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.     

An experimental study on thermal conductivity of concrete

Influencing factors on thermal conductivity of concrete are quantitatively investigated by QTM-D3—that is, a conductivity tester developed in Japan—and a prediction equation of thermal conductivity of concrete is suggested from the regression analysis of test results. To consider the interacted factors influencing thermal conductivity of concrete, mortar, and cement paste, seven testing variables such as age, water-cement (W/C) ratio, types of admixtures, aggregate volume fraction, fine aggregate faction, temperature, and humidity condition of specimen were adopted in this test.According to experimental results, aggregate volume fraction and moisture condition of specimen are revealed as mainly affecting factors on the conductivity of concrete. Meanwhile, the conductivities of mortar and cement paste are strongly affected by the W/C ratio and types of admixtures. However, age hardly changes the conductivity except for very early age. Finally, the conductivity of concrete is represented in terms of the aggregate volume fraction, fine aggregate fraction, W/C ratio, temperature, and humidity condition of specimen.     

Temperature dependence of thermal induced mesocracks around limestone aggregate in normal concrete

This paper presents the results of thermal expansion measurements of mortar and limestone aggregate and thermal induced mesocracks around limestone aggregate in normal concrete exposed to high temperature, up to 800°C. The former was measured by a fused silica Workhorse Dilatometer. The later was observed by the viewfinder of Polarizing Optical Microscope. The results showed that the difference in coefficient of thermal expansion between mortar and limestone exhibited the temperature dependence. Thermal induced tensile stress around limestone aggregate first took place, following the hydrostatic compression under higher temperature. During the whole heating process, the tangential, radical and linkage mesocracks in concrete gradually initiated and propagated for the response of thermal induced stress around limestone aggregate. Copyright © 2009 John Wiley & Sons, Ltd.     

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.     

Observations on dedolomitization of carbonate concrete aggregates,implications for ACR and expansion

Some carbonate aggregates used in concrete are unstable in a high alkaline solution, which is present also in pore solution of cement binder. This paper investigates the process of dedolomitization of carbonate aggregate rocks and mortar bars. Selected aggregates, limestone and dolostone are of high purity without reactive silica involvement confirmed by the XRD and the XRF. For the process of dedolomitization the effect of various temperatures, solutions and time was examined. In this investigation, measurements of expansion, optical and scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction were used. Te results indicate that the process of dedolomitization occurred not only in the NaOH solution but also in the water on the mortar bar with virgin dolostone aggregate. Elevated temperature, 60 °C, increased the rate of reaction. Furthermore, the rate of reaction significantly correlates with time, which has also been confirmed through the Rietveld analysis.     

Effect of transient creep on compressive strength of geopolymer concrete for elevated temperature exposure

The strength and transient creep of geopolymer and ordinary Portland cement (OPC)-based material (paste and concrete) were compared at elevated temperatures up to 550 °C. The strength properties were determined using an unstressed hot strength test and unstressed residual strength test for paste and concrete, respectively. At 550 °C, compared with the original strength, the strength of geopolymer was increased by 192% while the strength of OPC paste showed little change. However, after exposure to 550 °C, the residual strength percentage of both geopolymer and OPC concretes was similar. Transient creep data show that geopolymer had little change in transitional thermal creep (TTc) between 250 and 550 °C while OPC paste developed significant TTc in this temperature range. In comparison with OPC concrete, a higher strength loss of geopolymer concrete is thus believed to be due to the absence of TTc to accommodate nonuniform deformation during thermal exposure.     

Modelling of thermal cracking effects in a two-phase concrete-like solid — a parametric investigation

Incompatibility of the physical properties of concrete constituents — the aggregate and the cement paste — gives rise to microcracking when the material is cooled to very low temperatures. The phenomenon is complex and affected by a large number of factors. An analytical model of the cracking process could be instrumental in identifying parameters for experimental investigation. A preliminary study is presented, which models concrete as a two-phase medium consisting of the coarse aggregate as inclusions in the mortar matrix. A simplified finite element procedure is employed to evaluate the effects of four parameters on crack initiating temperature and on crack volume. The parameters include two physical properties — thermal expansion coefficient and elastic modulus — and two mix factors — inclusion/matrix volume fraction and inclusion particle size. Thermal coefficient and inclusion volume fraction emerge as the major parameters affecting the fracture process, but the relationship of the two mix factors is complex.     

