Effect Of Temperature Cycling Research Articles

Effects of Temperature Cycling on the Mechanical Properties of GFRP at Elevated Temperatures

Effects of Temperature Cycling on the Mechanical Properties of GFRP at Elevated Temperatures

Effects of freeze-thaw cycles on the tensile properties of geogrids and shear behavior of geogrid-soil interface

The tensile strength of geogrids and the shear strength of the reinforced soil interface are critical technical indicators for designing reinforced soil structures. This study developed a temperature-controlled apparatus to examine the tensile properties of geogrids subjected to freeze-thaw cycles within the soil, and the effect of interface temperature and freeze-thaw cycles on the shear behavior of the geogrid-soil interface. Eight and four groups of direct shear and tensile tests were conducted on the geogrid-soil interface and geogrids, respectively, under varying conditions. The peak tensile strength of geogrids remained similar to that of non-freeze-thaw samples after experiencing freeze–thaw cycles within the soil. tensile properties of geogrids were independent of the number of freeze–thaw cycles. At constant water content (w=6%), the peak shear stress at the geogrid-soil interface decreased by 24% (7 freeze-thaw cycles) compared to the non-freeze-thaw group. The cohesion and friction angle of the geogrid-soil interface decreased with increasing freeze-thaw cycles but tended to stabilize after 4 freeze-thaw cycles. The reinforcement provided by the geogrid enhanced with decrease in the interface temperature. The shear stress was higher at interface temperature under 0 °C, whereas in a non-frozen state, the interface exhibited lower shear stress with a consistent stable value.

Centrifuge modelling of end-bearing slender energy pile performance under temperature cycles

Slender energy pile shows a buckling deformation behavior under the coupling effect of temperature cycles and external loads, significantly affecting the long-term serviceability and even leading to the buckling failure. To investigate the thermo-mechanical performance of end-bearing slender energy piles under the long-term effect of temperature cycles, a centrifuge modelling experiment was conducted in this work, in which the temperature distribution, displacement, axial force and bending moment of the energy pile during the long-term temperature cycle were measured. The results unveiled that the axial force of the end-bearing slender pile increases as temperature rises, potentially leading to a displacement neutral point in the lower part of the pile. Meanwhile, there was an embedding effect in the lower part of the slender pile, with a preliminary calculated embedment depth of 36.2 meters. Temperature fluctuations can soften the soil at the pile end over time, diminishing the embedding effect on slender piles, and causing a reverse bending point to emerge at a depth of 45.5 meters. The appearance of bending moments in the pile signaled the horizontal displacement encountered in the pile service life, warning the bearing capacity loss problem from changes in pile deflection.

Synergistic Effect of Temperature Cycling and Atomic Oxygen on the Parallel Gap Resistance Welding Joint in GaAs Space Solar Panel

Synergistic Effect of Temperature Cycling and Atomic Oxygen on the Parallel Gap Resistance Welding Joint in GaAs Space Solar Panel

Temperature Cycling Effects on Infrared Radiation-Induced Attenuation of Silica-based Optical Fibers

Temperature Cycling Effects on Infrared Radiation-Induced Attenuation of Silica-based Optical Fibers

Experimental Study on the Compressive Strength and Fatigue Life of Cement Concrete under Temperature Differential Cycling

Concrete, as an engineering material with extremely wide applications, is widely used in various infrastructure projects such as bridges, highways, and large buildings. However, structures such as highways and bridges often need to be situated in variable and harsh service environments for long periods. They not only face cyclic reciprocating vehicle loads but also have to contend with the effects of temperature cycling. Therefore, studying the impact and mechanism of temperature differential cycling on the compressive strength and fatigue life of cement concrete has certain theoretical significance and practical value. This study employed a comprehensive experimental design to investigate cement concrete specimens subjected to typical temperature variations (20–60 °C) and different numbers of temperature differential cycling (0, 60, 120, 180, 240, 300). Axial compressive strength tests, ultrasonic tests, and compressive fatigue tests were conducted. The axial compressive strength test measured the compressive strength of the cement concrete. It was found that with an increase in the number of temperature differential cycling, the compressive strength exhibited a trend of an initial increase followed by a decrease: at 60 cycles, the strength increased by 10.8%, gradually declined; returned to near-initial strength at 120 cycles, and continued decreasing, reaching a decline of 19.4% at 300 cycles. The ultrasonic test measured the ultrasound velocity of the concrete specimens after different temperature differential cycling. It revealed a decreasing trend in ultrasound velocity with an increase in times of temperature differential cycling, showing a strong linear relationship between the ultrasound velocity loss and strength loss, confirming the correlation between the degree of concrete strength degradation and internal damage. The compressive fatigue test analyzed the fatigue life variation in cement concrete under different times of temperature differential cycling and stress levels, showing good adherence to the Weibull distribution pattern. Based on the approximation assumptions of log-normal distribution and the Weibull distribution, the Weibull distribution parameters for the compressive fatigue life of cement concrete under temperature differential cycling were obtained.

