Experimental Research On Bond Strength Of UHPC-NC Interface

The bond of new and old normal concrete(NC)exists widely in architectural structure,such as structural reinforcement and perfabricated structure etc.But the deteriorated bond of old and old normal concrete has brought many adverse effects on the structure,which seriously affects the strength,stiffness and durability of the structure.Ultra-high performance concrete(UHPC)has the advantages of high strength,good toughness.and perfect durability.Using UHPC as bonding material can significantly improve the strength,crack resistance and durability of the structure.The effects of age,moisture index,curing temperature and different NC substrate interface processed on the bond strength of UHPC-NC interface were studied by slant shear test,splitting tensile test and direct tension test,respectively.The experimental results show that:(1)Although the bond strength of UHPC-NC interface rises with the increase of curing age,the increase is not unlimited.95% of the bond ultimate strength has been completed by the age of 7 days.The speed of the strength development before the age of 7 days was the fastest and became very slow after the age of 7 days with the value being zero.This trend is basically consistent with slant shear bond strength,splitting tensile strength and direct tension strength.(2)The bond strength of UHPC-NC interface also has an upward trend with the increase of moisture index.Compared with the bond strength in case of dry interface,the bond strength in case of wetting and wet saturation interface can be increased by 0.1 to 0.45 times and 0.28 to 0.59 times,respectively.(3)The bond strength obtained by steam curing at 60? is not different from that of 28 d age in normal temperature curing.The steam curing at 60? for 48 h only promotes the hydration reaction and gelation in UHPC,and accelerates the strength development of UHPC.But the bond strength can not be improved obviously.By contrast,steam curing at 90? not only does not improve the bond strength,but also reduces the bond strength of the interface of UHPC-NC due to the large shrinkage stress caused by high temperature.(4)Compared with six different substrate interface,the bond strength of the rough interface is the largest,and the effect of smooth interface is the worst.In slant shear test,the combined interface did not show better bond performance compared with the the rough interface,and the slant shear strength of specimens of each group was almost the same.However,for splitting tensile test and direct tension test,the bond strength of the combined interface is slightly lower than that of the rough interface.