attention must be given to the water absorption capacity of RA and their potential influence on the fresh and hardened properties of concrete. As stated in Chapter 5, this property depends on the porosity of the constituent materials, which, in the case of RCA, is potentiated by the adhered mortar content, whereas in RMA, it is normally due to the inherent porosity of ceramic particles. This prompted the use of RA in the saturated and surface-dry (SSD) state (Hansen, 1992), to avoid excessive absorption of the mixing water and maintain reasonable workability levels. Eventually, variations of this approach have appeared, based on a pre-saturation of the RA for a given period of time before mixing the constituents (Amer et al., 2016; Bizinotto et al., 2017; GonzálezTaboada et al., 2017) or compensating with additional water during the mixing process (Leite, 2001; Amorim et al., 2012; Evangelista and de Brito, 2007, 2010; Ferreira et al., 2011; Fonseca et al., 2011; González-Taboada et al., 2017). A comparison of the two approaches was carried out by Ferreira et al. (2011), with interesting and unambiguous findings. The water compensation approach during mixing is capable of making RAC mixes with more predictable workability levels and slightly enhanced mechanical behaviour compared with mixes made with pre-saturated RA. This improvement can be explained by two factors, one of them being the greater envelopment of the RA’s surface and pores by the cement paste, thereby improving the mechanical bond at the ITZ. The other reason for the better performance was the fact that these aggregates’ water absorption capacity was not completely compensated for and thus resulted in mixes with slightly lower effective w/c ratio with passing time. Several others have used this approach and observed equivalent consistence levels and minimum strength loss compared with the control mixes (Amorim et al., 2012, Evangelista and de Brito, 2007, 2010, Ferreira et al., 2011, Fonseca et al., 2011, Soares et al., 2014a,b, Pacheco et al., 2015a,b, 2017, Pedro et al., 2014a, 2015, 2017). Another perspective of the mixing approach was evaluated in several studies by Tam et al. (Tam et al., 2005, Tam and Tam, 2007, 2008) and variations of it (Eckert and Oliveira, 2017; Güneyisi et al., 2014), wherein attention was called to the fact that dividing the procedure into different stages, rather than using a single one, will benefit

238

Sustainable Construction Materials: Recycled Aggregates

the mechanical performance of RAC. The procedure followed by ready-mix concrete plants generally consists of placing the constituents in the mixer all at once, which, in these studies, was considered as the normal mixing approach. Tam et al. (2005) have proposed dividing this process into two stages (two-stage mixing approach), wherein the aggregates are first introduced along with part of the mixing water, which allows them to absorb a portion of the water, and, after a given period of time, cement is added with the rest of the mixing water. This process was found to result in mixes wherein the RA’s absorption capacity is partly or fully compensated for and its surface is more effectively enveloped with the cement paste, thereby providing enhanced ITZs and lower compressive strength loss (Tam et al., 2005) or even strength gain (Güneyisi et al., 2014). The reason for this improvement and the necessity to adjust current procedures of ready-mix concrete plants is that, when the binder is added at a later time during the mixing procedure, it will envelop the aggregate after it has absorbed the compensating water, and thus it is likely not to act as a pore sealant (Leite, 2001; Neville, 2011). Otherwise, it would lead to the presence of a greater than expected water content in the binder, thereby increasing the effective w/c ratio. Even in cases in which the total amount of water is the same, the moisture state of RA at the beginning of the mixing process will have a notable influence on the mechanical performance of concrete. In the study of Poon et al. (2004b), the use of RCA in an airdried state led to significantly lower strength loss compared with specimens made with RCA in an SSD state (Table 8.4), even though these mixes were made with the same total water content. It was considered that the RCA at an SSD state may have resulted in some bleeding around the particles, thereby decreasing the mechanical bond at the ITZ. As shown in Section 8.2.1, in some cases, increasing RA replacement levels led to similar and even improved mechanical performance (Ridzuan et al., 2005; Ho et al., 2013; Domingo-Cabo et al., 2009, 2010; Salesa et al., 2017; Zhou and Chen, 2017; Thomas et al., 2016; Carneiro et al., 2014). This increase in compressive strength can be explained by the reduction of the effective w/c ratio prompted by the use of partly dry RA in mixes with the same amount of initial free water as that of the corresponding control mixes. With elapsing time, the uncompensated RA absorbs the Table 8.4  Compressive strength of concrete with aggregates in different moisture states AD

OD

SSD

RCA Content, %

fcm, MPa

fcm,RAC/ fcm,Control

fcm, MPa

fcm,RAC/ fcm,Control

fcm, MPa

fcm,RAC/ fcm,Control

0

48.3

1.00

40.2

1.00

46

1.00

20

44.9

0.93

43.2

1.07

43

0.93

50

44.7

0.93

39.7

0.99

38.1

0.83

100

46.8

0.97

43.3

1.08

39.1

0.85

AD, air-dried; OD, oven-dried; RAC, recycled aggregate concrete; SSD, saturated and surface-dried. Data from Poon et al. (2004b).

Strength Development of Concrete

239

free mixing water, thereby resulting in a cementitious matrix with lower effective w/c ratio, ultimately leading to greater temperature release during the hydration process (Koenders et al., 2014), a finer porous microstructure (García-González et al., 2015; Gonzalez-Corominas et al., 2016) and higher compressive strength (Carneiro et al., 2014). This phenomenon may also be observed to some extent when using RMA, but, at the same time, will result in mixes that are very dry and difficult to compact (Khalaf, 2006). However, the use of completely dry RCA may annul the effect of superplasticisers, leading to unworkable mixes (Amer et al., 2016; Gupta et al., 2016), and may also cause a decline in compressive strength, though to a lesser extent than when using saturated RCA (Pepe et al., 2016; Koenders et al., 2014). This decline in mechanical behaviour may be partly explained by the potential formation of macropores around the RCA’s ITZ, as a result of air bubble release prompted by the lingering water absorption mechanism after the concrete’s compaction process (Leite and Monteiro, 2016).

