Effect of elevated temperatures and cooling regimes on normal strength concrete

The compressive strength of normal strength concrete at elevated temperatures up to 700^°C and the effect of cooling regimes were investigated and compared in this study. Thus, two different mixture groups with initial strengths of 20 and 35 MPa were produced by using river sand, normal aggregate and portland cement. Thirteen different temperature values were chosen from 50 to 700^°C. The specimens were heated for 3 h at each temperature. After heating, concretes were cooled to room temperature either in water rapidly or in laboratory conditions gradually. The residual strengths were determined by an axial compressive strength test. Strength and unit weight losses were compared with the initial values. Throughout this study, ASTM and Turkish Standards were used. It was observed that concrete properties deteriorated with the heat; however, a small increase in strength was observed from 50 to 100^°C. Strength loss was more significant on the specimens rapidly cooled in water. Both concrete mixtures lost a significant part of their initial strength when the temperature reached 700^°C. Copyright © 2008 John Wiley & Sons, Ltd.     

Thermal and residual mechanical profile of recycled aggregate concrete prepared with carbonated concrete aggregates after exposure to elevated temperatures

Thermal and residual mechanical performance of recycled aggregate concrete (RAC) prepared with recycled concrete aggregates (RCAs) after exposure to high temperatures has so far received less attention than that of conventional concrete prepared with natural aggregates (NAs). This study experimentally investigated thermal and residual mechanical performance of RAC prepared with different replacement percentages of non‐carbonated and carbonated RCAs after exposure to high temperatures. The residual mechanical properties, including compressive strength, modulus of elasticity, and peak strain at the maximum strength, were measured for evaluating the fire resistance of RAC. The experimental results showed that although the fire‐resistant ability of natural granite aggregates was high, thermal deterioration of the conventional concrete after exposure to 600°C, presented by thermal induced mesocracks, was more serious than that of RAC due to thermal incompatibility between NAs and mortar. Using the carbonated RCAs can reduce the width of thermal mesocrack in RAC. The residual mechanical properties of RAC after exposure to 600°C can be obviously improved by incorporating 20% to 40% of the carbonated RCAs. For the RAC made with the 100% carbonated RCAs, the ratio of residual to initial compressive strength after exposure to above 500°C was even higher than that of the conventional concrete.     

Residual strength of hybrid-fiber-reinforced high-strength concrete after exposure to high temperatures

Residual strengths of high-strength concrete (HSC) and hybrid-fiber-reinforced high-strength concrete (HFRHSC) after exposure to high temperatures were investigated in the paper. The results showed that normal HSC is prone to spalling after exposure to high temperatures, and its first spalling occurs when the temperature approaches 400 °C. For HSC reinforced by high melting point fibers, the first spalling occurs when the temperature reaches to approximately 800 °C, while there is no spalling during exposing to high temperatures for HSC reinforced by polypropylene (PP) fiber with a low melting point. Mixing high melting point fiber (i.e., carbon or steel fiber) with low melting point fiber (i.e., PP fiber) HSC greatly improves the properties of HSC after exposure to high temperatures.     

Compressive strength of fly‐ash‐based geopolymer concrete at elevated temperatures

This paper presents the compressive strength of fly‐ash‐based geopolymer concretes at elevated temperatures of 200, 400, 600 and 800 °C. The source material used in the geopolymer concrete in this study is low‐calcium fly ash according to ASTM C618 class F classification and is activated by sodium silicate (Na₂SiO₃) and sodium hydroxide (NaOH) solutions. The effects of molarities of NaOH, coarse aggregate sizes, duration of steam curing and extra added water on the compressive strength of geopolymer concrete at elevated temperatures are also presented. The results show that the fly‐ash‐based geopolymer concretes exhibited steady loss of its original compressive strength at all elevated temperatures up to 400 °C regardless of molarities and coarse aggregate sizes. At 600 °C, all geopolymer concretes exhibited increase of compressive strength relative to 400 °C. However, it is lower than that measured at ambient temperature. Similar behaviour is also observed at 800 °C, where the compressive strength of all geopolymer concretes are lower than that at ambient temperature, with only exception of geopolymer concrete containing 10 m NaOH. The compressive strength in the latter increased at 600 and 800 °C. The geopolymer concretes containing higher molarity of NaOH solution (e.g. 13 and 16 m ) exhibit greater loss of compressive strength at 800 °C than that of 10 m NaOH. The geopolymer concrete containing smaller size coarse aggregate retains most of the original compressive strength of geopolymer concrete at elevated temperatures. The addition of extra water adversely affects the compressive strength of geopolymer concretes at all elevated temperatures. However, the extended steam curing improves the compressive strength at elevated temperatures. The Eurocode EN1994:2005 to predict the compressive strength of fly‐ash‐based geopolymer concretes at elevated temperatures agrees well with the measured values up to 400 °C. Copyright © 2014 John Wiley & Sons, Ltd.     

