*3.2. Hardened Properties*

#### 3.2.1. Compressive Strength

For each mix, three concrete cubes were tested as per specifications made in [48] and the results presented are the average values for 7, 28 and 90 days of curing. As expected, the results in Table 5 and Figure 5 reveals that addition of crumb rubber particles in the recycled aggregate concrete reduces its compressive strength. The reduction of the compressive strength was attributed to the low elastic modulus and high poisons ratio of the crumb rubber capable of initiating premature cracking under loading condition [23,49,50]. Weak bonding between the rubber particles and cement paste could also lead to reduction in compressive strength [51]. However, the recycled aggregate concrete maintains most of its properties when the crumb rubber concentration is limited to 5%.

**Table 5.** Experimental results of the concrete hardened properties.


The results of the ultrasonic pulse velocity in Table 5 were used to ascertain the quality of already cast concrete. The ultrasonic pulse velocity tests were based on the specifications made in [52]. The pulse velocity of the recycled aggregate concrete drop by 6.9% compared to reference concrete mainly due to the micro cracks sustained by the recycled aggregates during extraction. Addition of crumb rubber to the recycled aggregate concrete further drop the pulse velocity in Table 5 and is attributed to the entrapped air by the crumb rubber particles, as well as the low ultrasonic wave velocity in crumb rubber. However, based on the criteria made in Table 6, the quality of the rubber recycled aggregate concrete in Table 5 is questionable if the rubber content is more than 5%.

**Figure 5.** Influence of crumb rubber content on the compressive strength of recycled aggregate concrete.


**Table 6.** Concrete classification based on Ultrasonic pulse velocity [52].

Figure 6 shows the compressive strength of the concrete mixes subject to axial monolithic loading. The stress strain results in Figure 6 shows higher ductility of the concrete with crumb rubber particles compared to the conventional concrete.

The failure mode of the concrete mixes without crumb rubber is brittle. However, addition of crumb rubber enhances the ductility of the recycled concrete mixes. This due to the higher deformability of the crumb rubber particles used in the replacing the recycled aggregates.
