Elevated Temperature Exposure Research Articles

Effects of perlite/fly ash ratio and the curing conditions on the mechanical and microstructural properties of geopolymers subjected to elevated temperatures

This paper investigated the mechanical and microstructural properties of geopolymer mortars before and after elevated temperature exposure. The mortars were based on blends of finely ground raw perlite (RP) and Class F fly ash (FA). RP–FA blends were alkali-activated with 10 M NaOH solution. The synthesis of geopolymer mortars was conducted at 90 °C during 4, 8, and 24 h, with different RP/FA mass ratios (100/0; 75/25; 50/50; 25/75; 0/100). Several experiments regarding the hardened properties were performed after curing for 7, 28 and 90 days. After curing, the geopolymer mortars were heated up to elevated temperatures of 400 °C, 600 °C, and 800 °C, separately. Influence of elevated temperature on the geopolymer properties were determined in terms of the mass loss, strength loss, and alterations in the microstructures after exposure to elevated temperatures. The best results were observed on the geopolymer mortars made with the RP/FA mass ratio of 25/75. The flexural and compressive strength values of the mortars after exposure to 800 °C were higher than the values after exposure to 600 °C. New crystalline phases were detected in the mortars after being subjected to 800 °C.

Gene Expression, Activity and Localization of Beta-Galactosidases during Late Ripening and Postharvest Storage of Tomato Fruit

Beta-galactosidases (β-GALs) hold a key role in both fruit softening and the increase of total soluble solids during maturation. Despite determining both quality and potential postharvest longevity, β-GAL activity during ripening, with a special focus on the postharvest period, has not been adequately addressed in a spatial and temporal manner. This study focused on the regulation of gene expression in relation to the total β-GAL enzyme activity during the ripening of tomato fruit attached on the plant, as well as harvested fruit ripened for 5 d at 4, 10, or 25 °C. The transcription of genes coding for β-GAL isoenzymes was significantly affected by both the fruit maturation stage (unripe vs. red ripe) and postharvest storage temperature. Cold stressed tomatoes (4 °C) exhibited a remarkably higher transcription of most β-GAL genes compared to on-plant red ripe fruit and to fruit exposed to either 10 or 25 °C, indicating a low temperature response. However, enzymatic activity and water-soluble pectin content increased with elevated temperature exposure, peaking in fruit stored at 25 °C. β-GAL activity was present in the pericarp, while it was less detected in locular parenchyma. These findings highlight the dual role of β-GAL not only in maturation, but also in the metabolism during postharvest homeostasis and cold acclimation of tomato fruit.

Mechanical performance evolution and life prediction of prestressed CFRP plate exposed to hygrothermal and freeze-thaw environments

The long-term prestressed loss of anchorage system and mechanical property evolution play the significant role for the steel/concrete structures strengthened with prestressed CFRP plates. In the present study, a wedge-extrusion bond anchorage system was applied to provide the reliable anchorage load-bearing capacity for CFRP plate. A new prestressed tension device was put forward to realize the coupling exposure of elevated temperature (20 °C, 60 °C and −20 to 30 °C), distilled water and sustained loading (20%, 40% and 60%). The mechanical and thermal properties of CFRP plate were conducted to obtain the long-term evolution exposed to the above environments. The mechanical analysis and tensile tests of anchorage system showed the tensile failure modes under the static and cyclic loads were the CFRP burst, and there was no debonding occurring in the anchor. The higher anchorage capacity was attributed to the uniform stress distribution of CFRP plate parallel to the greatest dimension of cross section in the anchor. The new prestressed tension device can realize the maximum prestressed level (∼70%) with an allowable prestressed loss (∼300 με). Meanwhile, the prestressed loss of CFRP plate exposed at the maximum temperature (60 °C) and longest time (90 days) was found to be ∼300 με. Immersed at higher temperature and prestressed level brought about an additional degradation rate of tensile strength (5%∼10%). The minimum strength retention of CFRP plate was found at the freeze-thaw exposure for 90 days. The long-term life prediction showed the residual tensile strength retentions of CFRP plates were more than 50% for the maximum prestressed level (60%) for the service life of 30 years in civil engineering structures.