Effects of Temperature and Diurnal Cycle in the Molting Schedule of Mangrove Crab, Scylla serrata (Forskål, 1775)

Soft-shell crab farming is gaining popularity since all crab parts can be eaten with almost no discards. However, its biggest challenge is the tedious monitoring every 4-hour interval to check for molting. A total of 90 mangrove crabs weighing 80-100 g were stocked in individual crab trays, recording 24-hour water temperature and incidence of hourly molting for 57 days to provide another basis for monitoring and cues in the molting schedule. Results showed that molting (82.24 %) happens at optimum temperature (Topt) levels, between 27–31°C, with a high incidence of molting (85.29 %) at nighttime when the water temperature is lower and within the Topt. Findings suggested that aside from 4-hour intervals, monitoring of molting for soft-shell crab farming is recommended at nighttime and early morning.

Effects of temperature cycles on human thermal comfort in built environment under summer conditions

Dynamic temperature control strategies are feasible for enhancing energy flexibility and reducing energy consumption in buildings. However, guidelines for designing such dynamic thermal environments are lacking. In this study, 30 participants were recruited to undergo four experimental cycles formed by combining two temperature ranges (25–28 °C and 26–29 °C) and two temperature change rates (3 °C/h and 6 °C/h). Variations in the subjective perception and physiological responses with time were recorded throughout the experiments. The participants reported cooler thermal sensation and better thermal comfort for the same temperature during the ramp-down phase than during the ramp-up phase, which was more pronounced at faster temperature changes. The limits on temperature variations in the current standards underestimate the thermal acceptability of people. Although the temperature cycles exceeded the limits in the standards, sustained thermal comfort and high thermal acceptability were achieved when the temperature changed within 25–28 °C. At a rapid 6 °C/h change rate, the thermal sensation briefly deviated from the comfort zone when Top changed within 26–29 °C, suggesting that the limits should be set relative to the temperature change span. The comfortable temperature ranges for change rates of 3 °C/h and 6 °C/h in summer conditions were 22.8–28.7 °C and 22.8–28.4 °C, respectively, which are broader than the recommended indoor temperature range for summer in the Chinese standard. These findings indicate the potential of temperature variations to extend the thermal comfort zones while consuming less energy without requiring additional cooling devices.

Effect of High Temperature and Thermal Cycle of 4043 Al Alloy Manufactured through Continuous Casting Direct Rolling.