Dynamic compressive behavior of basalt fiber reinforced concrete after exposure to elevated temperatures

This paper investigated the dynamic behavior of basalt fiber reinforced concrete (BFRC) after elevated temperatures by using a 100‐mm‐diameter split Hopkinson pressure bar apparatus. Changes in weight and ultrasonic pulse velocity (UPV) were also studied. The results indicate that the weight losses of BFRC before cooling increase with temperature, while a reduction in weight loss value is observed after water cooling. The UPV values of BFRC decrease constantly as temperature increases, and the measured velocities under the same temperature increase with fiber content as temperature exceeds 200 °C. For a given temperature, the strain rate, dynamic strength, critical strain, and impact toughness of BFRC increase with impact velocity. For a given impact velocity, the increasing temperature generally leads to an increase in strain rate and critical strain and results in a decrease in dynamic strength and impact toughness except in the case of 200 °C. At 200 °C, however, a marginal reduction, even an improvement in dynamic strength is observed, and the impact toughness initially decreases, then increases with loading rate when compared with that at room temperature. Basalt fiber is effective in improving the strength performance, deformation capacity, and energy absorption property of concrete after high temperature. Copyright © 2015 John Wiley & Sons, Ltd.     

Residual compressive behavior of alkali‐activated concrete exposed to elevated temperatures

This paper reports the effect of elevated temperature exposures, up to 1200^°C , on the residual compressive strengths of alkali‐activated slag concrete (AASC) activated by sodium silicate and hydrated lime; such temperatures can occur in a fire. The strength performance of AASC in the temperature range of 400–800^°C was similar to ordinary Portland cement concrete and blended slag cement concrete, despite the finding that the AASC did not contain Ca(OH)₂ , which contributes to the strength deterioration at elevated temperatures for Ordinary Portland Cement and blended slag cement concretes. Dilatometry studies showed that the alkali‐activated slag (AAS) paste had significantly higher thermal shrinkage than the other pastes while the basalt aggregate gradually expanded. This led to a higher thermal incompatibility between the AAS paste and aggregate compared with the other concretes. This is likely to be the governing factor behind the strength loss of AASC at elevated temperatures. Copyright © 2008 John Wiley & Sons, Ltd.     

Effect of cooling methods on residual compressive strength and cracking behavior of fly ash concretes exposed at elevated temperatures

This paper presents the effects of cooling methods on residual compressive strength and cracking behavior of concretes containing four different class F fly ash contents of 10%, 20%, 30% and 40% as partial replacement of cement at various elevated temperatures. The residual compressive strength of the aforementioned fly ash concretes is measured after being exposed to 200, 400, 600 and 800 °C temperatures and two different cooling methods, for example, slow cooling and rapid water cooling. Results show that the residual compressive strengths of all fly ash concretes decrease with increase in temperatures irrespective of cooling regimes, which is similar to that of ordinary concrete. Generally, control ordinary concrete and all fly ash concretes exhibited between 10% and 35% more reduction in residual compressive strength because of rapid cooling than slow cooling except few cases. Cracks are observed over concrete specimens after being exposed to temperatures ranging from 400 to 800 °C. Samples that are slowly cooled developed smaller cracks than those rapidly cooled. At 800 °C, all fly ash concretes that are exposed to rapid cooling showed the most severe cracking. X‐ray diffraction analysis shows reduction of Ca(OH)₂ peak and formation of new calcium silicate peak in concretes containing 20% and 40% fly ash when subjected to 800 °C in both cooling methods. Thermo gravimetric analysis and differential thermal analysis results show increase in thermal stability of concrete with increase in fly ash contents. The existing Eurocode also predicted the compressive strength of fly ash concretes with reasonable accuracy when subjected to the aforementioned elevated temperatures and cooling methods. Copyright © 2014 John Wiley & Sons, Ltd.     

