Investigation On The Assessment Method Of Cracking Resistance For Fiber Reinforced Concrete Subjected To Restrained Shrinkage At Early Ages

Restrained shrinkage cracking is very extensive in the structures with a large exposed surface area-to-volume ratio,such as concrete pavements,bridge desks and roof,in particular when concrete is at early ages.The exist of shrinkage cracks gives an access for harmful substance to the interior of concrete,accelerating deterioration,increasing maintenance costs and shortening the service life of concrete structures.The addition of fiber is one of the effective methods to improve cracking resistance and ductility of concrete structures.The difference in shrinkage cracking resistance is quite obvious for concretes with different types of fiber.Therefore,it is significant to reasonably assess the cracking resistance of fiber reinforced concrete at early ages in order to choose appropriate fiber volume in practical engineering.In line with this,some researches carried out in this paper are presented as follows:(1)Experimental investigation on both material properties of concrete at early ages and cracking resistance of fiber reinforced concreteMaterial properties of concrete were measured at different ages(3d,7d,14 d,21d,28d),including splitting tensile strength,elastic modulus,initial fracture toughness and fracture energy.Based on experimental results,fitting curves and fomulas were attained,which present variations of material properties of concrete with increase of ages.And the free shrinkage strains were measured through free shrinkage test on prisms of fiber reinforced concrete with 1.0% fiber volume fraction.The curve presenting variation of shrinkage strains of concrete with increase of ages were fitted.According to the standard ring test recommended by ASTM,restrained circular ring tests were conducted for fiber reinforced concrete with three different steel fiber volumes,e.g.0.5%,1.0% and 1.5%.In experiments,strains on the inner surface of each steel ring,the cracking ages,numbers,and widths for cracks were recorded.Experimental results indicate that the cracking resistance for the concrete with high fiber volumes is greater than that with lower fiber volumes.And in the case of a high fiber volume fraction,e.g.1.5%,there is no visible crack initaiton observed in the tests.(2)Calculation on the moisture distribution for concreteMoisture distribution for concrete was calculated based on an integrative model.In the modeling,both cement hydration and moisture diffusion resulted humidity variations were taken into account synchronously.For simplifying calculation process,it was assumed that cement hydration and moisture diffusion take place in succession during a short duration time dt.Firstly,the initial moisture at the time t for moisture diffusion was equal to that total humidity minus the humidity reduction produced for self-desiccation.Then,according to the second Fick's law,the residual humidity at the moment t+dt was deduced.Finally,the relative humidity at different position of concrete cross-section,including nodes on suface,interior,and bottom of concrete,was calculated using finite differential method.(3)Numerical simulation on the whole fracture process for fiber reinforced concrete under restrained shrinkageFirstly,based on calculated moisture distribution and measured free shrinkage strains,a fictitious temperature drop that can reflect the shrinkage characteristic of concrete was derived.Secondly,the fictitious crack model was applied in numerical simulation.After cracking,the cracking resistance was provided both by the interface stress between concrete matrix and fiber and the cohesive stress of concrete matrix itself.Finally,by introducing a crack propagation criterion,the initiation and propagation of cracks can be simulated for fiber reinforced concrete under restrained shrinkage.Results obtained from numerical simulation indicate that the amounts of fibers added in concrete will essentially determine the fracture behavior in the restrained shrinkage ring tests.In the case of a low fiber amount,e.g.0.5%,the crack would propagate throughout the cross-section of the concrete wall once its initiation.In the case of a high fiber volume fraction,e.g.1.5%,there is no visible crack initaiton observed in the tests.In the case of a medium volume fraction,e.g.1.0%,three cracks initiated and propagated in sequence.The final obvious drop of the strain values represents the crack propagates throughout the FRC wall,and the drop before it indicates the new cracks initiation and propagation.It has been found that numerical results accord well with experimental results,indicating that the fracture mechanics-based numerical approach adopted in this paper is effective to predict the cracking potential of fiber reinforced concrete under restrained shrinkage.