Performance of metakaolin concrete at elevated temperatures

An experimental investigation was conducted to evaluate the performance of metakaolin (MK) concrete at elevated temperatures up to 800 °C. Eight normal and high strength concrete (HSC) mixes incorporating 0%, 5%, 10% and 20% MK were prepared. The residual compressive strength, chloride-ion penetration, porosity and average pore sizes were measured and compared with silica fume (SF), fly ash (FA) and pure ordinary Portland cement (OPC) concretes. It was found that after an increase in compressive strength at 200 °C, the MK concrete suffered a more severe loss of compressive strength and permeability-related durability than the corresponding SF, FA and OPC concretes at higher temperatures. Explosive spalling was observed in both normal and high strength MK concretes and the frequency increased with higher MK contents.     

High strength blended cement concrete incorporating volcanic ash: Performance at high temperatures

The strength and durability of high strength blended cement concretes incorporating up to 20% of volcanic ash (VA) subjected to high temperatures up to 800 °C are described. The strength was assessed by unstressed residual compressive strength, while durability was investigated by rapid chloride permeability (RCP), mercury intrusion porosimetry (MIP), differential scanning calorimetry (DSC), crack pattern observations and microhardness testing. High strength volcanic ash concrete (HSVAC) exhibited better performance showing higher residual strength, chloride resistance and resistance against deterioration at high temperatures compared to the control high strength OPC concrete. However, deterioration of both strength and durability of HSVACs increased with the increase of temperature up to 800 °C due to weakened interfacial transition zone (ITZ) between hardened cement paste (hcp) and aggregate and concurrent coarsening of the hcp pore structure. The serviceability assessment of HSVACs after a fire should therefore, be based on both strength and durability considerations.     

Residue strength,water absorption and pore size distributions of recycled aggregate concrete after exposure to elevated temperatures

In this paper, the effects of high temperature exposure of recycled aggregate concretes in terms of residual strengths, capillary water absorption capacity and pore size distribution are discussed. Two mineral admixtures, fly ash (FA) and ground granulated blast furnace (GGBS) were used in the experiment to partially replace ordinary Portland cement for concrete production. The water to cementitious materials ratio was maintained at 0.50 for all the concrete mixes. The replacement levels of natural aggregates by recycled aggregates were at 0%, 50% and 100%. The concretes were exposed separately to 300 °C, 500 °C and 800 °C, and the compressive and splitting tensile strength, capillary water coefficient, porosity and pore size distribution were determined before and after the exposure to the high temperatures. The results show that the concretes made with recycled aggregates suffered less deteriorations in mechanical and durability properties than the concrete made with natural aggregates after the high temperature exposures.     

Effects of slag and cooling method on the progressive deterioration of concrete after exposure to elevated temperatures as in a fire event

In the present work OPC and OPC/slag concretes were exposed to elevated temperatures, 400 and 800°C. The critical temperature of 400°C has been reported for OPC paste. Above 400°C, the paste hydrate Ca(OH)₂ dehydrates into CaO causing the OPC paste to shrink and crack. After cooling and in the presence of air moisture, CaO rehydrates into Ca(OH)₂, resulting in disintegration due to re-expansion of OPC paste. Therefore, the present work assessed whether this also applies to OPC concretes. Two cooling methods were used: furnace and water cooling. Following the heat treatment/cooling method, compressive tests and Infrared (IR) spectroscopic studies were conducted. Results showed that after 400°C, water cooling caused all concrete, regardless of the type of blended cement binder, a further 20% loss in the residual strength. After 800°C, water cooling caused OPC concrete a further 14% loss while slag blends presented around 5% loss. IR indicated that the further loss observed in the OPC concrete is due to the accelerated CaO rehydration into Ca(OH)₂. Afterward, the non-wetted furnace cooled specimens were exposed to air moisture for one week, resulting in further strength loss of 13%. IR results suggested that slow rehydration of CaO occur with exposure to air moisture. In conclusion, water cooling caused more damage in OPC concrete, while the concrete that has not been wetted undergoes progressive deterioration. This indicates a need to monitor the non-wetted concrete after a fire event has occurred for potential further deterioration.     