An empirical model for confined concrete after exposure to high temperature

In this study, constitutive relationships have been developed for confined concrete subjected to elevated temperature to specify the fire‐performance criteria for concrete structures after exposé to fire. This study extends over a total of 63 circular hoop confined concrete specimens that were casted and tested under concentric compression loading after exposure to high temperature. The test variables studied are the yield strength of transverse reinforcement, spacing of the hoop, and exposure to temperatures from ambient to 800°C. It is shown that all of these variables have significant influence on concrete behavior at different temperatures and further an improvement in the thermal resistance of concrete when confined using transverse steel reinforcement. On the basis of experimental results, a model for confined concrete after exposed to high temperature is proposed to predict the results of residual behavior after thermal cycles. The proposed empirical stress‐strain equations are suitable to predict the postfire behavior of confined normal strength concrete in compression. The predictions were found to be in good agreement and well fit with experimental results.     

Residual mechanical resistance of concrete blocks and laying mortars after high temperatures

This paper reports an experimental campaign to evaluate the residual mechanical resistance after high temperatures of two structural masonry components: block and mortar. Residual compressive strength and deformation modulus of four different hollow concrete blocks and two different mortar mixes after heating at high temperatures are investigated. The test method used was the one recommended by RILEM TC 200 for mortars and an adaptation of the same method proposed by Medeiros et al. suitable for the geometry of hollow blocks. Despite the sharp drop in the deformation modulus after heating blocks and mortar, no different behaviours are observed in the deformability of the materials caused by the variables studied. The same cannot be said in relation to the variation of the residual compressive strength of the blocks, which is affected by the variables: initial nominal compressive strength and width of the concrete block. Regarding laying mortars, the results confirmed the small influence of compressive strength on the evolution of residual mechanical strength. The data and analyses reported here on the residual mechanical properties of hollow concrete blocks produced from a concrete mixture of very dry consistency, vibro-pressed and with normal weight aggregates are relevant, since the data found in the literature generally refer to the wet cast concrete material and in cylindrical bodies.     

Properties of pumice aggregate concretes at elevated temperatures and comparison with ANN models

The mechanical properties and thermal conductivity of concretes including pumice aggregate (PA) exposed to elevated temperature were analyzed by thermal conductivity, compressive strength, flexure strength, dynamic elasticity modulus (DEM) and dry unit weight tests. PA concrete specimens were cast by replacing a varying part of the normal aggregate (0–2 mm) with the PA. All concrete samples were prepared and cured at 23 ± 10C lime saturated water for 28 days. Compressive strength of concretes including PA decreased that reductions were 14, 19, 25 and 34% for 25, 50, 75 and 100% PA, respectively. The maximum thermal conductivity of 1.9382 W/mK was observed with the control samples containing normal aggregate. The tests were carried out by subjecting the samples to a temperature of 0, 100, 200, 300, 400 500, 600 and 700 °C for 3 h, then cooling by air cooling or in water method. The results indicated that all concretes exposed to a temperature of 500 and 700 °C occurred a significant decrease in thermal conductivity, compressive strength, flexure strength and DEM. An artificial neural network (ANN) approach was used to model the thermal and mechanical properties of PA concretes. The predicted values of the ANN were in accordance with the experimental data. The results indicate that the model can predict the concrete properties after elevated temperatures with adequate accuracy. Copyright © 2016 John Wiley & Sons, Ltd.     

Residual stress-strain relationship for concrete after exposure to high temperatures

An experimental research is performed on the complete compressive stress-strain relationship for concrete after heating to temperatures of 100-800 °C. All concrete specimens are ?15 cm × 30 cm standard cylinders, made with siliceous aggregate. The heated specimens are tested at 1 month after they are cooled to room temperature. From the results of 108 specimens with two original unheated strengths, a single equation for the complete stress-strain curves of heated concrete is developed to consider the shape varying with temperature. Through the regression analysis, the relationships of the mechanical properties with temperature are proposed to fit the test results, including the residual compressive strength, peak strain and elastic modulus. Compared with the experimental curves, the proposed equation is shown to be applicable to unheated and heated concrete for different temperatures. In addition, the split-cylinder tests of 54 specimens are also carried out to study the relationship of splitting tensile strength with temperature.     

