Thaw Damage Research Articles

X-ray computed tomography analysis of internal voids in steel slag asphalt mixture under freeze–thaw damage and microwave healing process

In this work, the variation characteristics of different scale voids under freeze–thaw damage and microwave healing were studied by X-ray computed tomography (CT). Firstly, the scale classification of voids was carried out to analyze the initial distribution state of voids. Then the distribution of the horizontal plane and vertical direction was quantitatively characterized, and the source of the damage-healing effect was analyzed by using iterative variation indexes. Finally, the critical void was judged by combining the total and relative iterative variation rates. The results show that the area of small voids (0.15 ∼ 2.0 mm2) accounts for 55.7 %, and the number accounts for 90.17 % of the whole, serious interference with overall development when voids are not classified. The initial distribution of voids at different scales is quite different. The void growth is relatively uniform in the horizontal plane. From the distribution of the number of voids, the inner region is more prone to coherent vertical cracks, and the outer region is more prone to discrete micro-cracks. The role of voids at different scales in the damage-healing process is different. Small voids are the critical voids in the freeze–thaw damage stage. Both small and big voids are critical voids in the healing stage. This work is helpful to better understand the damage-healing process of steel slag asphalt mixture at mesoscale and contribute to the sustainability of asphalt pavement.

Study on the Influence of Saturation on Freeze–Thaw Damage Characteristics of Sandstone

In order to explore the evolution mechanism of freeze-thaw disasters and the role of water in the freezing-thawing cycles of rocks, the macro mechanical indexes and microstructural characteristics of seven different saturation sandstones after certain freeze-thaw cycles were analyzed. Electron microscope scanning, nuclear magnetic resonance, and uniaxial compression tests were employed to study the migration law of water in the rock, the crack growth law, and the damage mechanism during freeze-thaw cycles. The results showed that when the saturation was 85%, the peak load curve of sandstone with different saturation appeared at the minimum point, and the porosity of sandstone reached the maximum. The damage variable increased sharply when the saturation was 75-85%. This proves that 85% saturation is the critical value of sandstone after five freeze-thaw cycles. The water migration freezing model is established, and the migration direction of capillary film water during freezing is micropore → mesopore → macropore. The migration of water is accompanied by the expansion and generation of cracks. Then we study the mechanism and law of crack expansion, and the crack propagation rate is positively related to the theoretical suction. The theoretical suction and theoretical ice pressure increased linearly with the decrease in temperature, which accelerated the crack propagation. The crack propagation rate in decreasing order is Vmacropore > Vmesopore > Vmicropore. The research results can provide a theoretical basis for evaluating the stability of rocks under the action of freeze-thaw cycles in cold regions.

Evaluation of long-term fracture behavior of hot mix asphalt modified with Nano reduced graphene oxide (RGO) under freeze–thaw damage and aging conditions

Asphalt mixtures suffer from different types of cracking such as low-temperature cracks (LTCs) and intermediate-temperature cracks (ITCs). These two types of cracking may occur in the short- or long-term due to conditions such as temperature cycles, freeze–thaw cycles (FTC), and the aging process. Therefore, it is necessary to investigate the effect of these factors on the fracture behavior of LTC and ITC. This study was used the fracture mechanics method to evaluate the short-term and long-term fracture behavior of hot mix asphalt (HMA) containing Nano reduced graphene oxide (RGO) under mode I loading conditions. To this purpose, the geometries of symmetric specimen SCB and classical-modified specimen SCB-2 containing vertical and angular cracks were selected. In order to simulate short-term fracture behavior, the SCB samples were subjected to 0 FTC (un-condition) and to simulate long-term behavior, the SCB samples were subjected to 3 FTC and long-term aging (LTA) process. The results showed that the Nano RGO additive enhanced the fracture resistance of the HMA mixture containing vertical and angular cracks at temperatures of + 15 and −15 °C by increasing the fracture energy and fracture toughness indices. Also, the results showed that by increasing the stiffness of the HMA mixture, the additive increased the resistance against elastic deformations. Compared to the LTA process, the application of 3 FTCs to the HMA mixture (with and without Nano RGO), by reducing Pmax and increasing the amount of displacement at the moment of fracture, caused a decrease in the stiffness of the samples (at temperatures of + 15 and −15 °C). For mixtures with and without Nano RGO that were subjected to the LTA process, the increase in stiffness resulted from the increase in Pmax and the decrease in displacement at the moment of fracture (at temperatures of + 15 and −15 °C). Finally, the results showed that the application of 3 FTCs had the greatest impact on reducing fracture indices such as KIC, GF, and Pmax at temperatures of + 15 and −15 °C (for HMA and Nano RGO modified HMA mixtures). This result indicated the importance of using 3 FTC instead of LTA to simulate 12 years of damage on asphalt pavement.

