1. Introduction
An engineered cementitious composite (ECC) is a type of high-performance fiber-reinforced cementitious composite (HPFRCC) with a unique property of high ductility with medium fiber content. ECCs are built to combine strong mechanical, physical, and toughness qualities even under normal or harsh conditions by using micromechanics-dependent theoretical methods [
1,
2]. An ECC is a composite material developed to allow the concrete industry to maximize the effective use of products, minimize waste, deliver economic and environmental benefits, and induce structural durability [
3,
4]. An ECC also shows high resilience to cracks, good ductility, and the ability to control crack depth, rendering it the ideal composite to improve the durability of civil infrastructures [
5]. This is because ECCs are able to form steady and multiple microcracks that considerably improve its durability in the aspects of tensile strength and ductility compared to other forms of concrete [
6]. An ECC has an efficiency of around 3–5% (1.03–1.05 times) more than the strength of conventional concrete, with respect to high tensile strength [
7]. Studies have shown that its compressive strength varies from 20 to 95 MPa, its tensile strength ranges from 4 to 12 MPa, and its compressive strain is between 0.4% and 0.65% [
8].
Although an ECC has a number of superior qualities as compared to those of conventional concrete, there are also some drawbacks that come with using this technology. First, the drying shrinkage of the ECC matrix is lower, leading to eigenstress in the composite when restrained compared to that of normal concretes. Next, regarding compression behavior, due to the absence of coarse aggregates in the ECC the elastic modulus will be lesser than that of conventional concrete as well, resulting in more strain when it attains its compressive strength [
9]. When exposed to elevated temperatures, the performance of the ECC degrades as well. Fire is one of the most dangerous structural threats, as high temperatures contribute to physical and chemical modifications that weaken the ECC’s mechanical qualities, such as strength and elasticity modulus [
10]. This is because of the physical–chemical changes of the cement paste and the aggregate, the alteration of pores, and the thermal incompatibility between the aggregate and the concrete paste causing internal microcracking.
The incorporation of crumb rubber (CR) into an ECC was observed to reduce the effects of explosion and spalling because, when rubber is melted up to 170 °C, it allows space for the escape of water vapor and helps relieve pore strain. This in return decreases the destruction on the concrete structure [
11]. Hernández-Olivares and Barluenga [
12] indicated that CR was added to lower the danger of explosive spalling at extreme temperatures. In addition, crumb rubber has a lower specific gravity, ranging from 0.51 to 1.2, a bulk density ranging from 524 kg/m
3 to 1273 kg/m
3, as well as lower water absorption, strength, and stiffness compared to fine aggregates [
13]. The adverse consequence of rubberized concrete is a reduction in strengths due to the hydrophobic action of the particles of crumb rubber which repels water and causes air pitfalls to the surface [
14]. The cumulative findings of studies revealed a significant decline in strength and stiffness properties of concrete after the application of tire rubber cement. In fact, increasing the density of concrete will also decrease as drying shrinkage is decreased [
5].
The use of rubberized aggregates in eco-friendly cementitious materials improved workability, deflection capacity, cracking behavior, impact energy, and acoustic properties [
15,
16,
17,
18]. In addition, concretes prepared with rubber aggregates were lightweight compositions with from 2% to 13% lower density than that of the control mixtures [
16,
19]. However, CR enhances dynamic resistance and durability but posts a negative impact on compressive strength, water absorptivity, and workability [
20,
21,
22,
23]. Hesami et al. [
24] showed that the ECC’s 28-day compressive strength reduces and the lower adhesion between the paste and CR is attributed to it. Aslani [
25] defined CR as voids that result in a poor linkage force between CR and the fresh paste surrounding the degenerated interfacial transition zones (ITZs). Because of its water repelling nature, CR tends to cause air voids to be entrapped and increases the number of ITZs in the cement mixture.
Previous investigations have concluded that the use of crumb rubber (CR) in an ECC harms the mechanical and durability properties due to the hydrophobic behavior of CR, which repels water around its surface [
5]. Due to CR’s hydrophobic nature and air entrapment, the bonding between the cementitious materials’ matrices and CR becomes weak and less dense and thus the quality of the ECC made with CR is inferior compared to that of the normal ECC [
26]. The resistance to chemical ingress in the ECC may be comparatively low when it is incorporated with CR as the residual mortar contains old ettringite and calcium monosulfoaluminate cement hydrates. Gesoğlu and Güneyisi [
27] reported a progressive increase in the chloride ion penetration on the partial replacement of coarse aggregate and fine aggregate by crumb rubber and rubber chips, respectively. Sofi [
28] concluded that rubberized concrete is highly resistant to aggressive environments and can be implemented in areas where there are chances of acid attack. Yung, et al. [
29] investigated the durability properties of self-compacting concrete containing waste tire rubber, which indicated that the anti-sulfate corrosion was improved with the increase of rubber content from 5% to 20% of the volume ratio.
