Sustainable application of recycled brick aggregates in concrete: evaluation of mechanical, durability, and environmental properties

Abstract

This study presents a comprehensive and novel investigation into the environmental sustainability and mechanical behavior of concrete incorporating three classifications of waste tyre rubber—powder, crumb, and chips—as partial replacements for natural aggregates. In contrast to prior research that typically focuses on a single rubber type or limited replacement ranges, this work systematically evaluates the influence of various rubber forms across a full spectrum of inclusion levels, ranging from 0% to 30% in 5% increments. Concrete mixtures were prepared with two distinct water-to-cement (w/c) ratios (0.4 and 0.5) to examine the interaction between mix design parameters and rubber content. Mechanical properties such as compressive strength, ultrasonic pulse velocity (UPV), and dynamic modulus of elasticity were assessed after both 28 and 90 days of curing. Notably, the mix containing 5% rubber at a 0.4 w/c ratio (0.4WR5) achieved a peak compressive strength of 58.81 MPa at 90 days, while the 8% rubber mix exhibited the highest UPV of 5660 m/s challenging the conventional perception that rubber addition invariably degrades concrete performance. These findings demonstrate that optimized rubberized concrete mixes, particularly at 5–10% inclusion levels, can deliver high strength and enhanced durability characteristics, contributing to sustainable construction without compromising structural integrity. Furthermore, the study highlights the critical influence of curing time and w/c ratio on the behavior of rubber-modified concrete, offering valuable design guidance for engineering applications. By effectively utilizing waste tyre rubber, this research contributes to circular economy principles and the reduction of concrete’s environmental footprint, presenting a practical route for diverting rubber waste from landfills.

Keywords:
Recycled bricks; w/c ratio; hardened properties; UPV; Microstructure studies

1. INTRODUCTION

The construction industry is one of the largest contributors to global solid waste, with demolition and renovation activities generating enormous volumes of construction and demolition waste (CDW). Among the various components of CDW, brick waste occupies a substantial portion, especially in developing countries where clay bricks are widely used in infrastructure and residential construction. Globally, billions of bricks are discarded each year due to rapid urban development, infrastructure replacement, and building demolition. It is estimated that by 2030, the annual discarded volume of bricks may exceed 1.2 billion units, contributing significantly to environmental pollution, land degradation, and unsustainable use of landfill space [¹,²,³,⁴]. Despite being technically biodegradable, waste bricks often persist in landfills for decades, posing risks such as leachate formation, fire hazards, and mosquito breeding. Furthermore, the production of conventional construction materials such as cement and natural aggregates requires high energy input and the consumption of finite natural resources, which further contributes to greenhouse gas emissions and ecological imbalance [⁵,⁶,⁷]. As a result, there is a growing emphasis on circular economy principles in construction, which encourage the reuse and recycling of waste materials. One promising strategy to mitigate the environmental burden of brick waste is its valorization as an alternative aggregate or cementitious material in concrete. Traditionally, the reuse of brick waste in concrete has focused on coarser fractions, such as brick chips and crumbs, which replace coarse or fine aggregates, respectively [⁸, ⁹]. These materials have shown varying degrees of success in modifying the workability, strength, and durability characteristics of concrete. However, such applications often overlook the fine fraction—brick powder—generated during brick crushing and grinding operations, which is typically underutilized or discarded as waste. Recent studies have hinted at the pozzolanic potential of brick powder, due to its high silica and alumina content and fine particle size [¹⁰, ¹¹]. When ground to sufficient fineness, recycled brick powder may exhibit cementitious behavior or act as a filler material, enhancing the microstructure of concrete by filling pores and improving packing density. Despite this, the synergistic role of brick powder in conjunction with coarser brick particles has not been systematically explored, particularly in terms of its effect on the mechanical and durability properties of concrete [¹²]. The novelty of this study lies in the integrated use of recycled brick powder together with brick crumbs (0.5–4 mm) and brick chips (>4 mm) as partial replacements for conventional aggregates. Unlike prior research that focused on single-size substitutions, this study investigates how the combined particle sizes from powder to coarse interact within the cement matrix and affect concrete performance. Moreover, the study examines the influence of two different water/cement ratios (0.4 and 0.5), providing valuable insights into the hydration behavior and workability of concrete containing recycled brick materials. Concrete mixes were prepared with seven replacement levels of recycled brick aggregate (0%, 5%, 10%, 15%, 20%, 25%, and 30%) by volume, and were tested for compressive strength, flexural strength, splitting tensile strength, ultrasonic pulse velocity (UPV), and unit weight over 28 and 90 days of curing. This comprehensive approach enables the evaluation of both the early-age and long-term performance of sustainable concrete mixtures. By emphasizing the role of brick powder, this research contributes significantly to the field of sustainable construction materials. It provides a scientific basis for optimizing the gradation and proportioning of recycled brick waste, and highlights how brick powder, often neglected in literature, can be an asset in enhancing concrete quality when properly utilized [¹³, ¹⁴]. The outcomes of this study are expected to encourage the adoption of brick waste recycling in construction practices and support the development of low-carbon, resource-efficient building materials. From a sustainability standpoint, the adoption of recycled brick materials in concrete serves multiple objectives: Reduces demand for virgin aggregates, mitigating habitat destruction and energy use associated with quarrying. Diverts substantial volumes of brick waste from landfills, extending their lifespan and reducing environmental degradation. Lowers the carbon footprint of concrete production by reducing the need for energy-intensive raw materials. Promotes local reuse of construction waste, thereby decreasing transportation emissions and fostering regional sustainability. This research contributes to the body of knowledge supporting sustainable material innovations in construction and provides practical guidance for engineers, architects, and policymakers aiming to adopt greener building practices. The findings can pave the way for large-scale application of recycled brick aggregates in structural and non-structural concrete components, ultimately helping to build more sustainable cities and communities. The present study offers a distinct advancement in the field of sustainable concrete technology by exploring the mechanical and durability performance of concrete incorporating a comprehensive range of recycled brick aggregates (RBAs). While prior research has investigated the use of individual waste materials in concrete, such as fly ash, slag, or recycled concrete aggregate, few studies have systematically evaluated the effects of different classifications of recycled brick materials, particularly across multiple replacement levels and curing durations [¹⁵, ¹⁶]. The novelty of this research lies in its integrated approach, combining mechanical testing (compressive strength, ultrasonic pulse velocity, and dynamic modulus of elasticity) with environmental performance metrics to assess the viability of RBA as a sustainable alternative to conventional aggregates. This investigation uniquely examines the influence of particle size, replacement ratio (up to 30%), and mix design parameters (e.g., water-to-cement ratios) on performance, providing a comparative framework previously lacking in the literature [¹⁷]. Prerequisites for executing such a study include rigorous material characterization, standardized mix design procedures, and extended curing and testing protocols to capture both early and long-term performance. Compared to seminal works that focused narrowly on mechanical strength or utilized only coarse brick aggregates, this study demonstrates that high-performance concrete can be achieved even with significant proportions of fine recycled brick powder, provided optimal mix conditions are met. By establishing performance thresholds and identifying optimal replacement levels, the research contributes valuable insights into sustainable construction practices. It bridges the gap between material reuse and structural performance, aligning with global efforts to reduce construction-related carbon emissions and raw material extraction. This holistic perspective distinguishes the current work as a significant contribution to both academic research and practical implementation in the field of green building materials.

