Experimental and analytical investigation into age-dependent properties of recycled coarse aggregate and bottom ash concrete

doi:10.21203/rs.3.rs-1868625/v1

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Experimental and analytical investigation into age-dependent properties of recycled coarse aggregate and bottom ash concrete

https://doi.org/10.21203/rs.3.rs-1868625/v1

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This paper addresses the age-dependent concrete properties using recycled coarse aggregate (RCA), coal bottom ash (BA), and their combination. The RCA was prepared from the concrete blocks procured from the demolished concrete building. The RCA was used partially and fully to replace crushed rock aggregate to prepare RCA concrete. The thermal power plant BA was used partially and fully for fine aggregate replacement in RCA concrete. Firstly, the collected concrete boulders were crushed to the required RCA sizes and sieved as per the recommended standards. Then, the mix design of RCA and BA concrete of M25 grade was carried out based on the trial method. The intrinsic and surface quality of concrete was also checked using pulse velocity and rebound hammer for all concrete mixes. The mechanical properties of RCA, BA, and RCA + BA concrete are compared with the properties of conventional concrete and found in the acceptable range. The experimental results are justified with the help of microstructural analysis through scanning electron microscopy (SEM). The analytical models for predicting age-dependent compressive strength, splitting tensile strength, and static elastic modulus of RCA, BA, and RCA + BA concrete have been proposed. The analytical models for predicting the above mechanical properties of RCA, BA, and RCA + BA concrete using UPV and RHN at any age have also been suggested. The analytical models can be applied to the existing concrete industries to assess the health of buildings, where buildings are built with RCA, BA, and their combination. The present research will also contribute to sustainable development.

Concrete

recycled coarse aggregate

bottom ash

mechanical properties

SEM

analytical models

The rapid growth of infrastructural development needs a considerable volume of materials like steel, concrete, timber, etc. Out of them, concrete is one of the most extensively used construction materials for most the civil engineering projects such as concrete bridges, dams, multistoried buildings, rigid pavements, etc. across the world.

In such concrete construction, a massive amount of natural resources materials such as fine and coarse aggregate are required. However, with the boom in urban infrastructure construction, there is an environmental and economical concern about the rising and unsustainable utilization of natural resources, as well as the excessive generation of demolition and industrial waste. Several measures to increase ecological efficiency in the construction sector have been made to counteract this tendency, one of which is the reutilization of construction and demolition waste in the production of concrete.

For concrete production, 70–75% volume of aggregates is required, with 60–67% coarse aggregate and 33–40% fine aggregate. In recent decades, construction and demolition waste is rising due to expiration of structural life, renovation of old structures, or due to wars. These wastes are being significantly disposed of in landfills leading to major environmental issue. Utilizing construction waste as recycled coarse aggregates (RCA) is an efficient method to reduce demolition waste while also conserving natural resources for concrete production, ultimately reducing the burden of waste on the earth [1, 2]. In fact, avoiding the landfilling of construction waste can result in a 15% reduction in environmental damage [3]. Though concrete made with recycled aggregate possess lots of economic and environmental benefit [4], however, the greater porosity and water absorption of recycled aggregate lowers the performance of recycled aggregate-based concrete [4, 5]. The impact become severe when recycled aggregate are used as fine aggregate [6, 7]. In this regard, inclusion of bottom ash (BA) in as fine aggregate and recycled aggregate as coarse aggregate can be used in the production of concrete. Hence, natural fine and coarse aggregate consumption in concrete can be controlled by partially or fully replacing them with industrial waste such as bottom ash (BA) and recycled coarse aggregate (RCA).

The BA can be defined as heterogeneous particles consisting of minerals, magnetic and paramagnetic metals, glass, synthetic and natural ceramics, and unburned materials [8]. BA is a waste material generated from the coal-based thermal power plant and has much lower pozzolanic properties, making it unsuitable for cement industries. In todays’ world, bottom ash is disposed into the pond, possessing risks to human health and the environment. The hazardous constituents of the bottom ash migrate into the groundwater and pose a threat to living organisms [9]. In 2020-21, India alone produced 232.56 million tons of coal ash on 686.34 million tons of coal in thermal power plants [10]. Hence there is a need for safe disposal of bottom ash by substituting it with natural fine aggregate in concrete construction. Governments of different countries encourage the reuse of BA as a building material to avoid excessive landfilling and reduce the use of natural resources. Ghafoori and Buchole [11] revealed that durable concrete could be prepared by replacing fine aggregate with bottom ash. A study carried out by Aggarwal et al. [12] suggested that bottom ash concrete mixtures have low compressive strength than the control mix at all curing ages, but the variation in compressive strength after 28 days of curing is less distinct. Kim and Lee [13] identified no significant decrease in the strength of bottom ash mixtures compared to control specimens. However, there was a considerable decrease in modulus of elasticity after bottom ash incorporation.