Neural networks analysis of compressive strength of lightweight concrete after high temperatures

When concrete, one of the most important structural materials, is exposed to elevated temperatures generally strength loss is observed. Decrease ratio in the compressive strength depends on many materials and experimental factors. An artificial neural network (ANN) approach was used to model the compressive strength of lightweight and semi lightweight concretes with pumice aggregate subjected to high temperatures. Model inputs were the target temperature, pumice aggregate ratio and heating duration and the output was the compressive strength of pumice aggregate concrete. Data on the compressive strength of pumice aggregate concrete after the effects of high temperatures was obtained from a previous experimental study. The predicted values of the ANN are in accordance with the experimental data. The results indicate that the model can predict the compressive strength with adequate accuracy.     

Comparison of compressive and splitting tensile strength of autoclaved aerated concrete (AAC) containing water hyacinth and polypropylene fibre subjected to elevated temperatures

This paper described the results of an extensive experimental study on the comparative between compressive and splitting tensile behavior of autoclaved aerated concrete (AAC) containing water hyacinth fibre (WHF) with AAC mixed with polypropylene (PP) fibre. The specimens of AAC-WHF and the AAC-PP were subjected to elevated temperatures (100, 200, 400, 800 and 1000 °C). Test results indicated that an optimum water hyacinth and PP fibre dosage was at 0.5 and 0.75 % by volume respectively. The maximum residual in compressive strength and the splitting tensile strength of AAC-WHF and AAC-PP were 0.43 and 0.16 N/mm² and 0.51 and 0.18 N/mm² respectively. In addition, the loss in residual strength of AAC mixed PP fibre was slower than AAC mixed WHF. The splitting tensile strength of AACs was more sensitive to high temperatures than the compressive strength. A severe strength loss was observed for all of the AAC after exposure to 800 °C. Based on the test results, it can be concluded that the addition of PP fibers can significantly promote the residue mechanical properties of AAC during heating.     

纤维对超高性能混凝土残余强度及高温爆裂性能的影响

采用普通原材料制备56 d龄期抗压强度为140~160 MPa的空白组超高性能混凝土、钢纤维超高性能混凝土及混杂纤维超高性能混凝土,测定其遭受高温作用后的残余抗压强度和劈裂抗拉强度,并对100%含湿量的混凝土试块进行高温爆裂试验。此外,测定大小2种加热速率对超高性能混凝土高温爆裂行为的影响。结果表明:所配制混凝土的残余抗压强度均随着目标温度的升高呈现先增大再降低的趋势,800℃高温后的残余抗压强度约为常温强度的30%。钢纤维与混杂纤维混凝土的残余劈裂抗拉强度亦呈现先升高再降低的趋势,800℃高温后的残余劈裂抗拉强度分别为常温强度的15.1%和35.4%。空白组混凝土的残余劈裂抗拉强度随着目标温度的升高而单调下降,800℃高温后的强度值约为常温强度的20.3%。7.5℃/min加热速率下,100%含湿量的3种混凝土试块均发生了严重高温爆裂,单掺钢纤维可以改善超高性能混凝土的高温爆裂,但不能避免爆裂的发生,而混杂纤维对超高性能混凝土高温爆裂的改善效果并未显著优于钢纤维。2.5℃/min加热速率下,混杂纤维可避免部分超高性能混凝土试块发生爆裂。      

Experimental study on the dynamic compressive mechanical properties of concrete at elevated temperature

Strain rate effect and temperature effect are two important factors affecting the mechanical behavior of concrete. Each of them has been studied for several years. However, the two factors usually work together in the engineering practice. It is necessary to understand the mechanical responses of concrete under high strain rate and elevated temperature. A self-designed high temperature SHPB apparatus was used to study the dynamic compressive mechanical properties of concrete at elevated temperature. The results show that the dynamic compressive strength and specific energy absorption of concrete increase with strain rate at all temperatures. The elastic modulus decreases obviously with strain rate at room temperature and stabilizes at a level with slightly decrease at elevated temperature. The dynamic compressive strength of concrete at 400 °C increases by nearly 14% compared to the room temperature. However, it decreases at 200 °C, 600 °C and 800 °C with the decrease ratio of 20%, 16% and 48%, respectively. The dynamic elastic modulus decreases largely subjected to elevated temperature. The specific energy absorption at 200 °C, 400 °C and 600 °C is higher than room temperature and decreases to be lower than room temperature at 800 °C. Formulas are established under the consideration of mutual effect of strain rate and temperature.     

