*5.1. Particle Size Distribution*

At present, during the development of fragments of destroyed buildings and structures, a large amount of unfractionated crushing concrete waste with a size of 0–5 mm is formed [67,92,93], the use of which in the production of concrete is difficult due to the presence of a significant amount of dusty fraction in their composition (Table 3). However, judging by the chemical composition, the fine fraction of concrete scrap can be used as an additional cementitious material [94]. The comminuted form of the particles, as well as the high content of silica and clinker minerals in them, will contribute to their high activity, which is confirmed by the studies of other authors [95–97]. RCA have very rough and angular due to the crumbling of the concrete and the presence of grout adhered to the original rough surfaces of the aggregate (Figure 2). Consequently, recycled concrete aggregate has better adhesion to the cement matrix than natural one. Depending on the size of the aggregates, RCA particles typically contain 30 to 60% of mortar. RCAs are identical in particle shape to crushed stones, but the types of grinding tools affect the gradation and other characteristics of fine concrete. Also, Figure 3 shows the fundamental components of demolition wastes [80].



**Figure 2.** Original rough surfaces of the RCAs.

**Figure 3.** Fundamental components of demolition wastes [80]. Reprinted with permission from Elsevier [80].

#### *5.2. Specific Gravity and Bulk Density*

The specific gravity and bulk density of RCA are often lower, and the pore volume and absorption, respectively, are higher than those of natural aggregate. The lower specific gravity of RCA is due to the presence of aggregate particles in the old cement mortar/paste, which makes it less dense than NCA due to its higher porosity [98]. In saturated dry conditions, RCA distinctive gravity ranges from 2.10 to 2.50, which is 5–10% less than NCA as shown in Table 2. Experimental data from Aliabdo et al. [99] found that RCA bulk densities are 9.80% less than primary gravel aggregates. Compared to virgin concrete aggregate, the higher pore volume of RCA makes it less dense and fragile.

### *5.3. Aggregate Grounded, Abrasion and Effect Values*

Aggregate ground values (AGV) are measures of the aggregate resistance to commination under progressively applied compressive loads. It is reported that the lower the value, the stronger the aggregates. It was found that ACV for NCA (14% to 22%) is significantly less than for ACV for RCA (20–30%) from the literature [54,100–102] and as indicated in Table 3. This is expected due to the relatively weak mortars and cement pastes attached to the RCA particles.

The aggregate abrasion values (AAV) are a measure of the aggregate wear resistance. When material loss due to wear becomes higher, a higher AAV is obtained. Generally, the cumulative abrasion of RCA is higher than NCA. Classic RCA values range from 20% to 45%, which is higher than the values for pure concrete aggregates, as shown in Table 3 [54,100–102]. Despite its origins, RCA's cumulative abrasion rates are nevertheless generally below the acceptable optimum limits for structural applications (50% by weight).

Furthermore, aggregate effect values (AEV) are the aggregate strength values exposed to impact. AEVs indicate the aggregate resistance to dynamic loads. As shown in Table 4, AEV of NCA (15% to 20%) are lower than RCA (20% to 25%) [54,100–102]. Attached cementitious and mortar pastes make RCA less durable and thus lead to higher toughness values for RCAs that were frozen or wetted. About 0.150% by weight RCA is the acceptable limit for organic matter.

**Table 4.** The main RCA and NCA mechanical properties [54,100–102].


### **6. Fresh Properties**

RCA can affect the performance of fresh concrete due to their higher porosity, absorption, surface roughness, and angularity [103]. The higher size and angularity of RCAs will reduce the workability of the concrete and make it difficult to lay it properly [104].

#### *6.1. Workability*

Workability decline rates increase with the improvement in the proportion of RCA in concrete mixes [105]. Thus, RCA concretes require more water to obtain similar workability to NCA concretes [106]. Concrete mixes that integrate RCA generally meet the initial settlement requirements [107]. Also, Table 5 summaries the RCA effects on the concrete hardened features. However, greater RCA uptake can lead to a rapid loss of workability, which limits the time required for paving and completing concreting [108]. Problems associated with rapid loss of workability should be addressed by changing and controlling the RCA water content before mixing or increasing the amount of superplasticizer, rather than adding more water on construction sites [109]. In general, the greater sharpness and surface coarseness of RCA particles reduce the concrete workability and lead it more problematic to finish appropriately.

