BEHAVIOUR OF HIGH PERFORMANCE CONCRETE EXPOSED TO ELEVATED TEMPERATURES

Abstract

When the concrete is submitted to high temperatures, a series of physical and chemical changes take place, which result in a complex thermal behaviour in the multi-phase nature of hardened concrete. The unwanted heating may cause large volume changes due to thermal dilation, shrinkage due to moisture migration, eventual spalling due to high thermal stresses. The differential thermal volume changes produce stresses that results micro cracking and large fractures which may finally lead to structural failure. Previous studies indicate that the properties of concrete at high temperatures are influenced by many internal and external parameters, such as properties of mix constituents, mineral and chemical admixtures, type of aggregates, concrete strength and grade, mixture constituents, heating rate, cooling rate, peak temperature, size and shape of member and testing methods. Among the notable revolutionary developments in the field of concrete technology over the last few decades, high performance concrete (HPC) is considered to be a construction material with variety of applications. The HPC has found its use in concrete structures such as nuclear reactor vessels, clinker silos of cement plants, metallurgical and chemical industrial structures, glass making industrial structures, storage tanks for hot crude oils, coal gasification and liquefaction vessels, reinforced concrete chimneys, high rise buildings etc. Often these modern concrete structures may get subjected to elevated temperatures due to exposure to an aggressive fire or other heat source. A review of the existing literature indicates that only few studies were undertaken on the influence of elevated temperature on the behaviour of HPC and the research in this area is inconclusive. It is now well established that the inclusion of mineral admixtures in a concrete mix is generally an essential condition for obtaining high performance concrete (HPC). However, a review of the existing literature shows that the behaviour of HPC is more sensitive to high temperature conditions than a normal concrete mix, though there are conflicting conclusions about the role of mineral admixture on the performance of heated high performance concrete. While some studies report that the inclusion of mineral admixtures, especially silica fume, enhances the chances of sudden explosive spalling during heating, some other studies report that the addition of fly ash, ground granulated blast furnace slag (GGBFS) etc. may be beneficial to the post-fire strength properties of HPC. Most of the earlier investigations related to fire induced spalling of concrete were iii undertaken on small scale un-reinforced cubes or cylinders without any pre-load. It may be mentioned here that the presence of a pre-load and the reinforcing bars, especially confining reinforcement, are expected to influence the spalling and other mechanical properties of concrete exposed to fire scenarios. Thus there is a need to investigate the spalling and mechanical properties of high performance concrete under stressed residual conditions. This study was aimed to investigate the role of various concrete mix parameters on the residual compressive strength of pozzolanic concretes after exposing to various elevated temperatures. The experimental variables of the study were the type of high performance concrete (plain HPC, silica fume HPC, fly ash HPC and GGBFS HPC), their mix constituents and five different target temperatures (room temperature, 200 °C, 400 °C, 600 °C and 800 °C). A large number of trial experiments are usually required to deal with such cases where the number of variables and mix combinations become worth investigating. However, a good mix proportioning procedure has to minimize the number of trial mixes and achieve an economical and satisfactory mixture with desired properties. In view of this the design of experiment (DoE) methods and optimization tools are generally used to fix-up a suitable mixture combination for getting the targeted requirements. For this purpose, the design of experiments based on Taguchi method was formulated considering four parameters (mix constituents) at three levels with an aim to achieve maximum compressive strength using larger-the-better criterion. A total of 756 cubic specimens (100 mm × 100 mm × 100 mm) of different types of high performance concretes were cast and tested under this study. The cubic specimens were first exposed to temperatures ranging from room temperature to 800 °C and then they were tested under axial compression after complete cooling. Using the resulting mix parameter design, the experiments were carried out and the results were analyzed statistically by analysis of variance (ANOVA) to find out the significant factors affecting the residual compressive strength of HPC. While the best mono mix combination for each target temperature was established by Taguchi’s technique, an overall most excellent single concrete mix combination was obtained by using the utility concept. The results show that the residual strength of HPC largely remains unaffected and rather it increases up to a temperature of exposure of 400 °C irrespective of the type of pozzolana and other mix parameters considered in this study. It is only in the temperature range of 600 to 800 °C that a noticeable degradation in the compressive strength of high performance concrete is iv observed. The results also showed that the mix parameters change their influence with the change in temperature of exposure on the residual compressive strength. The results of this study would help in designing HPC mixes for concrete structures liable to be exposed to elevated temperatures. The experimental program was planned to explore the effects of high temperatures on the spalling, residual strength and deformation behaviour of reinforced high performance concrete short