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Citation |
Yang H, Lin Y, Hsiao C, Liu JY. Fire Safety J. 2009; 44(1): 121-130. |
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Copyright |
(Copyright © 2009, Elsevier Publishing) |
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DOI |
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unavailable |
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Abstract |
aDepartment of Civil Engineering, National Chung Hsing University, 250 Kuo-Kung Road, Taichung 402, Taiwan bDepartment of Interior Design, National Taichung Institute of Technology, 129 Sanmin Road, Sec. 3, Taichung 404, Taiwan In this paper, the ultrasonic pulse velocity (UPV) is used to quantitatively evaluate the residual compressive strength of concrete subjected to elevated temperatures. A series of tests were performed to examine the relationship between the residual UPV and strength of concrete with different mixture proportions at elevated temperatures. Cylindrical specimens were made of concrete with water–cement ratios of 0.58 and 0.68, and heated in an electric furnace at temperatures ranging from 400 to 600 °C. After exposing to the elevated temperature, the concrete specimens were cooled down in the ambient air and tested on different days. For each test, the pulse velocity and compressive strength were measured. Experimental results show that change in mixture proportion of concrete does not have a significant effect on the residual strength and UPV ratios of concrete subjected to elevated temperatures. This important finding considerably enhances the feasibility of using UPV for quantitative evaluation of the residual strength of fire-damaged concrete structures. The relationship between the residual strength ratio and the residual UPV ratio was developed and a general equation was proposed for residual strength prediction. Finally, this paper verifies the suitability of the proposed equation for predicting the residual strength ratios of different concrete specimens with the measured residual UPV ratios. Keywords: Residual compressive strength; Concrete; High temperature; Pulse velocity Damage assessment is the first and the most important job for structural safety evaluation of a concrete building subjected to fire [5]. The use of ultrasonic pulse velocity (UPV) for assessment of concrete residual strength is one of the most interesting subjects in the field of nondestructive testing of concrete after exposure to high temperatures. Several experimental studies had been carried out to investigate how the pulse velocity was affected by the damage of concrete caused by various high temperatures [6], [7], [8], [9] and [10]. Their experimental results showed that the pulse velocity decreases by increasing the exposure temperature for all the concrete specimens with different mixture proportions. In a previous paper by Chiang and Yang [11], neural network analysis based on the relationship between the residual strength and pulse velocity was used to predict the normalized residual strength of concrete at elevated temperatures, in which the normalized residual strength was defined as the ratio of compressive strength of the heated specimen to that of the unheated specimen. However, because of heterogeneous nature of concrete, it is still hard to make use of the UPV method for quantitative evaluation of fire-damaged concrete structures based on the scattered data of these previous studies. In this paper, a series of tests were performed to examine the changes in pulse velocity and strength of concrete subjected to elevated temperatures. The experimental parameters include concrete mixture proportion, exposure temperature, exposure time, and post-fire age. The objectives of this paper are to find a clear relationship between the residual strength and UPV of concrete with different mixture proportions and to evaluate the feasibility of using the UPV method for quantitatively estimating the residual compressive strength of concrete subjected to elevated temperatures. Materials used for making specimens include cement, fine aggregate (FA), coarse aggregate (CA), and superplasticizer. The cement used was Portland Type I. River sand with a saturated-surface dry (SSD) density of 2.62 and crushed stone with a maximum nominal size of 12 mm and a SSD density of 2.60 were used as fine and coarse aggregates, respectively. The pulse velocity of sand was measured by using the two-phase model [12] to be about 4960 m/s. The pulse velocity of the coarse aggregate was measured to be about 5100 m/s. Table 1 shows the mixture proportions of concrete used in this study. Two water/cement (W/C) ratios of 0.58 and 0.68 were considered. One hundred and forty-seven concrete specimens were produced for each mixture proportion. All the specimens were cast in steel molds (100 mm in diameter, 200 mm in height) and demolded 24 h after casting. Subsequently, the concrete cylinders were cured in water at 20 °C. After 28 days of water-curing, the specimens were placed in ambient air. At an age of 90 days, the specimens were heated in an electric furnace. The average heating rate of the electric furnace used in the experiments was 2.5 °C/min. Fig. 1 is a schematic illustration of the temperature–time curve showing heating time and exposure time of the peak temperature. Two heating parameters including the peak temperature and the exposure time were considered in the studies. The peak temperatures of 400, 500, 550, and 600 °C, and the exposure times of 0, 1, and 2 h were adopted in experiment. The studies performed by Nassif et al. [8] investigated the temperature distribution in a concrete core at elevated temperature and it was found that there is no temperature difference in the concrete core soon after the heating temperature reaches the peak temperature. After exposing to the elevated temperature, the concrete specimens were cooled down in the ambient air (natural