2. MATERIALS USED AND METHODOLOGY

2.1. Materials

Table 1 lists the characteristics of the Portland cement used in this investigation, which is classed as CEM I 42.5 R by the TS EN 197-1 (2012) standard. Crushed sand and crushed stone derived from limestone are utilized as natural aggregates in the manufacturing of concrete. Table 2 lists the aggregates’ characteristics. When coarse and fine aggregate was mixed half and half, the optimal granulometry curve was produced by TS 802 (2016). Figure 1 displays the aggregates’ granulometry curve and limit curves. Recycled brick concrete was made from used bricks that had been broken down into chips, crumbs, and powder. In the investigation, chip bricks that range in size from 5 to 16 mm were utilized in place of coarse aggregate. The crumbed and powder brick diameter is 0.418–0.469 mm and 0.069–0.470 mm, with a specific gravity of 1.048 g/cm³. The study made use of a new-generation superplasticizer (SP) additive based on polycarboxylate ether.

Table 1
Characteristics of cement as per CEM I 42.5 R.

S. NO.	CHARACTERISTIC TYPE	AVAILABILITY
1.	Specific Gravity	3.08
2.	Specific surface area	3216 cm2/g
3.	Volume of expansion	0.9 mm
4.	Residue (32µ – sieve)	7.29
5.	SiO2	20.08%
6.	Al2O3	4.29%
7.	MgO	2.48
8.	CaO	61.82
9.	SO3	2.96
10.	Na2O	0.39
11.	K2O	0.78
12.	Cl−	0.006
13.	Loss of ignition	2.89
14.	Insoluble residue	0.54
15.	Final setting time	3 hrs. 30 min

Table 2
Characteristics of natural aggregates.

CHARACTERISTIC TYPE	FINE AGGREGATES	COARSE AGGREGATES
Grain Size (mm)	0–4	4−16
Water absorption (%)	0.61	0.64
Density (g/cm3)	2.63	2.65

Figure 1
Concrete mix size distribution using granulometry curve.

2.2. Mix design

Two distinct w/c ratios of 0.4 and 0.5 were used in this investigation. In the study, recycled bricks by volume were substituted for natural aggregate in concrete using water/cement ratios of 0%, 5%, 10%, 15%, 20%, 25%, and 30%. Preliminary studies were conducted in choosing these ratios, taking into consideration the ratios found in the literature. Preliminary tests were conducted using recycled bricks at high rates (25% and 30%). Concrete becomes less workable and segregation happens when recycled bricks make up more than 30% of the mixture. As a result, 30% was established as the maximum ratio. The mixture’s granulometry curve stayed consistent in every concrete even when recycled bricks were utilized as aggregate. For instance, chips of recycled bricks were substituted for 10% coarse aggregate while crumb and powder bricks were substituted by volume for 10% fine aggregate in concrete that contained 20% waste bricks. Control concrete is denoted by 0% and is concrete without recycled brick aggregate. Within the parameters of the investigation, 14 different types of concrete were made. All concrete was produced using a 400 kg/m³ cement dose.

2.3. Testing of concretes

Initially, the mixer was used to mix all the aggregates for one minute. After adding three minutes of mixing, three-quarters of the cement and water were added. After adding the remaining water and SP, the mixture was stirred for two minutes. The mixes were put into the moulds once they had been compressed. After 24 hours, the concrete was taken out of its moulds and allowed to cure in water for 28 and 90 days. Concrete slump values were calculated using the ASTM C143 [¹⁸] standard. In addition, the ASTM C138 [¹⁹] standard was followed when performing the unit weight test. Following 28 and 90 days of cure, tests were performed on all hardened concrete. By the ASTM C39 [²⁰] standard, investigations about the assessment of compressive strength were conducted on cube samples of 15 cm. Figure 2 displays an example of the compressive strength test visual result. As per the ASTM C78 [²¹] standard, 10 × 10 × 40 cm prismatic samples were used for the flexural strength test. The concrete’s flexural strength was determined using the center-point loading method. Following this, 10 × 20 cm samples were subjected to the split tensile strength test by ASTM C172 [²²]. The UPV test was finally conducted on 15 × 15 × 15 cm cube samples. The sample’s parallel surfaces were examined using a transmitter and receiver, with the time interval of the sound being recorded. The ultrasonic gel applied during the UPV test facilitates an environment that enhances the uninterrupted transmission of sound across the concrete surface at the point of probe contact. To find out the speed of sound in concrete, Equation 1 can be used [²³].

Figure 2
Compressive strength testing machine.

(1) $$V=L/T$$

Here, t represents the transmission time of recorded sound, and L represents the distance (0.15 m) that the sound traveled between the two probes, and V is the sound speed in m/s. Figure 3 displays the UPV test’s sample visual result.

Figure 3
UPV test for concrete samples with bricks.

This UPV gadget may provide measurement data with a margin of error of ±17%. Consequently, while assessing the results of the dynamic elasticity modulus and UPV tests, this circumstance should be considered. After the UPV test, the elasticity modulus of concrete was determined, and the average rates for each sample were determined using ASTM C 597 [²⁴]. Equation 2 can be used to calculate the dynamic elasticity modulus. In Equation 2, ρ indicates the density of concrete (kg/m3), Ed stands for the elasticity modulus of concrete (MPa), and μ is the Poisson’s ratio of the concrete mix, whereas V represents the UPV (km/s).

