EXPERIMENTAL INVESTIGATION ON STRESS STRAIN BEHAVIOR OF CONCRETE SUBJECTED TO ELEVATED TEMPERATURE BY STANDARD FIRE

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 10, October 2018, pp , Article ID: IJCIET_09_10_167 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed EXPERIMENTAL INVESTIGATION ON STRESS STRAIN BEHAVIOR OF CONCRETE SUBJECTED TO ELEVATED TEMPERATURE BY STANDARD FIRE T. Daniel Paul, G. Ruskin Samuel, N. Anand and G. Prince Arulraj Department of Civil Engineering, Karunya Institute of Technology and Sciences, Coimbatore , Tamil Nadu, India ABSTRACT Aim of the research is to investigate the stress-strain behaviour of concrete under elevated temperature. Key parameters that affect the stress strain behavior of concretes are grade of concrete, rate of heating, temperature and age of concrete were considered during the study. Experiments are conducted for four different grades of concrete such as M20, M30, M40 and M50. Specimens were exposed to elevated temperatures for duration of 1 hour, 2 hour, 3 hour and 4 hour as per standard fire. Test results showed that the reduction in ultimate compressive strength for the duration of 1 hour was about 56.35% of the original strength for M20 grade concrete. For the specimens heated upto 4 hours, a significant reduction was observed in compressive strength, reaching to a low level of 5.51% of initial strength. M50 grade concrete loses a significant amount of its compressive strength compared to other grades of concrete when exposed to heating for 1 hour and attains a strength loss of about 62.19% and when heated for 4 hours, the strength loss was about 96.86%. It was found that there was a reduction in young s modulus as the duration of heating increases. The Stress Strain curves of different grades of concrete showed a decrease in stress with an increase in strain as the duration of heating was increased. The weight of the concrete specimens reduced significantly as the temperature increased. Key words: Stress Strain Behavior, Elevated Temperature, Standard Fire, Concrete and Weight Loss Cite this Article:, T. Daniel Paul, G. Ruskin Samuel, N. Anand and G. Prince Arulraj, Experimental Investigation On Stress Strain Behavior Of Concrete Subjected to Elevated Temperature by Standard Fire International Journal of Civil Engineering and Technology, 9(10), 2018, pp editor@iaeme.com

2 T. Daniel Paul, G. Ruskin Samuel, N. Anand and G. Prince Arulraj 1. INTRODUCTION Concrete is a composite material produced from aggregate, cement and water. Generally concrete structures exhibit good performance under fire situations. In case of fire, concrete is exposed to high temperature that induces a material degradation such as strength loss, cracking, and in certain conditions spalling also occurs. Appropriate fire resistance should be provided for structural members and to be considered as a major safety requirement in building design. The properties of concrete may vary after exposure to elevated temperatures. It is essential, to understand the behavior of concrete materials under elevated temperatures. A significant physical and chemical changes observed on the fire affected concrete, resulting in the deterioration of the mechanical properties [1, 2]. It is therefore essential to investigate the variations in the mechanical properties of concrete after exposed to high elevated temperature. When concrete exposed to high temperature, strength and stiffness of concrete reduces significantly due to loss of moisture, dehydration of cement paste and decomposition of aggregate [3]. Due to these changes in the micro-structure of concrete, compressive, tensile and bending strength of the concrete reduces significantly [4, 5]. It is essential to understand the effect of temperature on the mechanical properties of concrete, especially the stress-strain behaviour which determines the behavior of structural members under different loadsto examine and repair the fire affected concrete members [14]. The considerable reduction in compressive strength, tensile strength, flexural strength and Young s modulus has been observed for concrete specimens exposed to elevated temperatures [6 14]. It was understood, from the existing literatures that, further investigations to be carried out on stress-strain behaviour of concrete subjected to elevated temperature by standard fire rating. The behaviour of concrete specimens subjected to elevated temperatures as per ISO rate of heating may be different from that of concrete specimens subjected to conventional heating. Hence an attempt has been made to determine the stress strain behaviour of concrete under elevated temperatures. 2. EXPERIMENTAL INVESTIGATION 2.1. Materials Mix design carried out for concrete with various strength grades following IS10262:2009. The concrete cylinders were cast of size with 150mm diameter x 300mm height. The specimens were cured for 28 days and after curing it was kept for drying prior to testing. The details of the material properties are given in Table 1 and details of mix proportion are shown in Table 2. Table 1 Material Properties Material Density (kg/m 3 ) Specific gravity Surface moisture Cement Fine aggregate Nil Coarse aggregate Nil editor@iaeme.com

