Increasing Exposure Temperature Research Articles

Bond behaviour between steel fibre and cement mortar exposed to elevated temperature

In order to accurately evaluate the deterioration of the mechanical properties of concrete reinforced with steel fibre (SF) after exposure to elevated temperature, it is particularly important to study the bonding properties between the SF and the concrete matrix. In this work, four types of SFs and five cement mortars with different strengths were used to investigate the bond behaviour of the SF–mortar interface after exposure to elevated temperature. The results showed that the compressive and flexural strengths of each mortar decreased significantly after elevated temperature treatment, and the attenuation degree of flexural strength was greater than that of the compressive strength. In addition, the ultimate pull-out load of specimens with straight SFs gradually decreased with an increase in exposure temperature. However, the ultimate pull-out load of specimens with hooked-end SFs showed no significant change between exposures to 20°C and 400°C. When the exposure temperature exceeded 400°C, the ultimate pull-out load gradually decreased with an increase in temperature. The damage mechanism of the SF–mortar interface under elevated temperature was investigated by analysing the interfacial microstructure and phase change of cement pastes. Finally, a formula to related the ultimate bond strength with temperature was established.

Research on residual behavior of tensile stiffened welded hollow spherical joints after exposure to fire

Space grid structures are subject to fire like other building structures, but they rarely collapse as a whole after exposure to fire. The welded hollow spherical joints are the main joint types of space grid structures. It is particularly critical to accurately identify the mechanical behavior of welded hollow spherical joints after exposure to fire to confirm whether the grid structure needs reinforcement or demolition for reconstruction. Therefore, the mechanical behavior of tensile stiffened welded hollow spherical joints after exposure to fire without load was studied by experiment. The failure modes of stiffened welded hollow spherical joints after exposure to fire are pull-out failures and oblique splitting pull failures. The initial axial stiffness of stiffened welded hollow spherical joints declines with the increase of exposure temperature, and the cooling method has little effect on it. When the exposure temperature is low, the cooling method has no impact on the load-bearing capacity of the joint. The load-bearing capacity of the air-cooling joints decreases linearly with the progress of exposure temperature, while the water-cooling joints present an opposite trend. The ductility of the air-cooling joints increases rapidly with the growth of exposure temperature, while the water-cooling joints fluctuate up and down. The strain of the tensile stiffened welded hollow spherical joints closely relates to its location. The numerical model of the joint after exposure to fire without load and with load was established, and the model was verified against experimental results. Through the numerical simulation, a method for calculating the residual mechanical behavior of stiffened welded hollow spherical joints after exposure to fire without load and with load was presented.

Effects of Thermal Exposure on the Microstructure and Mechanical Properties of a Ti-48Al-3Nb-1.5Ta Alloy via Powder Hot Isostatic Pressing.

Research on how thermal exposure affects the microstructure and mechanical properties of the Ti-48Al-3Nb-1.5Ta (at. %) alloy, which is prepared via powder hot isostatic pressing (P-HIP), is essential since this low-density alloy shows promise for use in high-temperature applications, particularly for aero-engines, which require long-term stable service. In this study, a P-HIP Ti-48Al-3Nb-1.5Ta (at. %) alloy was exposed to high temperatures for long durations. The phase, microstructure and mechanical properties of the P-HIP Ti-48Al-3Nb-1.5Ta alloy after thermal exposure under different conditions were analyzed using XRD, SEM, EBSD, EPMA, TEM, nanomechanical testing and tensile testing. The surface scale is composed of oxides and nitrides, primarily Al2O3, TiO2, and TiN, among which Al2O3 is preferentially generated and then covered by rapidly growing TiO2 as the thermal exposure duration increases. The nitrides appear later than the oxides and exist between the oxides and the substrate. With increasing exposure temperature and duration, the surface scale becomes more continuous, TiO2 particles grow larger, and the oxide layer thickens or even falls off. The addition of Ta and Nb can improve the oxidation resistance because Ta5+ and Nb5+ replace Ti4+ in the rutile lattice and weaken O diffusion. Compared with the P-HIP Ti-48Al-3Nb-1.5Ta alloy, after thermal exposure, the grain size does not increase significantly, and the γ phase increases slightly (by less than 3%) with the decomposition of the α2 phase. With increasing thermal exposure duration, the γ phase exhibits discontinuous coarsening (DC). Compared with the P-HIP Ti-48Al-3Nb-1.5Ta alloy, the hardness increases by about 2 GPa, the tensile strength increases by more than 50 MPa, and the fracture strain decreases by about 0.1% after thermal exposure. When the depth extends from the edge of the thermally exposed specimens, the hardness decreases overall.