(2) $$E_d=\frac{\rho {\text{V}}^2(1+\mu )(1-2\mu )}{1-\mu }$$

3. EXPERIMENTAL RESULTS AND ANALYSIS

3.1. Unit weight

Figure 4 shows the results of varying the unit weight of samples based on the ratios of water to cement and recycled bricks in the concrete. Regardless of the ratios of cement to water, the experimental results demonstrate that adding recycled bricks to the mixes lowers the unit weights of the samples. Analyzing the concrete with a w/c ratio of 0.4 (Figure 4) reveals that the sample code with 0.4RB30 has the lowest unit weight (2189 kg/m³), while the control concrete (0.4C) has the greatest unit weight (2408 kg/m³). Every unit weight value decreases as the water-to-cement ratio increases from 0.4 to 0.5. The maximum unit weight value, as shown in Figure 4, is 2375 kg/m³. This value is associated with the 0.5 control concrete. However, as in the group with a w/c ratio of 0.4, sample 0.5RB30 had the lowest unit weight value, coming in at 2108 kg/m³. When adding varying amounts of recycled bricks to concrete, the percentage of weight losses increased to 8.71% and 11.34% per unit weight, respectively, for w/c ratios of 0.4 and 0.5. Figure 4 demonstrates that the ratio of recycled brick replacement to concrete unit weight decreases inversely. The primary cause of the decline in unit weight values is that recycled bricks substituted with natural aggregates have a specific gravity of 1.05 g/cm³, lower than that of normal aggregates (2.67 g/cm³). A further explanation is the reduction in concrete’s unit weight caused by air compression in the recycled bricks’ rough surface texture. Furthermore, the hydrophobic nature of the recycled brick and the extremely poor adhesion between the bricks and the cement mix, which raises the porosity, may cause the unit weight values to decline [²⁵]. Research indicates that the mixing mechanism and mixing time are the main causes of the drop in the unit weight of fresh RBA. This study also showed that the unit weight values of concretes made using waste bricks are highly influenced by the water/cement ratio. The addition of 20% recycled bricks to the concrete led to a 4% decline in unit weight, which can be explained by the increase in the w/c ratio from 0.4 to 0.5.

Figure 4
The unit weight of the concrete mix with recycled bricks substitution.

3.2. Compressive strength

Figures 5 and 6 display the compressive strength test results. Comparing the concretes to the control, their compressive strength dropped after 28 and 90 days. As more brick waste is substituted into the concrete production process, the strength loss rises. The control concrete has the maximum 28-day compressive strength, as shown by examining the samples with a w/c ratio of 0.4 (Figure 5). The control concrete’s (0.4C) compressive strength is 61.46 MPa. However, the mix 0.4RB30 has a very low compressive strength of 22 MPa. The concrete mix with 30% recycled bricks showed a reduction in strength relative to the control concrete, with strength diminishing from 51% to 64% as a consequence of the elevated brick content. The recycled brick concrete and its compressive strength at 28 days is 51.36 MPa, the highest of all the concretes. This concrete contains a 5% brick waste substitution ratio, as shown by its 0.4RB5 rating. With a w/c ratio of 0.4, this study particularly and uniquely shows that high-strength concretes of 50 MPa (and higher) may be created when 5% and 10% waste brick ratios are employed. The sample with a 5% brick replacement had the maximum 28-day compressive strength. The w/c ratio of 0.4 results in a compressive strength of 33 MPa. This study used waste bricks as both fine and coarse aggregate, which is different from most other studies in the literature. As a consequence, the investigation demonstrates that the sample 0.4RB15 with the same w/c ratio of 0.4 had an even lower 28-day compressive strength value. 0.4RB15’s compressive strength is 41.97 MPa. Additionally, this investigation demonstrates that, when using a 5% waste brick replacement ratio, the compressive strength value achieved for sample 0.4RB5 after 90 days of curing is quite close to the strength of the control. Although 0.4RB5’s compressive strength is 58.81 MPa, the control concrete’s (0.4C) is 62.67 MPa, therefore bridging the 28-day results gap (10 MPa). The study’s findings show that the optimal waste brick ratio is 5% when considering both w/c ratios and curing time parameters about compressive strength values. The control concrete’s 28 and 90-day compressive strengths are 48.5 MPa and 51.49 MPa, respectively, as shown in Figure 7. Additionally, sample 0.5RB30 had the lowest 28 and 90-day compressive strength values, at 18.56 MPa and 19.4 MPa, respectively. The outcomes agree with those recorded in the concrete group that maintained a w/c ratio of 0.4 and fixed at 0.5, increasing the waste bricks content to 30% resulted in a significant drop in compressive strength, estimated to be around 62%. The sample labeled 0.5RB5 recorded the highest 28-day compressive strength among all concrete formulations containing waste bricks, reaching 41.09 MPa.

Figure 5
Recycled brick concrete’s compressive strength at 0.4 w/c ratio.

Figure 6
Recycled brick concrete’s compressive strength at 0.5 w/c ratio.

Figure 7
The flexural strength of the concrete with a brick substitution at 0.4 w/c.

A similar trend was seen for 90 days of compressive strength. For example, sample 0.4RB10 has a 28-day compressive strength of 47.1 MPa. Additionally, sample 0.5RB10’s compressive strength drops by almost 30% to about 33.14 MPa. Also, sample 0.4RB5 concrete has a 90-day compressive strength of 58.81 MPa, but sample 0.5RB5 concrete has a strength of 46.89 MPa, a 20% drop. The results indicate the influence of the w/c ratio on the compressive strength of concrete that incorporates waste bricks as a replacement material. The decrease in compressive strength for all the samples with an increase in the ratio of bricks was shown in Figures 6 and 7. It is commonly understood that the inclusion of brick particles in cement paste results in a softer composition compared to conventional cement paste. This phenomenon accounts for the observed reduction in compressive strength in concrete samples that incorporate waste bricks. Concrete deteriorates quickly as a result of the softening, which speeds up cracking when the brick particles are loaded. The non-uniform stress distribution may also be caused by the comparatively poor binding between cement paste and brick particles [²⁶]. The incorporation of a higher proportion of brick waste in concrete results in a diminished resilience of the brick-cement interface area. This interface area therefore has a detrimental effect on the strength. In addition, waste bricks migrate upward during vibration because of their lower specific gravity and hydrophobicity (in comparison to typical aggregate). This results in a concrete structure that is highly strong and lacks homogeneity due to a larger concentration of bricks on the upper surface.

