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Effect of Elevated Temperatures on Performance of RCC Beams
Figure 7: Tested Sample Showing Flexural Crack
Figure 8: Tested Samples
Other Observations Sundry observations (Ref Table 3) too revealed minor damage up to 550°C & major damage only above 750°C. S. C. Chakrabarti et al.(5) too have reported the phenomenon of concrete cubes becoming fragile & friable at & above 800°C. In an actual fire affected structure, the changed color could be utilized to assess the temperature attained by the member during fire. Table 3: Comparison of Damage to Beams at Different Temperatures Observation Cracks Color Spalling Distortion Scaling
Room Temperature No Normal Grey None None No
550°C
750°C
950°C
No No change Minor No No
Moderate Blackish Grey Localized to corners Slight but insignificant No
Major Buff Corners & edges spalled Slight Yes, Delamination
CONCLUSIONS To sum up, up to 550°C, the weight loss was negligible & the flexural strength got reduced by 1/3rd. No cracking, spalling or scaling was observed up to this stage. It could be concluded that up to 550°C the fire affected structure was only mildly affected & would require rapid cooling & minor repairs. At @ 750°C, there was further drop in weight & flexural strength, cracks did appear but there was hardly any spalling or scaling. The fire affected members at this point would require rapid cooling & retrofitting after proper evaluation. Factor of safety would come down but the structure would still be serviceable. Beyond this stage, all the parameters dropped alarmingly. Weight loss at 950°C exceeded 10%, flexural strength came down by 2/3rd, major cracking, spalling & scaling could be observed. Concrete became friable during cooling. The fire affected portion at this stage would require major retrofitting or might need replacement after detailed assessment. Based on the experimental work carried out in the laboratory, the following conclusions could be drawn:
Up to 550°C the fire affected rcc member would hold its forte & would require only minor repairs.
At higher temperatures the rcc beam would lose its fidelity rapidly and at or around 750°C it would require rapid cooling & retrofitting after proper evaluation. Factor of safety would come down but the structural member would be serviceable.
At and around 950°C the fire affected member would require major retrofitting or might need replacement.