Mechanical properties and modified binary–medium constitutive model for red–bed soft rock subjected to freeze–thaw cycles

For the deformation and evaluation of red–bed landslides in seasonally frozen ground regions, it is of great significance to study the mechanical properties of red–bed soft rocks subjected to freeze–thaw cycles and to establish an applicable constitutive model. Triaxial tests are conducted on red–bed soft rocks at conditions of different confining pressures and freeze–thaw cycles. The test results demonstrate that the freeze–thaw cycles significantly reduce the strength of the rock and make the ductile damage characteristics more significant. Based on the breakage mechanics for geological materials, the red–bed soft rocks are abstracted into two parts: the structural block and the weak zone. The structural block is transformed into the weak zone when the internal cementation is lost. The binary–medium structure composed of structural blocks and weak zones can reasonably explain the residual deformation mechanism and freeze–thaw damage mechanism of rocks. A constitutive model considering initial freeze–thaw damage is proposed based on the binary–medium structure and homogenization theory. The initial damage caused by the freeze–thaw cycle is reflected by introducing the initial breakage ratio parameter in the model. In addition, the inhomogeneous deformation characteristics of the structural block and the weak zone are reflected by introducing the strain concentration parameter. The model validation results demonstrate that the proposed model can accurately predict the deformation characteristics of red–bed rocks and can successfully reflect the deterioration characteristics caused by freeze–thaw cycles.

Flexural fatigue behavior and damage evolution analysis of aeolian sand concrete under freeze–thaw cycle

Aeolian sand is abundant and is widely distributed in western China and thus can reduce the project cost, alleviate the consumption of river sand resources, as well as protect the ecological environment in the application of concrete. Concrete structures, such as roads, bridges, and airport runways in northern cold regions, are exposed to the combined action of freeze–thaw cycle and fatigue load, and the damage mechanism is more serious and complex than that under a single factor. In the present study, the flexural fatigue behavior and damage evolution of aeolian sand concrete under freeze–thaw cycles are studied. Results show that freeze–thaw damage aggravates the flexural fatigue failure of concrete with aeolian sand, and the addition of aeolian sand enhances the average fatigue life of concrete after freeze–thaw cycle. The fatigue life of concrete that has aeolian sand content of 100% is improved more significantly than that of concrete with aeolian sand content of 50%. A double logarithmic fatigue equation is established for varying numbers of freeze–thaw cycles and different failure probabilities. Additionally, the course of flexural fatigue strain and fatigue modulus of aeolian sand concrete under freeze–thaw cycle action follows a typical three-stage development course. The effects of freezing and thawing action on fatigue behavior, such as fatigue strain growth, stiffness degradation, stress–strain relationship, and dissipated energy of 50% aeolian sand concrete (ASC50), are more significant, whereas the effects of 50 and 100 freeze–thaw cycles on the fatigue properties of 100% aeolian sand concrete (ASC100) are not evident. The research results are of great significance for the popularization and use of aeolian sand concrete in cold regions.

Optimization and Performance Evaluation of Steel Slag Asphalt Mixture Modified with Fibers under Freeze–Thaw Cycles

