Optimal Replacement Ratio of Recycled Concrete Aggregate Balancing Mechanical Performance with Sustainability: A Review

Abstract

Significant construction and demolition waste (CDW) is produced by many useless concrete buildings, bridges, airports, highways, railways, industrial mining, etc. The rising need for new construction has increased the use of natural materials, impacting the ecosystem and incurring high costs from mining natural aggregates (NA) and processing CDW. The concept and implementation of recycled aggregate concrete (RAC) offer a sustainable solution for the concrete industry. Crushed concrete, made from recycled concrete, can be used instead of natural aggregates in structural concrete. This sustainable byproduct, recycled concrete aggregate (RCA), has the potential to replace natural aggregate. This paper examines the benefits of RAC from economic, social, environmental, and technological perspectives and discusses the replacement ratio (RR)—the weight percentage of natural aggregate replaced by recycled aggregate—which is crucial to RAC performance. A collection of used data on mechanical properties and economic performance, national specifications, standards, and guidelines is reviewed to determine the optimal replacement ratio for RCA, which was found to be 20%. Finally, we discuss the challenges and future of using RAC in structural concrete.

1. Introduction

Except for a few specific use cases, most useless buildings, streets, highways, bridges, utility plants, piers, dams, etc., are demolished. Demolition generated 90% of construction and demolition waste (CDW) [1], while, in contrast, new construction activities generated 10%, according to the 2018 EPA information sheet published in December 2020 [1]. Global CDW production in 2018 is depicted in Figure 1, showing that developing China had the largest CDW production (i.e., around 2360 million tons in 2018) [2]. Although this statistic differs from that of Chinese media [3], as shown in Figure 2, there is no doubt that China’s CDW production has increased yearly. Figure 2 shows that 2025 production will exceed 4 billion tons.

Concrete is the material with the greatest demand for new construction, and solid waste concrete (SWC) is the largest proportion of solid waste in CDW, as shown in Figure 3 [4]. Kabirifar et al. estimate that 35% of the CDW worldwide was landfilled [5]. With landfill space becoming increasingly scarce and global demand for aggregate reaching a staggering 40 billion tons annually [6], effectively utilizing SWC and reducing environmental impact have emerged as crucial research areas. Fortunately, recycled aggregate concrete (RAC) opens up SWC solutions. Studies suggest that employing recycled aggregate (RA) from SWC instead of natural aggregate (NA) in RAC benefits the economy and environment [2]. It could save 10–20% of material costs [7], reduce greenhouse gas emissions by 65%, and reduce non-renewable energy use by 58% [8]. The use of RAC can be traced back to the last century after World War II [9]. RA and RAC are being studied more due to environmental concerns and sustainable development. Over 10,000 RAC publications have been published in the past 20 years [2].

Recycled concrete aggregate (RCA) is essential for RAC, with its properties significantly affecting RAC performance. RCA differs from NA as it includes NA and old cement mortar. The size of RCA is inversely related to the amount of adhering mortar [10,11,12], and RCA absorbs more water than NA [10,13,14,15]. Larger RCA particles have even higher water absorption [15,16] due to more attached cement mortar [17]. Standards often prohibit RCA in structural concrete if its water absorption exceeds 7–10% [16]. RCA also has higher aggregate crushing values [11] but lower mechanical strength than NA [14], primarily due to the adhering mortar. Sulfate soundness losses in RCA vary from 29.1% to 49% [18], and it typically contains higher chloride levels, though usually within safe limits [14,19]. In summary, RCA comprises old NA, attached and detached mortar, brick, and minor debris, leading to lower density and higher water absorption than NA.

In the past two years, several review articles on RCA or RAC have been published [20,21,22,23,24,25,26,27,28]. Fanijo et al. [20] reviewed using RCA for pavement construction, comparing the physical, chemical, mechanical, and durability properties of RCA and natural aggregate (NA) pavements. Varshney et al. [21] explored using bacillus species and fibers to enhance specific physico-mechanical properties of RAC. Zaidi et al. [22] reviewed RAC properties modified with nano-silica (nS). Gu et al. [23] examined various fiber properties and recycled aggregates, summarizing the impact of fibers on RAC performance. Liu et al. [24] systematically reviewed RAC’s frost resistance, identifying factors affecting its performance during freeze–thaw cycles (FTCs). Muhammad et al. [25] analyzed the effects of bonded mortar on RCA and the resulting concrete properties, providing prediction equations for mechanical and durability properties. Wu et al. [26] discussed RA modification, RAC maintenance, and application, concluding that chemical, physical, biological, and composite modifications can improve RA performance. Zhang et al. [27] reviewed research on seawater–sea sand recycled coarse aggregate concrete (SSRAC), focusing on its durability and time-dependent properties. Zhao et al. [28] examined the impact of various parameters on RAC’s workability, mechanical properties, and durability, focusing on machine learning algorithms used to predict RAC performance.

In the research and review papers [2,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28] mentioned above, one crucial indicator is the replacement ratio (RR) of RCA, which is the weight percentage of natural aggregate (NA) replaced by RCA. This ratio significantly impacts both the economics and mechanical properties of structural concrete. Is there an optimal RR that balances mechanical properties and economics? Currently, there is no review paper specifically addressing the optimal RR. This paper aims to review the existing literature to draw valuable conclusions and enhance the use of RCA.