**Table 5.** The RCA effects on the concrete hardened features [7,110,111].


### *6.2. Wet Density*

Several studies have been conducted to investigate the effect of RCA on the wet density of concrete [112]. Typically, the wet density of RCA concrete is lower than that of virgin concrete aggregates, as noted in Table 4 [7,110,111]. Compared to NCA concrete, it has been observed that the wet density of RCA concrete is 5–15% less [111]. This RCA contains adhered old cement pastes or mortars that are less dense than NCA [113]. As a rule, the density of hardened RCA concrete is 5–15% less than that of concrete with natural aggregates [114]. This is due to the used solutions attached to the RCA [115]. Depending on the size of the aggregate, the amount of mortar attached to the re-concrete aggregates ranges from 30% to 60% by volume RCA [116]. The density of the secondary mortar is much lower than that of most natural concrete materials [117]. This results in a lower density of RCA concrete [118].

#### *6.3. Stability*

The stability of the concrete mix enhanced as a result of abridged bleeding and augmented cohesiveness. Therefore, the resistance to segregation of NCA concrete could be equivalent to that of RCA concrete. As a rule, the soaking of NCA concrete is greater than that of RCA concrete [6]. During mixing, some of the ancient cement pastes are wiped off the RCAs and form additional fines in the concrete mix. Thus, these fine particles minimize the soaking of the concrete after some water has been adsorbed in the mixture. With lower free water content, more fine particles also increase the adhesion of the concrete

mix. In addition, the increased surface roughness and angularity at higher RCA levels contribute to better concrete adhesion. The stability of the concrete mix is enhanced by improved cohesiveness and minimal soaking [119].

#### *6.4. Air Content*

The air content in concrete is significantly affected by the volume of their mortars. RCA influences the air content of concretes as they have higher mortar content. Fresh concrete having RCA is usually 60% more than the air content of fresh NCA concrete [62]. This is due to air entrained in recycled RCA mortars. Thus, when determining the target air content in RCA concrete, the existing air content in the mortar must be taken into account.

#### **7. Mechanical Properties**

The mechanical properties of concrete depend on the properties of the aggregate. It was found that the mechanical characteristics of RCA are lower in comparison with the mechanical characteristics of primary concrete aggregates from the available literature [87,98,99]. The main mechanical characteristics of RCA and NCA are listed in Table 3 and then briefly explained [120–122].

#### *7.1. Compressive Strength*

Silva et al. [123] studied the effect of adding a small percentage of waste plastic fiber and waste building material on some of the mechanical properties of concrete. In his study, the volume fraction of waste was 0.1–0.2% by volume. The results obtained proved the improvement in compressive and flexural strength. The results also showed an increase in the density of the fiber-reinforced concrete specimens compared to the control mix. As shown in Figure 4, the compressive strength of RCA concrete is often 5–10% lower than that of virgin concrete [124].

**Figure 4.** Compressive strength versus level of replacements of NCA by RCA [124]. Reprinted with permission from Elsevier [124].

Depending on the RCA quality, it can also be reduced by up to 25%. Typically, higher air content in concrete mixes with RCA can also lead to lower strength values [99]. RCA concretes, however, can have the same and sometimes higher compressive strength than NCA concretes if the reclaimed concrete is obtained from old sources of concrete that were originally produced with a lower water to cement ratio than new concretes.

Duan et al. [125] found that RCA does not have any effect on the compressive strength of concrete up to a degree of substitution of 30% by weight, after which it decreases. It is also showed that the compressive strength of concrete was much lower when RCA was used in dry state [87]. The reduction in compressive strength from 20% to 30% was realized due to the use of RCA in the case of high strength concretes. Similar results have been noted by other researchers [87,98,126]. Seethapathi et al. [127] studied the selfcompacting properties of concretes made with RCA and compared them with those of NCA concretes. Nitesh et al. [128] found that at the same age, the variation in compressive strength was negligible.

In addition, fine RCAs can affect the compressive strength of concrete. The compressive strength of RCA concrete depends on the ratio of the coarse aggregate to the fine aggregate of the original RCA concrete according to [129]. A lower ratio of coarse aggregate to fine aggregate results in more mortar adhered to the coarse RCA particles, and therefore a decrease in the strength of RCA concrete. This reduction is even greater when using recovered fines. Thus, the use of fine RCA in concrete is generally not recommended. Nevertheless, according to Maria et al. [130], the reduction in compressive strength did not occur for concretes containing up to 20% fine RCA. Strength decreases with increasing RCA content above this level.