Effect of thermal exposure on the microstructure and mechanical properties of Ti60 alloy

The effect of thermal exposure on the microstructure and mechanical properties of Ti60 alloy was investigated in the present study. Meanwhile, the fatigue fracture microscopic appearance characteristics at elevated temperature were analyzed compared, providing the basis to further improve the performance of the series of high temperature titanium alloy. The results show that the yield strength and tensile strength were basically stable under long-term thermal exposure below 600°C, while the elongation decreased with the increase of exposure temperature. Thermal exposure below 800°C did not change the microstructure type of Ti60 alloy, but at higher temperature local β coursing occurred at the grain boundary. When the alloy was exposed above 600°C, Si was obviously concentrated in the grain boundary region, and the maximum concentration was up to 2.5%. With the increase of thermal exposure temperature, the characteristics of high-temperature fatigue fracture of Ti60 alloy change from trans-granular toughness to intergranular brittleness.

Bonded Behavior of Hybrid-Bonded CFRP to Heat-Damaged Concrete Interface

The reliable bond of CFRP to heat-damaged concrete is fundamental to ensuring cooperative working when the CFRP is applied for strengthening fire-damaged concrete structures. In this paper, the bond performance of hybrid-bonded (HB) CFRP to a heat-damaged concrete surface was investigated by using single shear tests. The concrete blocks were initially heat-damaged to temperatures of 100 °C, 200 °C, 300 °C, and 400 °C. The heat-damaged blocks were subsequently bonded with CFRP using the HB technique. The primary experiment parameters were the exposure temperature, the number of mechanical fasteners, and the bonded layers of CFRP. The test results show that the external-bonded (EB) and HB-CFRP joints with concrete exposed to a temperature of 400 °C were prone to fail with concrete shear-tension due to the decreased shear strength of concrete at high temperatures. The EB- and HB-CFRP joints with heat-damaged concrete anchored with no more than three fasteners present a higher bond than the reference joints with unheated concrete, while the HB-CFRP joints anchored with three fasteners provide a decreased bond capacity with the increase in exposure temperature. The utilization rate of single-layer CFRP joints with unheated concrete increased by 57.9%, 139.5%, and 136.8% with the mechanical fasteners in numbers of one, two, and three compared with the reference specimen. Accordingly, the bond capacity increased by 111.8%, 128.4%, and 186.7%. Finally, a model was proposed to estimate the bond strength of HB-CFRP joints with heat-damaged concrete.

Properties of polyoxymethylene fibre-reinforced seawater sea sand concrete exposed to high temperatures

Using seawater and sea sand to prepare concrete can reduce the adverse effects from the resource shortage and environmental pollution caused by the exploitation of freshwater and river sand. In addition, polyoxymethylene (POM) fibres have excellent alkali resistance and durability, which may considerably improve the mechanical properties of seawater sea sand concrete (SWSSC). Fires are a common structural disaster. Thus, the post-fire mechanical properties of POM fibre-reinforced SWSSC (POM-SWSSC) were experimentally studied herein to evaluate the residual bearing capacity of the POM-SWSSC structure after a fire. After the heating and cooling procedures, the apparent morphology and mass loss of POM-SWSSC were recorded, and the compressive and tensile properties of the POM-SWSSC were investigated. The test results indicate that the colour of the SWSSC changed from cyan to yellow with an increase in exposure temperature. When the temperature exceeded 400 °C, the mechanical properties of the POM-SWSSC deteriorated sharply. Compared with compressive strength, high temperature had a more significant effect on the tensile strength of SWSSC. The addition of POM fibres could alleviate this issue. Compared with other fibre-reinforced concretes, POM-SWSSC had a higher cube compressive strength and splitting tensile strength after exposure to 400 °C. A prediction model was proposed to quantify the effect of different high temperatures on the mechanical properties of NF-SWSSC and POM-SWSSC.

