1. Introduction
Waste tyre disposal is currently causing serious environmental issues all over the world. Every year, approximately I billion waste tyres are generated globally, with 1.6 billion new tyres being produced [
1]. Likewise, the amount of construction and demolition (C&D) wastes generated as a result of the increasing demolition of existing infrastructures is a thing of concern. It also is estimated that the UK generated 67.8 million tonnes of non-hazardous C&D waste, of which 62.6 million tonnes (92.3%) was recovered [
2]. Studies have tried to utilize recycled aggregates and crumb rubber particles in making new concrete for structural applications [
3,
4,
5], however a limited number of the studies combine both recycled materials to produce concrete [
6,
7,
8]. To tackle the twin challenges of improving properties of recycled aggregate concrete, with or without crumb rubber, and replacing concrete using natural aggregates with recycled aggregate, it is necessary to understand the properties of recycled aggregate concrete with and without crumb rubber for structural applications.
The performance of recycled aggregate concrete generally decreases [
9,
10,
11,
12] and the reduction depends on many factors such as the quality of recycled aggregates [
13], replacement level in concrete [
9,
12], water cement (w/c) ratio [
14,
15], etc. The strength reduction was also attributed to the amount of attached cement matrix on the recycled aggregates [
10,
12,
16], which causes a weak interfacial bond between the attached old cement matrix and the surrounding concrete matrix. Different methods have been devised by authors to enhance the performance of the recycled concrete. Some of these techniques include the addition of extra amounts of cement, use of super plasticizers, incorporation of fly ash, silica fume [
9,
16,
17,
18] and the two stage mixing approach [
19].
The research results so far also indicate that incorporating crumb rubber in concrete decreases the resulting concretes’ compressive and tensile strengths. This is attributed to the lower strength of rubber particles, and their weak bonding with cement paste [
20,
21,
22,
23,
24]. However, some researchers have demonstrated that if a small amount of crumb rubber (not more than 5% in volume according to [
21,
25] and not more than 3% according to [
26]) is used to replace mineral aggregates in concrete, then the rubberized concrete could maintain the same mechanical properties as concrete without crumb rubber. It is also possible to enhance the mechanical properties of rubberized concrete as suggested in [
27,
28,
29] by using silica fume. The ultrafine silica fume is believed to create a good bonding between the rubber particles and the surrounding cement paste matrix. Pre-treatment of the rubber particles by soaking in sodium hydroxide NaOH solution before incorporating them into concrete is another method of enhancing the mechanical properties of rubberised concrete, as demonstrated by [
6,
30]. This was attributed to the benefit of the NaOH solution dislodging the zinc stearate on the rubber surface thereby enhancing the bonding between the rubber powder and the concrete substrate. It was also reported by [
23,
25,
31,
32] that rubber particles with small sizes gave higher strength than coarse rubber particles. This was attributed to the formation of larger air voids in the concrete when coarse crumb rubber particles were used [
31]. Reference [
32] investigated the durability properties of rubberized concrete with up to 30% rubber content. The carbonation depth of rubberized concrete was also greater than that of conventional concrete, and it increased with the rubber content, indicating greater corrosion susceptibility. The study’s findings suggest that rubberized concrete with a rubber content of up to 15% can be used for structural components with sufficient strength and service life. Rubberized concrete is frequently used in low-value applications such as road barriers, concrete paving blocks and playground concrete works. However, using rubberized concrete in structural members is an effective way to improve ductility, which is critical for structural members, particularly in seismic areas. When compared to conventional concrete, the use of crumb rubber in precast concrete panels is also beneficial in terms of sound absorption [
33].
The current study assesses the performance of concrete made with recycled aggregate and crumb rubbers at different replacement levels. Different studies have been conducted on concrete performance either made with recycled aggregates or crumb rubber, but very limited studies utilize both recycled materials to make concrete [
6,
7,
8]. Furthermore, there is a lack of information on the durability performance of rubber recycled aggregate concrete; thus, this study aims to fill these knowledge gaps.
4. Conclusions
The effect of crumb rubber on the strength and penetration of chloride ion in recycled aggregate concrete was investigated using the surface resistivity test method. Different concentrations (5%,10%, 15% and 20%) of crumb rubber in recycled aggregate concrete were considered in the study. The results clearly shown that addition of crumb rubber reduces the compressive strength of the recycled aggregate concrete moderately when its concentration is limited to 5%. The study also observes the resistivity of chloride ions for recycled aggregate concrete with 5% crumb rubber concentration. Based on the ultrasonic pulse velocity results, the use of recycled aggregate concrete with crumb rubber content beyond 5% is questionable.
The results of the surface resistivity tests shows that the resistivity of the recycled aggregate concrete to chloride ion penetration is moderate under the air dried curing technique. However, using the water bath technique, the resistivity was found to be low based on the specifications listed in
Table 7. Hence, the study suggests the application of recycled aggregate concrete with crumb rubber in the construction industry where the exposure conditions are not extreme. The rubber recycled aggregate concrete could be useful, for example, in making concrete wall panels, concrete lintels and road kerbs where the criteria for exposure conditions for durability are not so critical.
Furthermore, the ductility of recycled concrete with crumb rubber is found to be greater than that of recycled concrete without crumb rubber. As a result, using rubberized concrete for structural members subject to seismic loads is a promising potential application. Rubberized recycled concrete is an alternative solution for meeting sustainability targets and reducing embodied carbon in the construction industry. However, its limitations are the reduction in strength and the willingness of contractors to use the concrete product on new projects due to the previously mentioned associated drawbacks.
Further Research
Further research on the durability performance of the recycled concrete with crumb rubber concentration using different mix ratios is needed. Pre-treatment of the crumb rubber particles could also be considered when assessing the durability of the recycled concrete.