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Article

Recycled Concrete in Foundations: Mechanical and Environmental Insights

by
Tereza Pavlů
*,
Jan Pešta
,
Kristina Fořtová
and
Jakub Řepka
University Centre for Energy Efficient Buildings, Czech Technical University in Prague, Třinecká 1024, 273 43 Buštěhrad, Czech Republic
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(8), 1237; https://doi.org/10.3390/buildings15081237
Submission received: 14 February 2025 / Revised: 16 March 2025 / Accepted: 26 March 2025 / Published: 9 April 2025
(This article belongs to the Special Issue Low-Carbon Concrete Structures Incorporating Sustainable Pozzolan and Recycled Materials)
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Abstract

:

Highlights

What are the main findings?
  • A total of 26 recycled aggregate concrete (RAC) mixtures with different mix proportions are comprehensively evaluated.
  • The two-stage recycling process improves aggregate quality, enhancing concrete properties.
What is the implication of the main finding?
  • The mechanical performance assessment confirms RAC viability for structural foundation applications.
  • The Life Cycle Assessment (LCA) shows a nearly 50% reduction in environmental impact with full natural aggregate replacement.

Abstract

Recycled aggregate concrete (RAC) has significant potential for sustainable construction; however, concerns regarding its mechanical performance and environmental impact persist. This study evaluates 26 RAC mixtures with varying cement content, water–cement ratios, and recycled aggregate replacement levels (30%, 50%, and 100%) using two distinct recycling processes. The results confirm that while RAC exhibits a decline in mechanical properties compared to natural aggregate concrete (NAC), lower-strength concrete classes maintain acceptable performance. The Life Cycle Assessment (LCA) indicates that fully replacing the natural aggregate with a high-quality recycled aggregate reduces environmental impact by nearly 50%, primarily due to lower resource depletion and transportation emissions. The study demonstrates that RAC can be optimized for structural applications, particularly in foundation structures, without compromising functional integrity. Unlike previous studies, this research provides a systematic evaluation of how a two-stage recycling process enhances aggregate quality, leading to improved RAC performance, and introduces practical strategies for optimizing RAC durability and mix design for real-world foundation applications. Future research should explore alternative mix designs and durability improvements to enhance RAC’s viability for broader construction applications.

1. Introduction

Concrete is the most widely used construction material globally due to its versatility and durability. However, its production relies heavily on natural aggregates (NAs), which are finite resources. The increasing demand for NAs, growing by approximately 5% annually, has raised concerns about resource depletion. Meanwhile, vast amounts of waste concrete are generated, accounting for about 63% of global construction and demolition waste (CDW), yet only a small fraction—less than 5%—is effectively reused in new concrete production [1]. Despite extensive research on the mechanical properties of recycled aggregate concrete (RAC), its application in structural elements remains limited, primarily due to concerns regarding reduced strength and durability.
Most waste concrete undergoes downcycling, where recycled aggregates (RAs) are mainly used for backfilling or landscaping rather than as a direct replacement for NAs in structural concrete. One of the primary challenges in using RAs for concrete production is their contamination with dust, clay, and other impurities, which negatively affect the mechanical performance [2]. Additionally, the presence of residual cement mortar leads to increased porosity and water absorption, further weakening the interfacial transition zone (ITZ) in RAC. To address these limitations, improvements in selective demolition techniques and optimized recycling processes, including multi-stage crushing, are necessary to enhance the quality of RAs and expand their potential structural applications.
The increasing demand for sustainable construction materials necessitates a shift toward circular economy practices, where RAC plays a key role in reducing resource depletion, landfill waste, and CO2 emissions associated with aggregate extraction and cement production. Despite concerns regarding its mechanical performance, advances in aggregate processing and mix optimization have made RAC a competitive option for specific structural applications, particularly in foundation structures. However, a clear engineering framework is required to facilitate the broader adoption of RAC in construction projects, ensuring that structural integrity and environmental benefits are balanced with economic feasibility.
This study aims to evaluate the structural feasibility of RAC in building foundation applications, where high material volumes are required, but lower concrete strength is often acceptable. By conducting a comprehensive experimental and environmental assessment, this research examines the effects of various cement contents, water–cement ratios, and aggregate replacement rates (30%, 50%, and 100%) on the mechanical performance of RAC. Two types of RAs, produced through different recycling processes, are analyzed to determine their impact on concrete properties. The study also applies the Life Cycle Assessment (LCA) method to compare the environmental footprint of RAC and conventional concrete, particularly in terms of CO2 emissions, resource depletion, and landfilling reduction.
The findings of this study contribute to the advancement of innovative approaches in sustainable concrete production by demonstrating how optimized recycling technologies and novel mix design strategies can enhance RAC performance. By leveraging multi-stage recycling processes, tailored aggregate treatments, and advanced material characterization, this research paves the way for a new generation of high-performance, structurally viable recycled concrete. Furthermore, by providing practical engineering recommendations for RAC applications and outlining strategies for further optimizing its properties, this study reinforces its potential to transform conventional construction practices. Through a science-driven, circular economy approach, these insights challenge conventional perceptions of recycled materials, positioning RAC as a sustainable, cost-effective, and structurally reliable alternative for modern construction.