The strength of RCA concrete can be increased either by soaking up a portion of the reconstituted aggregates for mixing without or with pozzolanic fluids during mixing, or by soaking RCA in mixtures of pozzolanic fluids such as colloidal silica or water before mixing the concrete. It is expected that microcracks in RCA will be filled with absorbed pozzolanic fluids or water absorbed cement gels during pozzolanic reactions or cement hydration. Consequently, the strength of RCA concrete can be increased.

#### *7.2. Splitting Tensile and Flexural Strengths*

There is limited literature on the effect of RCA on the tensile strength of concrete [39,46,100]. As shown in Figure 5 [124], tensile strength at cracking of RCA concrete is lower than that of NCA concrete. Various researchers have reported that the splitting tensile strength of RCA concrete is 0–10% less than that of NCA concrete [39,46,100]. Over a period from 90 to 365 days, there was no statistically significant decrease in tensile strength. Guo et al. [46], in contrast, noted that RCA concretes have higher tensile strength than NCA concretes. Therefore, more research is needed to investigate the effect of RCA on concrete cracking toughness.

**Figure 5.** Splitting tensile strength versus level of replacement of NCA by RCA [124]. Reprinted with permission from Elsevier [124].

Typically, as shown in Figure 6, the flexural strength of RCA concrete is less than that of NCA concrete [124,131–133]. However, the flexural strength of 3 day RCA concrete was higher than that of NCA concrete, but at 28 days the strength was lower according to [131]. In their studies, NCA concrete gradually increased in strength and had greater flexural strength than RCA concrete at a later age. It is noted that RCA has never had a noticeable negative effect on the flexural strength of concrete [133]. However, given sufficient strength, RCA concretes can be produced for various purposes, sometimes even with 100% NCA replacements.

**Figure 6.** Flexural strength versus level of replacement of NCA by RCA [124]. Reprinted with permission from Elsevier [124].

#### *7.3. Bond Strength and Impact Strengths*

The bond strength of concrete is an indicator of the interrelated properties of pastes and aggregates. Rough RCA surfaces provide better adhesion than pure concrete aggregates. To test the bonds between reinforcement and concrete, which included 100%, 50% and 0% RCA, in [134], cylindrical specimens 150 mm × Ø100 with fixed soft and ribbed reinforcement were used (diameter = 12 mm and embedment length = 150 mm). The data obtained showed that RCA connections between reinforcement and concrete are not strongly influenced by RCA inclusions in concrete. It is, however, showed that the bond strength of NCA concrete was 9–19% higher than that of RCA concrete [135]. These conflicting results mean more research is needed to investigate the effect of RCA on concrete bond strength.

The impact of RCA on the hardened concrete performance can be significant or negligible depending on their physical characteristics, grades, content, types and sources [136]. The performance of RCA hardened concrete declines with the NCA substitution rate due to reclaimed concrete aggregates [3]. Without significant effect on the properties of hardened concrete, up to 30 wt. % natural filler can be replaced with RCA. As noted in the available literature, various changes in the performance of RCA hardened concrete are presented in Table 4.

#### *7.4. Elasticity Modulus*

The elasticity modulus of concrete is increased by an aggregate with a higher elasticity modulus. As shown in Figure 7, the elasticity modulus of concrete thus decreases with increasing RCA content in the concrete [129,136,137]. As a rule, the elasticity modulus of RCA concrete is 10–33% less than that of NCA concrete. It is demonstrated that the use of 30% RCA in concretes led to a decrease in the elasticity modulus by about 15% [129]. However, compared to NCA concrete, RCA concrete's elasticity modulus can be as low as 46%. The decrease in the modulus of elasticity of concrete is due to the fact that RCA generally have a lower elasticity modulus compared to NCA. In addition, the decrease in the elasticity modulus of concrete is associated with an improved total content of mortars (recycled and new), which have lower elastic moduli than most concretes based on natural aggregates.

**Figure 7.** Modulus of elasticity versus level of replacement of NCA by RCA [124]. Reprinted with permission from Elsevier [124].

#### *7.5. Creep and Thermal Expansion*

Limited research has been done on the creep of RCA concrete. As a rule, the creep of RCA concrete is greater than the creep of NCA concrete [76,86,87]. This is because creep depends on the paste content, which can be 51% higher in RCA concretes.