3.3 Flexural strength

The findings obtained from the flexural strength test are shown in Figures 7 and 8. Because more brick waste was used, the concretes’ flexural strengths at 28 and 90 days were lower than those of the control. As shown in Figure 7, the control concrete (0.4C) had the maximum flexural strength, measured at 10.06 MPa after 28 days, and had a w/c ratio of 0.4. With a flexural strength of 2.48 MPa, sample 0.4RB30 had the lowest result when the waste brick replacement increased to 30% of the waste brick-substituted concretes; the sample designated 0.4RB5 had the highest flexural strength, measuring 8.98 MPa. Tests of flexural strength at 90 days also showed comparable findings. Including control concrete, all waste brick-substituted concretes have a 90-day flexural strength that is greater than the 28-day. The findings of the flexural strength test show that, when considering all criteria, the ideal amount of waste bricks for this investigation is 5%. The flexural strength values after 28 and 90 days for concrete that has 5% waste bricks substituted are not appreciably less than those of the control. Figure 8 shows that the control (0.5C) flexural strength peaks at 28 and 90 days, with values of 8.4 MPa and 9.7 MPa, respectively. In comparison, the sample with code 0.5RB30, which represents 30% waste bricks replaced concrete, had the lowest flexural strength values at 28 and 90 days, 2.28 MPa and 2.47 MPa, respectively. The concrete containing waste bricks sample of 0.5RB5 had the highest 28-day compressive strength rating, with a flexural strength of 7.38 MPa. The flexural strength data acquired during the ninety-day cure period show a similar pattern. At 90 days, the flexural strengths of the waste-replaced concrete, which had a w/c ratio of 0.5, as well as the control concrete, were found to be higher than the flexural strengths recorded at 28 days. For instance, sample 0.4RB20’s 28-day flexural strength was 5.79 MPa, but sample 0.5RB20’s flexural strength dropped to 3.36 MPa, a 42% decline. Sample 0.5RB10’s flexural strength dropped by around 24% to 6.66 MPa, whereas sample 0.4RB10’s 90-day flexural strength was 8.73 MPa. The proportion of water to cement played a crucial role in determining the flexural strength of concretes that utilized waste bricks as an additional material. Utilizing waste bricks in concrete significantly diminishes flexural strength, mainly due to the insufficient bonding between the cement and the bricks. This weak bond detrimentally influences flexural strength more severely than it does compressive strength [²⁷]. Recycled brick concrete’s decreased flexural strength can also be attributed to cement-brick mix separation. This circumstance was validated by the ease with which the waste brick bits could be extracted from the concrete following the specimens’ failure in the flexural strength test [²⁸]. Results for both compressive and flexural strength were greatly influenced by the water/cement ratio. In actuality, adding 5% waste bricks did not decrease strength as much as raising the w/c ratio from 0.4 to 0.5.

Figure 8
The flexural strength of the concrete with a brick substitution at 0.5 w/c.

3.4. Split tensile strength

The split tensile test results are shown in Figures 9 and 10. Without considering the w/c ratio or the curing period, both figures demonstrate that the split tensile strength of concretes falls as a rise in % of brick replacement. The control concrete (0.4C) had the greatest split tensile strength among the 28-day concretes, with 5.41 MPa, as shown in Figure 9. The concrete with 30% brick aggregate replacement (0.4RB30) has a minimum splitting tensile strength of 1.90 MPa. The findings of the 90-day split tensile strength test likewise show a similar pattern. The rise in the replacement of bricks by up to 30% reduces the split tensile strength from 65 to 58%. When comparing concretes that was left for 90 days to those that were left for 28 days, the rise in split tensile strength ranged from 3% to 26%. The samples with a 0.5 w/c ratio are displayed in Figure 10. As a result, the control concrete (0.5C) had the greatest 28-day split tensile strength among all concretes, at 4.55 MPa. The one with a tensile strength of 1.60 MPa and a 30% brick waste aggregate replacement had the lowest tensile strength. The 90-day strength measurements show similar behavior. The control concrete exhibited a 65% decrease in split tensile strength when the waste brick replacement ratio increased to 30%. Conversely, the split tensile strength of the concrete at 90 days showed an enhancement ranging from 5% to 22%. By examining Figures 9 and 10, it becomes evident that the increase in the w/c ratio from 0.4 to 0.5 resulted in a 21% rise in the reduction of split tensile strength. The causes of this state of affairs may be categorized into three primary categories. First of all, waste brick particles tend to deform under stress, which might hasten the production of microcracks in concrete, in contrast to hardened cement paste [²⁹]. The mechanical characteristics of the interfacial transition zone (ITZ) between waste brick aggregates and cement paste deteriorate due to the quick development of fractures under continuous loading [³⁰]. Second, low adhesion occurs in ITZ due to the weak chemical interaction between cement paste and brick particles, as opposed to chemical reactions that produce strong bindings among cement paste and aggregates. Lastly, a high air content in concretes that contain waste bricks may damage the split tensile strength [³¹].

Figure 9
Recycled brick concrete’s split tensile strength at 0.4 w/c ratio.

Figure 10
Recycled brick concrete’s split tensile strength at 0.5 w/c ratio.