8.2.11  Influence of Chemical Admixtures As explained in Chapter 7, the higher water absorption and rougher surface of RA will lead to stiffer and less workable mixes. Therefore, by adjusting the WRA content, the production of workable and high-strength RAC containing (partly) dry RA becomes possible (Prakash and Krishnaswamy, 1998). A WRA basically modifies the fresh behaviour of a concrete mix by making it as workable as a homologous mix without the admixture, but with higher water content. The mechanism behind this, which varies according to the chemical base of the admixture, will act mostly on cement and other finer solid particles. Thus, the efficacy of a WRA on the hardened properties of concrete is not likely to be affected by the incorporation of coarse RA. An analysis of the results of RAC and NAC mixes presented in Figure 8.6 shows that the average relative compressive strength slightly increases for concrete specimens made with WRA and increasing coarse RA. Nevertheless, given the high scatter involved and the low coefficients of determination, the only conclusion that can be drawn from this analysis is that the strength development of concrete with increasing coarse RA content is marginally affected by the presence of superplasticisers. Moreover, Figure 8.7(a) presents an additional example of this in a more controlled study by showing negligible influence on the compressive strength loss of RAC, with varying contents of WRA, as the coarse RCA content increases (Juan and Gutiérrez, 2004). However, it is possible that the water-reducing capacity of some admixtures may be affected by the presence of fine RCA, as suggested by the results of Pereira et al. (2012a,b), presented in Figure 8.7(b). In that investigation, the regular WRA was lignosulphonate based, whereas the high-range WRA was a combination of modified polycarboxylates. Due to the specific mechanism of the former admixture, it is possible that its efficacy in reducing the amount of water was hindered by the higher surface of area of the fine RCA compared with that of the replaced sand. For this reason, the effective w/c ratio had to increase, with higher replacement levels ultimately leading to slightly higher compressive strength losses.

240

Sustainable Construction Materials: Recycled Aggregates



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Figure 8.6  Influence of the presence of a water-reducing admixture (WRA) on the relative compressive strength of concrete mixes with increasing coarse recycled aggregate content. NAC, natural aggregate concrete; RAC, recycled aggregate concrete.

Although the effectiveness of WRA is apparently not affected by the increasing inclusion of coarse RA, this is likely to occur only in mixes with water-compensated RA. Indeed, if these are incorporated at a (partly) dry state, part of the mixing water will be absorbed, decreasing the mixes’ effective w/c ratio, thereby affecting the admixtures’ activity. Even though a comprehensive study evaluating the mechanics of this process does not exist to the best of the authors’ knowledge, the use of WRA can, nonetheless, produce highly flowing mixes with relatively high compressive strength. DomingoCabo et al. (2009, 2010) evaluated the mechanical performance of concrete containing 100% coarse RCA that had not been compensated for and with higher WRA content compared with that of the control mix (1.4% vs. 0.7%, respectively). The control and RAC mixes presented initial slumps of 170 and 190 mm, respectively, and 28-day compressive strength values of 45.3 and 54.8 MPa, respectively. This demonstrates the possibility of achieving significantly improved mechanical performance for the same total w/c ratio and slightly higher WRA content. Regarding the effects of using air-entraining admixtures on the compressive strength of RAC, findings suggest that incorporating RCA has marginal influence on the expected air content produced with the use of air-entraining admixtures, and thus its effect on the mechanical behaviour should remain unaffected (Dhir et al., 1999; Otsuki et al., 2003; Salem et al., 2003). In the study by Dhir et al. (1999), the authors noticed that, for mixes with the same target strength (achieved by maintaining the cement content and varying the effective w/c ratio), the air content was mostly unaffected (Table 8.5)

Strength Development of Concrete

241

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IF03D

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IF03D



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Figure 8.7  Compressive strength of recycled aggregate concrete with (a) varying contents of water-reducing admixture (WRA) and (b) different water-reducing capacities. HRWRA, highrange water-reducing admixture; RCA, recycled concrete aggregate. (a) Data sourced from Juan and Gutiérrez (2004); (b) data sourced from Pereira et al. (2012a,b).

242

Table 8.5  Effect of air-entraining admixtures on the air content of concrete mixes with target strength of 35 MPa RCA Content (%)

Air Content (%)

Cement Content (kg/m3)

Effective Water/ AEA Content Cement Ratio (%)

Control Air Content (%)

Fine

15 min

60 min

0

0

165

325

0.51

0.40

1.2

6.0

6.0

20

0

165

325

0.51

0.40

1.3

5.8

5.6

30

0

165

325

0.51

0.40

1.2

5.9

5.6

50

0

159

325

0.49

0.40

1.3

6.1

5.8

100

0

150

325

0.46

0.40

1.4

5.7

5.7

0

20

166

325

0.51

0.40

1.3

5.9

5.6

0

30

156

325

0.48

0.37

1.6

5.8

5.7

0

50

146

325

0.45

0.37

1.7

5.7

5.7

100

20

146

325

0.45

0.40

1.4

5.8

5.6

100

50

127

325

0.39

0.40

1.5

5.7

5.5

AEA, air-entraining admixture; RCA, recycled concrete aggregate. Values sourced from Dhir et al. (1999).

Sustainable Construction Materials: Recycled Aggregates

Coarse

Water Content (kg/m3)

Strength Development of Concrete

243

for a constant air-entraining admixture content (100 mL per 100 kg of Portland cement). Similar observations were reported elsewhere, wherein for the same mix design and airentraining admixture content, the air content of mixes containing 100% coarse RCA was not affected (Otsuki et al., 2003). Furthermore, in a study on the freeze–thaw resistance of RAC, it was observed that the relative strength loss between RAC and NAC was the same, regardless of the use of air-entraining admixtures (Salem et al., 2003).

