Dynamic compressive behavior of a novel ultra-lightweight cement composite incorporated with rubber powder

Abstract

This paper develops a novel rubberized ultra-lightweight high ductility cement composite (RULCC) with added rubber powder and low content PE fiber (0.7%), and investigates the dynamic compressive response and failure mechanism of the RULCC both experimentally and analytically. The test program examines the dynamic compressive stress-strain relationship of the RULCC through Split Hopkinson Pressure Bar (SHPB) impact tests. The results show that the rubber powder aggregates have significant effect on the compressive strength, stress-strain relations and failure mechanism of the RULCC. A volume replacement of fine aggregates with 5%, 10% and 20% rubber power results in a reduction in static compressive strength by 29.5%, 47.7% and 60.3%, respectively. The RULCC with a low fiber content of 0.7% in volume exhibits a 3% direct tensile strain, and a 4–5% tensile strain can still be achieved after 10% rubber powder is added to the RULCC, showing a high ductility of the material. The SHPB impact test shows that the compressive strength increases with strain rate. An empirical model, taking into account of the replacement ratio of the rubber powder aggregates in the RULCC, is developed in this paper to evaluate the Dynamic Increasing Factor (DIF). The experimental and analytical studies are essential to better understand the fundamental dynamic behavior of the RULCC for its further applications in engineering applications, such as protective structures, etc.

Introduction

Concrete using lightweight aggregates, such as expanded clay/shale [1], fly ash aggregates [2], fly ash cenospheres [3], [4], perlite [5], pumice [6], are classified as Lightweight aggregate concrete (LWAC). As summarized in Huang et al [4], LWAC has an apparent density of less than, e.g., 2000 kg/m³ with a compressive strength of 8–80 MPa, 1950 kg/m³ with a compressive strength of 10–38.5 MPa and 1850 kg/m³ with a compressive strength of 17–35 MPa, respectively, as specified in JGJ 51-2002 [7], CEB-FIP 2010 and BS EN 13055-2016 [8], [9], ACI 213R-14 and ASTM C330 [10], [11]. Lightweight aggregates mainly reduce self-weight and improve thermal performance of concrete [12], [13]. LWAC can be used in industrial and building structures to reduce structural weight and the materials used in construction. LWAC also reduces the transportation and hoisting cost during construction, the gravity load on the foundation, thus the reinforcement and labor cost [1], [2], [3], [4], [5], [6], [12], [13], [14], [15]. Due to the superior performance of LWAC, it has been used in, e.g., bridges [16], prefabricated construction [17] and offshore structures [18]. To further reduce the self-weight of offshore structures, Huang et al. [3], [4], Chia et al. [19] and Wu et al. [20] developed a novel ultra-lightweight cement composite (ULCC) using fly ash cenospheres. The apparent density of the ULCC is only 1450 kg/m³ with a 28-day compressive strength of 60 MPa. To further downsize the design, they developed a novel steel-ULCC-steel sandwich composite [21], and studied the bending, shearing, compression and dynamic impact resistance of beams, plates, shells and walls made of the sandwich composite experimentally and theoretically [15], [18], [21], [22], [23], [24]. A set of design methods were also proposed. The above studies have demonstrated that the ULCC has obvious advantages, though the brittleness of the ULCC has limited its wider applications.

With the rapid development of the global economy and the automobile industry, the annual increase of waste tires over the world is currently about 8% to 10%. It is estimated that by 2020, the output of waste tires in China will reach 20 million tons, which has become an emerging issue of environmental concerns. Traditional landfill and incineration not only cause huge environmental pollution, but also are energy inefficient. In 1996, Fedroff et al. [25] pioneered in producing rubberized concrete by mixing rubber powder made from grinding waste tires, which offered a new approach to recycle waste tires and started a new research topic on rubberized concrete. Many studies have since shown that, compared with the ordinary concrete, rubberized concrete has good resistance to crack and abrasion and energy dissipation capacity [26], [27], [28], [29], [30], [31], [32], [33], [34]. As an elastomer, rubber aggregates in concrete can restrain the generation and development of cracks, thus improve the energy dissipation capacities of concrete [25], [26]. However, adding rubber may reduce compressive strength, flexural strength and workability of concrete. To achieve improved energy dissipation, while still maintain sufficient material strength, adding additional mineral admixture such as silica fume and steel or polypropylene fibers are considered as commonly used and effective methods [27], [29], [37]. Nili et al. [28] conducted drop hammer impact tests on hooked steel fiber reinforced concrete with or without silica fume using the test method specified in ACI 544 [35]. The impact resistance of the concrete with 1% steel fiber and silica fume was twenty times higher than that of the plain concrete and 2.4 times higher than that of the concrete with silica fume only. Fiber-bridging plays a significant role in preventing crack and energy dissipating in the damage process. Similar findings were also concluded by Ali et al. [27] and Gupta et al. [29]. Guo et al. [37] reported that steel slag increased the stiffness and brittleness of the concrete in the static and SHPB impact tests. Yoo and Banthia [36] also studied the impact resistance of fiber reinforced concrete. Strain-rate sensitivities of fiber reinforced concretes depend on the types of loading and the strength of matrix. Tensile impacts are more sensitive to strain rate than compressive and flexural impacts are. Higher strength concrete is less sensitive compared to lower strength concrete. Liu et al. [30] evaluated the impact behavior of rubberized concrete of different rubber particle size and content through Splitting Hopkinson Pressure Bar (SHPB) tests. The results showed that with a fixed content of rubber, the dynamic compressive strength increased as the increase of the rubber particles size. However, when the rubber content exceeded 10% of the fine aggregate by weight, the energy dissipation capacity of the concrete started to decline.

The observations and conclusions made from the previous research have suggested that rubberized concrete is particularly beneficial to structures that require high impact resistance, such as high-rise buildings, long-span bridges, offshore platforms and other mega constructions. Moreover, these structures are normally very heavy due to their large cross sections that require more reinforcement. Thus, lightweight high strength concrete is a compromising alterative due to its unique advantages, such as low density, good thermal insulation and durability [13], [14]. Unfortunately, most of the existing lightweight concrete has low strength and is brittle, which has limited their applications. Hence, demands for new lightweight cement-based materials that have high ductility and good energy consumption are increasing. Naturally, using rubber aggregates to replace fine or coarse aggregates can reduce the weight and increase impact resistance, thus have the potential to meet the demand and recycle waste tires at the same time. To the authors’ best knowledge, the research on the dynamic responses of rubberized LWAC is rare. The failure mechanism and strain rate sensitivity of the promising material also remain unclear.

This paper reports an experimental study on the development of an ultra-lightweight, high ductility cement composite with rubber powder and PE fibers. The mechanical properties of the new material, such as compressive strength, elastic modulus and damage modes under different strain rates are evaluated through static and SHPB impact tests. Furthermore, this paper proposes and validates a modified equation to predict the Dynamic Increasing Factor (DIF) of the new material.