Thermal cycles, microstructures and mechanical properties of AA7075-T6 ultrathin sheet joints produced by high speed friction stir welding

0.5 mm thick AA 7075-T6 ultrathin sheets were butt jointed by conventional friction stir welding (C-FSW) and high speed friction stir welding (HS-FSW) under a constant rotational speed ( ω )/welding speed ( v ) ratio of 6.67 rad/mm using a pinless tool. In this study, the conventional ω of 2000 rpm and conventional v of 300 mm/min were adopted by C-FSW, and the high ω of 8000 rpm and high v of 1200 mm/min were adopted by HS-FSW. Compared with C-FSW, HS-FSW presented a fast heating and cooling rate thermal characteristic with a slightly higher peak temperature and significantly shorter elevated-temperature exposure time. Kissing bond defect was appeared in the weld bottom for C-FSW. While, no obvious defect was found in the weld obtained by HS-FSW. Compared with C-FSW, the proportion of deformed grains in the center of nugget zone (NZ) fabricated by HS-FSW was increased. For the joint fabricated by HS-FSW, the upper NZ was dominated by recrystallized grains, and the lower NZ was dominated by deformed grains. The ultimate tensile strength of the joint fabricated by HS-FSW can reach 87% of base metal (BM), and its elongation, yield strength and ultimate tensile strength were increased by 58.8%, 23.3% and 22.8% respectively compared with C-FSW. Both the joints fractured in the center of the NZ. The joint fabricated by C-FSW broke due to the kissing bond defect. While the joint fabricated by HS-FSW may be broken due to the weld thickness reduction. • The thermal cycle characteristics of HS-FSW were revealed. • The effect of the thermal cycle on microstructure was revealed. • The microstructures of the joint produced by HS-FSW were investigated by EBSD. • The tensile properties of the joint produced by HS-FSW were superior to by C-FSW.

Utilization of hybrid sisal and steel fibers to improve elevated temperature resistance of ultra-high performance concrete

This paper investigated the mechanical properties and elevated temperature resistance of sisal fibers and steel fibers hybridization reinforced ultra-high performance concrete (UHPC). The compressive strength, flexural strength, and toughness of UHPC before and after exposure to elevated temperature were measured. Moreover, the spalling behavior, gas permeability, and sorptivity of hybrid fiber reinforced UHPC after the elevated temperature exposure was also studied. The spalling resistance mechanism was elucidated by thermal analysis and microstructure observation. The result shows that hybridization of sisal fibers and steel fibers exhibits an excellent synergy effect on improving the mechanical properties of UHPC before and after elevated temperature exposure. The addition of sisal fibers significantly increases the gas permeability of UHPC, hence the spalling of UHPC at elevated temperatures was prevented efficiently. A sisal fiber content of 0.6 vol% was most favorable in terms of improving the residual mechanical properties. Sisal fibers can increase the porosity of UHPC after elevated temperature exposure as indicated by sorptivity results, therefore excessive sisal fibers impaired the residual mechanical properties. This study provides new insights into the utilization of natural sisal fibers to produce sustainable and cost-effective UHPC with high spalling resistance characteristics at elevated temperatures.

Shear performance of reinforced concrete deep beams using different coarse aggregates under the effect of elevated temperatures

This paper describes experimental tests of the shear performance of high-strength reinforced concrete deep beams produced with different coarse aggregates, conducted at an ambient temperature of 25 °C and after heating to 600 °C. Three types of coarse aggregates of 10 mm maximum size were considered: limestone-based normal weight aggregate, quartzite-based normal weight aggregate, and steel slag-based heavyweight aggregate. Additionally, 20 mm maximum size aggregate was used for the steel slag concrete only. Eight beam specimens were designed to fail in shear, eliminating the possibility of flexural failure. Four beam specimens were tested at ambient temperature, and the corresponding four specimens were exposed to a temperature of 600 °C for 3 h. The results showed that the shear capacity of the deep beams tested at the ambient temperature was closely correlated with the abrasion resistance of the coarse aggregate. Exposure to an elevated temperature led to variable reduced shear strength, depending upon the type of aggregate. Compared to the beams tested in ambient conditions, the best performance was achieved by the limestone aggregate, followed by steel slag aggregate then quartzite aggregate, recording shear strength losses of 23.1%, 37.9% and 40.5%, respectively. This reduction in shear capacity was due to the reduction in compressive strength by 53–61% for the concrete exposed to 600 °C. The mid-span deflection at peak load was in general reasonably comparable for the beams tested at 25 °C and after exposure to 600 °C. The trend of stiffness loss after elevated temperature exposure for the various types of aggregate was very consistent with shear strength.

