Experimental study of mechanical properties of normal-strength concrete exposed to high temperatures at an early age

ARTICLE IN PRESS Fire Safety Journal 44 (2009) 997–1002 Contents lists available at ScienceDirect Fire Safety Journal journal homepage: www.elsevier.com/locate/firesaf Experimental study of mechanical properties of normal-strength concrete exposed to high temperatures at an early age Bing Chen , Chunling Li, Longzhu Chen Department of Civil Engineering, Shanghai Jiaotong University, 800 Dongchuan Road, Shanghai 200240, PR China a r t i c l e in fo abstract Article history: Received June 2008 Received in revised form 17 June 2009 Accepted 19 June 2009 In China, accidental fires are known to occur during construction, causing concrete to be exposed to high temperatures when it is at an early age (i.e ‘‘young’’) In this paper, compressive and splitting tensile strengths of concretes cured for different periods and exposed to high temperatures were obtained The effects of the duration of curing, maximum temperature and the type of cooling on the strengths of concrete were investigated Experimental results indicate that after exposure to high temperatures up to 800 1C, early-age concrete that has been cured for a certain period can regain 80% of the compressive strength of the control sample of concrete The 3-day-cured early-age concrete was observed to recover the most strength The type of cooling also affects the level of recovery of compressive and splitting tensile strength For early-age concrete, the relative recovered strengths of specimens cooled by sprayed water are higher than those of specimens cooled in air when exposed to temperatures below 800 1C, while the changes for 28-day concrete are the converse When the maximum temperature exceeds 800 1C, the relative strength values of all specimens cooled by water spray are lower than those of specimens cooled in air & 2009 Elsevier Ltd All rights reserved Keywords: Compressive strength Concrete Curing High temperature Mechanical properties Introduction Fire has become one of the greatest threats to buildings Being a primary construction material, the properties of concrete after exposure to high temperatures have gained a great deal of attention since the 1940s [1–3] General reviews of concrete behavior at high temperature can be found in the literature [4–6] Although concrete is generally believed to be a fireproof material, studies have shown extensive damage and even catastrophic failure at high temperatures The mechanical properties of concrete at high temperature degrade mainly because of two relevant mechanisms: mechanical and physico-chemical damage [7–14] In the case of elevated heating conditions, when the temperature reaches about 300 1C, the interlayer calcium silicate hydrate (C–S–H) water and some of the combined water from C–S–H and sulfoaluminate hydrates will evaporate Calcium hydroxide (Ca(OH)2), which is one of the most important compounds in cement paste, dissociates at around 530 1C, resulting in the shrinkage of concrete The chemical changes in the aggregates occur at temperatures above 600 1C The mechanical degradation of concrete takes place due to the formation of cracks and microcracks They appear in concrete because of self-  Corresponding author Tel.: +86 21 54705110 E-mail address: hntchen@sjtu.edu.cn (B Chen) 0379-7112/$ - see front matter & 2009 Elsevier Ltd All rights reserved doi:10.1016/j.firesaf.2009.06.007 equilibrating stresses (caused by incompatible thermal strains) when concrete is rapidly heated up or cooled down and the temperature gradients are high According to prior research [15], the factors that influence the strength of concrete at high temperature can be divided into two groups: material properties and environmental factors Some research has studied the effect of material properties, such as properties of the aggregate, cement paste, aggregate–cement paste bond and their thermal compatibility with each other, on the resistance of concrete In addition, heat resistance of concrete is affected by environmental factors such as heating rate, duration of exposure to maximum temperature, cooling rate, loading conditions and moisture regime In China, large fire accidents occur every year during construction Normally, in these situations the concrete in the field is at its early age (i.e ‘‘young’’) The inner structure and chemical composition of early-age concrete are different from that in service due to uncompleted hydration at an early age There are few or no studies in the technical literature to determine how these early-age concretes behave after exposure to high temperatures and whether they are able to recover to the design strength after certain