Repair of Second-Generation Recycled Fine Aggregate of Waste Concrete from Freeze-Thaw Environment by Carbonation Treatment

The reuse of waste recycled concrete from harsh environments has become a research hotspot in the field of construction.This study investigated the repair effect of carbonation treatment on second-generation recycled fine aggregate(SRFA)obtained from recycled fine aggregate concrete(RFAC)subjected to freeze-thaw(FT)cycles.Before and after carbonation,the properties of SRFA were evaluated.Carbonated second-generation recycled fine aggregate(CSRFA)at five substitution rates(0%,25%,50%,75%,100%)to replace SRFA was used to prepare carbonated second-generation recycled fine aggregate concrete(CSRFAC).The water absorption,porosity and mechanical properties of CSRFAC were tested,and its frost-resisting durability was evaluated.The results showed after carbonation treatment,the physical properties of SRFA was improved and met the requirements of II aggregate.The micro-hardness of the interfacial transition zone and attached mortar in CSRFA was 50.5%and 31.2%higher than that in SRFA,respectively.With the increase of CSRFA replacement rate,the water absorption and porosity of CSRFAC gradually decreased,and the mechanical properties and frost resistance of CSRFAC were gradually improved.Carbonation treatment effectively repairs the damage of SRFA caused by FT cycles and improves its application potential.

Coupling Effect of Cryogenic Freeze-Thaw Cycles and Chloride Ion Erosion Effect in Pre-Cracked Reinforced Concrete

Chloride (Cl−) ion erosion effects can seriously impact the safety and service life of marine liquefied natural gas(LNG) storage tanks and other polar offshore structures. This study investigates the impact of different low-temperaturecycles (20°C, –80°C, and −160°C) and concrete specimen crack widths (0, 0.3, and 0.6 mm) on the Cl−ion diffusion performance through rapid erosion tests conducted on pre-cracked concrete. The results show thatthe minimum temperature and crack width of freeze-thaw cycles enhance the erosive effect of chloride ions. TheCl− ion concentration and growth rate increased with the increasing crack width. Based on the experimental modeland in accordance with Fick’s second law of diffusion, the Cl− ion diffusion equation was modified by introducingcorrection factors in consideration of the freeze-thaw temperature, crack width, and their coupling effect.The experimental and fitting results obtained from this model can provide excellent reference for practical engineeringapplications.

Failure Characteristics of Rock-like Mortar Specimens with Arc-shaped Flaws under Freezing-thaw Cycles and Uniaxial Compression

To investigate the freeze-thaw(F-T)damages and failure characteristics of rock mass with arc-shaped joints in cold regions,three types of cement mortar specimens with different central angles and prefabricated arc-shaped flaws are subjected to uniaxial compressive tests under different F-T cycles.Experimental observations show that the uniaxial compressive strength of specimens are significantly influenced by F-T cycles and their failure modes are mainly affected by the central angleαof the prefabricated flaws.Unlike the specimens with a central angle of 60°,the specimens with a central angle of 120°and 180°have greater curvature of flaws,so tensile cracks occur in the arc-top area of their prefabricated flaws.According to experimental images observed by environmental scanning electron microscope(ESEM),as the number of F-T cycles increases,the deterioration effect of the specimen becomes more obvious,which is specifically reflected in the increase of the mass loss,peak stress loss,and damage variables as a power function,and the peak strain decreases as a quadratic polynomial.According to numerical results using two-dimensional particle flow code(PFC2D),it is found that F-T cycles cause more damage to the specimen in the early stages than in the later ones.The area of the concentrated compressive stress zone in the middle is decreased due to the increased number of F-T cycles,while the area of the surrounding tensile-shear stress zone is increased.The models appear different failure modes due to the release of concentrated stress in different tensile-shear zones.

