Concrete structures can be demolished through various methods, ranging from mechanical machines to chemical processes [21,22,23]. Among the most common demolition techniques are pneumatic and hydraulic breaking, pressure bursting, dismantling, the ball and crane methods, and implosion as shown in Figure 2.

A concrete recycling plant crushes concrete through primary and secondary crushers. Initially, the size of the concrete is reduced to 8-10 cm (3-4 in) by primary crushers, and then secondary crushers further reduce the materials to the desired maximum coarse aggregate size. Jaw, cone, and impact crusher designs are used in concrete recycling plants [29]. The selection of crushers relies on material size, operating cost, the desired end product, and environmental considerations [30,31]. Jaw crushers tend to leave higher levels of cement paste adhered to recycled aggregates, while impact cone crushers are more effective in removing adhered hardened cement paste [32]. Different crushing processes remove different amounts of mortar from the original aggregate particles, and they produce different RCA gradations. Gradation describes the particle size distribution of the aggregate, and its maximum size is limited by equipment capability, dimensions of construction members, clearance between reinforcing steel, and layer thickness [33]. The maximum aggregate size is referred to as the smallest size sieve that 100% of the aggregates pass through. The degree of mortar removal and gradation also depends on the properties of the natural aggregates used in concrete. Recycled aggregates can be specified to the same gradation as virgin aggregates [34]. The desired gradation of aggregates for both virgin and recycled can be produced similarly by programming crushing equipment and using proper screening [35].

Despite available comprehensive literature related to the use of recycled concrete aggregate, there are a very limited number of studies related to the economic and environmental life cycle impacts of replacing RAs with NAs [36,37]. Gangolells et al. [38] surveyed 74 Spanish construction companies based in Catalonia and found that most of the opportunities identified by the construction firms are within the scope of the government and related to the combined system of bonuses and penalties.

Duran et al. [39] developed a model for assessing the economic viability of construction and demolition waste. They identified several factors to influence the economic feasibility of recycled concrete aggregates. The decision of which aggregate to use (RCA or NA) was based on the following expression [39]:

Where Q_p is the price per ton of NA at the quarry gate; T_q is the cost of transportation from the quarry to the site (per ton); E_ru refers to any additional cost per ton created by using the recycled concrete (for example, the addition of cement content); RC_p the price per ton of recycled concrete at the recycling center and T_ru is the cost per ton of transportation from recycling center to the site. Martinez-Large et al., [40] used the above model to study the economic viability of concrete with ranges of compressive strength under different scenarios. They concluded that the use of recycled aggregates instead of natural aggregate may increase the cost under certain conditions, particularly when additional cement or processing is required to meet performance criteria. Therefore, they suggested that the government should take necessary measures to facilitate the use of recycled concrete aggregates. This can be achieved by imposing additional penalties and taxes for landfilling [41], increasing the price for NA, or subsidizing recycled aggregates. The Netherlands is reported to be the country with the highest recovery rate of construction demolition waste due to long-standing landfill restrictions [42]. The introduction of the landfill tax in 1995 and the landfill ban in 1997 in the Netherlands contributed to the reduction of landfilling of concrete demolition waste significantly [43].

Based on Equation (1) and literature, the economic viability of RCA is strongly influenced by processing, transportation, and avoided disposal costs. While processing operations such as crushing and screening are generally comparable to those used for natural aggregates production, their contribution to the overall cost is often secondary relative to other factors [42,44,45]. Instead, transportation cost is consistently identified as a dominant parameter, as it is highly sensitive to the relative distance between the construction site, quarry, and recycling facility [39,40,45]. Additional cost may arise from mix design modifications, such as increased cement content or the use of admixtures to achieve comparable mechanical performance when RCA is used [9,40]. However, these potential increases are often offset by the economic benefits associated with avoided landfill disposal, which eliminates tipping fees and reduces environmental burden, thereby improving the competitiveness of RCA [41,42,45]. As shown in Table 1, the economic advantages of RCA are therefore not derived solely from construction cost, but rather from the combined effects of transportation logistics and landfill cost avoidance. At the project level, these benefits become more pronounced when recycling is performed on-site, where reduced hauling distances and disposal requirements can significantly decrease total project costs while generating reusable aggregate. Consequently, transportation distance and local landfill conditions are identified as dominant parameters governing the economic feasibility of RCA, consistent with Equation (1).