CCDR 4043 Al alloys are an outstanding candidate for producing mechanical components for automotive or aircraft engines. Two experimental environments-sustained high temperature and repeated heating-cooling-were simulated in the laboratory to replicate the actual operating conditions of engine components. This research investigated the microstructural evolution, mechanical properties, and fracture characteristics of the 4043 Al alloy manufactured through the continuous casting direct rolling (CCDR) process under different post-processing conditions. The CCDR process combines continuous casting, billet heating, and subsequent continuous rolling in a single equipment of production line, enabling the mass production of Al alloy in a cost-effective and energy-efficient manner. In the present work, the 4043 alloy was subjected to two environmental conditions: a sustained high-temperature environment (control group) and a cyclic heating-cooling environment (experimental group). The maximum temperature was set to 200 °C in the experiment. The experimental results show that, in a sustained high temperature working environment, the strength and elongation of the CCDR 4043 Al alloy tend to be stable. The overall effect involves the Al matrix softening and the spheroidization of eutectic Si caused by prolonged exposure to high temperature. This can enhance its ductility while retaining a certain level of mechanical strength. Comparatively, in the working environment of cyclic heating-cooling (thermal cycle), the direction of Si diffusion was different in each cycle, thus leading to the formation of an irregular Ai-Si eutectic structure containing precipitated Si particles of different sizes. The two compositions of Al and Si with very different thermal expansion coefficients may induce defects at the sharp points of Si particles under repeated heating-cooling, thereby reducing the strength and ductility of the material. The results of this work can confirm that the fracture behavior of 4043 Al alloys is obviously controlled by the morphology of the precipitated eutectic Si. In addition, CCDR 4043 Al alloys are not suitable to be used in working environments with a thermal cycle. In practical applications, it is necessary to add traces of special elements or to employ other methods to achieve the purpose of spheroidizing the precipitated eutectic Si and Al-Fe-Si phases to avoid the deterioration of strength and ductility under cyclic heating. To date, no other literature has explored the changes in the microstructure and mechanical properties of CCDR 4043 Al alloys across various time scales under the aforementioned working environments. In summary, the findings provide valuable insights into the effect of thermal conditions on the properties and behavior of CCDR 4043 Al alloys, offering potential applications for it in various engineering fields, such as the automotive and aerospace industries.

Dry–wet cycles induce the decoupling of carbon and nitrogen mineralization at high temperatures in semi-arid grassland soil

Soil carbon (C) and nitrogen (N) transformations are typically interlinked because of the conserved elemental stoichiometry of microbes, which can be remarkably altered by intensified temperature variations and dry–wet (DW) cycles. However, the interactive effects of temperature and DW cycles on soil C and N transformations and how microbial communities mediate these processes remain largely unknown. In this study, we subjected a semi-arid grassland soil from northern China to four successive DW cycles at 15, 25, and 35 °C. The soil respiratory rates during incubation, labile C and N fractions, activities of C and N acquisition enzymes, community-level physiological profiles, and microbial community composition at the end of the incubation were determined. Soil respiration rates decreased with drying but sharply increased with wetting, particularly at higher temperatures. Soil DW cycles did not significantly affect cumulative C mineralization and net N mineralization at 15 °C. However, they decreased cumulative C mineralization by 24.5% and 23.4% and increased net N mineralization by 48.6% and 41.7% at 25 and 35 °C, respectively, indicating a decoupling of C and N mineralization at higher temperatures. As the temperature increased, soil DW cycles decreased cellobiohydrolase and peroxidase activities and the microbial use of amino acids and lipids but increased N-acetyl-β-glucosaminidase activities and the microbial use of saccharide polymers and recalcitrant amines. Moreover, soil DW cycles increased the abundances of gram-positive bacteria and actinomycetes and gram-positive-to-gram-negative bacteria ratios with increasing temperature, particularly at 35 °C. Overall, our findings suggest that DW cycles at higher temperatures decelerated the mineralization of organic soil C by decreasing cellobiohydrolase and peroxidase activities. However, DW cycles at higher temperatures improved N mineralization by shaping the microbial communities to excrete more recalcitrant-organic-N acquisition enzymes, thereby meeting their high N requirements.

Combined effects of temperature and emersion-immersion cycles on metabolism and bioenergetics of the Pacific oyster Crassostrea (Magallana) gigas

Life on tidal coasts presents physiological major challenges for sessile species. Fluctuations in oxygen and temperature can affect bioenergetics and modulate metabolism and redox balance, but their combined effects are not well understood. We investigated the effects of intermittent hypoxia (12h/12h) in combination with different temperature regimes (normal (15 °C), elevated (30 °C) and fluctuating (15 °C water/30 °C air)) on the Pacific oyster Crassostrea (Magallana) gigas. Fluctuating temperature led to energetic costly metabolic rearrangements and accumulation of proteins in oyster tissues. Elevated temperature led to high (60%) mortality and oxidative damage in survivors. Normal temperature had no major negative effects but caused metabolic shifts. Our study shows high plasticity of oyster metabolism in response to oxygen and temperature fluctuations and indicates that metabolic adjustments to oxygen deficiency are strongly modulated by the ambient temperature. Co-exposure to constant elevated temperature and intermittent hypoxia demonstrates the limits of this adaptive metabolic plasticity.