Thermal expansion coefficients mainly depend on the content and types of aggregates. It is noted that the coefficients of thermal expansion of RCA concrete are usually 10–30% higher than that of NCA concrete [138]. However, further studies of the effect of RCA on the thermal expansion behavior of concrete are required to confirm these results.

#### *7.6. Drying Shrinkage*

Drying shrinkage commonly depends on the ratio of water to cement and paste content and is controlled by the aggregate particles. In some works, it was found that shrinkage is 20–50% higher than that of NCA concretes, because RCA concretes have a high paste content [45,86,87,139]. On the other hand, several studies have reported relatively lower RCA drying shrinkage values. It is reported low shrinkage strain at different curing ages of concrete when replacing 30% virgin concrete aggregate with recycled concrete aggregate, as shown in Figure 8 [140]. Conflicting results suggest that more research is needed to investigate the effects of RCA on concrete shrinkage when drying.

**Figure 8.** (**a**) Drying shrinkage versus time of exposure, and (**b**) drying shrinkage versus type of mix (ID) [140]. Reprinted with permission from MDPI [140].

#### **8. Durability and Functional Properties**

Concrete's ability to resist abrasion, chemical attack, weathering, and other adverse operating conditions is durability. Even when RCA is constructed from concretes with strength problems, RCA concretes can be significantly durable, provided the quality is maintained during construction and the mix ratio is correct [86,87,118,141]. The various strength characteristics of RCA concrete are described below.

#### *8.1. Permeability*

As shown in Figure 9, the porosity of the aggregate was less than that of the control concretes with aggregates from virgin concrete [55,142–144]. It is also reported that for all concretes the total porosity was increased at 50% RCA [143]. However, at 100% NCA substitution, class 20 concrete had a slightly lower porosity than traditional class 20 concrete. The porosity of RCA concrete can be 10–30% higher than that of NCA concrete, depending on the strength class according to [142]. These characteristics of RCA concrete are due to differences in the composition of concrete mixes such as RCA and cement content, aggregate amount and percentage of pozzolan.

**Figure 9.** Total porosity versus level of replacement of NCA by RCA, with (**a**) Water/cement (w/c) ratio of 0.55; (**b**) w/c ratio of 0.45 [144]. Reprinted with permission from Elsevier [144].

The permeability of concrete depends both on the permeability of the concrete matrix (binder and cement paste) and on the absorptive capacity of the aggregate. In addition, the permeability of concrete is influenced by pore continuity, distribution, size, and porosity. The coefficients of intrinsic gas permeability of RAC are shown in Figure 10. The gas permeability of RAC decreased significantly with the carbonated RCAs; it is also showed an increase on the carbonation pressure from 0.1 Bar to 5.0 Bar still helped to reduce the gas permeability of RAC with new RCAs [72].

**Figure 10.** Gas permeability versus curing age [72]. Reprinted with permission from Elsevier [72].

#### *8.2. Water Absorption*

The water absorption of RCA concrete is expected to be greater than that of NCA one. This is due to the very high air permeability and water absorption of RCA. As shown in Figure 11, water absorption of concrete increased with increasing RCA content [145]. The water absorption of RCA concrete is 0–40% higher than that of NCA concrete, depending on the strength grades according to [146]. The author did not explain the reason for the lower water absorption at 20% RCA content, but suggested further research.

#### *8.3. Sulfate and Chloride Resistance*

Limited research has shown that RCA concrete's sulfate resistance is about the same or slightly lower than that of natural concrete aggregates [147–149]. In general, the sulfate resistance of RCA concrete can be improved by using silica fumes, fly ash and fine-grained blast-furnace slags and using the correct mixing ratio. Good quality control of the construction and correct dosing of mixes can reduce the corrosion rate of steel reinforcement in RCA concretes. It is applied polarization approaches to determine the corrosion behavior of steel anchored in RCA concretes [148]. The researchers reported that the corrosion rate never depended on aggregates for lower chloride levels (0.50%) and binder type. Compared to NCA concrete, the researchers also reported that RCA concrete with 65% fine blast furnace slag and 30% pulverized fuel ash was more successful in mitigating corrosion reactions at higher chloride levels.

**Figure 11.** Water absorption versus level of replacement of NCA by RCA [145]. Reprinted with permission from Ozbakkaloglu et al. [145].