In addition to these conventional economic considerations, recent studies incorporated multi-objective optimization frameworks that simultaneously evaluate mechanical performance, cost, and environmental impact in sustainable concrete design [46,47]. These approaches extend traditional cost-based analyses by integrating parameters such as CO₂ emissions and material efficiency into the decision-making process [46]. A notable example in sustainable cementitious systems involves the application of machine learning-assisted mix design to optimize trade-offs between compressive strength, cost, and carbon emissions. This is demonstrated for waste glass powder concrete using predictive modeling and the NSGA-II multi-objective optimization algorithm [48]. Although developed for a glass powder system rather than recycled aggregate concrete, the methodological framework is directly transferable to RCA-based mixes, where variability in material properties and the need for mix adjustments introduce comparable complexity [47,49]. Data-driven, multi-objective strategies, combining ML-based strength prediction with simultaneous optimization of CO₂ emissions and cost, have demonstrated practical applicability in RCA mix design process [46,49], providing a promising framework for selecting optimal RCA-based mixtures and enabling more balanced and performance-oriented decision-making compared to conventional single-parameter economic evaluations.

The shape of the RCA greatly depends on the crushing process to break the aggregates, similar to the NA. Common aggregate shapes include rounded, flaky, elongated, and angular [61]. The surface texture or morphology of the aggregates can be influenced significantly by the hardness, grain size, and pore characteristics of their parent materials. Smooth surfaces improve the workability of fresh concrete, whereas angular surfaces create a stronger bond between the mortar and aggregates, leading to higher strength. This is particularly important in applications where enhanced flexural strength or higher compressive strength is required. There are several test methods to determine the shape properties of aggregates, such as flakiness and elongation indices, angularity number of coarse aggregates, and index of aggregate particles’ shape and texture [62].

Elongated and flaky aggregates require more water and sand in the mix to obtain suitable workability. They may also break during compaction. These adversely affect the strength of concrete. Thin and flat particles can reduce the strength of concrete when a load is applied to the flat side of the aggregate and are also prone to segregation and breakdown during compaction and mixing. The breaking of aggregates results in the creation of additional fine particles. Higher flakiness and elongation also reduce the workability and durability of concrete [63]. Elongated particles reduce the strength of concrete due to the reduction of bond strength between the aggregate and cement paste. In addition, higher cement content will be required to achieve a similar strength [64,65].

Particle density and water absorption are important properties for both natural and recycled aggregates in quality control. In general, the particle density of aggregates varies from 2000 to 2500 kg/m³ depending on the composition of the crushed materials. The particle density also depends on aggregate size. Limbachiya [66] reported that RCA has 7-9% lower relative density compared to the natural aggregates in the saturated surface dry state. Sago-Crentsil et al. [67] reported a bulk density of 2394 kg/m³ for RCA and 2890 kg/m³ for natural aggregates. The decrease in density is due to the adhered mortar, which is lightweight compared to the natural aggregates.

Water absorption of RCA is generally higher compared to natural aggregates due to the higher porosity of the old mortar adhered to the recycled aggregates. Water absorption of natural aggregates ranges from 0.8 to 3.7%, whereas the corresponding absorption for recycled aggregates ranges from 3.7 to 8.7% (Table 1). Other studies showed a water absorption of 4.9-5.6 for RCA compared to 1.0-2.5% for natural aggregates [66,67]. Li et al. [68] reported a decrease in the water absorption rate for RCA after carbonation. This was consistent with the results claimed by Li et al. [69].

Gradation or size distribution of the aggregates is an important property that directly affects the fresh and hardened properties of the concrete, such as workability, porosity, permeability, strength, degree of compaction, and durability of the concrete. Similar to the virgin aggregate, RCA needs to be a continuous gradation to allow greater interaction between particles and provide greater compactness and improved mechanical properties. Gradation or size distribution of RCA is similar to natural aggregate and is based on general standards such as ASTM C33 [70] or EN 12620 [12]. Aggregate gradation or particle size distribution is also an important factor in selecting the pavement preservation method [71] or geotechnical projects [72].