curing Thus, an experimental study on the strength properties of early-age concrete after exposure to high temperature was carried out in this paper The compressive and splitting tensile strengths of concrete after different curing ages, where the specimens were kept in the curing room for another 28 days after exposure to high temperatures, were examined The effects of ARTICLE IN PRESS 998 B Chen et al / Fire Safety Journal 44 (2009) 997–1002 curing age, type of cooling and the maximum temperature on the relative recovered strengths of concrete after exposure to high temperatures were investigated Experimental study 2.1 Materials The cement used in this experiment was ordinary Portland cement provided by Shanghai Hailuo Cement Plant with a 28-day compressive strength of 57.5 MPa The chemical and physical properties of cement and type C fly ash (FA) produced by the Shanghai Wujin Power plant are presented in Table Natural siliceous river sand (fineness modulus of 2.8) and nature granite crushed stone (maximum size of 19 mm) were used as aggregates In this paper, the designed compressive strength of concrete is 35 MPa at 28 days, which is a typical value for concrete for most civil engineering structures in China The designations, proportions and properties of the concrete mixtures are given in Table Different curing-age specimens were exposed to target temperatures of 200, 400, 600, 800 and 1000 1C For each exposure, the target temperature was maintained for h in the furnace to achieve thermal steady state [17,18] In order to ensure that the specimens reached a uniform temperature, the center temperature of the specimen was measured by another identically sized specimen with a thermocouple inside Afterwards, the hot specimens were cooled by two different ways until their temperature decreased to 20 1C One group of specimens was left at laboratory conditions for slow cooling, while the other group was cooled by water spray for rapid cooling The cooling periods varied from 20 to h depending on heating temperatures and cooling method After the cooling period, the specimens were placed into the curing room at a temperature of 20 1C and relative humidity of 9075% for another 28 days At the end of the curing, the specimens were taken out for compressive and splitting tensile strength tests The compressive strength was measured by a testing machine of 2000 kN capacity at a loading rate of 2.5 kN/s The splitting tensile strength test was conducted on cubes as per ASTM C 496-89 Six companion specimens were tested for each property, and average values were recorded 2.2 Test procedure The specimens were cast in 100  100  100 mm3 After casting, the specimens were left in the molds for 24 h at room temperature of 20 1C After demolding, the specimens were placed in a curing room at a temperature of 20 1C and relative humidity of 9075%; the specimens were then tested after different curing ages, i.e 1, 3, 7, 14 and 28 days Before testing, the surface water of the specimens was wiped off with a damp cloth All samples were heated in an electrical resistance furnace at a rate of 10 1C/min, which was controlled by a computer The heating rate of 10 1C/min was determined based on prior works [15,16] and in order to simulate the fire accidents, in which the temperature rises rapidly Table Physical, chemical and mechanical properties of cement and fly ash Chemical composition (%) SiO2 Fe4O3 Al2O3 CaO MgO SO3 Free CaO Loss on ignition Cement Fly ash 22.48 3.15 4.69 64.40 1.47 1.63 0.77 1.92 40.04 7.08 21.13 18.60 1.96 1.36 6.02 1.92 Physical and mechanical properties of cement Specific gravity Initial setting time/min Final setting time/min 3.0 55 200 Compressive strength (MPa) of cement days 28 days 25.7 47.5 Table Concrete mix design Portland cement (kg/m3) Fly ash (kg/ m3) Coarse aggregate (kg/m3) Sand (kg/m3) Water (kg/ m3) Superplasticizer (kg/m3) Slump (mm) 330 60 1110 690 155 2.65 145 Experimental results and analysis 3.1 Testing phenomena During the testing, different curing-age concrete samples heated to different temperatures showed different phenomena When the temperature increased at the rate of 10 1C/min, some amounts of water mist appeared near the entrance of the furnace before the temperature reached 200 1C Meanwhile, the amount of water mist decreased with increasing curing age For those concrete specimens with only and days of curing, large amounts of white vapor appeared around the entrance of the furnace The main reason is that the cement inside the earlyage concrete is not fully hydrated and most of the water is in a free state When the temperature reached 400 1C, for concrete samples with curing age over days, a great deal of moisture began to evaporate, which