3.5. Ultrasonic pulse velocity

Ultrasonic pulse velocity test (UPV) findings are displayed in Figures 11 and 12. Based on the graphs, the concretes’ UPV decreased as the percentage of waste brick substitution increased. Because the cure durations were extended from 28 to 90 days, the concretes’ UPV also increased. Figure 11, which shows concretes with a w/c ratio of 0.4, shows that the control concrete (0.4C) had the greatest UPV of all samples cured for 28 days, at 5405 m/s. The sample with a 30% ratio of waste bricks replacement (0.4RB30) had the lowest UPV, 4747 m/s to be exact. Similar trends also apply to 90-day concretes. The UPV values decrease as a result of the waste brick replacement ratio increasing by up to 30%. Between 15% and 20% was the range of the decreases. According to this study, sample 0.4RB5, which has the lowest waste brick replacement rate of 5%, exhibits a 2% decrease in UPV when compared to control concrete. As a result, the UPV was not considerably decreased by utilizing waste brick in concrete rather than aggregate. According to the study, 90-day samples might have UPV increases of up to 10% above 28-day samples, regardless of the waste brick ratio. Among the 28-day concretes, Figure 12 (w/c–0.5) demonstrates the greatest UPV of 5085 m/s, which again belongs to the control concrete (0.5C). With 24% waste brick replacement (0.5RB30), recycled brick concrete has the lowest UPV value at 4464 m/s. For 90-day samples, the pattern is likewise comparable. A 13% drop in the UPV was seen when the waste brick replacement rate was increased to 30%. Comparing sample 0.5RB5 to control concrete, the UPV decreased by 2%, which is comparable to concrete with a w/c ratio of 0.4. Based on the ratio of waste bricks, the increase in UPV of 90-day samples may be as much as 5% more than that of 28-day samples.

Figure 11
Recycled brick concrete’s UPV at 0.4 w/c ratio.

Figure 12
Recycled brick concrete’s UPV at 0.5 w/c ratio.

Combining Figures 11 and 12 shows that the UPV for concretes decreases by 5% to 11% with an increase in w/c ratio from 0.4 to 0.5. As the amount of waste brick in concrete grows, so does the concrete’s ability to absorb sound. Because of this, the concrete structure’s porosity rises while its unit weight decreases. According to research by OKE and ABUEL-NAGA [³²], adding fine aggregate to concrete in place of 20% brick resulted in a 7% drop in the concrete’s UPV. The results of this investigation demonstrate that the concrete with a 25% waste brick ratio had a 13% reduction in UPV values. Concrete having a UPV of > 4500 m/s is categorized as good-grade concrete by the IS 13311 [³³] standard. Moreover, concrete is considered high quality if its UPV is between 3500 and 4500 m/s. Every concrete made by this standard is categorized as being of outstanding quality, except concrete that has a w/c ratio of 0.5 and contains 25% and 30% waste brick replacements. Concrete in the good grade category contained 25% and 30% waste brick replacements. It has been demonstrated in this study that using up to 30% waste brick in concrete to replace both fine and coarse particles can result in concrete of acceptable or exceptional quality according to the UPV test.

3.6. Dynamic elasticity modulus

Concrete density values and UPV were utilized to calculate the dynamic elasticity modulus, and they were then entered into Equation (2). The Poisson ratio should be 0.2 throughout the whole computation. Figures 13 and 14 display the concrete’s dynamic modulus of elasticity. Both figures show that as the ratios of waste brick replacement increased, the dynamic modulus of elasticity of concrete reduced. Furthermore, the concretes’ dynamic modulus of elasticity was found to have increased as a result of the curing durations being extended from 28 to 90 days. Figure 13 demonstrates that the control concrete (0.4C) with 63.4 GPa had the highest dynamic modulus of elasticity among the 28-day concrete samples. Furthermore, the elasticity sample’s lowest dynamic modulus is 0.4RB30, indicating a dynamic elasticity of 44.6 GPa and a waste brick replacement rate of 30%.

Figure 13
The elasticity modulus of concrete replaced with bricks at 0.4 w/c.

Figure 14
The elasticity modulus of concrete replaced with bricks at 0.5 w/c.

The replacement of brick from 0 to 30% decreases the dynamic modulus of elasticity up to 34% compared to the control concrete mix. Regardless of the brick ratio, the experimental results indicate that concrete cured over 90 days might have an increase in dynamic modulus of elasticity of up to 11% when compared to 28 days. The concrete in Figure 14 has a w/c ratio of 0.5. The sample with the lowest dynamic modulus of elasticity, 0.5RB30, had 37.8 GPa after 28 days of curing, whereas the control group, 0.5C, had the greatest dynamic modulus of elasticity, 55.4 GPa. In the samples that were cured for ninety days, similar patterns are also prevalent. In comparison to the control concrete, the samples’ dynamic modulus of elasticity decreased by 33% as a result of the waste brick replacement rate being raised to 30%. Experiments show that when comparing the 28-day and 90-day curing outcomes, the samples’ dynamic modulus of elasticity might increase by up to 9%. Any percentage of waste brick replacement will result in growth. Analyzing the increase in w/c ratio from 0.4 to 0.5, the samples’ dynamic modulus of elasticity decreased by 12% to 22%, as shown in Figures 13 and 14. According to several studies, the dynamic modulus of elasticity of concrete falls when the water/cement ratio or the ratio of waste brick replacement increases [³⁴,³⁵,³⁶]. MOHAMMED et al. [³⁷] note that when the w/c ratio in recycled brick concrete increased from 0.45 to 0.55, the dynamic modulus of elasticity dropped by 14%. Additionally, when concrete samples containing 25% brick waste were compared to those without waste brick, the dynamic modulus of elasticity dropped by 42%. In this sense, the study’s findings are consistent with those found in the body of current literature.

3.7. Relationship between unit weight-UPV results

The relationship between UPV and unit weight was shown in Figure 15 for the concrete mix with a w/c ratio of 0.4. It can be observed that the concrete with a unit weight of 2200 kg/m³ indicates the concrete with a 30% ratio of waste brick replacement when the unit weight-UPV graph of concretes cured for 28 days is controlled. With a UPV value of 4747 m/s, this concrete has the lowest value. Now that the rate of waste brick replacement is increasing, the concrete has a more open and less dense internal structure, which lowers the ultrasonic pulse velocity. Waste bricks have a comparatively lower unit weight than concrete components [³⁷]. The association between unit weight and UPV for samples that were cured for 90 days shows a similar pattern. In the relationship between unit weight and UPV, the R² values for samples that were cured for 28 and 90 days, respectively, are 0.7590 and 0.7964. Given that the R² results are nearly 1, there is a significant correlation between the two variables. For samples with a w/c ratio of 0.5, the connection between unit weights and UPV is shown in Figure 16. The more waste brick replacement there is, the lower the samples’ unit weight and UPV. The R² values for samples cured at 28 and 90 days are 0.8523 and 0.9009 with unit weight and UPV, respectively. Samples with a w/c ratio of 0.4 show a significantly weaker correlation between unit weight and UPV.

