Spalling Resistance of Hybrid Polyethylene and Steel Fiber-Reinforced High-Strength Engineered Cementitious Composite

Abstract

We analyzed the effect of elevated temperatures on the integrity of high-strength engineered cementitious composite (ECC) made with a hybrid combination of polyethylene (PE) and steel fibers. The 50 mm cube specimens were subjected to temperature ranging from 200 to 800 °C at three different heating rates: 1, 5, and 10 °C/min. Five different types of mixes with varying content of supplementary cementitious materials and fibers were evaluated. No spalling was observed at 1–5 °C/min heating rate and <400 °C. However, at a heating rate of 10 °C/min for temperature 600–800 °C, all ECC specimens with a PE fiber volume of 1.25 and 1% steel fiber spalled explosively. Moreover, cementitious matrix with silica fume was more prone to spalling at 800 °C and the use of slag or quaternary blend of slag and dolomite at an optimum content was effective in maintaining the integrity of the ECC specimens even at very high heating rates. Thus, the type of cementitious matrix is equally important to consider, as well as fiber type and content, while analyzing the spalling resistance of ECC.

1 Introduction

High-strength concrete may undergo severe spalling when exposed to elevated temperatures. The use of polypropylene (PP) fibers in concrete is the most widely recognized method of preventing explosive spalling from both the economic and technical perspective. Though existing literature validates the suitability of PP fibers in spalling prevention, their sole addition may not be very effective in post-fire strength retention. In addition, the use of PP fibers in concrete may not lead to any significant improvement in the tensile performance to prevent durability-related damage. Engineered cementitious composite (ECC) is an alternate material type with superior tensile performance at normal temperatures and has also been found promising in elevated temperature scenarios. Existing studies of the fire performance of ECC have considered individual or combined roles of steel, PP, and polyvinyl alcohol (PVA) fibers [1]. The melting of PP or PVA fibers prevents the build-up of vapor pressure and, simultaneously, steel fibers can provide the required resistance to prevent mechanical decay in the higher temperature range. However, PVA fibers are not suitable for the development of ultrahigh-performance engineered cementitious composites due to their relatively low tensile strength and hydrophilic nature [2]. Another type of low-melting fiber, polyethylene (PE) fibers, have higher strength and modulus of elasticity than PP or PVA fibers and cementitious composites with PE fibers have exhibited superior ductility, strength, and energy absorption capacity.

There has been limited work on the fire performance of PE fiber-reinforced ECC and therefore, this study is a further step towards understanding the effect of elevated temperatures on the integrity or spalling resistance of high-strength ECC made with a hybrid combination of PE and steel fibers.

2 Methods

Five different types of mixes were considered to simultaneously study the effect of fiber content and type of supplementary cementitious material (SCM) on spalling resistance. The constituents of the main mix (Mix 1) used in the study were first optimized using the Grey Taguchi method and the Taguchi method with Utility Concept to obtain optimum compressive and tensile performance. More details about the optimization methodology and other mix parameters can be found Rawat et al. [3]. In addition, four other mixes were considered to study the effect of heating rate on mixes with varying ratios of SCM (slag, dolomite, and silica fume) and steel and PE fiber content as shown in Table 1. The specimens were exposed to different temperature ranges and heating rates to analyze their effects on spalling resistance and residual compressive strength.

Table 1 Mix proportions used in the present study

Fiber content is expressed as volume fraction of the mix, whereas all other constituents’ ratios are expressed as weight proportion of the cement content. FA, fly ash; HRWR, high range water reducer.

3 Brief Discussion

Figure 1 shows the normalized compressive strength of mixes 1 and 5 after exposure to different temperatures and heating rates considered in the study. In general, the residual compressive strength decreased with increase in heating rate for all types of mixes. Nevertheless, the effect was not significant, and the difference diminished at higher heating rates. Both Mix 1 and 5 had a different cementitious matrix but the same fiber content. However, all specimens of Mix 5 spalled at 800 °C, indicating that the role of thermally better performing SCMs as adopted in Mix 1 may help in improving the spalling resistance if the fiber content is sufficient.

Fig. 1

Normalized residual compressive strength of Mix 1 and Mix 5 at different temperatures and heating rates

In addition, all the specimens of Mix 3 spalled at 600 and 800 °C at 10 °C/min. Figure 2 shows the specimens of Mix 3 spalled inside the furnace, which further confirmed the efficiency of PE fibers in mitigating spalling in ECC specimens. Mix 3 had a lower content of PE fiber, which may not be sufficient to mitigate the sudden increase in vapor pressure due to the very high heating rate.

Fig. 2

Explosive spalling in Mix 3 specimens at 800 °C and 10 °C/min heating rate

4 Conclusions

We investigated the effect of heating rate on the spalling resistance of hybrid fiber-reinforced ECC and found that a mix with 1.5% PE and 0.75% steel fibers may be effective in preventing spalling even at very high heating rates. Moreover, this spalling resistance was dependent on the type of SCM used. We also observed that the use of thermally better performing SCM such as slag and dolomite can greatly improve both spalling resistance and residual compressive strength. However, a more systematic study including the role of specimen size is needed to obtain a deeper understanding of the effect of PE fiber content and the mix proportions on the spalling resistance of ECC.