Dynamic mechanical characteristics of frozen subgrade soil subjected to freeze-thaw cycles

As a widely-applied engineering material in cold regions, the frozen subgrade soils are usually subjected to seismic loading, which are also dramatically influenced by the freeze-thaw(F-T)cycles due to the varying temperature. A series of dynamic cyclic triaxial experiments were conducted through a cryogenic triaxial apparatus for exploring the influences of F-T cycles on the dynamic mechanical properties of frozen subgrade clay.According to the experimental results of frozen clay at the temperature of-10℃, the dynamic responses and microstructure variation at different times of F-T cycles(0, 1, 5, and 20 cycles) were explored in detail.It is experimentally demonstrated that the dynamic stress-strain curves and dynamic volumetric strain curves of frozen clay are significantly sparse after 20F-T cycles. Meanwhile, the cyclic number at failure(Nf) of the frozen specimen reduces by 89% after 20freeze-thaw cycles at a low ratio of the dynamic stress amplitude. In addition, with the increasing F-T cycles,the axial accumulative strain, residual deformation,and the value of damage variable of frozen clay increase, while the dynamic resilient modulus and dynamic strength decrease. Finally, the influence of the F-T cycles on the failure mechanisms of frozen clay was discussed in terms of the microstructure variation. These studies contribute to a better understanding of the fundamental changes in the dynamic mechanical of frozen soils exposed to F-T cycles in cold and seismic regions.

Mechanical and acoustic emission characteristics of anhydrite rock under freeze-thaw cycles

To study the damage mechanisms of anhydrite rock under freeze-thaw cycles, the physicalmechanical properties and the microcracking activities of anhydrite rock were investigated through mass variation, nuclear magnetic resonance, scanning electron microscope tests, and uniaxial compression combined with acoustic emission(AE) tests. Results show that with the increase of freeze-thaw processes,the mass, uniaxial compression strength, and elastic modulus of the anhydrite specimens decrease while the porosity and plasticity characteristics increase.For example, after 120 cycles, the uniaxial compression strength and elastic modulus decrease by 46.54% and 60.16%, and the porosity increase by 75%. Combined with the evolution trend of stressstrain curves and the detected events, three stages were labeled to investigate the AE characteristics in freeze-thaw weathered anhydrite rock. It is found that with the increase of freeze-thaw cycles, the proportions of AE counts in stage Ⅰ and stage Ⅱ show a decaying exponential trend. Contrarily, the proportion of AE counts in stage Ⅲ displays an exponential ascending trend. Meanwhile, as the freeze-thaw cycles increase, the low-frequency AE signals increase while the intermediate-frequency AE signals decrease. After 120 cycles, the proportion of low-frequency AE signals increases by 168.95%, and the proportion of intermediate-frequency AE signals reduces by 81.14%. It is concluded that the microtensile cracking events occupy a dominant position during the loading process. With the increase of freeze-thaw cycles, the b value of samples decreases.After 120 cycles, b value decreases by 27.2%, which means that the proportion of cracking events in rocks with small amplitude decreases. Finally, it is proposed that the freeze-thaw damage mechanism of anhydrite is also characterized by the water chemical softening effect.

Dynamic behaviors of water-saturated and frozen sandstone subjected to freeze-thaw cycles

In high-altitude cold areas,freeze-thaw(F-T)cycles induced by day-night and seasonal temperature changes cause numerous rock mass slope engineering disasters.To investigate the dynamic properties of rock in the natural environment of a high-altitude cold area,standard specimens were drilled from the slope of the Jiama copper mine in Tibet,and dynamic compression tests were performed on watersaturated and frozen sandstone with different numbers of F-T cycles(0,10,20,30,and 40)by the split Hopkinson pressure bar(SHPB)system with a cryogenic control system.The influence of water-saturated and frozen conditions on the dynamic performance of sandstone was investigated.The following conclusions are drawn:(1)With increasing strain rate,the attenuation factor(la)of water-saturated sandstone and the intensifying factor(li)of frozen sandstone linearly increase.As the number of F-T cycles increases,the dependence factor(ld)of water-saturated sandstone linearly decreases,whereas the ld of frozen sandstone linearly increases.(2)The prediction equation of the dynamic compressive strength of water-saturated and frozen sandstone is obtained,which can be used to predict the dynamic compressive strength of sandstone after various F-T cycles based on the strain rate.(3)The mesoscopic mechanism of water-saturated and frozen sandstone’s dynamic compressive strength evolution is investigated.The water softening effect causes the dynamic compressive strength of water-saturated sandstone to decrease,whereas the strengthening effect of pore ice causes it to increase.(4)The decrease in the relative dynamic compressive strength of water-saturated sandstone and the increase in the relative dynamic compressive strength of frozen sandstone can be attributed to the increased porosity.

