Influence of Environmental Factors and Aggregates on Concrete Elasticity: Ultrasonic and Strength Testing

Abstract

This study investigates the effects of recycled aggregates (RAs), temperature, humidity, and freeze–thaw cycles on the modulus of elasticity in concrete, using a comparative approach between ultrasonic pulse velocity (UPV) and compressive strength tests. A range of RA proportions (0%, 25%, 50%, 75%, and 100%) was explored. These mixes were subjected to environmental conditions, including temperatures of 25, 0, and −18°C in both dry and wet states, to assess their impact on the mechanical properties of concrete rigorously. The findings reveal that the modulus of elasticity decreases by up to 35% with higher recycled content and more severe environmental exposure, with significant variations observed under different temperatures (25, 0, and −18°C) and moisture conditions. Notably, samples with 25% recycled content maintained higher elasticity, suggesting an optimal threshold for incorporating recycled materials. The study also validates using UPV as a nondestructive method to efficiently evaluate concrete’s structural integrity. These insights advocate for the tailored use of RA concrete (RAC) in construction, enhancing durability and sustainability in varying climatic conditions. This research contributes to the development of concrete formulations that are both durable and environmentally adaptive, promoting sustainable construction practices.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Currently, the employment of recycled aggregates (RAs) in concrete is a central research topic, aiming to substitute natural aggregates [1, 2]. For instance, Datta et al. [3] investigated the generation of construction wastes in Bangladesh and highlighted the significant role of effective waste management strategies in mitigating environmental impacts. This study emphasizes the necessity of understanding and managing construction and demolition (C & D) waste to improve sustainability in the construction industry. The findings underscore the importance of proper waste management practices in ensuring the quality and sustainability of RA concrete (RAC). These RA, particularly the coarser types, significantly impact the concrete’s characteristics, including its modulus of elasticity [4].

Research indicates that the modulus of elasticity in concrete containing RAC is generally lower than in traditional concrete. Studies by Júnior, Silva, and Ribeiro [5], Rahal [6], Al-Hindawi et al. [7], Hansen [8], and Ghorbel and Wardeh [9] show a reduction ranging from 5% to 35% when substituting natural coarse aggregate with recycled concrete aggregate entirely. Additionally, Datta et al. [10] evaluated the effects of recycled concrete aggregate size and concentration on the properties of high-strength sustainable concrete. Their research demonstrated that different sizes and concentrations of RAs significantly influence the modulus of elasticity and overall mechanical properties of concrete. This study offers a more nuanced understanding of how specific variables such as aggregate size and concentration affect concrete performance, thereby providing detailed guidance for optimizing RAs in high-performance concrete applications. This variability in the modulus of elasticity is influenced by several factors, including the aggregates’ properties, the cement paste’s attributes, the transition zone’s characteristics, and the testing procedures and construction practices [11].

Determining the elastic modulus in concrete is crucial for designing reinforced concrete structures, as it can be dynamically assessed using nondestructive methods that involve longitudinal vibrations [12]. Pulse velocity measurement, a straightforward and quick method, helps illustrate material properties and detect cracks in concrete [13, 14]. Standards such as British Standards (BS) 1881 [15], C 597-16 [16], and Normas Brasileiras Regulamentadoras (NBR) 8802 [17] outline procedures for determining the dynamic modulus of concrete through ultrasound devices and compressive strength tests. However, they lack comprehensive guidelines on crucial concrete properties like RA, temperature, and moisture content, which considerably influence the outcomes [18]. Furthermore, the modulus of elasticity is a fundamental property of concrete that affects how reinforced concrete structures respond under load. It is crucial to understand how recycled materials impact the elastic properties of RAC, as these effects can significantly influence the structural performance and longevity of concrete infrastructures. Recent research has expanded on this understanding. Datta et al. [19] investigated the effects of different aggregate sizes and the impact of sulfate attack on RAC. Their findings show that the size and concentration of RAs significantly influence the mechanical properties, including the modulus of elasticity, and that sulfate conditions can further impact these properties. Similarly, Sobuz et al. [20] demonstrated how nondestructive testing methods, such as ultrasonic pulse velocity (UPV), can effectively assess the properties of RAC, highlighting the importance of considering aggregate characteristics and environmental conditions in the evaluation. Although these recent studies underscore the need for detailed investigations into how specific factors like aggregate size, sulfate attack, and nondestructive testing methods can be optimized to improve the performance of RAC. The lack of specificity in standard codes makes it challenging to consistently correlate different methods for determining or estimating the elastic modulus, especially without prior knowledge of the concrete’s moisture or temperature content. The specificities required for different concrete types mean that creating a universal prediction model is impractical [21, 22]. As a result, studies often present conflicting results regarding the dynamic modulus of elasticity in concrete that uses RA, highlighting the need for careful consideration of these variables. For instance, moisture affects the modulus of elasticity more significantly in low-strength concrete due to its greater porosity than in high-strength concrete [16]. In RAC, the larger pore volume amplifies the impact of moisture, affecting not only the strength and mechanical characteristics but also the durability of the concrete in certain applications. Therefore, it is essential to simultaneously investigate the effects of humidity, low temperature, and freeze–thaw cycles on the durability of recycled concrete [23].