8.2.12  Strength Development With Time Concrete exhibits progressively enhanced mechanical performance with time, the rate of which depends on the mix design (i.e., admixtures, additions and cement type used). Concerning the influence of RA in this respect, regardless of its content, the literature points towards a parallel strength development in comparison with corresponding control mixes, which suggests that RA generally has an inert role in the hydration reactions of concrete. One of the most relevant experimental studies undertaken was that by Poon and Kou (2010), in which the 10-year mechanical performance of concrete with increasing coarse RCA and fly ash content was evaluated (Figure 8.8). Tests were carried out 28 days and 1, 3, 5 and 10 years after casting. Initially (28 days after casting), concrete mixes without fly ash achieved the highest compressive strength values. These values began to decrease as the fly ash content increased; after 28 days, the average strength loss of mixes with fly ash relative to the control was 5%, 15% and 33% for mixes with fly ash content of 25%, 35% and 55%, respectively. The use of additions, such as fly ash or other materials exhibiting pozzolanicity, typically results in specimens with a slower initial strength development rate, but, after some time, they achieve equivalent or even higher compressive strength than mixes without the additions. After one year, there was a higher strength development of concrete mixes with 25% fly ash, having acquired a slightly higher compressive strength than specimens without additions. This greater strength development was due to the pozzolanic activity between the cement and the fly ash. All mixes with 25% fly ash began an almost parallel increase, ending with similar compressive strength after ten years. Mixes with 35% fly ash showed a similar higher rate of strength development, but over a longer period; performance comparable to that of control specimens was achieved after three years and a similar development was exhibited thereafter. After ten years, mixes with 55% fly ash content not only had almost the same compressive strength as mixes without fly ash, but also still showed higher strength development trends than any other mixes. This indicates that, further ahead, 55% fly ash-containing specimens will probably present similar or even higher compressive strength than the control mixes. Although all mixes with increasing replacement levels had progressively lower compressive strength 28 days after casting, ten years later, the difference between RAC and NAC was insignificant, except for the mix with 100% coarse RCA and 55% fly ash. These findings indicate that, apart from the residual cementing properties of unhydrated cement particles existing in RCA (Amin et al., 2016), further pozzolanic reactions may occur between the adhered mortar and the fly ash leading to an improvement of the ITZ between the coarse RCA and the new cement matrix.

244



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Figure 8.8  Strength development of concrete mixes with increasing content of coarse recycled concrete aggregate and varying content of fly ash. (a) No fly ash; (b) 25% fly ash (F25); (c) 35% fly ash (F35); (d) 55% fly ash (F55). Adapted from Poon and Kou (2010).

Sustainable Construction Materials: Recycled Aggregates

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5) 5) 5)

  

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Strength Development of Concrete



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Figure 8.8 Cont’d

245

246

Sustainable Construction Materials: Recycled Aggregates

The use of fine RMA may induce an increase in the rate of strength development of concrete over time (Wild et al., 1996; Khatib, 2005; Vieira et al., 2016) compared with the RCA-containing concrete, which may be attributed to their pozzolanicity (Awoyera et al., 2017; Asensio et al., 2016; Letelier et al., 2017a). Taking into consideration the higher water absorption of RA, some studies have been carried out on the potential benefits of this feature in different environments. Most authors have reported a parallel strength development between RAC and NAC, regardless of the environmental condition (Amorim et al., 2012; Dhir et al., 1999; Fonseca et al., 2011; Buyle-Bodin and Hadjieva-Zaharieva, 2002). However, it has been suggested that RCA (Gonzalez-Corominas and Etxeberria, 2016; Gayarre et al., 2014) and RMA from crushed ceramic bricks (Meddah and Sato, 2010) may have some benefits in terms of internal curing.

8.2.13  Multiple Recycling In light of increasing, albeit rare, use of RA in the production of new concrete products, an additional issue may arise in the near future with the multiple recyclability of concrete. With progressing generations of RAC, the RCA originating from its recycling will gradually exhibit lower content of original NA and higher content of adhered mortar, which can significantly affect the quality of RA. Indeed, in the study by De Brito et al. (2006), it was observed that the water absorption of coarse RCA increased from 6.3% to 7.6% and then to 8.5%, after three consecutive crushing cycles of concrete. The resulting RAC exhibited 42, 40 and 46 MPa, respectively, in 28-day compressive strength tests. These fairly constant values were the result of using semi-dry RCA, which absorbed part of the mixing water and resulted in mixes with decreased effective w/c ratio and less workability. The results from another study (Huda and Alam, 2014) showed a similar increase in the water absorption of RCA after three recycling cycles (i.e., 5.2%, 7.1% and 9.4%). Naturally, the increased adhered mortar content led to an increase in the water requirement of RAC mixes, to obtain comparable consistence (workability), and this also affected the mechanical behaviour of the resulting RAC. Although the first and second generations of RCA resulted in similar 28-day compressive strength values of about 34 MPa (15% lower compared with the control NAC), the third generation led to a compressive strength closer to 24 MPa (40% lower), thus showing a considerable decline in the mechanical performance of the resulting concrete. However, concrete made with RCA from repeated recycling exhibited a higher rate of strength development and, after 120 days, the RAC made with the third-generation RCA showed a compressive strength just 12% lower than that of the control concrete. Wang and Huang (2003) produced an original concrete with a compressive strength of around 60 MPa, the RCA of which was used to produce concrete specimens with increasing replacement levels with an intended target strength of 50 MPa. This approach was followed for two more generations of RAC with target strengths of 40 and 30 MPa. Within each generation, the cement and total water contents were maintained