2. Recycling Challenges and Solutions

2.1. Recycling of Concrete Waste and Properties of Recycled Aggregate

Concrete is composed of cement, water, and aggregates, with aggregates making up 55–80% of its volume. While recycling construction and demolition waste (CDW) offers environmental benefits, the main challenge is maintaining the mechanical properties of recycled aggregate concrete (RAC) [2,3]. The natural aggregate (NA) consumption grows by 5% annually, while concrete waste accounts for 63% of global CDW, yet its recycling covers less than 5% of the total aggregate demand [4].
Recycling processes typically include sorting, crushing, and sieving, but the presence of residual cement mortar increases porosity and weakens the interfacial transition zone (ITZ), reducing concrete durability [5,6]. Multi-stage crushing can improve RCA quality, though it increases energy demand [7,8]. Alternative methods, such as chemical treatments, can enhance RCA properties, but they have environmental drawbacks [9]. The Particle Packing Method (PPM) and equivalent mortar volume (EMV) approach optimize mix designs to mitigate these challenges [10,11].
Recent studies have explored innovative methods to enhance RAC properties. For instance, accelerated carbonation has been shown to improve mechanical performance by increasing compressive strength and durability [12]. Additionally, physics-informed modeling has been used to examine the fracture properties of RAC, identifying key parameters affecting the split tensile strength [13]. A systematic review emphasized the importance of automated mix design and standardization to improve the consistency and applicability of RAC [14].
RCA retains over 90% of the original aggregate, but its shape, density, and contamination influence workability and performance [15,16]. Fine RCA (fRCA) is particularly challenging due to a high water absorption and lower strength, making its use in structural concrete problematic [17].

2.2. Standards and Structural Use of RAC

International standards regulate RCA usage based on exposure conditions and mechanical requirements. European standards permit up to 50% RCA for non-exposed applications and 30% for low-risk environments, aligning with experimental findings. In Italy, RCA use is restricted to C30/37, while Brazil allows up to 20% RCA in any strength class [18].
Despite these regulations, RAC is mainly used in non-structural applications, such as road bases, backfilling, and masonry mortars. The most studied structural application concerns beams, where flexural performance aligns with Eurocode predictions, but the shear strength is lower [19]. However, foundation structures offer a promising application for RAC, as they require high material volumes but do not demand high-strength concrete.

2.3. Life Cycle Assessment (LCA) of RAC

LCA is crucial for assessing RAC’s environmental impact, yet inconsistencies in functional units (FUs), system boundaries, and recycling methods create challenges for direct comparisons [20]. Cement production is the largest contributor to CO2 emissions (74–81%), followed by transportation [21].
Earlier LCA studies compared 1 m³ of concrete, but newer approaches assess structural performance and durability [22]. In aggressive environments, RAC may require thicker covers or additional cement, impacting sustainability [23]. This study evaluates foundation structures, ensuring a function-based LCA approach for more accurate environmental assessments.
A key question in LCA is system boundaries. A cradle-to-gate approach excludes use-phase and end-of-life impacts, whereas cradle-to-grave or cradle-to-cradle approaches consider recycling benefits [24]. RAC absorbs 13–48% of CO2 over its life cycle, potentially offsetting emissions [25]. This study adopts a comprehensive life cycle approach, incorporating material production, transport, construction, and disposal.

3. Materials and Methods

3.1. Materials

3.1.1. Recycled Concrete Aggregate

Generally, the quality of the RA is influenced by the demolition process, the recycling technique, the separation process, the crushing method, the number of crushing stages, and the properties of the parent concrete. These aspects influence the number of unwanted impurities in RCA, such as soil, dust, and clay, the amount of cement mortar contained in RCA, and finally, the number of cracks that develop from crushing. Similarly to the previous studies, a higher water absorption and lower density of RCA were identified. As it is generally known, the higher water absorption affects the effective water–cement ratio and workability of the concrete mix. For this reason, RCA properties must be known before its use in concrete. It was presented that the water absorption of coarse RCA ranges between 0.5% and 14.75%, and the dry density of coarse RCA ranges from 1900 to 2700 kg/m3 [26]. The dry densities of fRCA have been between 1630 and 2560 kg/m3, and water absorption varies between 2.38% and 19.3% [15,16,17,27,28,29,30,31,32,33]. Furthermore, the use of fRCA is mainly connected to doubts due to the finest content, which influences the effective water–cement ratio and properties of fresh and hardened concrete. Furthermore, if the basic recycling process is used, it could contain contaminants such as soil, dust, and clay. It was found [2] that RCA containing clay obtained the worst properties of concrete. The clay covers the particles of the RA and makes a barrier between the RCA and the new cement paste. Moreover, the mixing water is absorbed by clay, so it is necessary to increase the amount of mixing water to achieve the same workability. This may be eliminated by the multi-stage recycling process.
In this study, the RA originated from waste concrete and was prepared from demolition waste in a recycling center in the Czech Republic. Two different recycling processes were used for preparing the RA: recycled aggregate type 1 (RA1) was produced using a one-stage crushing process, while recycled aggregate type 2 (RA2) underwent a multi-stage crushing process. In both cases, the reinforced concrete was first pre-crushed using hydraulic shears, and the steel reinforcement was removed by a magnetic separator. All crushing stages were performed using diesel-powered crushers. In the first stage, a jaw crusher reduced the concrete fragments into fractions of 0/4, 4/8, 8/16, and 16/128 mm. In the second stage, the 16/128 mm fraction was further crushed using a jaw or impact crusher and subsequently sieved into fractions of 0/4, 4/8, and 8/16 mm (see Figure 1 and Figure 2). RA1, produced through a single-stage crushing process, contains higher levels of contaminants such as soil and dust. In contrast, RA2 undergoes an additional crushing stage, effectively removing contaminants in the first stage and further refining aggregate properties. RA1 is obtained through a single-stage crushing process, where particles larger than 16/128 mm are rejected. RA2, however, undergoes an additional crushing stage, where these larger particles are further broken down into finer fractions (0/4, 4/8, and 8/16 mm). Due to this additional processing, RA2 is expected to have higher hardness compared to RA1, as finer crushing reduces weaker adhered mortar and enhances the strength of individual particles. According to the Czech European standard, the RCA meets recycled aggregate class A requirements [34]. The properties of manufactured RA are summarized in Table 1.