The Los Angeles (LA) abrasion test measures the pulverization that takes place for a given aggregate when subjected to impact, abrasion, and grinding. In general, a softer aggregate is expected to have a higher abrasion loss. A higher percentage loss indicates less resistance to abrasion while a load is applied. Virgin aggregates tend to have LA abrasion of about 10-20% mass loss; however, the corresponding values for RCAs are reported to be higher and between 20-45%. The higher value is due to the presence of cement paste which is more susceptible to abrasion than most aggregates [9,54]. However, based on ASTM C131, aggregates for construction are required to have an abrasion loss of less than 50% for general construction and less than 40% for crushed stone for pavement. Despite the lower abrasion resistance for RCA, typically, their resistance is within the acceptance of the ASTM C131.

Recycled concrete aggregate includes the coarse aggregate from the original concrete partially or fully surrounded by mortar from the crushed concrete, as shown in Figure 3. The amount of cement mortar attached is one of the significant factors affecting the material properties of RCA [52]. Adhered mortar content is affected by the number of crushing processes in the production plants. Increasing the number of crushing processes reduces the mortar content and improves the quality of aggregates [73]. Nevertheless, this also increases the production cost; therefore, a balance between the number of crushing stages and the quality of aggregate needs to be considered. There is concern that the original cement attached to the RCA may not have completely reacted with water during mixing. Crushing of concrete can expose unhydrated cement, which can react with water when RCA is used for new concrete, as suggested by Manzi et al. [74]. This may have positive and negative effects on new concrete. The cement can increase the strength of the new concrete by lowering the water-to-cement ratio. However, it can also increase the drying shrinkage, which may result in premature cracking in new concrete. Due to the importance of the adhered mortar content on the properties of the RCA, several test methods have been developed to quantify the adhered mortar content. These methods involve soaking aggregates in chemical solutions and/or subjecting aggregates to freeze-thaw cycling. These two methods remove the mortar content by chemical degradation and degradation due to mechanical stress allowing the quantity of adhered mortar to be measured. For example, ASTM C88-05 [75] uses sodium or magnesium sulfate solutions to remove mortar by chemical degradation. There are other methods, such as subjecting RCA to freeze-thaw cycling while soaked in sodium chloride in order to remove mortar through mechanical stress [76].

New concrete produced by RCA provides two interfacial transition zones (ITZ), one refers to the new ITZ between RCA and the new mortar matrix, and the other ITZ lies between the original aggregate in the RCA and old adhered mortar. Tam et al. [77] conducted microstructure studies of the RCA and provided SEM images highlighting these ITZs. The original cement content attached to NA in recycled aggregate is more porous than NA and therefore results in more water absorption. Etxeberria et al. [64] found that ITZ is the weakest region in concrete with RCA due to its higher porosity. However, Otsuki et al. [78] conducted Vickers micro hardness testing and concluded that the characteristic of the ITZ depends on the quality rather than the quantity of the adhered mortar.

It has been claimed that using a pozzolanic solution to treat RCA can enhance the strength of adhered mortar [15]. RCA immersed in a pozzolanic material, especially containing silica fume, can fill the pores and voids of adhered mortar [77].

The attachment of old mortar to the RCA results in a higher porosity compared to the natural aggregates. Higher porosity directly influences other mechanical properties such as water absorption, density, and strength properties of the concrete. Higher water absorption will affect the water volume available for the chemical reactions. Several authors [79,80] suggested that in the case of RCA, an additional 10-15% water in the total mixing water may be required to improve workability. However, the addition of water consequently increases the w/c ratio and adversely affects the mechanical properties of concrete [81]. Some researchers [66,82] proposed complementing the mixture with an equivalent volume of extra cement. Nevertheless, Laserna and Montero [83] showed that the strength of the recycled concrete is clearly influenced by the richness of the cement content.

There have been several investigations [84,85,86,87,88] to study the influence of pre-wetting recycled aggregates on the rheological behavior of concrete by varying the initial moisture of the RCA. Some researchers [84,89] claimed that prewetting may introduce a potential solution to the high-water absorption of RCA. Malešev et al. [90] suggested increasing the water volume corresponding to 30 minutes of RCA water absorption in the middle of the mixing process. Rodrigues et al. [91] recommended washing the aggregate to remove the fine fraction adhered part of the soluble sulfate content to decrease the water absorption. Otsuki et al. [78] and Ryu [92] recommended using a double-step mixing method which they reported as adding water or aggregates in two stages to improve the interfacial transition zone surrounding RCA. Despite the fact that these methods may improve the strength of concrete, they introduce complexity to the mixing process and are difficult to implement in the current concrete manufacturing procedures.