Changes of wettability of shale exposed to supercritical CO2-water and its alteration mechanism: Implication for CO2 geo-sequestration

The wettability of shale are the key factors determining the shale gas production efficiency and CO2 storage safety. However, the effects of the supercritical CO2 (ScCO2)-water-shale interactions on the wettability in shale is unclear. To address this issue, water wettability alterations in shale at different ScCO2-water exposure pressure and temperature conditions were evaluated based on a nuclear magnetic resonance (NMR) test method, and the mineral composition, element mobilization and pore of shale were analyzed to interpret the alterations. After ScCO2-water exposure, the water wettability of shale was reduced, and the reduction enhanced with the increase of exposure pressure while weakened with the increase of exposure temperature. The weakening of water wettability in shale is closely related to the exposure pressure and temperature dependent mineral and pores alteration in shale. ScCO2-water exposure induced the increase in the content of hydrophobic quartz, and the decrease in the contents of hydrophilicity clay and carbonate minerals, leading to the weakening of water wettability in shale. In addition, the micro and mesopores gradually transfer to larger pores after ScCO2-water exposure, reducing capillary pressure and the hydrophilicity of shale, also responsible for the decrease of water wettability in shale. The weakening of water wettability may enhance the sweep efficiency of CO2 in shale reservoirs or reduce the capillary trapping capacity of CO2 when shale served as caprocks. Thus, this study contributes to the evaluation of CO2 storage capacity and security in shale reservoirs.

Residual performance of compression stiffened welded hollow spherical joints after exposure to elevated temperatures

The post-fire residual behavior of the joints and their accurate identification are critical issues for the assessment of the fire damage in the commonly used grid structures. Accordingly, it will be determined, whether the structure is directly used, disassembled, or used after repair. In this paper, the residual performance of compression stiffened welded hollow spherical joints after a fire is studied using the methods of air cooling and water cooling. The failure mode, load-displacement curves, and residual mechanical properties of the joints are analyzed. The results show that the residual performance of the joints after a fire is mainly related to the exposure temperature and the cooling mode. In air-cooled joints, the initial axial stiffness and bearing capacity decrease with the increase of exposure temperature, while the ductility increases significantly. In water-cooled joints, the initial axial stiffness and ductility decrease with the increase of exposure temperature, and the bearing capacity shows a decreasing trend before 500 °C and gradually increases after 500 °C. When conducting the FEM simulation of the experimental specimens, three-dimensional hexahedral elements were adopted with the relationship of the steel material after elevated temperature consisting of a combination of straight lines. The validity of the FEM model was verified by comparison with the experimental results. Based on the results of the parametric analyses, a calculation formula is proposed for predicting the residual bearing capacity and initial axial stiffness of compression stiffened welded hollow spherical joints after a fire.