Recycling waste steel slag for asphalt pavement construction can reduce energy consumption and preserve nonrenewable natural resources. However, the moisture susceptibility of steel slag asphalt mixture (SSAM) in severe freeze–thaw regions should be taken into consideration. In light of this, three types of fibers (basalt fiber, polyester fiber, and lignin fiber) and rubber powder were blended into the SSAMs, and their freeze–thaw damage characteristics were investigated. First, the optimum bitumen and fiber content of SSAMs were determined based on Marshall design using response surface methodology. Then, the air voids of SSAMs were identified through X-ray computed tomography (CT) technology, while the effects of the fiber and freeze–thaw cycles on the moisture stability of SSAMs were investigated. Moreover, the enhancement effects of the fibers were analyzed using scanning electron microscopy (SEM). The results show the optimum fiber content of different fiber-reinforced SSAMs are 0.35%, 0.29%, and 0.23%, respectively, and the optimum bitumen content is 6.7% for all cases. The effect of freezing–thawing leads to an increase of the air voids in the middle part of the specimen, whereas the incorporation of fibers weakens the adverse action of freezing–thawing and effectively improves the moisture stability of the SSAMs. Furthermore, the network structure formed by basalt fibers in asphalt mastic significantly improves the integrity of the SSAM. Additionally, basalt fiber reinforced SSAM shows the best resistance to moisture damage.

Freeze-thaw failure characteristics and strength loss of non-penetrating fractured rock mass with different fracture densities

Rock mass usually contains cracks, joints and other defects with different density and spatial combination characteristics in the diagenetic process. Under the freeze–thaw effect, they continue to expand and connect, which continuously weakens the strength property and poses a serious threat to the stability of rock mass. Previous studies paid more attentions to the strength reduction of rock, failing to directly reflecting the freeze–thaw damage characteristics of rock mass. In this paper, the freeze–thaw failure characteristics and strength loss of non-penetrating fractured rock mass with different fracture densities were studied by means of theoretical derivation and laboratory tests. Firstly, non-penetrating fractured rock mass samples with different fracture conditions were prepared. Then the freeze–thaw samples were subjected to laboratory compression tests and the loss of uniaxial compressive strength was measured. On this basis, the theoretical values of geological strength index GSI was calculated for the samples, and the rough prediction empirical model of GSI for non-penetrating fractured rock mass treated with different freeze–thaw cycles was obtained through data fitting. Finally, by combining the apparent observation method with acoustic emission monitoring technique, the effects of freeze–thaw cycles and fracture density on the failure characteristics of non-penetrating fractured rock mass samples were analyzed comprehensively from both the macro and micro perspectives.

Experimental study on the hydro-thermal-deformation characteristics of cement-stabilized soil exposed to freeze–thaw cycles

The exploration of the hydro-thermal characteristics and deformation behaviors of cement-stabilized soils is important for the prevention and control of freeze–thaw damage in cold region engineering. This study used six groups of cement-stabilized soil samples with different cement contents (i.e., 3%, 6%, 9%, 12%, 15%, and 18%) to investigate the variations in soil temperature, volumetric unfrozen water content, deformation, freezing temperature, and dry density. The results showed that the temperatures of the cement-stabilized soil samples during the freezing and thawing processes can be categorized into three stages and that the freezing temperature decreased with increasing cement content. Moreover, the cement content and ambient temperature significantly affected the volumetric unfrozen water content of the cement-stabilized soil samples during the freeze–thaw cycles, and the soil temperatures corresponding to the peak hysteresis degree were relatively consistent with the freezing temperature. The residual volumetric unfrozen water content primarily depended not only on the cement content but also on the freezing condition. Although the variations in volumetric unfrozen water contents during the freezing and thawing processes were similar, the ranges in temperature change differed significantly, particularly in the drastic phase transition zone. Additionally, adding cement into soils effectively inhibited deformation, mainly due to the dual positive effects of the liquid water reduction owing to hydration reaction and structure compaction owing to the filling of hydration products.

Influence of Fiber Mixing Process on the Cracking Resistance of Cold Recycled Asphalt Mixture