In addition to these conventional approaches, recent studies have explored data-driven and machine learning (ML) methods to address the RCA variability and optimize mix design. These approaches aim to balance workability, mechanical performance, and durability by leveraging large data sets and predictive modeling techniques. For example, recent work on low-carbon recycled concrete has demonstrated the use of synthetic data augmentation techniques (e.g., CTGAN) combined with machine learning models to improve prediction accuracy under limited or heterogeneous datasets. These emerging methodologies provide a promising complement to traditional empirical and experimental approaches, particularly for the design of sustainable concrete systems where material variability remains a critical challenge [93].

The constituent materials of recycled concrete aggregate have a large effect on the fresh properties of concrete, such as workability, air content, and density. Type of aggregate, aggregates maximum size, water absorption of aggregate, etc., have different effects on the fresh properties of concrete. The workability of fresh concrete refers to how easily it can be placed and finished without segregation and is affected by surface texture, aggregate size, and aggregate shape. In general, the use of RCA will require more water compared to mixes with natural aggregates to achieve desired workability [94,95]. For example, Tabsh and Abdelfatah [96] concluded that concrete mixtures with recycled aggregates required an additional 10% water compared to the NA to provide similar workability. The moisture content of the RCA is another parameter that affects the workability of concrete mixtures with RCA. RCA has higher water absorption and, therefore, absorbs the water from the fresh mix, which in return, adversely affects the workability [64,94,97]. Poon et al. [98] observed bleeding of concrete for concrete mixes with surface-saturated dry RCA, and they concluded that the bleeding was responsible for the reduction of compressive strength compared to concrete with dry RCA or NA.

Shi et al. [15] discussed several pretreatment methods for RCA that include either removing or strengthening the adhered mortar from RCA. Typical methods to remove adhered mortar include mechanical grinding, heat grinding, and pre-soaking in acid. Other methods, such as polymer impregnation, pozzolanic slurry, immersion, accelerated carbonation curing, and bio-deposition methods, are claimed to strengthen the RCA [99]. Dimitrou et al. [10] reported that RCA with a rounder shape had higher workability than with more angular recycled aggregates. A similar conclusion is reported by Sagoe-Crentsil et al. [67].

Belangra et al. [100] concluded that the addition of admixtures at various percentages could increase the workability and hence decrease the w/c ratio in order to enhance the mechanical properties of concrete with RCA. The air content of fresh concrete containing RCA is more variable and often 0.6% higher compared to the fresh concrete with NA [101,102,103]. This is due to the air entrained and entrapped in the RCA.

Compressive strength, perhaps, is the most important property of the concrete with RCA and often is the primary factor in determining the classification of RCA. The compressive strength requirement of concrete is based on its application. For example, concrete with compressive strength less than 15 MPa is suitable to be used as concrete fill, 20 MPa for basement and foundation, 20-25 MPa for driveways, garages and industrial floor slabs, and higher than 70 MPa is for high-rise buildings [104].

The compressive strength of RAC depends on the water-to-cement ratio, the level of aggregate replacement, and the resistance of the aggregates to crushing [98,105]. Based on available literature, the normalized compressive strength typically ranges from 0.70 to 1.00 for partial replacement and decreases to 0.40-0.90 for 100% RCA, depending on mix design and aggregate quality.