It is noted that the high chloride penetration rates in concrete made with and without RCA were very similar [139]. The use of 100% RCA in concretes reduces their ability to resist the penetration of chloride ions by about 30% compared to NCA concrete [143,150]. However, the chloride penetration resistance of RCA concrete can be significantly improved by increasing additional binders such as crushed granular blast furnace slag and fly ash. It is disclosed that, in comparison with fly ash, crushed granular blast furnace slag reduced the diffusion coefficient of chloride in concrete by about 80–200% [147,150,151] (See Figures 12 and 13).

**Figure 12.** Chloride-ion penetration at 28 days and 90 days [151]. Reprinted with permission from Silva et al. [151].

**Figure 13.** Chloride-ion penetration with 55% of GGBS at 28 days [151]. Reprinted with permission from Silva et al. [151].

#### *8.4. Carbonation Resistance*

With regard to concrete carbonation, existing research points to conflicting effects of RCA. The researchers [102] reported that carbonation depth decreases with increasing RCA content, resulting in better performance except for 100% replacement. RCA concretes require a higher cement content to achieve identical strength to NCA concrete at the same water to cement ratio [102]. The higher the cement content, the higher the alkali reserves, which affect the depth of concrete carbonization. In contrast, it is found that at the same water-to-cement ratio, RCA concrete has a greater carbonization depth than NCA concretes [152,153]. In addition, the carbonization rate of RCA concrete is four times that of NCA concrete. Increased carbonation can increase the risk of reinforcing steel corrosion in RCA concretes. Increased concrete pavement, appropriate additional cementitious materials, corrects curing, and a lower water to cement ratio can counterbalance such risks. On the other hand, previous studies have reported that concrete containing the RA requires to more supplementary materials with high fineness to act as pore fillers. Thus, this will reduce the diffusion of carbon dioxide and slow down the speed of carbonation [154,155] (See Figures 14 and 15). However, more research is needed to analyze the effect of RCA on concrete carbonation resistance.

#### *8.5. Resistance to Alkalis and Acid*

Resistance of RCA concrete to alkali-silica reactions depends on the RCA sources [12,139,156]. It is noted that RCA obtained from old concretes containing alkalis affects the resistance of newly created RCA concretes to alkali-silica reactions [12]. The results of the researcher showed that the amount of new concretes made using RCA increased disproportionately due to alkaline-silica reactions. The use of low-lime grade F fly ash can significantly minimize expansion due to alkaline silica reactions in RCA concretes. Ziyi Peng and others suggested a method in their study to improve RCA quality by absorbing silica fume slurry into residual mortar for RCA [157] (See Figures 16 and 17).

There is insufficient information available to draw specific conclusions about the alkali-silica reactivity of RCA concrete. Several studies recommend the use of SCMs to dilute AAR in recycled and RCI concrete [157].

It is noted that the acid resistance of RCA concrete is similar or slightly lower than that of NCA concrete [86]. Scientists immersed concrete samples in test baths containing sulfuric acid at pH = 2 to test the acid degradation rate of RCA concrete. Compared to NCA concrete, the penetration of acids into RCA concrete was slightly higher. This may be due to the correlation of acid penetration with high of the porosity and absorption of the RAC and residual mortar compared to that of natural aggregates in concrete [158]. This caused a higher penetration of the acid ions into the concrete, thus caused the dissolution of calcium hydroxide and the destruction of the gel calcification (CSH) in the concrete 8However, researchers have shown that the acid attack resistance of RCA concrete can be improved by adding cement supplementary materials. Kazmi et al. demonstrate the possibility of treating RAC by immersion in the lime with accelerated carbonization and immersion in acetic acid with friction techniques [92] (See Figure 18). This method can be used to improve the acid resistance of RAC-containing concrete in chemically aggressive environments. More studies of RCA concrete under acidic conditions are needed to test their durability due to the small amount of research.

**Figure 14.** Relation between carbonation depth and coarse recycled concrete aggregates (%) [155]. Reprinted with permission from Elsevier [155].

**Figure 15.** Effect of addition of RCA, FA and MK on concrete carbonation depths an exposure period of 4 weeks [154]. RA0F: without RCA with 30% of FA, RA25F: 25% of RCA with 30%of FA. RA50F: 50% of RCA with 30%FA, RA100F: 100% of RCA with 30%FA, RA25FM: 25% of RCA with 30% of FA & 10% of MK, RA50FM: 50% of RCA with 30% of FA & 10% of MK, RA100FM:100% of RCA with 30% of FA & 10% of MK. Reprinted with permission from Elsevier [154].