2. The Benefits of Using RCA

2.1. Environmental Benefits

2.1.1. Reduction in CDW Landfill

Recycling RCA minimizes landfill CDW and protects the environment. Most CDW, like soil, wood, and SWC, do not disintegrate. CDW is often dumped untreated in landfills. The global average is 35% CDW landfilled [5]. StoneCycling [29] reports that the United States (US) landfilled 24.3% of its CDW, the United Kingdom (UK) 23.6%, and the European Union (EU) 40–60% of CDW either landfilled or backfilled. Landfill rates are less than 3% in the Netherlands and 11.46% in Germany [29]. Croatia’s CDW landfill ratio is 40%, per the World Bank [30]. Qiao et al. [31] found that 98% of China’s CDW was landfilled untreated before 2020. These numbers highlight a significant disparity in landfill proportions between developed and developing countries. Furthermore, CDW constitutes a considerable proportion of total waste in landfills, occupying a substantial amount of landfill space. Evidence [5,32,33] indicates that the construction industry generates about 65% of landfill waste in China, 44% in the UK, 44% in Australia, 40~50% in New Zealand (NZ), 40% in Brazil, 29% in the US, 27% in Canada, and 25% in Hong Kong. The demand for landfill space is immense. Accommodating the increasing CDW requires substantial landfill capacity, potentially encroaching on farmland. This shortage of landfill space could lead to heightened environmental concerns and increased waste disposal costs. Additionally, many nations grapple with ecological damage from CDW disposal [34,35]. Some materials within CDW potentially contaminate soil and groundwater, exacerbating these environmental issues [36].

2.1.2. Conservation of Natural Resources

Recycling RCA can significantly reduce the demand for natural aggregates in construction. The past 50 years have seen increased concrete use due to population growth, with aggregates comprising 70–80% of concrete volume [37]. As indicated in Figure 4, global aggregate consumption peaked at 48.3 billion metric tons in 2015, primarily in Asia and the Pacific [38], growing annually by 5% [39]. Some countries, like China, now face shortages in quality aggregates, and mining them harms the environment. Thus, RCA, a sustainable alternative, can effectively replace natural aggregates, mitigating environmental impact [40].

2.1.3. Lower Carbon Footprint

Recycling RCA can significantly reduce the carbon footprint compared to natural aggregates, as shown in Figure 5 [41]. Natural aggregate production relies on non-renewable fossil energy, contributing to global warming [42]. In Brazil, the carbon footprint for coarse aggregate is 1.50 kg CO_2-e per ton, with contributions from extraction, transport, and crushing [43]. Brazil has the ninth largest economy in the world, so this survey’s results can represent the carbon footprint of processing natural coarse aggregates in most developing countries. Global carbon emissions from natural coarse aggregate production are astonishing. Nayana and Kavitha [41] found that producing one ton of natural aggregates emits 0.046 tons of CO₂, whereas recycled aggregates emit just 0.0024 tons, reducing emissions by 23–28%. A similar experiment [44] showed that the 50% RR cut 19.78% of emissions. Also, using recycled materials in infrastructure can cut GHG emissions by 47–98% [45], greatly benefiting the environment.

In summary, using recycled aggregates lowers CO₂ emissions and mitigates environmental impacts from construction waste [46].

2.2. Economic Benefits

Using recycled aggregates in infrastructure projects offers several economic benefits:

• Lower waste disposal costs: less waste sent to landfills reduces fees, transportation costs, and long-term monitoring expenses.
• Material cost savings: recycled aggregates are typically cheaper than natural aggregates, leading to significant cost savings in construction.
• Job creation: the recycling industry expands with increased demand for recycled aggregates, creating more job opportunities.

According to ARRB [45], using recycled materials in road and rail infrastructure can save 2% to 83% in costs, and recycling creates more jobs per 10,000 tons of recovered material than waste disposal. Additionally, USD 62.5 million in savings was predicted from using recycled materials in highway construction [47].

In summary, recycled aggregates reduce costs, create jobs, and support environmental sustainability.

2.3. Social Benefits

The benefits of using recycled aggregates in infrastructure projects extend to the environment, health, economy, and social aspects [45]:

• Improving welfare by generating jobs: expanding the recycling industry generates new employment opportunities, supporting community welfare and the local economy.
• Public health: reduced greenhouse gas emissions and pollutants from the use of recycled aggregates improve public health outcomes.
• Resource conservation: using recycled aggregates preserves natural resources for future generations, promoting intergenerational fairness and stewardship.
• Civic pride and satisfaction: sustainable building practices enhance community satisfaction and pride in resource management and environmental care.

These advantages collectively contribute to the overall social sustainability of communities.

2.4. Technological Benefits

The use of RCA provides significant technological benefits:

• Materials development: enhances RCA quality and performance through improved processing techniques [48,49,50], new cementitious materials [51,52,53], and composite effects with other materials [21,22,23,27].
• RAC performance assessment: requires novel methods for evaluating mechanical and durability properties, focusing on the interfacial transition zone (ITZ) [24,25,28,54,55,56].
• Construction technology: advances in mix design [57] and admixture studies [58] optimize RAC performance in construction.
• Structural design: updates to design standards and methodologies to accommodate RAC properties, improving structural design theories [59,60].

3. The Replacement Ratio of RCA of Structural RAC in Standards and Guidelines

RCA offers significant benefits, including reduced landfill waste, resource conservation, and lower carbon emissions. Economically, the use of RCA lowers waste disposal costs and material expenses while creating job opportunities in the recycling sector. Socially, RCA enhances community welfare and public health by reducing pollutants. Technologically, RCA drives advancements in concrete research and construction technology. Given these advantages, maximizing the RCA replacement ratio (RR) is advantageous across these dimensions. However, higher RRs may challenge mechanical performance and durability. Therefore, design standards or guidelines place limits on RR.

Information obtained from previous studies [6,61,62,63,64] indicates that the countries or organizations that have formed standards or guidelines for RCA and structural RAC are generally developed and developing countries with large economic bodies. So far, no national standards or guidelines exist in the Middle East and Africa.

This paper compares international standards for using RCA in structural concrete, highlighting differences in replacement ratios and application conditions. Countries or organizations are listed in alphabetical order.

