Research On The Resistance Performance Of HTRB630 Reinforced UHPC-CA Eccentric Compression Columns

Ultra high performance concrete is a fiber reinforced cement-based composite material produced from active powder materials such as cement and mineral admixtures,as well as fine aggregates,admixtures,steel fibers or organic synthetic fibers,and water.It combines ultrahigh impermeability and mechanical properties(compressive strength not less than 120 MPa,tensile strength not less than 5MPa),abbreviated as UHPC.At present,extensive research has been conducted on ultra-high performance concrete in the field of materials.In recent years,ultra-high performance concrete structures have received increasing attention and achieved certain results.However,related research mainly follows the approach of studying reinforced concrete structures.For example,in most studies,the steel bars used are still ordinary reinforced concrete bars,and structural measures such as protective layer thickness and cracking loads are also taken The calculation methods for stiffness,cracks,and so on are all based on the theory of ordinary reinforced concrete structures.This article considers that HRB500 and below steel bars do not match with UHPC,and only the combination of the two can give full play to the advantages of ultra-high performance concrete structures.Therefore,on the one hand,this article considers reducing material costs as much as possible and reducing shrinkage,Coarse Aggregate Ultra-High Performance Concrete(UHPC-CA)was designed,and on the other hand,higher strength steel bars of HTRB630 were added to study the failure mode and resistance performance of eccentrically compressed columns.The main conclusions are as follows:1)Compared to ordinary concrete columns,the failure modes of ultra-high performance concrete eccentric compression columns can still be divided into three categories: large eccentric compression failure,boundary eccentric compression failure,and small eccentric compression failure.However,after reaching the ultimate bearing capacity state,the compression zone of the UHPC-CA column remains intact without severe peeling,and the longitudinal reinforcement does not buckle into a lantern shape.The decreasing section of the load deflection curve is smoother,and both large and small eccentric compression columns show obvious ductility.2)Compared to ordinary concrete columns,the increase in crack width in the tensile zone of UHPC-CA columns is smaller with increasing load.Within the limit state of normal use,the maximum crack width of UHPC-CA columns is about 0.2mm,which is equivalent to the crack width limited by Class II environment and below in the ordinary concrete specification,and far less than the 0.3mm limit of Class I environment.Moreover,its crack resistance and bearing capacity are greatly improved.3)The stress process of UHPC-CA columns is similar to that of ordinary concrete columns,and can still be divided into three stages: elastic stage,working stage with cracks,and crack development stage after yielding.The ductility of UHPC-CA eccentric compression columns is much better than that of ordinary concrete eccentric compression columns;Under the same eccentricity,the ductility of UHPC-CA columns with large eccentricity is 1.62 times that of ordinary concrete columns with large eccentricity,and the ductility of UHPC-CA columns with small eccentricity is more than 1.3 times that of ordinary concrete columns with small eccentricity.4)The strain distribution of the UHPC-CA column section conforms to the assumption of a flat section,which means that the normal section strain is distributed linearly.Compared to ordinary concrete,the slip between HTRB630 grade steel bars and UHPC-CA is very small,resulting in better bonding.5)By comparing the tensile constitutive relationship of UHPC,the internal force balance method is more accurate in calculating the large eccentric cracking load,but there are certain errors in calculating the small eccentric cracking load,and the formula is complex and difficult to provide convenience for practical engineering.However,in axial tensile tests,the peak stress is not reached during cracking,so it is not necessary to consider the section resistance moment plastic coefficient.Therefore,a new cracking load calculation formula for HTRB630 grade UHPC-CA columns is proposed using the equivalent section method.6)A formula for calculating the bending stiffness of HTRB630 grade UHPC-CA columns was proposed;A formula for calculating the crack width of HTRB630 grade UHPC-CA columns under normal service limit state has been proposed;By comparing various specifications,it was found that the results obtained by the ultimate bearing capacity calculation method in this article are more consistent with the experimental values.A formula for calculating the ultimate bearing capacity of HTRB630 grade UHPC-CA columns is proposed,which can provide reference for practical engineering.