Study On Salt Freezing Durability Of Acrylic Fiber Concrete In Cold Regions

In the cold western regions represented by the Qinghai-Tibet Plateau,concrete structures are subject to harsh weather conditions and strong corrosive media such as high concentrations of chloride and sulfate all year round.The understanding of structure and material durability damage and deterioration induced by special working environment is a weak aspect of the current research on concrete in such areas.Based on a project in Tibet,this paper takes acrylic fiber concrete with different fiber contents as the main research object,and carries out the basic mechanical test,double salt erosion freeze-thaw test and nuclear magnetic resonance pore structure test of hardened acrylic fiber concrete(PAN/PC),etc.In the experimental work,combined with the grey entropy correlation analysis theory,the influence of pore structure characteristics on the deterioration law of macroscopic performance was analyzed in detail.The relevant results of this paper can provide a reference for the re search on the durability of concrete(PC)/acrylic fiber reinforced concrete(PAN/PC)engineering under the salt-freezing environment in the cold regions of the west,and also provide suggestions for the application and promotion of acrylic fiber reinforced concrete in cold regions.The main research contents and results are as follows:main research contents and results are as follows:(1)The effect of PAN content on the working performance and mechanical properties of concrete was studied.The results show that increasing PAN can reduce the slump of concrete,but too much will cause agglomeration;with the increase of PAN content,the air content increases first and then The decline trend is generally 4%～4.5%;Under the conditions of the same age,compared with the standard curing,there is an obvious gap in the mechanical properties of each group of specimens under natural curing,PAN can improve the mechanical properties,and the tensile strength is the most obvious;Under different curing conditions,the transforpmation reationship between surface rebound hardness,compressive strength and dynamic elastic modulus was established,all of which were in line with the power function relationship.(2)The performance degradation law of PAN/PC under the action of salt-freezing was studied,and the results showed that the mass of the specimen increased slightly in the early stage of the salt-freezing cycle,and the surface spalling degree of the specimen gradually deepened in the middle and later stages,and the mass loss rate increased;The dynamic elastic modulus decreased,and the strength decreased compared with that before salt-freezing.From the obtained salt-freezing deterioration model,the deterioration rate of PAN/PC is lower than that of PC.The test results can prove that PAN fiber can improve the salt-freezing resistance of concrete.(3)The thickness of the damaged layer,the compressive strength of the damaged layer and the evolution law of internal defects of PAN/PC under the action of salt freezing are studied.The results show that:with the increase of the number of salt freezing cycles,the thickness of the damaged layer of concrete increases gradually,and the compressive strength of the damaged layer increases.The strength gradually decreases;under the same number of salt-freezing cycles,the thickness of the damaged layer and the loss of compressive strength of the damaged layer of PAN/PC are always smaller than those of PC,and PAN can effectively delay the surface damage process of concrete under the action of salt-freezing.The mathematical relationship between the compressive strength of the damaged layer and the overall compressive strength is obtained by the calculation model of the thickness of the PAN/PC layer damaged by salt freezing;the obtained relationship between the density and the number of cycles of salt freezing can further explain the deterioration of concrete by PAN fibers due to salt freezing.The effect of PAN fiber is due to the positive effect of PAN fiber on the reduction of the internal defect area of concrete during the salt freezing process.(4)The evolution law of pore characteristics of PAN/PC under the action of salt-freezing was studied.The results show that:in the early stage of the salt-freezing cycle,the pore changes of PAN/PC in each pore size range are smaller than that of PC;in the middle and later stages of the salt-freezing cycle,the change range begins to increase,and the increase rate of PAN/PC is slower than that of PC;harmless pores and less harmful pores The rate at which pores are transformed into harmful pores is greater than that of new small pores,and the proportion of harmful pores and micro-cracks increases gradually,resulting in the gradual deepening of the internal damage of concrete.The effect of PAN on salt-freezing damage is mainly reflected in the micro-pore level.Proper introduction of small pores can improve the salt-freezing durability of concrete.In addition,the transformation of small pore throat connections into large pore throats is also an important factor in the change of internal damage of concrete.(5)Based on the grey entropy correlation analysis method,the correlation between the pore structure characteristics and macroscopic properties of PAN/PC under the action of salt freezing was studied.It is relatively close,and it can be considered that the increase in the thickness of the damaged layer is the internal cause of mass loss(surface spalling).The internal cause of performance degradation also proves from the pore structure that a mathematical relationship can be established between the indicators.