Physical and mechanical properties of expanded vermiculite (EV) embedded foam concrete subjected to elevated temperatures

Expanded vermiculite (EV) is an innovative alternative to conventional fire-resistance material due to exceptional thermal performance and non-combustible property. In this study, the effect of elevated temperatures on microstructure, mechanical and physical properties of foam concrete containing EV as lightweight aggregates (LWA) was investigated. Foam concrete samples with 10%, 15% and 20% EV as sand replacement were developed and exposed to 300 ℃, 600 ℃, and 900 ℃. Scanning electron microscopy (SEM) test was conducted to identify microstructure changes in the cement matrix and fine aggregates after elevated-temperature exposure. The enhanced close-fitted bond in the interfacial transition zone (ITZ) of EV-based foam concrete was observed in SEM, which might be due to the higher water absorption of EV granules. Residual compressive and flexural strength, relative porosity and water sorptivity coefficient of foam concrete were also obtained. The 15% EV reduced compressive strength loss of samples by 1.84%, 11.68% and 69.1% at 300 ℃, 600 ℃ and 900 ℃, respectively, compared with that of the reference as 9.81%, 33.7%, and 72.28%, respectively. Enhanced sorptivity-related durability and lower residual porosity were also obtained at elevated temperatures when 15% EV was added.

Effects of different elevated temperature and long-term exposure on microstructural evolution and mechanical characteristics of IN617 Ni-based superalloy

The microstructural changes and mechanical properties of service-exposed IN617 Ni-based superalloy were studied at temperatures of 750, 800, 850 and 900 °C and different time ranging from 4250 to 105000 h. Some microscopic techniques and mechanical experiments, including Optical Microscopy (OM), Field Emission Scanning Electron Microscopy (FESEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS), Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), impact, hardness and room/elevated temperatures tensile tests were employed to investigate the microstructure and mechanical performance of the superalloy. In the samples operated at 750 °C/105000 h and 800 °C/98000 h conditions, the γ′ strengthening phase was observed, leading to noticeable improvement in mechanical features of the material. At 850 °C/24000 h and 900 °C/4250 h, the γ′ weight percentage was very low to be detected, so that no significant variation in the mechanical properties of the superalloy was observed. Moreover, metallographic evaluations showed that carbide particles nucleated and grew along the grain boundaries and also within the grains with increasing exposure time during service condition, while their growth rate depended upon the operating temperature. Changes in microstructural characteristics and mechanical properties were assessed using Larson-Miller parameter. As the value of P parameter went up, the area fraction of carbides and the weight percentage of M23C6 carbide increased in return, whereas weight percentage of M6C carbide revealed a decreasing trend. This is due to the fact that a part of M6C carbide was transformed to equilibrium state of M23C6 carbide by increasing P parameter. Impact energy, ultimate tensile strength at room temperature and ultimate tensile strength at 750 °C drastically reduced with increasing P parameter. Although elongation percentage and yield strength showed a decreasing trend in terms of the P parameter, they did not follow regular changes contrary to other mechanical attributes. Measurements indicated that the average hardness value remained constant at around 215 HV with increasing P parameter up to 27500 and then decreased with a steep slope down to about 178 HV.