Effect of PVA Fiber on the Dynamic and Static Mechanical Properties of Concrete under Freeze-thaw Cycles at Extremely Low Temperature(-70℃)

In order to study the effect of PVA fiber on the dynamic and static mechanical properties of low-temperature freeze-thaw concrete under the saturated surface dry state,different contents of PVA fiber were added to prepare concrete in this experiment.The concrete was subjected to compression,flexural and SHPB impact tests combined with scanning electron microscopy for microstructure analysis,after different times of freeze-thaw cycles in the temperature range of 20-70℃.The experimental results show that the compressive strength of the PVA fiber reinforced concrete first increases and then decreases after freeze and thaw cycles,and the compressive strength is positively correlated with the fiber content.The flexural strength gradually decreases with freeze-thaw cycles.The flexural strength of the concrete with 1.2 kg/m^(3) of PVA fiber presents the lowest strength loss after 45 freeze and thaw cycles,which is about 14%.The dynamic failure strength gradually decreases with the increase of freeze-thaw times,and the reduction amplitude decreases with the increase of PVA fiber content.The best impact resistance is achieved when the PVA fiber dosage is 1.2 kg/m^(3).

Effect of High Temperature Curing on the Frost Resistance of Recycled Aggregate Concrete and the Physical Properties of Second-Generation Recycled Coarse Aggregate under Freeze-Thaw Cycles

With the emphasis on environmental issues,the recycling of waste concrete,even recycled concrete,has become a hot spot in the field of architecture.But the repeated recycling of waste concrete used in harsh environments is still a complex problem.This paper discusses the durability and recyclability of recycled aggregate concrete(RAC)as a prefabricated material in the harsh environment,the effect of high-temperature curing(60℃,80℃,and 100℃)on the frost resistance of RAC and physical properties of the second generation recycled coarse aggregate(RCA_(2))of RAC after 300 freeze-thaw cycles were studied.The frost resistance of RAC was characterized by compressive strength,relative dynamic elastic modulus,and mass loss.As the physical properties of RCA_(2),the apparent density,water absorption,and crushing value were measured.And the SEM images of RAC after 300 freeze-thaw cycles were shown.The results indicated that the frost resistance of RAC cured at 80℃ for 7 days was comparable to that cured in the standard condition(cured for 28 days at 20℃±2℃ and 95%humidity),and the RAC cured at 100℃ was slightly worse.However,the frost resistance of RAC cured at 60℃ deteriorated seriously.The RAC cured at 80℃ for 7 days is the best.Whether after the freeze-thaw cycle or not,the RCA that curd at 60℃,80℃,and 100℃ for 7 days can also meet the requirements of Grade III RCA and be used as the aggregate of non-bearing part of prefabricated concrete components.RCA_(2) which is cured at 80℃ for 7 days had the best physical properties.

The Effect of Different Freeze-Thaw Cycles on Mortar Gas Permeability and Pore Structure

Two different freeze-thaw cycles(FTC)are considered in this study to assess the related impact on gas permeability and micro-pore structure of a mortar.These are the water-freezing/water-thawing(WF-WT)and the air-freezing/air-thawing(AF-AT)cycles.The problem is addressed experimentally through an advanced nuclear magnetic resonance(NMR)technique able to provide meaningful information on the relationships among gas permeability,pore structure,mechanical properties,and the number of cycles.It is shown that the mortar gas permeability increases with the number of FTCs,the increase factor being 20 and 12.83 after 40 cycles for the WF-WT and AF-AT,respectively.The results also confirm that gas permeability hysteresis phenomena occur during the confining pressure loading and unloading process.

Influence of relative compaction and degree of saturation on the deformation characteristics of bentonite under freeze-thaw cycles

Bentonite,consisting of clay minerals of the montmorillonite group,has been widely used as an adsorbent and backfill material in nuclear waste disposal and groundwater remediation.It is challenging to use bentonite as a filling material in cold regions since bentonite is highly sensitive to thermal environmental changes,during which its bulk volume and microstructure change significantly.In this study,a series of one-dimensional and three-dimensional freeze-thaw tests were carried out within a closed system to investigate the influencing factors of the deformation of bentonite under freeze-thaw cycles.Results show that the initial soil water content greatly impacts bentonite's deformation during freeze-thaw cycles.For an initial higher degree of saturation(Sr),the expansion caused by the formation of ice lenses has a greater impact than the shrinkage induced by dehydration,ice-cementation,and so on.Conversely,bentonite tends to shrink at a lower degree of saturation during freezing.And the critical degree of saturation that determines bentonite's behavior of frost heave or frost shrinkage seems to be roughly 0.8.As the number of freeze-thaw cycles rises,initially uncompacted bentonite clay becomes more compacted,and initially compacted bentonite clay remains unchanged.