The effect of alkali-aggregate reactivity on the mechanical properties of high and normal strength concrete

In this investigation, a potentially highly reactive aggregate, and a potentially moderately reactive aggregate (identified by accelerated mortar bar testing and petrographic examination) were used in the preparation of concrete of normal and high strength concretes. After the initial 28 day curing period, the specimens were equally divided, and then submerged in a holding tank containing either a solution of a sodium hydroxide or de-ionised water at 80 °C for a period of 12 weeks. Normal strength concrete specimens containing the potentially highly reactive aggregate and exposed to the sodium hydroxide solution experienced more losses in mechanical properties than the concrete specimens prepared with potentially moderately reactive aggregates. However, in high strength concrete specimens exposed to the sodium hydroxide solution, there was a minimal loss in mechanical properties for both the specimens containing the highly reactive or moderately reactive aggregates. The superior performance of high strength concrete can be explained by the improved micro-structure and decreased permeability due to the formation of secondary calcium silicate hydrate formed as a result of the pozzolanic reaction.     

The effect of realistic curing temperature on the strength and E-modulus of concrete

The strength and E-modulus of concrete are decisive parameters when it comes to ultimate limit state design, serviceability limit state design, and early age crack assessment. The properties of concrete are generally determined in the laboratory under 20 °C isothermal conditions and then used as the basis for calculations under realistic temperature conditions. It is well-known, however, that the curing temperature affects both the rate of property development in concrete and the “final value” of a given property. The current study investigated the effect of a realistic temperature history on the compressive cube strength, the tensile strength, and the tensile E-modulus for two concretes, a reference concrete and a fly ash concrete. Concrete specimens were subjected to either (1) 20 °C isothermal curing conditions, or (2) realistic temperature curing conditions for 14 days and then 20 °C isothermal conditions, until they were tested after 28 and 91 days. Parallel tests performed in a Temperature-Stress Testing Machine were also used to evaluate the results. The reference concrete showed a general reduction in strength and E-modulus when subjected to a realistic curing temperature, whereas the fly ash concrete showed an 11% increase in the 28-day E-modulus when cured under realistic temperature conditions. Furthermore, in both isothermal and realistic curing temperature conditions, the fly ash concrete showed a pronounced property development beyond 28 days, which could not be described by the material model currently used.     

Residual compressive strength and microstructure of high performance concrete after exposure to high temperature

Experimental programs were carried out to study compressive strength and microstructure of high performance concrete (HPC) subjected to high temperature compared with normal strength concrete (NSC). After the concrete specimens were exposed to a peak temperature of 800°C, the compressive strength was tested. Changes of porosity and pore size distribution of the concrete were measured by using mercury intrusion porosimetry (MIP). Test results show that high performance concrete had higher residual strength although the strength of high performance concrete degenerated much more than the normal strength concrete after high temperature exposed. Variations in pore structure of high performance concrete after high temperature indicated the degradation of the mechanical properties. A model by optimizing the parameters in Ryshkewitch model was developed to predict the relationship between porosity and compressive strength of the high performance concrete.

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Résumé Une série de programmes expérimentaux a été réalisée afin d'étudier la résistance à la compression et la microstructure des bétons à haute performance (BHP) soumis à de fortes températures par comparaison aux bétons ordinaires. Les bétons ont été soumis à une température extrême de 800°C, puis la résistance à la compression a été testée. Les changements de porosité et la répartition de la taille des pores dans le béton ont été mesurés par la technique de porosimétrie au mercure. Les résultats des essais ont montré que les bétons à haute performance présentaient des niveaux de résistance résiduelle mais que la résistance des bétons à haute performance se dégradait beaucoup plus que celle des bétons ordinaires après exposition à haute température. Des variations dans la structure des cavités des bétons à haute performance après exposition à de hautes températures ont indiqué la dégradation des propriétés mécaniques. L'étude a donné lieu au développement d'un modèle par optimisation des paramètres du modèle Ryshkewitch afin de prévoir les relations entre la porosité et la résistance à la compression du béton à haute performance.

     

The strength and deformation characteristics of high early strength structural concrete

There are many possible structural applications of concrete with a compressive strength of about 100 N/mm². In practice, however, the development of early strength is far more important, but the combination of both can bring considerable economic benefits to the construction industry. Tests are reported on the strength and deformation characteristics of high early strength structural concrete. Tests using an ultra fine cement with expanded slate lightweight aggregate and granite produced concretes with a strength of 30–40 N/mm² and 60–70 N/mm² respectively in 24 hours. Tests with aluminous cement produced better aggregate-matrix bond and developed strengths of about 95 N/mm² in the same time. Equations are presented to predict the tensile strength and elasticity of the high early strength concrete. It is shown that the rapid hydration results in a high rate of shrinkage and creep initially but the long-term deformation characteristics are comparable to normal concrete. It is suggested that it is worth exploring methods to minimise the effects of conversion.     

