Volume 1 Issue 8 - October 12, 2007
Residual Stress-Strain Relationship for Concrete after Exposure to High Temperature

Detartment of Architecture, NCKU gcyao@mail.ncku.edu.tw
Cement and Concrete Research, Vol. 36, No.10, pp.1999-2005, Oct., 2006

Fire in buildings often occurs in Taiwan. The statistical data colleted by the Ministry of Interior shows that about 40 building fire events occur per day in Taiwan in the last ten years. Concrete is a good fire-resistant material due to its inherent non-combustibility and poor thermal conductivity. It has never been heard in Taiwan that any RC building collapsed during a fire. Since concrete structures usually have good fire performance, post-fire repair is preferable to demolition. Concrete buildings can be safely reused after appropriately retrofitted.

When concrete is exposed to heat, chemical and physical reactions occur at elevated temperatures, such as loss of moisture, dehydration of cement paste and decomposition of aggregate. These changes will bring a breakdown in the structure of concrete, affecting its mechanical properties. Therefore, when analyzing a concrete structure after a fire, it is essential to understand the residual mechanical properties of concrete after fire damage, especially the stress-strain relationships used to predict the entire behavior in a future strong earthquake.

Many studies have been made on the residual mechanical properties of concrete after exposure to elevated temperatures such as compressive strength, splitting tensile strength and elastic modulus. The results obtained by different works in different countries are not easy to compare quantitatively with each other, because of the differences in the materials, specimen sizes and test conditions. In addition, very little information is available on the compressive stress-strain curves of concrete after exposure to elevated temperatures.

The purposes of this paper are to establish a database of the mechanical properties of concrete after heating to temperatures up to 800˚C and to propose a single equation of the complete stress-strain curve applicable to unheated and heated concrete for different temperatures. As shown in Figs.1 and 2, the compression tests and split-cylinder tests are carried out to examine the validity of the relationships of the residual compressive strength, corresponding peak strain, elastic modulus, splitting tensile strength and stress-strain curves with temperature.

Fig. 1 Compression test
Fig. 2 Split-cylinder test
All tests are carried out on the standard concrete cylinders of 15cm diameter by 30cm height, which is more representative of the quality of normal concrete at room temperature. The specimens are made with the Portland cement of Type I and the siliceous aggregate commonly used in Taiwan. The heated specimens are tested at one month after they are cooled to room temperature.

Fig. 3 The shape of stress-strain curve varies with the temperature
From the compression tests, the shape of the stress-strain curves varies with temperature as shown in Fig. 3. As the temperature increases, the difference between initial tangent elastic modulus Eo and the secant modulus at peak stress Ep decreases. The ascending curves become more linear. When the temperature increases to above 600˚C, Eo may be less than Ep. There could be a pronounced concaveup curve at the beginning of loading due to the closing of pre-existing cracks caused by heating and cooling. Therefore, the shape of ascending curves for heated concrete is different from that for unheated concrete, and the shape varies with the temperature. Besides, as the temperature increases, the descending curves become flatter.

This paper proposes a single equation for the complete stress-strain curves of heated concrete to consider the shape varying with temperature. Compared with the experimental curves, the proposed equation is applicable to unheated and heated concrete for different temperatures as shown in Fig. 4. The relationships of the mechanical properties with maximum exposed temperature are also proposed to fit the test results, including the residual compressive strength, corresponding peak strain and elastic modulus. In addition, the relationship of splitting tensile strength with temperature is proposed to fit the results of split-cylinder tests.
Fig. 4 Comparison of proposed stress-strain curves with experimental results for fc′=40Mpa
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