This study investigates the dynamic modulus of elasticity in RAC by examining the effects of RA proportions, moisture content, surface temperature, and freeze–thaw cycles. The research provides new insights into how these factors collectively influence the mechanical properties of RAC, using a dual-method approach that compares the modulus of elasticity obtained through both UPV and compressive strength tests under varying environmental conditions. This approach highlights the effectiveness of UPV as a potential alternative for assessing compressive strength and dynamic modulus, especially in cold regions prone to freeze–thaw cycles. However, it also underscores the practicality and cost-effectiveness of the research methods. These findings have significant implications for sustainable construction practices, encouraging the use of durable and eco-friendly building materials.

2. Materials and Methods

2.1. Material and Mixture Proportion

In this study, we developed five distinct concrete mix designs by replacing natural aggregates with RA at 0%, 25%, 50%, 75%, and 100% replacement levels. Recycled fine aggregate (RFA) and recycled coarse aggregate (RCA) were replaced based on mass. Each mix utilized 437 kg/m³ of cement, suitable for compressive strength and freeze–thaw testing of cube samples. As the percentage of RAs increased, we proportionally added more super-plasticizers (SP) to maintain the workability of the concrete, compensating for the lower efficiency of the RAs. The air-entraining (AE) additive amount was kept constant across all mixes to ensure uniform air content. The aggregates were initially oven-dried to remove any moisture before mixing. The beach sand, fine aggregates, and coarse aggregates were mixed for 3 min. Subsequently, water and SP were gradually added, with mixing continuing for another 5 min. Detailed mix proportions are presented in Table 1.

Table 1

Detailed mix proportions.

Mix code	AE(%)	SP(%)	Water(kg/m3)	BS(kg/m3)	Cement(kg/m3)	FA(kg/m3)	RFA(kg/m3)	CA(kg/m3)	RCA(kg/m3)
RA 0	1.5	0.8	195.62	108.10	437.00	1083.30	—	532.38	—
RA 25	1.5	0.85	195.62	108.10	437.00	812.48	270.83	399.29	133.10
RA 50	1.5	0.95	195.62	108.10	437.00	541.65	541.65	266.19	266.19
RA 75	1.5	1.05	195.62	108.10	437.00	270.83	812.48	133.10	399.29
RA 100	1.5	1.15	195.62	129.70	437.00	—	1083.30	—	532.38

Abbreviations: AE, air-entraining; BS, British Standards; CA, coarse aggregate; FA, fine aggregate; RA, recycled aggregate; RCA, recycled coarse aggregate; RFA, recycled fine aggregate; SP, super-plasticizers.

2.2. Gradation Curves of Aggregates

The mechanical and physical properties of aggregates are fundamental for their performance in concrete applications [24, 25]. We classified crushed aggregates into coarse (CRA, up to 12.5 mm) and fine (FRA, up to 4.75 mm) categories according to American Society for Testing and Materials (ASTM) D75 [26]. The gradation curves, which show the distribution of these aggregates, are displayed in Figures 1 and 2 as per ASTM C136 [27].

[figure(s) omitted; refer to PDF]

2.3. X-Ray Diffraction (XRD) Analysis

Figures 3 and 4 present the XRD analysis results for recycled and natural aggregates. Significant peaks of quartz, feldspar (anorthite, albite), and calcite were observed for the natural sand mix, consistent with the mineralogy expected in standard concrete. The RA samples displayed quartz and calcium-rich minerals, suggesting the presence of old mortar. The calcium peaks are typically associated with the cement matrix, while quartz peaks correspond to sand and aggregates [28–30].

[figure(s) omitted; refer to PDF]

3. Experimental Procedures and Testing Methods

This section outlines the experimental procedures and testing methods used to investigate the impact of RA content and environmental conditions on the modulus of elasticity in concrete. The study involved a systematic approach to material selection, mix design, and the application of various testing techniques, including UPV and compressive strength tests. The following subsections provide a detailed description of the materials and mix designs used, followed by an explanation of the testing procedures and conditions under which the experiments were conducted.

3.1. Materials and Mix Design

The concrete samples were prepared by pouring the mix into molds immediately after mixing, ensuring no impact or vibration was applied to maintain the integrity of the mixture. This step is crucial as it affects the uniformity and consistency of the samples, as depicted in the flowchart shown in Figure 5. This flowchart details each stage of the sample preparation process. After the samples were molded, they were placed through a standard curing period, which was crucial to reaching the necessary hardness and durability for tests. The concrete’s mechanical properties, including compressive strength and modulus of elasticity, were then examined under freeze–thaw conditions. The performance of RAC is highly dependent on the ratio of RA used, as established in prior research [31–33], highlighting the importance of precise and controlled sample preparation and curing.