Effect of Freeze-Thaw Cycles on Chloride Transportation in Concrete: Prediction Model and Experiment

This research aims to investigate the effect of frost damage on chloride transportation mechanism in ordinary andfiber concrete with both theoretical and experimental methods.The proposed theoretical model takes into account the varying damage levels caused by concrete cover depth and freeze-thaw cycles,which are the two primary parameters affecting the expression of the chloride diffusion coefficient.In the experiment,three types of concrete were prepared:ordinary Portland concrete(OPC),polypropylenefiber concrete(PFC),and steelfiber concrete(SFC).These were then immersed in NaCl solution for 120 days after undergoing 10,25,and 50 freeze-thaw cycles.The damage coefficient of the tested concrete was determined by measuring the dynamic elas-tic modulus.The results indicated that the relative dynamic elasticity modulus of the specimens decreased with each freeze-thaw cycle,and the chloride diffusion coefficient of the specimens increased as the degree of frost degradation increased.Samples containing steel and polypropylenefibers exhibited greater resistance to cyclic water freezing compared to the controlled concrete withoutfibers.A model has been also developed that takes into account the damage caused by freezing-thawing cycles and the depth of the concrete,which can predict variations in free chloride concentration at different depths.The calculated values were in good agreement with the test results for depths between 10 to 30 mm.This new damage-induced diffusion model can helpfill the gap in research on the effects of freeze-thaw cycles on chloride diffusion.

NMR research on deterioration characteristics of microscopic structure of sandstones in freeze-thaw cycles

In order to study the deterioration characteristics of the microscopic structure of sandstones in freeze-thaw cycles, tests of180 freeze-thaw cycles were performed on sandstone specimens. The nuclear magnetic resonance （NMR） technique was applied tothe measurement of sandstone specimens and analysis of the magnetic resonance imaging. Then, the fractal theory was employed tocompute the fractal dimension values of pore development of rocks after different freeze-thaw cycles. The results show that the massand porosity of rocks grow with the increase of freeze-thaw cycles. According to the NMR T2 distribution of sandstones, the poresizes of rock specimens increase after 180 freeze-thaw cycles, especially that of the medium-sized and small-sized pores. The spatialdistribution of sandstone pores after freeze-thaw cycles has fractal features within certain range, and the fractal dimension ofsandstones tends to increase gradually.

Effects of freeze-thaw cycles on fracture behavior of epoxy asphalt concrete

According to the winter temperature of Peking,the freeze-thaw（FT） condition in laboratory was determined.Seven groups of epoxy asphalt concrete（EAC） specimen were exposed to different FT cycles.The flexural modulus and fracture energy（G_F） of EAC exposed to different FT cycles were obtained through the 3-point bending test.Meanwhile,the plane strain fracture toughness（K_（IC）） of EAC was obtained through numerical simulation.The results show that the flexural modulus of the FT conditioned EAC samples decreases with the increase of FT cycles.The FT damage of flexural modulus is 60%after 30 FT cycles.Nevertheless,with the increase of FT cycles,the G_F and K_（IC） of EAC decrease first and then increase after 15 FT cycles.

Ultimate load bearing capacity evaluation of concrete beams subjected to freeze-thaw cycles

A theoretical prediction method based on the change of concrete material is proposed to evaluate the ultimate bending moment of concrete beams which have undergone freeze-thaw cycles （PTCs）. First, the freeze-thaw damage on concrete material is analyzed and the residual compressive strength is chosen to indicate the freeze-thaw damage. Then, the equivalent block method is employed to simplify the compressive stress-strain curve of the freeze-thaw damaged concrete and the mathematical expression for the ultimate bending moment is obtained. Comparisons of the predicted results with the test data indicate that the ultimate bending moment of concrete beams affected by FTC attack can be predicted by this proposed method. However, the bond-slip behavior and the randomness of freeze-thaw damage will affect the accuracy of the predicted results, especially when the residual compressive strength is less than 50%.

