Compressive strength of lightweight aggregate concrete exposed to high temperatures

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 11, February 2004, pp Compressive strength of lightweight aggregate concrete exposed to high temperatures A Ferhat Bingöl * & Rüstem Gül Civil Engineering Department, Atatürk University, Erzurum, Turkey Received 14 July 2003; accepted 27 November 2003 In this paper, effects of high temperatures on the compressive strength of concrete were investigated with the aim to produce a fire resistant concrete. Thus, the mixture groups were determined by replacing pumice for ordinary aggregate in the ratios of 25, 50, 75 and 100% in volume. The temperature values were chosen as 150, 300, 450, 600 and 750 C. The effects of heating duration over the compressive strength were also examined, and the different types of concrete mixtures were heated for one hour, three hours and five hours periods for each temperature values. It was observed that concrete properties were deteriorated at 150 C and the specimens began to lose some of their initial strengths at this temperature. Though a considerable strength lose was not seen between C, all types of concrete mixtures continued to lose their compressive strength after 300 C. Every concrete mixture lost a significant part of their initial strength when the temperature is reached up to 750 C. When the lightweight aggregate ratio is increased for each temperature value, the loss of compressive strength of the concrete compared to the initial strength was decreased. It was found that the heating duration does not affect the strength loss significantly but a high temperature is a more significant parameter on the strength loss. In recent years, there has been renewed interest in lightweight aggregate concretes (LWAC), in particular for production of high strength concretes (HSC) that can be used for bridge construction and special applications. For the production of such high quality concretes there is a need for a better understanding of the mechanism by which strength is generated in such systems. This is necessary for making the correct choice of aggregates 1,2. The most popular way of lightweight concrete (LWC) production is by using LWA 3. Research on using the lightweight aggregate (LWA) to produce concrete as a material of construction is important in concrete technology. Obviously it has economical and technical advantages over ordinary concrete, such as construction saving due to the reduction of dead weight, thermal isolation, freeze-thaw resistance, lower handling cost and fire protection but have disadvantage of having low mechanical properties 4,5. The influence of elevated temperatures on the mechanical properties of concrete is important for fire resistance studies 6. Heat resistant materials are increasingly being used for structural purposes. The need for such building materials is particularly great in chemical and metallurgical industries and for the thermal shielding of nuclear power plants. In such For correspondence ( afbingol@atauni.edu.tr) installations structural members may be subjected to sustained and cyclic thermal exposures at the lower heat levels, at which the use of refractory materials is not essential 7. Concrete must at times resist the effects of induced high temperatures, but in most instances it is desired to avoid deterioration of the concrete s physical properties, as much as possible 8. Physical and chemical transformations take place in concrete during first heating which can result in significant loss in strength. Many factors combine to influence the strength of concrete during first heating. Therefore, representing the typical strength behaviour of concrete at high temperatures with an average curve can be misleading unless the specific mix and environmental conditions are specified 9. The effects of high temperature on the mechanical properties of concrete have been investigated over 80 years. Lea and Stradling 10 explored the factors which can influence the strength of concrete at elevated temperatures, in 1920s. Based on the experimental data, it has been found that the effect of elevated temperatures on the mechanical properties vary with a number of factors, some of which are external and some internal. External factors include the test methods and heating rates. Internal factors include original compressive strength, porosity or permeability and the moisture content at start of testing 11. In recent years, HSC specimens are began to use commonly in high