[figure(s) omitted; refer to PDF]

3.2. Testing Procedure

Mechanical tests, including ultrasonic testing and compressive strength assessments, were carried out to assess the influence of RA on concrete properties under freeze–thaw conditions. Ultrasonic tests, adhering to ASTM C597-16 [16], measured pulse velocity as a nondestructive means to estimate concrete’s compressive strength. This study phase involved 80 cubic samples, divided into Alpha and Beta groups, to evaluate concrete strength under freeze–thaw conditions according to ASTM C192/C192M [34]. After a 28-day curing period, samples in the Beta group were subjected to freeze–thaw cycles in line with ASTM C666/C666M [35], while the Alpha group served as the control. Each sample, measuring 100 mm × 100 mm × 100 mm, was temperature-conditioned to achieve a specific freezing point within a tolerance of 1–2°C before beginning the cycles. Their weights were monitored to detect any signs of deterioration. The freeze–thaw cycles, which fluctuated the temperature of samples from 4 to −18°C over approximately 3 h, were designed to simulate severe winter conditions typical in northern climates. These conditions and their effects on the samples are depicted in Figure 6A, providing insights into the durability challenges such infrastructures face. The UPV test, shown in Figure 6B, employs a device utilizing piezoelectric traits to convert electrical energy into mechanical ultrasonic energy, determining the transit time of a wave train between two points. The device measures the time “t” the wave requires to traverse from the transmitter to the receiver. Using the measured distance between the transmitter and receiver, the velocity “ν” of the wave in the material is computed [36]. The device operates at a frequency of 54 kHz, emitting pulses whose transfer duration, measured in microseconds (μs) with an accuracy of 0.1 μs, is displayed digitally. Polygel is applied to minimize the gap between the transducers and the concrete surface, ensuring reliable readings across various points on the surface, thus covering the entire volume of the sample. A numerical value for the pulse transfer time is recorded, and the test results are subsequently consolidated.

[figure(s) omitted; refer to PDF]

The speed (ν) of the pulse, essential for determining the dynamic modulus of elasticity, is defined as follows [37]:$$\begin{array}{}\text{(1)}& V=L/T,\end{array}$$where V is the pulse velocity (m/s), L is the path length (m), and T is the pulse duration (s). The modulus of elasticity is derived from the propagation velocity of the longitudinal waves generated by the shock excitation of a piezoelectric crystal [37]:$$\begin{array}{}\text{(2)}& VL=\sqrt{\frac{E1-\mu }{\rho 1+\mu 1-2\mu }},\end{array}$$where VL is the longitudinal velocity, ρ is the medium’s density, E indicates the modulus of elasticity, and μ corresponds to the Poisson’s ratio. In relation to the link between the velocity of ultrasonic sound and concrete’s compressive strength [38], following the nondestructive tests, compressive strength was measured on the same samples, as shown in Figure 6C, following the ASTM C39/C39M [39] test method. The results were used to determine the modulus of elasticity according to the method described in [28]:$$\begin{array}{}\text{(3)}& E=0.043{\rho }^{1.5}\sqrt{R_c},\end{array}$$where R_c is the compressive strength of the concrete, and ρ is the average specific weight of the concrete, estimated at 2260 kg/m³.

4. Results and Discussion

This section presents the findings of our study on the influence of RA content and environmental conditions on concrete’s modulus of elasticity. By examining results from both UPV and compressive strength tests, we explore the interplay between RA content, moisture, temperature, and freeze–thaw cycles on the mechanical properties of RAC. The discussion will highlight key trends and insights, emphasizing the implications of using RAC in sustainable construction.

4.1. Elastic Modulus of Samples During Freeze–Thaw Cycles

The results of this experiment and the calculation of the samples’ elastic modulus based on the ultrasonic device’s output are shown in Table 2. Additionally, the percentage reduction in modulus with increasing cycles and variations with substitutions at different recycled percentages are presented in Figure 7. As observed, consistent with findings from Çavdar [41] and Xie, Yang, and Xie [42], our results also indicate a decline in the concrete’s elastic modulus as the number of cycles increases. Moreover, similar to the findings by Datta et al. [10] on the influence of coarse aggregate size and content, the reduction in modulus of elasticity becomes more pronounced with higher RCA content. This reduction is attributed to the increased porosity and weaker interfacial transition zones (ITZ) in concrete containing RAs, as also noted by Sobuz et al. [20] in their study of sustainable concrete. Furthermore, the investigation of recycled samples reveals a decrease in the elastic modulus with increasing amounts of RA, a phenomenon also noted by Sharma [43], where RA contributed to variations in mechanical properties. As depicted in the graph, the high elastic modulus, similar to other parameters in concretes containing recycled materials, is due to microsilica and wollastonite. By filling voids and improving the ITZ, these additives mitigate some of the adverse effects of RA. The fine particle size of these additives and their performance in filling the voids and cracks in concrete enhance the quality and durability of the samples against freeze–thaw cycles [44, 45]. After completing the cycles, samples containing 25% recycled material exhibit the highest elastic modulus, followed by the reference sample, with remaining recycled percentages experiencing the most significant reduction in elastic modulus.

[figure(s) omitted; refer to PDF]

Table 2

Elastic modulus of samples during freeze–thaw cycles [40].

Mix code (%)	Modulus of elasticity (MPa)
	0 Cycle	40 Cycle	80 Cycle	160 Cycle	240 Cycle
RA 0	28,960.48	27,210.38	26,601.57	25,756.39	23,689.85
RA 25	32,606.81	32,153.01	31,276.43	30,435.22	29,627.50
RA 50	32,000.44	31,553.30	30,735.22	28,210.38	26,505.62
RA 75	31,430.44	31,010.23	30,725.54	29,832.56	27,672.07
RA 100	30,081.23	30,772.88	29,523.19	27,240.90	25,288.04

Abbreviation: RA, recycled aggregate.