3.1.2. Concrete Mixtures

For comparison, the RAC mixtures and NAC mixtures were designed to optimize the use of the RA for the same structural use. Various replacement ratios used one type of NA and different types of RAs (fRA1, RA1, fRA2, RA2). The mixtures are considered with increasing grade, where the lowest is labelled I, contains 240 kg/m3 of CEM I 42.5 R, and an effective water–cement ratio of 1.0; mixtures labelled II contain 260 kg/m3 of CEM I 42.5 R and an effective water–cement ratio of 0.65. Mixtures labelled III contain 300 kg/m3 of CEM I 42.5 R and an effective water–cement ratio of 0.55, and mixtures labelled IV contain 320 kg/m3 of CEM I 42.5 R and an effective water–cement ratio of 0.50; this is considered the highest-grade concrete. The replacement ratio of the coarse fraction was 30% (C30), 50% (C50), and 100% (C100); the complete or partial replacement of natural sand by fRA is labelled by F. The mix types were designed considering the strength class requirements according to Eurocode 2 (EN 1992-1-1) and CSN EN 206 [34], exposure classes X0 and XC1, and the Bolomey particle size distribution curve. The replacement ratios (30%, 50%, and 100%) were selected based on previous research optimizing RAC compositions for structural applications. The two-stage mixing approach from Tam [35] was applied to enhance the performance of the RAC by improving the interfacial transition zone (ITZ). In the first stage, the RA was inserted into a part of the water (the water estimated to be absorbed) and mixed for 10 min; consequently, the remaining constituents were placed. The additional water was calculated according to the water absorption capacity of fRA and RA and current levels of aggregate saturation before mixing. The composition of concrete mixtures per cubic meter is shown in Table 2.

3.2. Methods

Concrete, as a material with a wide range of properties, can be designed to perform different functions. Thus, the methodology to characterize the properties and approach to design a concrete element with the demanded functions are described in Section 3.2.1 and Section 3.2.2, respectively. Furthermore, different processes during the life cycle of concrete elements can potentially influence the environment. LCA was used in several studies to describe this potential environmental impact [36,37,38]. Although LCA is a standardized method, individual case studies can differ in some parameters. Parameters considered in the environmental assessment in this study are described in Section 3.2.3. The overall methodology adopted in this study is summarized in Figure 3, which outlines the key steps from material selection to data interpretation.

3.2.1. Concrete Properties Evaluation Methodology

The laboratory verification of fundamental physical properties of concretes with various mix proportions of NAs and RAs was performed. The properties were evaluated on three samples for each mixture, and the average values with standard deviations are presented. The mechanical properties were tested on Controls MCC8 50-C8422/M according to the following standards: EN 12390-3 [39] compressive strength (2003); EN 12390-5 [40] flexural strength (2009); EN 12390-13 [41] static modulus of elasticity (2014); and (2005) dynamic modulus of elasticity EN 12504-4 [42]. To ensure reproducibility and accuracy, all mechanical tests were conducted on at least three specimens per mixture, and the results are presented as average values with standard deviations. The selection of mechanical tests was based on standard procedures for evaluating structural concrete performance, following EN 12390 guidelines [39,40,41]. Additionally, water absorption and capillary absorption tests were included to assess the potential impact of recycled aggregate porosity on long-term durability.
The water absorption capacity by immersion, which describes the material’s behavior, especially in terms of open pore structure, was obtained on a cubic specimen 100 × 100 × 100 mm3 in size. The samples were immersed in a water chamber and, after stabilizing the weight, dried in an oven at 105 ± 2 °C as long as their weight stabilized all over. The capillary water absorption of concrete specimens of size 100 × 100 × 100 mm3 with time was determined by conditioning the samples at 105 °C in the oven until their weight stabilized. The stabilized samples were placed on a support device by exposing one of the surfaces to water. The change in mass of samples was noted at 0, 1, 10, 30, and 60 min, and 2, 4, 24, 36, and 72 h. The measurement is performed for 72 h or until the weight stabilizes. The slope of the line obtained by plotting absorption against the square root of time gives the sorptivity of the concrete as per ASTM C1585-20 [43].

3.2.2. Foundation Structural Element

Based on the characteristics of the mixture, the element of plain concrete for the foundation structure of the building was considered to have the same effective load area [m2] to carry the same load under the same geological conditions. The element’s height varied according to the concrete properties for the same utility properties. It follows that the lower strength was compensated for by the height, leading to the higher concrete volume. The volume of each concrete structural element (presented in Table 3) was used to define a functional unit for environmental assessment.
The following equation designs the foundation structure:
σ = N E d + G 0 A e f f ≤ R d
where σ is the stress in the foundation joint, NEd is the loading from the upper part of the building, G0 is the estimated load from the foundation structure, Aeff is the effective loading area of the foundation structure, and Rd is the load capacity of the soil given by geology conditions.
The height of the foundation structure is designed by the following equation:
h ≥ a 0.85 3 σ f c t d
where h is the height of the foundation structure, a is ½ of the width of the foundation structure, 0.85 is the coefficient of shear, σ is the stress in the foundation joint, and fctd is the flexural strength of concrete according to the target strength class.

3.2.3. Environmental Assessment

The LCA case studies comparing RAC with NAC for structural use have been conducted [11,20,25,36,37,44,45,46]. In this study, LCA was performed according to ISO 14040:2006 [47], which describes four main stages of this assessment: the goal and scope definition, life cycle inventory (LCI), life cycle impact assessment (LCIA), and life cycle interpretation [48].