It is generally accepted that recycled concrete aggregates reduce the strength of concrete compared to comparable NA mixes. The reduction is higher when RCA replaces 100% of the NA. Henson and Narud [106] reported a decrease in compressive strength up to 30% when 100% NA was replaced with RCA. Similarly, other researchers [64,107] observed a reduction in compressive strength between 12 to 25% with total replacement of RA. Nevertheless, there are cases where the reduction in compressive strength is more significant, reaching up to 60% [108] and even 76% [109] for 100% RCA. This reduction in strength could be partially attributed to the presence of adhered mortar, increased porosity, and weaker ITZ. However, there is no clear trend in the reduction of compressive strength and total replacement of RA. In some cases, comparable strength to NA is reported. Shayan and Xu [110] conducted a comprehensive study to investigate strength development and durability-related properties of RCA of 50 MPa strength-grade concrete. They used one commercially available source for RCA and investigated several different treatment methods, and also used silica fume in order to achieve similar mechanical and durability properties as concrete with natural aggregates. They concluded that RCA could be used to produce 50 MPa structural concrete with durability properties similar to concrete made with natural aggregates. In another investigation, Padmini et al. [111] crushed laboratory concrete with a compressive strength ranging from 31-58 MPa and used the resulting aggregate in new concrete. They concluded that the water absorption of recycled aggregates increases with an increase in the strength of the original concrete used to produce RCA. They also concluded that the strength of RCA increases with an increase in the maximum size of the aggregate. However, they reported that reducing the w/c ratio and increasing cement content is needed for RCA to achieve a similar compressive strength as NA. Similar conclusions were also claimed by Limbachiya et al. [112]. These findings highlight the critical role of mix design optimization and quality of sourced concrete in determining performance.

Flexural strength is another important property of concrete, particularly for pavement and slab applications. The properties of coarse aggregate have a greater influence on flexural strength than on compressive strength. Recent studies [113] have shown that aggregates with a rough surface and angular shapes provide a higher flexural strength compared to smooth and round shaped coarse aggregates due to the enhanced mechanical bond between the cementitious paste and the aggregate. The flexural strength of concrete is often estimated using empirical relationships with compressive strength. For concrete made with smooth and round shaped aggregates, the modulus of rupture (MOR) can be approximated as [114]:

For the case of concrete made with rough textured and angular shaped aggregate, Equation 2 will be changed to Equation 3.

The purpose of presenting Equations 2 and 3 is to provide a baseline predictive framework for estimating flexural strength based on compressive strength and aggregate characteristics. These relationships can also be used to evaluate the deviation of RCA behavior from conventional concrete. Comparison with experimental results from the literature indicates that RCA generally follows a similar trend, although slight variations may occur due to weaker ITZ and higher porosity. Available studies suggest that normalized flexural strength typically ranges from 0.80 to 1.00, even at high RCA replacement levels, indicating that flexural strength is less sensitive to RCA incorporation than compressive strength. For example, Ravindrarajah and Tam [115] reported no significant difference in the flexural strength of conventional concrete and concrete made with RCA. Other researchers [90,105] reported a reduction of up to 10% in split tensile strength for 100% RAC. The quality of the concrete used to produce RCA has a strong influence on flexural strength which relies on the bond strength of the mortar content of RCA [116].

The elastic modulus of concrete is influenced by the cement paste, types of aggregate, the interfacial transition zone, and the strength of concrete [117]. In addition, similar to the other properties of concrete, elastic modulus also varies with the age of the concrete. In general, a lower elastic modulus for concrete with RCA is reported. Some researchers [118,119] claimed that the effect on the elastic modulus is minimal with up to 30% replacement of NA with RCA. Other researchers reported a reduction of 20-40% when 100% RCA is used [120]. This reduction is related to the fact that the elastic modulus of concrete made with RCA is generally lower than for concrete made with natural aggregates because of the mortar on the aggregate surface.

Cementitious materials may exhibit significant drying shrinkage during their service life [121]. Shrinkage is related to volume change, and it can be a detrimental property of concrete, especially if the concrete is restrained. Shrinkage depends on the mix design parameters such as mix proportions, cement content, cement replacement materials, and admixtures used in the mix [121]. Shrinkage occurs in the form of plastic shrinkage at the early stages of curing, and then drying shrinkage occurs throughout the service life of concrete. The reduction is due to the loss of water in the form of vapor from concrete. In general, concrete with RCA shows higher shrinkage, which is attributed to water absorbed by the mortar content attached to RCA. Some studies [122,123] suggested that long-term strain is higher for concrete with RCA compared to similar concrete with NA. They claimed that the higher strain is produced by shrinkage where concrete is not subjected to load, and shrinkage and creep for concrete subjected to load. There is some ongoing research in order to improve concrete with recycled aggregate using alkali-activated binder treatment to reduce shrinkage [124].