Effect of thermal exposure on the microstructure and strength of an alumina‐silica ceramic fiber

AbstractThe microstructure and mechanical properties of an alumina‐silica ceramic fiber after thermal exposure at 1100–1300°C were investigated by X‐ray diffraction, nuclear magnetic resonance, scanning electron microscopy, transmission electron microscopy analyses and room temperature tensile strength test. The results showed that the fiber was composed of γ‐A12O3 and amorphous SiO2. A phase reaction of γ‐A12O3 and amorphous SiO2 occurred when thermal exposure temperature exceeded 1150°C, and a new mullite phase formed. The grain size of the newly formed mullite increased with the increase of exposure temperature. Both the phase transition and grain growth of mullite had a significant impact on the mechanical properties of the fiber. Tensile strength of the fiber decreased slightly when thermal exposure temperature was below 1150°C, while the strength retention of the fiber decreased sharply to 65.36% as exposure temperature rose to 1200°C. A higher dispersion of tensile strength was also observed at higher exposure temperatures, as revealed by the Weibull statistical model.

Bond performance between FRP bars and geopolymer concrete after elevated temperature exposure

The use of geopolymer concrete structure reinforced with fiber reinforced polymer (FRP) bars is an effective approach to solve the problems of high energy consumption, large CO2 emission, and steel corrosion caused by traditional reinforced concrete structures. This paper presents an experimental study on the effect of elevated temperatures on bond performance between FRP bars and geopolymer concrete. The influence factors include exposure temperature, surface treatment of FRP bars, fiber type of FRP bars, geopolymer concrete strength and type of concrete. The test results indicate that the bond strength between FRP bar and geopolymer concrete increases first and then decreases gradually with the increase of exposure temperature. After exposure to 350 °C, the bond strength retention ranges from 56.0% to 83.3%, while after exposure to 400 °C, the residual bond strength retention is not more than 17.7%. The bond strength retention of FRP bars in geopolymer concrete is basically higher than that in ordinary concrete from 25 °C to 350 °C. Moreover, the ribbed FRP bars possess a higher bond strength retention than the sand-coated FRP bars after elevated temperature exposure. With the increase of the geopolymer concrete strength, the high temperature resistance of geopolymer concrete decreases, resulting in a corresponding decreasing in the bond strength retention. In addition, the fiber type of FRP bar has no significant effect on the interfacial bonding properties compared with surface treatment of FRP bar and concrete strength. Finally, the available equations for calculating the development length of FRP bar in geopolymer concrete after elevated temperature exposure was proposed, which may be used for the design of fire prevention and fire safety assessments.

Predicting the mechanical properties of E-glass fiber-reinforced polymer bars after exposure to elevated temperature

This paper presents the experimental investigations on the tensile, flexural, and shear performance of E-glass fiber-reinforced polymer (GFRP) bars after exposure to elevated temperatures from ambient to 500 ℃. Two types of GFRP bars with nominal diameters of 12 mm and 16 mm were directly exposed to several target temperatures for 0.5 h, and their mechanical properties were tested after cooling. The experimental results showed that the GFRP bars gradually loosened, and their surface color darkened with the increasing exposure temperature. After experiencing high temperatures, the bars’ tension, bending, and shear failure modes remained unchanged. However, their mechanical strengths showed a phased decline in three typical temperature ranges of 20 to 120 ℃, 120 to 270 ℃, and 270 to 420 ℃. In addition, no significant tensile and flexural elastic modulus degradation was observed. The measured strengths were compared with the results of several existing prediction models. A simplified model was proposed to predict the residual strength of GFRP bars after high-temperature exposure. Based on fire safety requirements, a fire-resisting reduction factor, CF, was proposed to determine the design strength of GFRP bars used in concrete slabs. For GFRP bar-reinforced concrete slabs with a cover thickness of 60 mm, the suggested CF for tensile strength and the other two strengths should be 0.65 and 0.75, respectively, to achieve structural safety after 2-h fire disasters.