These findings suggest that while RAC may be less suited for high-load-bearing structures, its performance in residential constructions or nonload-bearing walls could be optimal. This aligns with Dutkiewicz, Yucel, and Yildizhan [46], who observed enhanced performance properties in various concretes produced with wollastonite as a pozzolanic additive. Similarly, the resilience of RAC to freeze–thaw cycles could make it a suitable choice for pavement applications in regions that experience severe winter conditions [47]. These findings highlight the potential for RAC in specific construction scenarios, promoting its use where high durability and moderate load-bearing capacity are required. Our latest research [40] found that adding 10% microsilica and 7.5% wollastonite to the recycled mix significantly boosts the strength of all samples. Notably, samples with 25% recycled material displayed superior strength and durability, even as strength generally declines and expansion increases with higher recycling percentages and more freeze–thaw cycles. This corroborates the observed trend in the current experimental setup, reinforcing the feasibility of achieving durable and high-performance self-compacting recycled concrete by optimizing additive levels.

4.2. Assessing Concrete’s Elastic Modulus

In addition to investigating the concrete’s elastic modulus through ultrasonic tests, this study delves into calculating the elastic modulus based on relationships with compressive strength and specific weight. The results of these calculations are described in Table 3, and Figure 8 provides a visual representation of the increase or decrease in elastic modulus. This section is based on the strength of the samples with various percentages of recycled, the reference sample, and the average specific weight of the concrete, which is 2260 kg.

[figure(s) omitted; refer to PDF]

Table 3

Presents the concrete’s elastic modulus determined by compressive strength and specific weight.

Mix code (%)	Concrete’s elastic modulus calculated from compressive strength (MPa)
	0 Cycle	40 Cycle	80 Cycle	160 Cycle	240 Cycle
RA 0	27,172.23	27,046.69	26,468.84	24,930.29	22,561.57
RA 25	31,738.81	31,707.05	31,147.24	30,117.95	28,237.08
RA 50	31,370.34	30,857.17	30,359.44	28,926.72	26,384.39
RA 75	30,697.59	30,180.59	29,192.11	27,759.01	26,322.77
RA 100	28,960.08	28,929.81	28,182.45	27,580.59	23,418.50

Abbreviation: RA, recycled aggregate.

As a result, the magnitude of the samples’ elastic modulus reduction depends on their strength. Consistent with Noguchi et al. [48], who provided a practical equation for predicting the modulus of elasticity from compressive strength and unit weight, our results confirm that these parameters are crucial for understanding concrete’s mechanical behavior under various load conditions. The maximum decrease in modulus occurs at 240 cycles, and control designs with a 100% recycled material composition highlight the inverse relationship between RA content and concrete’s elastic modulus.

Comparing these findings with those of Datta et al. [10], the impact of aggregate size and content on the modulus of elasticity is evident. The study showed that smaller aggregate sizes in RAC lead to a higher modulus of elasticity due to better compaction and fewer voids, which is consistent with our observation that the modulus of elasticity decreases with higher RA content. The presence of larger RCA particles exacerbates the reduction in modulus due to increased porosity and weaker ITZ, further validating the importance of aggregate size and mix optimization in maintaining concrete’s mechanical integrity.

4.3. Discrepancy Between Ultrasonic Testing and Compressive Strength in Determining Concrete Elastic Modulus

This part of the study presents results from experiments designed to determine the elastic modulus using ultrasonic testing and calculations based on concrete’s mechanical properties, such as compressive strength and specific weight, shown in Table 4. It also highlights the discrepancies between these two approaches for varying levels of recycled materials and cycle numbers. Figure 9 illustrates that samples containing 25%, 50%, and 75% recycled material differ from the reference sample the least, while the reference sample and samples containing 100% recycled material differ the most. This difference changes with increased cycles and does not follow a specific pattern.

[figure(s) omitted; refer to PDF]

Table 4

Illustrates the percentage difference in the elastic modulus of the samples obtained through ultrasonic testing and compressive strength.

Mix code (%)	The difference in the modulus of elasticity (%)
	0 Cycle	40 Cycle	80 Cycle	160 Cycle	240 Cycle
RA 0	6.17	0.60	0.50	3.21	4.76
RA 25	2.66	1.39	0.41	1.04	4.69
RA 50	1.97	2.21	1.22	−2.54	0.46
RA 75	2.33	2.68	4.99	6.95	4.88
RA 100	6.82	5.99	4.54	−1.25	7.39

Abbreviation: RA, recycled aggregate.

4.4. Impact of Temperature on Ultrasonic Testing at Various Recycled Concrete Percentages (Average of Three Samples)

Table 5 and Figure 10 detail the effects of different testing temperatures on ultrasonic testing for recycled and reference samples. As observed, the wave velocity decreases with a decrease in temperature, and a test was conducted at −18°C. This trend is evident in all samples except for the 100% recycled material sample, which experiences the most significant reduction at 0°C. Consistent with our observations, Hager, Carré, and Krzemień [49] found that temperature variations significantly affect the UPV in concrete, indicating a decrease in mechanical properties as temperature decreases. Their study provides a valuable comparison point for understanding how temperature impacts the ultrasonic properties of concrete, reinforcing the trends observed in our experiments.