Determination of Optimum Conditions for Cell-wall Destruction of Auricularia auricula Mycelia under the Synergistic Effect of Ultrasonic Waves and Repeated Freeze-thaw Cycles

We aimed to investigate the synergistic effects of ultrasonic waves and repeated freeze-thaw cycles on cel-wal destruction of Auricularia auric-ula mycelia, and determine the best combination of conditions for cel-wal destruc-tion of Auricularia auricula mycelia. [Method] The effects of destruction time, added water, destruction times, freeze time and number of freeze-thaw cycles during ultra-sonic treatment on polysaccharide yield were investigated by single-factor test in our research. The optimum conditions for cel-wal destruction of Auricularia auricula mycelia by the synergistic effect of ultrasonic waves and repeated freeze-thaw cycles were ascertained by orthogonal test. [Result] The results of single test indicated the best combination of conditions was as fol ows： freeze time, 30 min; destruction time, 20 min; added water, 15 times; destruction times, 2 times; number of freeze-thaw cycles, 3 cycles. The results of orthogonal test indicated the influencing factors ranked as destruction time 〉 destruction times 〉 freeze time. The best combination of conditions was as fol ows： freeze time, 30 min; destruction time, 25 min; destruc-tion times, 2 times. Under above conditions, the polysaccharide yield reached 57.76 mg/g. [Conclusion] This research would provide a basis and reference for practical production.

Frost heaving force considering synchronous damage to tunnel lining and surrounding rocks under freeze—thaw cycles

In areas with seasonal freezing,when the tunnel lining concrete is saturated with water infiltrating the interior,the lining and the surrounding rocks will simultaneously freeze.However,the current calculation of the frost heaving force fails to consider the synchronous damage to the lining and surrounding rocks under freeze-thaw cycles.Therefore,as per the elastic calculation model of the frost heaving force and model of steady-state heat transfer of circular tunnels,this study introduces the frost heaving rate of lining and surrounding rocks.First,the analytical solution of frost heaving force is obtained for simultaneous frost heaving of lining and surrounding rocks under any steady-state temperature field.Then,based on the fracture theory and meso-damage mechanics,the damage variables of lining and surrounding rocks under freeze-thaw cycles are extracted,representing their elastic modulus and porosity.Finally,the formula of frost heaving force for synchronous damage to the lining and surrounding rocks at any steady-state temperature field is obtained.The calculation results demonstrate that the lower the temperature inside the lining,the greater the frost heaving force.With the increasing number of freeze-thaw cycles,frost heaving force tends to gradually increase initially,reaching a peak value at 85 freeze-thaw cycles,decreasing to 80%of the peak value at 140 cycles before reaching a constant value.The lining participates in frost heaving,increasing the frost heaving force.The initial increase rate of frost heaving force is 15.7%.Changing the fitting coefficients s1 and s2 of the lining and surrounding rocks can effectively control the magnitude of the frost heaving force in the tunnels.

Effect of Freeze-Thaw Cycles on Mechanical Properties and Permeability of Red Sandstone under Triaxial Compression

Geological disasters will happen in cold regions because of the effects of freeze-thaw cycles on rocks or soils, so studying the effects of these cycles on the mechanical characteristics and permeability properties of rocks is very important. In this study, red sandstone samples were frozen and thawed with o, 4, 8 and 12 cycles, each cycle including 12 h of freezing and 12 h of thawing. The P-wave velocities of these samples were measured, and the mechanical properties and evolution of the steady-state permeabilities were investigated in a series of uniaxial and triaxial compression tests. Experimental results show that, with the increasing of cyclic freeze-thaw times, the P-wave velocity of the red sandstone decreases. The number of freeze-thaw cycles has a significant influence on the uniaxial compressive strength, elastic modulus, cohesion, and angle of internal friction. The evolution of permeability of the rock samples after cycles of freeze-thaw in a complete stress-strain process under triaxial compression is closely related to the variation of the microstructure in the rock. There is a highly corresponding relationship between volumetric strain and permeability with axial strain in all stages of the stress-strain behaviour.