Differential Physiological Response and Antioxidant Activity Relative to High-Power Micro-Waves Irradiation and Temperature of Tomato Sprouts

Among the various types of stress, microwaves and temperature can induce major impacts on plant growth. There is information describing the thermal impact of microwaves on living organisms, but it is necessary to segregate the warming effect and direct impact of microwaves irradiation on plants. It was detected that High Power Microwaves (HPM) (9.3 GHz) and elevated temperature exposure upon tomato seeds and sprouts in primary ontogenetic stages showed a slightly incentive effect on plant-growing indicators such as dry mass, fresh mass, plants height, and assimilation area. Such a positive effect on plant growing parameters could be related to saccharides distribution by microwaves in seeds or plants and nutrients mobilization. Moreover, tomato plants (+R) and seeds (R) irradiation significantly reduced the content of non-structural carbohydrates (raphinose, glucose, fructose, and sucrose). Obtained results confirm that a common plant acclimatization response to various environmental elements is the concentration of secondary metabolites and antioxidants.

Hydrolysis susceptibility and carbamate formation for a low moisture-absorbing, siloxane-modified cyanate ester resin matrix (TC410) material used for composite space applications

Siloxane-modified cyanate ester resins are ideal matrix materials for next-generation, high-precision composites used for radomes and satellite structures. They provide extremely low moisture uptake, excellent microcracking resistance, and protection from atomic radiation. However, cyanate ester resins have been shown to be susceptible to hydrolysis during cure, which may significantly impact both mechanical and thermal performance. In this investigation, we evaluate the cure kinetics and hydrolysis susceptibility of a siloxane-modified prepreg system (TC410/M55J). DSC tests verified that the Ea of polymerization for the TC410 system is 65 KJ/mol. Samples were also exposed to moisture during cure at several temperatures, and FTIR was used to follow changes in the carbonyl peak absorption intensity to compare the rate kinetics and activation energy for hydrolysis leading to carbamate formation (52 KJ/mol). DMA tests of composites exhibited significantly reduced Tg’s with increasing moisture levels during cure. TGA of these cured samples exhibited both a significant decrease in the onset of the thermal decomposition temperature as well as an increase in the relative degree of volatiles generated at low temperature. Flatwise tension strength tests showed a linear reduction in strength with decreasing Tg, at an equivalent trend to previous work on an M55J/RS3C system indicating a similar mechanism. However, the added margin provided from the higher initial FWT strength in the TC410 system allows for larger decreases in Tg before significant degradation occurs. Laminates manufactured on composite mandrels resulted in parts with larger decreases in the tool-side Tg of the part when compared to the bag-side Tg, with the differential depending on the initial moisture content of the tool. AFM was shown to selectively identify these carbamate-affected regions by showing that the recession behavior of these less cross-linked areas was greater than in areas where the resin was properly cured. Composites with increased carbamate formation resulted in increasing warpage of the part with elevated temperature exposure due to variations in stress relaxation. This effect should be considered when manufacturing precision, high-dimensional stability composite hardware.

Acute exposure to perfluorooctane sulfonate exacerbates heat-induced oxidative stress in a tropical coral species.

Perfluorooctane sulfonate (PFOS) is among the most commonly per- and poly-fluoroalkyl substances (PFAS) found in environmental samples. Nevertheless, the effect of this legacy persistent organic contaminant has never been investigated on corals to date. Corals are the keystone organisms of coral reef ecosystems and sensitive to rising ocean temperatures, but it is not understood how the combination of elevated temperature and PFOS exposure will affect them. Therefore, the aims of the present study were (1) to evaluate the time-dependent bioconcentration and depuration of PFOS in the scleractinian coral Stylophora pistillata using a range of PFOS exposure concentrations, and (2) to assess the individual and combined effects of PFOS exposure and elevated seawater temperature on key physiological parameters of the corals.Our results show that the coral S. pistillata rapidly bioconcentrates PFOS from the seawater and eliminates it 14 days after ceasing the exposure. We also observed an antagonistic effect between elevated temperature and PFOS exposure. Indeed, a significantly reduced PFOS bioconcentration was observed at high temperature, likely due to a loss of symbionts and a higher removal of mucus compared to ambient temperature. Finally, concentrations of PFOS consistent with ranges observed in surface waters were non-lethal to corals, in the absence of other stressors. However, PFOS increased lipid peroxidation in coral tissue, which is an indicator of oxidative stress and enhanced the thermal stress-induced impairment of coral physiology. This study provides valuable insights into the combined effects of PFOS exposure and ocean warming for coral's physiology. PFOS is usually the most prevalent but not the only PFAS defected in reef waters, and thus it will be also important to monitor PFAS mixture concentrations in the oceans and to study their combined effects on aquatic wildlife.