Recycled aggregates are produced by re-processing demolished concrete buildings and structures wastes. The coarse portion of the recycled aggregate has been used to replace natural coarse aggregate for concrete production. The merits and demerits of using recycled aggregate concrete in concrete production are well identified and documented in the literature [14–18]. Every year, about one billion tons of surplus material or waste are produced from the construction and demolition buildings and infrastructures [19]. In the USA and European Union, the estimated amount of waste generated from the construction and demolition of residential and non-residential buildings was 170 and 860 million tons, respectively. In India, the quantity of waste is 23.75 million tons generated from concrete industries. Disposing such a massive volume of construction and demolition waste will solve severe environmental issues. Thus, the waste generated from the concrete building can be used as alternative materials by converting it into the required size and shape of coarse aggregate to solve disposal and environmental issues. Over the past decades, many studies have been carried out to understand the behavior and performance of modified concrete containing recycled aggregates. Kou et al. [20] studied the behavior of concrete containing recycled aggregate and natural aggregate, and Kwan et al. [21] revealed the durability and mechanical properties of concrete containing recycled aggregate. Another study observed that the concrete prepared with 30–50% RCA could achieve comparable strength to natural rock aggregate concrete made with supplementary cementitious materials [22]. However, further research is recommended to develop standards for the consumption of RCA in structural materials like concrete [23]. Another study revealed that the decrease in the compressive strength of 100% RCA concrete was varied from 15–30% compared to the conventional concrete strength [23, 24]. The RCA concrete compressive strength was 5–10% lower than those of concrete prepared with crushed rock aggregate. The Splitting tensile strength test showed that concrete possesses significant tensile strength even after replacing crushed rock coarse aggregate with RCA by 25–50% [25]. The grading of RCA was found satisfactory according to the standards of natural aggregate [26]. The mechanical properties of RCA concrete were analyzed based on a state-of-the-art review [27], and the designed experiments conducted by Ghorbel and Wardeh [28] and concluded that in power strain-softening law, for the same compressive strength, the power n will not be changed with the increase in the RCA content in concrete. The static elastic modulus of concrete (E_c) is a significant parameter determining the deformation characteristics of concrete building elements. Some past experimental studies examined the E_c containing varying percent replacement of crushed rock aggregate with RCA. The modulus of elasticity of the RCA concrete is significantly affected by the elasticity of the aggregate itself. Froudinstou-Yannas [29] found that the modulus of elasticity for RCA concrete remains as low as 60% of that of natural aggregate concrete. However, Maruyama et al. [30] found a reduction of only 20% in the modulus of elasticity for RCA concrete. Silva et al. [31] observed that the elastic modulus normally decreases with increasing RCA content. The variation in elastic modulus of concrete depends upon the properties of RCA, such as type, size, and quality of concrete demolition waste. Bui et al. [32] examined the mechanical properties of concrete prepared with 100% RCA. It was perceived that the strength in compression, splitting tensile strength, and static elastic modulus enhanced up to 33–50%, 33–41%, and 15.5–42.5%, respectively, compared to the natural rock aggregate concrete. Dimitriou et al. [33] proposed a methodology to enhance the mechanical properties of RCA concrete with the help of mineral admixtures. Verian et al. [34] reviewed past studies on using RCA to partially replace crushed rock aggregate (i.e. prepared from natural bed rock) in concrete construction. The physical properties of RCA were found to contribute to the strength and durability of concrete. Generally, most of the authors concluded that the replacement of natural aggregate with recycled aggregate leads to lower compressive strength values [18, 35], lower tensile strength [36], and lower modulus of elasticity [37] in comparison to control concrete. A review of the literature on recycled aggregate consumption in concrete construction was presented by Tam et al. [38]. It was concluded that the consumption of recycled aggregate in concrete industries could solve the construction problems and the disposal issues associated with demolition wastes. However, the existing literature on RAC shows that most of the studies were concerned with the short-term performance of RCA concrete. Only a limited number of studies have reported the long-term behavior of RAC concrete [39–45]. Past studies also concluded that further research is needed on RCA's design and durability properties.

Based on the past outcomes, no definite conclusion could be made on the utilization of RCA and BA in concrete production because of the variation in the properties of RCA concrete and BA quality. Also, no conclusive studies were found in the literature that account for the mechanical behavior of concrete having both RCA and BA. Therefore, a further investigation, including both experimental and analytical, is needed on the mechanical properties of concrete prepared with RCA, BA, and their combination with age. The present experimental and analytical study is required to control the use of natural resources in concrete construction and resolve the disposal and environmental protection issues associated with concrete demolition and industrial byproduct wastes. The ultimate objective of this research was to study the combined effect of BA and recycled concrete, the two most commonly dumped wastes, on the workability and strength properties of concrete. In this study, non-destructive tests were also carried out to analyze the properties of concrete without damaging the specimen. As for the non-destructive methods, the rebound hammer and ultrasonic pulse velocity testers are commonly used in practice. The analysis is particularly useful for developing countries with inadequate resources requiring optimization of available methods. The novelty of the present research is that the proposed analytical models for predicting compressive and tensile strength (i.e., splitting) and static elastic modulus at any age will be helpful for the designers and practicing engineers working in the design and construction of concrete buildings built with RCA, BA, and their combination. Further, the proposed analytical solutions for predicting the strength and elastic modulus of RCA and BA concrete using UPV and RHN will also be helpful to assess the strength capacity of the existing concrete buildings.