Fiber reinforcement is often used to improve the road performance of cold recycled asphalt mixture (CRAM). The purpose of this research is to evaluate the impact of fiber mixing process on the cracking resistance of CRAM from multiple perspectives. Four kinds of fiber mixing processes, named A, B, C, and D, were designed by changing the order of fiber addition during the mixing process. Then, semicircle bending tests and indirect tensile tests were conducted to characterize the low-temperature cracking behavior of fiber CRAM. Freeze–thaw cycle tests under both dry and water-saturated conditions were performed to investigate the freeze–thaw damage behavior of fiber CRAM. Furthermore, the fiber dispersion in CRAM was observed using scanning electron microscopy (SEM). The results show that the fiber mixing process has a significant effect on the cracking resistance of CRAM. The CRAM specimens prepared by process C have the largest fracture energy, splitting strength, and fracture work, while the specimens made by process D have the smallest value. Specially, the fracture energy of the specimens prepared by process C is 77.23% larger than that of the specimens prepared by process A, while the fracture energy of the specimens prepared by process D is 5.6% smaller than that of the specimens prepared by process A. The reason for this phenomenon is that the fiber is well dispersed in the specimens prepared by process C, which contributes to obtain a better crack resistance. For all CRAM specimens, with the increase of freeze–thaw cycles, splitting strength and fracture work of fiber CRAM decrease. However, there is an obvious difference in the reduction rate of splitting strength and fracture work, especially for the specimens under the water saturation condition. The specimens made by process C have the smallest reduction rate, which indicates a better resistance to freezing and thawing damage. According to the analysis of fiber macro-distribution state in loose CRAM, the fiber dispersion is affected by the humidity conditions in the mixing environment. The best humidity conditions are obtained for fiber dispersion in process C. Based on the SEM observation, the overlapping bridging network structure can be observed in the microstructure of the specimens prepared by process C, allowing the mixture to better transfer and disperse stress.

Study on critical slowdown characteristics and early warning model of damage evolution of sandstone under freeze–thaw cycles

Freeze–thaw damage of rock mass poses a great threat to the safety of rock engineering, ground buildings, and low-temperature storage of liquefied natural gas (LNG) in cold regions. By collecting acoustic emission (AE) signals of sandstone during uniaxial compression failures, this paper analyzed the critical slowdown phenomenon of different types of sandstone during the freeze–thaw failure. According to the auto-correlation coefficients and the variance of AE signals under different windows and steps, the precursors were determined and a warning model of rock engineering failure precursors based on the critical slowdown principle was proposed. Then the Grey Wolf Optimizer (GWO) algorithm was used to optimize the initial weights and thresholds of the back propagation (BP) neural network, and the influence factors of rock engineering failure under different working conditions were input as training sets to train the network. The results showed that the correlation coefficients between the predicted value and real value of the GWO-BP neural network reached 99.90% and 98.81% respectively, indicating that the accuracy of the BP neural network prediction was improved. This study provides a new method for rock engineering failure early warning, and has great theoretical and guiding significance for enriching and improving the rock mass AE monitoring technology.

Seismic Fragility Assessment of RC Columns Exposed to the Freeze-Thaw Damage

Freeze–thaw damage is one of the primary causes deteriorating the seismic resistance of reinforced concrete (RC) structures. This paper proposed a freeze–thaw damage deterioration model for C30 concrete, and it can be employed to study the time-varying seismic performance of aging RC columns. Next, this study developed a seismic fragility analysis framework for deteriorating RC columns considering the effect of freeze–thaw damage. Considering the geometric parameters of the case-study bridge, the deterioration characteristics of material, and the uncertainties involved in structural modeling and ground motions, a probabilistic seismic fragility analysis on aging RC columns was conducted. The results indicate that the influence of freeze–thaw damage cannot be ignored in studying the seismic performance of aging RC structures. The seismic fragilities of deteriorating RC columns shown a nonlinear increase trend as the increased of freeze–thaw cycles and severity of the damage state. In the early stage of freeze–thaw cycles, the seismic fragilities of RC columns increased slowly. However, the closer to the later stage of freeze–thaw cycles, the more significant of the increase in the seismic fragilities of the columns.

Evaluation of long-term fracture behavior of amorphous poly alpha olefin (APAO)-modified Hot Mix Asphalt (HMA) under modes I and II at low and intermediate temperatures

