2.1.3. Superplasticizer

2.1.3. Superplasticizer Ultra-Super Plast 470 was used throughout the casting of the TSC. The Ultra-Super Plast 470 was procured from Ultra Chemicals, Peshawar, Pakistan. The product was light Ultra-Super Plast 470 was used throughout the casting of the TSC. The Ultra-Super Plast 470 was procured from Ultra Chemicals, Peshawar, Pakistan. The product was light brown, and the specific gravity was 1.15.

### brown, and the specific gravity was 1.15. *2.2. Mixture Proportions*

*2.2. Mixture Proportions*  Six groups of concrete mixtures were prepared in total as shown in Table 3. Two mixtures were prepared with the conventional method. Four different mixtures of TSC were made. All six groups were prepared with a ratio of 1:1:2.7 (cement: fine aggregate: Six groups of concrete mixtures were prepared in total as shown in Table 3. Two mixtures were prepared with the conventional method. Four different mixtures of TSC were made. All six groups were prepared with a ratio of 1:1:2.7 (cement: fine aggregate: coarse aggregate). Control mix-N was prepared using the conventional method and contained 100% natural coarse aggregate (NCA). Control mix-R was prepared using the conventional method and contained 100% RCA. Control mix-TSCI was a TSC and was made with 100% NCA. Control mix-TSCII was a TSC and was made with 100% RCA. The fifth mix was a TSC and was prepared with 10% bagasse ash as a fractional substitution

of the cement and 100% RCA. The sixth mix was a TSC and was made with 20% bagasse ash as a fractional substitution of the cement and 100% RCA. The water-to-cement ratio (W/C) used in this experimental research was 0.5. This ratio was used for all six of the mixes. Superplasticizer was added at a rate of 1% by the weight of the cement in TSC10BA and TSC20BA.

**Table 3.** Mix types with identification based on replacement ratio.


### *2.3. Specimen Casting and Curing*

CM-N and CM-R were normal concrete mixes. The RCA used during casting of the normal concrete and the TSC were prewet. Fine aggregate and coarse aggregates were first dry mixed for one minute in a mixer. Cement was then added and dry mixed for a further minute. Finally, water was added, and the total mixing time was five minutes. Cylindrical molds of 6 inches (152.4 mm) in diameter and 12 inches (254.8) in height were used for casting as per ASTM C 31/C 31M-03. For the TSC, a pipe with a diameter of 1 inch (25.4 mm) and a height of 2 m (2000 mm) was placed in the middle of a mold. Then, a mold was filled with coarse aggregates. Following this, grout was injected from the top via a pipe. The grout was injected with the help of pressure created by gravity. As the voids in the coarse aggregate were filled, the pipe was raised. After the appearance of grout at the top of a mold, the pipe was removed from the mold. This procedure was used for all the specimens and was also used by Abdelgader [24]. After 24 h, the specimens were taken out of the molds and were kept in a water tank. Ninety specimens were prepared in total; CM-N and CM-R had three specimens each, while each mix of the TSC had twenty-one specimens.

### **3. Results and Discussions**

## *3.1. Compressive Strength*

The compressive strength of all the concrete mixes is shown in Figure 1. Each compressive strength is an average of three measurements. The compressive strength of CM-N and CM-R was determined at day 28 of the curing process, whilst the compressive strength of all the TSC mixes was determined at days 7, 28, and 56 of the curing process. The figures show that the compressive strength of CM-TSCI is at its highest among all mixes at days 7, 28, and 56 of the curing process. The compressive strength of CM-TSCI is 3.32% more than that of CM-N at 28 days Thus, the compressive strength of the TSC is greater than that of the conventional concrete. The increase in strength is due to the point-to-point contact of coarse aggregates in the TSC; stresses in the TSC are first passed through the skeleton of the coarse aggregates and then through the grout, while in normal concrete, stresses are passed through a non-homogenous matrix. Due to this phenomenon, the crack mechanism and the ultimate strength of the TSC were different [25].

The compressive strength of CM-TSCI was at its highest at 56 days of curing. When 100% RCA is used, compressive strength is decreased. The compressive strength of CM-TSCII is reduced by 14.14% when compared with CM-TSCI at 28 days, while the compressive strength of CM-R is reduced by 22.13% when compared with CM-N at 28 days [26–28]. Noritaka Morohashi [29] evaluated the compressive strength of TSC by replacing 30% of the NCA with RCA in TSC, which resulted in a reduction in compressive strength of 12.6% at 28 days. RCA has a porous structure due to the adhered mortar and absorbs more water, which makes it weaker than NCA and causes a reduction in compressive strength [30]. This

*Crystals* **2021**, *11*, x FOR PEER REVIEW 5 of 11

with compressive strength.