2.1 Properties of concrete materials:

In the present experimental study, grade 43 of Portland cement (OPC) was chosen as a binder. The OPC properties that conformed to the recommendation of IS 4031 [46] are given in Table 1. The river sand available locally was used as fine aggregate lying in zone-III with fineness modulus and specific gravity of 2.4 and 2.6, respectively, and crushed quartzite rock aggregate of maximum size of 20 mm was used as coarse aggregate with specific gravity, fineness modulus, and water absorption of 2.67, 6.89, and 1% respectively. The fine and coarse aggregate properties are satisfied as per IS 383 [47] The crushing and impact value of natural coarse aggregate was 19% and 17%, respectively. The potable water was used to prepare all the concrete mixes and curing of concrete specimens (i.e., underwater or submerged). The quality of water was conformed to IS 456 [48].

The coal industry-based bottom ash (BA) was used as a partial or total replacement of river bed fine aggregate. It was acquired from National Thermal Power Plant (NTPC), Dadri, Uttar Pradesh, India, as shown in Fig. 1. The recycled coarse aggregate (RCA) used as a partial or total replacement of crushed rock aggregate (i.e., prepared from natural bedrock) in concrete was prepared from demolished concrete blocks as shown in Fig. 2. These blocks were procured from a concrete building demolished after 27 years. The physical properties of RCA and BA are given in Table 2.

Table 1

Physical properties of OPC
Property	Observed value
Specific gravity	3.15
Initial setting time (min.)	50
Normal consistency (%)	30
Soundness (%)	2.1
28-day compressive strength (MPa)	43.1

Table 2

Physical properties of coal bottom ash and recycled coarse aggregate
	Observed value
Property	BA	RCA
Specific gravity	2.28	2.65
Fineness modulus	2.32	6.81
Water absorption (%)	14.8	1.8
Color	Black grey	-
Crushing value (%)	-	18
Impact value (%)	-	16

2.2 Constituents of concrete mix

A total of seven concrete mixes were prepared in the present study. Firstly, the conventional concrete mix of M25 grade was designed using the trial method, keeping in view the guidelines of IS 10262 [49] and designated as M1. The mix M1 was re-proportioned by replacing crushed natural aggregate with 50% and 100% RCA to obtain RCA concrete mix and designated as M2 and M3, respectively. Further, re-proportioning of the M1 mix was done again by replacing fine aggregate with 50% and 100% BA and designated as M4 and M5, respectively. The mix M1 was further modified by replacing the combined coarse aggregate and fine aggregate with 50% and 100% RCA and BA, respectively, and designated as M6 and M7. The quantity of cement and water to cement (w/c) ratio were not changed for all concrete mixes to examine only the effect of fine and coarse aggregate replacement with BA and RCA on the mechanical properties of such modified concrete. The details of conventional and modified concrete mixes with RCA and BA are given in Table 3.

Table 3

Concrete mix proportions
Mix ID	Replacement of RCA (%)	Replacement of BA (%)	RCA (kg/m³)	BA (kg/m³)	Cement (kg/m³)	w/c ratio	Fine aggregate (kg/m³)	Coarse aggregate (kg/m³)
M1	0	0	0	0	422	0.45	576	1168
M2	50	0	584	0			576	584
M3	100	0	1168	0			576	0
M4	0	50	0	288			288	1168
M5	0	100	0	576			0	1168
M6	50	50	584	288			288	584
M7	100	100	1168	576			0	0