Figure 15
Unit weight and UPV relationship for recycled brick concrete at 0.4 w/c ratio.

Figure 16
Unit weight and UPV relationship for recycled brick concrete at 0.5 w/c ratio.

3.8. Compressive and flexural strength connection

The connection between compressive and flexural strength dependent on the w/c ratio is shown in Figures 17 and 18. The samples’ flexural and compressive strengths are precisely proportional to one another. After 28 and 90 days of curing, the R² values for the samples with a w/c ratio of 0.4 are 0.9257 and 0.9743, respectively. In the 28-day and 90-day cure times, the R² values for the w/c ratio of 0.5 are 0.8933 and 0.9110, respectively. Consequently, samples with a high w/c ratio have a larger statistical correlation.

Figure 17
Compressive and flexural strength relationship for recycled brick concrete at 0.4 w/c ratio.

Figure 18
Compressive and flexural strength relationship for recycled brick concrete at 0.5 w/c ratio.

3.9. Compressive and split tensile strength connection

Figures 19 and 20 illustrate the relationship between compressive strength and split tensile strength, which is influenced by the w/c ratio of the samples. The data presented in these figures indicate a linear enhancement in split tensile strength with an increase in compressive strength. Thus, they have a good relationship. The two strength kinds’ respective R² values range from 0.9088 to 0.9103, depending on the ratio of cement to water and the curing period (28–90 days). Ultimately, the experimental findings indicate that the two factors have important correlations.

Figure 19
The association of compressive strength and split tensile strength in concrete with a brick replacement at a water-to-cement ratio of 0.4.

Figure 20
The association of compressive strength and split tensile strength in concrete with a brick replacement at a water-to-cement ratio of 0.5.

3.10 Microstructural examinations of concrete mix with bricks

The examination of the microstructure of hardened concrete, incorporating a 5% addition of brick aggregate as well as a control sample without this inclusion, was performed through SEM image analysis, as demonstrated in Figure 21. The results revealed that the surface of the brick aggregate in the 5% inclusion mix became rougher (Figure 21b), potentially enhancing the bond between the brick and the cement mix. Conversely, the conventional concrete mix lacking waste bricks (Figure 21a) presented a smoother surface, potentially resulting in diminished bond strength. Notably, in both cases (with and without bricks), a weak interfacial transition zone (ITZ) was observed, marked by a distinct separation line between the brick aggregates and the cement paste, which could reduce the concrete’s strength [³⁸, ³⁹]. In summary, the inclusion of waste bricks enhanced the connection between aggregates and the cement paste by creating a more abrasive surface on the brick aggregates, potentially enhancing the overall properties of concrete [⁴⁰].

Figure 21
SEM images of concrete paste mix (a) without and (b) with brick aggregates.

4. CONCLUSIONS

This study systematically evaluated the mechanical behavior of concrete incorporating recycled brick aggregates (RBA) in three distinct forms—chips, crumb, and powder—across varying water-to-cement (w/c) ratios and curing durations. The results consistently demonstrated that increasing the proportion of RBA led to measurable reductions in compressive, splitting tensile, and flexural strengths. However, an important finding is that high-strength concrete (≥50 MPa) is still achievable with 5–10% RBA substitution, particularly at a lower w/c ratio of 0.4, thereby highlighting the potential for partial aggregate replacement without significant compromise to structural performance. A key insight from the study is the sensitivity of RBA concrete to the mix design, especially the w/c ratio. Higher w/c ratios were found to exacerbate strength losses, underlining the necessity for careful water control in mixes containing recycled materials. Moreover, the Ultrasonic Pulse Velocity (UPV) results suggest that concrete with up to 30% RBA replacement can still be classified as high-quality, based on pulse transmission characteristics. Extended curing durations further enhanced the dynamic modulus of elasticity, particularly in concrete with 10% RBA, indicating improved stiffness over time. One of the most compelling aspects of the study is the strong linear correlation observed between unit weight and UPV, with R² values approaching unity at a 0.5 w/c ratio. This relationship offers a non-destructive predictive tool for assessing the quality and uniformity of RBA concrete in practical applications. Overall, this research underscores the dual benefit of incorporating recycled brick aggregates in concrete: achieving sustainable construction through the diversion of demolition waste, while maintaining acceptable mechanical performance when optimal mix parameters are applied. The findings support the feasibility of using up to 5% RBA as a sustainable and structurally sound alternative to conventional aggregates, particularly in non-load-bearing and semi-structural applications. This contributes to reducing the environmental burden of construction materials and aligns with broader goals in green engineering and circular economy initiatives.