[figure(s) omitted; refer to PDF]

Table 5

Impact of temperature on ultrasonic testing at various recycled concrete percentages (average of three samples).

Mix code (%)	25°C	0°C	−18°C
0	27.93	28.17	29.97
25	25.43	26.40	27.23
50	26.60	26.93	27.13
75	25.30	25.27	26.23
100	27.50	25.93	28.23

4.5. The Difference in the Concrete’s Elastic Modulus at 25, 0, and −18°C

Figure 11 illustrates the variations in the elastic modulus of concrete under the influence of different temperatures, as measured by ultrasonic tests. As observed, in all scenarios, a decrease in temperature below zero leads to a reduction in elastic modulus. These findings are consistent with those reported by Chen et al. [50], who reported a marked reduction in the dynamic elastic modulus of concrete subjected to simulated periodic temperature-humidity variations, particularly under extreme cold conditions. Their study provides empirical support for our observations, highlighting the influence of temperature on concrete’s mechanical properties.

[figure(s) omitted; refer to PDF]

4.6. Impact of Moisture Levels on the Elastic Modulus in Recycled Concrete

Considering Figure 12, moisture significantly affects the determination of the elastic modulus in both recycled concrete and the reference sample. As is evident, the modulus of saturated samples is approximately 7%–10% higher than that of dry samples, suggesting that moisture presence can significantly affect the elastic properties of concrete. This observation is supported by Zhang et al. [51], who found that the elastic modulus of concrete increases with humidity, confirming the importance of moisture conditions in evaluating concrete properties. Our recent study [40] also explored this aspect, which confirmed moisture’s substantial effects on recycled concrete’s durability and mechanical properties, particularly under freeze–thaw conditions.

[figure(s) omitted; refer to PDF]

5. Microstructure Studies by Scanning Electron Microscope (SEM)

SEM imaging revealed differences between concrete samples with 0% and 100% RA before and after 240 freeze–thaw cycles (Figure 13). The ITZ, the most vulnerable part of the concrete structure, showed marked degradation in recycled concrete, especially after freeze–thaw exposure, with increased micropores and microcracks due to the old mortar’s reduced cohesion [52–54]. Conventional concrete maintained better ITZ cohesion but still displayed surface microcracks.

[figure(s) omitted; refer to PDF]

The presence of air voids (as shown in Figure 13A) within the control sample before freeze–thaw exposure is due to using an AE additive. These voids are intentionally introduced and are relatively small and uniformly distributed, which is crucial for improving freeze–thaw resistance. The evenly dispersed air voids help relieve internal pressure caused by the freezing of water within the concrete, enhancing the material’s durability under freeze–thaw conditions. In contrast, the ITZ in the RAC (Figure 13B) is more porous, with visible microcracks even before exposure to freeze–thaw cycles. This increased porosity weakens the overall structure, making it more susceptible to further degradation under cyclic freeze–thaw conditions. Post-freeze–thaw exposure, the control sample (Figure 13C) maintains a relatively intact microstructure, with only minor microcracks observed. However, the RAC (Figure 13D) shows extensive spalling, pore expansion, and the formation of new microcracks. These changes are primarily due to the weaker ITZ in RAC, which is more prone to cracking and deterioration under thermal stress. The RAs, having already undergone significant mechanical processing, contribute to this vulnerability by introducing pre-existing micro defects into the concrete matrix.

These observations are consistent with existing research [53, 55] and are further supported by findings from Nath et al. [56], which investigated the effect of recycled steel fibers (RSF) on the microstructure of RAC. Nath et al. [56] found that the addition of RSF significantly improved the ITZ in RAC by bridging microcracks and reducing the formation of new cracks under mechanical stress. The fibers’ presence helps to redistribute stresses across the concrete matrix, thereby enhancing the overall cohesion and durability of the concrete, even after multiple freeze–thaw cycles.

In summary, the SEM analysis reveals that the microstructural integrity of concrete, particularly the ITZ, plays a crucial role in determining the material’s resistance to freeze–thaw cycles. The addition of RAs compromises this integrity by increasing porosity and the likelihood of microcrack formation. However, the introduction of RSF, as suggested by Nath et al. [56], could potentially mitigate these effects, leading to a more durable RAC suitable for use in harsh environmental conditions.

6. Conclusions

This study explored how environmental factors and RA content affect the modulus of elasticity in concrete using UPV and compressive strength tests. Higher RA content generally reduces the modulus, especially under freeze–thaw conditions. However, adding microsilica and wollastonite improves durability and mechanical properties, making RAC a sustainable option for less demanding applications when properly enhanced. The following conclusions can be drawn from this experimental investigation:

1. The proportion of RA significantly influences the dynamic elastic modulus of concrete. Establishing a link between the dynamic elastic modulus and compressive strength necessitates individual consideration of each concrete variant, focusing on the specific aggregate type.

2. Microsilica and wollastonite added to recycled concrete samples improve the quality and durability of the specimens, particularly their resistance to freeze–thaw cycles.

3. It was observed that after completing cycles, samples containing 25% recycled material exhibited the highest elastic modulus, followed by the reference sample, and the remaining recycled percentages experienced the most significant reduction in elastic modulus.