3.1. Australia

The application of RCA in Australia is guided by the “Guide to Using Recycled Concrete and Masonry Materials (HB 155:2002) [65]”. This guideline recommends a 30% coarse aggregate RCA substitution limit in concrete, a 1% mass limit on contaminants (e.g., bricks, metal, wood), and a 6% water absorption limit for coarse RCA. Due to high water demand, fine RCA is deemed unsuitable for concrete manufacturing. The guideline specifies Class 1A for well-graded RCA with up to 0.5% brick content and Class 1B for RCA with up to 30% crushed brick. If the quantity of material finer than 75 μm remains within acceptable limits as outlined in AS 2758.1:2014 [66] or comparable standards for fine or manufactured natural aggregates (not surpassing 5% or 20%, respectively), these fine particles should not present an issue. These details are outlined in

Table 1. Requirements in Australia Standards for RCA.

Requirement	AS 2758.1:2014	HB 155:2002
Water absorption	No hard limit.	Class 1A: ≤6% Class 1B: ≤8%
Saturated dry density (kg/m3)	≥2.1, <3.2 t/m3 (normal weight aggregate)	Class 1A: ≥2100 kg/m3 Class 1B: ≥1800 kg/m3
Chloride content	Combined chloride salt content <0.04% in reinforced concrete Total water-soluble chloride salt content <0.03%
Sulfate content	Sulfate content of concrete mix ≤5% by mass of Portland cement
Deleterious fines index	≤150
Soundness	6% (Exposure class C)
Light particles	≤1% (fine) ≤3% for vesicular aggregates
Material finer than 75 μm	Coarse: ≤2%; Fine: ≤20%
Material finer than 2 μm	≤1% (fine)
Flakiness index	≤35%
Replacement ratio		Class 1A: ≤30%

.

3.2. Brazil

The current Brazilian standard NBR 15.116 [67] has significantly revised the requirements [6] for using RCAs. The most critical revision is to use recycled concrete aggregates in structural concrete. NBR 15.116 [67] defines recycled aggregate as predominantly concrete waste. It is also stipulated that only Class A of CDW can be used for recyclable or reusable aggregate. The limit RR is 20% for structural Portland cement concrete. The details of the requirements are shown in

Table 2. Requirements in Brazil Standards for RCA.

Requirement	Limitation
Undesirable materials content	<1%
Clay in lumps content	<3%
Sulfate content	<0.1%
Chloride content for simple concrete	<0.2%
Chloride content for reinforced concrete	<0.1%
Chloride content for prestressed concrete	<0.01%
Water absorption	<7%
Fine (<0.075 mm) content for concrete protected from surface wear	<12%
Fine (<0.075 mm) content for concrete subjected to surface wear	<10%
Unpolished red or white ceramic content	0%
Replacement ratio	Structural Portland cement concrete: ≤20%

.

3.3. China

Chinese specifications categorize RCA into three quality classes, Class I, II, and III [68], with guidelines for structural RAC [69].

Table 3. Requirements in Chinese Standards for RCA.

Requirement	Class I	Class II	Class III
Water absorption	<3%	<5%	<8%
Apparent density (kg/m3)	>2450	>2350	>2250
Content of clay by mass	<1%	<2%	<3%
Content of clay lumps by mass	<0.5%	<0.7%	<1.0%
Porosity	<47%	<50%	<53%
Content of elongated and flaky particles	<10%
Content of organic	Standard
Sulfide and sulfate by mass	<2%
Chloride by mass	<0.06%
Other impurities	<1%
Mass loss	<5%	<10%	<15%
Crushing Index	<12%	<20%	<30%
Replacement ratio	50~100%	30~50% for multi-story and high-rise recycled concrete buildings 50~100% for low-rise recycled concrete houses

outlines RCA specifications. Class I aggregates are of the highest quality and suitable for structural and prestressed concrete. Class III aggregates are of the lowest quality and are used only in non-structural applications.

For recycled concrete buildings, Class I RCA replacement should be 50–100%. Class II and Class III RCA should follow these ratios: (1) 30–50% for multi-story and high-rise buildings; (2) 50–100% for low-rise buildings.

3.4. Germany

The German Committee for Reinforced Concrete (DAfStb, 2010) [70] allows using aggregates over 2 mm from Type 1 (concrete waste) or Type 2 (demolition waste) in structural concrete. Up to 25% recycled aggregate is allowed for concrete with strength up to B35 (35 MPa) and up to 35% for strength below B25 (25 MPa).

The amended German standard DIN 42226-100 [71] permits RCA in new concrete if it meets specific class requirements (see

Table 4. Requirements in German Standards for RCA.

Requirement	Constituent by Mass [%]
	Type 1	Type 2
Concrete and natural aggregates	≥90	≥70
Clinker, no porous clay bricks	≤10	≤30
Calcium silicate bricks	≤10	≤30
Other mineral materials (e.g., porous brick, lightweight concrete, plaster, mortar, porous slag)	≤2	≤3
Asphalt	≤10	≤30
Foreign substances (e.g., glass, plastic, metal, wood)	≤0.2	≤0.5
Oven dry density	≥2000 kg/m3	≥2000 kg/m3
Maximum water absorption (in 10 min)	10%	15%
Aggregate composition	Aggregate > 90% Bricks + Sandstone < 10	Aggregate ≥ 70% Bricks + Sandstone ≤ 30%
Replacement ratio	≤25%, with concrete strength ≤ 35 MPa ≤35%, with concrete strength ≤ 25 MPa

). RCA is classified into higher-quality Type 1 and lower-quality Type 2, with stricter use limitations for Type 2 in structural concrete.

