Effects of silica fume addition and water to cement ratio on the properties of high-strength concrete after exposure to high temperatures
Introduction
Since the 1950s [1], high-strength concrete is widely produced as an appropriate substitute for normal-strength concrete (NSC). The lower water to cement ratio (w/c) and higher content of binder are needed to produce HSC. Consequently, high-range water-reducing admixtures (HRWRA) are used to achieve the required workability. Investigations of the performance of silica fume (SF) in concrete began since the 1950s [2]. During the previous decades, enormous researchers evaluated the effects of the partial replacement of cement by SF on the properties of concrete. Silica fume is a by-product resulting from the reduction of high-purity quartz with coal in electric arc furnaces in the manufacture of ferro-silicon alloys and silicon metal [3]. Nowadays, it has been well known that the use of SF can significantly improve the mechanical properties as well as durability of HSC. The very high content of amorphous silicon dioxide and very fine spherical particles are the main reasons for its high pozzolanic activity [2]. The advantages of SF caused SF being the most well-known additive material for HSC in recent years. Several research works have been done to investigate the behaviour of HSC containing SF after exposure to elevated temperatures [4], [5], [6], [7], [8], [9], [10], [11]. The main aim of these works was evaluating the potential of HSC containing SF for explosive spalling. Hertz [4] suggested 10% of cement replacing by SF as an upper limit of SF content to avoid spalling. However, different researchers have reported inconsistently the relative residual compressive strength of concretes containing SF after subjecting to high temperatures [5], [6], [7], [8], [9], [10], [11]. Hertz [5] reported that the residual compressive strengths of the special 170-MPa concretes containing 14–20% SF increased after heating up to 350 °C and then decreased rapidly at higher temperatures. Phan and Carino [6] reported that concretes containing 10% SF showed better performance than OPC concretes at 100 and 200 °C, whereas higher relative strength losses were observed in the SF concretes in comparison with the OPC concretes after subjecting to 300 and 450 °C. They showed that the relative residual strengths of concretes were increased approximately 13.6% and 6.1% by replacing of cement with 10% SF at w/c of 0.33 after heating to 100 and 200 °C, respectively, whereas the relative strengths of the SF concretes were 9.1% and 7.3% lower than those of the OPC concretes at 300 and 450 °C, respectively. Sarshar and Khoury [7] reported no significant advantages of the cement replacing by 10% SF at elevated temperatures. Poon et al. [8] observed the higher relative residual strengths of the SF concretes than those of the OPC concretes at 200 °C, whereas the relative residual strengths of all of the concretes at 400 °C were approximately the same. Above 400 °C, the SF concretes showed significant strength losses in comparison with the OPC concretes. Some investigators reported that the OPC concretes showed superior behaviour in comparison with the SF concretes under elevated temperatures [9], [10]. In terms of w/c, Phan and Carino [6] reported that the relative residual strengths (the ratio of the strength at a desired temperature to initial strength at room temperature) of concretes with w/c of 0.22 and 0.33 at the same content of SF (10% of cement) were almost similar at 100 °C. The relative residual strengths of concretes with w/c of 0.22 were 22% and 30% higher than those of concretes with w/c of 0.33 at 200 and 300 °C, respectively, whereas concrete specimens with w/c of 0.22 were exploded at 450 °C and the relative residual strength of concrete with w/c of 0.33 was 25.6% of room-temperature value. Chan et al. [11] concluded that the moisture content has a dominant influence on spalling. In general, there is conflicting data on the effect and optimum amount of SF and w/c regarding the relative residual compressive strength of HSC after exposure to elevated temperatures and consequently, more researches are needed to identify the role of SF and w/c. Thus, this experimental study was carried out to evaluate the residual compressive strength of HSCs containing different dosages of SF with different levels of w/c.
Section snippets
Materials and mix proportions
The fine aggregate was river sand with a water absorption of 0.8% and a specific gravity of 2.70. Limestone coarse aggregate with a nominal maximum size of 12.5 mm was used in this study. The coarse aggregate met the grading requirements of ASTM C 33 for 12.5–4.75 mm size aggregates; its water absorption was 0.6% and its specific gravity was 2.65. Type I ordinary Portland cement (OPC) meeting the requirements of ASTM C 150 was used in the preparation of the concrete specimens. A commercial silica
Specimen preparation and test method
The specimens were cast in the cylindrical molds of 102 mm diameter and a height of 204 mm in two layers; each layer being consolidated using a vibrating table. Mixing was according to ASTM C 192. Coarse aggregate was first added to the mixer, followed by approximately one-third of mixing water, and then the mixer was started. Fine aggregate, cement, SF, and the remaining water were added to the running mixer in a gradual manner. The mixing time for mixtures continued for 3-min. After 3-min as
Test results and discussion
The results of the compressive strength tests for all of the concretes before and after heating up to 600 °C are presented in Table 3 and Fig. 1. The relative residual compressive strengths of all five mixes after exposure to high temperatures are plotted in Fig. 2.
Conclusions
Based on the results of this experimental investigation, the following conclusions are drawn:
- 1.
The rates of strength loss were significantly higher in SF concretes, especially for the W30SF10 concretes, than those of the OPC concretes. As indicated, the residual compressive strengths of all three concretes were approximately the same at 600 °C, whereas the relative compressive strengths of concretes containing 6% and 10% SF were 6.7% and 14.1% lower than those of the OPC concretes, respectively,
Acknowledgements
The authors thank Souri, head of fireproof materials laboratory, and Safar Yousefi for his assistance in preparing and testing the specimens for the research reported in this paper. The first author wishes to extend his sincere thanks to Prof. S. Mindess, Dr. F. Behnood, Dr. A. Ansar and Dr. F. Rajabipour for their valuable guidance and advice.
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