Study On Autogenous Shrinkage Performance Of PVA Fiber Reinforced Aeolian Sand Cementitious Composite

Polyvinyl alcohol fiber cement-based composite material with its crack harmless diffusion characteristics,strain hardening characteristics and superior energy dissipation capacity came into being and promoted.The desert area in Northwest China is vast and the aeolian sand resources are abundant.If it can be used in the fine aggregate of building materials,it can not only respond to the concept of green development and the idea of adapting to local conditions,but also save the cost.In this paper,Polyvinyl Alcohol Fiber Reinforced Aeolian Sand Cementitious Composite(PVA-FRASCC)was prepared by replacing some fine aggregates with aeolian sand by internal mixing method and adding fiber to enhance the matrix.As an important factor causing non-load cracks in structures,autogenous shrinkage has attracted much attention in the application of high-performance concrete and the design of large-scale projects(such as subgrade and pavement projects,dams,bridges).The low water-binder ratio of high-performance cement-based materials makes the matrix produce large autogenous shrinkage deformation at early ages,and the shrinkage deformation is often larger than that of high-performance concrete,and the cracking risk is significantly improved.Based on this,four research variables of aeolian sand replacement rate,fiber volume rate,water-binder ratio and sand-binder ratio were set up to study the early autogenous shrinkage performance of PVA-FRACSCC,in order to provide reference for engineering disaster protection,improvement of structural durability and popularization and application of aeolian sand resources.The main conclusions are as follows:(1)The working performance of the substrate was evaluated by the fluidity index.When the water-binder ratio is 0.25,the fluidity increases first and then decreases with the increase of aeolian sand replacement rate.When the water-binder ratio is 0.35 and 0.45,the change of aeolian sand replacement rate has little effect on fluidity.With the increase of water-binder ratio or the decrease of fiber volume fraction,the fluidity of the substrate increases.Increasing the sand-binder ratio,the fluidity of the substrate increases first and then decreases.(2)The compressive mechanical properties of the specimens were evaluated by compressive strength index.When the water-binder ratio is 0.25,the compressive strength increases first and then decreases with the increase of the replacement rate of aeolian sand.When the water-binder ratio is 0.35,the compressive strength of the specimens decreases after the incorporation of aeolian sand,but the compressive strength is not much different when the replacement rate is 30% and 60%.When the water-binder ratio is 0.45,the change of aeolian sand replacement rate has little effect on the compressive strength of the specimen.Under different aeolian sand replacement rates,the compressive strength of the specimens decreases with the increase of water-binder ratio.In the range of this test,the compressive strength of the specimen increases with the increase of fiber volume ratio and sand-binder ratio.(3)The tensile mechanical properties of the specimens were evaluated by the number of cracks,initial crack strength,initial crack strain,ultimate tensile strength and ultimate tensile strain.The ultimate tensile strain of the specimen increases after the incorporation of aeolian sand,but when the water-binder ratio is 0.45,the ultimate tensile strength of the specimen with 60% aeolian sand replacement rate is significantly reduced.The increase of water-binder ratio is beneficial to the development of multiple cracks and strain hardening characteristics,but it will significantly reduce the ultimate tensile strength of the specimen.Compared with the specimens without fibers,the tensile strength of the specimens with fibers is improved.Continue to increase the fiber volume ratio has little effect on the tensile strength,but it can significantly increase the ultimate tensile strain of the specimens.High sand-binder ratio can improve the tensile strength,but it is not conducive to the increase of tensile deformation.The increase of sand-binder ratio enhances the micro-crack healing ability of the specimen.(4)The shrinkage properties of the specimens were evaluated by autogenous shrinkage strain,total shrinkage strain and mass loss rate.When the water-binder ratio is0.25,with the increase of the replacement rate of aeolian sand,the autogenous shrinkage,total shrinkage and mass loss rate of the specimens at 7d age increase.When the waterbinder ratio is increased to 0.35 and 0.45,the autogenous shrinkage and total shrinkage of the specimens with 30% aeolian sand replacement rate are the largest at 7d age.The replacement rate of 60% aeolian sand caused the expansion of the specimen to increase at1d～2d.The increase of water-binder ratio will reduce the autogenous shrinkage of the specimen,but will increase the total shrinkage risk of the specimen in the later stage.With the increase of water-binder ratio,the drying water loss of the specimen increases and the mass loss rate increases significantly.With the increase of fiber volume fraction,the autogenous shrinkage and total shrinkage of the specimen decrease.The mass loss rate increases first and then decreases.The mass loss rate of the specimen with a fiber volume ratio of 0 is close to that of the specimen with a fiber volume ratio of 1%,and the mass loss rate of the specimen with a fiber volume ratio of 0.5% increases significantly.After the sand-binder ratio increases,the 7d autogenous shrinkage and total shrinkage increase,and the mass loss rate decreases.(5)Through the pore fluid saturation,porosity and pore size distribution and other indicators,the mechanism of autogenous shrinkage is analyzed based on the characteristics of pore evolution.The 7d bound fluid saturation from high to low is the aeolian sand replacement rate of 60%,30% and 0,which corresponds to the potential autogenous shrinkage risk,and the autogenous shrinkage test results also verify this conclusion.With the increase of aeolian sand replacement rate,the porosity of smaller capillary pores increases and the autogenous shrinkage increases.With the increase of age,the overall porosity of the substrate decreased,the porosity of large pores and large pores decreased significantly,the porosity of small pores remained at a high level,and the autogenous shrinkage still maintained a rapid growth trend at 7d.The evolution characteristics of pore structure can better reveal the mechanism of autogenous shrinkage deformation of specimens.(6)The early autogenous shrinkage prediction model of PVA-FRASCC was established by modifying the E.Tazawa model.The modified model can accurately predict the early autogenous shrinkage strain of the material,and the experimental value is in good agreement with the calculated value of the model.