Strength Development of Concrete

247

Table 8.6  Compressive strength of three generations of recycled aggregate concrete containing increasing coarse recycled concrete aggregate content 28-Day Compressive Strength, MPa

Replacement Level, %

First Generation

Second Generation

Third Generation

0

63.4

60.8

40.0

30

60.1

61.3

42.0

50

65.5

61.9

41.4

70

61.0

60.1

38.4

100

58.5

58.0

38.5

Data sourced from Wang and Huang (2003).

as the replacement level of dry RCA increased. As a result of this, the effective w/c ratio decreased, having obtained mixes with a gradual decline in consistence (workability). Nevertheless, minimal strength loss was reported for all mixes (Table 8.6). The maximum registered strength loss was 7% for the first generation of concrete with 100% coarse RCA, whilst, by the end of the third cycle, mixes with increasing amounts of coarse RCA exhibited strength similar to that of the corresponding NAC.

8.3  Tensile and Flexural Strength In the brittle fracture mechanics of concrete, failure of a specimen is initiated through the largest crack oriented in the direction normal to the applied load. This makes the occurrence of such a crack a stochastic problem, in which the size and shape of the specimen are factors that affect strength, since there is a higher probability of a larger specimen containing a greater number of critical cracks that can initiate failure (Neville and Brooks, 2010). The aggregate’s shape has a greater influence on the flexural strength of a specimen in comparison with its compressive or splitting tensile strength. This is probably because of a stress gradient that delays the progress of cracking, leading to ultimate failure. Therefore, concrete with angular-shaped aggregates will normally exhibit higher flexural strength than when round-shaped aggregates are used, especially in mixes with low w/c ratios. However, to achieve the same workability, round-shaped aggregates require less water than angular-shaped aggregates, and thus the flexural strength of the two concrete mixes may be similar (Neville and Brooks, 2010). The improved flexural strength of concrete containing more angular aggregate may also be explained by the improved ITZ between the angular-shaped crushed aggregate and the cementitious matrix. However, this is not the case with aggregates presenting glassy surfaces, which usually lead to lower tensile strength due to a poor bond at the ITZ (Bamforth et al., 2008).

248

Sustainable Construction Materials: Recycled Aggregates

Typically, RA presents a rougher surface in comparison with NA, which improves the bond strength at the ITZ as a result of the greater surface area in contact with the new cementitious matrix and the formation of new C-S-H further into the pores of the old adhered mortar (Li et al., 2016b). However, as research has shown, the rupture of RAC is likely to occur through the RA or the old ITZ instead of the new one (Xiao et al., 2015), even though it usually exhibits lower tensile strength in comparison with that of a corresponding NAC. It is clear that the failure mechanism of RAC is a complex phenomenon depending on a number of factors inherent to the RA, including its content, size, porosity, quality and moisture state.

8.3.1  Influence of Recycled Aggregate Content Similar to the study of the effect of RA on compressive strength, a statistical analysis was carried out on the most important factors influencing the tensile strength of concrete containing increasing amounts of RA. Figure 8.9 depicts the relative splitting tensile strength of concrete with increasing RA content of different sizes, types and quality classes (results of 683 mixes from 44 publications). Despite the considerable scatter, in several cases, RAC with coarse RA exhibited similar and even slightly higher splitting tensile strength compared with the control mix. This was mostly observed for specimens made with RCA, wherein there may be enhanced bond strength at the ITZ as a result of the greater surface area in contact with the new cementitious matrix. Nevertheless, the inclusion of coarse and fine RA (Figure 8.9(a) and (b), respectively) usually leads to lower splitting tensile strength. In the authors’ previous study (Silva et al., 2015), the analysis of a smaller-sized sample available at the time showed that there was a probability of 95% that RAC containing 100% coarse or fine RA could exhibit a loss in splitting tensile strength up to 60% or 54%, respectively, in comparison with corresponding control NAC mixes. However, it was also argued that some of the results were potentially unrepresentative because of the propagation of error to higher replacement levels (Silva et al., 2015), and thus a more in-depth analysis of RACs with specific RA content was carried out. Table 8.7 presents the statistical indicators of the values plotted in Figure 8.9(a) and (b) for given replacement levels of coarse and fine RA. Its contents suggest that the splitting tensile strength of RAC is expected to fall within the interquartile range (Q3–Q1). For example, the use of 100% coarse RA is expected to result in a decline of 5%–18% in the splitting tensile strength, whereas the use of 100% fine RA will likely cause a 16%–32% loss. However, from a structural design perspective, using the fifth percentile (P5) would constitute a practical and conservative approach to estimate the design splitting tensile strength of RAC with a given percentage of RA. In this case, using 100% coarse or fine RA, it is likely that the maximum loss in splitting tensile strength of RAC would be 66% or 65%, respectively, compared with that of the corresponding NAC. To further understand the influence of using coarse RA on the splitting tensile strength of concrete, the data were split according to type and quality class, as shown in Figure 8.9(c) and (d), respectively. As expected, there was a greater loss in splitting tensile strength with increasing MRA content in comparison with concrete made with increasing RCA content. Since MRA is more porous and weaker than RMA, there is a higher probability of critical cracks that can lead to failure occurring at lower loadings (Dhir and Paine,



