ASSESSMENT OF FIRE DAMAGED STRUCTURAL ELEMENTS

Abstract

Fire losses in terms of deaths and direct fire losses are huge all around the globe. Precautionary measures are adopted to avoid the initiation of fire, but the probability of occurrence of fire in a building can't be ruled out. Active and passive fire protection measures are being designed and provided to minimize fire losses. After the fire in an occupancy reparability of fire damaged structural elements is a important issue. Before taking any decision regarding the repair, the proper assessment of fire damage to the structural elements is to be made. A large amount of data is available in the literature on the behavior of concrete at high temperature. Relatively less information is available on the behavior of RCC members when subjected to standard time - temperature curve. The available information on the behavior of concrete at high temperature does not conclude any clear pattern as mechanical properties show a wide variation at a particular temperature. This variation is mainly due to the combination of different variables responsible for governing the behavior of concrete at high temperature chosen in various research program. Further for the assessment of fire damage, the approach is to find out the maximum temperature attained at different locations in the structural element. Based on the indication of maximum temperature to which a building element is exposed during real fire situation estimation about the degradation of material properties is made. The reconstruction of the time-temperature history of the real fire is a complex problem. But if the reconstruction of real fire history is correct then the data on the behaviour of RCC members at high temperature can be used for the assessment of the fire damage. Besides this non-destructive testing is carried out for studying the residual strength of the structural members after the fire. Experimental and analytical studies have been carried out for the assessment of fire damage and results have been discussed in this study. The various approaches for the assessment of fire damage includes: a. Estimation of maximum temperature of real fire exposure b. Non destructive testing and visual observations c. Reconstruction of real fire history with respect to temperature variation with time d. Post fire behaviour ofRCC members For determination of maximum temperature attained TGA and XRD studies have been carried out on the samples exposed to different duration of standard fire, i.e., 30,60 and 90 minutes. In all the cases XRD and TGA studies were carried out on the specimen collected from the beam specimen. The XRD and TGA studies revealed that the exposure of specimen up to 90 minutes of standard fire exposure did not show appreciable change in mineralogy however a little difference in weight loss was observed. Therefore it was difficult to differentiate in the duration of fire exposure of the specimen based on the XRD and TGA results. Further ultrasonic pulse velocity (UPV) examination was carried out on aerated concrete blocks, self-compacting concrete beams of M60 and M30 normal concrete beams at grade M30 and M 60 and columns ofM 30 grade by using Portable Ultrasonic Non Destructive Digital Indicating Tester. The UPV examination was carried out on unexposed and exposed beams. In case of Sec beams the duration of standard fire exposure was kept 90 minutes. Due to spalling of concrete at few locations it was not possible to carry out UPV testing without surface preparation by using 50 mm transducers hence UPV examination was carried out by using both 50 mm and point type transducers. Besides this observations w.r.t nature of spalling, degree of spalling and change in colour were also made. 50mm transducers measured the variation of pulse velocity at different locations for M30 and M60 concrete beams. The variation in pulse velocity was not appreciable. The majority of results falls into a velocity range of 4.0 to 4.5 km/sec. for unexposed beams Based on the pulse velocity results it was difficult to get an idea or to differentiate M30 and M60 beams, however uniformity is confirmed which was essential to study the results. Further the UPV examination of specimen of beams after 90 minutes of standard fire was carried out. Two different type of transducers, i.e., 50 mm diameter and point type with 5mm diameter were used. A variation in the values of ultrasonic values of velocity was observed as measured by using 50 mm diameter and pin type transducers. / For unexposed specimen, both of M30 and M60 grade of Sec beam the values of UPV varied within a range of 4-4.5 km/sec as measured by 50mm transducer to 2.3 to 2.5 km/sec as measured by pin type transducers. Therefore the values of ultrasonic pulse velocity measured by pin type transducer are lower by about 55 %. Further it is observed that the degree of damage as per the values of ultra sonic pulse velocity is more at bottom in M60 grade concrete beams. A clear gradation in degree of damage from bottom to top surface of M60 grade concrete beam was visible. While in case of M30 grade concrete beams the damage at the bottom and middle depth of the beam was not appreciable as depicted by UPV values. The values ofUPV obtained after the patch up repair of the specimen were on higher side. A clear indication of repaired (surface patch up) details can be obtained by plotting the difference in the values of un-repaired and repaired portion. The results of ultrasonic pulse velocity examination showed that the degree of damage was more in M 60 self compacting concrete beams as compared to M 30 grade self compacting concrete beam after 90 minutes of standard fire exposure. It was observed that degree of spalling was more in M60 concrete beams as compared to M30 concrete beams for the same duration of fire exposure, i.e., 90 minutes of fire exposure as per standard heating conditions. The average value of degree of spalling in M60 beam was 15.38 % as compared to 9.5% in M30 beams. As during post fire investigations the change in colour of concrete after fire gives an indicative idea of the temperature attained in that particular area. Similarly the nature and extent of damage by spalling is recorded in the present study can be used for assessing the degree of fire severity in terms of standard fire exposure. Based on this an estimate can be made weather the severity of fire was more than, equal to or less than 90 minute standard fire severity. Based on this further assessment of damage can be made by correlating the results/data already available on the behavior of different building components/ systems under different fire exposure duration as per standard heating conditions of definite durations. The two-wall specimen of size 3000 mm x 3000 mm x 200 mm were constructed in the wall furnace specimen frame holder using aerated concrete blocks of size 600 mm x 200 mm x 200 mm with cement sand mortar of 1: 6 ratio. The specimens were exposed to the standard heating conditions. The wall specimen was in dismantled carefully and exposed blocks of aerated concrete blocks were taken out from the specimen of the exposed wall. The exposed blocks were examined for assessing the fire damage by measuring the ultrasonic pulse velocity. A relationship was derived between pulse velocity reduction factor and standard fire duration. Further the observations with respect to cracking pattern and change in colour were also made. The experimental results of the study have been analysed with respect to variation in fire severity at different locations in the core of the aerated concrete blocks at locations where the ultrasonic pulse velocity was recorded. A simple way of predicting the fire history of the fire exposed occupancy in terms of standard fire duration with the help of experimental data generated by carrying out ultra sonic pulse velocity examination on aerated concrete blocks is tried. A relationship between pulse velocity/ pulse velocity reduction factor with respect to duration of standard fire was derived. Further to study the post fire behaviour of RCC members under load, the experimental work is carried out on columns and beams. As besides earthquake also there may be certain other reasons for the damage of structural members that may be encountered during their service life. One of the probable reasons is the occurrence of fire and subsequent repair, as it is not always possible to repair very minute cracks in the core of concrete member. To keep this into consideration this study has been planned to quantify the effect of fire on the structural elements with some degree of induced damage. The data available in the literature contains no information on the behaviour of structural elements subjected to high temperature exposure with induced degree of damage. The data is analyzed for the stress strain behavior and variation in strength ratio and ductility ratio of columns with same spacing, same duration of fire exposure but different degree of damage and with different spacing but same degree of damage and fire exposure is analyzed. Further the experiments were also carried out on beams. The moment - curvature relationships for unexposed beams and exposed to fire with and without induced damage are studied. Further based on UPV results relationship between UPV values and standard fire exposure time for normal strength concrete was dev