| In cold regions,concrete materials are damaged by repeated freeze-thaw cycles for a long time,which seriously affects their long-term service performance and service life.However,the mechanism of freeze-thaw failure of concrete has not been completely revealed for many years.However,various hypotheses and theories put forward by scholars have shown that the failure of freeze-thaw cycle is caused by changes in the internal temperature of concrete materials,and the more severe the temperature changes,the more positive and negative temperature alternating times,and the more obvious the destructive effect.Therefore,if the internal temperature of concrete can be controlled within a certain range or the temperature change amplitude can be reduced,the impact of freeze-thaw cycles can be reduced or eliminated.The phase of the phase change material changes under the induction of the external temperature,and a large amount of latent heat can be stored or released in the process,and the temperature of the material itself remains almost unchanged until the phase change is completed.The addition of phase change materials to concrete can achieve its temperature control.To this end,a series of indoor experimental studies,outdoor model experimental studies and numerical simulation analysis are carried out,and the main research contents and conclusions obtained are as follows:(1)Phase change material(PCM)is often mixed into concrete as artificial aggregate.Developing artificial phase change aggregates(APCA)with excellent freeze-thaw resistance is a key for the high-performance phase change concrete.In this paper,the APCA with controllable particle size,large apparent density,high strength,and well frost resistance was obtained by using a disc granulator.The microencapsulated phase change material(MPCM)was completely dispersed in the aggregates.The cement shell further protects the MPCM from leakage.The enthalpy,specific heat,microstructure,and product composition of APCA were measured and analyzed by differential scanning calorimeter,scanning electron microscope(SEM),and X-ray diffraction(XRD).The effects of the amount of MPCM on the strength and freeze-thaw resistance of the APCA were analyzed through single particle compressive strength test and freeze-thaw cycle test.The results indicate that the artificial aggregates with more MPCM have higher enthalpy.The MPCM can absorb the cement hydration heat,reduce the cement hydration rate,and then slow down the development of early strength of the artificial aggregates.With the increasing content of the MPCM,the apparent density and the strength of the APCA decrease,and the water absorption increases.At the same time,a large number of MPCM are added to increase the pore number of the APCA,which promotes the carbonation to develop inside the aggregates and generate a large number of calcite crystals,reducing the strength reduction caused by the increase of the pores.When the content of the MPCM is 10–30% of the mass of cement,the APCA have high freeze-thaw resistance,which is helpful to improve the freeze-thaw resistance of the phase change concrete.(2)Replace coarse aggregate crushed stone with artificial phase change aggregate,and the replacement amounts are 0,20%,50% and 80% of the mass of coarse aggregate,respectively,and prepare phase change concrete specimens.The water absorption characteristics,mechanical properties,frost resistance and microstructure characteristics of phase change concrete were analyzed by micro and macro tests.The results show that the porous characteristics and water absorption characteristics of concrete are changed by artificial phase change aggregate,and the saturated water absorption rate of concrete gradually decreases with the increase of artificial phase change aggregate replacement.The amount of different artificial phase change aggregates did not change the main phases in concrete,and the types of phases did not change with age.The larger the amount of artificial phase change aggregate replacement,the greater the reduction of the compressive strength of concrete,but when the replacement amount is 20% and 50%,the compressive strength of concrete in 28 days is 35.81 MPa and 30.65 MPa,respectively,which still meets the strength requirements of C30 concrete.The compressive strength of phase change concrete increased with age.The splitting strength of phase change concrete increased rapidly in the early stage and relatively slowly in the later stage,and the strength values of 28 days could reach more than 2.88 MPa.Artificial phase change aggregates help to improve the frost resistance of concrete,and as the number of freeze-thaw cycles increases,the strength of phase change concrete decreases.L20-BF0 has good frost resistance,and its strength is still 96% of the strength of 28 d after 100 freeze-thaw cycles.The CT test results show that artificial phase change aggregates are beneficial in inhibiting pore and crack propagation under the action of freeze-thaw concrete.The original pores and cracks of ordinary concrete specimen L0-BF0 gradually expand,the pore structure becomes loose,the surface of the specimen is damaged and falling off,and L80-BF0 is less affected by freeze-thaw damage.(3)By adding basalt fibers to improve the tensile performance of phase change concrete,the frost resistance of phase change concrete is further improved.The content of basalt fiber was 0.1%,0.2% and 2% of the mass of concrete,and the replacement amount of artificial phase change aggregate in phase change concrete was 0,20%,50% and 80% of the mass of the required coarse aggregate,respectively,and seven mix ratios containing basalt fiber were prepared for phase change concrete specimens,and four mix ratio groups without basalt fiber were used as control groups.The effects of basalt fibers on the physical and mechanical properties,microstructure and frost resistance of phase change concrete were studied.The results show that the basalt fiber surface is smooth,the appearance is relatively complete,and there is no sign of corrosion by electron microscopy.In concrete with high content of basalt fibers,basalt fibers clump together to form a weak zone of concrete strength.Basalt fiber will slightly reduce or do not change the compressive strength of phase change concrete,can improve its splitting tensile strength,whether in ordinary concrete or phase change concrete,mixed with a reasonable number of basalt fiber can improve its frost resistance,of which 0.2% basalt fiber content is more reasonable,after 240 freeze-thaw cycles,its strength is 73%,75% and 73% of the strength of 28 days,respectively.(4)Considering the actual temperature characteristics of cold areas,a composite structure concrete pier containing phase change concrete was designed,including phase change concrete cladding(MPCMC)material,phase change microcapsule layer(MPCM)material,thermal insulation layer(Insulation)material and structural concrete pier(SC)material from outside to inside,and two reference concrete piers were designed at the same time.Through the physical model test,the following conclusions are obtained: the ability of each composite structure to adjust temperature is different,and the effect is different.The phase change concrete cladding played a role in smoothing the oscillation of the external ambient temperature,and the daily maximum temperature of the layer decreased,the minimum temperature temperature increased,and the daily maximum and minimum temperature changes decreased.The phase change microcapsule layer plays a crucial role in reducing the amplitude of the temperature change of the pier,and the daily average temperature of the phase change microcapsule layer increases significantly,and the time of the occurrence of the minimum temperature and the maximum temperature is significantly lagged.The insulation slightly reduces the amplitude of peak temperature variation,resulting in a significant lag between the maximum and minimum temperatures of the day.(5)Concrete structures in cold regions are often damaged by freeze-thaw action,which can seriously reduce their service life.How to actively reduce the freeze-thaw cycles suffered by the structures is key to enhance their freeze-thaw durability.In this paper,a composite pier protective technique with microencapsulated phase change material concrete(MPCMC)was proposed,including the MPCMC layer,the pure microencapsulated phase change material(MPCM)layer,the insulation layer,and the structure concrete(SC).To analyze the thermal control performance of the composite MPCMC pier,a 3-D heat transfer model with phase change were established.Combing the laboratory and field tests,a series of numerical simulations were conducted.The results indicated that the key parameters,including the thickness,phase change range,and thermal conductivity of the MPCMC layer and MPCM layer,and the thickness and thermal conductivity of the insulation layer,can determine the thermal control performance of the composite pier.An optimal composite structure was obtained considering the engineering cost.The optimal structure can effectively reduce the freeze-thaw cycles and extend its service life.Compared to the ordinary concrete pier,the maximum degree of reduction for freeze-thaw cycles can reach94% and the maximum extension for service life can be 1470% at the freezing point of 0°C.The study can provide some theoretical basis for the design of the freeze-thaw resistance of the concrete structure in cold regions and related researches. |
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