Study On Mechanics And Durability Of Steel-polypropylene Hybrid Fiber Lightweight Aggregate Concrete

Lightweight aggregate concrete,with its light weight,excellent thermal insulation and seismic performance,has gradually replaced ordinary concrete in high-rise buildings,soft soil foundation and large-span structures,and has become a widely used new civil engineering material.However,with the improvement of structural design strength and the weak characteristics of lightweight aggregate,the shortcomings of low tensile strength and brittle fracture seriously affect the development of lightweight concrete structures in many fields.Fiber and lightweight aggregate concrete are doped and compounded to alleviate the problems of low tensile strength and high brittleness on the basis of reducing the dead load of the structure.In this paper,the steel fiber with high elastic strength(i=0.5%,1%,1.5%)and polypropylene fiber with low elastic strength and high toughness(j=0.6kg/m3,0.9kg/m3,1.2kg/m3)are comprehensively combined and mixed with lightweight aggregate concrete.The slump value and dry apparent density of the mixture were measured,and the mixing ratio parameters were adjusted.By observing the micro-structure of the transition zone inside the lightweight aggregate and around the fiber,the influence of the change of mixed fiber content on the basic mechanical strength and tensile failure mode of the lightweight aggregate concrete was analyzed base on the fiber strengthening mechanism.In addition,rapid freeze-thaw test and early age free shrinkage test were carried out to study the improvement effect of fiber on the durability of lightweight aggregate concrete.The main research work and results are as follows:(1)The slump and apparent dry density of mixing materials were studied with the mixture of hybrid fiber as the only variable.The results show that:the slump value of lightweight aggregate concrete decreases to different degrees with the addition of fiber,fluctuates between 50mm-80mm,and decreases by 12%-40%compared with the base group.The apparent dry density increased slightly with the addition of fiber,but the increase range was small.(2)The experimental results of the basic mechanical properties of lightweight aggregate concrete with different fiber content show that,the addition of fiber can inhibit the extension and development of cracks in the process of tension and compression,and improve the failure morphology and mechanical properties of lightweight aggregate concrete.The improvement rates of compressive strength,splitting tensile strength and bending strength were 26%,116%and 37%respectively.The change rules of the them were similar,and all show a trend of increases first and then decreases with the increase of fiber content.(3)The microstructure of the aggregate and the transition zone were observed by SEM.The results show that the interfacial bonding performance of aggregate and cement mortar is good,and the porous structure of aggregate and the fiber-mortar interface constitute a relatively weak area inside the matrix,the phenomenon of mechanical strength decreasing with too much fiber was explained.(4)Based on freeze-thaw failure mechanism and capillary tension theory,the durability of hybrid fiber lightweight aggregate concrete was studied.The results show that the addition of fiber can restrain the free shrinkage deformation of lightweight aggregate concrete to a certain extent,reduce the mass loss after freezing-thawing and the relative dynamic modulus decline rate,improve the freezing-thawing damage damage morphology of specimens,and enhance the frost resistance.