Study On Mechanical Properties Of PVA Fiber Reinforced Fast-Hardening Concrete Under Freezing-Thawing

Freeze-thaw damage is one of the main factors affecting the dura bility of concrete structures.In the severe cold regions of China,especially in the Northeastern region,due to the freeze thaw cycles caused by temperature difference and the harsh climate,most of the hydraulic structures have suffered serious damage from freezing and thawing damage.Therefore,the solutions to improve the frost resistance of concrete structures is of importance in nowadays.Polyvinyl alcohol(PVA)fiber has many excellent properties,such as bonding interface with cement matrix.At present,the freeze-thaw characteristics of PVA fiber reinforced concrete are mainly concentrated on the short-term mechanical behaviors of well cured specimens,such as tensile,compressive,toughness,impact resistance etc.The topic of anti-freeze durability of early-strength fiber reinforced fast-hardening concrete with cold-thaw environment under cold conditions is insufficient.In order to reveal the mechanical properties and deterioration of fast-hard PVA fiber concrete under freezing and thawing effects,four groups of specimens with different volume contents were investigated.The compressive strength,flexural strength,relative dynamic elastic modulus and mass loss rate after different freeze-thaw cycles were measured,and the early strength PVA fiber was hardened.Freeze-thaw damage model of concrete based on macro relative dynamic modulus was proposed as well.The main research contents and conclusions of this paper are listed as follows:(1)The freeze-thaw cycle experiments were carried out on the early curing age of fast-hardening concrete.And the effects of fiber volume contents on deterioration of compressive,flexural strength and macroscopic failure modes before and after freezing and thawing of concrete were obtained.The experimental results show that the incorporation of fibers leads to the increase of the compressive strength and flexural strength of PVA fiber reinforced fast-hardening concrete,which is higher than that of unincorporated fiber specimens,and the compression failure mode turns to ductile failure.With the increase of the freeze-thaw cycles,the compressive and flexural properties of PVA fiber reinforced fast-hardening concrete exhibits a downward trend.And the frost resistance property of such materials can be enhanced while fiber volume content lower than 0.6%.(2)The frost resistance of PVA fiber reinforced fast-hardening concrete was analyzed by ultrasonic wave velocity and mass loss rate.The results show that the initial ultrasonic wave velocity of specimens with PVA fiber are larger than that of ordinary concrete under freeze thaw effects.With the increase of freeze thaw cycles,the longitudinal wave velocity decreased.After 200 freeze-thaw cycles,the decreases of PVA-0,PVA-2,PVA-4 and PVA-6 were 14.8%,9.2%,10.6% and 13.8%,respectively.It is proved that the overall compactness and freeze-thaw resistance of concrete can be improved remarkably with incorporation of 0.2%-0.4% PVA fiber.(3)Based on the Damage Theory,a cubic polynomial evolution equation and a two-parameter Weibull probability distribution prediction model were proposed to predict the damage revolution of PVA reinforced fast harden concrete under freeze thaw cycles.Moreover,a freeze-thaw damage prediction model based on relative dynamic modulus was discussed as well.The modified exponential model possess both acceptable applicability and fitting accuracy,which can reflect the evolution of freeze-thaw damage of PVA fiber fast-setting concrete properly.