Behaviour of similar strength crumbed rubber concrete (CRC) mixes with different mix proportions

Highlights

• A number of similar strength Crumbed Rubber Concrete (CRC) mixes were developed.

• Mechanical properties of these CRCs were experimentally measured and compared.

• Applicability of existing guidelines for ordinary concrete on CRC was investigated.

Abstract

This paper presents the results of a series of tests aimed at comparing the mechanical behaviours of various crumbed rubber concrete (CRC) mixes of similar strength. For this purpose, a number of similar strength CRC mixes using different mix proportions and crumbed rubber content were developed. Shredded scrap truck tyre was used as the crumbed rubber for these CRC mixes. Mechanical properties such as compressive and tensile strength and their variation with time, elastic modulus, stress-strain relationships of the developed mixes were experimentally measured and compared. These experimental results were also used to examine the appropriateness of using available standard guidelines for predicting mechanical properties of ordinary concrete to CRC. The measured stress-strain behaviours were also compared with the constitutive models for ordinary concrete available in the design guidelines. The results of this study show that regardless of rubber content, similar strength CRC mixes exhibit similar mechanical properties and available standard guidelines for normal concrete can be used to predict their behaviour with similar level of accuracy of normal concrete.

Introduction

As one of the most widely used materials in the world, concrete production consumes significant amount of natural resources every year. Sources of these natural resources are gradually depleting, which is a big rising concern for the concrete industry. Extraction of natural aggregates from river beds, lakes and other water bodies for prolonged period of time have resulted in enormous environmental problems in some parts of the world [1], [2], [3]. The huge yearly consumption of concrete means that even if a small percentage of natural aggregate is replaced with recycled material, it will result in considerable savings of natural resources; this led researchers attempted to use different recycled materials in concrete during the last few decades.

Scrap tyre rubber is produced from used tyres in the automobile industry. Significant numbers of used tyres are discarded all over the world every year, and appropriate use of this ever growing waste has now become an environmental concern worldwide [4]. It is estimated that 1000 million tyres reach to end of life condition annually all over the world [5]. Therefore, use of scrap tyre particles in concrete provides twofold benefit to the environment as it helps to reduce both natural resource demand for concrete production, and supports the waste management problem of scrap tyre. Over the last two decades, many researchers have investigated the material properties of rubberised concrete. A comprehensive literature review on rubberised concrete showed that addition of rubber from scrap tyre and proportional subtraction of mineral aggregates from concrete mix reduces its compressive strength [6], [7], [8], [9], [10], [11], [12], [13], [14], tensile strength [10], [13], [15] and elastic modulus [7], [9], [11], [14], [16], [17], [18], [19]. On the other hand, addition of scrap tyre rubber in concrete is reported to improve the energy absorption capacity [20], [21], [22], [23], resistance to abrasion [9], resistance to freeze-thaw condition [9], [24] and performance under fire [9], [14] when compared against those of ordinary concrete.

Researchers have used recycled tyre in three different forms in concrete, as rubber powder [7], [10], as crumb rubber [6], [9], [13], [19], [23], [25], [26], [27] and as rubber chips [6], [9], [10], [27], [28], [29]. Rubber powder works as binder material, and can be partially replaced for cement in concrete. Crumb rubber is granulated rubber usually less than 6 mm in size, and can be used as replacement for sand in concrete manufacturing. Rubber chips are also granulated rubber but their size is larger than 6 mm and can be used as replacement for coarse aggregate in concrete. To convert used tyre to these forms of tyre particles some mechanical shredding process is required, hence, the cost of shredding increases with decrease in particle size [18]. However, it is reported that the size of the tyre particle has influence on the strength of the resulting concrete, where larger size particles produce lower strength concrete [10], [20]. Cement replacement also reported to lower energy absorption and damping ratio [20]. When coarse aggregate replacement comes to the picture, it lowers the strength considerably compared to fine aggregate replacement but enhances energy absorption capacities [30], [31]. Thus, to find an acceptable balance between cost of production and reduction of compressive strength, use of crumb rubber as a replacement for fine aggregate is recommended by many research studies [6], [11], [25]. Concrete produced from partially or fully replacing fine aggregate with similar size scrap tyre particles is usually called crumbed rubber concrete (CRC).

Despite its potential benefits, CRC is not widely adopted in the industry largely due to the perceived notion that the strength of concrete is significantly reduced by the introduction of the rubber particles [6], [7], [8], [9], [10], [11], [12], [13], [14]. The actual compressive strengths of CRC in some recently reported studies showed sufficient strengths justifying its potential for structural applications; in particular, Thomas et-al, Lijuan et-al and Liu et-al [5], [32], [33] reported rubber concrete with strength range from 40 MPa to 25 MPa. In addition to its useful compressive strength, CRC also offers enhanced energy absorption capacity [20], [21], [22], [23] which could be advantageous in designing structures subjected to dynamic and impact loading. Recently in Australia researches have investigated the performance of CRC columns under seismic loads and showed that use of CRC increases the damping ratio and energy dissipation [34]. They also showed that Confined CRC columns increase the peak strength compared to bare CRC columns [35]. The lack of appropriate design guidelines, however, doesn’t allow practicing engineers to incorporate CRCs in their design. Appropriate design rules for CRC structures are very much required to promote its use in the construction industry.

In reinforced concrete design, it is customary to assume that the structural members produced using similar strength concrete will behave similarly under structural loads irrespective of the mix proportion used. This underlying assumption has been shown to be reasonably valid for ordinary concrete through extensive research studies conducted over the last century, and all available design guidelines are developed based on this assumption as most of their equation for predicting concrete mechanical properties uses concrete strength as the only input. CRC is a new material and as such there is no research evidence currently available in the literature to assess the suitability of this principle. The current study examines whether this assumption is applicable for CRC so that the performance of the existing design guidelines available for normal concrete can be evaluated for CRC. This comparison should pave the way to develop appropriate design rules for CRC, and will allow easy adoption of this material in construction. However, comprehensive investigations are required to validate the appropriateness of the use of existing design guidelines available for normal concrete to be used for designing CRC structural members.

In this context, the present study aims to compare the mechanical properties of similar strength CRC mixes containing different proportion of rubber content. For this purpose, a number of similar strength CRC mixes using different mix proportions and crumbed rubber content were developed. The mechanical properties such as compressive and tensile strength and their variation with time, elastic modulus and stress-strain relationships of the developed mixes were experimentally measured and compared. The experimental results were also used to examine the appropriateness of using available standard models for predicting mechanical properties of ordinary concrete to CRC. The measured stress-strain behaviours were also compared with the constitutive models for ordinary concrete available in design guidelines.