Determination Of Initial Fracture Toughness Of Concrete And Its Application

Crack initiation and propagation of concrete are widespread problems in engineering.The formation and extension of crack reduce the load capacity and durability of concrete structures.The initiation and prediction of crack are of significant importance to form a well early warning system of cracking and avoid the accidents caused by dangerous cracks.This dissertation took initial fracture toughness as the criterion of crack initiation.Simplified theoretical methods of determining initial fracture toughness of concrete were proposed.The proposed method was adopted to study the crack initiation of steel fibre reinforced cementitious composites(SFRC).The mechanism of the effect of addition and alignment of steel fibres on initial fracture toughness of SFRC was explained.The prediction of unstable fracture toughness from initial fracture toughness was proposed.The major contributions of this dissertation are summarized as follows: 1?A simplified extreme method using experimental peak loads to obtain the initial fracture parameters of concrete for three-point bending beams was proposed.This approach was based on the traditional extreme method and weight function method.Complex numerical integration and measurement of critical crack opening mouth displacement are avoided.Only one specimen is required to be tested for determining the initial fracture parameters.The calculation is easier,and the results obtained by the proposed method are more stable.2?Based on the linear regression theory,a simplified method for determining the initial fracture toughness of concrete was studied.The initial cracking loads were determined by using a load-crack mouth opening displacement curve which is linear before crack initiation.The new method is superior to the experimental methods using surface measurement and theoretical methods using numerical integration.Compared to other methods,it is easier to operate and insensitive to artificial factors.3?Crack initiation of SFRC and aligned steel fibre reinforced cementitious composite(ASFRC)was studied based on the linear regression method.The enhancement of ASFRC to initial fracture toughness was explained by taking into account the fact that the inclusion embedded at the crack tip of composite would change the crack tip stress intensity factor.Results show that,the initial fracture toughness of ASFRC is significantly higher than that of SFRC.The initial fracture toughness increases with the steel fibre volume fraction and specimen size.4?Based on the numerical simulation of the whole fracture process and the simplified extreme method of three-point bending notched beams,the predictions of unstable fracture toughness of concrete from initial fracture toughness were studied.Implicit expression of the relationship between double-K fracture parameters was given.According to the proposed relationship,the two parameters could be obtained from each other without experimental measurement and complex numerical integration.