resulted from dehydrations of C–S–H and AFt White water mist was emitted from the entrance of the oven and thickened when peak temperature was maintained After the temperature exceeded 400 1C, higher temperature led to thicker water mist release When maintaining a peak temperature of 600 1C for 50 min, water mist around the entrance of the oven almost dissipated, indicating the completion of the dehydrations of C–S–H and AFt The damage to the concrete after being subjected to high temperatures can be roughly evaluated by observing the concrete surface Thus, assessment of fire-damaged concrete usually starts with visual observation of color change, cracking and spalling of concrete surface Fig illustrates the concrete surfaces after exposure to 200 and 400 1C temperatures The color and appearance of those specimens was the same between those cooled in air and ones cooled by water spray The concrete started to crack when the temperature increased to 600 1C, but the effect was not significant at that temperature level At this temperature, the color of the specimens cooled in air became white, while those cooled by water spray became yellow The cracks became very pronounced at 800 1C and extensively increased at 900 1C The color of the specimens became white and even red in some cases, as shown in Fig After 28-day curing, some cracks in the specimens disappeared, especially those concrete specimens with only 1, and days of prior curing ARTICLE IN PRESS B Chen et al / Fire Safety Journal 44 (2009) 997–1002 999 Fig Specimens after exposure to (a) 200 1C and (b) 400 1C at a curing age of days Fig Specimens after exposure to (a) 600 1C, (b) 800 1C and (c) 1000 1C at a curing age of days Table Compressive and splitting tensile strengths of all mixtures Curing age (day) Compressive strength (MPa) 20 1C 200 1C Type of cooling 14 28 – – – – 35.0 Splitting tensile strength (MPa) 400 1C 600 1C 800 1C 1000 1C Type of cooling Type of cooling Type of cooling Type of cooling Air Water Air Water Air Water Air 33.0 39.5 37.5 37.0 36.5 34.5 42.6 41.0 40.7 34.5 34.1 38.2 37.1 35.6 26.5 33.0 37.2 35.6 33.1 23.0 31.0 34.0 33.5 33.0 29.5 28.5 33.5 33.0 31.5 26.5 Water Air 16.0 16.8 26.5 27.2 25.0 26.1 24.5 25.3 16.5 15.0 3.2 Strengths The cooled specimens were placed into a curing room for another 28-day curing and then removed for strength tests Compressive and splitting tensile strengths of air-cooled and sprayed-water-cooled concrete specimens heated to 20 1C (control specimen), 200, 400, 600, 800 and 1000 1C are given in Table 3.2.1 Compressive strength The relative recovered compressive strength values of aircooled and sprayed-water-cooled concrete specimens are shown in Figs and 4, respectively The relative recovered strength was calculated as the percent retained concrete strength with respect to the strength of the unheated specimen at an age of 28 days As shown in Fig 4, the recovered compressive strength of all concrete specimens except for those initially aged for day increased by about 10–15% at 200 1C in comparison with the control specimens The strength gain under 200 1C can be partially 8.5 13.0 11.5 10.0 13.5 20 1C 200 1C Water 6.5 7.0 8.0 8.5 9.0 – – – – 5.4 400 1C 600 1C 800 1C 1000 1C Type of cooling Type of cooling Type of cooling Type of cooling Type of cooling Air Water Air Water Air Water Air Water Air Water 5.1 6.0 5.8 5.7 5.6 5.3 6.15 5.8 5.8 5.4 4.8 5.8 5.5 5.3 5.0 5.0 6.0 5.6 5.4 4.9 4.6 5.2 5.1 5.0 4.0 4.7 5.4 5.2 5.1 4.0 2.5 4.3 4.2 4.1 2.6 2.4 4.4 4.3 4.2 2.4 1.0 1.1 1.2 1.2 1.2 0.6 0.8 0.9 1.0 1.0 due to strengthened hydrated cement paste during the evaporation of free water, which leads to greater van der Waal’s forces as a result of the cement gel layers moving closer to each other [5,16–20] Because the transportation of moisture in concrete is rather gradual, residual moisture in concrete allowed for accelerated hydration at the early stage of heating the concrete to high temperatures Further hydration of cementitious materials is another important cause of the hardening of hydrated cement paste For concrete specimens aged for day, the strength was too low to be damaged by high temperature because of the unhydrated cement materials After heating up to 800 1C, the recovered compressive strengths of early-age concrete specimens, except those aged for day, were much higher than that of specimens aged for 28 days The early-age concrete specimens, except ones aged for day, showed a negligible strength loss at this temperature level, which indicates that the strength of earlyage concrete can recover after certain curing The main reason for this is the existence of some unhydrated cement grains in the early-age concrete specimens Those unhydrated cement