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Figure 8.9  Relative splitting tensile strength of recycled aggregate concrete (RAC) with increasing levels of (a) coarse recycled aggregate (RA), (b) fine RA, (c) different RA types and (d) RA of different quality classes. MRA, mixed recycled aggregate; NAC, natural aggregate concrete; RCA, recycled concrete aggregate; RMA, recycled masonry aggregate. Data taken from Ajdukiewicz and Kliszczewicz (2002, 2007), Akbarnezhad et al. (2011), Alves et al. (2014), Arezoumandi et al. (2014), Bravo et al. (2015), Cachim (2009), Cakir (2014), Correia et al. (2006), Dhir and Paine (2007), Dilbas et al. (2014), Duan and Poon (2014b), Evangelista (2014), Evangelista and de Brito (2007), Folino and Xargay (2014), Fonseca et al. (2011), González and Etxeberria (2014), Gonzalez-Corominas and Etxeberria (2014), González-Fonteboa et al. (2011), Kim et al. (2013b), Kou and Poon (2009b, 2013), Kou et al. (2004, 2007, 2008, 2011a, 2012), Manzi et al. (2013), Mas et al. (2012a,b), Matias et al. (2013), Medina et al. (2012), Pedro et al. (2014a,b, 2015, 2017), Pereira et al. (2012b), Radonjanin et al. (2013), Schubert et al. (2012), Sheen et al. (2013), Thomas et al. (2013a), Vaishali and Rao (2012), Wang et al. (2013), Adams et al. (2016), Vieira et al. (2016), Anderson et al. (2016), Huda and Alam (2014). 249

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Table 8.7  Statistical indicators of the relative splitting tensile strength of recycled aggregate concrete for specific replacement levels Coarse RA

Fine RA

Statistical Indicator

20%

50%

100%

10%

50%

100%

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1.02

0.96

0.95

0.95

0.92

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RA, recycled aggregate.

2007; Correia et al., 2006; Mas et al., 2012b). However, the results corresponding to concrete containing increasing RMA content suggest that its splitting tensile strength will vary little. This is mostly due to the fact that most of the data belong to one investigation, wherein the RMA had been incorporated in an SSD state and thus resulted in reduced effective w/c ratio and increased strength (Anderson et al., 2016). Since RAC produced for practical purposes (i.e., with predictable and sufficient consistence) is not likely to exhibit such behaviour, this trend should not be considered as representative. The data in Figure 8.9(d) are divided according to the quality classes proposed by the authors in their previous study (Silva et al., 2014b). The results indicate a logical decrease in splitting tensile strength with increasing content and decreasing quality of RA. These findings are further supported by those of individual studies without the variability of data seen here, in which similar splitting tensile strength loss was observed for concrete containing RA exhibiting very similar oven-dried density and water absorption values (Yang et al., 2008). Furthermore, a division of RAs according to their physical properties makes sense in light of the considerable influence of the more porous adhered mortar on the performance of concrete, which may be present at different percentages of the total mass of RA. Studies on the effects of applying further processing stages to RCA have reported enhanced splitting tensile strength for RCA subjected to more crushing stages or acid-based treatments (i.e., lower adhered mortar content and rounder shape) (Pedro et al., 2014b; Otsuki et al., 2003; Nagataki et al., 2004; Katkhuda and Shatarat, 2017; Wang et al., 2017).

8.3.2  Strength Development with Time The development of splitting tensile strength of concrete progresses in a manner similar to that of its compressive strength. However, it is possible that the interaction between the new cement paste and the adhered mortar of RCA may create an improved bond strength, consequently leading to a lower probability of the presence of critical areas where failure can occur (e.g., ITZ between NA and mortar). This is suggested by the results of a long-term study by Poon and Kou (2010), in which the mechanical performance of RAC with increasing fly ash and coarse RCA content was evaluated. Figure 8.10 presents the splitting tensile strength of RAC tested at 28 days, one year

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and ten years after casting. After 28 days, all RACs exhibited decreasing splitting tensile strength with increasing RCA content (i.e., incorporation of 100% RCA led to a reduction of 8%–12% compared with the control mix). However, after one year, all RCA-containing specimens exhibited similar or even higher splitting tensile strength compared with the corresponding control specimens. It is possible that, as a result of the higher surface area of coarse RCA compared with NA, superficial pores may be filled up with the new cement paste, thereby improving the bond strength at the ITZ.

8.3.3  Influence of Mineral Additions As observed in Section 8.2.2, the use of mineral additions is expected have an effect in RAC similar to that in NAC, regardless of the coarse RA content (Berndt, 2009; Kou et al., 2007, 2011b; Poon and Kou, 2010; Salem et al., 2003). Indeed, the results of studies that evaluated the influence of replacing part of the cement with different types of mineral additions (i.e., silica fume, metakaolin, fly ash and ground granulated blastfurnace slag) suggest that the expected relative increase or decrease in splitting tensile strength was not hindered by the increasing amount of RCA (Berndt, 2009; Kou et al., 2011b). Furthermore, the strength development is also not likely to be affected by the combined presence of the mineral additions and RA (Poon and Kou, 2010). Figure 8.10(b)–(d) shows plotted the splitting tensile strength of concrete containing increasing RCA and fly ash content of 25%, 35% and 55%, respectively. The effects of adding fly ash in concrete caused, as expected, a reduction in the splitting tensile strength, except for mixes with 25% fly ash, the strength development of which was similar to that of mixes without additions. In fact, as opposed to what typically occurs with the compressive strength, where higher strength gain rate is observed compared with mixes without the addition, the results indicate strength development over time despite the high fly ash content (Poon and Kou, 2010). Nevertheless, the aforementioned negligible effect between RA and mineral addition is likely to occur only with coarser RA fractions. RA exhibiting high specific surface area may react with mineral additions existing in the binder, which can give rise to the formation of further products of hydration and improvement at the ITZ between RA and new mortar. Indeed, it has been observed that the combined use of a high volume of fly ash and fine RCA may lead to the production of RAC specimens with lower than expected (Kurda et al., 2017a,b) or negligible tensile strength loss (Kou and Poon, 2009b).

