Experimental Study On Durability Of Pier Concrete With UHPC Jacket

With the development of construction projects,the construction of cross-sea bridges has achieved considerable progresses.Substantial conveniences can be brought to transportation by the cross-sea bridge,which is across straits/bays and connects different regions.However,cross-sea bridges are facing with a severe challenge of durability.In particular,serious problems of durability in tough marine environment the concrete pier may be developed due to the generation of properties of concrete,which even may further threat safety of the bridge engineering.In this study,UHPC(ultra-high performance concrete)jacket was proposed to solve the durability problem of concrete pier in the Changxing Island and Jiaoliu Island cross-sea bridge.The UHPC jacket can act as a barrier against ingress of aggressive elements.Meanwhile,the compressive behavior of concrete pier can be improved due to the confinement effect of UHPC jacket.To investigate the influences of UHPC jacket on concrete pier,an experimental study on 18 UHPC-confined concrete specimens and 129 specimens with UHPC jacket after freezing-thawing cycles,wetting and drying cycles and seawater immersion was carried out.The influence of the thickness and shape of the UHPC jacket as well as type and time of environmental action were revealed and reported.Anlysis on the influences of UHPC jacket on compressive behavior of the stub concrete column and the durability of concrete with UHPC jacket under three typical marine environments was also carried out.Experimental results showed that the ductility index,peak stress and corresponding strain of UHPC-confined concrete increase with the thickness of UHPC jacket in room environment.The confinement effect of UHPC jacket on square specimens is significantly weaker than that on cylindrical specimens.Under freezing-thawing cycles/wetting and drying cycles,the axial stress-strain curve of the stub concrete column with UHPC jacket is full,while that of plain concrete column is flattening.The durability of concrete with UHPC jacket is improved significantly.Exposed to the same environmental type and action time,the strength retention rate of specimens increases,while the mass loss rate decreases with the increasing of the UHPC jacket thickness.Based on comparison of mass loss rate and strength retention rate of specimens under wetting and drying cycles and seawater immersion,it can be concluded that the wetting and drying cycles can accelerate degradation of concrete.After long-term immersion in seawater,the strength retention rate of square specimens is lower than that of cylindrical specimens due to the ion concentration in the corners of square specimens.Moreover,the theoretical model of stress-strain behavior of concrete with UHPC jacket and strength degradation model of concrete in marine environment are developed in this study.Finally,based on the environmental conditions and design of the bridge,a design method,which determined the thickness,shapes and construction scheme of UHPC jacket,was proposed.This design method can be used for the application of UHPC jacket in similar projects in future.