The compressive strength of CM-TSCI was at its highest at 56 days of curing. When

The compressive strength of the TSC mixes that had 100% RCA was improved by the addition of bagasse ash as a partial substitution of cement. The compressive strength of TSC10BA and TSC20BA was increased by 4.57% and 8.47%, respectively, when compared with CM-TSCII at 56 days of curing. The increase in strength is due to the pozzolanic reaction between calcium hydroxide and bagasse ash, which forms a calcium silicate hydrate gel [33]. Figure 2 shows the development of compressive strength of all four mixes of the TSC. Figure 2 clearly shows that the increase in compressive strength was rapid until day 7. CM-TSCII, TSC10BA, and TSC20BA show a very similar compressive strength at day 7. From day 7 to day 28, CM-TSCI shows more of an increase in compressive strength than that of the other mixes. From day 28 to day 56, the increase in compressive strength was low for all four of the mixes of the TSC. Kou, Poon, and Agrela [12] reported that there was an increase in compressive strength in RCA concrete mixes when mineral

100% RCA is used, compressive strength is decreased. The compressive strength of CM-TSCII is reduced by 14.14% when compared with CM-TSCI at 28 days, while the compressive strength of CM-R is reduced by 22.13% when compared with CM-N at 28 days [26,27,28]. Noritaka Morohashi [29] evaluated the compressive strength of TSC by replacing 30% of the NCA with RCA in TSC, which resulted in a reduction in compressive strength of 12.6% at 28 days. RCA has a porous structure due to the adhered mortar and absorbs more water, which makes it weaker than NCA and causes a reduction in compressive strength [30]. This adhered mortar also decreases the density of RCA. Tavakoli and Soroushian [31] reported that the compressive strength of concrete made with RCA is less than that of concrete made with NCA when the same w/c ratio is used. Li [32] observed in his study that the percentage of RCA in concrete has an inverse relation

adhered mortar also decreases the density of RCA. Tavakoli and Soroushian [31] reported that the compressive strength of concrete made with RCA is less than that of concrete made with NCA when the same w/c ratio is used. Li [32] observed in his study that the percentage of RCA in concrete has an inverse relation with compressive strength. admixture was used. This is mainly due to the porous nature of the RCA; mineral admixture penetrates into the pores of the RCA, which improves the ITZ bond strength between the cement paste and the RCA. Another reason for the increase in compressive strength is the filling of cracks present in the RCA with hydration products.

**Figure 1.** Compressive strength of the concrete mixes at 7, 28, and 56 days. **Figure 1.** Compressive strength of the concrete mixes at 7, 28, and 56 days.

The compressive strength of the TSC mixes that had 100% RCA was improved by the addition of bagasse ash as a partial substitution of cement. The compressive strength of TSC10BA and TSC20BA was increased by 4.57% and 8.47%, respectively, when compared with CM-TSCII at 56 days of curing. The increase in strength is due to the pozzolanic reaction between calcium hydroxide and bagasse ash, which forms a calcium silicate hydrate gel [33]. Figure 2 shows the development of compressive strength of all four mixes of the TSC. Figure 2 clearly shows that the increase in compressive strength was rapid until day 7. CM-TSCII, TSC10BA, and TSC20BA show a very similar compressive strength at day 7. From day 7 to day 28, CM-TSCI shows more of an increase in compressive strength than that of the other mixes. From day 28 to day 56, the increase in compressive strength was low for all four of the mixes of the TSC. Kou, Poon, and Agrela [12] reported that there was an increase in compressive strength in RCA concrete mixes when mineral admixture was used. This is mainly due to the porous nature of the RCA; mineral admixture penetrates into the pores of the RCA, which improves the ITZ bond strength between the cement paste and the RCA. Another reason for the increase in compressive strength is the filling of cracks present in the RCA with hydration products.

### *3.2. Compressive Strength at 250* ◦*C*

The compressive strength of the TSC mixes at 20 ◦C and 250 ◦C is demonstrated in Table 4. The compressive strength at 250 ◦C is compared with the compressive strength of the mixes at 20 ◦C to determine the decrease in compressive strength due to high temperature. All mixes of the TSC demonstrated a loss of strength at 250 ◦C. The compressive strength loss in all of the mixes was less than 5.2%. The highest strength loss was 5.13% in TSC10BA and the lowest strength loss was 3.53% in CM-TSCI. The decrease in compressive strength at high temperature was mainly due to a loss of water and dehydration of the calcium silicate hydrate. Maanser et al. reported a 4% decrease in compressive strength at 200 ◦C [34]. The decrease in compressive strength in the mixes that had RCA was slightly

more than that of CM-TSCI. This is mainly due to the weak interfacial bond between the RCA and the hardened paste in the concrete matrix. Otherwise, there is no significant difference in the strength loss between the TSC that has NCA and RCA. *Crystals* **2021**, *11*, x FOR PEER REVIEW 6 of 11

**Figure 2.** Development of compressive strength of TSC mixes. **Figure 2.** Development of compressive strength of TSC mixes.