3 Experimental Investigation On Stress Strain Behavior Of Concrete Subjected To Elevated Temperature By Standard Fire Materials Table 2 Details of Mix Proportion Units Grade of Concrete M20 M30 M40 M50 Cement kg/m Coarse Aggregate kg/m Fine Aggregate kg/m W/C Ratio Super plasticizer l Heating Procedure The concrete specimens that are kept for drying, is taken and kept inside the furnace for heating at different elevated temperature and different time duration as per ISO 834 fire curve. The details are shown in Table 3. The specimens are placed in the furnace through a boogie and the temperatures are set for the required time. The furnace consists of coils on four sides which heats the specimens through radiation. Control panel is used to set the temperature and rate of heating. After reaching the target temperature, the furnace will stop its function and starts cooling. Then the specimens are taken out from the furnace and stored in an area for air cooling about 24 hours. Figure 1 shows the view of electrical furnace. Table 3 Standard Time Temperature Fire Curve Time (min) Elevation of furnace temperature ( C) Figure 1 High Temperature Electrical Furnace 2.3. Experimental Methodology The details of the experimental investigation are shown in Figure 2. The Stress - Strain behavior of reference and heated specimens were found by experimental investigation. The specimen with the compressometer was placed on the Universal Testing Machine and load was applied at a rate of 140/kg/cm 2 /min. At each interval of load, the compressometer readings were noted down. These readings were used to determine the strain corresponding to the stress. Stress Strain graph was plotted to find out the Modulus of Elasticity of concrete editor@iaeme.com

4 T. Daniel Paul, G. Ruskin Samuel, N. Anand and G. Prince Arulraj Stress strain behavior of concrete under elevated temperature M20 M30 M40 M50 Duration of heating (0hr,1hr,2hr,3hr,4hr) 3. EXPERIMENTAL RESULTS Air cooling Peak stress, Elastic modulus, Weight loss & Spalling Figure 2 Methodology 3.1. Stress Strain Behaviour The mechanical behaviour of concrete subjected to compressive load is usually expressed in the form of stress-strain relations, which are often used as input data in mathematical models for evaluating the fire resistance of concrete structural members. Generally, because of a decrease in compressive strength and increase in strain of concrete, the slope of stress-strain curve decreases with increasing temperature. The strength of concrete has a significant influence on stress-strain behaviour of concrete at room and elevated temperatures. The shape of ascending curves for heated concrete is different from that for unheated concrete, and the shape varies with the temperature. There could be a pronounced concave-up curve at the beginning of loading due to the closing of pre-existing cracks caused by heating and cooling. Figure 3 shows the stress strain graph of different grades of concrete of 28 days compressive strength. Figure 3 Stress-Strain Behaviour of Different Grade of Concrete It can be seen from the figures that the compressive strength decreases and peak strain increases with an increase in temperature. In other word, concrete is softening with increasing temperature [8]. WenzhongZheng [15] found that when the elevated temperature is less than 300 C, there was no change on the shape of stress strain curve compared with the original unheated specimen. After exposure to C, the damage of specimen increases gradually, thus editor@iaeme.com