2 BINGÖL & GÜL: COMPRESSIVE STRENGTH OF LIGHTWEIGHT AGGREGATE CONCRETE 69 temperature tests and in these studies explosive spalling was pointed as the main problem The lowest temperature at which explosive spalling occurred was reported to be about 300 o C and the highest was about 650 o C in different studies. On the other hand, the studies about the fire resistance of LWAC are insufficient. For this reason the compressive strength of LWAC, produced with pumice aggregate, was investigated. Materials and Methods ASTM Type I Portland cement (PC) is used in this study. Natural aggregate (NA) and pumice aggregate (PA) both with maximum sizes of 16 mm were obtained from Karasu river in Erzurum and Kocapınar Table 1 Chemical constitution of PC and PA (%) Component PC ASTM Req. for PA PC SiO Fe 2 O Al 2 O CaO MgO 2.71 max SO max K 2 O 5.00 Na 2 O 3.40 TiO Undetermined 0.91 Free CaO 0.45 LOI 3.84 max region in Van-Erciş in Turkey, respectively. Tap water was used for mixing and curing. The chemical composition of PC and PA and standard requirements for PC according to ASTM C are summarized in Table 1. The physical and mechanical properties of PC are given in Table 2 and physical properties of NA and PA are shown in Table 3. The cement content and slump were kept constant at 300 kg/cm 3 and 3±1 cm respectively throughout the investigation. Five main groups of mixes of NA and PA were produced. The first group, produced by using 100% NA, was the control group and in the other groups 25, 50, 75, and 100% PA ratios were used instead of NA to determine the effect of PA on the mixtures compressive strength. The full details of these mixes are given in Table 4. The concrete mixes were prepared in a laboratory countercurrent mixer. Hand compaction was used. Precautions were taken to ensure homogeneity and full compaction. After casting, specimens were cured Table 2 Physical and mechanical properties of PC Specific gravity (g/cm 3 ) 3.01 Setting time start (h) 3.30 Setting time end (h) 4.20 expansion (Le Chatelier, mm) 4 Compressive strength (kg/cm 2 ) ASTM limits 1 day 83 3 days days Table 3 Physical properties of NA and PA NA (fine agg) NA (coarse agg) PA (fine agg) PA (coarse agg) Max size (mm) Specific gravity (gr/cm 3 ) Unit weight (gr/cm 3 ) Water absorption ratio (%) Modulus of fineness Table 4 Mix proportions of concretes used in this study Mix Group I (100% NA) II (75%NA-25%PA) III (75%NA-25%PA) IV(75%NA-25%PA) V(100% PA) Target Dosage Real Dosage Water Air PA PA NA NA

3 70 INDIAN J. ENG. MATER. SCI., FEBRUARY 2004 in lime-saturated water at 23±3 o C until the 27 th day according to ASTM C The effect of high temperatures over the compressive strength of the produced lightweight and semilightweight concretes was investigated. The temperature values were chosen as 150, 300, 450, 600 and 750 C in this study. The effects of heating duration over the compressive strength were also examined and the different types of concrete mixes were heated for 1 h, 3 h and 5 h periods for each temperature values. For heating process laboratory type furnace, capacity of 1000 o C was used. For each temperature and heating duration group, three specimens mm cylinder were prepared for each mix group. After heating specimens up to the target temperature, the furnace temperature was kept constant during the reference heating duration. All groups were cooled to room temperature in air in laboratory conditions. Compressive strength tests were conducted using cylinders capped with a sulphur compound and then specimens tested in accordance with ASTM C Results and Discussion The change of mechanical properties of concrete subjected to high temperature is influenced by many factors. Generally, for mature concrete increase in exposed temperature causes concrete to gradually loose its compressive strength. Table 5 shows the test results of fresh concrete. The theoretical unit weight was calculated as the sum of weights of the materials in the mix groups and the real unit weight was determined by the weights of specimens which were cured in water for 27 days and then kept in laboratory conditions for 24 h for drying. The compressive strengths of different groups subjected to elevated temperatures in a range of o C are shown in Figs 1-3 according to their heating durations 1 h, 3 h and 5 h, respectively. For the specimens which were kept for 1 h in the furnace after reaching the reference temperature, when the temperature is 150 o C the most significant loss in compressive strength was seen in group I. With the in- Fig. 1 Residual compressive strength of concrete specimens after exposure to elevated temperatures for 1 h Fig. 2 Residual compressive strength of concrete specimens after exposure to elevated temperatures for 3 h Fig. 3 Residual compressive strength of concrete specimens after exposure to elevated temperatures for 5 h PA Ratio (%) Target slump (cm) Table 5 Test results of fresh concrete Measured slump (cm) Theoretical unit weight (kg/m 3 ) Measured unit weight (kg/m 3 ) 25 3± ± ± ±