Thermally induced shear displacement of sand-concrete interface under different stress levels

Energy geostructures are modern technologies developed to exploit shallow geothermal energy. Their installation has recently increased rapidly throughout the world. Their use is devoted to the heating and cooling of buildings, and their operation can induce thermal cycles at the soil-structure interface in the range of 5–40 °C [1]. In addition, their activation would produce thermally-induced stress and strain in structure. Over the last few decades, several full-scale tests have been performed to study the geotechnical aspects of energy piles [2,3]. The main results show that the thermal operations may slightly influence the mechanical performance of the energy pile by increasing the additional thermal compressive stresses and changing the pile axial strain. Numerical studies [4,5] concluded that the temperature increment can lead to thermal expansion of the pile, causing its uplift and the generation of negative shear forces at the pile–soil interface near the pile head. Considering that the thermal operation of energy geostructures leads to temperature change along the piles as well as in the neighbouring soil due to the heat exchange phenomenon, many researchers have investigated the temperature effects on the behaviour of soil and soil–structure interface [6,7,8,9]. Others investigated thermally induced soil deformation using modified triaxial or oedometric apparatuses allowing temperature control [10,11,12]. These results show expansive deformation of coarse-grained soils upon heating and contractive deformation upon cooling for all relative densities. A modified geothermal direct shear box (MGSB) developed at Gustave Eiffel University [13] was used to investigate the effects of temperature cycles on soil–concrete interface. The preliminary results showed that the sand expands upon heating and contract upon cooling. An accumulated contraction and shear displacement were observed after each thermal cycle. However, these preliminary results should be carefully re-analysed since the device was not fully calibrated under various thermo-mechanical loads.
 In this study, the MGSB was utilized after its calibration [13], to investigate the thermally induced vertical and lateral displacements of sand–structure interface. It consists of two half-shear boxes with dimensions of 200 mm x 200 mm x 80 mm. A Peltier module was used to heat and cool water connected to a pipe embedded in the concrete plate, which was used to simulate the pile surface. Normal and shear stresses were applied to represent the geostatic stress of soil and pile load, respectively. Four linear variable differential transformers (LVDTs) were placed in contact with the lower and the upper boxes of the device to measure the shear displacement. One LVDT was placed at the top of the box to measure the vertical displacement of soil. To prevent heat losses, the system was insulated using an insulating foam. Two series of test were performed. The four tests of the first series were conducted on dense (D) sand and the three tests of the second series were on loose (L) sand, with relative densities of about 90% and 30%, respectively. For each density, three normal stress levels were tested: 25, 50 and 100 kPa. For each normal stress, a shear stress equal to 40% of normal stress (45% of the shear strength) was applied. Test D_50 was repeated because of technical problems that happened during the first test. After samples preparation, two phases were carried out. The first phase corresponds to the
 application of normal and shear stresses. Regarding the second phase, 20 thermal cycles were applied, varying the soil temperature between 14 °C and 32 °C.
 The mechanical results showed a good agreement compared to standard direct shear tests. The thermo-mechanical results showed that the succession of soil expansion and contraction upon heating and cooling respectively lead to an accumulated irreversible contraction and continuous shearing displacement after each thermal cycle with different magnitudes, depending on the soil density and stress level. In addition, the rate of these incremental irreversible displacements decreases when the cycle number increases. Moreover, the increase in the stress levels will lead to an increase in the accumulated lateral displacement after 20 cycles, and this increment is significant in loose sand compared to dense sand under the same mechanical conditions. Overall, the thermally induced sand-concrete interface displacements are affected by stress level and strongly depend on the sand density. The thermally induced settlement may be critical in the case of floating energy piles under high mechanical load, and greater than 45% may lead to greater in the case of loose soil would be greater than 0.3 mm. Finally, this study suggests that using a homogeneous stress design method for energy geostructures may adversely affect the design.