**Figure 16.** Mitigation of AAR expansion of RCA by sucking fine pozzolanic material into residual mortar [157]. Reprinted with permission from Elsevier [157].

**Figure 17.** Expansions of RCA with various cementitious material blends [157]. Reprinted with permission from Elsevier [157].

#### *8.6. Freeze-Thaw Resistance*

Various researchers have noted that RCA concrete has sufficient resistance to freezethaw cycles [141,159,160]. There is evidence that repeated recycling of RCA concrete further improves frost resistance [161]. For RCA concretes, some researchers have reported similar or slightly reduced frost resistance compared to NCA concretes 10 In addition, concretes made from dry and water-saturated RCA have a lower resistance to freeze-thaw. RCA Concretes prepared using water-saturated concretes have shown better results due to

improved anchoring at joints between pastes and aggregates. Previous studies showed that the effect of freeze-thaw was greater in mixtures with higher w/b ratio and higher RCA content. However, the effect of w/b ratio on freeze-thaw through percentage of weight change is more significant. The increase in the w/b ratio increased the number and size of the capillary pores as well as the freeze water in the cement paste, causing mainly the extended internal pressure during freezing. In addition, a strong relationship was observed between freeze-thaw damage and water absorption of mixtures [159,162] (See Figures 19–21). However, more research is needed to confirm the effect of RCA on concrete resistance to freezing and thawing.

**Figure 18.** Loss in weight of NAC and RAC having untreated and treated RA after acid immersion [92]. Reprinted with permission from Elsevier [92].

**Figure 19.** Residual compressive strengths of the mixes after 300 freeze-thaw cycles [159]. FNA: Fine aggregate, RC: Normal strength concrete, HC: High-strength concrete and HCAE: HC with air entraining agent. Reprinted with permission from Elsevier [159].

**Figure 20.** Relationship between water absorption and weight change percentages of after 300 freezethaw cycles [162]. Reprinted with permission from Elsevier [162].

**Figure 21.** Weight change percentage after 300 freeze-thaw cycles [162]. Reprinted with permission from Elsevier [162].

#### **9. Improvement Methods for RCA Concretes**

#### *9.1. RCA Quality Improvements and Adjusting the Ratio of Water and Cement*

It is tested heat treatment methods to improve RCA quality [71]. It was noted that RCAs are rationally comparable to the traditional used aggregate removed from the river after heat treatment at 800 ◦C. It is found that the strength of concrete was adversely affected by unwashed RCA used in concrete mixes [79]. However, the reduction in strength was offset by the use of rinsed RCAs. It is reported that the correct adjustment of the water balance of cement mixes for concrete mixes can increase the strength of RCA concrete [6]. It is have proposed a higher cement content in RCA concrete and a lower water to cement

ratio than NCA concrete to achieve similar compressive strength [11]. It is similarly noted that lowering the water-to-cement ratio to certain levels was very beneficial for RCA concretes to establish an equivalent thaw and freeze resistance to those NCA concretes [43].

#### *9.2. Pozzolanic Materials Integration and Soaking of RCA in Pozzolanic Liquids*

The strength and durability of RCA concrete can be increased through the use of appropriate pozzolanic substances [87,98,99]. It is showed that the use of 65% fine granular blast furnace slag and 30% pulverized fly ash improved the RCA compressive strength of concrete to control levels of concrete pouring with clean granite gravel [98]. In addition, fine-grained blast-furnace slags and pulverized fuel ash have been successful in increasing the resistance to chloride ion penetration into RCA concrete. Also, silica fumes have been found to significantly improve chloride permeation resistance for RCA concrete. The RCA concrete strengths may be boosted by either allowing reclaimed aggregates to soak up parts of mixing water without or with pozzolanic liquids during mixings or soaking the RCAs in mixes of pozzolanic liquids such as colloidal silicas or water before mixing of concretes. The micro-cracks in RCAs are expected to be filled up by the absorbed pozzolanic liquids or absorbed water with cement gels during pozzolanic reactions or cement hydrations. Therefore, the RCA concrete strengths can be increased.

