1. Introduction
The biggest problem facing the world now is the climate emergency, and it has been ignored for decades. Professor Raymond Pierrehumbert’s words stress the crucial situation: “Let’s get this on the table right away, without mincing words. With regard to the climate crisis, yes, it’s time to panic. We are in deep trouble.” [
1] Ice melts by billions of tonnes and this leads to a rise in the sea levels all over the world (sea levels are rising about three millimetres a year); as a result, many coastal cities will be partially or totally underwater between 2030 and 2040 [
2,
3]. Rainforests burned at a record rate and estimates showed that about 20% of the oxygen produced by the Earth’s land comes from the Amazon rainforest. Moreover, climatic emergency is one of the main reasons of flood risk [
4]; it could concentrate the hydrological cycle, which causes more intense rainfall, leading to increased frequency and severity of floods. (In the UK, the rainfall in 2015 broke records, receiving 341 mm within one day, breaking the 2009 record of 316.4 mm). Floods not only caused structural damage in roads, sewage treatment plants, and energy supplies, but they also caused economic losses—for example, flood damage costs the UK £1.3 billion every year [
5].
Pervious concrete could mitigate flood runoff effects. A pervious or porous concrete is capable of capturing water on the surface and then allowing it to infiltrate into the subgrade layer and groundwater, which is one of the best storm water management systems [
6]. The main difference between the traditional concrete and the pervious concrete mixes is eliminating or using a small fraction of fine aggregate which is responsible for the high porosity and void ratio. The strength of pervious concrete is affected by various factors (strength mainly depends on the bonding between cementitious paste and aggregate particles) including the cement content, water-to-cement ratio, type and level of compaction, and the quality and the gradation of used aggregates. The key factor of optimization of pervious concrete mix design is the balance between the strength and permeability [
7,
8]. The porosity of concrete is the reason for its permeability; the higher the porosity, the higher permeability which is defined by the infiltration rate test [
9]. However, increasing the porosity of the mixture negatively affect the compressive strength of concrete [
10]. Despite the benefits of pervious concrete, the clogging issue represents an important limitation for pervious concrete from a hydrologic perspective—for example, silt, clay-sized materials, algae, and plant roots, which block the pores created in the previous concrete leading to a reduction on the voids ratio and, consequently, the permeability of the concrete. Clogging also reduces the effective service life and impedes the widespread application of pervious concrete [
11,
12].
On the other hand, the Earth is running out of natural resources. One way to overcome this issue is recycling the waste materials into existing industries to replace the natural resources. Waste plastic and recycled rubber are considered the most abundant waste materials generated globally. Waste plastic increases from only two million tonnes annually in 1950 to about 381 million tonnes in 2015, and account for 12% of the world total municipal waste annually in 2016 [
13,
14]. In 2005, about 10 billion rubber tires were generated worldwide. In the US, four million tonnes of waste tires are generated every year [
15]. The problem is that these wastes can potentially take over a hundred years to break down when deposited in landfill sites [
16]. Another serious issue is that the plastic and rubber wastes end up getting dumped in the globe’s oceans. Polluting oceans can have catastrophic effects on marine life and ecosystems, all of which are essential for a balanced and functioning planet [
17]. However, we can lessen the negative effect of that waste by incorporating it in various industries.
Many researchers have studied various types of waste materials to be used as an alternative to natural aggregates in concrete. For example, recycled aggregate from demolished concrete [
18,
19,
20], recycled rubber tires [
21,
22,
23], post-consumer glass [
24,
25], steel slag by-products [
26,
27,
28,
29], and recycled waste plastic [
30,
31,
32,
33,
34,
35,
36,
37,
38]. Using waste plastic aggregate on concrete has been evaluated in many studied. Rahim, N.L. et al. replaced the coarse aggregate with high-density polyethylene (HDPE) by 10%, 20% and 30%, and the results showed that increasing the waste plastic replacement decrease the compressive strength by 6%, 19%, and 35%, respectively [
39]. Azad A. Mohammed et al. studied the effect of using plastic waste from PVC waste sheets to partially replace coarse aggregate or fine aggregate up to 85%. The results indicated a significant decrease in the compressive strength from 41.5 to 16.4 MPa for 85% fine aggregate replacement and from 41.5 to 8.3 MPa for 85% coarse aggregate replacement [
35]. For all studies, it was reported that using waste plastic aggregates decreased the compressive strength of the manufacturing concrete [
40,
41,
42,
43,
44,
45]. The relation between the increasing of infiltration rate and the decreasing of compressive was also reported.
Researchers also studied the effect of using recycled rubber on the properties of concrete. Sanjeev Kumar et al. studied various sizes of discarded tyre rubber (from powder to 4 mm) as aggregates replacement at different levels (from 2.5% to 20%) with different water-to-cement (W/C) ratios. Compressive strength for all mixes decreased with the addition of discarded rubber; with replacing 20% of rubber, the values declined from 33, 30, and 26.5 N/mm
2 to 20, 20, and 17 N/mm
2 with 0.40, 0.45, and 0.50 W/C, respectively [
46]. Hanbing Liu et al. used two types of waste crumb rubber as coarse and fine aggregates with four replacement levels (2%, 4%, 6%, and 8%), and the compressive strength results at 8% replacement decreased by 34% with coarse aggregate replacement, while with fine aggregate, it was 11%. A slight decrease in the permeability coefficient was also reported [
47]. Eshmaiel Ganjian et al. replaced the coarse aggregate with scrap tyre rubber by 5%, 7.5%, and 10%. The strength lost 21% by replacing 10% of scrap rubber, while the permeability increased by 150% [
48].
The main purpose of this study was to investigate the effect of using waste plastic and/or recycled rubber as a coarse aggregate replacement on the compressive strength and permeability of pervious concrete.