Graphene oxide (GO) (C140.H40.O20) [
30] has been considered one of the most superior graphene derivatives for cementitious composites, because of its significant bonding to various oxygen functional groups and thus exhibiting higher reactivity with cement due to its large surface area [
31,
32]. The presence of hydrophilic functional groups in GO implies that the composites are still best dispersed [
9]. Graphene is an excellent nanofiller, but is not quite durable and is expensive to modify cement products.
Previous literature has suggested that GO–cement composites had significantly higher compressive and bending forces (over 100% depending on the formulation used) in the same mixture proportions than those of conventional cement composites [
33,
34]. This is because the ECC’s durability is enhanced by including GO, thereby optimizing the composition of the micropore, avoiding the initiation and proliferation of microcracks at the beginning, enhancing transport properties (water permeability, gas permeability, and tolerance to chloride penetration), and increasing the freezing and tanning process tolerance [
9]. The overall porosity of cement composites of 1 wt% GO was also found to be decreased from 25.21% to 10.61% [
35]. For these reasons, GO is intended to help increase the product density of calcium silica hydrate (C-S-H), reduce the porosity of the microstructure, and stabilize the composites [
36]. Research has found that specimens incorporating GO and at elevated temperature displayed increased specimen weight and dimensional stability and tolerance to spalling [
9].
Concrete is prone to certain types of chemical attacks that can severely impact its mechanical and physical properties. The penetration of different chemicals into concrete members may lead to failure such as strength loss, cracking, and corrosion of the cement paste of concrete [
37]. In the case of acid attacks on concrete, the degradation of concrete members exposed to aggressive sulfuric acid environments is a crucial durability issue that affects the life cycle performance and maintenance costs of vital civil infrastructure. Acids present in the environmental groundwater or in chemical wastewater reduce strength and corrode concrete [
38]. Moreover, concrete structures in industrial zones are susceptible to deterioration due to acid rain, of which sulfuric acid is a chief component. Portland cement concrete hydration items are alkaline and have a pH rating of 12 to 13.5 [
39]. For sulphate attacks on concrete (sodium sulphate), there are two types, namely external and internal. An external sulphate attack is the penetration of salt-bearing solutions into the concrete and forms ettringite [
40]. As for the internal attack, it occurs when the mixing components of concrete are exposed to sulphate. The gypsum present in the concrete reacts with monosulfates to form ettringite. The development of ettringitis is believed to be the primary source of the expansion and destruction of sulphate attack concrete systems [
41]. Furthermore, a chemical sulphate attack occurs when sulphate penetrates the concrete and reacts with the hydration products of the concrete. It occupies a more noteworthy volume, causing expansion inside the cement mix and bond, which at that point creates an interior and concentrated tensile stresses in hardened concrete [
42]. The inclusion of GO presents an effective solution as compared to traditional fibers as it constructs modifications and achieves better performance at the nanoscale due to its higher specific surface area and availability of larger functional groups [
43]. The addition of GO provides planes for the reaction of cement hydration products and the formation of strong covalent bonds [
44]. As a result, it enhances the structural interface and strengthens the performance of cementitious composites [
9].
Graphene oxide (GO) has been used to improve the hydration of cementitious material to make the concrete denser and more durable [
45]. The oxygenated functionalities attached make GO a highly dispersible reinforcing agent in cement matrix compared to other carbon-based nanomaterials such as carbon nanotubes, carbon nanofibers, etc., which easily agglomerates in the cement-based composites [
46,
47]. Mohammed et al. [
48] confirmed that the GO inclusion led to improvement in porosity of GO-reinforced cement-based composites, thus the resistance to chloride ion penetration increases and the sorptivity value reduces with percentage increments of GO. Previous studies have concluded that GO incorporation accelerates the hydration in cement. This may be attributed to the oxygenated functional groups attached to GO nanosheets, which makes them more approachable to cement particles, further boosting the reaction of cement with water by acting as nuclei for the cement phases [
49]. Hence, GO addition in concrete seems a promising nanomaterial for enhancing cement-based composites. Zhao, et al. [
50] reported that graphene oxide (GO) is a derivative of graphene, which can be viewed as a layer of graphene with grafted oxygen functional groups. These active functional groups prefer to participate in chemical or physical interactions, which can improve the interfacial bonding with the host materials. Lin, Wei and Hu [
44] concluded that the addition of GO provides planes for the reaction of cement hydration products and the formation of strong covalent bonds. Furthermore, Zheng, Han, Cui, Yu and Ou [
9] found that GO enhances the structural interface and strengthens the performance of cementitious composites.
Therefore, the inclusion of GO has the potential of minimizing the challenges related to the full-scale utilization of an ECC and the adverse effects of CR in an ECC by improving the weak bond between the CR and the cementitious materials’ matrices. The main aim of the current study was to determine the durability of an ECC incorporating crumb rubber and GO with respect to resistance to acid and sulphate attacks.