Freeze-thaw resistance is a measure of concrete’s ability to resist internal cracking due to forces inside the concrete generated during freeze-thaw cycles. ASTM C666 [125] is a common standard used to determine the freeze-thaw resistance of concrete. Concrete subjected to freeze-thaw cycles requires air entrainment based on the aggregate size and the degree of exposure. ACI 330R [114] categorizes concrete into three types based on exposure: mild, moderate, and severe. Mild exposure refers to climates where the concrete is not exposed to freezing or deicing salts. Severe exposure, on the other hand, refers to climates where concrete is expected to be exposed to deicing chemicals or continuous moisture before freezing. The ACI 330R [114] recommends air content percentages of 3.0, 4.5, and 6.5% for mild, moderate, and severe exposure, respectively, for concrete with a maximum aggregate size of 25 mm (1 in).

The freeze-thaw resistance of RCA is a major durability concern in cold regions due to its higher absorption, porosity, and adhered mortar. Further research is required to fully understand the freeze-thaw durability performance of RCA. Šeps et al. [126] reported a decrease in freeze-thaw resistance in concrete containing RCA. They suggested that RCA should only be used in concrete when it can be protected from freezing in moist conditions [126]. However, Huda and Alam [127] reported that the durability performance of RCA was satisfactory for concrete made with recycled coarse aggregates at a replacement rate of 50%.

Yamato et al. [128] also reported lower freeze-thaw resistance for concrete with RCA. However, they also suggested that this reduction in durability can be mitigated by partial use of recycled aggregate, reducing W/C, and increasing entraining air.

Alkali-Silica Reaction (ASR) occurs due to the reaction of alkaline pore solution of hydrated cement paste and reactive silica phases within some aggregates [129]. The ASR mechanism results from several sequential reactions as described by Rajabpour et al. [130] as follows: dissolution of metastable silica, formation of nano-colloidal silica sol, gelation of the sol, and swelling of the gel. Brantley et al. [131] claimed that silica dissolution is often the slowest reaction among these reactions, and therefore, it controls the rate of ASR in concrete. It is very common to use high temperatures and higher alkalinity in order to speed up the ASR in concrete in lab settings. The extent of gel swelling relies on the availability of moisture and mass transport properties of concrete, with damage occurring at a limited rate in dry and dense concrete [132].

In order for an alkali-silica reaction to occur, several specific conditions must be met. These include the presence of reactive forms of silica in the aggregate, a high alkali (pH) pore solution, and sufficient surface area [133,134]. If any of these conditions are absent, ASR will not take place. When the reaction occurs in the presence of water, it creates a gel, leading to expansion. This expansion induces internal stress, ultimately resulting in a reduction in the modulus of elasticity, flexural strength, and compressive strength.

There are a number of test methods for assessing the potential alkali reactivity of aggregates in mortar or concrete [135]. The accelerated mortar-bar test method (AMBT) as outlined in ASTM C1260 [136], and the concrete prism test (CPT) conducted at 38°C following ASTM C1293 [137], are the most common tests.

In general, the heterogeneity that may exist in the RCA with different origins add complexity to accurately assessing its potential for ASR [135]. One possibility is that the higher porosity of RCA due to the adhered mortar can accommodate some of the formed gel and can result in hiding or delaying the expansion. One can also use slag in mixes with RCA to help mitigate the potential for ASR [135].

The carbonation of concrete depends on factors such as the concentration of carbon dioxide that concrete structures are exposed to, pore distribution of the hardened concrete, moisture content, temperature, and the presence of the microcracks. In the case of concrete with normal aggregates, carbonation is widely studied. However, there is limited study for concrete with RCA. Resistance to carbonation for concrete with RCA has two main differences compared to concrete with natural aggregates [138]. First, RCA may have already carbonated before its use as concrete aggregate, subsequently leading to the local increase of the carbonation depth. Second, adhered mortar to the aggregate, which is porous, may carbonate considerably faster than the cement paste of the new concrete, which results in a local increase of the carbonation depth as well. On the other hand, Leemann [138] showed that new concrete made with RCA obtained from demolishing a strong concrete would result in a lower carbonation depth.

