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.     

Effect of nano silica and fine silica sand on compressive strength of sodium and potassium activators synthesised fly ash geopolymer at elevated temperatures

Environment friendly geopolymer is a new binder which gained increased popularity due to its better mechanical properties, durability, chemical resistance, and fire resistance. This paper presents the effect of nano silica and fine silica sand on residual compressive strength of sodium and potassium based activators synthesised fly ash geopolymer at elevated temperatures. Six different series of both sodium and potassium activators synthesised geopolymer were cast using partial replacement of fly ash with 1%, 2%, and 4% nano silica and 5%, 10%, and 20% fine silica sand. The samples were heated at 200°C, 400°C, 600°C, and 800°C at a heating rate 5°C per minute, and the residual compressive strength, volumetric shrinkage, mass loss, and cracking behaviour of each series of samples are also measured in this paper. Results show that, among 3 different NS contents, the 2% nano silica by wt. exhibited the highest residual compressive strength at all temperatures in both sodium and potassium‐based activators synthetised geopolymer. The measured mass loss and volumetric shrinkage are also lowest in both geopolymers containing 2% nano silica among all nano silica contents. Results also show that although the unexposed compressive strength of potassium‐based geopolymer containing nano silica is lower than its sodium‐based counterpart, the rate of increase of residual compressive strength exposed to elevated temperatures up to 400°C of potassium‐based geopolymer containing nano silica is much higher. It is also observed that the measured residual compressive strengths of potassium based geopolymer containing nano silica exposed at all temperatures up to 800°C are higher than unexposed compressive strength, which was not the case in its sodium‐based counterpart. However, in the case of geopolymer containing fine silica sand, an opposite phenomenon is observed, and 10% fine silica sand is found to be the optimum content with some deviations. Quantitative X‐ray diffraction analysis also shows higher amorphous content in both geopolymers containing nano silica at elevated temperatures than those containing fine silica sand.     

Fire resistance of geopolymer concrete produced from Elazığ ferrochrome slag

This paper presents the effect of elevated temperatures up to 700 °C on compressive strength and water absorption of two alkali‐activated aluminosilicate composites (one of them is river sand aggregate geopolymer concrete; the other one is crushed sand aggregate geopolymer concrete) and ordinary Portland cement based concretes. To obtain binding geopolymer material, Elaz?? ferrochrome slag was ground as fine as cement, and then it was alkali activated with chemical (NaOH and Na₂SiO₃). Geopolymer concrete samples were produced by mixing this binding geopolymer material with aggregates. At each target temperature, concrete samples were exposed to fire for the duration of 1 h. Fire resistance and water absorption of geopolymer and ordinary Portland cement concrete samples were determined experimentally. Experimental results indicated that compressive strength of geopolymer concrete samples increased at 100 °C and 300 °C temperatures when compared with unexposed samples. In geopolymer concrete samples, the highest compressive strength was obtained from river aggregates ones at 300 °C with 37.06 MPa. Water absorption of geopolymer concrete samples increased at 700 °C temperature when compared with unexposed samples. However, a slight decrease in water absorption of concrete samples was observed up to 300 °C when compared with unexposed samples. SEM and X‐ray diffraction tests were also carried out to investigate microstructure and mineralogical changes during thermal exposure. Copyright © 2016 John Wiley & Sons, Ltd.     

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.     

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.     

Effects of elevated temperature on pumice based geopolymer composites

Abstract

Aluminosilicate type materials can be activated in alkaline environment and can produce geopolymer cements with low environmental impacts. Geopolymers are believed to provide good fire resistance so the effects of elevated temperatures on mechanical and microstructural properties of pumice based geopolymer were investigated in this study. Pumice based geopolymer was exposed to elevated temperatures of 100, 200, 300, 400, 500, 600, 700 and 800°C for 3?h. The residual strength of these specimens were determined after cooling at room temperature as well as ultrasonic pulse velocity, and the density of pumice based geopolymer pastes before and after exposing to high temperature was determined. Microstructures of these samples were investigated by Fourier transform infrared for all temperatures and SEM analyses for samples that were exposed to 200, 400, 600 and 800°C. Specimens, which were initially grey, turned whitish accompanied by the appearance of cracks as temperatures increased to 600 and 800°C. Consequently, compressive strength losses in geopolymer paste were increased with increasing temperature level. On the other hand, compressive strength of geopolymer paste was less affected by high temperature in comparison with the ordinary Portland cement. As a result of this study, it is concluded that pumice based geopolymer is useful in compressive strength losses exposed to elevated temperatures.     