3.5. RILEM

RILEM’s 1998 standard [72] classifies coarse recycled aggregates into three types: Class I (from concrete rubble), Class II (from masonry rubble), and Class III (a mix of RCA and natural aggregates). Class II can replace up to 20% of aggregates, with brick content limited to 10%. Class I and Class III can be used as 100% replacements.

Table 5. Requirements in RILEM Standards for RCA.

Requirement	Class I	Class II	Class III
Water absorption	20%	10%	3%
Saturated dry density (kg/m3)	1500	2000	2400
Maximum content of material with SSD * < 2200 kg/m3	-	10%	10%
Maximum content of material with SSD * < 1800 kg/m3	10%	1%	1%
Maximum content of material with SSD * < 1000 kg/m3	1%	0.5%	0.5%
Maximum content of foreign materials (metals, glass, soft materials, bitumen)	5% (by volume)	1% (by volume)	1% (by volume)
Maximum content of metals	1% (by mass)	1% (by mass)	1% (by mass)
Maximum content of organic material	1% (by mass)	0.5% (by mass)	0.5% (by mass)
Maximum sulfate content	1% (by mass)	1% (by mass)	1% (by mass)
Replacement ratio	≤100% (Coarse > 4 mm)	≤20% (Coarse > 4 mm)	≤100% (Coarse > 4 mm)

* Note: SSD—saturated surface-dry.

shows the standards for each RCA class. The guidelines specify strength and durability requirements based on RCA type.

3.6. United Kingdom (U.K.)

The BS 8500-2: 2015 + A1:2016 standard [73] specifies general requirements for using coarse recycled aggregate (as detailed in

Table 6. Requirements in the U.K. Standards for RCA.

Requirement	Limitation
Maximum fines	≤4% by mass of particles passing the 0.063 mm sieve
Maximum acid-soluble sulfate (SO3)	≤0.8% by mass acid-soluble sulfate
Content of: concrete, concrete products, mortar, concrete masonry units	≥90% by mass
Content of: concrete, concrete products, mortar, concrete masonry units, unbound aggregate, natural stone, hydraulically bound aggregate	≥90% by mass
Content of: clay masonry units (i.e., bricks and tiles), calcium silicate masonry units, aerated non-floating concrete	≤10% by mass
Content of bituminous materials	≤5% by mass
Content of other materials: Cohesive (i.e., clay and soil) Miscellaneous metals, ferrous and non-ferrous Non-floating wood, plastic, and rubber Gypsum plaster and glass	≤1% by mass
Floating material by volume	≤2 cm3/kg
Replacement ratio	≤20%

) but does not cover fine recycled aggregates. Recycled aggregate concrete can be used for unreinforced, internal, and external applications not exposed to chlorides or deicing salts, but it is unsuitable for foundations or paving. The standard allows up to 20% recycled concrete aggregate.

3.7. United States of America (U.S.A.)

The American Concrete Institute (ACI) 318-14 construction code does not explicitly include recycled concrete. However, ACI Technical Committee 555 advises using RCA in their state-of-the-art report [74], which discusses impurity content and RCA requirements (see

Table 7. Requirements in ACI 555-01R for RCA.

Impurities	Lime Plaster	Soil	Wood	Hydrated Gypsum	Asphalt	Paint Made Vinyl Acetate
Percentage of aggregate by volume	7%	5%	4%	3%	2%	0.2%

). According to ACI 221R [75], trial batches, comprehensive tests, chemical and petrographic investigations, and local performance records are crucial for RCA use decisions due to potential impurities like brick, glass, gypsum, reactive aggregates, or high chloride content.

Materials such as municipal or industrial waste, recycled materials, and marginal materials should be thoroughly tested as they may possess undesirable physical and chemical properties. Recycled materials should be specified and evaluated according to ASTM C33 [76], which allows recycled fine aggregates to have 0–3% to 0–5% finer material than the No. 200 sieve for abrasion-resistant concrete and 0–5% to 0–7% for non-abrasive concrete. ASTM C33 recommends using local, state, and federal test methodologies to evaluate air, water, and storage quality but does not provide specific guidance for RCA use in concrete applications.

AASHTO M80-13 [77] permits crushed concrete aggregates but does not specifically address RCA. AASHTO MP16 [78] categorizes RCA for non-structural applications into Class A (severe exposure), Class B (moderate exposure), and Class C (minimal exposure), with criteria detailed in

Table 8. Requirements in AASHTO MP 16 for RCA.

Requirement	Limitation
Maximum LA abrasion loss	50%
Soundness loss	12% (under sodium sulfate)
	18 (under magnesium sulfate)
Amount of material passing No. 200 sieve	1.5%
Chlorite ion content	0.6 lb/yd3 of concrete

. ACI Education Bulletin E1-07 [79] dedicates a chapter to introducing RCA, noting that recycled fine aggregate typically constitutes about 25% of the finished recycled material. New concrete formulations can include fine and coarse recycled aggregates, with coarse aggregates potentially being 100% recycled, while fine aggregates are often limited to 10–20%, with the remainder being virgin material.

In a saturated surface-dry (SSD) condition, the specific gravity of crushed recycled aggregate ranges between 2.2 and 2.5, and recycled sand ranges from 2.0 to 2.3. The absorption of recycled aggregates ranges from 2% to 6% for coarse aggregate and is higher for fine aggregate, which may reduce the workability of fresh concrete. Fine aggregate typically accounts for 10–20% of the mixture [79].

Economically developed countries like Australia, Brazil, China, Germany, the U.K., and the U.S. have established standards that balance mechanical performance and durability with the benefits of reduced environmental impact and lower material costs. However, gaps in standards remain in regions like the Middle East and Africa, highlighting the need for global collaboration to develop comprehensive RCA standards. Environmental, economic, and technical considerations influence the adoption of RCA in structural concrete. Establishing universally applicable guidelines through international cooperation and knowledge transfer can enhance sustainability and innovation, addressing regional challenges in concrete production and waste management. This holistic approach would ensure that the benefits of RCA are maximized globally, promoting a more sustainable and resilient built environment. However, the replacement ratio of RCA in structural concrete varies across countries, reflecting differing priorities in balancing RCA’s environmental and economic benefits with the technical challenges of maintaining concrete performance and durability. Finding the most effective replacement rate for RCA is still necessary and valuable.