Opening and in-plane sliding cracks are among the most important cracks in flexible pavements, resulting from tensile and sliding deformations, respectively. Loading and freeze–thaw damage (FTD) are the two main factors for the initiation and growth of these types of cracks, which may lead to two types of cracks, intermediate-temperature cracks (ITCs) and low-temperature cracks (LTCs), depending on the ambient temperature. Therefore, in order to simulate the tensile and sliding deformations of mixtures containing ITCs and LTCs, two geometries called symmetric sample SCB (containing vertical cracks) and classical-modified sample SCB-2 (containing angular crack) resulting from mode I loading, as well as two geometries named modified sample SCB-1 (containing vertical crack) and modified sample SCB-2 (containing vertical crack) resulting from mode II loading at temperatures of + 15 and −15 °C were evaluated. In order to control and reduce the cracking potential of Hot Mix Asphalt (HMA), the amorphous poly alpha olefin (APAO) additive was used in percentages of 3, 6, and 9 % and compared with the mixture without additives. Also, all specimens were under 0 and 3 FTD cycles. In order to measure and evaluate the fracture behavior results of unmodified and APAO-modified specimens, fracture toughness indicators (KIC and KIIC) and fracture energy (GF) were calculated. Eventually, to investigate the fracture behaviors of mixtures before the maximum loading, two indicators of tensile stiffness index (TSI) and tensile strength (TS) were evaluated. The results showed that at −15 °C and from the perspective of KIC (or KIIC) and GF (under modes I and II) indicators, the fracture resistance in specimens comprising 6 and 9 % APAO under 0 and 3 FTD cycles was higher compared to the base specimen. At + 15 °C and from the perspective of KIC (or KIIC) indicator, the fracture resistance of mixtures containing 6 and 9 % APAO in 0 and 3 FTD cycles was higher than the unmodified specimen; however, from the point of view of GF (under modes I and II) indicator, the mixture containing 9 % APAO represented greater fracture resistance. Also, it was indicated that mixtures modified with APAO represented higher TSI as well as TS values; therefore, APAO additive increased the resistance to elastic deformation by increasing the fracture performance before the maximum loading. Finally, mixtures containing 6 and 9 % APAO were chosen as the best mixture; however, the use of these mixtures in areas with temperatures lower than −15 °C should be done with caution due to the relative reduction in flexibility of the mixture containing APAO additive.

Restoring Force Model of Precast Segmental Reinforced Concrete Piers after Seawater Freeze–Thaw Cycles

Precast segmental reinforced concrete (RC) piers have been widely used in the construction of offshore bridges to speed up construction. Offshore bridges in cold regions are inevitably affected by the seawater freeze–thaw cycles under the periodic movement of tides, which could reduce the mechanical property of RC piers. Based on the low cyclic loading test on 12 specimens with different seawater freeze–thaw cycles, axial compression ratio, diameters of longitudinal reinforcement, and stirrup spacing, the hysteresis characteristics of precast segmental RC piers were analyzed. The test results show that the peak load decreased by 11%, while the peak displacement increased by 40% after 125 seawater freeze–thaw cycles. The hysteresis curves became fuller and the residual displacement became smaller with the accumulation of freeze–thaw damage. In the same 125 freeze–thaw cycles, the peak load increased by 15% and 27% while increasing the axial compression ratio and the longitudinal reinforcement diameter. Combined with the regression analysis of the experimental results, the restoring force model of RC piers considering the seawater freeze–thaw damage and design parameters was established, and the calculation method of each characteristic point in the model was given. The deviation values of flexural capacity are not more than 6.5%, and the deviation values of peak displacement are not more than 12%. The restoring force model determined in this paper could provide a reference for seismic response analysis of offshore bridges in cold regions.

Dynamic compressive behavior of freeze–thaw damaged roller-compacted concrete and establishment of constitutive model

Roller-compacted concrete (RCC) structures are frequently subjected to the combined effects of freeze–thaw (FT) damage and dynamic loads in cold and earthquake-prone areas, resulting in a decrease in the mechanical properties of RCC. In this paper, the mechanical properties of RCC under the combined action of FT damage and dynamic compression were studied through physical tests, and the dynamic compression constitutive model of freeze–thaw RCC (FTRCC) was established based on Weibull-Lognormal model. The results showed that 75 cycles were the threshold of FT damage of RCC, beyond which the relative dynamic elastic modulus (RDEM) decreases rapidly and the pores increases significantly. At the same strain rate, the dynamic compressive strength of RCC presents three different stages with the increasing of FT cycles: (I) slow decline (II) decline (III) rapid decline. The dynamic compressive strength of RCC increased with the increasing of strain rate under the same FT cycles. Meanwhile, the failure pattern of the specimen changed significantly. The variation law of mechanical properties of RCC under any FT cycles and strain rates can be quickly determined using the established dynamic constitutive model of FTRCC and the static compressive mechanical parameters of RCC under normal state. It can offer a theoretical support for assessing the dynamic mechanical behavior of RCC in cold and earthquake-prone areas.