Residual compressive strength of fire‐damaged mortar after post‐fire‐air‐curing

The effects of cooling regimes and post‐fire‐air‐curing on compressive strength of mortar were investigated. Mortars were made with CEN reference sand, CEM I 42.5 R cement and natural spring water. The sand–cement and water–cement materials' ratios were chosen as 3.0 and 0.50 for all mixtures, respectively. At 28 days, the specimens were heated to maximum temperatures of 400, 600, 800 and 1000°C. Specimens were then allowed to cool in the air, furnace and water. After cooling, the specimens were air‐recured. Compressive strength test was carried out before air‐recuring and after 7 days of air‐recuring. The highest reduction in compressive strength was observed at 1000°C regardless of cooling regime. Gradual cooling regime in air and furnace without post curing showed almost no difference in terms of compressive strength reduction for four elevated temperatures. Shock cooling in water caused significant reduction in compressive strength compared with both gradual cooling regimes without post curing. After air and furnace cooling regimes, 7 days air‐recured specimens showed further reduction in compressive strength for four elevated temperatures. Specimens cooled in water and subjected to 7 days air‐recuring showed significant strength gain approximately 39, 100 and 130% for 400, 600 and 800°C elevated temperature, respectively. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

Effect of polypropylene fibers on thermogravimetric properties of self‐compacting concrete at elevated temperatures

In this study, the effect of polypropylene (PP) fibers on thermogravimetric parameters of self‐compacting concrete (SCC) containing indigenous materials was investigated experimentally and statistically. The mixes containing cement, water, fly ash, fine aggregate, coarse aggregate, and super plasticizer, with the addition of PP fibers (0%, 0.05%, 0.1%, and 0.15%) by volume of the mixtures, were prepared. The physical properties of SCC were determined at elevated temperatures (200, 400, and 600 °C) after cooling in the laboratory. Regression models were developed to determine the responses, and the optimum amount of 0.05% PP fibers by volume was measured. Copyright © 2012 John Wiley & Sons, Ltd.     

Effect of elevated temperatures on geopolymer paste, mortar and concrete

Geopolymers are generally believed to provide good fire resistance due to their ceramic-like properties. Previous experimental studies on geopolymer under elevated temperatures have mainly focused on metakaolin-based geopolymers. This paper presents the results of a study on the effect of elevated temperature on geopolymer paste, mortar and concrete made using fly ash as a precursor. The geopolymer was synthesized with sodium silicate and potassium hydroxide solutions. Various experimental parameters have been examined such as specimen sizing, aggregate sizing, aggregate type and superplasticizer type. The study identifies specimen size and aggregate size as the two main factors that govern geopolymer behavior at elevated temperatures (800 °C). Aggregate sizes larger than 10 mm resulted in good strength performances in both ambient and elevated temperatures. Strength loss in geopolymer concrete at elevated temperatures is attributed to the thermal mismatch between the geopolymer matrix and the aggregates.     

The effect of high temperature on the residual mechanical performance of concrete made with recycled ceramic coarse aggregates

This paper presents an experimental study on the residual mechanical properties of concrete with recycled ceramic coarse aggregate (RCCA) after exposure to elevated temperatures. Four concrete mixes were produced: a control concrete and three concrete mixes with replacement ratios of 20, 50 and 100% of natural aggregate (NA) by RCCA. The specimens were subjected to temperatures of 200, 400 and 600°C, for a period of 60 min. After cooling down to room temperature, the following concrete properties were evaluated: (i) compressive strength; (ii) splitting tensile strength; (iii) modulus of elasticity; (iv) ultrasonic pulse velocity (UPV); and (v) water absorption by immersion. At ambient temperature, as expected, the replacement of NA by RCCA resulted in a performance reduction of concrete. After exposure to elevated temperature, in general, the results obtained indicated an improvement of the residual relative mechanical properties of the mixes with RCCA, particularly after exposure to 400 and 600°C. However, exposure to the highest temperature (600°C) tended to cause spalling in concrete mixes containing RCCA. Significant linear correlations were observed between the residual compressive strength of all concrete mixes and both the UPV and the water absorption by immersion. Copyright © 2014 John Wiley & Sons, Ltd.     