Study on mechanical properties and microstructure of aluminate cement-based materials incorporating recycled brick powder after exposure to elevated temperatures

The mechanical properties of aluminate cement-based materials (A-CM) that partially replaced by recycled brick powder (RBP) after exposure to elevated temperatures were studied in this paper. A total of 72 prism specimens were prepared, and the main parameters included RBP replacement ratio (0%, 5%, 10%, and 15%) and exposure temperature (20 °C, 200 °C, 400 °C, 600 °C, 800 °C, and 1000 °C). The compressive and flexural tests of specimens after exposure to elevated temperatures were conducted, and the phase of hydration products and the microstructure of hardened cement paste were systematically analyzed by microscopic tests. The influence mechanism of RBP on the mechanical properties of A-CM before and after high temperatures was revealed. Results showed that the A-CM specimen with RBP replacement ratio of 5% presented the highest compressive and flexural strength at any exposure temperature. The compressive strength of specimens with the same RBP replacement ratio first increased and then decreased with the increase of exposure temperatures, and the highest compressive strength was obtained after exposure to 200 °C. While the flexural strength of specimens at any RBP replacement ratio decreased all the time with the increase of exposure temperature. Moreover, the explosive spalling of A-CM specimens after exposure to 1000 °C could be inhibited when the RBP replacement ratio exceeded 10%. Finally, a calculation method that considered the effect of RBP replacement ratio to predict the compressive and flexural strength of A-CM specimens after exposure to elevated temperatures was proposed and validated.

Durability of seawater coral aggregate concrete under seawater immersion and dry-wet cycles

Coral aggregates replace traditional aggregate sources to prepare coral aggregate concrete (CAC) in remote island or reef areas contributing to reduced construction costs and periods of offshore projects. However, the high porosity of coral aggregates provides a channel for external corrosion solutions to penetrate into the concrete, rendering the CAC and its structures prematurely exposed to durability-related problems during service in marine environments. Therefore, this paper performed the mechanical properties and durability of CAC under seawater immersion and dry-wet cycle environments, and the deterioration mechanisms in the mechanical characteristics of CAC after seawater environment exposure were evaluated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The experimental results reported that the mechanical properties of CAC after exposure to seawater environments gradually declined with increasing exposure temperature and exposure age. After being exposed to seawater dry-wet cycle environments at 60 °C for 12 months, there was a reduction of approximately 14.4%, 13.0%, and 16.9% in the cubic compressive strength, elastic modulus, and axial compressive strength of CAC, respectively. This performance degradation was mainly attributed to the chemical reaction between the internal hydration products of the concrete and the corrosive ions in seawater. Additionally, compared with seawater immersion environments, the deterioration in mechanical properties of CAC was more significantly affected by seawater dry-wet cycle environments.

Static and dynamic tensile behaviors of BFRP bars embedded in seawater sea sand concrete under marine environment

Basalt fiber reinforced polymer (BFRP) bars can be used as an appropriate substitute for steel rebars in marine concrete structures due to their corrosion-free nature and high tensile performance. However, the long-term dynamic properties of BFRP bars in the marine environment are not well known, which is critical for structural safety when subjected to external dynamic loads (e.g., tsunamis and earthquakes). Therefore, this study investigates the static and dynamic tensile behaviors of BFRP bars after exposure to the seawater sea sand concrete (SWSSC) under marine environment. Accelerated corrosion tests were conducted under various exposure periods (60, 90, and 120 days) and temperatures (25, 40, and 55 °C). Both optical stereo microscope (OSM) and scanning electron microscope (SEM) were used to examine the failure patterns of BFRP bars, whereas the degradation mechanisms were investigated by X-ray micro-computed tomography (micro-CT), SEM, and Fourier transform infrared spectroscopy (FTIR). Results show that the tensile properties of BFRP bars are sensitive to strain rate. Both the tensile strength and the elastic modulus of BFRP bars increase with increasing strain rate, but the failure strain drops. The tensile strength and failure strain reduce with increasing exposure temperature and duration, whereas the elastic modulus does not change significantly. Microstructure analysis clearly reveals that failures of BFRP bars occur due to fiber breakage, matrix cracking, and interface debonding. The basalt fibers always exhibit brittle fracture, but the fracture surfaces change from relatively flat to rough when the strain rate increases. Moreover, the deterioration of BFRP bars is caused by resin hydrolysis, fiber-resin interfaces debonding and fiber damage after corrosion. These results can provide some insights to promote the application of BFRP composites in coastal and marine structures.

