Experimental Research On Sulfate Attack Resistance Of Hybrid Polypropylene Fiber Reinforced Concrete With Different Sizes

Sulfate attack is one of the most harmful environmental deteriorations that affect the long-term durability of concrete structures and can cause huge economic loss or even casualties.The source of sulfate ion is widely distributed in natural environment,including seawater,saline soils,groundwater,and urban sewage et al.The concrete structure which under sulfate attack will manifest degradation such as strength drop,cracking,spalling et al.As an excellent performance building material,polypropylene fiber(PPF)reinforced concrete has attracted wide attention from researchers around the world because of its superior mechanical performance(tensile strength and fatigue life).Besides,based on large number of early experiments,our group find that fiber hybridization between different scale of fibers shows good a synergy effect at different stage,however,research on durability of polypropylene fiber reinforced concrete(PFRC)is scarce.Therefore,the damage process of PFRC exposed to sulfate stack under drying-wetting cycles was investigated in this paper.Cubic compressive test,acoustic emission(AE)test,ion content test,SEM and XRD were used to investigate the degradation law of macro-mechanical properties,acoustic emission characteristics under compressive process,diffusion law of sulfate ion,the changing of micro-structure and corrosion products of PFRC.The effect of PPF blending into concrete on sulfate resistance of concrete was revealed.The main research contents and results are as follows:(1)One group of plain concrete as reference group and eight groups of PFRC as test group were designed by changing the polypropylene fiber hybridization.The natural drying-wetting cycle method was used to accelerate the speed of sulfate attack.The visual phenomenon and the mass loss were recorded at different ages of exposure.The result shows that: the degree of surface deterioration aggravated with the increasing exposure time,such as pores at surface become bigger and more.After sulfate attack,the surface deterioration the plain concrete was the most serious,cracking and spalling was occurred,the surface deterioration of multi-scale polypropylene fiber reinforced concrete(MPFRC)was not obvious.What’s more,the mass loss of all specimens increased firstly,then decreased with exposure time increasing.After 90 d,the mass of plain concrete begins to drop,and the speed of mass loss was much larger than MPFRC,the PPF can delay the drop rate of mass.(2)The damage evolution law of PFRC under compressive process and sulfate attack was investigated with AE technology.The result shows that: the compressive strength of all specimens increased first and then decrease which is similar with mass loss.The strength of plain concrete decreased by 8% compared to before erosion,however,after the sulfate attack,the strength of MPFRC was higher than before erosion,and 34.9% higher than the plain concrete.Similarly,incorporating PPF can delay the drop rate of strength and maintain the failure integrity of concrete.The proposed function to predict the strength of MPFRC under sulfate attack can fit well with the test result.The AE activity appeared in the whole the compressive process,which can be divided into three stages,and the AE activity of MPFRC was much more than plain concrete at different exposure time.The accumulative AE counts of plain concrete significantly decreased with exposing time increasing,which close to an order of magnitude.After corrosion,the initial damage cause by sulfate attack of MPFRC was-0.056 which was the smallest among the all specimens.(3)Gravimetric analysis was carried out to determine the sulfate ion content of each group specimen at special depth and different exposure time,the result shows that: with increasing depth of the concrete sulfate ion contents were decreased and with increasing exposure time sulfate ion contents were increased at the same depth,the surface sulfate ion content of MPFRC was minimum,namely 3.440%,after the exposure time,that of the plain concrete was very close to concrete blended with only macro PPF,namely4.062% and 4.028% respectively,it proved that incorporating with multi-scale PPF can efficiently slow down the speed of sulfate attack.Besides,the proposed sulfate ion content distribution function based on Fick law can well describe the diffusion of sulfate ion.(4)SEM,EDS and XRD were carried out to research the micro-structure and corrosion products of polypropylene fiber concrete,and the microscopic corrosion mechanism of sulfate attack was revealed.The result shows that: the corrosion products like ettringite accumulated at micro-crack and micro pores,each product can produce expansion stress at interior concrete,once they reach a certain amount,the pore wall would be squeezed by expansion stress,and new micro-crack would form.The PPF can bridge the pore and reduce the aperture size.On the other hand,after expose time,there was almost no characteristic peak of calcium hydroxide,however,for MPFRC there was still some characteristic peak of calcium hydroxide,that is the CH consumption rate of plain concrete at surface layer(2mm)is much larger than that of the MPFRC,and it also proved that multi-scale PPF resist the sulfate attack speed to concrete.