In-situ thermo-mechanical testing of fly ash geopolymer concretes made with quartz and expanded clay aggregates

The mechanical and microstructural properties of geopolymer concretes were assessed before, during and after high temperature exposure in order to better understand the engineering properties of the material. Fly ash based geopolymer concretes with either quartz aggregate or expanded clay aggregate were exposed to various temperatures up to 750 °C using a thermo-mechanical testing apparatus. Microstructural investigations were also undertaken to better understand the measured changes in the mechanical properties. It was found that dehydration of capillary water caused cracking and strength losses at temperatures ≤ 300 °C, an effect that was more severe in the quartz aggregate geopolymer due to its lower permeability. At higher temperatures (T ≥ 500 °C) sintering promoted strength increases which enabled both concrete types to yield significant strength advantages over conventional materials. Stress–mechanical strain curves, which form the basis of the fire design of concrete structures, are reported.     

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.     

Assessment of recycling use of GFRP powder as replacement of fly ash in geopolymer paste and concrete at ambient and high temperatures

Environmental issues caused by glass fiber reinforced polymer (GFRP) waste have attracted much attention. The development of cost-effective recycling and reuse methods for GFRP composite wastes is therefore essential. In this study, the formulation of the GFRP waste powder replacement was set at 20–40 wt%. The geopolymer was formed by mixing GFRP powder, fly ash (FA), steel slag (SS) and ordinary Portland cement (OPC) with a sodium-based alkali activator. The effects of GFRP powder content, activator concentration, liquid to solid (L/S) ratio, and activator solution modulus on the physico-mechanical properties of geopolymer mixtures were identified. Based on the 28-day compressive strength, the optimal combination of the geopolymer mixture was determined to be 30 wt% GFRP powder content, an activator concentration of 85%, L/S of 0.65, and an activator solution modulus of 1.3. The ratios of compressive strength to flexural strength of the GFRP powder/FA-based geopolymers were considerably lower than those of the FA/steel slag-based geopolymers, which indicates that the incorporation of GFRP powder improved the geopolymer brittleness. The incorporation of 30% GFRP powder in geopolymer concrete to replace FA can enhance the compressive and flexural strengths of geopolymer concrete by 28%. After exposure to 600 °C, the flexural strength loss for geopolymer concretes containing 30 wt% GFRP powder was less than that of specimens without GFRP powder. After exposure to 900 °C, the compressive strength and flexural strength losses of geopolymer concretes containing 30 wt% GFRP powder were similar to those of specimens without GFRP powder. The developed GFRP powder/FA-based geopolymers exhibited comparable or superior physico-mechanical properties to those of the FA-based geopolymers, and thus offer a high application potential as building construction material.     

Flexural behavior of hybrid PVA fibers reinforced ferrocement panels at elevated temperatures

Fire safety should consider not only the performance of the structure after the fire but also the behavior during the fire. The structural fire reliability performance of hybrid PVA fiber reinforced ferrocement (HFF) panels is experimentally determined based on its flexural characteristics and damage during the exposure to elevated temperatures. The residual compressive strength of 60 cubs was also tested after exposed to temperatures. In addition, 30 HFF panels were tested to evaluate their structural capacity by conducting an in‐situ binding test during the heating of up to 200°C, 400°C, 600°C, and 800°C, and compared with control samples tested at ambient (24°C) temperature condition. The main parameters investigated were the specimen thickness and the effect of using mineral admixtures (fly ash and silica fume) in the mortar mixtures. The results show a strength decline of both flexural and compressive strengths as temperature increases. The bending capacity at 800°C is reduced to about 90% of the ambient capacity only. In between the 2 temperatures, the reduction rate is found to be almost linear. A theoretical prediction of the moment capacity reduction shows a good agreement with the test results.     

Utilisation of steel furnace slag coarse aggregate in a low calcium fly ash geopolymer concrete

This paper evaluates the performance of steel furnace slag (SFS) coarse aggregate in blended slag and low calcium fly ash geopolymer concrete (GPC). The geopolymer binder is composed of 90% of low calcium fly ash and 10% of ground granulated blast furnace slag (GGBFS). Mechanical and physical properties, shrinkage, and detailed microstructure analysis were carried out. The results showed that geopolymer concrete with SFS aggregate offered higher compressive strength, surface resistivity and pulse velocity than that of GPC with traditional aggregate. The shrinkage results showed no expansion or swelling due to delayed calcium oxide (CaO) hydration after 320 days. No traditional porous interfacial transition zone (ITZ) was detected using scanning electron microscopy, indicating a better bond between SFS aggregate and geopolymer matrix. Energy dispersive spectroscopy results further revealed calcium (Ca) diffusion at the vicinity of ITZ. Raman spectroscopy results showed no new crystalline phase formed due to Ca diffusion. X-ray fluorescence result showed Mg diffusion from SFS aggregate towards geopolymer matrix. The incorporation of Ca and Mg into the geopolymer structure and better bond between SFS aggregate and geopolymer matrix are the most likely reasons for the higher compressive strength observed in GPC with SFS aggregate.     