Influence of fibers on bond strength of concrete exposed to elevated temperature

Concrete is a building material having good fire resistance and the resistance depend on many factors including the properties of its constituent materials. Fiber Reinforced Concrete (FRC) apart from improving mechanical properties has better fire resistance than conventional concrete. Bond strength of concrete is one of the important properties to be considered by structural engineers while designing reinforced concrete cements. In this research, an experimental investigation has been carried out to determine the effect of fibers on the bond strength of different grades (M20, M30, M40 and M50) of concrete subjected to elevated temperature. Different types of fibers such as Aramid, Basalt, Carbon, Glass and Polypropylene were used in the concrete with a volume proportion of 0.25% to determine the bond strength by pull-out test. Prior to the pull-out test, the specimens were kept in a furnace and subjected to elevated temperatures following standard fire curve as per ISO 834. Based on the test results of the investigations, type of fiber, grade of concrete and duration of heating were found to be the key parameters that affect the bond strength of concrete. The contribution of carbon fiber in enhancing the bond strength was found to be more significant compared to other fibers. An empirical relationship has been developed to predict the bond strength of FRC at a slip of 0.25?mm. This empirical relationship is validated with experimental results.     

High temperatures properties of barite concrete with cathode ray tube funnel glass

Heavyweight concrete has been used for different types of radiation shielding applications. Research studies have been performed to determine the mechanical properties of heavyweight concrete. However, relatively little information is available on the fire performance of heavyweight concrete after exposure to elevated temperatures. This study investigates the effects of elevated temperatures (25, 300, 500, 600 and 800 °C) on the residual density, compressive strength and water sorptivity of heavyweight barite concrete. A control mix of normal granite concrete was produced for comparison. The influence of using treated cathode ray tube (CRT) funnel glass cullets as a fine aggregate replacement in the barite concrete was also examined. The results show that the type of aggregate used had a significant influence on the residual properties of concrete mainly because of their physical and mineralogical transformation upon heating. Incorporation of CRT glass in barite concrete seemed to increase the risk of explosive spalling after exposure to 500 °C. However, as the temperature was increased from 600 to 800 °C, the loss of mechanical properties of the concrete containing CRT glass was smaller because of the molten glass being able to fill the pores/cracks, which was followed by resolidification upon cooling. Copyright © 2013 John Wiley & Sons, Ltd.     

粗骨料颗粒级配对混凝土抗折强度的影响

粗骨料最大粒级及颗粒级配影响混凝土的抗折强度,本文主要研究了粗骨料最大粒级、粗骨料单级配、二级配、三级配对混凝土抗折强度的影响。研究表明,粗骨料的最大粒级及颗粒级配对混凝土抗折强度影响很大,合理的最大粒级和级配能够提高混凝土的抗折强度。     

Compressive behavior of fiber reinforced high-performance concrete subjected to elevated temperatures

In this paper, the effects of elevated temperatures on the compressive strength stress–strain relationship (stiffness) and energy absorption capacities (toughness) of concretes are presented. High-performance concretes (HPCs) were prepared in three series, with different cementitious material constitutions using plain ordinary Portland cement (PC), with and without metakaolin (MK) and silica fume (SF) separate replacements. Each series comprised a concrete mix, prepared without any fibers, and concrete mixes reinforced with either or both steel fibers and polypropylene (PP) fibers. The results showed that after exposure to 600 and 800 °C, the concrete mixes retained, respectively, 45% and 23% of their compressive strength, on average. The results also show that after the concrete was exposed to the elevated temperatures, the loss of stiffness was much quicker than the loss in compressive strength, but the loss of energy absorption capacity was relatively slower. A 20% replacement of the cement by MK resulted in a higher compressive strength but a lower specific toughness, as compared with the concrete prepared with 10% replacement of cement by SF. The MK concrete also showed quicker losses in the compressive strength, elastic modulus and energy absorption capacity after exposure to the elevated temperatures. Steel fibers approximately doubled the energy absorption capacity of the unheated concrete. They were effective in minimizing the degradation of compressive strength for the concrete after exposure to the elevated temperatures. The steel-fiber-reinforced concretes also showed the highest energy absorption capacity after the high-temperature exposure, although they suffered a quick loss of this capacity. In comparison, using PP fibers reduced the energy absorption capacity of the concrete after exposure to 800 °C, although it had a minor beneficial effect on the energy absorption capacity of the concrete before heating.     