4. A Brief Review of the Mechanical Properties of RAC

4.1. Compressive Strength

The compressive strength of concrete mixed with RCA is influenced by various factors, including the replacement ratio [80,81,82], water-to-cement ratio [80,82], cement type and admixtures [83], aggregate size and shape [84], curing moisture conditions [85], temperature [86], and loading ratio during testing [87]. Early studies indicate that the compressive strength of RAC is typically 0–30% lower than natural aggregate concrete (NAC) due to the replacement ratio and water-to-cement ratio [13,88,89,90]. Recent studies [81,82] on the effect of substitution rate on compressive strength have reached similar conclusions, as shown in Figure 6 and Figure 7. Additionally, Li et al. [91] found that the compressive strength of GRAC decreases by about 4–30.4% with increasing RA content. The replacement rate of recycled concrete aggregate has a significant and non-negligible effect on the reduction in compressive strength in recycled aggregate concrete.

4.2. Tensile Strength

Concrete typically has a tensile strength of 8–15% of its compressive strength. Most studies indicate that RAC has lower tensile strength than those with natural aggregates [91,92,93,94]. For instance, Li et al. [91] found that the splitting tensile strengths of GRAC decrease by about 2.96–34.91% with increasing RA contents. However, Safiuddin et al. [95] found that RAC and NAC had comparable tensile strengths. Etxeberria [89] demonstrated that controlled NAs had lower splitting tensile strength than RCAs. Silva et al. [96] also mentioned this phenomenon.

Two factors can explain these results:

• The increased water-to-cement ratio due to the RCA particles’ absorption of the attached mortar.
• Forming a new interfacial transition zone creates a stronger link between the aggregate and cement paste.

4.3. Flexural Strength

For concrete pavements or slabs on grade, flexural strength or rupture modulus is a crucial hardened property. According to Abou-Zeid et al. [97], RACs have the same or slightly lower flexural strength than NACs. Mohammed et al. [39] also observed a slight decrease. This is likely because the angularity and rough surface of RCA particles form a stronger bond with the cement binder, potentially due to a chemical interaction between the RCA and the cement paste. However, Yaba et al. [98] observed a 10% decrease in flexural strength due to the weaknesses of the RCA.

4.4. Elastic Modulus

The elastic modulus of concrete, which depicts its response to an axial load, is influenced by the equivalent stiffness of the composite material in RACs, consisting of five different components. The presence of adherent mortar in RACs makes them weaker and more porous than NACs, resulting in a less promising elastic modulus. The density of the RCA is crucial in determining the elastic modulus of RAC [99,100]. As the aggregate density and maximum aggregate size decrease, the elastic modulus decreases due to the higher adherent mortar content and increased cement mortar paste area. Consequently, the elastic modulus of RAC is often 5% to 20% lower compared to concrete systems made with Nas [14,88,94,101].

5. A Brief Review of the Durability Properties of RAC

5.1. Shrinkage and Creep

Significant portions of RCA particles consist of porous adhered mortar, leading to high absorption capacities and relative pore water movement during hydration. Hansen reported that drying shrinkage in RAC can be up to 50% higher than in NAC. This increased shrinkage is due to the higher paste content (adhered mortar and cement paste) in RAC, which makes them less dimensionally stable [92,99,102,103]. In addition, the results given by Lv et al. [104] show that the shrinkage strain with replacement ratios of 50 and 100% at 180 days is 26% and 48% higher than that of ordinary concrete, respectively. Higher replacement levels mean higher shrinkage.

Adams et al. [105] experimentally tested the cracking susceptibility of RACs during drying shrinkage, finding that using RCA can reduce the cracking risk, especially when residual mortar content is less than 20%. Computational studies by Jayasuriya et al. [106] showed that increased adhered mortar content (up to 100%) could alleviate cracking potential due to better material stiffness compatibility.

Creep in RACs tends to increase with higher RCA replacement ratios. Gomez-Soberon [107] found that a 15% increase in RCA replacement can lead to 24% higher creep than NAC, attributed to the two-phase nature of the aggregate. The NA phase is strong and resists deformation, while the adhered mortar phase is less shrinkage-resistant [101,108]. Lv et al. [104] found the creep coefficient with replacement ratios of 50% and 100% increased by 19.6% and 39.6%. It shows that the creep increases by a magnitude comparable to the replacement ratio.

5.2. Abrasion Resistance

Dhir et al. [108] found that RACs with a 50% replacement ratio showed similar abrasion resistance to NAs. Increasing the RCA replacement level to 100% resulted in a 34% higher abrasion depth compared to NAC. Limbachiya et al. [109] conducted experiments on RAC abrasion resistance but found inconsistent results between replacement levels and abrasion depth. For instance, a 30% RCA replacement led to an abrasion depth of 0.81 mm, which decreased to 0.69 mm at 50% replacement and surprisingly increased to 0.79 mm at 100% replacement. Kumar [110] found that the complete replacement of NCA by RCA decreases the abrasion resistance of concrete from 18% to 22%.

These inconsistencies are primarily due to the variability in material properties of RCAs, where weaker adhered mortar phases can be more easily worn out during the abrasive testing cycles.

5.3. Permeability

Permeability in concrete refers to its ability to allow the passage of substances like carbon dioxide, chloride, sulfate, and moisture, which can lead to deterioration. Due to its porous nature, RCA typically results in higher porosity than concrete made with NA.

