# Feasibility of Developing Sustainable Concrete Using Environmentally Friendly Coarse Aggregate
## 4 Results and Discussion
### Mechanical Properties
On the other hand, the dry density of combined RCA concrete increased with time, increasing from 2207 and 2334 kg/m\({}^{3}\) between 7 to 90 days, Figure 2b. Both concretes displayed 5-10% lower density compared to quarried aggregate concrete which is characteristically cited as 2400 kg/m\({}^{3}\)[62]. The efficiency factor (i.e., compressive strength/mass ratio) of Combined RCA-25 and Combined RCA-40 concretes at 28 days are 11,655 and 17,901 Nm/kg. In comparison, BCA-25 and BCA-40 concretes have an efficiency factor of 11,029 and 17,563 Nm/kg at 28 days. Given the high efficiency factor, Combined RCA concretes could provide more utility space and architectural freedom in building construction. Tensile strength development of combined RCA concrete is presented in Figure 3. The flexural strength of the combined RCA-25 and RCA-40 concretes increased from 3.35-3.73 and 3.46-4.55 MPa between 7 to 90 days, respectively. Flexural strength generally determines the failure tensile stress of a concrete beam without steel reinforcement. The flexural strength increased with time and was approximately 14.7% and 9.1% of the respective compressive strength of combined RCA-25 and RCA-40 concretes. It was noted that the flexural strength/compressive strength ratio of combined RCA-25 concrete is slightly higher than the typical range of 9-12% cited for quarried aggregate concrete. Moreover, the splitting tensile strength of all combined RCA concretes increased with time. This ranged from 2.03 to 3.15 MPa for combined RCA-25 and 3.04 to 4.07 MPa for combined RCA-40 between 7 and 90 days and varied from 8 to 12% of the respective compressive strength. According to the AS 3600 [62], Equations (1) and (2), can be used to estimate the flexural and splitting tensile strengths as a function of compressive strength. The estimated flexural strength of combined RCA-25 and RCA-40 are 2.64, 2.99, 4.00 MPa and 3.62, 3.95, 4.67 MPa, respectively, at 7, 28, 90 days. Furthermore, the estimated splitting tensile strength of combined RCA-25 and RCA-40 are 1.76, 2.00, 2.67 MPa and 2.41, 2.64, 3.12 MPa, respectively, at 7, 28, 90 days. The experimental results of both combined RCA concretes showed close resemblance with the estimated flexural strength, whereas the experimental splitting tensile strength was marginally higher than the predicted tensile strength for quarried aggregate concrete. Figure 2: (a) Compressive strength and (b) density development of combined RCA concrete. \[f^{\prime}_{\alpha f}=0.6\sqrt{f^{\prime}_{c}}\;\text{MPa} \tag{1}\]  
\[f^{\prime}_{\alpha f.sp}=0.4\sqrt{f^{\prime}_{c}}\;\text{MPa} \tag{2}\]  
where \(f^{\prime}_{\alpha f,f^{\prime}}\), \(f^{\prime}_{\alpha f,sp}\) and \(f^{\prime}_{c}\) are characteristic flexural, splitting tensile, and compressive strength, respectively. In both combined RCA concretes, the observed flexural strength is higher than the splitting tensile strength, Figure 3. This is similar to the PC concrete produced with quarried aggregates [3]. In PC concrete, the flexural tensile strength overestimates the actual tensile strength [3]. Literature has reported that actual tensile strength of concrete is about 75% of the measured flexural strength [63]. This is primarily due to loading arrangement on the tensile stress distribution for two different test procedures. The splitting tensile test generates biaxial stress distribution, as a result, the larger volume of the specimen is exposed to tensile stress and the remaining volume is subjected to horizontal compressive stress. However, in the flexural strength test, the maximum fibre stress is at the bottom fibre which is higher than the splitting test. This is due to propagation of crack at flexure loading, which is blocked by lower stressed material near to the neutral axis. Hence, available energy is less than that required to form new crack surfaces, which results in a higher tensile failure resistance. The required stress to induce unit strain in concrete at elastic limit is denoted as the modulus of elasticity.