cooling) and tested at 7, 30, 90, and 180 days after heating. For each test, the pulse velocity and compressive strength of three specimens were measured according to the specification of ASTM C597 and ASTM C39, respectively. For comparison, unheated specimens also go through the test to investigate the changes in the pulse velocity and compressive strength of the concrete at elevated temperatures. To eliminate the variation in pulse velocity caused by the change in moisture content of concrete, the pulse velocity of the unheated specimens was determined by performing test on the saturated concrete specimens (right after 28 days of water-curing). Through a direct transmission mode as illustrated in Fig. 2, UPVs were measured by a commercially available pulse meter with an associated transducer pair. The transducer pair had a nominal frequency of 54 kHz. The principle of UPV measurement involves sending a wave pulse into concrete and measuring the travel time for the pulse to propagate through the concrete. The pulse is generated by a transmitter and received by a receiver. In the experimental studies, the transmitter and receiver were placed at the top and bottom surfaces of a cylindrical specimen, respectively. As a result, the path length of the ultrasonic pulse was the length of the specimen, which was measured by using a vernier with a minimum reading of 0.01 mm. Knowing the path length, the measured travel time (Δt) can be used to calculate the pulse velocity (υ) as follows: Fig. 3 shows the results obtained from the compressive strength tests of all the concrete specimens subjected to different high temperatures and cooled in ambient air to the 7th, 30th, 90th, and 180th day after heating. Each data point in the figure represents the average of three measurements. Fig. 3(a) and (b) shows the residual compressive strengths of the concrete specimens with W/C ratios of 0.58 and 0.68, respectively, for an exposure time of 0 h. In Fig. 3(a) and (b), it is shown that the residual compressive strength of the heated specimens decreases with an increasing temperature. In addition, reduction in compressive strength occurs shortly after the specimens are subjected to elevated temperatures above 400 °C and the strength does not recover apparently with the days of air-curing. Fig. 3(c) and (d) shows the residual compressive strengths of the concrete specimens with W/C ratio of 0.58 and 0.68, respectively, for an exposure time of 1 h. In the figures, it is clear that the residual compressive strength still decreases as the exposure temperature increases, and prolonging the air-curing days does not help restore concrete strength significantly. Similar results are also found in the experiment for an exposure time of 2 h as shown in Fig. 3(e) and (f). Fig. 4 shows the results obtained from the UPV measurements of all the concrete specimens subjected to different high temperatures. Each data point represents the average of three measurements. Fig. 4(a) and (b) shows the UPV values of the concrete specimens with W/C ratio of 0.58 and 0.68, respectively, for an exposure time of 0 h. In the figures, it is shown that the UPV of heated concrete decreases with an increasing temperature, and there is a notable reduction in UPV shortly after the specimens are subjected to elevated temperature over 400 °C. The UPV does not have a noticeable change with an extension of air-curing days. Similar results are also found in the experiment for an exposure time of 1 and 2 h as shown in Fig. 4(c) and (d) and Fig. 4(e) and (f), respectively. It should be noticed that there exist very low UPV values (slower than the sound velocity) labeled as star marks in Fig. 4(d) and (f). It is believed that this abnormal phenomenon is due to extensive cracks in heated concrete specimens that prevent stress wave from propagation. The extensive cracks were observed on the concrete specimens having a high W/C ratio (0.68) subjected to a temperature of 600 °C with an exposure time of over 1 h and placed in air more than 90 days after heating. To investigate the effect of exposure time on the compressive strength and UPV of concrete, Fig. 5 summarizes the experimental results of different exposure times from Fig. 3 and Fig. 4 for specimens tested on the 7th day after exposure to high temperatures. Fig. 5(a) and (b) shows the influence of exposure time on the residual compressive strengths of concrete specimens with W/C ratios of 0.58 and 0.68, respectively. The figures demonstrate that the reduction in the compressive strength of concrete specimens mainly comes from the peak temperature given in the experiment, and the increase of exposure time results in a slight decrease in the residual strength. Fig. 5(c) and (d) shows the UPV measurements of concrete specimens with W/C ratios of 0.58 and 0.68, respectively, which were cooled down naturally to the 7th day after heat treatment. The reduction in the UPV of concrete specimens at elevated temperatures is also dominated by the peak temperature and the effect of exposure time on the UPV reduction is insignificant. It is shown that the reduction trend in UPV is similar to that in compressive strength. To investigate the relative decrease in residual compressive strength of concrete specimens subjected to high temperatures as compared to the original strength before heating, the residual strength ratio is calculated. Fig. 6(a) shows the residual strength ratio of the specimens on the 7th day after exposure to various high temperatures with an exposure time of 2 h. In Fig. 6(a), the solid and dotted lines represent the residual strength ratio of concrete specimens with W/C ratios of 0.58 and 0.68, respectively. It is observed that both the solid and dotted lines go down as the temperature increases, i.e., the residual strength ratio decreases with an increasing temperature. The residual strength ratio of concrete reduces rapidly for heating temperature more than 