Studies [109,111] showed that carbonation effects on RACs vary with RCA replacement levels, and with 30%, 50%, and 100%, RCA replacements demonstrated negligible, better, and best performance regarding carbonation resistance. However, Sagoe-Crentsil et al. [112] found contradicting results, showing that RAC with 100% RCA replacement performed worse in carbonation resistance. Supporting this, Dhir et al. [108] reported that RCA replacements exceeding 50% exhibited poorer carbonation performance, while replacements below 50% showed better resistance.

Shayan and Xu found that RACs are very effective at resisting chloride penetration when silica fume is used as supplementary cementing material, creating denser concrete and preventing corrosion over time [113]. Otsuki et al. [114] reported that chloride penetration in RACs is slightly higher than in NACs but not significantly so.

Sulfate attacks can cause cracking, spalling, and loss of integrity in RACs due to the extensive network of interfacial transition zones between old and new concrete. This makes RACs more susceptible to internal pressure from ettringite formation. Limbachiya et al. [109] observed that higher RCA replacement levels led to severe exposure to sulfate attack in tested mortar bars. Dhir et al. [108] found that mortar bar expansions were similar to those in NACs up to a 30% RCA replacement level, but at 100% replacement, mortar bar expansion was about 68% higher than in NACs.

Tam et al. [115] reviewed previous studies and concluded that as the fine recycled aggregate (FRA) content in RAC increases from 20% to 100%, there is a corresponding increase in water penetration. However, 30% replacement was optimal, as it did not significantly compromise the durability properties of RAC.

5.4. Freeze and Thaw Resistance

Cracking due to freezing and thawing is more likely when saturated aggregates are used in concrete [99]. However, the freeze–thaw resistance of RACs is generally similar to NACs [108,109,116]. Air entrainment is crucial for enhancing freeze–thaw resistance in concrete with RCA. Even small amounts of non-air-entrained RCA can significantly reduce freeze–thaw resistance [116]. Therefore, it is recommended that appropriate air entrainment be selected based on the severity of freeze–thaw exposure and that RCA from previously air-entrained concrete be used. This approach helps balance the porosity of the RAC without compromising compressive strength. Zhang et al. [117] investigated the combined effects of sustained compressive loading and the salt frost resistance for recycled aggregate concrete (RAC) incorporating air-entraining agents (AEA) and nano-silica (NS). The results showed that the RAC mixed with a certain amount of AEA and NS dosages exhibited stronger salt frost resistance than the control group [117].

6. A Brief Review of the Cost-Benefit of RAC

Tam [118] studied the costs and benefits of dumping construction waste in landfills and producing new natural materials for concrete production, compared to recycling construction waste as aggregate for new concrete in Australia. The study found that the concrete recycling method results in significant cost savings, as shown in

Table 9. Comparison of the current method and the concrete recycling method [118].

	The Current Method (USD 1000/Year)	The Concrete Recycling Method (USD 1000/Year)
Total cost	44,097.16	6738.06
Total benefit	20.30	37,654.61
Net benefit	−44,076.84	+30,916.55

. The benefits of recycling construction waste as aggregate balance out the costs associated with the current practice.

A South African case study [63] found a negative net benefit for producing natural or recycled concrete aggregate, as shown in

Table 10. Comparison of costs and benefits of producing coarse NAs and coarse RCAs [63].

	NA (USD per ton)	RCA (USD per ton)
Total cost	−76.30	−45.77
Total benefit	0.73	22.36
Net benefit	−75.57	−23.41

. They observed that producing recycled concrete aggregates (RCA) is less expensive than producing natural aggregates (NA). For example, the long-term cost of producing one ton of coarse RCA was about 40% less than for coarse NA. Additionally, the environmental benefit of producing one ton of RCA was approximately 97% higher than for NA.

The study also compared the cost-benefit differences between Australia and South Africa for producing recycled concrete aggregates. This difference is attributed to varying economic policies and recycling costs in the two countries. For instance, the dumping charge for sending waste concrete to a landfill in Australia was about USD 41.00 [118], while in South Africa, it was USD 15.52 [63], representing 164.18% higher charges in Australia. By comparing the cost-benefit of RACs in Australia and South Africa, we can observe the differences in production costs between developed and developing countries.

Makul [119] investigated the cost-benefit of producing ready-mixed high-performance concrete with RCA. According to the analysis results in

Table 11. RCA incremental price range [119].

RCA Replacement (%)	FEL	OB
20	0 to 7.5	5.5 to 16.0
40	7.5 to 9.5	7.5 to 16.5
60	8.5 to 10.5	9.5 to 17.5
80	4.5 to 12.5	9.5 to 18.0
100	5.5 to 12.0	9.0 to 18.0

, the costs of RCA tend to rise when produced in overhead bin (OB) plants but decrease in front-end loader (FEL) plants. The cost-benefit gradually increases as the replacement ratio rises. In any manufacturing plant, it is unlikely that the pure cost of RAC will be lower than that of NA concrete.

In summary, while the production cost of recycled aggregate increases with the replacement ratio, the comprehensive benefits of replacing natural aggregate with recycled concrete aggregate are reasonable and align with environmentally sustainable development in the long run.

7. The Optimal Replacement Ratio of RAC in Structural RAC

Although many scholars [2,6,10,11,39,40,88,89,90,91,92,93,94,95,96,97,98,99,100,101], including the author [120,121], have studied the effect of RCA replacement ratios on the mechanical properties of RAC materials and components, determining the optimal RCA substitution ratio or range requires extensive data analysis. Fortunately, the ACI CRC 18.517 final report [62] gave an RCA concrete database with approximately 2250 mechanical property data points, including compressive strength (932 samples), elastic modulus (564 samples), flexural strength (252 samples), and splitting tensile strength (498 samples). The following results and analysis are derived from this report [62].