5 Experimental Investigation On Stress Strain Behavior Of Concrete Subjected To Elevated Temperature By Standard Fire the stress strain curves become flatter with the temperature increase. The compressive strength and elastic modulus decreased constantly, meanwhile the corresponding peak strain and ultimate strain increased rapidly [15]. In the present investigation, it was found that for concrete with different strength grades, the results showed that as the duration of heating increases, stress decreases with an increase in strain. For duration of 2, 3 and 4 hours, the descending curves of all the grades of concrete found to be flatter. The strain corresponding to peak stress starts to increase. The strains, corresponding to peak stress for 1 hour and 2 hours heating, were ten and fifteen times the strain at room temperature. The specimens of all grades heated for 3 hours and 4 hours showed a very brittle type of failure without much difference in strain value. Stress- Strain behaviour is complex in nature. Fu Ping Cheng et.al found the strain corresponding to peak strength did not significantly change up to about 400 C for High Strength Concrete (HSC). Above this temperature, the strains corresponding to peak strength, increased considerably. The strains attained, corresponding to peak strength at 600 C and 800 C, were twice and seven times the strain at room temperature [10]. From the above stress strain graphs, it is understood that for all the grades of concrete, strain values of 2, 3 and 4 hours are more pronounced. This is because the cylinder specimens undergo more spalling when subjected to high temperatures and causes a loss in bond due to weakening of material which leads to increase in strain. The increase in the peak strain can be attributed to the cracks caused by thermal incompatibility of aggregate and cement paste during heating and cooling [8] Peak Stress Generally, concrete structural members exhibit good performance under fire situations. But when subjected to high temperatures, critical deterioration takes place with reduction in strength and durability. It was found in all four grades of concrete, compressive strength decreases with increase in temperature and duration of heating. Initially, as the temperature increased, the strength decreased compared to the original strength. Figure 4 shows the peak stress of concrete subjected to elevated temperature. Figure 4 Peak Stress Chang Y.F [8] reported that the compressive strength decreased continuously with an increase in temperature. The reduction rate was found to be lower for the temperatures below 200 C. The residual strength at 200 C was around 90% of the original unheated value. However, the residual strength at 400, 600 and 800 C were around 65%, 40% and 15% of the strength of unheated specimens respectively. Fu-Ping Cheng et.al found that the strength at editor@iaeme.com

6 T. Daniel Paul, G. Ruskin Samuel, N. Anand and G. Prince Arulraj 200 C was about 80% of the original strength at 20 C. In the temperature range of 400 to 800 C, the strength drops sharply reaching to a low level of 45 and 20%, of initial strength at 600 and 800 C respectively [10]. As the specimens were subjected to very high temperatures by ISO rate of heating, the residual strength for a duration of 1 hour was found to be 56.35% of the original strength at 27 C for M20 grade concrete. Similarly, it was about % of the original strength for M50 grade concrete. With a further increase in temperature for a duration of 2 hours, there was a significant reduction in compressive strength, reaching to a low level of 17.73% and 10.52% of initial strength for M20 and M50 grade concrete respectively. When the concrete specimens were exposed to a high temperature for 3 hours, the ultimate compressive strength was about 12.5% and 7.16% for M20 and M50 concrete respectively. The residual strength, when the specimens were heated for 4 hours, the values reduced to about 5.51% and 3.14% for M20 and M50 concrete respectively. At these temperatures, the dehydration of the cement paste resulting in gradual disintegration. Since the paste tends to shrink and aggregate expands at high temperature, the bond between the aggregate and the paste is weakened, thus reducing the strength of the concrete [7, 8, 10] Loss in Peak Stress Figure 5 shows the loss in peak stress of different grades of concrete. Figure 5 Loss in Peak Stress The loss of strength in the temperature range of C is marginal and C is minimal. Above 400 C, High strength concrete losses its strength at a faster rate [10]. Fu-Ping Cheng [10] carried out an experimental investigation on Stress-Strain behaviour of high strength concrete at elevated temperatures. It was found that HSC at elevated temperatures loses a significant amount of its compressive strength above 400 C and attains a strength loss of about 75% at 800 C. It can be seen that M50 grade concrete loses a significant amount of its compressive strength compared to other grades of concrete. When concrete with M50 grade exposed to 1 hour duration of heating, attains a strength loss of about 62.19%. When heated for 2 hours, 3hours and 4 hours, the strength loss was about 89.48%, 92.84% and 96.86% respectively. A higher compressive strength is usually seen with more packing and less porosity, which may lead to higher pore pressures and spalling [16, 17]. This reduction in M50 grade concrete is because of the spalling of concrete under fire conditions due to the low porosity (permeability) in HSC. Unfortunate combinations of low permeability, low porosity, low thermal transmission and high moisture content were supposed to lead to increased tendency editor@iaeme.com

