**5. Conclusions**

This research analyzed the chloride penetration behavior of natural aggregate concrete and that of recycled aggregate from three different sources. An image processing technique was used to explain the results which could be summarized as follows:


**Author Contributions:** Conceptualization, W.Y.; methodology, W.Y.; software, K.N.; validation, W.Y. and K.N.; formal analysis, K.N.; investigation, K.N.; resources, K.N.; data curation, K.N.; writing—original draft preparation, K.N.; writing—review and editing, K.N.; visualization, K.N.; supervision, W.Y. project administration, K.N.; funding acquisition, K.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Robert B. Ataria \* and Yong C. Wang**

School of Mechanical, Aerospace and Civil Engineering, University of Manchester, Manchester M1 7JR, UK; yong.wang@manchester.ac.uk

**\*** Correspondence: robertataria@yahoo.com

**Abstract:** Despite extensive research studies, recycled aggregates and worn-out tyres of motor vehicles are still not fully reused and are hence disposed of in ways that are damaging to the environment. Several studies have been carried out on recycled aggregate and rubberized concrete, but very limited studies are conducted on rubber recycled aggregate concrete. This study focuses on the workability, mechanical properties and durability performance of concrete made with 100% recycled aggregates and crumb rubber at different replacement level (5%, 10%, 15% and 20%). The first stage of the study covers the effect of incorporating crumb rubber at different concentration on the workability and mechanical properties of recycled aggregate concrete. The results revealed that the workability and mechanical properties of the recycled aggregate concrete can be used for structural applications when 5% of crumb rubber are used to replace recycled aggregates. The 28-days compressive strength of the rubberized recycled aggregate concrete with 5% crumb rubber concentration is reduced by 21.1% and 32.8% when compared to recycled aggregate concrete and control concrete, respectively. The second stage of the study assesses the durability performance of the recycled aggregate concrete with 5% crumb rubber concentration. The 5% crumb rubber content for durability tests was considered because the ultrasonic pulse velocity tests revealed that the quality of the recycled aggregate concrete is questionable if the concentration of crumb rubber particles is beyond 5%. The durability performance using the surface resistivity test also shows that the chloride ion penetration of recycled aggregates concrete with 5% crumb rubber replacement is moderate using air dried curing technique and high using the water bath curing method. Hence the study suggests the use of rubber recycled aggregate concrete for applications were the exposure condition is not extreme.

**Keywords:** recycled aggregates; crumb rubber; recycled concrete; compressive strength; tensile strength; durability

## **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 [3–5], however a limited number of the studies combine both recycled materials to produce concrete [6–8]. 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 [9–12] 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

**Citation:** Ataria, R.B.; Wang, Y.C. Mechanical Properties and Durability Performance of Recycled Aggregate Concrete Containing Crumb Rubber. *Materials* **2022**, *15*, 1776. https:// doi.org/10.3390/ma15051776

Academic Editors: Daniel Ferrández Vega and Carlos Morón Fernández

Received: 18 December 2021 Accepted: 17 January 2022 Published: 26 February 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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 [9,16–18] 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 [20–24]. 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 [27–29] 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 [6–8]. 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.

#### **2. Materials**

#### *2.1. Material Used*

CEM 11/B-V 32.5N Portland fly ash cement complying with [34] was used for this study. Uncrushed natural aggregates of 10 mm size and recycled aggregates with composition shown in Table 1 were used in this work. The recycled aggregates used for this work sourced from Offerton Sand and Gravel (Manchester, UK). The recycled aggregates contain other impurities as detailed in Table 1. According to [35], the classification of recycled aggregates used in this study are low quality RC80 (recycled aggregates obtained from concrete products with 20% impurities). The water absorption rates and the densities of both the recycled and natural aggregates are shown in Table 2.

The grading of the natural aggregates, recycled aggregates are shown in Figure 1. Crumb rubber of 8 mm length and 2mm thickness with an aspect ratio of 4 from worn out vehicle tyres as shown in Figure 2 were supplied by SRC Products Ltd. (Stockport, UK).


**Table 1.** Composition of natural and recycled aggregates.

**Table 2.** Water absorption rates and densities of natural and recycled aggregates.


**Figure 1.** Grading of natural aggregates, recycled aggregates and fine sand [36].
