Effect of high temperatures on normal strength concrete and high performance concrete containing steel fibers

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Effect of high temperatures on normal strength concrete and high performance concrete containing steel fibers


Author: Lau, Alan
Title: Effect of high temperatures on normal strength concrete and high performance concrete containing steel fibers
Degree: M.Phil.
Year: 2003
Subject: Hong Kong Polytechnic University -- Dissertations
Concrete -- Effect of temperature on
Department: Dept. of Civil and Structural Engineering
Pages: 124, [77] leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1692728
URI: http://theses.lib.polyu.edu.hk/handle/200/3511
Abstract: This thesis reports a study of some of the mechanical properties of steel fiber reinforced concrete (SFRC) after being subjected to different elevated heating temperatures, ranging between 105 C and 1200 C, in relation to non-reinforced concrete properties. The properties evaluated consist of compressive strength, flexural strength, and elastic modulus. Three grades of concrete, included two normal strength concrete (NSC) mixes and one high performance concrete (HPC) mix, have been studied for their behavior after exposure to different heating temperatures, and those same mixes reinforced with 1% steel fibers by volume of concrete incorporated have also been studied. The main variables taken into consideration were maximum temperature of exposure, and moisture content of heated specimens at the start of heating. In addition, changes of color to the heated concrete were observed and changes in microstructure were also studied using mercury intrusion porosimetry (MIP) analysis. The results showed that the loss of concrete strength was significant and increased with the increase of maximum heating temperature and with initial moisture content. The percentage loss occurs over a wide range of maximum heating temperatures between 105 and 1200 C. For maximum exposure temperatures below 400 C, the loss in compressive strength was relatively small. Significant further reductions in compressive strength are then steadily observed, as maximum temperature increases, for all concrete mixes heated to temperatures exceeding 400 C. HPC mixes, however, started to suffer from higher compressive strength loss than NSC at maximum exposure temperatures of 600 C. It is suggested that HPC suffered both chemical decomposition and pore-structure coarsening of hcp where C-S-H is the main hydration product, which starts to decompose at this high temperature. For maximum exposure temperatures exceeding 800 C, HPC suffers a higher strength loss than NSC. Strengths for all mixes reached minimum values at 1000 or 1100 C. In this research study, no evidence of spalling was encountered. Concrete that incorporates steel fibers, at 1 per cent, retains better crack resistance and better mechanical properties than concrete without steel fibers if not heated to exposure temperatures above 1000 C. Flexural strength results also reflected that concrete with steel fibers performs better than concrete without steel fibers under temperature effects, at about 800 C, even though the residual strength was very low at this high temperature. From visual inspection, it is revealed that variations with color, which is mainly depending on the composition of aggregates, occurred in concrete is associated with maximum temperature of exposure and loss in mechanical properties. For steel fiber concrete, a network of hair cracks appeared after heating to maximum temperatures between 400 and 600 C, which become deeper when heated beyond 600 C. For ordinary concrete mixes, hair cracks first appeared in specimens heated to 300 C. It is concluded overall that (a) fiber reinforcing concrete has higher strength and better performance in crack resistance than non-fiber concrete after exposure to high temperature. (b) high performance concrete can withstand higher heating temperature, below 600 C, than normal strength concrete without spalling. (c) initial moisture content before firing affects the ultimate strength of concrete.

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