Influence of mix proportions and curing conditions on tensile splitting strength of high strength concretes

The effect of the composition of high strength concretes with low water to binder ratio and silica fume on the development of splitting tensile strength was studied. A statistical approach was employed to develop formulation which could adequately describe the relations between splitting tensile strength and the concrete composition, when cured in two different regimes: water curing at 20°C and sealed curing at 30°C. Autogenous shrinkage was induced in the second type of curing but was largely eliminated in the first one. The relations were presented as nomograms which could be used as a basis for mix design. The correlation between tensile splitting strength and compressive strength could not be described in terms of a simple linear relation with a characteristic constant. For the range of variables studied, the ratio between tensile and compressive strength varied over a large range of 0.08 to 0.12. As a result, the relations developed here for tensile strength are quite different in nature than those for compressive strength in a previous study. Analysis of the data suggest that tensile strength is sensitive to effects which induce autogenous shrinkage to a much greater extent than compressive strength. It is proposed that this may be the main reason for the different trends observed for the relations between the composition of the low water/binder ratio concretes and their compressive and tensile strength.     

Experimental investigations on polymer-modified concrete subjected to elevated temperatures

This paper presents the effect of elevated temperature and duration of exposures on polymer-modified concrete (PMC). Styrene Butadiene Rubber latex polymer solids were added in terms of 0, 5, 10 and 20 % by mass of cement. Curing of PMC specimen was done by the combination of wet and dry conditions. At appropriate ages, specimens were exposed to 200–800 °C for 1–3 h. The residual compressive strength was tested at 7 and 28 days. Micro structural properties were studied by XRD and SEM analysis. It was found that PMC and conventional concrete can be exposed to 400 °C for 3 h exposure without any adverse effect on strength properties. Addition of 20 % polymer was detrimental to concrete subjected to elevated temperature. Duration of exposure does not have much influence on the residual compressive strength properties of conventional concrete and PMC specimens.     

High strength concrete containing natural pozzolan and silica fume

Various combinations of a local natural pozzolan and silica fume were used to produce workable high to very high strength mortars and concretes with a compressive strength in the range of 69–110 MPa. The mixtures were tested for workability, density, compressive strength, splitting tensile strength, and modulus of elasticity. The results of this study suggest that certain natural pozzolan–silica fume combinations can improve the compressive and splitting tensile strengths, workability, and elastic modulus of concretes, more than natural pozzolan and silica fume alone. Furthermore, the use of silica fume at 15% of the weight of cement was able to produce relatively the highest strength increase in the presence of about 15% pozzolan than without pozzolan. This study recommends the use of natural pozzolan in combination with silica fume in the production of high strength concrete, and for providing technical and economical advantages in specific local uses in the concrete industry.     

Effects of fly ash fineness on the mechanical properties of concrete

The present study reviews the effects of fly ash fineness on the compressive and splitting tensile strength of the concretes. A fly ash of lignite origin with Blaine fineness of 2351?cm²/g was ground in a ball mill. As a consequence of the grinding process, fly ashes with fineness of 3849?cm²/g and 5239?cm²/g were obtained. Fly ashes with three different fineness were used instead of cement of 0%, 5%, 10%, and 15% and ten different types of concrete mixture were produced. In the concrete mixtures, the dosage of binder and water/cement ratio were fixed at 350?kg/m³ and 0.50, respectively. Slump values for the concretes were adjusted to be 100 ± 20?mm. Cubic samples were cast with edges of 100?mm. The specimens were cured in water at 20°C. At the end of curing process, compressive and splitting tensile strengths of the concrete samples were determined at 7, 28, 56, 90, 120 and 180?days. It was observed that compressive and splitting tensile strength of the concretes was affected by fineness of fly ash in short-and long-terms. It was found that compressive and tensile strength of the concretes increased as fly ash fineness increased. It was concluded that Blaine fineness value should be above 3849?cm²/g fineness of fly ash to have positive impact on mechanical properties of concrete. The effects of fly ash fineness on the compressive and splitting tensile strength of the concretes were remarkably seen in the fly ash with FAC code with fineness of 5235?cm²/g.     