8.3.4  Tensile Strength and Compressive Strength of Recycled Aggregate Concrete According to Eurocode 2 (EC2) (EN-1992-1-1, 2008), in the design of structural concrete, the tensile strength is estimated by means of its characteristic compressive strength. It was previously established that, regardless of the type, size and overall quality of RA, the relationship between the tensile strength and the characteristic compressive strength of concrete is the same as that proposed in EC2 (Silva et al., 2015). Using the same approach, but with a larger and more recent data population,

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the relationship between the mean 28-day tensile strength and the characteristic compressive strength of 635 concrete mixes sourced from 43 publications is shown plotted in Figure 8.11. The corresponding 95% confidence interval, put forward by EC2, for the characteristic axial tensile strength of concrete is included in Figure 8.11. This shows that the great majority (98.4%) of the samples are above the lower limit (fctk;0.05), whereas 10% are above the upper limit (fctk;0.95). These findings are encouraging from a practical point of view, since, as per EC2, the design tensile strength of concrete, which is normally calculated using the aforementioned lower limit, can still be used in the design of structural RAC. Similar associations have been made concerning the prediction of the flexural strength of reinforced RAC beams



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Figure 8.11  Relationship between mean tensile strength and characteristic cylinder compressive strength. Data taken from Ajdukiewicz and Kliszczewicz (2002, 2007), Akbarnezhad et al. (2011), Alves et al. (2014), Arezoumandi et al. (2014), Bravo et al. (2015), Cachim (2009), Cakir (2014), Correia et al. (2006), Dhir and Paine (2007), Dilbas et al. (2014), Duan and Poon (2014b), Evangelista (2014), Evangelista and de Brito (2007), Folino and Xargay (2014), Fonseca et al. (2011), González and Etxeberria (2014), Gonzalez-Corominas and Etxeberria (2014), González-Fonteboa et al. (2011), Kim et al. (2013b), Kou and Poon (2009b, 2013), Kou et al. (2004, 2007, 2008, 2011a, 2012), Manzi et al. (2013), Mas et al. (2012a,b), Matias et al. (2013), Medina et al. (2012), Pedro et al. (2014a,b, 2015, 2017), Pereira et al. (2012b), Radonjanin et al. (2013), Schubert et al. (2012), Sheen et al. (2013), Thomas et al. (2013a), Wang et al. (2013), Adams et al. (2016), Vieira et al. (2016), Anderson et al. (2016), Huda and Alam (2014).

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by using the method proposed in EC2. It was observed that there is no statistical difference between the measured/predicted flexural strength of RAC and NAC beams, which, from a practical point of view, means that some existing provisions can be used without alterations (Tosic et al., 2016). Nevertheless, to further understand the influence of various factors related to the use of RA on the tensile strength of concrete, the data were further segregated by size, content, type and quality class of RA (Figure 8.12). A parametric analysis of the results showed that there was no statistically significant difference between the relationships between the two properties, despite the data being divided into several variables. Since the tensile strength of concrete is affected by the same RA-related factors as those influencing the compressive strength, it is likely that the incorporation of RA leads to a proportional decline in the performance of both properties, the degree of which is parallel to the same relationship of conventional concrete. However, it has also been observed that the original NA may influence this relationship in RAC; specimens made with RCA containing original rolled pebble particles may show a higher than expected flexural strength for a given compressive strength, compared with concrete mixes made with RCA with crushed rock particles (Zhou and Chen, 2017).

8.4  Impact Loading Concrete structures, throughout their service life, may be subjected to combinations of different types of loads, apart from the usual gravitational-related ones. One such is the loading caused by impact (i.e., vehicle collisions, industry-related accidents, rock falls, military activity and terrorist attacks). Naturally, the resistance of concrete to impact loading is proportional to the resistance and stiffness of its constituents, which, in the case of RAC, can be of lower grade than that of a corresponding NAC (Nazarimofrad et al., 2017; Li et al., 2016a,d; Ismail and Ramli, 2014; Xiao et al., 2015; Rao et al., 2011). Rao et al. (2011) assessed the behaviour of RAC beams, containing different replacement levels of coarse RCA (0%, 25%, 50% and 100%), subjected to a drop hammer impact. The results of the impact test showed that, for a given impact energy, the resulting acceleration in RAC beams increased with the replacement level. The stiffness of the material influenced the vibration of the specimen, i.e., as the modulus of elasticity increased the acceleration decreased. As a result of the weight impact, the maximum displacement of the concrete beams also increased for concrete with higher coarse RCA content. These results indicate that increasing the amount of coarse RCA may lead to a lower resistance to impact loading. Xiao et al. (2015) investigated the compressive behaviour of RAC under quasi-static and high strain rate loading. The results indicated increased compressive strength and initial modulus of elasticity with increasing strain rate, but, compared with the control concrete, there was a decline in the compressive strength as the coarse RCA content increased. It was also observed that the propagation of cracks in RAC subjected to impact loading was different from that when subjected to quasi-static loading.

