Research On Cavity Mechanism Of Concrete Filled Steel Tube

The paper mainly analyses cavity problem of concrete-filled steel tube(CFST).Deep researches are made on the choose of material,maintenance environment and exterior surroundings(mainly for changes of temperature) which help solving this kind of practical projects.First,the rule that moisture affects shrinkage and expansion of concrete is studied.The result of concrete linear expansion expriments in different maintenance environments shows that concrete expansion is larger in un-sealing treatment than in sealing treatment.In sealing treatment,the result of different kinds of haydite concrete linear expansion expriments shows that when water absorbency rate is large, haydite concrete expansion of small grain diameter and high elastic modulus is lareger.The result of the same haydite concrete linear expansion expriments shows that there exists best blending amount.Concrete expansion is large and volume stability is good in best blending amount.So moisture is the main factor which affects core concrete expansion. Second,the rule that temperature affects cavity of CFST specimens is studied.The result of the high and low temperature circulating expriments of CFST specimens shows that radial shrinkage of different cross sections in core concrete diminishes when cross sectional area grows large.Radial shrinkage of the same cross section in core concrete diminishes when steel containing rate grows large.But after circle of high and low temperature,shrinkage of core concrete is large and the change of volume of outside steel is small.Last,the rule that temperature affects the formation and existence of ettringite and delayed ettringite is studied.Cement pastes in different maintenance environments are compared.After seven days room temperature(20℃) steam cured,then three days high temperature(80℃) steam cured and back to room temperature(20℃),formation amount of ettringite and delayed ettringite is studied.The result of X-ray diffraction expriment shows that there is plenty of AFm in cement paste by three days high temperature(80℃) steam cured in different maintenance environments.When back to 20℃,DEF appears less in cement paste in sealing maintenance environment.