4. The findings from ultrasonic testing and compressive strength suggest that conventional methods of estimating elastic modulus from compressive strength equations are unsuitable for concrete made with RA.

5. Except for the 100% recycled material sample, which experienced the most significant reduction at 0°C, all samples experienced a decrease in temperature, and conducting the test at −18°C resulted in a decrease in wave velocity and elastic modulus.

6. There was a noticeable difference in the dynamic elastic modulus between dry and saturated concrete, which persisted regardless of the RA content. This variation, consistently seen across all analyzed concrete groups, ranged from about 7%–10%. This indicates the importance of considering this factor when determining the dynamic modulus of concrete.

7. The practical implications of this research suggest that RAC could significantly impact sustainable construction practices. Given its varying performance under different conditions, further longitudinal studies are recommended to fully ascertain RAC’s effectiveness in specific construction scenarios, thus supporting its broader adoption in the industry.

Acknowledgments

For his fantastic help with R Programing and Plot Generation, I would like to thank Mr. Arash Bagherabadi sincerely.

References

[1] E. Najaf, H. Abbasi, "Using Recycled Concrete Powder, Waste Glass Powder, and Plastic Powder to Improve the Mechanical Properties of Compacted Concrete: Cement Elimination Approach," Advances in Civil Engineering, vol. 2022 no. 1,DOI: 10.1155/2022/9481466, 2022.

[2] J. Sivamani, M. Thiyaneswaranb, K. Navaneethanb, "Performance Studies on Glass Fiber Reinforced Recycled Aggregate Concrete," International Journal of Engineering, vol. 36 no. 5, pp. 904-913, DOI: 10.5829/IJE.2023.36.05B.07, 2023.

[3] S. D. Datta, M. J. Rana, M. N. Assafi, N. J. Mim, S. Ahmed, "Investigation on the Generation of Construction Wastes in Bangladesh," International Journal of Construction Management, vol. 23 no. 13, pp. 2260-2269, DOI: 10.1080/15623599.2022.2050977, 2023.

[4] L. Shehata, G. Isaia, "Immediate Deformations in Concrete," ISAIA, GC Concrete: Teaching, Research, and Accomplishments, 2005.

[5] N. A. Júnior, G. Silva, D. Ribeiro, "Effects of the Incorporation of Recycled Aggregate in the Durability of the Concrete Submitted to Freeze-Thaw Cycles," Construction and Building Materials, vol. 161, pp. 723-730, DOI: 10.1016/j.conbuildmat.2017.12.076, 2018.

[6] K. Rahal, "Mechanical Properties of Concrete With Recycled Coarse Aggregate," Building and Environment, vol. 42 no. 1, pp. 407-415, DOI: 10.1016/j.buildenv.2005.07.033, 2007.

[7] L. A. A. Al-Hindawi, A. M. Al-Dahawi, A. S. J. Al-Zuheriy, "Use of Waste Materials for Sustainable Development of Rigid Pavement," International Journal of Engineering, vol. 36 no. 10, pp. 1919-1931, DOI: 10.5829/IJE.2023.36.10A.16, 2023.

[8] T. C. Hansen, "Recycled Aggregates and Recycled Aggregate Concrete Second State-of-the-Art Report Developments 1945–1985," Materials and Structures, vol. 19 no. 3, pp. 201-246, DOI: 10.1007/BF02472036, 1986.

[9] E. Ghorbel, G. Wardeh, "Influence of Recycled Coarse Aggregates Incorporation on the Fracture Properties of Concrete," Construction and Building Materials, vol. 154, pp. 51-60, DOI: 10.1016/j.conbuildmat.2017.07.183, 2017.

[10] S. D. Datta, M. H. R. Sobuz, A. S. M. Akid, S. Islam, "Influence of Coarse Aggregate Size and Content on the Properties of Recycled Aggregate Concrete Using Non-Destructive Testing Methods," Journal of Building Engineering, vol. 61,DOI: 10.1016/j.jobe.2022.105249, 2022.

[11] I. Battagin, Modulus of Elasticity of Concrete: Standards, Influence Factors and Relationship With Precast Concrete, 2008.

[12] J. Ahmadi, M. H. Feirahi, S. Farahmand-Tabar, A. H. K. Fard, "A Novel Approach for Non-Destructive EMI-Based Corrosion Monitoring of Concrete-Embedded Reinforcements Using Multi-Orientation Piezoelectric Sensors," Construction and Building Materials, vol. 273,DOI: 10.1016/j.conbuildmat.2020.121689, 2021.

[13] L. C. Cossolino, A. H. A. Pereira, "Elastic Moduli: Overview and Characterization Methods," ATCP Physical Engineering, Technical Review ITC-ME/ATCP, pp. 10-11, 2010.

[14] B. Patel, U. K. Dewangan, "The Behaviour of Wave Propagation for Structural System Identification: A Comparative Study," International Journal of Engineering, vol. 35 no. 12, pp. 2305-2312, DOI: 10.5829/IJE.2022.35.12C.05, 2022.

[15] I. Lawson, "Non-Destructive Evaluation of Concrete Using Ultrasonic Pulse Velocity," Research Journal of Applied Sciences, Engineering and Technology, vol. 3 no. 6, pp. 499-504, 2011.