Mechanical Performances and Frost Resistance of Alkali-Activated Coal Gangue Cementitious Materials

The coal gangue after composite activation treatment is considered a potential low-carbon and green cementitious material, so the feasibility of employing composite-activated gangue to partially or entirely replace cement for building materials is systematically studied in this paper. The effects of alkali content, slag content, and water-to-binder ratio on the mechanical properties and frost resistance of alkali-activated coal gangue mortar (AACGM) were experimentally investigated. An ESEM was employed to observe the microstructure of the AACGM. Moreover, the microstructure damage to the AACGM was analyzed by a pixel-based image processing algorithm. The research was carried out in accordance with standards JGJ/T70-2009 and GB/T 50082-2009. Experimental results indicated that the mechanical properties and frost resistance of the AACGM were superior to those of ordinary Portland cement mortar (OPCM). Compared with the OPC group, the compressive and flexural strength of the W0.50 group increased by 16.01% and 14.19%. Moreover, the loss rate of mass, flexural strength, compressive strength, and microstructure damage of the AACGM were less than those of the OPCM. Between freeze–thaw cycles 25 and 100, the cracks and pores of specimens gradually grew, and the maximum crack width increased by 277.78%. In addition, the slag was beneficial in improving the flexural strength, compressive strength, and frost resistance of the AACGM. Finally, the freeze–thaw damage mechanism of the CGBG mortar was systematically analyzed.

Initial freeze–thaw damage detection in concrete using two-dimensional non-contact ultrasonic sensors

Freezing and thawing cause concrete deterioration such as micro-cracking and surface scaling. An effective method for initial damage detection in concrete is required to prevent further deterioration of structures. In this study, initial freeze–thaw damage (small-size damage) was detected on a concrete surface using developed two-dimensional non-contact ultrasonic sensors based on a micro-electromechanical system (MEMS). The wavefield of ultrasonic waves passed through damaged concrete surfaces contain propagating and scattered waves, caused by refraction and reflection. To measure the wavefield, multi-channel non-contact ultrasonic sensors was developed including 128 MEMS array. Then, the amplitude of scattered waves was extracted in the frequency-wavenumber domain. The proposed method was experimentally validated through freeze–thaw cycle testing and compared to the conventional dynamic modulus reduction. The results demonstrated that the proposed method was at least 15 times more sensitive than the conventional method toward initial freeze–thaw damage on concrete surfaces.

Experimental Investigation of the Influence of Freeze–Thaw Mode on Damage Characteristics of Sandstone

The influence of rock weathering caused by freezing–thawing on stone cultural relics cannot be ignored. For immovable stone cultural relics, different parts under different environmental conditions will be under different freeze–thaw actions and suffer different degrees of damage. In this paper, three typical freeze–thaw cycle tests of sandstone are designed, namely immersion test, capillary action test, and periodic saturation test. The macroscopic and microscopic morphologies of rock samples under different freeze–thaw cycles were analyzed. Weathering indicators such as porosity, water content, wave velocity, and surface hardness were tested, as well as uniaxial compressive strength. The variation law of weathering index and uniaxial compressive strength under different freezing–thawing cycles was obtained, and the quantitative relationship between each index parameter was further analyzed. The results show that under different freezing–thawing conditions, the apparent morphology of rock samples is different, and the trend of weathering indexes is similar, but the rate of change is different. The water content of rock has a great influence on the test results of wave velocity but has little influence on the surface hardness. The function relationship between weathering index and compressive strength under different freezing–thawing modes is similar, but the fitting parameters are different. Finally, the strength and wave velocity damage factors were used to quantitatively evaluate the degree of rock weathering. The results show that the immersion freeze–thaw damage is the highest, the periodic saturated freeze–thaw damage is the second highest, and the capillary freeze–thaw damage is the least highest. This is consistent with the field observation results. The conclusion of this paper can provide reference for the detection of stone cultural relics and provides a scientific basis for the anti-weathering protection of stone cultural relics.