500 °C. More importantly, it is also found that these two lines are very close to each other. In other words, the relative reduction in residual strength of specimens with different mixture proportions has no evident difference. Similarly, the relative decrease in residual UPV of concrete specimens subjected to high temperatures is also investigated. Fig. 6(b) shows the residual UPV ratio of the specimens on the 7th day after exposure to various high temperatures with an exposure time of 2 h. In Fig. 6(b), the solid and dotted lines represent the residual UPV ratio of concrete specimens with W/C ratios of 0.58 and 0.68, respectively. The residual UPV ratio decreases with an increasing temperature. It is also shown that the relative reduction in UPV of specimens with different mixture proportions has no big difference. To further investigate whether there is still coincidence in the reduction ratios of residual strength and UPV between these two types of concrete specimens when subjected to elevated temperatures, all the calculated residual strength and the residual UPV ratios are plotted in Fig. 7 and Fig. 8. Fig. 7(a)–(c) shows the residual strength ratios for the specimens having exposure times of 0, 1, and 2 h, respectively, and the solid and dotted lines in the figures represent the experimental results of concrete specimens with W/C ratios of 0.58 and 0.68, respectively. Fig. 7(a) shows that, for a specific exposure temperature, the solid and dotted lines are getting close to each other, indicating similarity in the strength reduction ratio of concrete having different mixture proportions. The similar results are also observed in Fig. 7(b) and (c). Fig. 8(a)–(c) shows the residual UPV ratios for the specimens having exposure times of 0, 1, and 2 h, respectively. Fig. 8 also shows the coincidence in the residual UPV ratio of concrete specimens with different mixture proportions for a specific elevated temperature. Fig. 7 and Fig. 8 clearly demonstrate that both the residual strength ratio and the residual UPV ratio are dominated primarily by different exposure temperatures and exposure times. The mixture proportion of concrete does not have a significant effect on the residual strength and UPV ratios of the concrete specimens subjected to elevated temperatures. This is a very important observation that can help establish a clear relationship between the residual strength and UPV of concrete and, thus, enhance the feasibility of using UPV for practical evaluation of the residual strength of fire-damaged concrete structures. This section examines the relationship between the residual strength and the residual UPV of concrete subjected to elevated temperatures. The change in the mixture proportion of concrete results in a variation of its strength and UPV. The residual strength and UPV ratios used to evaluate the damage degree of heated concrete are relative scales that help to mitigate the influence of the basic difference in strength and UPV of concrete due to the variation in mixture proportion. Thus, in this study, the relationship between the residual strength ratio and the residual UPV ratio is developed. Fig. 9(a) shows all experimental data pairs (the residual strength ratio vs. the residual UPV ratio) of the concrete specimens with a W/C ratio of 0.58 that were subjected to various elevated temperatures with different exposure times and tested on different days after heating. In Fig. 9(a), a positive relationship between the residual strength ratio and the residual UPV ratio is clearly observed and a linear regression method was used to correlate the experimental data. Eq. (2) is the regression equation with a coefficient of determination (R2) of 0.94 for concrete specimens with a W/C ratio of 0.58: Similarly, Fig. 9(b) shows the regression result of all experimental data pairs of the concrete specimens with a W/C ratio of 0.68 and Eq. (3) is the regression equation with a coefficient of determination (R2) of 0.92: To verify the validity of the proposed residual strength–UPV ratio relationship as presented in Eq. (4), additional 180 specimens were constructed of concrete having 12 different mixture proportions as listed in Table 2. The cement paste occupies 36% of the total concrete volume. Three S/A ratios of 36%, 44%, and 52% and four water/cement ratios of 0.43, 0.53, 0.58, and 0.63 were considered. For each mixture proportion, 15 specimens including 12 heated specimens and 3 unheated specimens were produced. The heated specimens were subjected to high temperatures of 300, 500, 600, and 800 °C with an exposure time of 2 h and their residual UPV and compressive strength were measured 30 days later after heating. The average UPV of three unheated saturated specimens was used as a base to calculate the residual UPV ratio of heated specimens, and so did the residual strength ratio. The measured residual UPV ratio of each heated specimen was used to predict the corresponding residual strength ratio with Eq. (4) and, then, the predicted residual strength ratio was compared to the measured one. Fig. 11 shows the comparison results of 144 heated specimens. In Fig. 11, the difference between the predicted and measured strength ratios of most specimens (more than 93%) is within 0.1 and the maximum difference is 0.17. As a result, the proposed general equation is suitable for estimating the residual strength ratio of concrete with various mixture proportions subjected to elevated temperatures. In this paper, a series of tests were performed to examine the changes in pulse velocity and strength of concrete subjected to elevated temperatures ranging from 400 to 600 °C and to establish the relationship between the residual UPV and strength ratios of concrete. Based on the experimental results presented in this paper, the following conclusions are drawn: This work was sponsored by the National Science Council of Taiwan, ROC, under Grant No. NSC-91-2211-E-167-001. |