*2.2. Concrete Mix and Specimen Preparation*

This study designed a reference concrete of 40 MPa cylindrical compressive strength as shown in Table 3. Crumb rubber was used to replace the coarse aggregates in different concentrations (5%, 10%, 15% and 20%). The experimental programme in Table 4 allows for the systematic investigation and comparison of the effects of different proportions of crumb rubber in recycled aggregate concrete to those of natural aggregate concrete (NAC). The percentages of replacement of recycled aggregates by crumb rubber were carefully

selected based on the recommendations made by [22,37,38] for the rubberised recycled aggregate concrete to achieve a substantial proportion of the mechanical properties of the reference concrete. In order to achieve a workable concrete, superplasticizer (1% of cement weight) was added to the recycled aggregate concrete with and without crumb rubber.

**Figure 2.** Crumb rubber (8 mm length) particles from worn out tyres.

**Table 3.** Concrete mix composition of the reference concrete.


**Table 4.** Experimental programme.


NAC- Natural aggregate concrete; RAC- Recycled aggregate concrete; RA- Recycled aggregate; RRAC5, RRAC10, RRAC15 and RRAC20- Rubber recycled aggregate concrete with 5, 10, 15 and 20 percent of crumb rubber content respectively of recycled aggregate weight.

The recycled aggregates and fine sand were incorporated into the concrete mixer and mixed for 60 s followed by crumb rubber and 50% of water for another 60 s. Cement was then added for another 30 s and finally the remaining quantity of water was added in 120 s to attain a uniform concrete mix.

#### *2.3. Concrete Resistivity of Specimens*

For regular reinforced concrete structures using recycled aggregate concrete, durability is a particular concern [9,11,39] because recycled aggregates in their natural crushed state have higher porosity and permeability than natural aggregates and are therefore more susceptible to corrosion of the reinforcement.

In this study, the concentration of crumb rubber particles in recycled aggregate concrete is limited to 5% in order to preserve its mechanical properties. Hence, the durability assessment of the recycled aggregate concrete with 5% of crumb rubber concentration will be conducted.

The surface resistivity test was used in this study to measure the durability performance (chloride ion penetration) of the recycled aggregate concrete with crumb rubber. The resistivity tests were conducted on cylindrical specimen of size φ100 × 200 mm based on the specifications presented in [40]. The four probes are placed on the surface of the cylindrical specimen to produce electrical contact as shown in Figure 3. The two external probes generate pulse of alternating current through the concrete sample and the inner probes measure the electrical potential created. The surface resistivity measurements were taken with a fixed probe spacing of 38 mm with an alternating current frequency of 13 Hz [41,42].

**Figure 3.** Surface resistivity measurements of cylindrical concrete samples. (**a**) Resistivity measurement on cylinders. (**b**) Sample markings.

#### **3. Experimental Results**

#### *3.1. Fresh Properties*

For each mix, slump tests were carried out and results presented to ascertain the workability of the concrete. The workability of all mixes was assessed by means of slump test according to [43]. The concrete was placed in a cone of 300 mm in three layers. At each layer, the concrete is compacted with 25 strokes of a tamping rod. After filling and compacting the top layer, the surface of the concrete is struck off by means of sawing and the rolling action of the compacting rod. The mould is then carefully lifted within a time interval of 2 to 5 s. Figure 4 shows the effect of the crumb rubber and recycled aggregates on the workability of concrete.

The workability of the recycled aggregate concrete was enhanced to level comparable to the reference concrete (165 mm) by adding superplasticizer (1% of the cement weight). The added superplasticizer compensates for the water absorbed by the recycled aggregates during mixing process [18,44,45]. However, adding crumb rubber reduces the workability of the recycled aggregate concrete. This can be attributed to the crumb rubber surface texture and water absorbability of the crumb rubber particles. The observed results are similar to those reported by [23,42,46].There is no significant difference in workability when 5% of the recycled aggregates were replaced with crumb rubber particles in recycled aggregate concrete. This is due to the crumb rubber's slightly higher water absorption than the replaced recycled aggregates. According to [47], the water absorption of crumb rubber less than 50mm is 6.7%, while recycled aggregates, as shown in Table 2, have a water absorption of 5.77%. Hence, 5% crumb rubber replacement level did not have influence on workability. However, a sharp reduction in workability was observed for rubber recycled aggregate concrete when more than 5% crumb rubber particles were incorporated into the recycled aggregate concrete.

**Figure 4.** Influence of crumb rubber concentration on the workability of recycled aggregate concrete.