Goal and Scope Definition, Functional Unit, and System Boundaries

To assess potential environmental impacts and estimate the impact caused by the type of RA in the comparison of concrete mixtures, the functional unit was described as one foundation structure with a defined function described in Section 3.2.2. Using this functional unit, the strength reduction was compensated by a change in the structure’s height. Different final volumes of elements affect the number of materials used, which is described in Table 2.
In the comparison, all life cycle phases (materials production and transport, concrete manufacturing, deconstruction, and landfilling as the regular end-of-life of the structure) were considered to characterize the system boundaries as cradle-to-grave boundaries. The use phase of the concrete was assumed to have no measurable environmental impact. Material production includes excavating primary raw materials or the recycling of construction and demolition waste, since the waste is unloaded in a recycling plant. For the basic scenario, the distances for transporting resources and waste were assumed to be 50 km.
In this study, two outputs of the recycling process are considered. The first output is RA1, produced in the one-stage crushing and sorting process. In the two-stage recycling process, RA1 is consumed as an input, and after another grinding and sorting process, RA2 is produced as an output with better properties and a lower amount of clay particles. Reinforcement steel bars are separated during the one-stage recycling process, producing three aggregate fractions. Except for the fraction of RA1, which is used for the second stage, the fraction of the RA from the one-stage recycling process is mainly used for landscaping.

Life Cycle Inventory (LCI)

A concrete production model was performed based on Fiala [49]. To The inventory analysis was conducted using Gabi 9 software version 9 [50]. describe the production of resources under the conditions of the Czech Republic, specific processes were used and supplemented with generic data from the GaBi 9 database version 9 [50]. The parameters that describe the recycling process were provided by the Czech manufacturer of recycled aggregate.

Life Cycle Impact Assessment, Normalization, and Weighing

The environmental assessment was carried out using the environmental footprint (EF) version 3.0 characterization method [51]. The results of the potential environmental impacts were normalized by relating the results of environmental indicators to the global contribution [48]. The normalization factors were based on the personal equivalents EF 3.0 included in Gabi software version 9 [50]. Then, the normalized results were weighted to emphasize a specific value of each category using EF 3.0 weighing factors, which were also included in Gabi software version 9 [50].

4. Results and Discussion

4.1. Potential of Concrete Mixtures for Foundation Structure Element

The basic properties (mechanical and physical) of the compared structures are shown in Table 3. The target concrete strength class was examined due to the characteristic compressive cube strength according to the Eurocode [52] and ISO 12491 [53] due to considering the number of samples to eliminate their influence. The target strength classes for NAC mixtures were C25/30 (NAC I), C25/30 (NAC II), C30/37 (NAC III), and C35/45 (NAC IV). The target concrete strength classes for RAC mixtures ranged from C8/10 to C25/30, depending on the type of used aggregate, replacement ratio of coarse and fine aggregate, and target strength class determined by the amount of cement and water–cement ratio. The flexural strength used for the structural design was considered according to the Eurocode based on the target strength class. The flexural strength verified by experimental measurements was higher than that used for calculation in all cases.
According to the European standard EN 206+A2, the lower strength class, which is possibly used for foundation structures, is C12/15 (with a characteristic cube compressive strength of 15 MPa), and usually, the strength class C16/20 (with a characteristic cube compressive strength of 20 MPa) is used for foundation structures made of plain concrete. Higher strength classes are mainly used for reinforced concrete foundation structures. However, for the most precise possible comparison of structural use, only one type of structural element needs to be chosen. Despite a reduction in mechanical properties, RAC remains suitable for foundation structures, as verified by experimental results and compliance with Eurocode 2 (EN 1992-1-1) [52]. Structural compensation strategies, such as increased cross-sections, can effectively mitigate strength reductions, making RAC a practical and sustainable alternative to NAC. For this reason, it was decided to use the plain concrete foundation structure for environmental comparison, even if concretes higher than strength class C20/25 would not be used for this application.

4.2. Life Cycle Impact Assessment

The results of the potential environmental impacts are shown in Supplementary Table S1. Considering the potential impact described using the Climate Change total indicator, the II C100F RA2 reached the lowest impact (705 kg CO2 eq). This mixture combines a high percentage of RAC and optimal volume of the foundation element. Similarly, other mixtures, marked as F, in which fine RA 2 was used, had better properties, so the volume of its foundation was smaller.
On the other hand, using recycled aggregate does not lead to a lower impact in the Climate Change category. For example, a mixture with only a partial replacement of natural aggregate (30% and 50%) containing RA1 has a higher impact on Climate Change indicators than the reference NAC.
In this study, NAC II as a reference foundation has never reached the best result in any indicator. However, the NAC III and NAC IV mixtures reached the lowest impact in the indicators Climate Change and Land Use and Land Use Change (LULUC), which are affected mainly by the smaller volume of the concrete mixture. Reference foundations are not supplemented with the contribution of the recycling process, where the recycling of iron steel scrap and additional transport have a dominant negative influence, specifically in the resource use (minerals and metals).
Supplementary Table S1 shows that some mixtures (e.g., II C100F RA2, III C100F RA2, and Iv C100F RA2) even have negative results. The negative result of the impact indicator represents the beneficial environmental impact caused by avoiding environmental damage. For instance, II C100F RA2 potentially has a beneficial impact on human toxicity (cancer total) and the particular matter category. Due to recycling, all RACs have a beneficial impact in the categories of resource use (mineral and metals) and water use.
To conclude, RAC II C100F RA2 and RAC IV C100FRA 2 have the lowest environmental impact for the considered function. Both contain fine RA2, so more CDW is used for their production.