Thin fly ash/ ladle furnace slag geopolymer: Effect of elevated temperature exposure on flexural properties and morphological characteristics

The flexural properties and thermal performance of 10 mm-thin geopolymers made from fly ash and ladle furnace slag were evaluated before and after exposure to elevated temperatures (300 °C, 600 °C, 900 °C, 1100 °C and 1150 °C). Class F fly ash was mixed with liquid sodium silicate (Na2SiO3) and 12 M sodium hydroxide (NaOH) solution using aluminosilicate/activator ratio of 1:2.5 and Na2SiO3/NaOH ratio of 1:4 to synthesise thin fly ash (FA) geopolymers. 40 wt% of ladle furnace slag was partially replacing fly ash to produce fly ash/slag-based (FAS) geopolymers. Thermal treatment enhanced the flexural strength of thin geopolymers. In comparison to the unexposed specimen, the flexural strength of FA geopolymers at 1150 °C and FAS geopolymers 1100 °C was increased by 161.3% to 16.2 MPa and 208.9% to 24.1 MPa, respectively. A more uniform heating was achieved in thin geopolymers which favoured the phase transformation at high temperatures and contributed to the substantial increase in flexural strength. The joint effect of elevated temperature exposure and the incorporation of ladle furnace slag further improved the flexural strength of thin geopolymers. The calcium-rich slag refined the pore structure and increased the crystallinity of thin geopolymers which aided in high strength development.

Mechanical behavior of high-strength concrete incorporating seashell powder at elevated temperatures

Cement-based composites may experience higher temperatures during their service period due to fire. After fire exposure, the concrete properties may be significantly affected in terms of safety and serviceability. Therefore, to improve the fire resistance of high-strength concrete, it was proposed to add seashells in cementitious composites for improved fire resistance. In this study, the seashell powder was used for concrete (0, 10, 20, and 30%) and plastering mortar (20%, 40%, and 60%) as a replacement of fine aggregate to develop high-strength concrete (HSC). Different properties, such as compressive strength, stress-strain response, elastic modulus, compressive toughness, strain ductility, mass loss of modified and controlled samples, and the deterioration caused by elevated temperatures exposure, were determined. The analyzed formulations were heated to a temperature of 200 °C, 400 °C, 600 °C, and 800 °C at a heating rate of 5 °C/min and then tested for residual conditions. Seashell modified samples showed a higher compressive strength, elastic modulus, and compressive toughness than the control sample at elevated temperature with less spalling sensitivity. According to visual inspection, SS-HSC, compared to HSC, showed less cracking at higher temperatures. Moreover, plastering high-strength concrete with seashell showed greater strength with more fire resistance to the concrete core. Conclusively, the utilization of seashells in high-strength concrete is efficient for fire-resistant concrete.

Effect of Elevated Ambient Temperature on Maternal, Foetal, and Neonatal Outcomes: A Scoping Review.

This scoping review provides an overview of the published literature, identifies research gaps, and summarises the current evidence of the association between elevated ambient temperature exposure during pregnancy and adverse maternal, foetal, and neonatal outcomes. Following the PRISMA extension for scoping reviews reporting guidelines, a systematic search was conducted on CINAHL, PubMed, and Embase and included original articles published in the English language from 2015 to 2020 with no geographical limitations. A total of seventy-five studies were included, conducted across twenty-four countries, with a majority in the USA (n = 23) and China (n = 13). Study designs, temperature metrics, and exposure windows varied considerably across studies. Of the eighteen heat-associated adverse maternal, foetal, and neonatal outcomes identified, pre-term birth was the most common outcome (n = 30), followed by low birth weight (n = 11), stillbirth (n = 9), and gestational diabetes mellitus (n = 8). Overall, papers reported an increased risk with elevated temperature exposures. Less attention has been paid to relationships between heat and the diverse range of other adverse outcomes such as congenital anomalies and neonatal mortality. Further research on these less-reported outcomes is needed to improve understanding and the effect size of these relationships with elevated temperatures, which we know will be exacerbated by climate change.