7 Experimental Investigation On Stress Strain Behavior Of Concrete Subjected To Elevated Temperature By Standard Fire to spalling. The lower water-cement ratio leads to lower porosity that makes HSC more brittle and makes it have less fire endurance at elevated temperatures as compared to Normal Strength Concrete (NSC) [10,18]. Figure 6 Spalling in M50 Grade Concrete Figure 6 shows the spalled portion of concrete. Spalling, which results in the rapid loss of moisture in concrete during a fire, exposes deeper layers of concrete to fire temperatures, thereby increasing the rate of transmission of heat to the inner layers of the member, including the reinforcement. It has been proposed that spalling is caused by the buildup of pore pressure during heating. M50 grade concrete is believed to be more susceptible to this pressure build-up because of its low permeability compared to other grades. The extremely high water vapour pressure, generated during exposure to fire, cannot escape due to the high density of HSC and this pressure often reaches the saturation vapour pressure. Such internal pressures are often too high to be resisted by the HSC. The porosity and mineralogy of the aggregates are also responsible for the strength loss [7, 19]. The loss of compressive strength of M20 grade concrete, when exposed to 1 hour heating was about 43.65%. When exposed to high temperatures for 2 hours, 3 hours and 4 hours the loss in strength was found to be 82.27%, 87.5% and 94.49% respectively Elastic Modulus Elastic modulus of concrete decreases with increase in temperature and it is mainly depends on water-cement ratio, age of concrete and the amount and nature of aggregates. The elastic modulus, defined as the ratio of the elastic modulus taken as the tangent to the stress-strain curve at the origin at a specified temperature. Figure 7 shows the elastic modulus of different grades of concrete. Figure 7 Elastic Modulus The elastic modulus of heated concrete in compression is based on the secant modulus at 50 % of the peak stress extrapolated from the experimental compressive stress strain curve [15]. Up to about 400 C the elastic modulus of all four types of HSC decreases in a similar fashion, reaching to about 50% of its initial values [10] editor@iaeme.com

8 T. Daniel Paul, G. Ruskin Samuel, N. Anand and G. Prince Arulraj Y.F Chang et.al [7] found that the elastic moduli at 200, 400 and 600 C are respectively about 80%, 40% and 6% of the original unheated value. Wenzhong Zheng et.al [15] also found that the initial tangent moduli at 200, 400, 600 and 800 C are 47.46%, 22.97%, 10.63% and 2.94 % of the original unheated value, respectively. It was found in this study that for all grades of concrete, elastic modulus of specimens exposed to high temperature for a duration of 1 hour was about 2-4 % of initial elastic modulus and for all specimens heated for a duration of 4 hours, there was a significant reduction in elastic modulus and was found to be marginal in the range of 0.2 % of initial elastic modulus. For the same stress compared to unheated concrete, strain was found to be very high which leads to very low elastic modulus i.e., lower stress and higher strain causes low elastic modulus. It was also found that the difference in loss in elastic modulus was found to be insignificant for all the grades of concrete which were heated upto 2 hours, 3 hours and 4 hours duration. The elastic modulus decreases with increasing temperature and the reduction in elastic modulus is greater than that of compressive strength. This is because at high temperature, disintegration of hydrated cement products and breakage of bonds in the microstructure of cement paste reduces elastic modulus and the extent of reduction depends on moisture loss, high temperature creep and type of aggregate as per literatures [20] Weight Loss Figure 8 shows the weight loss of different grade of concrete exposed to elevated temperature. Figure 8 Weight Loss An increase in weight loss observed as the temperature and duration of heating increases. Omer Arioz [21] investigated the effect of elevated temperatures on the weight loss of the concrete specimens. These losses were about 5% and 45% after heated to 200 C and 1200 C respectively. The reduction in weight gradually increased up to 800 C. Topcu et al. [22] found that the cement paste loses its binding property due to the evaporation of water in C S H structure. Somewhat higher losses were observed for concrete with higher w/c ratios prepared by both crushed limestone and river gravel aggregates. It was most probably caused by the fact that the water content of the mixture increases with respect to w/c ratio. It was found that there was an increase in loss of weight as the temperature and duration of heating increases. Higher loss of weight was found in M50 grade concrete heated upto 4 hours duration. The weight loss in concrete during exposure to elevated temperatures can be related to the change in the mechanical properties of the concrete. It is clear that loss in strength increases with increase in weight losses editor@iaeme.com