4.3. Properties of Recycled Aggregate Concrete

Generally, it was verified many times that the properties of RAC depend on the quality of the RA, especially the water absorption, content of contaminants, replacement ratio of aggregate, and considered concrete grades [54]. In the previous studies, it has been observed that the compressive strength of concrete decreases more intensively for higher concrete grades from the strength class C45/55, where the failure planes have occurred through the aggregate particles, which shows that the aggregate is a limiting factor of strength. On the contrary, for the concrete strength class C30/37, the failure planes have been observed around the aggregate; hence, in this case, the limiting factor is ITZ [55]. Due to this fact, it could be assumed that for higher strength classes, the use of RCA reduces the compressive strength more expressively, and with an increasing replacement ratio, the influence grows. On the other hand, in the case of low-strength classes of concrete, where the ITZ is more limiting, the increased replacement ratio should not be essential [54]. Additionally, it has been observed that the equivalent mortar volume (EMV), i.e., the total mortar volume considered as the sum of residual and fresh mortar volumes in concrete containing RCA, could lower the properties due to the high amount of ITZ. For this reason, it was verified that it is possible to reduce the amount of new cement in the mixture and improve the concrete’s mechanical properties and durability [56]. This could also be related to the previous findings that lower-strength classes have a lower amount of mortar, which is the probable reason that the decline in the properties is lower for the lower-strength concrete classes. Moreover, as mentioned before, the clay content in RCA influences the workability of fresh concrete due to its ability to absorb water and reduce the bond between the RCA and new cement mortar [2]. Finally, the replacement of natural sand by fRCA was also verified many times. The decline in the mechanical properties of concretes containing fRCA, likewise coarse RCA, relates to the old cement mortar, which leads to higher porosity and, consequently, higher water absorption. Furthermore, a higher specific surface area was found for the finest fraction (0–0.063) of fRCA, which is highly represented in fRCA [33].
The dependence of the quality of RA, replacement ratio, and considered concrete grade was shown by the evaluation of mechanical and physical properties, which corresponds with the previously published results. These results are compared and discussed in the following sections.

4.3.1. Physical Properties of Concrete

In [54,57], it was found that the density of RAC decreases and water absorption of RCA increases with the replacement of aggregate in the concrete mixture due to the higher porosity of RCA. This case study shows a decline in dry density for all tested RAC mixtures. The highest decrease was observed for low-grade mixtures with the complete replacement of fine and coarse aggregate by RA1, where the decrease was up to 25% compared to the control mix, corresponding with the concrete grade. On the contrary, the lowest decline was found for mixtures with a 30% replacement rate. This confirms the results of previous studies, which show that the density of RAC depends on the replacement rate and quality of the RCA. In the case of water absorption, it was reported that the WA of RAC with the complete replacement of coarse RCA increases up to 50% compared with the NAC [54]. However, in this case study, the WA increases many times more. The highest increase in water absorption was observed for mixtures with the complete replacement of fine and coarse aggregate by RA1, where the maximal increase is 3.3 times. On the contrary, a lower increase is shown for mixtures containing only 30% of RCA, where the maximal increase is 2%. Moreover, as reported in previous studies, no mixture with a complete aggregate replacement increased water absorption by up to 50%. The results showed that the WA depends on the replacement ratio and quality of RCA; furthermore, the influence of the concrete strength class could be observed. The impact of substitution by RCA grows with increasing grade concretes (see Figure 4). Water absorption by capillarity shows slightly different results than the WA by immersion, where capillary water absorption decreases with the increasing grade of concrete for NAC and RAC in general. The maximal increase, which is up to four times, of water absorption could be observed for mixtures with the complete replacement of aggregate by RA1, whereas in the comparison, in concretes containing only a coarse fraction of the RA and concretes with fRA1, the negative influence of fRA can also be observed. On the contrary, in the case of RA2, a positive effect could be found using fRA2. This could be caused by filling the pores with a fine aggregate in the concrete skeleton. In conclusion, it is necessary to mention that both water absorption and water absorption negatively influence the durability of concrete.

4.3.2. Mechanical Properties of Concrete

According to previous studies [57], the compressive strength decreases by up to 25%, the flexural strength decreases by up to 10%, and the modulus of elasticity ranges up to 45% for concretes with a complete replacement ratio. However, the results of the strengths and modulus of elasticity for RAC mixtures did not correspond with this assumption.
The decline in compressive strength confirmed the results of previous studies that the decline is more appreciable for higher-grade concretes. In this case study, the increasing deterioration in compressive strength, depending on the strength class of concrete, is shown. From this point of view, the maximal decline in compressive strength is shown for the mixture labelled RAC IV, with a drop of 67%. On the contrary, the mix with the same replacement ratio and aggregate type RA1 from mixtures labelled RAC I decreased “only” by about 55%. Furthermore, the influence of the aggregate quality is shown. All mixtures manufactured with RA2 achieved better results than those manufactured with RA1. This shows the negative impact of soil, which could be contained in aggregate manufactured by the one-stage crushing technique. Additionally, when the fRA2 is used in the mixture, the compressive strength increases compared to mixtures where only the coarse fraction of RA2 is used. This phenomenon could be caused by the filler effect of fRA, where the finest particles fill the pores and improve the structure of the mixture to be denser, reducing internal stresses and the early propagation of stress [58]. However, this phenomenon is essential to the quality of fRA, which needs to be composed only of crushed concrete without contaminants, as is shown by the negative influence of fRA1. Finally, the impact of the replacement rate was also confirmed by this study; the concretes with only a 30% replacement ratio showed a lower decrease in compressive strength. However, in the case of a 50% replacement ratio, it could be observed that the replacement ratio and quality of RA have a similar influence (see Figure 5 and Figure 6).
The result of flexural strength also shows a significant decline. However, in this case, the decreasing trend related to the growing grade of concrete was not observed. However, the influence of the aggregate quality, replacement rate, and the improvement made by using high-quality fRA can be seen. In the case of flexural strength, the angular shape and rough surface texture of fRA particles could lead to better interlocking between particles [58]. The highest decline in flexural strength was measured for mixtures containing only RA1 (fine and coarse fraction), where the maximal deterioration was 56%. In conclusion, the results of flexural strength, examined by experimental verification, were compared with those listed in the Eurocode for the corresponding concrete strength class. This comparison found that although the decline in flexural strength was significant, the examined flexural strength was still higher than in the Eurocode (see Figure 7).
In this study, a similar decline in the modulus of elasticity compared with the compressive strength was shown, which is a slightly different result than has been generally reported; according to previous reports, the modulus of elasticity is a more affected concrete property [54]. However, the decline in the modulus of elasticity is essential for future use for all tested properties except for concrete with a 30% replacement rate of a coarse fraction, where the decrease was only 4%. Like the compressive and flexural strength results, the modulus of elasticity is negatively affected by the low-quality aggregate and substitution level of the aggregate. The concrete grade, like the flexural strength, did not significantly influence the declines. However, contrary to both strengths, the positive influence of the high-quality fRA must be verified. This phenomenon corresponds with the results of previous studies dealing with replacing natural sand in concrete mixtures, where it has been reported that fRA negatively influences the modulus of elasticity for low replacement ratios, contrary to the compressive strength, where a replacement ratio of up to 30% was defined as usable. For this reason, using concrete with fRA is not recommended for structural concretes such as beams [58].

