Comparison of compressive and splitting tensile strength of high-strength concrete with and without polypropylene fibers heated to high temperatures

This paper presents the results of an extensive experimental study on the compressive and splitting tensile strength of high-strength concrete with and without polypropylene (PP) fibers after heating to 600 °C. Mixtures were prepared with water to cementitious materials ratios of 0.40, 0.35, and 0.30 containing silica fume at 0%, 6%, and 10% cement replacement and polypropylene fibers content of 0, 1, 2, and 3 kg/m³. A severe strength loss was observed for all of the concretes after exposure to 600 °C, particularly the concretes containing silica fume despite their good mechanical properties at room temperature. The range of 300–600 °C was more critical for concrete having higher strength. The relative compressive strengths of concretes containing PP fibers were higher than those of concretes without PP fibers. The splitting tensile strength of concrete was more sensitive to high temperatures than the compressive strength. Furthermore, the presence of PP fibers was more effective for compressive strength than splitting tensile strength above 200 °C. Based on the test results, it can be concluded that the addition of 2 kg/m³ PP fibers can significantly promote the residual mechanical properties of HSC during heating.     

Effect of elevated temperature on bond between steel reinforcement and fiber reinforced concrete

The bond behavior between fiber reinforced concrete and 20-mm reinforcing steel rebars was evaluated under elevated temperatures. Fifty modified pullout specimens (100×100×400 mm) were prepared using high strength concrete with basalt aggregate and different volumetric mixtures of three types of fibers, namely brass-coated steel fibers, hooked steel fibers, and high modulus polypropylene fibers, before being cured for 28 days at 40 °C. Specimens, designated for heat-treatment, were then subjected to elevated temperatures, ranging from 350 to 700 °C, whereas unheated (control) ones were left in laboratory air. The overall response of control and heat-damaged specimens, pulled out up to failure, and cracking extent and continuity were described. Standard cubes (100 mm³) were cast, cured, and heat treated under similar conditions, then tested to evaluate compressive and splitting strengths. The results showed marked reductions in residual compressive, splitting and steel–concrete bond under high temperatures with dramatic changes in bond stress–free-end slip trend behavior. Use of fibers minimized the damage in steel–concrete bond under elevated temperatures and hence the reduction in bond strength. Specimens which incorporated hooked steel fibers attained the highest bond resistance against elevated temperatures followed, in sequence, by those prepared with the mixture of hooked and brass-coated steel, the mixture of hooked steel and polypropylene, and brass-coated steel fibers. Statistical models for bond stress versus free-end slip and bond strength versus exposure temperature were developed. These showed excellent agreement with the trend behavior of present experimental data.     

Residual bond strength between steel bars and concrete after elevated temperatures

The effects of elevated temperatures and cooling regimes on the residual (after cooling) bond strength between concrete and steel bars are investigated. For this study, ribbed steel bars of 8 mm diameter are embedded in to C20 and C35 concrete blocks with embedment lengths of 6, 10 and 12 cm. Unsealed specimens are heated to 12 different temperatures ranging between 50 and 700 °C and then cooled in water or in air. Pull-out tests are carried out on the specimens, and the effects of elevated temperatures on the residual bond strength are investigated by comparing the results against unheated specimens.     

The effect of post-fire-curing on strength-velocity relationship for nondestructive assessment of fire-damaged concrete strength

This paper investigates the residual compressive strength and ultrasonic pulse velocity (UPV) of concrete, which has been water-cured after exposure to high temperatures. The relationship between the residual strength ratio and the residual UPV ratio was developed. Cylindrical specimens were made of concrete with water-cement ratios of 0.58 and 0.68 and, after 90 days, the specimens were heated in an electric furnace to temperatures ranging from 400 to 1000 °C. The concrete specimens exposed to elevated temperatures were cured in a water tank for 72 h and tested after 4, 27, 87 and 177 days. The ultrasonic pulse velocity and compressive strength of each post-fire-curing specimen were measured. Experimental results show that water curing of the concrete specimens after exposure to high temperatures has noticeable effects on the residual strength and UPV recovery. It is also shown that a change in the mixture proportion of concrete does not have a significant effect on the residual strength ratio and the residual UPV ratio of concrete subjected to elevated temperatures. The relationship between the residual strength ratio and the residual UPV ratio was developed and a general equation is proposed for predicting the residual strength of post-fire-curing concrete. Finally, this paper verifies the validity of the proposed equation for predicting the residual strength ratios of post-fire-curing concrete with the measured residual UPV ratios.     