 







 

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Ismail and Ramli (2014) evaluated the low-velocity impact loading of RAC containing treated and untreated RCA. The resistance to impact was assessed by means of the energy absorbed by the specimens’ fracture. The treatment consisted of soaking them in an acidic solution and subsequently impregnating them with a calcium metasilicate solution. The results suggested decreased resistance to impact loading with the presence of RCA but were better if these samples were treated; the energy absorption values of the control specimens and those containing untreated and treated RCA were 1.49, 1.00 and 1.25 kN∙mm, respectively, 28 days after casting. A visual analysis of the surface cracks indicated that the control specimens exhibited more tortuous crack path patterns compared with RAC, which presented more linear patterns, with cracks occurring across the RCA itself. It is possible that the NA’s relatively higher stress capacity may have prevented crack propagation. Li et al. (2016d) studied the resistance to impact loading of RAC mixes with 100% coarse RCA and with part of the cement replaced with nano-SiO2 and nano-CaCO3. Nanoparticle-bearing RAC mixes under impact loading showed improved compressive strength compared with the control RAC, the rate of which increased with increasing impact velocity. Still, no distinct differences were witnessed in the failure patterns of nanoparticle-modified RAC under impact loading. Nazarimofrad et al. (2017) also reported decreased resistance to impact of RAC. The number of weight impacts until the formation of the first crack decreased with increasing coarse RCA content and less so with increasing adhered mortar content. The authors observed enhanced resistance to impact loading with the use of silica fume and steel fibres; the number of impacts until the first crack of RAC incorporating 100% coarse RCA was 27–38 (30%–42% that of the control), whereas the combined inclusion of silica fume and steel fibres increased the number of impacts to 230–443 (39%–76% that of the control).

8.5  Resistance to High Temperatures The deterioration of the mechanical performance of concrete subjected to high temperature is due to considerable alterations in its constituents, specifically the physicochemical changes in the mortar and aggregates, as well as the thermal incompatibility between the two (Vieira et al., 2011). Figure 8.13 presents the relationship between the compressive strength of concrete specimens, made with varying RA content and w/c ratios, exposed to high temperatures and that of the corresponding mixes at ambient temperature (between 20 and 25°C). An assessment of the results of individual studies on the mechanical behaviour of concrete with coarse RCA after high temperature-induced damage suggests that it is likely to exhibit a performance comparable to that of the corresponding control mix (Etse et al., 2016; Yang et al., 2016; Chen et al., 2014; Kou et al., 2014a; Liu et al., 2016a,b; Guo et al., 2014; Zega and Di Maio, 2006; Eguchi et al., 2007; Vieira et al., 2011). Furthermore, products containing increasing coarse RMA content may present slightly higher resistance to fire damage compared with the corresponding conventional concrete (Khalaf and DeVenny, 2004a; Martins et al., 2016;

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Figure 8.13  Residual compressive strength as a function of temperature for concrete compositions with different recycled aggregate replacement levels compared with the curves corresponding to limestone and siliceous aggregates proposed by EC2. CC, control concrete; EC2, Eurocode 2; RCA, recycled concrete aggregate; RMA, recycled masonry aggregate.

Sustainable Construction Materials: Recycled Aggregates

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Strength Development of Concrete

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Hachemi and Ounis, 2015). This phenomenon can be explained by the lower coefficient of thermal conductivity (Hansen, 1992) and of thermal expansion of RMA, which is more compatible with that of the cement matrix in comparison with NA (Martins et al., 2016). This leads to lower thermal stresses within the cementitious microstructure and thus less cracking. Nevertheless, its inherently higher moisture content as a result of the mixing process may lead to severe explosive spalling when heated to temperatures above 600°C (Hansen, 1992; Martins et al., 2016). An overall analysis of the results presented in Figure 8.13 indicates considerable scatter in the residual compressive strength of concrete specimens, particularly after they have been heated to temperatures between 400 and 600°C. A comparative analysis of the results with the relationships proposed in EC2 (EN-1992-1-2, 2004), for concrete containing limestone and siliceous aggregates, shows that a considerable number are below the two curves and not necessarily limited to the RAC mixes (Liu et al., 2016a,b; Guo et al., 2014; Yang et al., 2016; Hachemi and Ounis, 2015; Martins et al., 2016). The use of RAC in structural applications may convey improved resistance to high temperatures. Assuming equivalent compressive strength classes, the resistance of RAC columns to high temperatures was found to be better than that of those made with conventional concrete and with the same cross section (Dong et al., 2014). This can be explained by the lower density of RAC, compared with NAC, thus resulting in a lower temperature penetration rate and delayed crack occurrence. Indeed, the incorporation of increasing RCA content in concrete was found to have little influence on the failure mechanism and peak strain of concrete after high temperature exposure (Liu et al., 2016a,b). Furthermore, even though there is a comparable decline in compressive strength for NAC and RAC, the absolute modulus of elasticity of NAC may tend to decrease faster than that of the corresponding RAC up to temperatures of 400°C, after which equivalent values may be seen between the two concretes (Zega and Di Maio, 2006; Liu et al., 2016a,b; Guo et al., 2014). However, it should be noted that conflicting findings have also been reported (Gales et al., 2016). According to EC2 (EN-1992-1-2, 2004), the tensile strength of concrete exposed to high temperatures should be ignored from a conservative design point of view. However, whenever it is needed to be considered, the simplified method adopted in EC2 may be used (plotted in Figure 8.14). The results, though limited, indicate that all RACs may comply with the simplified method of EC2 for estimating the tensile strength of concrete at high temperatures, regardless of the replacement level.

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Sustainable Construction Materials: Recycled Aggregates



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Figure 8.14  Residual tensile strength of concrete with varying recycled concrete aggregate (RCA) content exposed to high temperatures. CC, control concrete.