Using artificial neural networks for predicting mechanical and radiation shielding properties of different nano-concretes exposed to elevated temperature

This study aims to investigate and predict the effects of adding different Nano additives (Nano alumina, Carbon Nanotubes, and hybrid mix of both materials) within normal strength concrete on both mechanical and gamma ray radiation shielding properties of concrete after exposure to elevated temperatures. Both residual compressive strength (RCS) and gamma ray’s linear attenuation coefficient (µ) tests were used as an indicator of the change in properties after exposure to elevated temperature. While the results showed remarkable positive effects for all Nano additives at all replacement ratios, the hybrid mix of 1.5% Nano alumina (NA) along with 0.1 Carbon Nano tubes (CNT) showed the optimum enhancement for both the mechanical and radiation shielding properties. A smart modelling Artificial Neural Networks (ANN) was also developed for predicting the values of RCS and µ for the different concrete mixtures. The type and the proportion of Nano additives, the temperature, and the elevated temperature exposure time were input variables for the neural networks. The predicted values of RCS and µ are the outputs. The prediction results obtained from the developed ANN showed a high agreement with the experimental results and the predictive capability of developed models was better compared to regression analysis.

Investigation of Thermal Effects on Nailed Connection of Mass Ply Panels

Abstract Mass ply panels (MPP), a relatively new mass timber product, has been utilized in several construction projects as diaphragm and wall panels. Connection for MPP is a crucial structural component that requires a better understanding. This article presents an experimental investigation into elevated temperature exposure–driven property degradation of MPP nailed connections, which is important for both the design of new structures in terms of fire resistance and the rehabilitation of structures partially damaged by fire. One control group and 32 exposure groups, which were combinations of eight elevated temperatures and four exposure durations, were investigated. The failure modes and yield strength of the nailed connection were analyzed as a function of elevated temperature and exposure time and compared with the prediction from the National Design Specification and existing literature. The results show a decrease of up to 45 percent in initial stiffness and ultimate load; meanwhile, there was no statistical evidence for the change in yield load in the majority of testing groups. Two analytical models, namely, multilinear regression and first-order kinetics model, were proposed to model the degradation of initial stiffness and ultimate strength. The kinetics model provided a better prediction and suggested that the initial stiffness and ultimate strength of the nail connection degraded over time at rates depending on the exposure temperature.

A state-of-the-art on development of geopolymer concrete and its field applications

Massive growth in the infrastructure industry and development has led to a phenomenal increase in cement usage. The cement industry emits the greenhouse gas (CO2) and cement production requires more embodied energy. There is an increasing research interest in geopolymers (GP) for developing an alternate binder for civil engineering applications because of its low energy consumption and eco-friendly nature. GP contain alkali-activated material, which is relatively new, and Geopolymer concrete (GPC) has superior mechanical and durability properties compared to ordinary Portland cement (OPC) concrete. The objective of the review article is to make all-encompassing review of long-term (LT) and short-term (ST) properties of GP, their chemistry, reaction mechanism, effect of reaction parameters, fresh properties, stability under elevated temperature exposure, bond between steel and reinforcement and various field implementations. It is concluded that GPC can achieve structural grade strength when different industrial waste materials as precursors are used in its preparation by varying the alkaline concentration, curing method and curing temperature. It is also concluded that GP gives better thermal resistance at elevated temperature exposure up to 1000 °C whereas OPC fails. Various research and development works carried out in India by utilising the locally available industrial waste-based source materials are also discussed. Applications of GP technology in current scenario have also been reviewed. The gap in GP technology pertaining to corrosion, carbonation and leaching effects of GPC need to be focussed in future research.