The effects of high temperature on compressive and flexural strengths of ordinary and high-performance concrete

The variation of compressive and flexural strengths of ordinary and high-performance micro-concrete at high temperatures was examined. Compressive and flexural strengths of ordinary and high-performance micro-concrete which were exposed to high temperatures (200, 400, 600, 800 and 1000 °C) and cooled differently (in air and water) were obtained. Compressive and flexural strengths of these concrete samples were compared with each other and then compared with the samples which had not been heated. On the other hand, strength loss curves of these concrete samples were compared with the strength loss curves given in the codes. Experimental results indicate that concrete strength decreases with increasing temperature, and the decrease in the strength of ordinary concrete is more than that in high-performance concrete. The type of cooling affects the residual compressive and flexural strength, the effect being more pronounced as the temperature increases. Strength loss curves obtained from this study agree with strength loss curves given in the Finnish Code.     

Properties and behavior of self-compacting concrete produced with GBFS and FA additives subjected to high temperatures

This paper presents an experimental investigation on the performance of self-compacting concrete (SCC) subjected to high temperatures. For this purpose, Portland cement was replaced with fly ash (FA) and granulated blast furnace slag (GBFS) in various proportions with and without polypropylene (PP) fibers and the PP fiber content was 2 kg/m³ for the mixtures that contained fibers. When the specimens were 56 days old, they were heated to elevated temperatures (200, 400, 600 or 800 °C). Afterword, tests were conducted to determine the weight loss and the compressive strength. Moreover, the change in the ultrasonic pulse velocity (UPV) was determined, and observations for surface cracks were made after the specimens were exposed to elevated temperatures. A severe strength loss was observed for all of the concretes after 600 °C, particularly for the concretes that contained PP fibers; however, the fibers reduced and eliminated the risk of explosive spalling. Based on the test results, it can be concluded that the performance of FA concrete is better than that of the GBFS concrete.     

Performance of olive waste ash concrete exposed to elevated temperatures

This study explores the influence of olive waste ash (OWA) on the performance of concrete exposed to elevated temperatures in the range from 400 to 600 °C. The performance of concrete to elevated temperature was evaluated using compressive strength and electrical charge of concrete. Three OWA levels were used in the study: 7%, 15%, and 22% by weight of cement. The other experimental parameters investigated in the study were elevated temperature (400 and 600 °C), aggregate type (crushed basalt and volcanic tuff), w/c ratio (0.5 and 0.7), and air content (non-air and air entrained). After the initial moist curing period of 90 days, concrete specimens were exposed to elevated temperatures for a period of 2 h using an electric furnace.     

超高性能混凝土高温后残余力学性能试验研究

测定了抗压强度高于140MPa的含粗骨料超高性能混凝土和活性粉末混凝土遭受高温作用后的残余抗压强度、残余劈裂抗拉强度和残余断裂能。结果显示,两种超高性能混凝土的残余强度均随着目标温度的升高而呈现先增大再降低的趋势,而残余断裂能均随着目标温度的升高逐渐降低。各目标温度下,含粗骨料超高性能混凝土的残余抗压强度均高于活性粉末混凝土,但其残余劈裂抗拉强度和断裂能低于后者。活性粉末混凝土在300℃临界温度下的峰值残余抗压强度和峰值残余劈裂抗拉强度分别比常温时提高了26.8%和19.3%,800℃高温后的强度损失率分别为72.3%和81.4%。含粗骨料超高性能混凝土在400℃临界温度下的峰值残余抗压强度和在300℃目标温度下的峰值劈裂抗拉强度分别比常温时提高了34.0%和6.8%,800℃高温后的强度损失率分别为70.2%和84.9%。所以,对于有抗火灾高温要求的工程结构,含粗骨料超高性能混凝土适合用于受压构件,而活性粉末混凝土适宜于抗弯构件。     

The effect of high temperatures on the mechanical properties of concrete made with different types of aggregates

The main objective of this paper was to assess the benefits of using materials that were formed at high temperatures as an aggregate for concrete that was exposed to high temperature. The fire resistance of concrete made with some locally available, potential “fire-resistant” aggregates, such as diabase, steel slag, crushed bricks and crushed tiles, was investigated. The specimens of measurements 4×4×16 cm³ were kept in molds for 24 h and, after demolding, were kept in water at room temperature of about 20±2 °C until testing. At the age of 28 days, the specimens, with moisture content within the limits of 3-5%, were exposed to high temperatures in a previously heated test furnace. The residual mechanical properties (compressive and flexural strengths) of these concretes after natural cooling were compared with the residual mechanical properties of concrete made with commonly used river and dolomite aggregates. The replacement of natural concrete aggregates with brick and steel industry waste materials was justified, not only in terms of increased fire resistance, but also in terms of more responsible waste disposal.     