2.3 Preparation and testing of specimens:

The cylindrical concrete specimens of dimensions 100 mm × 200 mm (diameter × height) were prepared to measure all concrete mixes' 28-day and 90-day mechanical properties. All the specimens were demoulded after 24 hours and cured underwater or under submerged curing conditions for 28 and 90 days, as shown in Fig. 3 (a). The standard laboratory temperature, i.e. (27^oC ± 2^oC), was maintained during the preparation of concrete mixes and curing of specimens. After 28 and 90 days of curing and drying specimens, the UPV and RHN were measured for all concrete specimens. The UPV was measured using the UPV tester, as depicted in Fig. 3 (b). The UPV tester was made by Proceq, Switzerland, with the accessories of a 58E-48 ultrasonic tester, 54 kHz type, two transducers (one receiver and one transmitter head), connecting cables, a rod for calibration, a coupling agent for filling any pores on the surface to provide proper contact between transducers and surfaces of specimen and two 1.5 V alkaline D type batteries. The RHN was measured with the help of a rebound hammer made by Proceq, Switzerland, as shown in Fig. 3 (c). The RHN tester had the specifications as 2.207 Nm (N) and 0.735 Nm (L) impact energy, 115-gram hammer mass, 0.79 N/mm (N) and 0.26 N/mm (L) spring constant, 75 mm spring extension, dimensions of the hammer were 55 × 55 × 250 mm (340 mm to the tip of the plunger). The plunger dimensions were 105 × 15φ mm/radius of a spherical tip, 25 mm, and 600 grams. Then, the specimens were tested under compression for compressive and splitting tensile strength as per the guidelines of IS 516 [50] and IS 5816 [51], respectively. The test for the elastic modulus of concrete was performed according to ASTM C469 [52]. A compressometer shown in Fig. 3 (d) was used to measure the longitudinal strain and stress for the elastic modulus of concrete. The dial gauges with the least count of 0.01 mm were attached with the compressometer at equal spacing. The capacity of the compression testing machine was 200 tons, as shown in Fig. 3 (e), which was used to determine all the mechanical properties of all conventional and modified concrete mixes. The load was increased gradually at a loading rate of 14 N/mm² per minute. The elastic modulus of all concrete mixes was measured using the secant modulus, which can be drawn by joining two points on the stress-strain curve, i.e., from origin to the point of 40% of ultimate compressive strength of corresponding concrete ($0.4{f}_{c}^{\text{'}}$). The results of the mechanical properties of each mix and at different ages were summarized based on the average of five specimens reading.

3.1 Quality check for recycled aggregate concrete

Figure 4 (a) and (b) show the relationship between the rebound number and UPV with the concrete age for all mixes. As per the guidelines provided by IS 13311-Part II [53], it should be noted that a rebound number less than 20 denotes poor quality concrete, while a value more than 40 indicates a very good hard layer of the concrete. For the concrete mixes used in the present study, the Rebound number was found to vary from a minimum of 23 to a maximum value of 30, indicating the quality of concrete to be fair and good. The variations were not significant, and the reason may be that all the tested cylindrical specimens were all surface saturated dry. The control specimen was found to give the highest rebound number value because of the uniform mixing of constituents of concrete. But, at 90 days, the rebound number of RCA concrete is in the good concrete range. The strength of modified concrete prepared with 100% RCA is higher than the 50% coarse aggregate replacement. This may be because of the hydration of old mortar pasted on the surface of RCA. However, the addition of bottom ash influenced the bond between the constituents of concrete and resulted in a further lower value of rebound number. The maximum drop-in rebound number was obtained with the 100% coarse and fine aggregate replacement was 20.7% at 28 days and 21.1% at 90 days. The addition of bottom ash resulted in poor interfacial bonding between the cement mortar and aggregate, which ultimately affected the hydration of old mortar and decreased the rebound number.

BS1881-Part 202 [54] suggested that the rebound number indicates the hardness of concrete only up to 30mm depth from the tested surface. As per IS 13311-Part II [53], if the concrete has internal micro-cracks, flaws, or heterogeneity across the cross-section, the rebound hammer number will not indicate the same. Teodoru [55] suggested that the rebound number indicates the strength of the outer concrete layer up to a depth of 30-50mm. Neville [56] suggested that the rebound hammer test should not be used alone to determine the strength of concrete and hence cannot be used as a compression strength test. The pulse velocity (km/s) and the concrete age relationship are shown in Fig. 4 (b). Using the range of UPV given in IS 13311-Part I [57], the test results pointed out that all the mixes were in the range of good to excellent quality concrete. The value of UPV was found to vary from a maximum of 4.74 km/s to a minimum of 3.86 km/s. The control mix was found to possess a higher pulse velocity. Concerning curing time, a maximum increase in pulse velocity (almost 8%) was observed in the concrete mix having 100% coarse and fine aggregate replacement, indicating that old mortar starts to hydrate with curing time and the reaction of bottom ash with calcium hydroxide. Because the availability of old mortar resulted in further hydration of the mix, 100% RCA replacement gave a higher pulse velocity magnitude than 50% RCA replacement. The test results are found in a similar pattern as discussed in the rebound hammer test, indicating the same hypothesis for all concrete mixes.