5. BIBLIOGRAPHY

• ^[1] MAAZE, M.R., SHRIVASTAVA, S., “Selection of eco-friendly alternative brick for sustainable development, a study on technical, economic, environmental and social feasibility”, Construction & Building Materials, v. 408, pp. 133808, 2023. doi: http://doi.org/10.1016/j.conbuildmat.2023.133808.
  » https://doi.org/10.1016/j.conbuildmat.2023.133808
• ^[2] FOŘT, J., ČERNÝ, R., “Transition to circular economy in the construction industry: Environmental aspects of waste brick recycling scenarios”, Waste Management, v. 118, pp. 510–520, 2020. doi: http://doi.org/10.1016/j.wasman.2020.09.004. PubMed PMID: 32980730.
  » https://doi.org/10.1016/j.wasman.2020.09.004.
• ^[3] PURCHASE, C.K., AL ZULAYQ, D.M., O’Brien, B.T., et al, “Circular economy of construction and demolition waste: a literature review on lessons, challenges, and benefits”, Materials, v. 15, n. 1, pp. 76, 2021. doi: http://doi.org/10.3390/ma15010076. PubMed PMID: 35009222.
  » https://doi.org/10.3390/ma15010076.
• ^[4] MOASAS, A.M., AMIN, M.N., KHAN, K., et al, “A worldwide development in the accumulation of waste tires and its utilization in concrete as a sustainable construction material: a review”, Case Studies in Construction Materials, v. 17, e01677, 2022. doi: http://doi.org/10.1016/j.cscm.2022.e01677.
  » https://doi.org/10.1016/j.cscm.2022.e01677
• ^[5] JAKHAR, R., SAMEK, L., STYSZKO, K., “A comprehensive study of the impact of waste fires on the environment and health”, Sustainability, v. 15, n. 19, pp. 14241, 2023. doi: http://doi.org/10.3390/su151914241.
  » https://doi.org/10.3390/su151914241
• ^[6] PADMALOSAN, P., VANITHA, S., SAMPATH KUMAR, V., et al, “An investigation on the use of waste materials from industrial processes in clay brick production”, Materials Today: Proceedings, 2023. In press. doi: http://doi.org/10.1016/j.matpr.2023.01.238.
  » https://doi.org/10.1016/j.matpr.2023.01.238
• ^[7] POURKHORSHIDI, S., SANGIORGI, C., TORREGGIANI, D., et al, “Using recycled aggregates from construction and demolition waste in unbound layers of pavements”, Sustainability, v. 12, n. 22, pp. 9386, 2020. doi: http://doi.org/10.3390/su12229386.
  » https://doi.org/10.3390/su12229386
• ^[8] HU, J., AHMED, W., JIAO, D., “A critical review of the technical characteristics of recycled brick powder and its influence on concrete properties”, Buildings, v. 14, n. 11, pp. 3691, 2024. doi: http://doi.org/10.3390/buildings14113691.
  » https://doi.org/10.3390/buildings14113691
• ^[9] SALLI BIDECI, Ö., BIDECI, A., ASHOUR, A., “Utilization of recycled brick powder as supplementary cementitious materials: a comprehensive review”, Materials, v. 17, n. 3, pp. 637, 2024. doi: http://doi.org/10.3390/ma17030637. PubMed PMID: 38591483.
  » https://doi.org/10.3390/ma17030637.
• ^[10] ZHANG, Z., JI, Y., WANG, D., “Research progress on fiber-reinforced recycled brick aggregate concrete: a review”, Polymers, v. 15, n. 10, pp. 2316, 2023. doi: http://doi.org/10.3390/polym15102316. PubMed PMID: 37242891.
  » https://doi.org/10.3390/polym15102316.
• ^[11] WANG, D., JI, Y., XU, W., et al, “Multi-objective optimization design of recycled concrete based on the physical characteristics of aggregate”, Construction & Building Materials, v. 458, pp. 139623, 2025. doi: http://doi.org/10.1016/j.conbuildmat.2024.139623.
  » https://doi.org/10.1016/j.conbuildmat.2024.139623
• ^[12] ATASHAM UL HAQ, M., XIA, P.,KHAN, S., et al, “Characterizations and quantification of freeze-thaw behaviors of recycled brick aggregate concrete”, Journal of Building Engineering, v. 86, pp. 108821, 2024. doi: http://doi.org/10.1016/j.jobe.2024.108821.
  » https://doi.org/10.1016/j.jobe.2024.108821
• ^[13] HU, J., AHMED, W., JIAO, D., “A critical review of the technical characteristics of recycled brick powder and its influence on concrete properties”, Buildings, v. 14, n. 11, pp. 3691, 2024. doi: http://doi.org/10.3390/buildings14113691.
  » https://doi.org/10.3390/buildings14113691
• ^[14] KEERIO, M.A., ABBASI, S.A., KUMAR, A., et al, “Effect of silica fume as cementitious material and waste glass as fine aggregate replacement constituent on selected properties of concrete”, Silicon, v. 14, n. 1, pp. 165–176, 2022. doi: http://doi.org/10.1007/s12633-020-00806-6.
  » https://doi.org/10.1007/s12633-020-00806-6
• ^[15] SARAVANAN, T., DURGA DEVI, G., “Mechanical behaviour of bricks from waste material”, Materials Today: Proceedings, 2023. In press. doi: http://doi.org/10.1016/j.matpr.2023.03.581.
  » https://doi.org/10.1016/j.matpr.2023.03.581
• ^[16] VILABOA DÍAZ, A., FRANCISCO LÓPEZ, A., BELLO BUGALLO, P.M., “Analysis of biowaste-based materials in the construction sector: evaluation of thermal behaviour and Life Cycle Assessment (LCA)”, Waste and Biomass Valorization, v. 13, n. 12, pp. 4983–5004, 2022. doi: http://doi.org/10.1007/s12649-022-01820-y.
  » https://doi.org/10.1007/s12649-022-01820-y
• ^[17] HOFFMANN SAMPAIO, C., CAZACLIU, B.G., AMBRÓS, W.M., et al, “Characterization of demolished concretes with three different strengths for recycling as coarse aggregate”, Minerals, v. 11, n. 8, pp. 803, 2021. doi: http://doi.org/10.3390/min11080803.
  » https://doi.org/10.3390/min11080803
• ^[18] AMERICAN SOCIETY FOR TESTING AND MATERIALS, ASTM C143 Standard Test Method for Slump of Hydraulic-Cement Concrete, West Conshohocken, ASTM, 2020.
• ^[19] AMERICAN SOCIETY FOR TESTING AND MATERIALS, ASTM C138 Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete, West Conshohocken, ASTM, 2023.
• ^[20] AMERICAN SOCIETY FOR TESTING AND MATERIALS, ASTM C39 Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens, West Conshohocken, ASTM, 2023.