Ana et al. [139] determined the carbonation depth of concrete made with 25, 50, 75, and 100% RCA. Their results showed that the higher the replacement of RCA, the higher the carbonation depth will be. Similar results are also reported by Silva et al. [140], claiming that concrete with 100% RCA can exhibit carbonation depths up to almost 2.5 times greater than that of a corresponding concrete with NA. In another study, the carbonation depth was between 1.2 to 2 times higher for concrete with RCA than concrete with NA [141].

Nevertheless, some researchers reported similar or higher resistance to carbonation for concrete with RCA than NA [138]. For example, Limbachiya et al. [112] reported a decrease in carbonation depth with concrete made with 30% RCA compared to concrete with NA. The compressive strength of concrete made with RCA and NA was reported to be 39 MPa and 33 MPa, respectively. Similarly, Xiao et al. [142] reported a decrease in the carbonation depth of concrete with recycled aggregate concrete with the increase in the strength grades of the original concrete. Xiao et al. [142] suggested that the existence of higher mortar content in concrete with RCA (old mortar and new mortar) provides a larger quantity of materials available for carbonation, and therefore, the carbonation resistance will be improved. The use of superplasticizer is reported to lower the carbonation depth of the RA concrete than RA concrete without superplasticizer [143].

Some studies [144,145,146] show that carbonated RCA can increase the compressive strength of new concrete. For example, Lu et al. [144] showed that using carbonated RCA in new concrete can increase compressive strength compared to non-carbonated RCA aggregates. The authors reported the compressive strength of concrete with 100% non-carbonated RCA of 40 MPa. The corresponding compressive strength for concrete with 100% carbonated RCA was reported to be about 54 MPa. Lower water absorption and higher apparent density of carbonated RCA compared to non-carbonated RCA were responsible for improving the compressive strength.

Incorporating RCA increases the chloride penetration of new concrete [147]. Limbachiya et al. [112] reported a similar or moderate level of chloride concentration using RCA up to 30%. They claimed that concrete with higher than 30% RCA substantially increases the chloride concentration compared to concrete with NA. Higher compressive strength of concrete containing RCA tends to decrease the chloride concentration. In general, pore characteristics and strength of concrete are the major properties governing the resistance to chloride penetration. Therefore, lowering the w/c ratio of new concrete, reducing capillary pore volume, or refining the pore network significantly improves the resistance to chloride penetration of RCA concrete. Matias et al. [143] reported an improvement in chloride penetration resistance by using a high-performance superplasticizer. Using silica fume is also reported to decrease chloride ion penetration for concrete with RCA [112,148]. Shicong and Poon [148] reported that the replacement of cement with 25% fly ash increases the resistance to chloride ion penetration of concrete with RCA.

X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) are analytical methods used to assess the mineralogical composition, microstructure, and surface properties of RCAs. XRD relies on the interaction between X-rays and crystalline materials. When X-rays interact with a crystalline object, they diffract at particular angles, resulting in a distinctive diffraction pattern. Analyzing this pattern allows for the identification of the crystalline phases of a material [149]. XRD helps the identification of the mineral composition of RCA and differentiates between crystalline phases, including quartz, calcite, and various silicate minerals [150]. This offers insights into the comprehensive mineralogical composition. XRD also reveals the amorphous content in RCA which helps understand the degree of hydration or amorphous phases present, influencing the overall performance of recycled aggregates.

SEM utilizes electron beams to scan the surface of a sample. The interaction of the electrons with the sample’s atoms generates signals that can be used to create details of the surface morphology and composition at a micro to nanoscale [151]. It provides high-resolution images of the microstructure of RCAs, helping visualize surface roughness, porosity, and the distribution of distinct phases, thereby exposing the quality and characteristics of recycled aggregates. It also facilitates the measurement and characterization of particle size and morphology, which is critical for understanding the performance of RCA in concrete mixes. SEM is often used to reveal surface defects, cracks, and other features that may impact the overall durability of RCAs. In particular, SEM observations consistently show that RCA particles are surrounded by adhered old mortar, which introduces a more porous and heterogeneous ITZ compared to natural aggregates. This weak ITZ is characterized by loosely packed hydration products, higher microcrack density, and increased porosity, which directly influence the mechanical behavior of concrete.