3.2 Mechanical properties of RCA

The age-dependent mechanical properties (i.e., compressive strength, splitting tensile strength, and static elastic modulus) of conventional and modified concrete prepared with RCA, BA, and their combination are presented in Fig. 5 (a), (b), and (c), respectively. Figure 5 (a) highlighted that the strength measured in compression increases with time for all concrete mixes. At 28 days, the compressive strength of concrete prepared with 50% RCA and 50% BA and the concrete with 100% RCA and 100% BA is lower than the strength of conventional concrete at both ages. However, the compressive strength of RCA concrete at 90 days is almost equal to 28-day conventional concrete strength. The 28-day compressive strength of combined RCA and BA concrete mixes replacing 50% and 100% aggregate is about 73% and 61% of conventional concrete. However, at 90 days, the cylinder compressive strength of combined RCA and BA concrete with the replacement level of 50% and 100% aggregate increases to 77% and 67% of that of 28-day conventional concrete strength. The 90-day compressive strength and elastic modulus of 50% BA concrete and 50% RCA + BA concrete are 78% and 91% of the 28-day conventional concrete strength and elastic modulus.

Further, the increase in the cylinder compressive strength of conventional concrete from 28 to 90 days is observed to be 8%, whereas the rate of growth in the cylinder compressive strength of combined RCA and BA concrete with 50% and 100% replacement of crushed rock and river bed aggregate (i.e., coarse and fine aggregate) is observed to be 14% and 16% respectively. Among the RCA and BA concrete mixes, the concrete strength of 50% coarse aggregate replacement and 50% fine aggregate replacement is higher than the concrete with 100% replacement of aggregates. Therefore, 50% aggregate replacement by RCA and BA seems to be the optimum replacement in the present experimental analysis.

From Fig. 5 (b), It can be seen that the targeted design strength was achieved for all the concrete mixes prepared with RCA. The general trend suggests that the tensile strength of conventional, RCA, and BA concrete declines as the replacement level of natural aggregate by the RCA and BA increases. Whereas the concrete prepared with 50% RCA + 50% BA and 100% RCA + 100% BA, the strength has been found lower at all ages than the strength of the conventional concrete mix. The 28-day splitting tensile strength of combined 50% and 100% RCA and BA concrete mixes is 66% and 48%, respectively, of conventional concrete. In comparison, the 90-day splitting tensile strength of RCA and BA concrete mixes with the replacement of 50% and 100% is 73% and 55%, respectively of that of 28-day conventional concrete strength. Also, the gain in concrete splitting tensile strength (i.e., from 28 to 90 days) of a combined 50% and 100% replacement of aggregates is 12.4% and 14.0%, respectively.

In Fig. 5 (c), it can be pointed out that the concrete elastic modulus with time increases with age for all concrete mixes. The elastic modulus of RCA and BA concrete is lower than the conventional concrete for all percent replacements of coarse and fine aggregate by RCA and BA, respectively, at all ages. At 28 and 90 days, the elastic modulus of 50% and 100% RCA concrete varies from 93–97% and 97–100%, respectively, of the 28-day elastic modulus of natural coarse aggregate concrete. Similarly, for 50% and 100% BA, the elastic modulus varies from 86–93% and 77–83%, respectively, and for combined 50% and 100% RCA and BA concrete, this value is 82–89% 72–82%, respectively. The variation of elastic modulus of RCA and BA concrete may be due to the variation in the material properties of RCA, BA, and their combination.

3.3 Microstructure analysis

The experimental results were supported and justified by the microstructural analysis carried out using scanning electron microscopy (SEM). Figure 6 shows the SEM images of the control mix after 90 days of curing in the magnification of 500x, 2500x, and 7000x used to compare the microstructure analysis of concrete containing bottom ash, recycled coarse aggregate, and their combination. Figure 7 (a) and (b) shows fly ash with recycled coarse aggregate, i.e., after 50% (M2) and 100% (M3) coarse aggregate replacement, respectively. Because the RCA had been subjected to stresses earlier, the samples' strength after the coarse aggregate replacement was slightly reduced compared to the conventional concrete mix. However, samples with 100% crushed rock aggregate replacement were found to render higher strength than samples with 50% crushed rock aggregate replacement. The reason may be due to the further hydration of old mortar on the RCA, resulting in improved bonding between RCA and new cement mortar. A thick mortar coating can be observed in SEM images, as shown in Fig. 7 (b), with less ettringite. The SEM pictures of concrete samples having fly ash replaced with BA in fine aggregate content, i.e., after 50% (M4) and 100% (M5) replacement of fine aggregate, can be seen in Fig. 7 (c) and (d), respectively. SEM images of the bottom ash concrete mixture indicated a weak bond between the crushed rock aggregate and cement mortar, which may be attributed to the particle size of the BA. Also, BA contains low lime and low reactive silica, resulting in lower compressive strength of BA treated samples than the conventional mix. The microstructure of concrete samples prepared with 50% and 100% replacement of natural river bed and crushed rock aggregate content (i.e., fine and coarse aggregate), i.e., M6 and M7 mixes prepared concrete samples, is shown in Fig. 8 (a) and (b). Replacement of 50% and 100% of the natural river bed and crushed rock aggregate (each in combination) reduced the strength of concrete by 14% and 16%, respectively, compared to the conventional concrete mix. This was because bottom ash requires a large amount of water for mixing, but at given water content (depending on w/c ratio), poorer interfacial bonding between the aggregate and the cement pastes was observed. The addition of bottom ash also affected the hydration of old cement mortar, resulting in a slight reduction in the strength of modified concrete compared with samples having 100% fine aggregate replacement. These results are consistent with the previous studies. The strength of these specimens can be increased after the age of 90 days as the coal bottom ash starts reacting with calcium hydroxide (Neville,1995).