4.4. Contribution of Recycled Aggregate to Impact on Climate Change

In previous studies, the environmental impact in the Climate Change category is mainly associated with cement production. On the contrary, recycled aggregate production can have a beneficial total impact on the foundation element, as presented in Figure 7. In this figure, the contribution of the RA and cement in the Climate Change category is described and related to the total results of each foundation element. The contribution of RA1 is relatively insignificant in comparison with RA2. This is affected by the lower amount of processed CDW in the one-stage recycling process. On the other hand, a higher amount of CDW is consumed in the production of RA2, as presented in Figure 8. The impact of the recycling process of CDW is beneficial because it considers the benefits of recycling steel scrap from CDW and the reuse of other fractions of the RA as a replacement for the primary aggregate. This phenomenon is also described in [59].
The impact of the cement and aggregate was relatively related to the impacts of individual concrete. The results show the almost linear dependence on the replacement ratio of the aggregate in a concrete mixture separately for individual types of RAs. Furthermore, the higher influence of the cement is shown for concrete with a higher quality RA (RA2).

4.5. Beneficial Impact of Recycling Process

The influence of benefits associated with aggregate and steel scrap recycling is presented in Figure 9. Both types of benefits are higher for mixtures containing higher amounts of RAs. Also, using a two-step recycling process, in which more CDW is consumed, leads to a more beneficial contribution compared to the production of RA1. The most beneficial impact was reached by foundation III C100 RA2—credits for steel recycling.
The influence of benefits associated with aggregate and steel scrap recycling is presented in Figure 8. Both types of benefits are higher for mixtures containing more RA. In addition, using a two-stage crushing process, in which more CDW is consumed, leads to a more beneficial contribution compared with the production of RA1. In the case of credits for steel recycling and credits for aggregate, the most beneficial impact was reached by foundation RAC III C100F RA2. However, in the total results, the most beneficial impact was found for RAC II C100F RA2, where a lower amount of cement than in RAC III C100F RA2 was used.
In conclusion, the result confirmed the previously presented results that the positive effect of replacing NA in concrete with RA is shown for concretes with a high replacement ratio [60]. The concrete with a replacement ratio of 30% showed a higher total impact than reference concretes for a lower quality aggregate and a slightly lower total impact for the higher quality.

4.6. Contribution of Transport

As was reported in previous studies, the transportation of the materials is the process with the second-highest impact from concrete production [60] due to the energy and emissions related to diesel production and consumption. Furthermore, concrete is a material with a high mass by volume, so its transportation is costly. For these reasons, the use of recycled materials in demolition sites, such as the partial replacement of primary materials, leads to the reduction in environmental burdens. In the case of aggregate, RAs could replace NAs for backfilling and landscaping, or the high-quality RA could partially replace the aggregate in concrete.
The increase in the normalized and weighted impact related to the transportation and landfilling of unused materials is shown in Figure 10. The results show the clear benefit of utilizing the materials on the demolition site. The increase in the impact connected with transportation and landfilling is dependent on the increasing amount of the RA in concrete. Furthermore, the impact is higher for concretes containing the lower-quality aggregate RA1.
The decrease in the normalized and weighted impact related to transportation to a shorter distance is shown in Figure 11. The reference scenario is considered to be a transport distance of 50 km, and the modified scenario is considered to be a transport distance of 25 km. The results show a linear decrease for all evaluated concretes, which is 8%.
These results also confirmed the lower impacts related to the use of higher-quality RAs in general.

5. Conclusions

This study provided a comprehensive evaluation of recycled aggregate concrete (RAC) for foundation applications, focusing on mechanical performance, environmental impact, and engineering feasibility. The findings confirm that RAC remains a viable alternative for foundations and non-load-bearing applications despite its reduced strength compared to natural aggregate concrete (NAC). The two-stage recycling process significantly enhances aggregate quality, reducing contaminants such as soil and clay and improving RAC’s mechanical performance, particularly when fine recycled concrete aggregate (fRCA) is used as a partial sand replacement.

5.1. Key Findings

  • Recycling Process Impact—The two-stage crushing procedure produces a higher-quality recycled aggregate (RA) compared to one-stage crushing, leading to improved mechanical properties and reduced contaminants.
  • Durability Considerations—RAC exhibits higher water absorption and lower density, which may negatively affect long-term durability and should be considered in structural design.
  • Mechanical Performance—While compressive and flexural strengths decline, the verified flexural strength values still meet Eurocode requirements, allowing for safe use in foundation structures.
  • Environmental Benefits—Life Cycle Assessment (LCA) confirms that cement production and transportation contribute the most to RAC’s environmental footprint, but replacing the natural aggregate (NA) with a high-quality RA can reduce the environmental impact by nearly 50%.