[16] Testing, A.S.f. and Materials, "Standard Test Method for Pulse Velocity Through Concrete," ,DOI: 10.1520/C0597-22, 2016.

[17] A. NBR, "8802: Hardened Concrete—Determination of Ultrasonic Wave Transmission Velocity," Brazilian Association of Technical, 2013.

[18] A. Davis, "Nondestructive Test Methods for Evaluation of Concrete in Structures," American Concrete Institute, ACI, vol. 228 no. 4, 1998.

[19] S. D. Datta, M. H. R. Sobuz, N. M. Sutan, S. Islam, "Structural Sustainable Recycled Aggregate Concrete Production Under Environmental Conditions," International Journal of Integrated Engineering, vol. 15 no. 6, pp. 64-71, DOI: 10.30880/ijie.2023.15.06.007, 2023.

[20] M. H. R. Sobuz, "Evaluating the Effects of Recycled Concrete Aggregate Size and Concentration on Properties of High-Strength Sustainable Concrete," Journal of King Saud University-Engineering Sciences,DOI: 10.1016/j.jksues.2022.04.004, 2022.

[21] M. Aguilar, "Analysis of Concrete Deformation: Young’s Modulus x Modulus of Elasticity," Congress of Engineering and Materials Science, 2006.

[22] V. Corinaldesi, "Mechanical and Elastic Behavior of Concretes Made of Recycled-Concrete Coarse Aggregates," Construction and Building Materials, vol. 24 no. 9, pp. 1616-1620, DOI: 10.1016/j.conbuildmat.2010.02.031, 2010.

[23] D. Kim, R. Kim, J. Min, H. Choi, "Initial Freeze-Thaw Damage Detection in Concrete Using Two-Dimensional Non-Contact Ultrasonic Sensors," Construction and Building Materials, vol. 364,DOI: 10.1016/j.conbuildmat.2022.129854, 2023.

[24] D. Soares, J. De Brito, J. Ferreira, J. Pacheco, "Use of Coarse Recycled Aggregates From Precast Concrete Rejects: Mechanical and Durability Performance," Construction and Building Materials, vol. 71, pp. 263-272, DOI: 10.1016/j.conbuildmat.2014.08.034, 2014.

[25] Z. H. Duan, C. S. Poon, "Properties of Recycled Aggregate Concrete Made With Recycled Aggregates With Different Amounts of Old Adhered Mortars," Materials & Design, vol. 58, pp. 19-29, DOI: 10.1016/j.matdes.2014.01.044, 2014.

[26] ASTM, ASTM D75/D75M-19, Standard Practice for Sampling Aggregates, 2019.

[27] ASTM, ASTM C136/C136M-19, Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates, 2019.

[28] T. Gonçalves, R. Silva, J. De Brito, J. Fernández, A. Esquinas, "Mechanical and Durability Performance of Mortars With Fine Recycled Concrete Aggregates and Reactive Magnesium Oxide as Partial Cement Replacement," Cement and Concrete Composites, vol. 105, 2020.

[29] Q. H. Xiao, Z. Y. Cao, X. Guan, Q. Li, X. L. Liu, "Damage to Recycled Concrete With Different Aggregate Substitution Rates From the Coupled Action of Freeze-Thaw Cycles and Sulfate Attack," Construction and Building Materials, vol. 221, pp. 74-83, DOI: 10.1016/j.conbuildmat.2019.06.060, 2019.

[30] M. Nataraja, H. Chakravarthy, R. Shivaprasad, S. R. Naganna, "Self-Compacting Concrete Incorporating Incinerated Biomedical Waste Ash: A Performance Assessment," Journal of Engineering and Applied Science, vol. 70 no. 1,DOI: 10.1186/s44147-023-00191-y, 2023.

[31] J. De Brito, N. Saikia, Recycled Aggregate in Concrete: Use of Industrial, Construction and Demolition Waste, 2012.

[32] M. Tuyan, A. Mardani-Aghabaglou, K. Ramyar, "Freeze-Thaw Resistance, Mechanical and Transport Properties of Self-Consolidating Concrete Incorporating Coarse Recycled Concrete Aggregate," Materials & Design, vol. 53, pp. 983-991, DOI: 10.1016/j.matdes.2013.07.100, 2014.

[33] S. Yazici, A. Mardani-Aghabaglou, M. Tuyan, "Properties of Self-Consolidating Concrete Incorporating Coarse Recycled Concrete Aggregate," 2017.

[34] A. ASTM, "C192/C192M-13a-Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory," ,DOI: 10.1520/C0192_C0192M-19, 2013.

[35] ASTM, ASTM C666/C666 M Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing, 2015.

[36] A. B. Espinosa, V. Revilla-Cuesta, M. Skaf, F. Faleschini, V. Ortega-López, "Utility of Ultrasonic Pulse Velocity for Estimating the Overall Mechanical Behavior of Recycled Aggregate Self-Compacting Concrete," Applied Sciences, vol. 13 no. 2,DOI: 10.3390/app13020874, 2023.

[37] AStm, Standard Test Method for Pulse Velocity through Concrete, .

[38] X.-Y. Lu, Z. Zhao, "Effect of Rape Straw Fiber on Mechanical Properties and Microstructure of Fly Ash Concrete," Advances in Civil Engineering, vol. 2022 no. 1,DOI: 10.1155/2022/3002430, 2022.