*3.2. Compressive Strength at 250 °C* **Table 4.** Mean compressive strength with standard deviation.

### *3.3. Tensile Strength*

The results of the split tensile strength of the TSC at any given curing age are demonstrated in Figure 3. A split tensile test was only conducted on the TSC samples. Each value is an average of three measurements. It is evident from Figure 3 that the tensile strength of the TSC mixes that have RCA is less than that of the TSC mix that has NCA. The tensile

strength of CM-TSCI is highest among all the mixes of the TSC throughout all curing days. There was a rapid decrease in tensile strength when 100% RCA was used. CM-TSCII tensile strength was reduced by 26.84% when compared with CM-TSCI at 56 days of curing. Tensile strength increased with the addition of bagasse ash as a partial substitution of the cement in TSC mixes that have 100% RCA. Tensile strength showed similar behavior as that of compressive strength. Lee and Choi [35] and Padmini, Ramamurthy, and Mathews [26] reported that the tensile strength of the concrete that had RCA was less than that of the concrete that had NCA. The tensile strength of TSC10BA and TSC20BA was increased by 22.1% and 26.86%, respectively, when compared with that of CM-TSCII at 56 days of curing.

The TSC mixes prepared with bagasse ash and 100% RCA showed better results in tensile strength as compared to the TSC mix that had only 100% RCA. *Crystals* **2021**, *11*, x FOR PEER REVIEW 8 of 11

**Figure 3.** Tensile strength of concrete mixes at 7, 28, and 56 days. **Figure 3.** Tensile strength of concrete mixes at 7, 28, and 56 days.

### *3.4. Mass Loss at 250 °C 3.4. Mass Loss at 250* ◦*C*

shown in Table 7.

**(MPa)**

All the TSC mixes exhibited a mass loss due to the evacuation of free water due to an increase in temperature from 20 to 250 °C, which is demonstrated in Table 5. The decrease in mass loss is expressed in percentage form. The maximum mass loss was found in CM-TSCII, which was roughly 4.35%. The lowest mass loss was found in TSC10BA, which was 2.6%. The water present in concrete comes in three forms, which are the free, adsorbed, and bonded forms. This water escapes at high temperature and causes a mass loss. From 20 to 150 °C, a small mass loss occurs, while from 150 to 300 °C, there is an increase in mass loss. This mass loss is mainly due to dehydration of C-S-H [36]. A lesser mass loss is mainly due to the superplasticizer in TSC10BA and TSC20BA, which is due to the effect All the TSC mixes exhibited a mass loss due to the evacuation of free water due to an increase in temperature from 20 to 250 ◦C, which is demonstrated in Table 5. The decrease in mass loss is expressed in percentage form. The maximum mass loss was found in CM-TSCII, which was roughly 4.35%. The lowest mass loss was found in TSC10BA, which was 2.6%. The water present in concrete comes in three forms, which are the free, adsorbed, and bonded forms. This water escapes at high temperature and causes a mass loss. From 20 to 150 ◦C, a small mass loss occurs, while from 150 to 300 ◦C, there is an increase in mass loss. This mass loss is mainly due to dehydration of C-S-H [36]. A lesser mass loss is mainly due to the superplasticizer in TSC10BA and TSC20BA, which is due to the effect of resistances. Mean tensile strength is shown in Table 6 while percentage mass loss is shown in Table 7.

of resistances. Mean tensile strength is shown in Table 6 while percentage mass loss is

**Compressive Strength at 250 °C**

**(MPa) % Decrease**

CM-TSCII 21.63 20.67 4.43 TSC10BA 22.63 21.47 5.13 TSC20BA 23.45 22.33 4.77

**Table 5.** Compressive strength at 20 °C and 250 °C.


**Table 5.** Compressive strength at 20 ◦C and 250 ◦C.

**Table 6.** Mean tensile strength with standard deviation.


**Table 7.** Mass loss of the TSC.


### **4. Conclusions**

The use of recycled coarse aggregate and bagasse ash in two-stage concrete would be beneficial for sustainable and environmentally friendly construction. The following conclusions are deduced from the experimental investigation:


**Author Contributions:** M.F.J., conceptualization, data analysis, writing original draft preparation; A.A.D., formal analysis and modeling, conceptualization, data analysis, writing original draft preparation; S.K.U.R., supervision, review and editing; F.A., investigation and review; H.A., methodology

and review and editing; A.M., review and editing, supervision. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not Applicable.

**Informed Consent Statement:** Not Applicable.

**Acknowledgments:** We are grateful to the Department of Civil Engineering, COMSATS University Islamabad, Abbottabad Campus for their support in this research.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