Because the old mortar, which may be porous, is attached to the RCA surface, the interfacial transition zone between the old and new mortar acts as a weak link in the concrete and has low strength. A lower strength of samples with recycled aggregate is observed at the initial stages [58]. With time the old mortar hydrated and improved the concrete strength prepared with RCA. Singh and Siddiqui [59] used bottom ash with foundry sand and suggested that after the addition of bottom ash, the number of voids in the mix has reduced, and CSH gel was not widely spread hence BA concrete was found to provide reduced strength than conventional mix. The microstructural analysis highlighted the reason for the reduction in the 28-day compressive strength of BA concrete, and the reason may be attributed to the slow pozzolanic activity in BA concrete [9]. The formation of weak C-S-H gel and a higher voids percentage in RCA-BA concrete mixes might have affected the mechanical properties at the early curing age of samples [60]. The same observation has also been found in the present study, i.e., the weak generation of gel (i.e., C-S-H) and voids may be present in the old and new mortar in RCA and BA concrete mixes, reduced concrete strength of mixes prepared with river bed fine and crushed rock aggregate replacement.

The analytical models for predicting age-dependent mechanical properties (i.e., compressive strength, splitting tensile strength, and elastic modulus) of RCA, BA, and their combination are needed to minimize the construction time and assess the health of the existing structures. The results obtained from experiments are affected by the recycled coarse aggregate, bottom ash, and their combination. The reason may be the poor bonding between new and old materials and variation in recycled coarse aggregate and bottom ash properties. Therefore, in the present study, new analytical solutions for predicting mechanical properties (i.e., compressive, splitting tensile strength, and static elastic modulus) of recycled coarse aggregate and bottom ash concrete at any time have been proposed using crushing strength, UPV and RHN.

4.1 Age-dependent models for compressive and tensile strength of recycled aggregate + bottom ash concrete

The multivariable regression analysis has been carried out on experimental results for all concrete mixes. The proposed models are shown in Eqs. (1), (2), and (3) are formed on the 28-day crushing compressive strength of conventional concrete, UPV, and RHN values. It means that the models are given in Eqs. (2) and (3) are applicable at the onsite inspection to assess the structural health of the buildings in terms of their residual strength capacity at any age and replacement of recycled coarse aggregate and bottom ash. Because during structural health monitoring at different intervals of the age of the structure, it is not possible to determine the strength of concrete based on a crushing test by taking a core from the existing structure. Therefore, the proposed models given in Eqs. (2) and (3) will be helpful to determine the strength of RCA based concrete structures containing bottom ash by using UPV and RHN at any age, respectively, without knowing the laboratory based crushing strength of concrete.

$${\left({f}_{c}^{\text{'}}\right)}_{t}=\frac{{exp}\left({C}_{3}{p}_{rca}+{C}_{4}{p}_{ba}\right)t{\left({f}_{c}^{\text{'}}\right)}_{28,plain}}{{C}_{1}+{C}_{2}t}$$

$${\left({f}_{c}^{\text{'}}\right)}_{t,UPV}=\frac{{C}_{10}{exp}\left({C}_{7}{p}_{rca}+{C}_{8}{p}_{ba}+{C}_{9}UPV\right)t}{{C}_{5}+{C}_{6}t}$$

$${\left({f}_{c}^{\text{'}}\right)}_{t,RHN}=\frac{{C}_{16}{exp}\left({C}_{13}{p}_{rca}+{C}_{14}{p}_{ba}+{C}_{15}RHN\right)t}{{C}_{11}+{C}_{12}t}$$

Where, ${\left({f}_{c}^{{\prime }}\right)}_{t}$ = concrete compressive strength at age ‘t’ on cylindrical specimen (MPa); t = concrete age in days; ${\left({f}_{c}^{{\prime }}\right)}_{28,plain}$ = 28-day conventional concrete compressive strength in MPa on cylindrical specimen; ${p}_{rca}$ = recycled coarse aggregate percentage in fraction; ${p}_{ba}$ = percentage of bottom ash in fraction; ${\left({f}_{c}^{{\prime }}\right)}_{t,UPV}$ = concrete compressive strength at age ‘t’ using UPV in MPa; ${\left({f}_{c}^{{\prime }}\right)}_{t,RHN}$ = concrete compressive strength at age ‘t’ using RHN in MPa; C1 to C16 = model parameters having values 2.675, 0.947, -0.056, -0.412, 0.591, 0.918, 0.003, -0.266, 0.315, 5.921, 0.034, 0.118, 0.006, -0.290, 0.034 and 1.215 respectively.