Shear behavior of the sand-concrete interface under cyclic thermal cy-cles

Energy piles - foundation elements also used as heat exchangers - are subjected to daily and seasonal heating and cooling cycles, which might result in the modification of thermo-mechanical behavior of the soil-pile interface. In order to evaluate the effect of the pile’s geothermal activation on a sand-concrete interface, at laboratory scale, a series of direct shear tests were performed using a device adapted for thermomechanical loading. The constant normal load (CNL) direct shear tests were carried out for three different thermal loading scenarios, i.e., at a constant temperature of 13°C, and for two different cooling-heating (8-18°C) cycles. The volumetric deformation after 10 thermal cycles is evaluated, and the shear results are compared.
 A lot of studies [1, 2, 3, 4, 5, 6, 7, 8] have been performed to investigate the thermal effects on the shear behavior of soil-structure interface through direct shear tests. However, in previous studies all the thermal cycles were performed before the shearing phase. Hence, the effect of temperature cycles on the sand-concrete interface after the shear strength has already been mobilized (i.e., interface shear test) has not been well investigated so far. In fact, in real exploitation conditions, the thermal loading is usually added after a part of the mechanical loading or deformation occurred at the soil-pile interface, given the fact that the geothermal exploitation starts only once the building is in operation. To better understand the effects of thermal cycles on the shear properties of sand-concrete energy piles interface after a prior shear phase, a series of interface direct shear tests were performed in laboratory: (1) Reference Test (RT), shearing under constant temperature of 13°C (undisturbed soil temperature below 5 m in the Parisian region, France); (2) Cyclic Test 1 (CT-1), displacement controlled shearing under constant temperature (13°C), 10 thermal cycles (8 to 18°C), finally displacement-controlled shearing until the constant volume state was generated; (3) Cyclic Test 2 (CT-2), shearing to 1/2 of the residual shear strength, 10 thermal cycles (8 to 18°C), finally displacement-controlled shearing until the constant volume state was generated (CT-2 test). The monotonic displacement-controlled direct shear tests were conducted under constant normal stresses equal to 50 kPa, 100kPa and 150kPa that correspond to the typical normal effective stresses acting on the soil-pile interface at different depths.
 Figure 1 presents the volumetric strain (εv) during the thermal cycles of CT-1 and CT-2. The sand samples are characterized by a contractive behavior during each heating phase and followed by an expansive behavior at the high temperature of 18°C. The samples continue to expand in the next cooling phase, then they start to contract when the low temperature of 8°C is constant. The volumetric strain changes are attributed to the thermal-induced particle rearrangement during the heating-cooling cycles. After the 10 thermal cycles, the sand samples show an overall contractive response. The thermal cycles induce more vertical strain on the samples of CT-2 than CT-1 (see Figure 1). The possible reason is that shearing to 1/2 residual shear strength develops more unstable particle rearrangement in CT-2 samples than shearing to the residual shear strength in CT-1 samples.
 The peak and residual interface friction angles of CT-1 and CT-2 after the 10 cyclic thermal cycles are compared with RT in Table 1. The peak interface friction angle of CT-1 is 28.9°, 0.6° lower than the one of RT (29.5°). Whereas a higher peak interface friction angle is mobilized in CT-2 (31.7°). CT-1 is characterized with a residual interface friction angle of 27.9°, which is 0.5° lower than the 28.4° of RT. However, the difference of residual interface friction angles between CT-2 (28.5°) and RT (28.4°) is only 0.1°. To conclude, the effect of the 10 cyclic thermal cycles on the sand-concrete interface is quite limited, it slightly decreases the peak as well as residual interface friction angles of CT-1 and increases these of CT-2.