grains ARTICLE IN PRESS 1000 B Chen et al / Fire Safety Journal 44 (2009) 997–1002 120 1d 3d 7d 14 d 28 d Relative strength / % 100 80 60 40 20 0 200 400 600 800 1000 Temperature (°C) Fig Relative recovered compressive strength of air-cooled concrete after 28-day curing 1d 3d 7d 14 d 28 d Relative Strength / % 120 100 80 60 40 were similar to those cooled in air When heated up to 800 1C, the recovered strengths of early-age concrete samples, with the exception 1-day-aged ones, were much higher than those of specimens aged for 28 days Among all specimens, the recovered strength of specimens aged for days was the highest According to the relative recovered strength value, the cooling type affected the recovered strength, as shown in Fig It can be found that the recovered strength of early-age concrete cooled by sprayed water was higher than those of specimens cooled in air up to 800 1C For concrete specimens aged for 28 days, the recovered strengths of specimens after air-cooling were higher than those of specimens cooled by water spray In the case of heating to 1000 1C, the recovered strengths of all specimens after air-cooling were higher than those of specimens cooled by water spray Previous research indicated that the residual compressive strength values of water-cooled concrete specimens are smaller than those of air-cooled specimens at all temperatures, which is consistent with our results for concrete aged for 28 days [21] This result can be attributed to the formation of microcracks due to large thermal gradients occurring within the concrete (thermal shock) and to the increment in the degree of water saturation of the specimen However, for early-age concrete, some of the cement grains remained unhydrated, and the loss was only from the free water when exposed to high temperatures less than 800 1C The sprayed water compensated for the evaporated free water and would take part in the chemical reaction At the same time, the high temperature of the specimens accelerated the hydration reaction Even though a large number of microcracks and damage appeared due to the thermal gradients 20 120 1d 3d 7d 14 d 28 d 200 400 600 800 1000 Temperature (°C) Fig Relative recovered compressive strength of sprayed–water-cooled concrete after 28-day curing continue to hydrate and gain strength after exposure to high temperature and then being kept in a curing room for some time After heating to temperatures below 800 1C, the relative recovered compressive strength tended to increase with increasing curing age, with the order being days47 days414 days 428 days41 day For concrete specimens aged for days, the hydration may be approximately 45% complete, and the specimens were strong enough to resist the high temperature With increasing cure time, the amount of unhydrated cement grains decreases Therefore, the recovered strength of concrete specimens aged for days was the highest However, when the temperature was elevated to 1000 1C, the recovered strength of concrete specimens increased depending on their curing age The relative recovered compressive strength values for 1-, 3-, 7-, 14- and 28-day specimens were found to be 24%, 37%, 33%, 28% and 38%, respectively Thus, 28-day specimens revealed the highest recovered compressive strength Previous research [9] showed that the decomposition of hydration products and the destruction of C–S–H gel generally occur at temperatures above 800 1C At the same time, chemical changes occur in the aggregates Therefore, the concrete specimens cannot revert to their original strength after being kept in a curing room following exposure to above 800 1C At these temperatures, it is more reasonable to refer to residual strength rather than to recovered strength Relative recovered compressive strength of water-cooled concrete specimens is shown in Fig It can be seen that the effects of curing age and maximum temperature on the recovered strength of concrete specimens cooled by water spray 100 80 60 40 20 0 200 400 600 800 1000 Temperature (°C) Fig Relative splitting tensile strength of air-cooled concrete after 28-day curing 120 Relative Strength / % Relative Strength (%) 1d 3d 7d 14 d 28 d 100 80 60 40 20 0 200 400 600 800 1000 Temperature (°C) Fig Relative splitting tensile strength of sprayed-water-cooled concrete after 28-day curing ARTICLE IN PRESS B Chen et al / Fire Safety Journal 44 (2009) 997–1002 Relative Strength (%) 140 1001 In air d 120 By water 1d 100 By water d 80 By water d 60 By water 14 d 40 By water 28 d In air d In air d In air 14 d In air 28 d 20 200 400 600 800 1000 Temperature (°C) Fig Effect of cooling method on the relative splitting strength of different curing-age concrete after exposure to high temperature within