有早期受荷经历的钢纤维混凝土强度试验研究

通过对钢纤维混凝土在未达到设计强度前曾受过荷载非破坏性试验的试件进行后期强度试验,研究了这种早期受荷经历对钢纤维混凝土抗折强度、劈裂抗拉强度和轴心抗压强度的影响,进行了改变早期加荷时间、早期荷载水平和钢纤维掺量等因素的比较,并与普通混凝土做对比。试验结果表明,早期受荷使钢纤维混凝土的后期强度降低,这种不利影响比对普通混凝土的影响小,但在低含纤率时要给予足够重视;在工程中如需要在较早时期作用较大荷载时,增加钢纤维掺量是减小其后期抗折和抗拉强度降低幅度的一项有效措施,并做了相关的机理分析。     

Evaluating residual compressive strength of concrete at elevated temperatures using ultrasonic pulse velocity

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.     

Properties of concrete made with recycled crushed glass at elevated temperatures

The effect of replacement of fine and coarse aggregates with recycled glass on the fresh and hardened properties of Portland cement concrete at ambient and elevated temperatures is studied. Percentages of replacement of 0–100% of aggregates with fine waste glass (FWG), coarse waste glass (CWG), and fine and coarse waste glass (FCWG) were considered. Soda-lime glass used for bottles was washed and crushed to fine and coarse aggregate sizes for use in the concrete mixes. Samples were cured under 95% RH at room temperatures (20–22 °C), heated in the oven to the desired temperatures, allowed to cool to ambient temperatures, and then tested for their residual compressive strength. The compressive strength of the concrete samples made with waste glass was measured at temperatures up to 700 °C. Moreover, the effect of the percentages of replacement with recycled glass on the slump values and initial and final setting time of concrete has also been measured.     

Effect of moisture and temperature on the mechanical properties of concrete

Concrete mechanical properties are determined under laboratory conditions of ideal air temperatures between 20 and 22 °C and relative humidity between 40% and 60%. This paper describes the development of concrete mechanical properties when cured under different environmental conditions. Tests to measure modulus of elasticity, compressive strength, and split tensile strength were conducted at varying temperatures and humidity conditions to examine their effects on normal concrete. An environmental chamber was constructed in the laboratory using available materials. The chamber works in conjunction with a freezer to provide chilled air and a heat gun to provide hot air. The heating and cooling functions were controlled via a microcontroller. The moisture content in the concrete specimens was controlled by massing the specimens. The results indicate that concrete strength and modulus of elasticity are inversely related to temperature as well as moisture content in the concrete. Concrete modulus of elasticity was directly related to concrete compressive strength in both temperature and moisture testing. Mathematical formulas were developed for modulus of elasticity, compressive strength, tensile strength, and Poisson’s ratio.     

Effect of Grade of Concrete on the Performance of Self-Compacting Concrete Beams Subjected to Elevated Temperatures

Self-compacting concrete (SCC) is a new generation concrete that consolidates without any external effort. Due to its advantages over the conventional concrete, the usage of SCC increases day by day. Understanding the behaviour of SCC is important in the design of structures subjected to elevated temperature. A study was carried out to understand the behaviour of SCC beams of various grades exposed to elevated temperatures under flexural loading. The beams were exposed to a temperature of 900°C. The heated specimens were cooled either by air or water. The research work was carried out for different grades of concrete. It is found from the results that the loss of strength of SCC beams of higher grades was more than that of the lower grade SCC beams. It was also found that the reduction in compressive, tensile and flexural strength of the specimens depends on type of heating and cooling conditions.     

The effect of high temperature on compressive strength and splitting tensile strength of structural lightweight concrete containing fly ash

In this study, the effect of high temperature on compressive and splitting tensile strength of lightweight concrete containing fly ash was investigated experimentally and statistically. The mixes incorporating 0%, 10%, 20% and 30% fly ash were prepared. After being heated to temperatures of 200, 400 and 800 °C, respectively, the compressive and splitting tensile strength of lightweight concrete was tested. This article adopts Taguchi approach with an L₁₆ (4⁵) to reduce the numbers of experiment. Two control factors (percentage of fly ash and heating degree) for this study were used. The level of importance of these parameters on compressive and splitting tensile strength was determined by using analysis of variance (ANOVA) method.     