7.1. Variation in Compressive Strength

The database included 28-day compressive strength data for all test specimens. A specific class boundary approach was used to evaluate the effect of RRs on the 28-day compressive strength of RACs: 0%, 1–10%, 11–20%, 21–30%, 31–40%, 41–50%, 51–60%, 61–70%, 71–80%, 81–90%, and 100%. The replacement levels were determined by mass replacement of aggregates using a single RCA source. Tight class borders were chosen to capture compressive strength trends over RCA replacement levels, as rapid fluctuations may be missed with wider class intervals. The dataset provided more abundant compressive strength data compared to other mechanical parameters.

Figure 8 shows that up to 20% RR increased the RAC’s mean compressive strength by 14.6% (from 44.7 MPa to 51.2 MPa). The 11–20% RR exhibited the best compressive strength performance. The mean compressive strength showed three local peaks at RRs of 11–20%, 41–50%, and 100%, indicating good stiffness compatibility due to aggregate interaction between attached mortar and cement paste.

The morphology, property, and spatial distribution of aggregate geometries within the concrete affect aggregate interaction. As RR increases, so does the content of adhering mortar, enhancing stiffness compatibility and aggregate texture roughness. This explains the improved strength properties at 11–20% RRs. Conversely, higher RRs increase the number of extensive Interfacial Transition Zones (ITZ), the weakest links in the concrete, due to random aggregate geometry orientations. This comprehensive distribution of ITZ phases and their geometric orientations can significantly limit compressive strength. Lower performance trends were observed at RRs of 1–10%, 21–30%, 31–40%, 51–60%, 61–70%, and 71–80%, while some replacements showed local peaks.

7.2. Variation in Splitting Tensile Strength

Figure 9 shows the mean splitting tensile strength variation in RACs at different RRs. As RCA substitution increased from 1% to 80%, a constant decrease in tensile strength was observed. This strength loss is primarily due to the weak bond between the main material phases (new and old ITZ) under indirect tensile loads. At 100% RR, strength improved compared to the 61–80% range but was still lower than the remaining class bins. The higher percentage of adhering mortar at 100% RR contributes to increased concrete system homogeneity, which delays the formation of indirect tensile stresses [106,122]. Therefore, at 100% RR, the material deformed more before failing under indirect tensile pressures, likely due to enhanced cracking resistance.

7.3. Variation in Flexural Strength

Most investigations used beam specimens to measure the flexural strength of RACs. As illustrated in Figure 10, when RR was at 1–20%, the mean flexural strength peaked. Although this replacement level improved performance over lower levels, the flexural strength demonstrated local peak responses. The mean flexural strength generally declined as the RR increased beyond 20%.

7.4. Variation in Elastic Modulus

Figure 11 shows the average elastic modulus and the associated RRs. The elastic modulus peaked between 1% and 20% RR and then clearly degraded as the replacement level increased. This decline in elastic modulus at higher RRs is attributed to the increased amount of attached mortar, which reduces the stiffness of the aggregate. As the adhering mortar content rises, the overall stiffness (elastic modulus) of the RACs significantly decreases.

7.5. Discussion on the Optimal RR of RCA

The purpose of studying recycled aggregate concrete (RAC) is to facilitate its application. For practical use, having a design guide—typically provided through standards, specifications, and guidelines—is essential. Section 3 indicates that while some standards address material requirements and replacement ratios for RAC, few directly guide the design of RAC structures. However, despite numerous studies on the structural behavior of RAC in laboratory and real-life applications, progress in adopting it as standard practice has been limited [123]. China has a relatively comprehensive design standard [69], and Europe is revising its older standards to include RAC design provisions [59,60].

Structural design generally includes load-bearing capacity design and serviceability design. Serviceability design typically involves material strength grade, construction details (e.g., protective layer thickness), crack control, and mix ratio requirements. Conversely, load-bearing capacity design requires direct mechanical indicators such as compressive strength, tensile strength, and elastic modulus. Therefore, as a structural material, recycled aggregate concrete (RAC) focuses on its mechanical properties, with the replacement ratio of recycled concrete aggregate (RCA) being crucial to maintaining optimal performance. Zaki et al. [124] investigated the optimal replacement rate for fine RCA concerning compressive and splitting tensile strength, concluding that a replacement rate between 10% and 30% is optimal for maintaining performance.

Supporting this, the ACI CRC 18.517 final report [62], which compiled 2250 test data points, highlighted that at a 1–20% RCA replacement ratio, RCA concrete exhibited higher average hardened properties, such as compressive strength, elastic modulus, and flexural strength. However, splitting tensile strength decreased consistently across the replacement levels. Therefore, a replacement rate of 10–20% is considered mechanically optimal.

Despite this, the environmental benefits and sustainable development goals of recycled concrete cannot be overlooked. Thus, the mechanical properties and the cost-benefit impact must be considered together. Basit et al. [125] studied the strength and cost of concrete using three different recycled coarse aggregates. For Recycled Concrete Stone Aggregate (RCSA), Figure 12 shows the combined analysis of strength and cost, identifying the optimal replacement rate between 20% and 35% at the intersection points of the cost and strength curves.

In conclusion, while the long-term trend favors using recycled concrete for sustainability, the short-term focus must balance mechanical properties and economic performance. Therefore, the 20% replacement ratio could be currently optimal for producing and using recycled concrete aggregates.