8.6  Conclusions The increasing incorporation of RA is expected to result in an overall strength reduction, the extent of which depends on the size, type and quality of the RA. Larger RAs, which typically present lower adhered mortar content, are likely to result in a lower strength loss compared with finer fractions. This is usually a direct result of the processing of the material, in which the application of consecutive crushing stages causes the breaking down of the old adhered mortar, consequently accumulating in an increase in the finer fractions of RA. The loss in performance, on the other hand, may not be as noticeable when using finer RA from ceramic bricks or other aluminosilicate materials, which have been found to develop pozzolanic reactions with the products of cement hydration, thus contributing to the mitigation of strength loss or even strength increase of RAC. Of the identified RA, the ones that are widely considered to be more suitable for use in concrete are those obtained from crushed concrete (RCA), masonry walls (RMA) or a mixture of the two (MRA). RCA is likely to produce concrete mixes with the lowest strength loss, followed by MRA and RMA. Despite this, RA may come from construction materials exhibiting widely varying mechanical performance, which exert considerable influence on the performance of the concrete. RA exhibiting lower quality (e.g., low-strength original material, higher adhered mortar content,

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high porosity), for a given w/c ratio, is likely to cause higher strength losses. This phenomenon is more perceptible for lower w/c ratios because the ultimate strength is conditioned by the strength of the RA, rather than that of the new cement matrix. Applying the performance-based classification, mentioned in Chapter 5, was found to be a practical and cost-effective approach to assess the quality of RA when designing a concrete mix. Not only does it provide a greater control of the quality of RA during the processing stage, but the properties of RA can be well correlated with the mechanical performance of concrete containing such aggregate. Therefore, by knowing the quality of the RA, one can establish a relationship between its increasing content and the strength development of the concrete. The compensation of water is vital for the consistence of concrete. However, fully compensating for the water absorbed by RA is likely to lead to further strength loss. Partial compensation has been widely acknowledged as the middle ground for producing RAC with comparable consistence (workability) and minimum loss in its performance. In some cases, strength gain has been reported, though usually due to the use of WRAs, which allow the production of concrete mixes with good consistence (workability) and compaction for lower effective w/c ratios. However, these admixtures may prove to have slightly less effectiveness with ensuing replacement levels of (semi-)dry RA, as a result of the absorption of the admixtures themselves alongside the mixing water. Regarding the influence of the use of mineral additions on the strength development of concrete, it is likely that there is marginal interaction between the additions and coarse RA. However, some reaction may occur between the addition and the products of hydration of finer RCA. Nevertheless, the additional products of hydration that may come from it will not significantly influence the mechanical performance of the concrete. Concrete containing RCA may show a greater rate of strength development over time, compared with the corresponding control mixes. This may be partially due to the latent cementitious properties of RCA (i.e., unhydrated cement particles), but increased performance is mainly the result of the ensuing reactions leading to the development of new products of hydration that precipitate in void areas, thereby improving the bond strength between the RCA and the cementitious matrix. The compressive strength is considered the main property of concrete and can usually be correlated with others. This type of association is particularly useful in the relationship between compressive and tensile strength, as it is unaffected by the increasing incorporation of RA. From a practical perspective, this is helpful as the existing structural codes use this relationship to estimate the tensile strength of concrete. Among the few existing studies on the impact resistance of concrete, it has been observed that this property is likely to decrease with increasing RA content. This was to be expected considering that the resistance to impact loading of the material is proportional to the resistance and stiffness of its constituent materials. Nevertheless, no distinct differences are expected to exist in the failure patterns between RAC and the corresponding NAC. Furthermore, the overall performance of concrete under impact loading can be improved with the incorporation of some mineral additions, which can further densify the material, and particularly with the use of fibres.

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The residual mechanical behaviour of concrete subjected to high temperatures was found to be unaffected by the incorporation of RCA. Furthermore, the high temperature-induced damage is likely to be less significant in coarse RMA-bearing concrete as a result of its lower coefficient of thermal conductivity and of thermal expansion, thereby leading to lower thermal stresses within the cementitious microstructure and hence less cracking.

References Abdel-Mohti A, Shen H and Khodair Y, 2016. Characteristics of self-consolidating concrete with RAP and SCM. Construction and Building Materials 102, 564–573. Abdollahzadeh G, Jahani E and Kashir Z, 2016. Predicting of compressive strength of recycled aggregate concrete by genetic programming. Computers and Concrete 18 (2), 155–163. Abdulla N A, 2015. Effect of recycled coarse aggregate type on concrete. Journal of Materials in Civil Engineering 27 (10). ACI-209, 2008. Guide for Modeling and Calculating Shrinkage and Creep in Hardened Concrete. Farmington Hills, Michigan, USA, 48 p. Adams M P, Fu T F, Cabrera A G, Morales M, Ideker J H and Isgor O B, 2016. Cracking susceptibility of concrete made with coarse recycled concrete aggregates. Construction and Building Materials 102, 802–810. Ajdukiewicz A and Kliszczewicz A, 2002. Influence of recycled aggregates on mechanical properties of HS/HPC. Cement and Concrete Composites 24 (2), 269–279. Ajdukiewicz A B and Kliszczewicz A T, 2007. Comparative tests of beams and columns made of recycled aggregate concrete and natural aggregate concrete. Journal of Advanced Concrete Technology 5 (2), 259–273. Akbarnezhad A, Ong K C G, Zhang M H, Tam C T and Foo T W J, 2011. Microwave-assisted beneficiation of recycled concrete aggregates. Construction and Building Materials 25 (8), 3469–3479. Akca K R, Cakir O and Ipek M, 2015. Properties of polypropylene fiber reinforced concrete using recycled aggregates. Construction and Building Materials 98, 620–630. Alves A V, Vieira T F, de Brito J and Correia J R, 2014. Mechanical properties of structural concrete with fine recycled ceramic aggregates. Construction and Building Materials 64, 103–113. Amer A A M, Ezziane K, Bougara A and Adjoudj M, 2016. Rheological and mechanical behavior of concrete made with pre-saturated and dried recycled concrete aggregates. Construction and Building Materials 123, 300–308. Amin A, Hasnat A, Khan A H and Ashiquzzaman M, 2016. Residual cementing property in recycled fines and coarse aggregates: occurrence and quantification. Journal of Materials in Civil Engineering 28 (4), 11. Amorim P, de Brito J and Evangelista L, 2012. Concrete made with coarse concrete aggregate: influence of curing on durability. ACI Materials Journal 109 (2), 195–204.

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