[39] C09 Committee, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens,DOI: 10.1520/C0039_C0039M-21, 2021.

[40] M. R. Hadidi, Y. Mohammadi, "The Effect of the Freeze-Thaw Cycle and Alkali-Silica Reaction on Self-Compacting Recycled Concrete," European Journal of Environmental and Civil Engineering, vol. 28 no. 11, pp. 2733-2759, DOI: 10.1080/19648189.2024.2326966, 2024.

[41] A. Çavdar, "Investigation of Freeze-Thaw Effects on Mechanical Properties of Fiber Reinforced Cement Mortars," Composites Part B: Engineering, vol. 58, pp. 463-472, DOI: 10.1016/j.compositesb.2013.11.013, 2014.

[42] R. Xie, J. Yang, E. Xie, "Investigation on Mechanical Properties Deterioration of Concrete Subjected to Freeze-Thaw Cycles," Scientific Reports, vol. 12 no. 1,DOI: 10.1038/s41598-022-27122-w, 2022.

[43] S. K. Sharma, "Properties of SCC Containing Pozzolans, Wollastonite Microfiber, and Recycled Aggregates," Heliyon, vol. 5 no. 8, 2019.

[44] S. A. Zareei, F. Ameri, P. Shoaei, N. Bahrami, "Recycled Ceramic Waste High Strength Concrete Containing Wollastonite Particles and Micro-Silica: A Comprehensive Experimental Study," Construction and Building Materials, vol. 201, pp. 11-32, DOI: 10.1016/j.conbuildmat.2018.12.161, 2019.

[45] S. Lotfollahi, A. Jaidari, P. Bakhtiari, M. Hosseini, M. Ghorbani, "Effect of Wollastonite Microfibers and Waste Tire Rubber on Mechanical Properties of Concrete," International Journal of Concrete Structures and Materials, vol. 17 no. 1,DOI: 10.1186/s40069-023-00595-3, 2023.

[46] M. Dutkiewicz, H. E. Yücel, F. Yıldızhan, "Evaluation of the Performance of Different Types of Fibrous Concretes Produced by Using Wollastonite," Materials, vol. 15 no. 19,DOI: 10.3390/ma15196904, 2022.

[47] A. Duan, W. Jin, J. Qian, "Effect of Freeze-Thaw Cycles on the Stress-Strain Curves of Unconfined and Confined Concrete," Materials and Structures, vol. 44 no. 7, pp. 1309-1324, DOI: 10.1617/s11527-010-9702-9, 2011.

[48] T. Noguchi, F. Tomosawa, K. M. Nemati, B. M. Chiaia, A. P. Fantilli, "A Practical Equation for the Elastic Modulus of Concrete," ACI Structural Journal, vol. 106 no. 5, 2009.

[49] I. Hager, H. Carré, K. Krzemień, "Damage Assessment of Concrete Subjected to High Temperature Using the Ultrasonic Pulse Velocity-UPV Method," Studies and Research, vol. 32, pp. 197-211, 2013.

[50] D. Chen, "Degradation of Dynamic Elastic Modulus of Concrete under Periodic Temperature-Humidity Action," Materials, vol. 13 no. 3,DOI: 10.3390/ma13030611, 2020.

[51] G. Zhang, H. Wei, M. Wang, "Influence of Humidity on the Elastic Modulus and Axis Compressive Strength of Concrete in a Water Environment," Materials, vol. 13 no. 24,DOI: 10.3390/ma13245696, 2020.

[52] T. Le, G. Le Saout, E. Garcia-Diaz, D. Betancourt, S. Rémond, "Hardened Behavior of Mortar Based on Recycled Aggregate: Influence of Saturation State at Macro-and Microscopic Scales," Construction and Building Materials, vol. 141, pp. 479-490, DOI: 10.1016/j.conbuildmat.2017.02.035, 2017.

[53] K. Liu, S. Wang, X. Quan, "Study on the Mechanical Properties and Microstructure of Fiber Reinforced Metakaolin-Based Recycled Aggregate Concrete," Construction and Building Materials, vol. 294,DOI: 10.1016/j.conbuildmat.2021.123554, 2021.

[54] S. M. S. Kazmi, M. J. Munir, Y.-F. Wu, "Effect of Different Aggregate Treatment Techniques on the Freeze-Thaw and Sulfate Resistance of Recycled Aggregate Concrete," Cold Regions Science and Technology, vol. 178,DOI: 10.1016/j.coldregions.2020.103126, 2020.

[55] S. Sunayana, S. V. Barai, "Partially Fly Ash Incorporated Recycled Coarse Aggregate Based Concrete: Microstructure Perspectives and Critical Analysis," Construction and Building Materials, vol. 278,DOI: 10.1016/j.conbuildmat.2021.122322, 2021.

[56] A. D. Nath, M. I. Hoque, S. D. Datta, F. Shahriar, "Various Recycled Steel Fiber Effect on Mechanical Properties of Recycled Aggregate Concrete," International Journal of Building Pathology and Adaptation, vol. 42 no. 3, pp. 448-468, DOI: 10.1108/IJBPA-07-2021-0102, 2021.