5.2. Engineering Implications and Future Research

RAC is best suited for foundation structures and secondary concrete elements, where lower strength requirements allow for a higher proportion of recycled aggregate. However, its application in high-strength or highly exposed structures (e.g., bridge decks, marine environments) is limited due to the reduced mechanical performance. To expand RAC’s practical use, future research will focus on two key areas:
  • Optimizing Aggregate Processing—Enhancing RA quality through water-based washing techniques to remove fine contaminants, though this increases processing costs and must be balanced with economic feasibility.
  • Reducing Cement Dependency—Replacing CEM I with CEM II and low-carbon supplementary cementitious materials (SCMs) (e.g., fly ash, silica fume, and ground granulated blast-furnace slag) to improve durability while reducing carbon emissions.
  • This study contributes to shifting the perception of RAC from a low-value material to a structurally viable and environmentally sustainable alternative, paving the way for broader adoption in construction.
The findings of this study underscore the transformative potential of RAC in reducing the environmental footprint of the construction industry while maintaining structural viability in suitable applications. By demonstrating that high-quality recycled aggregates can effectively replace natural aggregates in foundation structures, this research reinforces the role of RAC in sustainable construction. Furthermore, by identifying key engineering strategies for mix design optimization and future research directions, this study provides a pathway toward the broader adoption of RAC. As the industry moves toward decarbonization and circular construction practices, the insights presented here serve as a foundation for the further innovation and implementation of recycled materials in structural applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/buildings15081237/s1, Table S1: Results of selected impact category indicators for foundation structures, EF 3.0 impact assessment method.

Author Contributions

Conceptualization, T.P. and J.P.; methodology, T.P.; software, J.P.; validation, T.P., J.P. and K.F.; formal analysis, K.F.; investigation, J.Ř.; resources, T.P.; data curation, T.P. and J.P.; writing—original draft preparation, T.P., K.F. and J.P.; writing—review and editing, T.P.; visualization, T.P. and K.F.; supervision, T.P.; project administration, J.P.; funding acquisition, J.P. All authors have read and agreed to the published version of the manuscript.