Impact of high temperature on mortar mixes containing additives

The structures may be exposed to fire or high temperature conditionally or accidentally. Alteration in the behavior of concrete structure is prospective under the exposure of elevated temperature. There is an urge to find the materials which can resist the alteration in physiochemical and strength properties of cementitious materials under high temperature. In the present study, the effect of elevated temperature on cement mortar consisting of additives i.e. accelerating admixtures, and stone waste i.e. stone slurry powder, was investigated and compared with specimens at room temperature. The aim of study was to examine the practicability of these additives under exposure to high temperatures. The mortar specimens were exposed to various temperatures i.e. 1500C, 3000C, 4500C and 6000C for the duration of one hour and compared with unheated samples. The change in mass, strength and micro-structure of mortar specimens at elevated temperature was studied. The environmental assessment and performance evaluation of various mortar mixes were also evaluated. The mass of mortar specimens reduced as the exposure temperature of specimens was raised. The residual strength of mortar increased up to a certain temperature afterward, it decreased. Stone slurry powder and calcium nitrate can be used individually and in combination to resist thermal changes.

Flexural strength of silica fume, fly ash, and metakaolin of hardened cement paste after exposure to elevated temperatures

The mechanical properties of concrete based mainly on flexural and compressive bearing capacity. Generally, researchers have an interest in the evaluation of compression property through the importance of the flexural performance of the material in the constructions, namely the significance of each mechanical property based upon the position of the structural element. The present experimentally work is directed toward improving the flexural strengths performance of ordinary hardened cement paste (HCP) at ambient and after elevated temperatures exposure. The used parameters were different pozzolanic materials with different replacements ratios to cement mass and different levels of temperature. Results proved the significant contribution of pozzolanic material to enhance the flexural properties of HCP after being exposed to elevated temperatures. The low content of CaO, the high grinding fineness, and the physical morphology of the used pozzolanic materials, made their adoption effective to HCP after exposure to elevated temperatures. Using 3%, 12%, and 15% of silica fume (SF), metakaolin (MK), and fly ash (FA), respectively, showed the highest heat endurance among the other replacements. However, the optimum replacement of MK has shown a better heat endurance than the optimum replacements of SF and FA. On the other hand, the spalling has occurred at high replacements of SF. Finally, the results are supported by means of thermo-gravimetric, SEM, and computed tomography investigations.

Evaluation of fracture energy, toughness, brittleness, and fracture process zone properties for lightweight concrete exposed to high temperatures

Proper understanding of the fracture mechanism of lightweight concrete (LWC) after exposure to high temperatures has a significant role in LWC proportioning design which provides a better comprehension of the structural behavior during a fire event. In this study, the effect of high temperatures (250, 500, and 750 °C) on the fracture parameters of LWC and its brittleness compared to normal-weight concrete (NWC) was experimented using a total of 120 notched beams subjected to the three-point bending test. For this purpose, the size effect method (SEM) and the work of fracture method (WFM) were employed to analyze and explain the fracture parameters. The results demonstrate a reduction of 85 and 77% in the size-independent total fracture energy (GF) and a decrease of 37 and 28% in the characteristic length (Lch) based on WFM, respectively for NWC and LWC with increasing temperature up to 750 °C. As the temperature reaches 750 °C, the size-dependent initial fracture energy (Gf), fracture toughness (KIC), and fracture process zone length (Cf) based on SEM, are reduced by 73, 64 and 76% for NWC. The corresponding reduction values for LWC are 68, 47 and 69% compared to specimen at ambient temperature. However, the brittleness number (β) based on SEM is increased. The results of the parameters of effective size of the process zone (Cf) in SEM and characteristic length (Lch) in WFM indicated that normal-weight concrete is more brittle and its ductility is reduced compared to lightweight concrete exposed to high temperatures. Based on the size effect phenomenon, the behavior of normal and lightweight concrete became closer to the linear elastic fracture mechanics (LEFM) criterion with increasing temperature. Finally, the average ratio of GF obtained from WFM to Gf obtained from SEM for NWC and LWC at elevated temperature exposure show 1.82 and 1.7, respectively.