Strength and Durability Characteristics of High Strength Concrete made with Scrap Tire Crumb Rubber

Abstract

The emphasis on environmental protection highlights the need to incorporate the 3R principle i.e., recycle, reuse, and reduce into the construction sector to prevent environmental degradation and make effective use of waste materials. Unrestrained human activities have led to the overexploitation of natural resources, damaging the surrounding ecosystems. It is crucial to decrease our reliance on natural resources by minimizing their use and focusing on recycling waste. The handling of waste materials encompasses a wide range of challenges, including but not limited to efficient collection and transportation, environmentally safe disposal or recycling methods, minimising health and environmental risks, implementing effective waste management policies and regulations etc. The end-of-life tires, referred to as ELTs, is one such waste material. The present study aims to use ELTs, in the form of crumb rubber in high strength concrete. The study evaluates the feasibility of incorporating crumb rubber as a partial sand replacement in high-strength concrete, with substitutions of up to 30%. Metakaolin content of 15% of weight of cement was chosen as supplementary cementitious material to achieve high strength and counter the mechanical strength losses due to presence of the crumb in the concrete mix. This content was chosen based on the initial trials performed, as 15% of metakaolin provided the highest compressive strength performance.

Various tests were performed to evaluate fresh properties i.e. workability and density. Strength and durability properties were evaluated for up to a period of 365 days. Non-destructive tests i.e., ultrasonic pulse velocity and rebound hammer, were also performed to establish their relationship with compressive strength. The performance of rubberised high strength concrete under aggressive environment, which consisted of sulfate attack (sodium sulphate), chloride attack and acid attack (sulfuric acid) was also evaluated for up to a 365 days period. The microstructural examination was also performed by using scanning electron micrographs, X-ray diffraction phase identification and thermogravimetry analysis (TGA).

The presence of crumb rubber in the high strength concrete mix reduced workability. The crumb rubber increased air void content by retaining air during mixing, leading to higher porosity and lower density. The compressive strength of rubberized high-strength concrete decreased with increasing crumb rubber content, with the lowest values at 30%. Concrete mixes with less than 20% crumb rubber achieved compressive strengths over 65 MPa at 28 days, qualifying as high-strength concrete. Strength estimation via non-destructive methods can be made more reliable using the SonReb method, which combines UPV and rebound hammer test results. Splitting tensile strength and flexural strength followed a similar loss pattern but at a slower rate compared to compressive strength. The brittle nature of high strength concrete is reduced by introducing crumb rubber in the mix as suggested by the stress-strain relationship; however, the improvement was achieved at the cost of modulus of elasticity, which decreased proportionally to the increase in the crumb rubber content. The crumb rubber content of 20% was found to be ideal for achieving a balance between compressive strength and toughness

The water absorption and sorptivity increase was observed to be proportional to the crumb rubber content in the concrete mix. The water absorption of the concrete mixes ranged from 0.38 to 2.4%, for the concrete mixes with 0 to 30% crumb rubber content. The initial surface absorption test also suggested that rubberised concrete mixes performance was lower than the reference mix in preventing water ingress. Water permeability of concrete increased proportional to the crumb rubber content. However, the penetration depth was well below 30 mm even for the highest replacement level of 30% crumb rubber, which is considered low. The abrasion resistance was satisfactory for all the mixes with wear depth remaining under 2 mm for up to 30% of crumb rubber content in HSC. The concrete’s drying shrinkage was higher for the rubberised samples with highest shrinkage value of 385 microstrain in 30% rubberised mix at 365 days. A slight improvement was seen for all the tested properties with increased curing age, among each mix category due to the densification of cement matrix from the continuation of hydration reaction. The microstructural analysis by various techniques found similar phase formation in the reference mix as well as the rubberised mixes. This is because that crumb rubber is inert in nature an do not affect the hydration process of the cement and its effect on concrete properties is primarily through the physical bonding with the cement matrix.

The rubberised mixes possessed a very low potential for chloride attack for the tested rubber content of 30% in the study. The evaluation of sulfate damage to the concrete samples by means of compressive strength and mass loss seemed to be ineffectual in detecting sulfate damage as both the tested parameters increased with the testing age. The length increase from the sulfate attack was higher for the rubberised sample with 30% crumb rubber mix depicting about 2.2 times higher expansion at 365 days sulfate solution curing period. The acid attack testing found that while the weight loss of mixes with above 10% crumb rubber content at 365 days was lower compared to the reference mix, it didn’t provide any benefit in terms of strength performance because compressive strength loss was proportional to the replacement ratio of the crumb rubber. The compressive strength loss was found in the range of 37.3% to 42.7% for 0% to 30% crumb rubber concrete mixes.