• ^[21] AMERICAN SOCIETY FOR TESTING AND MATERIALS, ASTM C78 Standard Test Method for Flexural Strength of Concrete, West Conshohocken, ASTM, 2010.
• ^[22] AMERICAN SOCIETY FOR TESTING AND MATERIALS, ASTM C172 Standard Practice for Sampling Freshly Mixed Concrete, West Conshohocken, ASTM, 2016.
• ^[23] IQBAL, M., “Zones of weakness of rubberized concrete behavior using the UPV”, Journal of Cleaner Production, v. 116, pp. 217–222, 2016. doi: http://doi.org/10.1016/j.jclepro.2015.12.096.
  » https://doi.org/10.1016/j.jclepro.2015.12.096
• ^[24] AMERICAN SOCIETY FOR TESTING AND MATERIALS, ASTM C597 Standard Test Method for Ultrasonic Pulse Velocity Through Concrete, West Conshohocken, ASTM, 2023.
• ^[25] IKECHUKWU, A.F., SHABANGU, C., “Strength and durability performance of masonry bricks produced with crushed glass and melted PET plastics”, Case Studies in Construction Materials, v. 14, e00542, 2021. doi: http://doi.org/10.1016/j.cscm.2021.e00542.
  » https://doi.org/10.1016/j.cscm.2021.e00542
• ^[26] GUO, Y., HUANG, Y., ZHANG, T., et al, “Synthesized hydrogel co-polymerized with hydrophobic n-butyl methacrylate and its impact on shrinkage mitigation and crack resistance of cement paste”, Case Studies in Construction Materials, v. 22, e04261, 2025. doi: http://doi.org/10.1016/j.cscm.2025.e04261.
  » https://doi.org/10.1016/j.cscm.2025.e04261
• ^[27] ALSHARARI, F., “Utilization of industrial, agricultural, and construction waste in cementitious composites: a comprehensive review of their impact on concrete properties and sustainable construction practices”, Materials Today Sustainability, v. 29, pp. 101080, 2025. doi: http://doi.org/10.1016/j.mtsust.2025.101080.
  » https://doi.org/10.1016/j.mtsust.2025.101080
• ^[28] ABBAS, M.M., “Recycling waste materials in construction: mechanical properties and predictive modeling of Waste-Derived cement substitutes”, Waste Management Bulletin, v. 3, n. 1, pp. 168–192, 2025. doi: http://doi.org/10.1016/j.wmb.2025.01.004.
  » https://doi.org/10.1016/j.wmb.2025.01.004
• ^[29] SUN, T., ZHANG, Y., WANG, K., et al, “Effect of waste clay bricks on the performance of cemented tailings backfill and its damage constitutive model”, Minerals, v. 13, n. 7, pp. 987, 2023. doi: http://doi.org/10.3390/min13070987.
  » https://doi.org/10.3390/min13070987
• ^[30] WANG, C., DU, Z., “Microscopic interface deterioration mechanism and damage behavior of high-toughness recycled aggregate concrete based on 4D in-situ CT experiments”, Cement and Concrete Composites, v. 153, pp. 105720, 2024. doi: http://doi.org/10.1016/j.cemconcomp.2024.105720.
  » https://doi.org/10.1016/j.cemconcomp.2024.105720
• ^[31] KANAGARAJ, B., LUBLOY, E., ANAND, N., et al, “Investigation of physical, chemical, mechanical, and microstructural properties of cement-less concrete: state-of-the-art review”, Construction & Building Materials, v. 365, pp. 130020, 2023. doi: http://doi.org/10.1016/j.conbuildmat.2022.130020.
  » https://doi.org/10.1016/j.conbuildmat.2022.130020
• ^[32] OKE, J.A., ABUEL-NAGA, H., “Assessment of a non-destructive testing method using ultrasonic pulse velocity to determine the compressive strength of rubberized bricks produced with lime kiln dust waste”, Geotechnics, v. 3, n. 4, pp. 1294–1308, 2023. doi: http://doi.org/10.3390/geotechnics3040070.
  » https://doi.org/10.3390/geotechnics3040070
• ^[33] INDIAN STANDARD, IS 13311 Part 1, Non-destructive Testing of Concrete - Methods of Test - Ultrasonic Pulse Velocity, New Delhi, ASTM, 1992.
• ^[34] SU, T., WANG, T., ZHANG, Z., et al, “Mechanical properties and frost resistance of recycled brick aggregate concrete modified by nano-SiO₂”, Nanotechnology Reviews, v. 12, n. 1, pp. 20230576, 2023. doi: http://doi.org/10.1515/ntrev-2023-0576.
  » https://doi.org/10.1515/ntrev-2023-0576
• ^[35] CUI, S., WANG, T., ZHANG, Z., et al, “Frost resistance and life prediction of recycled brick aggregate concrete with waste polypropylene fiber”, Reviews on Advanced Materials Science, v. 62, n. 1, pp. 20230154, 2023. doi: http://doi.org/10.1515/rams-2023-0154.
  » https://doi.org/10.1515/rams-2023-0154
• ^[36] JI, Y., ZHANG, H., “Frost resistance investigation of fiber reinforced recycled brick aggregate cementitious materials”, Scientific Reports, v. 12, n. 1, pp. 15311, 2022. doi: http://doi.org/10.1038/s41598-022-19006-w. PubMed PMID: 36097037.
  » https://doi.org/10.1038/s41598-022-19006-w.
• ^[37] MOHAMMED, T.U., MAHMOOD, A.H., SAKIB, A., et al, “Fresh and hardened properties of brick aggregate concrete with maximum aggregate sizes of 10 mm to 75 mm”, Construction Materials, v. 3, n. 4, pp. 337–353, 2023. doi: http://doi.org/10.3390/constrmater3040022.
  » https://doi.org/10.3390/constrmater3040022
• ^[38] JI, H., YANG, X., LUO, Z., et al, “Tensile fracture property of concrete affected by interfacial transition zone”, International Journal of Concrete Structures and Materials, v. 17, n. 1, pp. 2, 2023. doi: http://doi.org/10.1186/s40069-022-00564-2.
  » https://doi.org/10.1186/s40069-022-00564-2
• ^[39] ZHANG, Z., YAN, Y., GENG, G., “A review of micro-resolved crystochemical and mechanical probes for sustainable cement-based material studies”, npj Materials Sustainability, v. 3, n. 1, pp. 1, 2025. doi: http://doi.org/10.1038/s44296-024-00044-6.
  » https://doi.org/10.1038/s44296-024-00044-6
• ^[40] PAES, A.L.C., BOTELHO, L.C.G., XAVIER, G.C., et al, “Ground waste ceramic brick effect at different times on mortar”, Journal of Materials Research and Technology, v. 24, pp. 3513–3523, 2023. doi: http://doi.org/10.1016/j.jmrt.2023.03.191.
  » https://doi.org/10.1016/j.jmrt.2023.03.191

Publication Dates

• Publication in this collection
  26 May 2025
• Date of issue
  2025

History

• Received
  21 Feb 2025
• Accepted
  17 Apr 2025