Temperature influence on water transport in hardened cement pastes

Describing water transport in concrete is an important issue for the durability assessment of radioactive waste management reinforced concrete structures. Due to the waste thermal output such structures would be submitted to moderate temperatures (up to 80 °C). We have then studied the influence of temperature on water transport within hardened cement pastes of four different formulations. Using a simplified approach (describing only the permeation of liquid water) we characterized the properties needed to describe water transport (up to 80 °C) using dedicated experiments. For each hardened cement paste the results are presented and discussed.     

Flexural behaviour of plain and reinforced lightweight aggregate concrete beams up to 600°C

Flexural tests were conducted on concrete and reinforced concrete beams (50 × 75 × 846 mm) made of lightweight aggregate and ordinary portland cement as part of a general investigation of the properties of concrete at elevated temperature. The beams were tested at steady states of temperature in the range 20 to 600°C. The performances of the beams were compared at different temperatures by measuring the changes of their properties. The properties investigated were loss of weight, thermal movements, ultimate strength, and flexural rigidity under uniform distribution of temperature within the specimens. The performance of reinforced concrete beams shows continous deterioration of flexural properties at elevated temperature. Plain concrete, on the other hand, lose in strength and rigidity at temperatures below 100°C and gain strength at higher temperature.     

Behaviour of mortar and concrete at extremely low temperatures

The behaviour of hydrated cement paste, mortar and concrete specimens (age ～ 110 days) was investigated at low temperatures and after cyclic temperature-time histories. The relative changes of thermal strains, compressive and tensile splitting strength as well as the alteration of pore structure were studied. The strength in the frozen state increases with decreasing temperature and rising moisture content. If the frozen water-saturated mortar or concrete is reheated to room temperature a loss of strength is registered. The loss of strength increases with the number of temperature cycles. Storing the specimens at rel. hum. below 85% no losses were registered and no irreversible expansion occured. Water-saturated mortar showed after only 12 cycles an irreversible positive strain of 2,7%, indicating internal damage which was also proved by mercury porosimetry. Thus, the moisture content seems to be one of the decisive factors affecting the sensitivity of mortar and concrete subjected to extremely low temperatures.     

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.     

Study on the high‐temperature behavior and rehydration characteristics of hardened cement paste

The high‐temperature behavior and rehydration characteristics of the hardened cement paste and their mechanisms have been studied in this paper. X‐ray diffraction and thermogravimetry are used to establish the effect of elevated temperatures on the mineralogical changes that occurred in the hardened cement paste. The change of microstructure was characterized by scanning electron microscopy. The results showed that with the temperature increased, the compressive strength of hardened cement paste first increased and then decreased. According to micromeasurements, at 400°C, the porosity and average pore diameter of hardened cement paste increased slightly, while at 800°C, the porosity and average pore diameter of hardened cement paste increased sharply. When hardened cement paste was cured after exposing to 400°C, its pore structure and phase composition had no change, while when hardened cement paste was cured after exposing to 800°C, there are new hydration products, and its pore structure may be finer, but it cannot fully recover to the original state. Copyright © 2014 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 the lowering of initial curing temperature on the strength of steam cured mortar

Studies were made of the compressive strength and bending strength of mortars of standard composition. The period of initial curing was 1, 3 or 6 hours and the temperature of steam curing was 65°C and 80°C. The initial curing temperature was 5° or 20° or lowered from 20° to 5°C. The strength of mortar cooled in the course of initial curing showed larger strength than the specimens cured at 20°C and 5°C. The strength increase is related with the ill-crystallized hydrates and the homogenous distribution of pore size in cement paste.     

