Impact resistance of basalt fiber strain-hardening cementitious composites exposed to elevated temperatures

https://doi.org/10.1016/j.conbuildmat.2020.120081Get rights and content

Highlights

  • SHCCs are developed using basalt fiber and calcium aluminate cement.

  • Tensile stress–strain relationship of SHCC at high temperature is studied.

  • Compressive stress–strain relationship of SHCC at high temperatures is studied.

  • Strain-rate effect and specific energy absorption of BF-SHCC are studied.

  • DIF and strain rate of BF-SHCC at elevated temperatures are calculated.

Abstract

In this study, a basalt fiber strain-hardening cementitious composite (BF-SHCC) with good high-temperature mechanical performance was developed, and its static and dynamic compressive properties at elevated temperatures were investigated. The BF-SHCC lost its strain-hardening behavior at 200 °C. At a constant strain rate, the dynamic compressive strength and specific energy absorption of the BF-SHCC showed an increasing trend over the temperature ranges of 20–200 and 400–600 °C. The tensile and compressive properties of the BF-SHCC depended on its microstructure.

Introduction

Over the past few decades, significant progress has been made in the development of engineered cementitious composites (ECCs) through the addition of moderate fiber contents [1]. ECCs, also known as strain-hardening cementitious composites (SHCCs), exhibit strain hardening properties that can be achieved by inducing multiple microcracks with self-controlled widths after crack initiation. SHCCs exhibit ductility comparable to that of steel fibers [2]. It has been demonstrated that the tensile strain capacity of ECC materials containing 2% polyvinyl alcohol (PVA) fiber reinforcement is 3–5% [3], [4], [5], [6], [7].The self-controlled microcrack width and high tensile ductility of ECCs, and their deformation compatibility with the existing concrete srenders them highly durable under various mechanical and environmental load conditions [1]. Therefore, ECCs are widely used for structural applications [1], [8].

With the increasing demand for ECCs in infrastructure and transportation applications, extensive efforts are being invested to improve their properties in high-temperature environments. This is because the properties of ECCs tend to deteriorate at high temperatures. It has been demonstrated that the mechanical properties of conventional cement-based composites change at high temperatures. The compressive strength and modulus of elasticity of conventional cement-based composites decrease with temperature increasing. Sakr and Hakim investigated the effects of temperature (250, 500, 750, and 950 °C) on the mechanical properties of heavy-weight concrete. They found that its residual mechanical properties were inversely proportional to the temperature increase [9]. The trend of other characteristics of ECC with temperature has also been investigated [10], [11], [12], [13], [14], [15]. Magalhães et al. [15]reported that at 250 °C, the tensile strength of SHCCs with 2% PVA fibers decreases from 2.90 to 0.92 MPa, while the strain capacity decreases from 2.98% to 0.24%. Bhatet al. [13] reported that the tensile strain capacity of ECCs decreases with a decrease in temperature to less than 200 °C with tensile strain-hardening. On the other hand, a 40% decrease is observed in the tensile strength at temperatures higher than 200 °C. Meanwhile, the compressive strength reduces significantly [11].

The deterioration of the mechanical performance of ECC/SHCCs at high temperatures is caused mainly by the constituent fibers and cement. Currently, synthetic fibers such as polyethylene (PE) [16], PVA [5] and polypropylene (PP) [17] fibers are widely used as the reinforcing and toughening agents in ECCs. However, these fibers do not exhibit sufficient temperature resistance at elevated temperatures. For instance, PVA fibers melt at about 230 °C and lose their tensile strain-hardening and porosity in cementitious composites [10], [13].On the other hand, the Portland cement is widely used as the cementitious component in ECC/SHCCs. Calcium silicate hydrate gel and calcium hydroxide are the main hydration phases that determine the performance of Portland cement slurry. These hydration products dehydrate over the temperature ranges of 105–1000 °C and 400–550 °C, respectively. The dehydration and evaporation of capillary water results in the formation of concrete with high pore pressure and explosive spalling [18]. Therefore, the melting of fibers and the dehydration of cement hydration products at elevated temperatures deteriorate the high-temperature mechanical performance of ECC/SHCCs.

Basalt fibers (BFs) have been demonstrated to show huge potential for application as the reinforcing agent in ECC/SHCCs owing to their high melting temperature (over 650 °C) and low thermal conductivity [19]. Moreover, these fibers exhibit high elastic modulus and tensile strength, and high-temperature resistance. Another approach to improve the high-temperature strength of ECCs is to replace the conventional Portland cement with novel cements exhibiting high-temperature resistance. Calcium aluminate cement (CAC) is known for its rapid strength gain, high durability, and excellent high-temperature resistance [20]. Therefore, CAC is a highly versatile cementitious material that can be used either as a binding material or a component of a blended system. The excellent high-temperature resistance of CAC can be attributed to the absence of calcium hydroxide in its hardened paste [20], [21]. As mentioned earlier, calcium hydroxide dehydrates to CaO at 400–550 °C. This transformation is a reversible reaction. Fast cooling and exposure to moisture finally lead to concrete spalling. However, the formation of calcium hydroxide in a hardened CAC paste is difficult. Although the hydration products of CAC are sensitive to temperature and dehydrate at high temperatures, the dehydrated products are relatively stable and do not react with moisture. A recent study has confirmed the feasibility of fabricating SHCCs using BFs and CAC. However, the mechanical properties of SHCCs fabricated using BFs and CAC at elevated temperatures is rarely addressed.

In this study, BFs and CAC were used to prepare BF-SHCCs. The static tensile and compressive behaviors of the BF-SHCCs at 20, 100, 200, 400, and 600 °C were investigated using a SANS universal testing machine and a 500 kN capacity MATEST system (Treviolo, Italy). The dynamic compressive behaviors of the BF-SHCCs at 20, 100, 200, 400, and 600 °C were investigated by carrying out Split-Hopkinson pressure bar (SHPB) tests. The response mechanisms of the BF-SHCCs were investigated by TGA-DSC, SEM, and XRD.

Section snippets

Mixture compositions

The composition of the BF-SHCCs is listed in Table 1. CAC, FA (class-I), SF, water, BFs, and polycarboxylate superplasticizer (used as the water reducing agent) were used. Table 2 lists the physical properties and chemical compositions of CAC, FA, and SF. The physical properties of the BFs lists in Table 3.

A Hobart HL800 mortar mixer was used to prepare the BF-SHCCs. After mixing all the ingredients evenly, the fresh mixture was cast into molds and covered with plastic sheets at 20 ± 2 °C for

Static tensile properties

Fig. 6 shows static tensile stress–strain curves of the BF-SHCC at 20 °C. The BF-SHCC consisted of 3% BFs and showed strain-hardening properties under tension. According to “Test Method for Mechanical Properties of High Ductility Fiber Reinforced Cement-based Composites” (JC/T 2461-2018) [23], cement-based composites with 3% BF exhibit the characteristics of high-ductility fiber-reinforced cement-based composites. Therefore, the BF3% samples matching material is BF-SHCC.

The static tensile

Conclusions

The main findings of the study are summarized below.

  • (1)

    The BF3% specimens showed strain-hardening (at 100 °C) and multi-slit cracking with an ultimate tensile strain of more than 0.5%.The sample lost its strain-hardening behavior with the temperature increasing to 200 °C.

  • (2)

    The static compressive strength, dynamic compressive strength, and SEA of the BF-SHCC specimens at various temperatures increased with strain rate increasing. With the temperature increasing, the dynamic compressive strength and

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (NSFC) – China under grant numbers (51878238 and 51878241).

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