The presence of adhered mortar leads to the formation of a multi-layer ITZ system, including the old ITZ from the original concrete, the new ITZ formed with fresh cement paste, and the interface between old and new mortar. This composite ITZ is generally weaker than that in conventional concrete and acts as a preferential zone for crack initiation and propagation. As a result, the reductions in compressive strength and elastic modulus discussed in Sections 4.2 and 4.4 can be directly attributed to the inferior quality of the ITZ. In addition, the higher water absorption of RCA may cause localized variations in the effective water-to-cement ratio near the aggregate surface, leading to non-uniform hydration and further weakening of the ITZ.

Yaowarat et al. [152] investigated the effect of polyvinyl alcohol to improve the compressive and flexural strength of concrete containing RCA. They used SEM and XRD in their investigation and found that polyvinyl alcohol forms a film around cement grains and prevents the water absorption for cement hydration process. This led to retard the hydration process and delay the initial and final setting times of polyvinyl alcohol cement. As a result, they reported a reduction in compressive strength, which can also be associated with changes in the microstructure and ITZ characteristics.

Jitsangiam et al. [153] used XRD and SEM to analyze high grade RCA and road base RCA. They observed a fabric hydration product (CSH), with a needle-like product (ettringite) on their surfaces and pores. They claimed that CSH and ettringite fill the pores and result in higher strength. Their result also showed that the introduction of nonactive materials such as bricks and tires reduces the amount of cohesive hydration products, weakens the microstructure, and consequently lowers the mechanical performance [153].

Aggregates are a major part of all airport runway constructions, such as aggregate base, cement-treated base, asphalt, concrete, and fill materials. Standard specification for airport construction [167] describes the requirement for aggregates and other properties of concrete for airport construction. The weight loss of the fraction retained of the aggregate from the No.4 mesh sieve should not exceed 18% by weight when subjected to five cycles of sodium sulfate soundness according to ASTM C88 [75]. Gradation is another important criterion for this document. The crushed gravel and crushed stone should meet the requirements of one of the three gradations provided in the document to be accepted for concrete subbase course.

Compressive strength is another important property of concrete that needs to be specified for airport construction. Standard specification for airport construction [168] requires a compressive strength of 3.5 MPa (500 psi) at 7 days and 5.2 MPa (750 psi) at 28 days for a concrete subbase course. The document also requires air entrainment of between 6 to 10% according to ASTM C231 for gravel and stone aggregates and ASTM C173 for slag and other highly porous coarse aggregates. The slump of the mixture also needs to be 50 mm (2 in) according to ASTM C143 at the time of placing.

Airport pavement requires a minimum flexural strength of 4.1 MPa (600 psi) and compressive strength of 30 MPa (4400 psi) at 28 days. The w/c ratio, which influences these mechanical properties, is allowed up to a maximum of 0.50. In the case of severe freeze/thaw, deicer, or sulfate exposure, it may be necessary to use a w/c ratio lower than 0.50.

The Federal Aviation Administration (FAA) requires a service life of 20 years for airport pavement structures supporting aircraft traffic. However, it should be noted that due to the increase in passenger loads and also minimizing the downtime of airport pavement facilities, airline industries are undoubtedly under pressure to increase the service life.

Vehicle parking lots offer opportunities for utilizing sustainable materials, with concrete containing RCA being a common choice [169]. These parking lots typically have a service life of 30 years or more, after which the concrete can be recycled into aggregates and pavement subbase materials. Typical examples of concrete parking lots are public facilities, commercial developments, and business and multifunctional projects. They are found in various settings such as public facilities, commercial developments, and multifunctional projects, serving as car parking areas, access lanes, shopping center entrances, bus parking areas, and truck parking areas. While the primary function of these lots is vehicle parking, they often also serve as maneuvering areas and access points for delivery trucks. In terms of design and construction.

The main purpose of these is to park vehicles, but they may also provide maneuvering areas or access for delivery trucks. In general, there are many similarities between the design and construction of streets and highways with parking lots. In terms of design and construction, there are many similarities between parking lots and streets or highways.

ACI recommends workability with a slump of 38 mm (1½ in) or less for slip-form paving or 100 mm (4 in) when placed by hand or with vibrating [170]. In addition, the maximum aggregate size should be no greater than 1/3 the thickness of the slab. In the case of heavy traffic loads or when durability is critical, compressive strength of at least 28 MPa (4000 psi) is required for parking lots.