Mechanical properties of concrete containing phase-change material

This study aimed to investigate the mechanical properties of concrete containing solid–liquid phase-change material (PCM) and focused on two key factors. First, a systematic study on the mechanical performance of PCM-modified concretes was conducted, including compressive, elastic modulus, and shrinkage tests. Second, because PCM provides high latent heat during the solid–liquid phase change, the effects of the solid phase and liquid phase on the mechanical properties of concrete were also explored. Results of this study showed that the solid–liquid phase of PCM affected the mechanical properties of concrete. For example, the compressive strength of 10% PCM concrete in solid phase (23 °C) and liquid phase (40 °C) at 28 days was 29.30 and 19.57 MPa, respectively. In addition, with increasing PCM content, the mechanical properties were degraded. For example, 10, 20, and 30% of PCM content lowered the compressive strength by 35.4, 58.4, and 74.3%, respectively. Therefore, concrete with PCM may not be suitable for structural elements. However, PCM is an important solution for optimizing energy consumption in modern buildings. It can absorb or emit large amounts of heat to store or release thermal energy. These properties can be used to control building temperatures resulting in energy saving and carbon reduction.     

The role of 0–2 mm fine recycled concrete aggregate on the compressive and splitting tensile strengths of recycled concrete aggregate concrete

The aim of this study is to investigate the role of 0–2 mm fine aggregate on the compressive and splitting tensile strengths of recycled concrete aggregate (RCA) concrete with normal and high strengths. Normal coarse and fine aggregates were substituted with the same grading of RCAs in two normal and high strength concrete mixtures. In addition, to keep the same slump value for all mixes, additional water or superplasticizer were used in the RCA concretes. The compressive and splitting tensile strengths were measured at 3, 7 and 28 days. Test results show that coarse and fine RCAs, which were achieved from a parent concrete with 30 MPa compressive strength, have about 11.5 and 3.5 times higher water absorption than normal coarse and fine aggregates, respectively. The density of RCAs was about 20% less than normal aggregates, and, hence, the density of RCA concrete was about 8–13.5% less than normal aggregate concrete. The use of RCA instead of normal aggregates reduced the compressive and splitting tensile strengths in both normal and high strength concrete. The reduction in the splitting tensile strength was more pronounced than for the compressive strength. However, both strengths could be improved by incorporating silica fume and/or normal fine aggregates of 0–2 mm size in the RCA concrete mixture. The positive effect of the contribution of normal sand of 0–2 mm in RCA concrete is more pronounced in the compressive strength of a normal strength concrete and in the splitting tensile strength of high strength concrete. In addition, some equation predictions of the splitting tensile strength from compressive strength are recommended for both normal and RCA concretes.     

Effect of fire exposure on cracking,spalling and residual strength of fly ash geopolymer concrete

Fly ash based geopolymer is an emerging alternative binder to cement for making concrete. The cracking, spalling and residual strength behaviours of geopolymer concrete were studied in order to understand its fire endurance, which is essential for its use as a building material. Fly ash based geopolymer and ordinary portland cement (OPC) concrete cylinder specimens were exposed to fires at different temperatures up to 1000 °C, with a heating rate of that given in the International Standards Organization (ISO) 834 standard. Compressive strength of the concretes varied in the range of 39–58 MPa. After the fire exposures, the geopolymer concrete specimens were found to suffer less damage in terms of cracking than the OPC concrete specimens. The OPC concrete cylinders suffered severe spalling for 800 and 1000 °C exposures, while there was no spalling in the geopolymer concrete specimens. The geopolymer concrete specimens generally retained higher strength than the OPC concrete specimens. The Scanning Electron Microscope (SEM) images of geopolymer concrete showed continued densification of the microstructure with the increase of fire temperature. The strength loss in the geopolymer concrete specimens was mainly because of the difference between the thermal expansions of geopolymer matrix and the aggregates.     

Mechanical properties of lightweight concrete made with coal ashes after exposure to elevated temperatures

This study investigated the thermal resistance of lightweight concrete with recycled coal bottom ash and fly ash. Specimens were exposed to temperatures up to 800 °C then cooled to room temperature before conducting experiments. Compressive strength test, FF-RC test, TG analysis, and XRD analysis were performed to analyze the physicochemical effects of coal ashes on the thermal resistance of concrete. Test results indicated that both bottom ash and fly ash were associated with a substantial increase in the residual strength of thermal exposed concretes. The results were attributed to the surface interlocking effect and the smaller amount of SiO₂ for bottom ash. For fly ash, the formation of pozzolanic C-S-H gel and tobermorite retained water at high temperatures, and the consumption of Ca(OH)₂ lowered stress from rapid recrystallization after exposure to 600 °C. It was concluded that the incorporation of coal ashes allows for lightweight concrete with good thermal resistance.