4 BINGÖL & GÜL: COMPRESSIVE STRENGTH OF LIGHTWEIGHT AGGREGATE CONCRETE 71 crease of PA ratio in the mixes the loss of strength was decreased. No significant strength loss has been observed for all groups when the temperature reached up to 300 o C. The compressive strengths of specimens at this temperature are nearly equal to the strength values at 150 o C. When the temperature reached to 450 o C, in group I a significant strength loss was observed. It is seen that by the increase of temperature all groups show strength losses but these losses decrease with the increase of LWA ratio in the mixes. It is noticed that all of the mix groups lost a significant part of their initial strengths when the temperature is reached up to 750 o C. Moreover, group V was again the group in which less strength loss was observed. An increase in PA ratio in the mix reduces loss in strength of concrete. In brief the concrete specimens, heated to o C range and kept at these temperature values for 1 h, lost some of their initial compressive strength by the increase of temperature, but less strength loss was observed in the groups which include more LWA. At temperature above 450 o C, the concrete looses significant part of its compressive strength. This can be explained by the decomposition of calcium hydroxide into lime and water vapour at elevated temperatures which may lead to serious damage due to lime expansion during cooling 20. When the concrete specimens exposed to high temperatures for 3 h and 5 h, the test results are similar with the concretes which exposed to high temperatures for 1 h. At 150 o C the groups include more ratio of ordinary aggregate, lost a significant part of their compressive strengths. Between o C no significant strength loss was observed in any groups. When the temperature reached up to 750 o C all groups lost a significant part of their initial strengths. Moreover, group V is the group which could maintain the ratio of its initial compressive strength for 3 h and 5 h exposures for all temperatures. For the specimens exposed to elevated temperatures for 3 h; when the temperature reached up to 750 o C, while group I lost 80% of its initial strength, this ratio is 55% for group V. Similarly, for 5 h heated specimens at 150 o C group I lost 21% ratio of its initial strength but group V lost only 4% of its original strength. Between o C the test results are similar with the other researcher s reports in the literature. Dias et. al. 6 pointed out that there was a significant drop in strength initially, giving a minimum at 120 o C the original strength is regained with a further increase in temperature and maintained until 300 o C. Also, Abrams 21 reported that concretes did not start loosing strength until 300 o C, irrespective of the aggregate type. The decrease of compressive strength of semilightweight and all-lightweight concretes is less than Zoldners and Wilson s 7 results. They observed that the strengths of the all-lightweight concretes, produced with expanded shale and slag aggregates, were reduced to about 80% of the non-heated strength after initial heating up to 300 o C. But in this study, this ratio is approximately 95% for all-lightweight concrete. They also reported that after exposure to 700 o C the strengths were gradually reduced to about 45%. The test results of this study are similar at that point. The ratios are 41% for 1 h and 5 h and 45% for 3 h exposed 750 o C temperature. Conclusions For each mix group, with the increase of temperature the compressive strength decreased but no significant strength losses were observed between o C. Moreover, some specimens gained strength at 300 o C as compared to 150 o C. The strength loss ratios are less in the groups which include more LWA for all temperatures. For all mix groups, 750 o C is the important temperature value at which all groups loose their initial strength significantly. The heating duration does not affect the strength loss significantly but a high temperature is a more significant parameter on the strength loss. At the same temperature the strength losses of specimens heated for different durations are near to each other. References 1 Wasserman R & Bentur A, Cem Concr Res, 27(4) (1997) Zhang M H & Gjorv O E, Am Concr Inst Mater J, 88 (2) (1991) Neville A M, Properties of Concrete, (ELBS 3 rd ed., Longman, Singapore), Chang T P & Shieh M M, Cem Concr Res, 26(2) (1996) Demirboğa R, Örüng İ & Gül R, Cem Concr Res, 31, (2001) Dias P S, Khoury G A & Sullivan P J, ACI Mater J, 87 (2) (1990) Zoldners N G & Wilson H S, ACI Publication SP39 (1973) Saad M, El-Enein A, Hanna G B & Kotkata M F, Cem Concr Res, 26 (5) (1996)

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