Effect of elevated temperatures on concrete incorporating ferronickel slag as fine aggregate

This study evaluates the effect of elevated temperature exposure on concrete incorporating ferronickel slag (FNS) as a replacement of natural sand. Concrete cylinders were exposed up to 800°C, and the changes in compressive strength, mass, ultrasonic pulse velocity (UPV), and microstructure were investigated. The concretes containing up to 100% FNS aggregate showed no spalling and similar cracking to that of the concrete using 100% natural sand. For exposures up to 600°C, the residual strengths of concretes containing 50% FNS were 7% to 10% smaller than the concrete with 100% sand. Use of 30% fly ash as cement replacement improved residual strength by pozzolanic reaction for exposures up to 600°C. An equation has been found from the correlation between residual strength and UPV. Therefore, UPV can be used as a nondestructive test to estimate the extent of postfire damage and residual strength of concrete incorporating FNS aggregate and fly ash.     

The effect of recycled concrete aggregate properties on the bond strength between RCA concrete and steel reinforcement

The purpose of this study was to investigate the influence that replacing natural coarse aggregate with recycled concrete aggregate (RCA) has on concrete bond strength with reinforcing steel. Two sources of RCA were used along with one natural aggregate source. Numerous aggregate properties were measured for all aggregate sources. Two types of concrete mixture proportions were developed replacing 100% of the natural aggregate with RCA. The first type maintained the same water–cement ratios while the second type was designed to achieve the same compressive strengths. Beam-end specimens were tested to determine the relative bond strength of RCA and natural aggregate concrete. On average, natural aggregate concrete specimens had bond strengths that were 9 to 19% higher than the equivalent RCA specimens. Bond strength and the aggregate crushing value seemed to correlate well for all concrete types.     

Residual response of fire‐damaged high‐strength concrete beams

This paper presents results from an experimental study on residual capacity of fire‐damaged high‐strength concrete (HSC) beams. Four reinforced concrete (RC) beams, fabricated with HSC, were first subject to structural loading and fire exposure with a distinct cooling phase and then loaded to failure upon cooldown to ambient conditions to evaluate residual capacity. Temperatures, deflections, and spalling in the beams were monitored during heating and cooling phases of fire exposure. Further, residual capacity, strains at critical section, and crack patterns (failure mode) of fire‐damaged beams were recorded during residual capacity tests. Results from experiments indicate that the load level during fire exposure, duration of heating phase, rate of cooling, extent (type) of spalling, and duration of postcooling storage influence residual deformations and also residual capacity of RC beams. Further, fire‐damaged HSC beams can recover 40% to 70% of their flexural capacity with respect to their room temperature design capacity provided they survive the entire duration of fire exposure.     

再生混凝土抗压强度试验研究

设计3种不同再生骨料取代率的再生混凝土立方体试件,分别在7 d、14 d、28 d、100 d龄期进行抗压强度试验,结果表明：长龄期再生混凝土立方体抗压强度有所增大,但高取代率比低取代率的抗压强度降低,再生混凝土后期强度增长速率高于普通混凝土。对试验室实测的再生混凝土立方体抗压强度与试验龄期进行拟合,拟合结果相关系数均大于0.97,吻合结果较好。     

Effect of elevated temperatures on mechanical properties of high‐strength concrete containing varying proportions of hematite

Aggregates typically constitute 70 to 80 wt% of concrete, and therefore their type, size, and structure play an essential role in modifying the properties of concrete. When concrete is used for shielding nuclear applications, temperature is also a key factor. This study investigates the effects of elevated temperatures (25 °C, 200 °C, 400 °C, 600 °C, and 800 °C), heating durations (1, 2, and 3 h), and cooling regimes (air, and water cooling) on mechanical properties of concrete containing different proportions of hematite. A sample of plain concrete was produced for comparison purposes by using river sand, crushed sand, and crushed aggregates. Replacement ratios of 15%, 30%, 45%, and 60% were used for hematite aggregates. The cement content and water–cement ratio were 450 kg/m³ and 0.38, respectively. Slump values of fresh concretes as well as unit weight, compressive strength, flexural strength, splitting tensile strength, and elasticity modulus values of hardened concrete were determined. The addition of hematite into concrete seems to improve its mechanical properties, and hematite concretes have better thermal stability at elevated temperatures than plain concrete does. Copyright © 2011 John Wiley & Sons, Ltd.