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Compressive strength and pore structure of high-performance concrete after exposure to high temperature up to 800°C

https://doi.org/10.1016/S0008-8846(99)00240-9Get rights and content

Abstract

An experimental program was carried out to study the mechanical properties and pore structure of high-performance concrete (HPC) and normal-strength concrete after exposure to high temperature. After the concrete specimens were subjected to a temperature of 800°C, their residual compressive strength was measured. The porosity and pore size distribution of the concrete were investigated by using mercury intrusion porosimetry. Test results show that HPC had higher residual strength, although the strength of HPC degenerated more sharply than the normal-strength concrete after exposure to high temperature. The changes in pore structure could be used to indicate the degradation of mechanical property of HPC subjected to high temperature. A model was developed by optimizing the parameters in the Ryshkewitch model to predict the relationship between porosity and the strength of HPC.

Introduction

The structure of concrete material can be approximately classified into micro- (less than 1 um), meso- (between 1 um and 1 cm), and macrolevels (greater than 1 cm) [1]. For concrete subjected to high temperature, with the increase in temperature, strength and Young's modulus decrease at macrolevel, internal structures degenerate, and microdefects develop at micro- and mesolevels. Therefore, studying the pore structure of concrete after high-temperature exposure helps to understand the mechanisms of concrete deterioration.

The addition of pozzolanic or supplementary cementing materials as partial replacements is one effective method of preparing high-performance concrete (HPC) 2, 3, 4, 5. In general, these blending materials enhance the performance of concrete through pozzolanic reaction, with microaggregate filling effect. Furthermore, the durability of concrete relating to ingress of aggressive ions (such as sulfate, chloride, etc.) is also improved due to the more compact microstructure of concrete, which slows down the diffusion of ions. However, the compact internal structure could probably lead to the reduction of fire resistance. In particular, there is a greater risk that HPC spalls at high temperature compared with conventional concrete 6, 7. Due to the potentially poor fire resistance of HPC, it might be recommended that the use of HPC should be limited in some cases unless future research is carried out to study and solve this problem. Thus, the investigation on performance of HPC subjected to high temperature is of great significance. There has been much research work on the change of macromechanical properties of concrete subjected to high temperatures 8, 9, 10, 11, 12, 13, 14; however, very little work has been carried out on the change of pore structure of HPC. Rostasy et al. investigated the pore structure of two cement mortars with compressive strength of 55 MPa at both extremely low and high temperatures [15]. Chan et al. measured the pore size distribution of hardened cement paste of one normal and two high-strength concretes after exposure to a temperature of 600°C [16], and confirmed the “coarsening” effect reported previously 11, 15. In this study, more in-depth questions are discussed about the effect of high temperature up to 800°C on the residual strength and the corresponding pore structure in several HPCs in comparison to conventional concrete.

Section snippets

Materials and methods

The materials used in this study were: ordinary Portland cement conforming to BS12:1991, river sand, crushed granite with maximum sizes of 20 and 10 mm, superplaticizer complying to the requirement of BS5075 Part3:1985, silica fume and fly ash, and steel and polypropylene fibre. The length and aspect ratio were 25 mm and 60 for the steel fibre and 19 mm and 360 for the polymer fibre. The mix designs for conventional concrete and HPC in this study are listed in Table 1.

The specimens (100 × 100 ×

Results and discussion

The change of mechanical properties of concrete subjected to high temperatures are dependent on material as well as environmental factors (such as the constituents, initial strength before exposure to high temperature, moisture content, and so on) 12, 17. Particularly, for HPC exposed to high temperature, moisture content in material or structure is one of the crucial factors, because spalling may occur due to the dense internal microstructure, which makes it difficult for water vapor transport

Conclusions

The results and conclusions are summarized as follows:

  • 1.

    Although the strength of HPC degenerated more sharply than the conventional concrete with the increase of exposed temperature, the HPC had higher residual strength.

  • 2.

    The variation of pore structure, including porosity and pore size distribution, could be used to indicate the degradation of mechanical properties of HPC subjected to high temperature.

  • 3.

    A model optimizing the parameters in the Ryshkewitch model was developed to predict the

Acknowledgments

The authors thank the Hong Kong Polytechnic University CRG project S610 for the financial support for this research work. This research was also a part of a key project supported by National Nature Science Foundation Grant No. 59938170.

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