#### *9.3. Uses of New Mixing and Curing Techniques*

It is used two-phase mixing approaches to obtain better quality RCA concrete (Table 5). These scientists used reclaimed aggregates treated with pozzolanic powders to improve the properties of RCA concrete [99]. In addition, the authors [87] have developed a two-phase mixing technique to ensure high-quality use of RCA concretes. These researchers found that, compared to NCA concrete, 100% replacement of virgin concrete aggregate is possible through their mixing approaches to create RCA concrete with suitable characteristics, although the optimal scenario is with 20% NCA replacement. The results of the determination of strength and slump showed that the new mixing methods significantly contributed to the achievement of high values of flexural and compressive strengths, as well as better workability. In addition, using scanning electron microscopes, interfacial transition zones between RCA concrete surfaces were realized. The scanning electron microscopes results established that the new mixing techniques promoted dense microstructures. Internal leaks in additives can be minimized by mixing techniques. The use of long-term cure in a humid environment is another approach to improving the performance of RCA concrete [54,100,102]. One of the most widely used approaches to reducing the carbonization rate of RCA concrete is the long-term cure. In RCA concretes, the carbonization depth is almost half when the concrete is cured with water. It is also reported that the common assumption that RCA is more sensitive to different curing conditions and therefore removes another obstacle to its massive use. [163]. Furthermore, Table 6 shows the ratio of RCA replacement criteria for making structural grades [34].

### *9.4. Microstructure of RAC*

Recycled aggregate used to produce concrete differs from natural aggregate in that it contains old mortar attached to the surface of the aggregate. The RCA recycling process often requires crushing of the concrete parts, so the microstructure of the RCA is exposed to many defects such as micro-cracks, porosity, as well as weakening of the ITZ. Damage to the aggregate microstructure will damage the engineering properties and durability of the concrete containing the recycled aggregate.

In fact, all types of aggregate produced of broken concrete are a composite material consisting of natural aggregate and cement mortar (See Figures 22–26). These broken parts use to completely or partially replace natural aggregates after crushing to small particles [164]. The RCA consists of NA and an old cement mortar, so the RAC consists of three ITZ regions, as follows [164]. First, between NA and old cement matrix, Second, between NA and the new cement matrix, third, between new and old cement matrix. Several studies focus on studying the transition zone and its relationship to concrete degradation and weakness. The degradation of concrete often depends on the entity of a filtering path across the interface (See Figures 22–26). The transition region, i.e., the interface between the particle assembly and the bulk cement matrix [165]. To reduce this problem, suggestions are made regarding the durability adopted in National Concrete Standards [165–167] that include limits on the minimum cement content as well as on the maximum water-to-cement ratio used for structural concrete. Further, it is also reported that the incorporation of pozzolanic materials, such as silica fume, fly ash and slag, with different mixing methods and accelerated carbonation can improve the microstructure of RCA [168].


**Table 6.** Ratio of RCA replacement criteria for making structural grades [34].

**Figure 22.** The different types of ITZs contained in concrete prepared with different fine aggregate: (**a**) NFA (quartz or river sand), and (**b**) in concrete with NFA partially replaced by RFA [167]. Reprinted with permission from Elsevier [167].

**Figure 23.** A low magnification image showing adhered cement in a RAC [87]. Reprinted with permission from Elsevier [87].

**Figure 24.** SEM images for polished section in concrete with RCA [87]. Reprinted with permission from Elsevier [87].

**Figure 25.** Sectional view of RCA in concrete including the old and new ITZ [169]. Reprinted with permission from [169].

**Figure 26.** SEM images of interfacial transition zones in RAC [169]. Reprinted with permission from Elsevier [169].

On the other hand, the micro-pores in the mortar attached to the old rubble can contribute to retaining additional quantities of water, which may help to provide a selftreatment that leads to the promotion of filling the pores with moisturizing products (gel) [170]. Thus RA behaves as an internal curing agent, which also improves the ITZ between particles of aggregate and cement paste afterwards reduces the size of the pores within the microstructure of the concrete [171,172] (See Figure 27). This improvement is caused by a better inter-bond between the cement paste and the aggregate particles providing cross-linking sites for the cement paste resulting in better cement wetting and denser ITZ more uniform. The fracture value of aggregate, size, and porosity of ITZ has paradoxical effects on concrete's transport properties.

**Figure 27.** The ITZ between RCA and New Mortar Mixes; (**a**) 100% natural aggregates and (**b**) 75% natural aggregates + 25% recycled aggregates [171]. Reprinted with permission from Sadek and El-Attar [171].