columns exposed to elevated temperatures ranging from room temperature to 800 °C. A total of 108 numbers of tie confined short cylindrical column specimens of size 150 mm diameter and 450 mm height were cast and tested. The three pozzolanic HPC mixes and one non-pozzolanic high performance concrete mix optimized in the study as reported in the previous section were used here to cast confined cylindrical concrete specimens. The experimental variables included type of HPC based on mineral admixture (silica fume (SF), fly ash (FA) and ground granulated blast furnace slag (GGBFS) and control plain HPC), the different target temperatures of exposures (Room temperature, 200 °C, 400 °C, 600 °C and 800 °C) and the two test methods ( unstressed residual and stressed residual strength test). The effects of variables were studied and quantified in terms of spalling, residual strength and ductility. The results indicate that the detrimental effects of temperature on the spalling behaviour of pozzolanic and non- pozzolanic reinforced HPC do not matter much up to a temperature of 400 °C. However, an exposure to further higher temperatures such as 600 °C and 800 °C, the thermal spalling of HPC occurs irrespective of the type of pozzolana. The silica fume HPC is more vulnerable to spalling than fly ash and GGBFS HPC. Non-pozzolanic HPC is least influenced by thermal spalling. The presence of axial load on HPC during heating results in to more severe spalling compared to the HPC specimens with no load during heating. The results show that though the severe explosive spalling of core concrete is saved by providing the confining reinforcement, the cover is still liable to be spalled. The results indicate that the exposure of up to 400 °C temperature does not affect much the residual strength of HPC irrespective of type of HPC and type of test (unstressed or stressed). Rather a strength gain of about 6 to 16% was noted in this temperature range in some of the specimens. Beyond the temperature of 400 °C, most of the concretes showed thermal spalling of cover concrete and resulting drop in compressive strength of specimens. The reinforced HPC specimens exposed to 800 °C temperatures showed 0.34 to 0.42 times of original unheated concrete strength. The influence of pre-load during heating was to v reduce the residual load carrying capacity of reinforced HPC short columns, especially at temperatures of more than 400 °C. The results indicated that the strain ductility of HPC specimens also got influenced when the temperature was increased from room temperature to 800 °C. The influence of high temperature was more pronounced at temperatures more than 400 °C. The specimens tested under unstressed residual test conditions showed higher average strains than those tested under preloaded test conditions. The fly ash HPC had shown highest strain values among all the HPCs, on the other hand the silica fume HPC showed the lowest strain values. The part of this study presents effects of high temperatures on the thermal properties of different types of high performance concretes. The thermal properties namely thermal conductivity, specific heat, thermal expansion and mass loss were employed in this study. For thermal conductivity test, a total of 12 sets of three different types of pozzolanic high performance concretes and one plain HPC were cast. The specimens were in the shape of rectangular prisms of size 200 mm ×100 mm × 75 mm. For coefficient of thermal expansion test, cylindrical cores of 8 mm diameter and 25 mm length were cut from 100 mm cubes using special core cutting tool. Twelve samples were prepared from these cylindrical cores for evaluating thermal expansion using the dilatometric apparatus. The differential scanning calorimetry (DSC) and thermo gravimetric analysis test (TGA) were used for measuring the specific heat capacity and mass loss of HPC mixes at elevated temperatures. The above said experiments were performed from room temperature to pre-defined target high temperatures. While the specific heat capacity, thermal expansion and mass loss tests were carried out up to 1000 °C temperature, the thermal conductivity experiments could be performed up to 700 °C. The results indicate that the thermal properties of high performance concrete change with the increase in temperature. With the gradual increase of temperature up to 400 °C, all HPC mixes indicated continuously decreasing thermal conductivity values. Further increase of the temperatures, ranging between 400 °C and 700 °C, all the pozzolanic and non-pozzolanic high performance concretes showed an increase in thermal conductivity. The differential scanning calorimetry plots of plain HPC and pozzolanic high performance concretes revealed that the specific heat capacity of all types of concretes indicated almost similar trend up to 750 °C, however above 750 °C, all types of concretes varied widely. Silica fume based HPC showed maximum specific heat in this temperature range up to 1000 °C. The dilatometric curves showed steady increase of thermal strain vi from room temperature to 550 °C; thereafter the thermal expansion became constant from 600 °C to 900 °C before rising again up to 1000 °C. The thermal expansion results show that the GGBFS HPC had the highest thermal expansion and the fly ash HPC exhibited lowest thermal strain values. When the temperature was increased to 1000 °C, all concretes showed rising trend and maximum thermal strain and expansion coefficients were found in this temperature range. The results of gravimetric analysis showed that the loss in mass increased and the resulting mass decreased as the temperature increased. All the four HPC mixes showed almost similar kind of trends and sudden loss in mass was observed in temperatures ranges of 100-200 °C, 400- 440 °C and 600-700 °C. The mass gradually decreased with the increase in temperature up to 700 °C and thereafter remained stable at higher temperatures.