8. Challenges and Thinking

8.1. Challenges to Using Recycled Aggregate Concrete (RAC)

8.1.1. Technical Challenges

First, recycled concrete aggregates (RCA) often vary in quality because of differences in the original concrete, contaminants, and old mortar. This can make recycled aggregate concrete (RAC) performance unpredictable. Second, RAC generally has lower compressive strength, tensile strength, and elastic modulus than natural aggregate concrete (NAC). This is due to the porous nature of RCA and weak interfacial transition zones (ITZ). Third, RAC tends to be less durable because it has higher permeability, making it more vulnerable to chemical attacks, freeze–thaw cycles, and abrasion. The old mortar increases porosity, worsening these issues. At last, processing RCA to remove impurities and improve quality requires advanced technologies, which may not be available or cost-effective everywhere.

8.1.2. Economic Challenges

The cost of processing and treating RCA to achieve a quality comparable to natural aggregates can be high. This includes expenses for sorting, crushing, screening, and treating the RCA. Additionally, in many regions, such as Africa, the cost of natural aggregates is relatively low, making it difficult for RCA to compete economically. The lack of financial incentives or subsidies for using recycled materials further exacerbates this issue.

8.1.3. Regulatory and Standardization Challenges

National and regional standards for using RCA in concrete are often outdated or overly conservative. This lack of standardized guidelines and specifications can hinder the acceptance and use of RAC in construction projects. Regulatory frameworks may not support or incentivize the use of recycled materials. Stringent regulations and approval processes can deter the use of RAC, especially in structural applications.

8.1.4. Perceptual and Behavioral Challenges

There is often a negative perception among stakeholders, including engineers, contractors, and clients, regarding the quality and reliability of RAC. This skepticism can stem from a lack of awareness or understanding of its benefits and performance. Moreover, the construction industry is traditionally conservative and risk-averse, leading to resistance against adopting new materials and methods, including RAC. Therefore, there is a need for greater awareness and education about the benefits, performance, and best practices for using RAC. Without proper knowledge, stakeholders may hesitate to adopt RAC.

8.2. Thinking

Using recycled concrete aggregates (RCA) in concrete production greatly enhances sustainable construction practices. Despite challenges in maintaining the mechanical properties of recycled aggregate concrete (RAC), the benefits of RCA usage are significant. Here are key considerations and prospects.

8.2.1. Balancing Performance and Sustainability

The optimal replacement ratio of RCA is typically identified as being between 20% and 30% to uphold optimal mechanical properties. It remains crucial to explore innovative methods to enhance RAC quality continuously. This involves refining the processing techniques of RCA to minimize impurities and strengthen the bond between RCA and the cement matrix.

8.2.2. Technological Advancements

Investing in research and development for advanced technologies in processing and treating RCA can result in higher-quality aggregates suitable for higher replacement levels without compromising concrete performance. Technologies such as advanced sorting, crushing, and screening methods, along with treatments like carbonation and the addition of pozzolanic materials, can significantly enhance the quality of RCA.

8.2.3. Economic and Environmental Impact

The economic benefits of using RCA, such as cost savings from reduced landfill use and natural aggregate extraction, must be considered alongside environmental benefits. Quantifying these impacts can encourage broader acceptance and implementation of RAC in the construction industry. Governments and industries should collaborate to provide incentives and subsidies to promote the use of RCA.

8.2.4. Standardization and Guidelines

Updating national and regional standards to align with the latest research findings is crucial. Current guidelines often suggest higher replacement ratios that may not be optimal. Revising these standards to reflect the latest evidence-based recommendations can better balance sustainability and performance.

8.2.5. Comprehensive Testing and Data Collection

Continuously collecting and analyzing data on RAC performance is essential. Establishing extensive databases and conducting thorough testing can offer deeper insights into the long-term behavior of RAC under diverse conditions. These data are instrumental in shaping improved practices and guidelines.

8.2.6. Education and Awareness

Raising awareness and educating stakeholders, including engineers, architects, and policymakers, about the benefits and best practices of using RCA is essential. Implementing training programs, workshops, and certification courses can effectively disseminate knowledge and expertise on this topic.

8.2.7. Future Research Directions

Future research should prioritize investigating the long-term durability of RAC, specifically its resistance to environmental factors like freeze–thaw cycles, sulfate attack, and carbonation. Additionally, exploring the integration of RCA into high-performance concrete and specialized applications can introduce new opportunities for its utilization.

9. Conclusions

Utilizing recycled aggregates from construction and demolition waste (CDW) in the concrete industry presents a transformative approach to sustainable construction. This practice conserves diminishing natural aggregate resources and significantly reduces the burden on landfills. Economically, using recycled aggregates enhances efficiency by lowering material costs and waste disposal fees and fostering job creation within the recycling sector. Socially, it contributes to public welfare by improving environmental health, reducing pollution, and promoting community pride in sustainable practices. Furthermore, reducing carbon emissions aligns with global efforts to mitigate climate change.

Extensive research has underscored the delicate balance required to determine the optimal replacement ratio for recycled concrete aggregate (RCA) in producing recycled aggregate concrete (RAC). Studies suggest that a 20% replacement ratio is often optimal, striking a balance between maintaining the mechanical properties of the concrete and realizing economic benefits. This ratio is widely supported by international standards, with notable exceptions in regions like China. Generally, a replacement range of 20% to 30% is deemed acceptable, backed by standards in many countries, except in specific contexts such as China, Brazil, and the United Kingdom, where local standards and conditions may necessitate different ratios.

Although the optimal replacement ratio for RCA in concrete has been identified, current replacement levels are insufficient for significant progress in sustainable development. Balancing performance with sustainability through continuous innovation and technological advancements is crucial to harnessing the benefits of RCA. Enhancing the replacement ratio involves overcoming technical, economic, and perceptual barriers, which require collaborative efforts across various sectors. This collaborative approach promotes the widespread adoption of recycled aggregates and drives the construction industry toward a more sustainable and resilient future. By integrating advanced processing techniques, developing new standards, and fostering greater industry and governmental support, we can maximize recycled aggregates’ environmental and economic benefits, ultimately contributing to global sustainability goals.