Stress–strain behaviour of fire exposed self‐compacting glass concrete

Concrete normally suffers from low stiffness and poor strain capacity after exposure to high temperatures. This study focused on evaluating the effect of recycled glass (RG) on the residual mechanical properties of self‐compacting glass concrete (SCGC) after exposure to elevated temperatures. RG was used to replace fine aggregate at ratios of 0%, 25%, 50%, 75% and 100% by weight. The residual properties were evaluated according to compressive strength, elastic modulus, stress–strain behaviour and strain at pre‐load and peak stress. A comparative assessment of different curing conditions on the SCGC was also conducted. The results showed that there were significant decreases in compressive strength, elastic modulus and concrete stiffness of the concrete with increasing temperature. The use of RG had little influence on the elastic modulus at ambient temperature; however, after exposure to 800°C, the mechanical properties of the concrete were greatly enhanced by incorporating RG. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis of fly ash based geopolymer mortar considering different concentrations and combinations of alkaline activator solution

Geopolymerisation is a process that can transform alumina and silica rich waste materials into valuable binding materials, having excellent mechanical properties. The present experimental study shed a light on the variation in compressive strength of fly ash based geopolymer mortar by varying the molarity of sodium hydroxide as 12 M, 14 M, 16 M and accompanying by sodium silicate (Na₂SIO₃) in 2:1 (Na₂SIO₃/ NaOH) with same molarities. All the geopolymer mixes were oven cured at 80 °C for 24 h and after that kept at room temperature up to the time of testing. The compressive strength was checked subsequently at the ages of 3, 7, 14 and 28 days. The experimental results reveal that the addition of sodium silicate enhances the strength development in geopolymer mortar. The ultimate compressive strength of 40.42 MPa was obtained by incorporating sodium silicate along with 16 M concentrated sodium hydroxide. Furthermore, increasing trend of the compressive strength was found with increasing molar concentration of sodium hydroxide and curing period.     

Effect of aggregate and water to cement ratio on concrete properties at elevated temperature

Properties of concrete during and after fire exposure are of significant importance for serviceability and rehabilitation of buildings. This article presents an experimental investigation on the effects of elevated temperature on physical and mechanical properties of concrete made using two types of locally available coarse aggregates (crushed and wadi aggregates) and water‐to‐cement (w/c) ratios of 0.50 and 0.70. Temperature range from 200 °C to 1000 °C was used with intervals of 200 °C. Test results indicate that the weight of concrete reduced with increase in temperature. This reduction was quite sharp beyond 800 °C. Minor spalling was observed in concrete with Wadi aggregates at temperatures beyond 800 °C. The results also reveal that relative strength of concrete decreased as exposure temperature increased. The effect of high temperatures on the strength of concrete was more pronounced in concrete with Wadi aggregates. w/c ratio had insignificant effect on weight loss after exposure to elevated temperatures, but it increased the rate of strength degradation irrespective of aggregate type used. Comparison of results with Eurocode (EC‐2) and American Concrete Institute (ACI) standards indicate that the concrete with both aggregate types can satisfy the limits of siliceous aggregates set by ACI, but concrete made with Wadi aggregates with w/c ratio of 0.50 failed to satisfy limits of EC‐2. Copyright © 2015 John Wiley & Sons, Ltd.     

粉煤灰掺量对再生混凝土力学性能和抗冻性的影响研究

研究粉煤灰掺量、再生粗骨料取代率对再生混凝土抗压强度和抗折强度的影响,并对再生混凝土在不同冻融循环次数下的抗压强度和质量损失率进行了研究.结果表明:随着粉煤灰掺量的增加,再生混凝土抗压强度呈先增大后降低的趋势,当粉煤灰掺量为15%,再生粗骨料取代率为30%时,再生混凝土的抗压强度达到最大;粉煤灰掺量对抗折强度提高幅度较小;在冻融循环低于50次时,试块抗压强度下降速度较缓,此后下降速度加快,当冻融循环达到150次时,强度损失最大;再生粗骨料取代率对试块的抗冻性影响高于粉煤灰掺量.建立了考虑再生粗骨料取代率、粉煤灰掺量因素的冻融循环作用下再生混凝土抗压强度指数衰减规律预测模型.