Effects of freeze-thaw cycle on engineering properties of loess used as road fills in seasonally frozen ground regions,North China

Compacted loess is widely used as fills of road embankments in loess regions of northern China.Generally, densely-compacted loess can satisfy the requirements of embankment strength and postconstruction deformation. However, uneven subsidence, pavement cracks and other related damages can affect the integrity of loess subgrade after several years of operation,and even cause some hazards, especially in North China, where the strong freeze-thaw erosion occurs. In this study, cyclic freeze-thaw tests for both densely and loosely compacted loess samples were performed to determine the variation in engineering properties such as volume, void ratio, collapsible settlement,microstructure, and the related mechanisms were addressed. The experimental results showed that an obvious water migration and redistribution occurred within the samples during freeze-thaw cycles. Ice lenses and fissures could be identified in the upper frozen layers of the samples. After freeze-thaw cycles,the dry densities of the upper layers of samples changed significantly due to strong freeze-thaw erosion. The dry densities decreased for the dense sample and increased for the loose sample. It can be found that dense samples become loose, while loose samples became dense with the increasing number of freeze-thaw cycles. Their related void ratios changed reversely. Both void ratios tended to fall into a certain range, which verified the concept of a residual void ratio proposed by Viklander. The loosening process of densely compacted samples involves the formation of large pores, volume increase and density reduction as well as the related changes in mechanical properties because freeze-thaw cycles may be important contribution to problems of loess road embankments.Adverse effects of freeze-thaw cycles, therefore,should be taken into account in selecting loess parameters for the stability evaluation of road embankment in seasonally frozen ground regions.

Damage mechanism of soil-rock mixture after freeze-thaw cycles

As a widely distributed geological and engineering material,the soil-rock mixture always undergoes frequentative and short-term freeze-thaw cycles in some regions.Its internal structure is destroyed seriously,but the damage mechanism is not clear.Based on the damage factor,the damage research of properties of soil-rock mixture after different times of freeze-thaw cycles is investigated.Firstly,the size-distributed subgrade gravelly soil samples are prepared and undergo different times of freeze-thaw cycles periodically(0,3,6,10),and indoor large-scale triaxial tests are completed.Secondly,the degradation degree of elastic modulus is considered as a damage factor,and applied to macro damage analysis of soil-rock mixture.Finally,the mesoscopic simulation of the experiments is achieved by PFC3D,and the influence on strength between soil-rock particles caused by freeze-thaw cycles is analyzed.The results show that freeze-thaw cycles cause internal damage of samples by weakening the strength between mesoscopic soil-rock particles,and ultimately affect the macro properties.After freeze-thaw cycles,on the macro-scale,elastic modulus and shear strength of soil-rock mixture both decrease,and the decreasing degree is related to the times of cycles with the mathmatical quadratic form;on the meso-scale,freeze-thaw cycles mainly cause the degradation of the strength between soil-rock particles whose properties are different significantly.

Experimental studies on the pore structure and mechanical properties of anhydrite rock under freeze-thaw cycles

To study the deterioration mechanisms of anhydrite rock under the freeze-thaw weathering process,the physico-mechanical characteristics and microstructure evolutions of anhydrite samples were determined by a series of laboratory tests.Then,a descriptive-behavioral model was used to measure the integrity loss in anhydrite samples caused by cyclic freeze-thaw.Finally,the freeze-thaw damage mechanisms of anhydrite rock were revealed from the macro and micro aspects.The results show that the pore size of the anhydrite rock is mainly concentrated in the range of 0.001-10μm.As the number of freeze-thaw cycles increases,there is a growth in the proportion of macropores and mesopores.However,the proportion of micropores shows a declining trend.The relations of the uniaxial compressive strength,triaxial compressive strength,cohesion,and elastic modulus versus freeze-thaw cycles can be fitted by a decreasing exponential function,while the internal friction angle is basically unchanged with freezethaw cycles.With the increase of confining pressure,the disintegration rates of the compressive strength and the elastic modulus decrease,and the corresponding half-life values increase,which reveals that the increase of confining pressures could inhibit freeze-thaw damage to rocks.Moreover,it has been proven that the water chemical softening mechanism plays an essential role in the freeze-thaw damage to anhydrite rock.Furtherly,it is concluded that the freeze-thaw weathering process significantly influences the macroscopic and microscopic damages of anhydrite rock.