Funding

This study was developed within the project “CirCon4Climate—Circular Construction Practices for Climate Action”, 72.3024.6-003.28, under the European Climate Initiative (EUKI) https://www.euki.de/en accessed on 28 March 2025 of the German Federal Ministry for Economic Affairs and Climate Action (BMWK). The opinions put forward in this study are the sole responsibility of the author(s) and do not necessarily reflect the views of the Federal Ministry for Economic Affairs and Climate Action (BMWK).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Buildings 15 01237 g001aBuildings 15 01237 g001b
Figure 1. The particle size distribution of fractions 0/4, 4/8 and 8/16 of the recycled aggregate from waste concrete.
Figure 1. The particle size distribution of fractions 0/4, 4/8 and 8/16 of the recycled aggregate from waste concrete.
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Figure 2. The recycled aggregate from waste concrete.
Figure 2. The recycled aggregate from waste concrete.
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Figure 3. Flowchart illustrating the methodology, including material selection, mix design, experimental testing, environmental assessment, and data analysis.
Figure 3. Flowchart illustrating the methodology, including material selection, mix design, experimental testing, environmental assessment, and data analysis.
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Figure 4. Density and water absorption of NAC and RAC.
Figure 4. Density and water absorption of NAC and RAC.
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Figure 5. Compressive strength of NAC and RAC for mixtures with complete replacement of coarse aggregate.
Figure 5. Compressive strength of NAC and RAC for mixtures with complete replacement of coarse aggregate.
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Figure 6. Compressive strength of NAC and RAC for mixtures with partial replacement of coarse aggregate.
Figure 6. Compressive strength of NAC and RAC for mixtures with partial replacement of coarse aggregate.
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Figure 7. Examined flexural strength and flexural strength of NAC and RAC used for mixtures for design of foundation structural element.
Figure 7. Examined flexural strength and flexural strength of NAC and RAC used for mixtures for design of foundation structural element.
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Figure 8. Contribution of aggregates and cement to the total result of foundation element in the Climate Change category (only contributions of RA and cement are presented).
Figure 8. Contribution of aggregates and cement to the total result of foundation element in the Climate Change category (only contributions of RA and cement are presented).
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Figure 9. Normalized and weighted results associated with credits for use of aggregates and steel scrap recycling; normalization and weighing performed according to PEF 3.0 (person equivalents). Total result represents the sum of all environmental benefits and burdens of foundation structures.
Figure 9. Normalized and weighted results associated with credits for use of aggregates and steel scrap recycling; normalization and weighing performed according to PEF 3.0 (person equivalents). Total result represents the sum of all environmental benefits and burdens of foundation structures.
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Figure 10. Summarized results for two scenarios considering full use of RA on site and transport and landfilling of unused RA (normalized and weighted according to EF 3.0).
Figure 10. Summarized results for two scenarios considering full use of RA on site and transport and landfilling of unused RA (normalized and weighted according to EF 3.0).
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Figure 11. Summarized results for two scenarios considering distances of 50 and 25 km (normalized and weighted according to EF 3.0 personal equivalents); NAC II (50 km) is 100%.
Figure 11. Summarized results for two scenarios considering distances of 50 and 25 km (normalized and weighted according to EF 3.0 personal equivalents); NAC II (50 km) is 100%.
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Table 1. Physical properties of used aggregates.
Table 1. Physical properties of used aggregates.
Types of
Recycled
Aggregate		Grading (mm)Content of the Finest ParticlesOven-Dried Particle Density Water Absorption
Capacity		Saturation Level
f (%)ρRD (kg/m3)σ WA24 (%)σ(%)
Natural aggregate
(NA)		0/40.32570811.00.00.0
4/80.32530121.70.30.0
8/160.42540121.90.20.0
Recycled concrete aggregate (RA1)0/43.62220806.90.52.5
4/80.323803207.00.24.5
8/160.024201509.00.44.5
Recycled concrete aggregate (RA2)0/41.02430603.60.81.6
4/80.324201507.00.32.5
8/160.124203206.00.33.7
Table 2. Concrete mixtures of NAC and RAC per cubic meter.
Table 2. Concrete mixtures of NAC and RAC per cubic meter.
CEMWATERNA (0/4)NA (4/8)NA
(8/16)		RCA
(0/4)		RCA
(4/8)		RCA
(8/16)		W/CEFF W/C RR
(kg/m3)(kg/m3)(kg/m3)(kg/m3)(kg/m3)(kg/m3)(kg/m3)(kg/m3)(-)(-)(%)
NAC I2402407555305540001.001.000
RAC I C100 RA124025044000024711021.041.0075
RAC I C100 RA2240248000013310601.031.00100
RAC I C100F RA1240282608004715225261.181.0063
RAC I C100F RA2240271248004113465531.131.0084
NAC II2601697365335700000.650.650
RAC II C30 RA12602016320656048500.770.6527
RAC II C30 RA22601846320656048500.710.6527
RAC II C50 RA1260206611031105062830.790.6546
RAC II C50 RA2260206611031105062830.790.6546
RAC II C100 RA126017941500023911340.690.6577
RAC II C100 RA22602115880005265380.810.6564
RAC II C100F RA126017700044413210940.680.65100
RAC II C100F RA2260200221004183465670.770.6586
NAC III3001657005386010000.550.550
RAC III C30 RA1300200615067404856150.670.5555
RAC III C30 RA2300183615067404856150.650.5555
RAC III C100 RA130017536400022511980.580.5582
RAC III C100 RA23002085490005335640.690.5575
RAC III C100F RA130017400039013111630.690.55100
RAC III C100F RA2300196169004333475930.690.5589
NAC IV3201606815416160000.500.500
RAC IV C100 RA132017033900021712300.530.5081
RAC IV C100 RA23202045290005375770.640.5068
RAC IV C100F RA132016900036313011980.530.50100
RAC IV C100F RA2320191143004403486060.600.5091
Table 3. Properties of concrete mixtures and foundation elements with NAC and RAC.
Table 3. Properties of concrete mixtures and foundation elements with NAC and RAC.
Type of ConcreteDensityWater abs. by ImmersionCapillary Water abs. Compress. str.Flexural str.Static Elastic Modulus Strength Class acc. Standard Flexural str. acc. StandardThe Volume of the Element
(kg/m3)(%)(kg/m2)(MPa)(MPa)(GPa)(-)(MPa)(m3)
NAC I21997.614.44015.04.522.7C8/10--
RAC I C100 RA 1186416.926.2678.62.39.8---
RAC I C100 RA 2194915.219.39311.02.512.2C8/10--
RAC I C100 F RA 1177720.530.6576.72.18.1---
RAC I C100 F RA 2198316.113.13311.8313.6C8/10--
NAC II22845.55.96737.85.630.1C25/301.83.96
RAC II C30 RA 121437.65.76021.94.123.6C12/151.15.06
RAC II C30 RA 221995.65.43332.45.628.9C25/301.83.96
RAC II C50 RA 1202313.45.76322.13.818C16/201.34.62
RAC II C50 RA 221686.16.50033.5525.4C25/301.83.96
RAC II C100 RA 119771515.59315.23.614.2C8/10--
RAC II C100 RA 2205411.88.41322.33.3-C16/201.34.62
RAC II C100 F RA 1188118.320.94713.63.211.9C8/10--
RAC II C100 F RA 2210012.53.73327.43.921.6C20/251.54.4
NAC III22775.44.65346.37.333.2C30/372.03.74
RAC III C30 RA 121419.4-24.94.722.7C16/201.34.62
RAC III C30 RA 222006.75.06732.45.428.5C25/301.83.96
RAC III C100 RA 1200614.312.53321.94.213.8C12/151.35.06
RAC III C100 RA 22109114.86732.04.021.1C25/301.83.96
RAC III C100 F RA 1190317.618.42716.83.5-C12/151.15.06
RAC III C100 F RA 2210412.33.16732.44.622.6C20/251.54.4
NAC IV23174.83.32056.58.235.7C35/452.23.52
RAC IV C100 RA 1200513.97.53323.64.514.5C16/201.34.62
RAC IV C100 RA 2212710.63.62730.53.623.5C25/301.83.96
RAC IV C100 F RA 1193317.69.56718.73.612.9C12/151.15.06
RAC IV C100 F RA 2210612.13.26735.45.323.5C25/301.83.96
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Pavlů, T.; Pešta, J.; Fořtová, K.; Řepka, J. Recycled Concrete in Foundations: Mechanical and Environmental Insights. Buildings 2025, 15, 1237. https://doi.org/10.3390/buildings15081237

AMA Style

Pavlů T, Pešta J, Fořtová K, Řepka J. Recycled Concrete in Foundations: Mechanical and Environmental Insights. Buildings. 2025; 15(8):1237. https://doi.org/10.3390/buildings15081237

Chicago/Turabian Style

Pavlů, Tereza, Jan Pešta, Kristina Fořtová, and Jakub Řepka. 2025. "Recycled Concrete in Foundations: Mechanical and Environmental Insights" Buildings 15, no. 8: 1237. https://doi.org/10.3390/buildings15081237

APA Style

Pavlů, T., Pešta, J., Fořtová, K., & Řepka, J. (2025). Recycled Concrete in Foundations: Mechanical and Environmental Insights. Buildings, 15(8), 1237. https://doi.org/10.3390/buildings15081237

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