Performance of lightweight concrete with silica fume after high temperature

In this study, the effect of silica fume on compressive and splitting tensile strength of lightweight concrete after high temperature was investigated experimentally and statistically. The mixes incorporating 0%, 10%, 20% and 30% silica fumes were prepared. After being heated to temperatures of 200, 400 and 800 °C, respectively, the compressive and splitting tensile strength of lightweight concrete was tested. This article adopts Taguchi approach with an L₁₆ (4⁵) to reduce the numbers of experiment. Two control factors (percentage of silica fume and heating degree) for this study were used. The level of importance of these parameters on compressive and splitting tensile strength was determined by using analysis of variance (ANOVA) method.     

高温后不同冷却方式下自密实混凝土力学性能研究

对54个标准立方体和27个标准棱柱体C40自密实混凝土试件高温后,采用自然冷却和喷水冷却两种方式下的力学性能进行研究,并观察试块的表观特征及测量高温后损失率,结果表明:两种冷却方式下,500℃时混凝土试块的表观特征均发生显著变化;高温后质量损失率和峰值应变随温度升高而增大,弹性模量随温度升高而下降;试块的抗压强度、劈拉...     

Flexural and Split Cylinder Strengths of HSC at Elevated Temperatures

This paper investigates the effect of elevated temperature on the flexural strength (FS) and split cylinder strength (SCS) of high strength concrete (HSC). Four concrete mixes of 50, 90, 110, and 130 MPa grade were prepared and subjected to elevated temperature exposure of 200°C and 400°C, and cooled under slow and quick cooling conditions. In addition, 130 MPa grade concrete specimens were also subjected to 100°C and 600°C exposure temperatures to compare FS and SCS under elevated temperatures. It was observed that with the increase in the elevated temperature, the FS and SCS experienced significant losses. The loss was found to be higher for richer concretes. FS was observed to experience a sharp loss at low temperatures that became gradual later at high temperatures. SCS, however, experienced a gradual loss, though sharper than FS, with the increase in temperature. The results indicated that cooling had a significant effect on the residual values and quick cooling caused greater loss in FS and SCS, than slow cooling at elevated temperatures. The quick cooling was noted to produce maximum loss over slow cooling at temperatures around 400°C.     

Development of a high-temperature-resistant mortar by using slag and pumice

The effects of high temperatures up to 900 °C on the mechanical properties and the microstructure of cement-based pumice mortars incorporating different amounts of ground granulated blast furnace slag (GGBFS) were investigated in this study. The residual compressive and flexural strength of mortar specimens were determined after exposure to high temperatures. The results have indicated that the effect of GGBFS incorporation on high-temperature resistance of pumice mortar is shown significantly at 900 °C. At this temperature level, the mortar containing 80% GGBFS exhibited only 23% and 28% compressive strength loss when cooled in air and water, respectively, where as mortars without GGBFS lost almost 70% of their strength. Furthermore, none of the GGBFS incorporated mortar specimens showed compressive strength loss up to 600 °C when cooled in air. The most severe conditions in terms of strength loss due to high temperatures were flexural loading and water cooling case.     

Effects of High Temperature on the Pore Structure and Strength of Plain and Polypropylene Fiber Reinforced Cement Pastes

The effect of high temperature on the residual properties of plain and polypropylene fiber reinforced Portland cement paste was investigated. Plain Portland cement paste having water/cement ratio of 0.32 was exposed to the temperatures of 20, 50, 75, 100, 120, 150, 200, 300, 400, 440, 520, 600, 700, 800, and 1000°C. Paste with polypropylene fibers was exposed to the temperature of 20, 120, 150, 200, 300, 440, 520, and 700°C. Residual compressive and flexural strengths were measured and pore structure of the pastes was determined by mercury porosimetry. The total porosity of the pastes more than doubled when exposure temperature was increased from 20°C to 1000°C. The gradual heating coarsened the pore structure. The most notable coarsening of pore structure—together with strength loss—took place at exposure temperatures exceeding 600°C. At 600°C, the residual compressive capacity (f_c600°C/f_c20°C) was still over 50% of the original. Strength loss due to the increase of temperature was not linear. Polypropylene fibers produced a finer residual capillary pore structure, decreased compressive strengths, and improved residual flexural strengths at low temperatures. According to the tests, it seems that exposure temperatures from 50°C to 120°C can be as dangerous as exposure temperatures 400–500°C to the residual strength of cement paste produced by a low water cement ratio.