Grey Correlation Analysis between Macro Mechanical Damage and Meso Volume Characteristics of SBS Modified Asphalt Mixture under Freeze-Thaw Cycles

The effect of freeze–thaw (F–T) in the seasonal frozen area would lead to damage to asphalt pavement. After water enters asphalt pavement, the water in voids would expand at a lower temperature, which could change the void content and number, affecting the macro mechanical properties of the asphalt mixture. The rapid development of CT scanning and digital image processing (DIP) provides powerful technical support for the research of asphalt mixture meso volume characteristics. In this paper, the mechanical properties of basalt fiber reinforced asphalt mixture subjected to F–T cycles were tested at different temperatures to clarify the decay law of mechanical properties under F–T cycles. Then, the meso images of the asphalt mixture under various F–T cycles could be obtained by using CT tomography. Based on DIP technology, the meso characteristic parameters of CT images for asphalt mixture were extracted, and the development of asphalt mixture freeze–thaw damage was further analyzed. The test results showed that with the F–T cycle, the macro mechanical properties of the asphalt mixture rapidly declined in the early stage of the F–T cycle and gradually tended to be flat. There would be serious damage inside the asphalt mixture in the late stage of the F–T cycle. The damage to the mechanical properties of the asphalt mixture under the F–T cycle can be attributed to the change in the internal mesostructure of the asphalt mixture. Based on the grey relational analysis theory, the formation of the connected void was the main factor affecting the damage in the early stage of the F–T cycle, while the formation of new voids mainly affected the later development of F-T damage.

Impact of Rice Husk Ash on the Mechanical Characteristics and Freeze–Thaw Resistance of Recycled Aggregate Concrete

With the accelerating growth of infrastructure construction, carbon emission and environmental pollution problems have become increasingly severe. In order to promote the sustainable development of the construction industry, using rice husk ash (RHA) in recycled aggregate concrete has aroused extensive interest. This study aims to investigate the impact of the partial replacement (0%, 10%, 20%, 30% of binder) of ordinary Portland cement (OPC) with RHA by equal mass on recycled concrete’s mechanical characteristics and freeze–thaw resistance. The workability, compressive strength, mass loss and dynamic elastic modulus of recycled concrete were tested, and the hydration products and microstructure were analyzed using scanning electron microscope (SEM) tests. The mechanism of the freeze–thaw damage deterioration of RHA recycled aggregate concrete was revealed. The results indicate that the incorporation of RHA has an adverse effect on the workability of fresh concrete. Its high specific surface area will provide a large number of nucleation sites for the hydration reaction, refining the pore structure in the paste and improving the weak bonding of the interfacial transition zone (ITZ) by enhancing the matrix’s pozzolanic reaction effect and filling effect, thus improving the compressive strength of concrete. Furthermore, the porous structure of the recycled aggregate attached mortar and mesoporous RHA will absorb a lot of water during the freeze–thaw cycles. With the continuous accumulation of expansion pressure, the interior pores and cracks will gradually expand and extend, leading to more severe damage to the concrete, and the degree of freeze–thaw damage deterioration grows as the RHA replacement ratios increase.

Experimental Study on the Stability and Distribution of Air Voids in Fresh Fly Ash Concrete.

The air void system purposely introduced by an air-entraining admixture (AEA) is of great significance for the protection of concrete from freeze-thaw damage. Fly ash has been globally used in concrete, while the unburnt carbon in fly ash can adsorb AEA molecules and, thus, increase the AEA demand. Previous studies primarily focused on the air content of fresh fly ash concrete. This paper aimed to explore the stability and distribution of air voids in fly ash concrete at the fresh state. To achieve this goal, eleven different fresh fly ash concrete mixtures with an initial air content of 6 ± 1% were prepared in the laboratory. Samples were taken at various times within 75 min after initial mixing to investigate the air content and air void distribution in fly ash concrete at the fresh state using a super air meter (SAM). The results indicated that there was no significant correlation between loss on ignition (LOI) of fly ash and AEA demand to achieve the initial air content of 6 ± 1%. Class C fly ash concrete tended to have a better air content retention than Class F fly ash concrete. Compared with LOI, AEA demand had a stronger correlation with air content retention. Most of the fly ash concrete mixtures had a satisfactory air void system immediately after mixing, but the SAM number showed an increasing trend over time, suggesting the coarsening of the air void system with time.