concrete specimens, those microcracks and damage can be healed and strength recovered to the reference strength after certain curing 3.2.2 Splitting tensile strength The relative splitting tensile strength values of air-cooled and sprayed-water-cooled concrete specimens are shown in Figs and 7, respectively From the figures, it can be seen that the trends in recovered splitting tensile strength were similar to those in compressive strength However, the deteriorating effect of elevated temperatures on splitting tensile strength of concrete specimens was more severe than that on compressive strength The main reason is that the existence of cracks caused by high temperature reduces the effective cross-sectional area, and the existence of tensile stress causes expansion of the cracks Due to this, microcracks that form at elevated temperatures are more destructive on splitting tensile strength than on compressive strength [22,23] Fig shows the variation of relative recovered splitting tensile strength for air-cooled concrete specimens It was shown that the recovered splitting tensile strengths of specimens aged for and days were higher than that of the control specimens, while those of the specimens aged for and 28 days were much lower than that of the control specimens, in the case of temperature exposure to below 400 1C When the maximum temperature reached 600 and 800 1C, the recovered splitting tensile strength of early-age concrete was much higher than that of concrete aged for 28 days For the maximum temperature of 1000 1C, the recovered splitting tensile strength of 28-day concrete was the highest, with a value of only 20% This indicates that the hydration products and aggregates in concrete have been decomposed and that the curing was completely ineffective Fig shows the variation of relative recovered splitting tensile strengths for concrete specimens cooled by sprayed water Similarly to concrete specimens cooled in air, it also indicates that the recovered strength of early-age concrete was much higher than that of 28-day concrete after heating up to 800 1C However, the values of concrete specimen cooled by water spray were higher than those of air-cooled specimens, with the exception of 28-day ones When the maximum temperature reaches 1000 1C, for all specimens, the recovered strength of air-cooled specimens was higher than that of water-sprayed ones Conclusions (1) For early-age concrete, 80–90% of its initial strength can be recovered when kept in a curing room for another 28 days (2) (3) (4) (5) after exposure to high temperatures up to 800 1C The recovered strength can be higher than that of the control specimen when the maximum temperature is only 200 or 400 1C Among the different early-age concretes exposed to high temperatures less than 800 1C, the recovered strength of the 3-day-cured specimens was the highest The order of the recovered strengths of the high-temperature exposed specimens was days47 days414 days41 day When the maximum temperature reached 1000 1C, the strength of early-age concrete specimens could not be recovered after curing, and the recovered strength was lower than that of concrete aged for 28 days Compared with the compressive strengths of the elevatedtemperature exposed specimens, a greater decrease was shown in the splitting tensile strength due to the more destructive microcrack and brittle microstructure formation that resulted from the tensile stress The recovered mechanical properties of concretes are noticeably affected by the cooling method In the case of maximum temperature being below 800 1C, for early-age concrete, the recovered strength of specimens cooled by sprayed water was higher than that of specimens cooled by air, while for concrete specimens aged for 28 days, the converse is true In the case of the maximum temperature being above 1000 1C, the recovered strength of all specimens cooled by air was higher than that of water-sprayed specimens Acknowledgements This research work was financially supported by the National Natural Science Foundation of China, Grant no 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Because the transportation of moisture in concrete is rather gradual, residual moisture in concrete allowed for accelerated hydration at the early stage of heating the concrete to high temperatures. .. degree of water saturation of the specimen However, for early- age concrete, some of the cement grains remained unhydrated, and the loss was only from the free water when exposed to high temperatures. .. strength of early- age concrete was much higher than that of 28-day concrete after heating up to 800 1C However, the values of concrete specimen cooled by water spray were higher than those of air-cooled