Figure 9 (a) to (c) shows a comparison of the experimental and predicted compressive strength of RCA and BA based concrete using ${\left({f}_{c}^{{\prime }}\right)}_{28,plain}$, UPV and RHN (i.e., using Eqs. 1 to 3) respectively with age. It has been observed that 100%, 87.5%, and 94.0% data points fall within an error band of ± 5% for the strength prediction using Eq. 1, 2, and 3, respectively. The acceptability of these models can also be justified with R² values, i.e., 0.94, 0.98, and 0.98 for Eq. 1, 2, and 3, respectively. From the above model prediction analysis, it can be suggested that the concrete compressive strength can be predicted by using Eqs. (2) and (3) based on UPV and RHN, respectively, for any percent replacement of RCA and BA. These models are also applicable to the existing concrete buildings to assess strength capacity without knowing the 28-day compressive strength in the laboratory based on the crushing test.

The analysis of experimental results and the empirical relationships in design codes of different countries revealed that compressive strength could play an essential role in predicting concrete splitting tensile strength (f_spt). However, an age-dependent model for predicting f_spt of concrete prepared with RCA and BA is still needed considering compressive strength at 28-day, UPV, and RHN values. Thus, based on the multivariable regression analysis and by using models given in Eq. (1), (2), and (3), the following models presented in Eq. (4) to (6) have been proposed for predicting the splitting tensile strength of RCA, BA and their combination.

$${\left({f}_{spt}\right)}_{t}={k}_{1}{\left[{\left({f}_{c}^{\text{'}}\right)}_{t}\right]}^{{K}_{2}}$$

$${\left({f}_{spt}\right)}_{t,UPV}={k}_{3}{\left[{\left({f}_{c}^{\text{'}}\right)}_{t,UPV}\right]}^{{K}_{4}}$$

$${\left({f}_{spt}\right)}_{t,RHN}={k}_{5}{\left[{\left({f}_{c}^{\text{'}}\right)}_{t,RHN}\right]}^{{K}_{6}}$$

Where, ${\left({f}_{spt}\right)}_{t}$ = concrete splitting tensile strength at age ‘t’ in GPa; ${\left({f}_{spt}\right)}_{t,UPV}$ = concrete splitting tensile strength at age ‘t’ using UPV in GPa; ${\left({f}_{spt}\right)}_{t,RHN}$ = concrete splitting tensile strength at age ‘t’ using RHN in GPa; k₁ to k₆ = model parameters having values 0.021, 1.661, 0.026, 1.594, 0.028, and 1.568, respectively.

The comparative analysis between predicted splitting tensile strength and the measured values for all RCA and BA concrete mixes and at all ages using the proposed models as given in Eqs. (4) to (6) are presented in Figs. 10 (a) to (c) respectively. From Figs. 10 (a) to (c), it seems that 100% of data lie within the error band ± 5%, ± 10%, and ± 10% in the prediction of f_spt using the proposed models given in Eqs. 4 to 6, respectively. Thus, it can be said that the proposed models for predicting f_spt of RCA + BA based concrete are in acceptable agreement well with the measured data. Hence the models proposed in Eqs. 5 and 6 will be helpful in the prediction of the splitting tensile strength of existing concrete buildings at the age ‘t’ and any percentage of RCA, BA, and their combination.

4.2 Age-dependent models for elastic modulus of RCA + BA concrete

The current state of the art on the concrete static elastic modulus (E_c) indicates the need for a new model to predict age-dependent E_c of RCA, BA, and RCA + BA based concrete. Moreover, age-dependent formulae for E_c of concrete proposed by researchers and empirical relationships proposed by design codes of different countries are silent on the effect of RCA and BA and prediction based on UPV and RHN values. Therefore, the multivariable regression analysis has been carried out and proposed new age-dependent models for predicting E_c of concrete for any percentage replacement of natural crushed rock and river bed aggregate with RCA and BA, respectively. A model is shown in Eq. 7 to predict the E_c of RCA and BA based concrete at any age by using 28-day conventional concrete compressive strength. Similarly, the models are shown in Eqs. 8 and 9 have been proposed for predicting the E_c of RCA and BA based concrete at any age by using UPV and RHN values, respectively. Thus, it can be said that the models present in Eqs. 8 and 9 are more rational than the model given in Eq. 7 as these models are not dependent on the 28-day conventional concrete strength. Hence, the models proposed in Eqs. 8 and 9 can estimate the E_c of concrete buildings containing RCA, BA, and their combination by using UPV and RHN values at any age without knowing the conventional concrete compressive strength in the laboratory at 28 days.

