3.2.2. Tensile Strength

Three split tensile strength tests were carried out for each mix based on the specifications in [53] and the average results are presented in Figure 7, which shows how the amount of crumb rubber influenced the splitting tensile strength of rubber recycled aggregate concrete. A linear reduction is the best fit, ranging from 14.3% at 5% crumb rubber to reductions of 21.4%, 35.7% and 45.4% at 10%, 15% and 20% of crumb rubber, respectively, for rubber recycled aggregate concretes.

#### *3.3. Durability of the Recycled Aggregate Concrete with Crumb Rubber*

The samples for the concrete resistivity tests were cured for 28 days prior to testing. Two curing techniques were adopted in this study; some samples are fully submerged in tap water at a temperature of (20 ± 2) ◦C and relative humidity ≥95% while others are wrapped with damp cloth at a temperature of (20 ± 5) ◦C. The samples cured in tap water were tested under surface saturation condition. All the samples are marked at four locations

equally spaced at 90 degrees as shown in Figure 3 prior to curing process. The surface resistivity tests for surface saturated samples and air-dried samples were carried out after 28 days curing. Excess water on the samples surface cured in water were wiped off with damped cloth. The probes of the resistivity equipment are dipped in water prior to tests for air dried samples to ensure proper surface contact. Resistivity measurements of all samples were repeatedly taken at four different locations (total of eight readings on each sample) to ensure quality control applications as specified in [40].

**Figure 6.** Compressive stress strain relationships of recycled aggregate concrete with different amounts of rubber particles.

**Figure 7.** Influence of crumb rubber content on the tensile strength of recycled aggregate concrete.

The surface resistivity of concrete mixes cured for 7 and 28 days are shown in Figures 8 and 9. The chloride ion penetration of concrete mixes for rubber recycled aggregate concrete under saturated surface dried state is high based on the specifications made in Table 7. This may be attributed to the voids in recycled concrete due to the presence of crumb rubber particles, hence creating a path for ingress of fluids into the concrete. This can also be illustrated with the ultrasonic pulse velocity tests in Table 5. The ultrasonic pulse velocity of the recycled aggregate concrete reduces with the increase in crumb rubber concentration, indicating the presence of voids in the concrete.

**&RQWUROPL[ 55\$&**

**Figure 8.** Surface resistivity of concrete mixes at saturated surface dried and air-dried state for 7 days curing.


**Table 7.** Comparison of chloride penetrability levels established for standards based on electrical resistivity (AASHTO TP 95) and charged passed (ASTM C1202) [40].

However, under air dried state the durability performance of the recycled concrete with 5% crumb rubber concentration is moderate based on the Table 7 specifications. The result of the recycled concrete is also comparable to the control mix under the air-dried curing method. Hence, the use of recycled concrete containing rubber content of limited amount (5%) could be useful for construction applications, especially where exposure condition is not extreme or critical.