Reducing Surface Roughness of Selective Laser Melting of 316 Stainless Steel Component by Electropolishing

Additive manufacturing (AM) is widely used in the fields of aerospace, automotive, and biomedical. Because it enables to fabricate and customize parts with complex geometrical features. However, due to the layer-by-layer deposition and the partially fused raw material, AM part suffers from the step and balling effects, which shows inadequate and poor surface quality. Conventional polishing methods such as grinding, milling, and sandblasting have been applied to meet the requirement of well surface quality. But these contact polishing techniques usually introduce extra residual stress and surface damage, which are unsuitable for workpieces with complex shapes. In this study, we propose an environmentally friendly electrochemical polishing method for AM 316L stainless steel parts based on a combination of positive and negative pulses. Unlike the traditional phosphoric acid or sulfuric acid electrochemical polishing liquid system, the sodium chloride electrolyte adopted in this study avoids the pollution problem of waste liquid. A negative pulse was introduced to avoid the impaction of Cl- ions to the passivation film generated during the electrochemical polishing process. The effects of voltage, polishing duration, temperature, and duty cycle were analyzed by orthogonal experiments. Under the optimum reaction conditions, a surface roughness Ra of 0.421μm without pitting corrosion is achieved and the balling effect was eliminated.

Effect of thermal cycles on volumetric and shear behavior of sand–concrete interface

Energy piles are dual-purpose foundation elements that transfer the building loads to the supporting soil and act as heat exchangers for heating and cooling. Energy piles are subjected to heating and cooling cycles over days and seasons, resulting in cyclic expansion and contraction. The cyclic expansion and contraction bring about the modification of vertical stress and the increase of vertical displacements on the piles, in other words, the thermo-mechanical behavior of the pile, the soil, and the soil–pile interface is changed by the cyclic thermal cycles. Investigation to know the effect of temperature cycles on soil–pile interface characteristics has high academic and significant engineering values. This paper intends to investigate the thermo-mechanical response of soil–pile interface under different types of cyclic thermal loadings, to better understand the behavior of energy piles and their surrounding soils after the buildings are constructed and the geothermal activation. Direct shear tests are performed by using a new interface direct shear test machine composed of a temperature controlling device to assess the influence of different cyclic thermal loadings on the shear behavior of Fontainebleau sand–concrete interface. Direct shear tests are carried out after three different thermal loadings, i.e., a constant temperature of 13 °C, and two different kinds of cyclic cooling–heating​ (8–18 °C) loadings with 10 cycles (namely CT-1 and CT-2). The horizontal stress and horizontal displacement are evaluated after the 10 thermal cycles in CT-1 and CT-2 programs, and the shear results are then compared. The results reveal that the cyclic thermal cycles increase the peak shear strength of CT-2, however they do not significantly affect the residual shear strength of both CT-1 and CT-2. The peak/residual interface friction angle of the reference test is 29.5∘/28.4°, after the 10 cyclic thermal cycles, it becomes to 28.9∘/27.9° for CT-1, and 31.7∘/28.5° for CT-2.

Temperature cycling effect on structural, optical and electrical properties of nanostructured sodium titanates

Sodium titanates represent a promising alternative for the construction of anodes for sodium-ion batteries. To study the electrical response of nanostructured sodium titanate samples under plausible temperature conditions, we evaluate the electrical impedance in the range of temperatures from −30 ∘C to 50 ∘C, and the effect of consecutive temperature cycling. Strong changes in electrical properties are observed during the first temperature cycle, followed by a stabilization in the consecutive cycles. This behavior is reversible when the sample is exposed to external conditions and room temperature for less than an hour. Probably a structural and surface water removal and a surface water reabsorption occur, affecting the charge carriers and with it the impedance. Structurally, no differences are detected before and after the cycles, nor when evaluating only the vacuum process. However, when the optical properties are evaluated we find non-reversible changes after the temperature cycles. The optical behavior after the temperature cycling becomes uniform, the same type of electronic defects become predominant and similar direct and indirect band gap values are obtained, regardless of the starting synthesis condition. These results have significant implications for potential technological applications of these materials.

Fusion Splicing and Testing of Photonic Crystal Fibers

Properties of two different photonic crystal fibers (PCF) were characterized, enabling comparisons. Properties investigated included spectral attenuation, polarization mode dispersion (PMD), optical time domain reflectometer characteristics, elastooptic factor describing transmission delay induced by axial strain plus effects of temperature cycling and fiber twist on PMD and loss. In particular, temperature and twist dependence of PMD was different for each fiber tested. For optical measurements, fibers were fusion spliced to pigtails with standard telecom single mode fibers. PCF splicing procedures and solutions adopted to minimize collapse of holes during arc fusion and splice loss are presented. It was found that fusion splicing procedure must be individually tailored to each combination of fibers.