9 Experimental Investigation On Stress Strain Behavior Of Concrete Subjected To Elevated Temperature By Standard Fire 4. CONCLUSION The ultimate compressive strength decreases with increase in temperature and duration of heating. The results showed that the ultimate compressive strength for duration of 1 hour was about 56.35% of the original strength for M20 grade concrete. Further when specimens were heated for 4 hours, there was a significant reduction in compressive strength, reaching to a low level of 5.51% of initial strength. Loss in compressive strength increases with increase in duration of heating for different grades of concrete. M50 grade concrete loses a significant amount of its compressive strength compared to other grades of concrete when exposed to heating for 1 hour and attains a strength loss of about 62.19% and when heated for 4 hours, the strength loss was about 96.86%. For different grades of concrete, as the duration of heating increases, elastic modulus decreases and increases with the increase in grade of concrete. It was found in this study that for all grades of concrete, elastic modulus of specimens exposed to high temperature for a duration of 1 hour was about 2-4 % of initial elastic modulus. For concrete with different strength grades such as M20, M30, M40, M50, it was found that the duration of heating increases, stress decreases with an increase in strain. For duration of 2, 3 and 4 hours, the descending curves of all the grades of concrete found to be flatter. M50 grade concrete has steeper and more linear stress-strain curves. The strains attained, corresponding to peak strength for 1 hour and 2 hours heating, were ten and fifteen times the strain at room temperature. The specimens of all grades heated for 3 hours and 4 hours showed a very brittle type of failure without much difference in strain value. The weight of the concrete specimens reduced significantly as the temperature increased. ACKNOWLEDGEMENT The authors wish to thank the Science and Engineering Research Board, Department of Science and Technology of the Indian Government for the financial support (YSS/2015/001196) provided for carrying out this research. REFERENCES [1] Chan, Y. N., Luo, X. and Sun, W. Compressive strength and pore structure of highperformance concrete after exposure to high temperature up to 800 C. Cement and Concrete Research, 30, 2000, pp [2] Diederichs, U., Jumppanen, U. M. and Schneider, U. High temperature properties and spalling behaviour of HSC. In: Proceedings of 4 th Weimar workshop on HPC, HAB Weimar, Germany, [3] Luccioni, B. M., Figueroa, M. I. and Danesi, R. F. Thermo-mechanic model for concrete exposed to elevated temperatures. EngStruct, 25, 2003, pp [4] Anagnostopoulos, N., Sideris, K. K. and Georgiadis, A. Mechanical characteristics of self compacting concretes with different filler materials exposed to elevated temperatures. Materials and Structures, 42, 2009, pp [5] Anand, N. and Prince Arulraj. Effect of grade of concrete on the performance of self compacting concrete beams subjected to elevated temperatures. Fire Technology, 50, 2014, pp [6] Bilodeau, A., Kodur, V. R. and Hoff, G. C. Optimization of the type and amount of polypropylene fibres for preventing the spalling of lightweight concrete subjected to hydrocarbon fire. Cement and Concrete Composites, 26, 2004, pp [7] Chan, Y. N., Peng, G. F. and Anson, M. Residual strength and pore structure of highstrength concrete and normal strength concrete after exposure to high temperatures. CemConcr Compos, 21, 1999, pp editor@iaeme.com

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