Post-fire performance of high-strength steel plate girders developing post-buckling capacity

Fire is a frequent disaster, which has lasting effects on mechanical properties of high-strength steel (HSS). For HSS structures after fire, it is important to assess if the structure can continue to service, should be repaired or demolished. The experimental and numerical investigations are performed to investigate the effects of exposure temperature on residual resistance of Q690 HSS plate girder considering the post-buckling capacity. In this study, the post-fire mechanical properties of Q690 HSS are investigated experimentally. The appearance characteristics of Q690 HSS with different exposure temperatures are obtained. The effects of exposure temperature on mechanical properties of Q690 HSS were discussed. To investigate the residual resistance of Q690 HSS plate girder after fire, the post-fire mechanical properties of Q690 HSS are introduced in the plate girder model, which has been strictly validated. Based on numerical results, the effect of exposure temperature on the initial stiffness of HSS plate girder can be ignored. The nonlinear properties of the Q690 HSS have the certain effect on the nonlinear characteristics of resistant curve of Q690 HSS plate girder considering the post-buckling capacity, which are influenced by the exposure temperature. The effect of exposure temperature on the buckling resistance of Q690 HSS plate girder is relatively small. With the raise of exposure temperature, the postbuckling capacity of Q690 HSS plate girder decreases. Effects of exposure temperature on the resistant properties of Q690 HSS plate girder were investigated, which were quantified by a suggested equation. The increase of exposure temperature has an adverse effect on the proportion of safety reserve of the Q690 HSS plate girder with post-buckling capacity.

The oxidation behavior of nickel‐based alloy Inconel 617 in supercritical water

AbstractThe corrosion behavior of Inconel 617 in supercritical water has been analyzed at 600°C–650°C and 25 MPa for up to 1000 h. The time dependence of the oxidation rate indicated that the oxidation kinetics follows parabolic law. Many fine oxide crystallites were observed at the surface of Inconel 617 while the thickness of the oxide increased with an increase of exposure temperature and time. The Inconel 617 exhibited excellent oxidation resistance due to the formation of Cr2O3. Furthermore, NiFe2O4, NiCr2O4, and CoO were also detected at 600°C while TiO2, NiFe2O4, and NiCr2O4 were detected at 650°C by using X‐ray diffraction, X‐ray photoelectron spectroscopy, and Raman spectroscopy. The formation mechanism of the oxide film is discussed.

The rheology of ultra-high molecular weight poly(ethylene oxide) dispersed in a low molecular weight carrier.

Gel spinning is the industrial method of choice for combining hydrophilic ultra-high molecular weight (UHMW) polymer resins with a hydrophobic support polymer to produce composite filaments for cytapheresis. Cytapheresis is a medical technique for removal of leukocytes from blood. Gel spinning is used to avoid high melt viscosity and thermal sensitivity of UHMW resins and the high melt temperature of the substrate resin but requires the recovery of toxic solvents. The UHMW resin is used because it forms a stable gel phase in the presence of water; a lower molecular weight resin (LMW) simply dissolves. UHMW and LMW resins were both poly(ethylene oxide) (PEO) and the substrate was polyarylsulfone (PAS). The literature indicated PEO undergoes non-oxidative thermal degradation above 200 °C and PAS is processed up to 350 °C. Dynamic oscillatory shear rheometry was used to study 0, 25, 40, 50, 60, and 75 wt. % UHMW PEO in LMW PEO to take advantage of the sensitivity of viscosity to changes in molecular weight and material configuration, indicating degradation. Samples were exposed to 220 °C, 230 °C, 240 °C, 250 °C, 275 °C, and 300 °C temperatures for 5 min to explore conditions that could result in sample degradation. The viscosity decreased less with increasing UHMW PEO content for samples exposed to the same temperature and the viscosity decreased more with increasing exposure temperature for samples with the same UHMW PEO content. Parameters were regressed from observed data to predict the change in molecular weight via empiricisms relating the viscosity to molecular weight, shear rate, temperature, and time.