Durability of MICP-reinforced calcareous sand in marine environments: Laboratory and field experimental study

As eco-friendly methods, microbial induced carbonate precipitation (MICP) method was used to reinforce the calcareous sand in the South China Sea in this paper. The durability characteristics and deterioration mechanism of MICP-reinforced calcareous sand under various environment factors were investigated synthetically based on the unconfined compressive strength, mass loss rate and microscopic morphology in laboratory and field experimental study. Results show that, the unconfined compressive strength value of the sample is only 35.19 % of the initial strength, while the mass loss rate is about 6.69 % after 30-days of field marine environment erosion. MICP-reinforced calcareous sand shows the strongest resistance to temperature cycles, followed by dry-wet cycles, coupling effect of temperature and dry-wet cycle and salt spraying with drying cycles. MICP-reinforced calcareous sand exhibits the worst resistance to the field marine conditions, but the integrity of the sample could still be maintained after 30-days of field tests. The deterioration mechanism of MICP-reinforced calcareous sand is consistent under the various environmental cycles. First, the weakly cemented calcium carbonate crystals on the sample surface fall off, and then the hard-shell layer on the sample surface became weaker under various erosion. Finally, the internal cemented structure of the sample was gradually destroyed. The results indicated the utilization value of the MICP method in ocean engineering, but it is necessary to enhance the performance of the MICP-reinforced calcareous sand to ensure its protective effect after a certain environmental impact cycle.

Effects of load level, dimensional effects and temperature conditions on the pressure sensitivity of short carbon fiber cementitious composites

Carbon fiber reinforced cementitious composites (CFRCC) are a kind of sensing materials in structural self-monitoring system. However, their performances still need to be enhanced. In this study, the effects of load level, dimensional effect, temperature and temperature cycles on the pressure sensitivity of CFRCC with hydroxyethyl cellulose (HEC) dispersant were systematically investigated. Simultaneously, the change rate of resistance was adopted to quantify the pressure and temperature sensitivity of CFRCC. It is found that the pressure sensitivity of CFRCC is stable as load level is below 50 per of the ultimate load. With the increasing size of the specimens, the compressive strength of CFRCC gradually declines whereas its change rate of resistance hikes quickly, both of which show obvious size effect. Furthermore, in the range of 5 °C to 80 °C, the resistivity of CFRCC demonstrates the effect of negative temperature coefficient (NTC), and the sensitivity coefficients of CFRCC tend to decrease with the increasing temperature. Compared with the sensitivity coefficient at 70 °C, the sensitivity coefficient of each specimen at 15 °C have increased by four times. In addition, the pressure sensitivity of CFRCC has significantly decreased after temperature cycles, and especially after 20 temperature cycles, CFRCC’s maximum change rate of resistance decreases by about 30% compared to the initial one. Therefore, CFRCC can be directly used in concrete structures to undertake the task of structural health self-monitoring.

Effect of Cyclic Temperature and Fatigue Load on the Performance of Bond Strength at High-Strength Concrete–UHPC Interface

One of the most important components of box beam bridges is the shear key that connects the box beams together. The shear keys in box beam bridges often fail at the interface. In recent years, ultrahigh-performance concrete (UHPC) has been successfully used to cast these shear keys. The current study investigates the effect of temperature cycles on bond performance between a high-strength concrete (HSC) box beam and UHPC shear key. The interfacial bond performance was evaluated by subjecting specimens to 60, 120, and 180 freeze-thaw (FT) cycles representing 1, 2, and 3 years of environmental exposure. Some direct tension specimens that were subjected to 180 FT cycles were additionally subjected to 1-Hz frequency fatigue cycles with stress reversals. Using the direct tension, pull-off, slant shear, and bishear assessment methods, the test results have demonstrated that the number of FT cycles did not affect bond performance in terms of strength and failure modes. The test results also showed that standard specifications underestimate the cohesion and friction coefficients evaluated using Mohr-Coulomb’s and Carol’s approaches for concrete placed against intentionally roughened surfaces. Furthermore, it was found that fatigue loading with stress reversals did not diminish the direct tensile bond strength of the specimens.