Permeability measurement on high strength concrete without and with polypropylene fibers at elevated temperatures using a new test setup

A new test setup for permeability measurement at room and high temperature is presented. The experimental results obtained by employing the new setup are reported and validated. The experiments are performed on high performance concrete, without and with addition of polypropylene fibers under temperatures ranging from 20 °C to 300 °C as well as after cooling of previously heated specimens to the room temperature. The results show that plain concrete exhibits steady increase in permeability with increasing temperature, whereas concrete with fibers exhibit a sudden increase of permeability at temperatures between 80 °C and 130 °C. The results confirm the governing role of permeability on explosive spalling and suggest the existence of mechanisms of pressure relief other than just melting of fibers. The microstructure of concrete with fibers is investigated using SEM before and after exposure to high temperature. It is observed that the melted polypropylene flows only into the micro-cracks and does not penetrate into cement paste.     

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.     

A volumetric technique for measuring the coefficient of thermal expansion of hardening cement paste and mortar

Knowledge of the coefficient of thermal expansion (CTE) is of paramount importance for the determination of the cracking risk of concrete structures at early ages. This paper presents a novel technique which is suitable to measure the CTE of hardening materials with high accuracy starting from casting time.The technique consists of casting a small amount of cement paste or mortar into flexible membranes. The specimens are immersed in an oil bath, whose temperature is rapidly changed and then kept constant in repeating cycles. By suspending the sample from a high-precision balance and reading the change of mass after each temperature step, the CTE is calculated with high accuracy from the measured temperature and strain.Results on cement pastes and mortars (water/cement 0.3) showed a good repeatability. In particular, a sudden decrease in the CTE at setting time, followed by a gradual increase as the cement paste self-desiccates, was measured.     

Several properties of mineral admixtured lightweight mortars at elevated temperatures

The improvement of thermal and mechanical properties of mortars including expanded perlite aggregate (EPA) containing either clinoptilolite, a type of natural zeolite (NZ), waste glass powder (GP) or blast furnace slag (BFS) cured at elevated temperature was analyzed using thermal conductivity, compressive strength, flexure strength and dry unit weight. EPA mortar specimens were prepared by replacing a varying part of the portland cement with the above minerals. All mortar samples were prepared and cured at 23±1°C lime saturated water for 28 days. The maximum thermal conductivity of 1.3511W/mK was determined with the control samples containing plain cement. GP has shown 1 and 4% decrease for both 10, 20% GP and 25% EPA, respectively. Both BFS and NZ have a decreasing effect on thermal conductivity. The experiments were carried out, in which the samples were subjected to temperature of 300, 500 and 800°C for 2 h, then cooled in air. The results indicated that all the mortars exposed to temperature of 500 and 800°C shown a significant decrease in thermal conductivity, compressive strength and flexure strength. However, compared with the mortars including 25% EPA, adding the other admixtures at all level replacement decreased thermal conductivity, compressive strength, flexure strength and dry unit weight as a function of replacement percent. Copyright © 2010 John Wiley & Sons, Ltd.     

A rapid method for identification of alkali reactivity of aggregate

A rapid method for the identification of alkali reactivity of aggregate within two days has beeb achieved. 1×1×4 cm mortar bars with cement:aggregate = 10:1, w/c = 0.3, size of aggregate = 0.15?0.75mm were demolded after one-day curing and subjected subsequently to 100°C steam curing for 4 hours, after which they were immersed in 10% KOH solution and autoclave-treated at 150°C for 6 hours. After each stage of curing expansion measurements were carried out. From the data of more than thirty species of rocks, the authors arrived at the conclusion that the rapid method could be used to distinguish reactive and non-reactive aggregate. The results of microscopic observation made clear that the expansion of mortar bars was caused by alkali-silica reaction. This method cannot only be used to identify the alkali reactivity of aggregate, but when combined with the use of optic and electron microscope, can be also used to study the mechanism of alkali-aggregate reaction.     

Assessment of the elastic moduli of cement paste and mortar phases in concrete from pulse velocity tests

This paper describes a method for estimating the elastic moduli of the cement paste, mortar and ‘coarse aggregate-paste mixture’ phases in concrete. It is concluded that the elastic moduli of these phases can be predicted without recourse to tests on neat mortar and cement paste mixes. The elastic modulus of a mortar phase in concrete can be significantly different from that of a ‘neat mortar’ having the same mix proportions. Factors such as age, mix proportions and aggregate types, influence the elastic moduli of the different phases.