Research On Shrinkage And Crack Resistance Of Strain Hardening Cementitious Composite (SHCC)

Shrinkage crack is one of the main causes of the concrete structural durability deterioration,which is often associated with defects of ordinary cement-based materials used for engineering,for example,low tensile strength,large brittleness,easy cracking and other defects.Effectively controlling the emergence and development of shrinkage crack of cement-based material and improving its durability are always issues of common concern in engineering field.Strain hardening cementitious composite(SHCC for short)has become a research hot spot in recent years because of its typical tensile strain hardening and multiple micro-cracking characteristics.As a new type of cement-based material,shrinkage and crack resistance of SHCC can not be avoided.So far,the domestic and foreign research results in this area are limited,and it is still in the exploratory stage.In this paper,the uniaxial tensile properties,shrinkage performance,restrained shrinkage cracking resistance of SHCC were systematically studied by testing,and the coordination properties of shrinkage deformation of SHCC repair old concrete beam were preliminarily explored.Specific work was as follows:(1)The tensile properties of SHCC under different curing environment(standard,indoor and outdoor)and different ages were studied by uniaxial tensile test.Test results show that in curing appropriate,standards,indoor and outdoor maintenance SHCC and curing age longer SHCC showed significant tensile strain hardening and multiple micro-cracking characteristics,ultimate tensile strain reached stably above 3%,and the average crack width corresponding to ultimate strain was well below 125?m.Compared with standard curing SHCC,tensile strength of indoor curing SHCC decreased,the hardening phenomenon was not obvious,but its toughness didn't decrease,crack width was below 60?m.Under the common influence of early full curing and late natural curing,performance of the outdoor curing SHCC were stable that tensile strength and toughness had not decreased significantly and crack width was below 80?m.Indoor curing environment,SHCC tensile strength increases with age that the tensile strength of 28 d more than 96% of that of 90 d.tensile strain capacity of SHCC reached its peak about on the 14 th day,subsequently declined slightly,ultimate tensile strain tended to be 3.5% at the age of 90 days.The average limit crack width increased gradually with age,gradually stabilized after 28 days,about 50±2?m.Logarithmic function and exponential function could be used to describe the relationship between SHCC tensile parameters and age.(2)The development of shrinkage deformation and moisture loss of SHCC under different curing environment(standard,indoor and outdoor)was studied by free shrinkage test.Test results show that the influence of curing environmental on the shrinkage of SHCC was far greater than that of swelling.Compared with the indoor curing SHCC,shrinkage deformation amplitude of outdoor curing SHCC was larger and the shrinkage value was larger.During early water curing period,outdoor curing SHCC shrinkage reduced,or even slight expansion.Moisture loss and shrinkage deformation of SHCC occurred mainly in the early stage,and mass loss rate and shrinkage strain of SHCC was linear relationship.The relationship between SHCC shrinkage strain and age could be described and predicted by hyperbolic function.By theoretical analysis,Higher negative p of SHCC for resistance to restrained shrinkage cause local failure provided a very safe limits.Compared with the indoor curing SHCC,anti-fracture ability of outdoor curing SHCC decreased slightly.Indoor curing SHCC easily appeared shrinkage crack in the early age.(3)The shrinkage cracking patterns of SHCC under different curing environment(indoor and outdoor)was studied by ring restrained shrinkage test.Test results show that SHCC showed multiple micro-cracking patterns under indoor and outdoor curing environment.The addition of PVA fiber greatly improved the shrinkage cracking resistance of SHCC that crack control rate of SHCC reached more than 70% that of the matrix.Compared with concrete,though SHCC shrinkage cracking time earlier,its the crack control ability was obviously superior to concrete that the average crack width of SHCC was about 23.0~38.1% of concrete.Compared with the indoor curing SHCC,crack number of outdoor curing SHCC increased and crack resistance decreased slightly,the maximum crack width(about 120?m)was significantly higher than that of indoor curing SHCC(about 90?m),but early water curing could delay shrinkage cracking time of outdoor curing SHCC.(4)The repair layer cracking and repair/old interface delamination patterns of SHCC repaired beams with different interface roughness,different repair layer thickness and under different environment and the development trend of restrained shrinkage strain of SHCC repair layer were studied by SHCC and old concrete bond shrinkage test.Test results show that compared with concrete,the SHCC repair layer appeared multiple micro-cracks rather than local fracture,and the interface delamination was effectively controlled.SHCC repaired beam with smooth bonding surface was prone to delamination failure.Cracking and delamination were easily occurred in SHCC repaired beams with thinner repair layer.Compared with the indoor curing SHCC repaired beam,the degree of the cracking and delamination of the outdoor curing SHCC repaired beam was relatively serious.From the perspective of degree of cracking and delamination,the interfacial bond performance was the main factor that affected shrinkage deformation compatibility of SHCC repaired system.Restrained shrinkage strain of SHCC repair layer continued to increase at a decreasing rate,and then gradually to maintain stable development stage.With the increase of repair layer thickness,stable period of restrained shrinkage strain delayed gradually.The local shrinkage strain near bonding interface was small,and the size of the local shrinkage strain away from the bonding interface was related to the cracking patterns of the repair layer.