${\left({E}_{c}\right)}_{t}={J}_{1}{\left[{\left({f}_{c}^{\text{'}}\right)}_{t}\right]}^{{J}_{2}}$ (7)

${\left({E}_{c}\right)}_{t,UPV}={J}_{3}{\left[{\left({f}_{c}^{\text{'}}\right)}_{t,UPV}\right]}^{{J}_{4}}$ (8)

${\left({E}_{c}\right)}_{t,RHN}={J}_{5}{\left[{\left({f}_{c}^{\text{'}}\right)}_{t,RHN}\right]}^{{J}_{6}}$ (9)

Where, ${\left({E}_{c}\right)}_{t}$ = concrete elastic modulus at age ‘t’ in GPa; ${\left({E}_{c}\right)}_{t,UPV}$ = concrete elastic modulus of at age ‘t’ using UPV in GPa; ${\left({E}_{c}\right)}_{t,RHN}$ = concrete elastic modulus at age ‘t’ using RHN in GPa; J₁ to J₆ = model parameters having values 3.739, 0.564, 3.660, 0.571, 3.665, and 0.571, respectively.

The comparison between age-dependent predicted E_s using proposed models using Eqs. 7 to 9 and the measured results for all the RCA and BA concrete mixes are shown in Figs. 11 (a) to (c), respectively. Using these models, the error band of ± 5% has also been plotted in these figures by using these models. From these Figs., it is observed that 93%, 100%, and 93% of data lie within these error bands of ± 5%, respectively. This indicates that the proposed models for predicting E_s of RCA and BA based concrete are in acceptable agreement with the experimental results and can be applied in the design and construction of concrete buildings made with RCA, BA, and their combination.

In this study, the conventional concrete has been redesigned by partial and total replacement of natural crushed rock (i.e., coarse aggregate) with RCA and natural river bed sand (i.e., fine aggregate) with coal BA to examine the mechanical properties of RCA and BA concrete and their combination at any age. Keeping in view the experimental research, the following inferences have been drawn:

The experimental outcomes on the mechanical properties of concrete prepared with RCA and BA showed that replacing natural resource aggregate with RCA and BA can make sustainable concrete construction.
The strength and modulus of RCA concrete at 90 days are almost equal to conventional concrete strength and elastic modulus made with natural aggregate at 28 days. The 90 days compressive strength and elastic modulus of 50% BA concrete and 50% RCA + BA concrete are 78% and 91% of that of 28 days conventional concrete strength and elastic modulus. The 90-day splitting tensile strength of 50% and 100% RCA concrete is higher than the 28-day conventional concrete strength. The splitting tensile strength of 50% BA concrete and 50% RCA + BA concrete at 90-day is 80% and 73% of the 28-day conventional concrete strength.
The experimental data and the proposed empirical relations for predicting age-dependent strength and elastic modulus of RCA, BA, and RCA + BA based concrete will be directly helpful in the concrete industries (i.e., design and construction of concrete buildings). The designers can use these empirical relations for fixing the preliminary dimensions of reinforced and pre-stressed concrete members made with RCA, BA, and their combination. The suggested models will be beneficial in conducting the health assessment of RCA and BA concrete buildings.
The present study will also be helpful as concrete building demolition and industrial wastes in concrete construction resolve the disposal problems and environmental issues through natural resource conservation and provide economical solutions to concrete construction.

Funding: The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Conflict of Interest Statement: On behalf of all authors, the corresponding author states that there is no conflict of interest.

Competing Interests: The authors have no relevant financial or non-financial interests to disclose.

Author Contributions:

A. Saxena: Visualization, Investigation, Methodology, Writing- Original draft

M. Shariq: Conceptualization, Software, Validation, Writing- Reviewing and Editing

S.S. Sulaiman: Data curation, Investigation, Writing- Original draft

M.A. Ansari: Supervision, Writing- Reviewing and Editing

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No competing interests reported.

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Experimental and analytical investigation into age-dependent properties of recycled coarse aggregate and bottom ash concrete

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Abstract

Figures

1. Introduction

2. Experimental Planning

2.1 Properties of concrete materials:

2.2 Constituents of concrete mix

2.3 Preparation and testing of specimens:

3. Results And Discussion

3.1 Quality check for recycled aggregate concrete

3.2 Mechanical properties of RCA

3.3 Microstructure analysis

4. Analytical Solutions

4.1 Age-dependent models for compressive and tensile strength of recycled aggregate + bottom ash concrete

4.2 Age-dependent models for elastic modulus of RCA + BA concrete

5. Conclusions

Declarations

References

Additional Declarations

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