Effect of temperature on stress strain behavior of high calcium fly ash and alccofine blended high strength concrete

In present study, the effect of temperature on stress–strain response under monotonic compressive loading is studied for two blended high strength concrete (HSC, viz., high calcium fly ash blended HSC (FC-HSC) and alccofine blended HSC (AL-HSC)) mixes. The experimental studies have been carried out with a 100 mm × 100 mm × 100 mm cube specimen and tested at 28 day of curing. For each mix, four sets of specimens, including control specimen and those exposed to elevated temperatures of 80 °C, 140 °C and 160 °C for 6 h were considered for the study. The stress–strain behavior of HSC mixes at elevated temperatures exhibits a distinct effect of material composition. The increasing exposure temperature increases peak stress for AL-HSC, whereas it initially decreases and then increases for FC-HSC. The results from the study confirm that elevated temperature contributes to stiffening of the pre-peak region, but the effect on peak stress is influenced by the material composition. The effect of elevated temperature on microstructural change is evaluated through thermo gravimetric analysis and the results are compared for FC-HSC and AL-HSC. The weight loss (30–160 °C) representing calcium silicate hydrate (CSH) degradation decreases with increasing exposure temperature for FC-HSC, whereas it remains range bound for AL-HSC. On the other hand, weight loss (400–500 °C) representing degradation of calcium hydroxide (CH) remains range bound for FC-HSC while it increases with increasing exposure temperature for AL-HSC. The observed results suggest that increasing exposure temperature contributes to CSH degradation in FC-HSC, whereas it enhances CH formation for AL-HSC. The study results provide useful insight on the effect of elevated temperature on the stress–strain behavior of blended HSC with flyash and alccofine.

Study on the high temperature microstructure stability of the high Al content Ni-based single crystal superalloy

The high-temperature structure stability of high Al content Ni-based single crystal superalloy was studied, the microstructure was observed, and the micro-mechanism of the thermal stability of the material was analyzed. The results show that the exposure time and exposure temperature have significant effects on the stability of the alloy’s high-temperature structure. With the increase of exposure temperature and/or exposure time, the γ’ phase in the material becomes coarser, gradually connects and grows, and the stability of the material structure is obviously reduced. Compared with standard heat-treated alloys after high temperature exposure, The absolute value of the γ’ and γ phases misfit increases significantly. With the increase of exposure time, although the absolute value of the two-phase misfit continues to increase, the increase of its absolute value gradually slows down. The long-term structural stability of the alloy under continuous exposure to high temperature environment is good.

Residual mechanical properties and numerical analysis of recycled pebble aggregate concrete after high temperature exposure and cooled by fire hydrant

In urban fire, the structure will not only be subjected to high temperature, but also cooled by fire hydrant. This paper presents the experimental results of the residual mechanical performance and the complete compressive stress–strain relationship of recycled pebble aggregate concrete (RAC) after exposure to high temperature of 20 to 600 ℃ and cooled by fire hydrant. In the experiments, five different concrete compositions were designed by substituting natural coarse aggregates with recycled coarse aggregates at the replacement ratios of 0%, 30%, 50%, 70% and 100%. Results exhibit that with the increase of exposure temperature, the surface color of the specimens changes from light to dark, the mass of RAC first increases and then decreases, and the residual mass of RAC at 200 ℃ is higher than that at room temperature. The compressive strength and the strain at the peak point decrease rapidly with the increase of temperature. The compressive strength and the strain at the peak point fluctuate with the increase of aggregate replacement percentage, and the temperature and aggregate replacement percentage have little effect on the ductility of RAC. The residual bearing capacity calculation method and constitutive equation of RAC after high temperature exposure and cooled by fire hydrant are proposed and verified. The accuracy of the constitutive equation is verified by ABAQUS finite element analysis software.