Determination Of Concrete Fracture Parameters From Three-Point Bending Tests

The mechanical behavior within the processing zone of concrete materialcan be well described by the fictitious cracking model (FCM). The materialproperties related to the crack bridging are cracking strength, tensile strengthand the stress-crack width relationship. Crack propagation is governed by thecracking strength. Based on FCM, a reversed analysis method to determine thefracture parameters, such as stress-crack opening relationship, tensile strengthand fracture energy from bending test is adopted. The procedure of calculationis accomplished by computer program and a windows-based close program isdesigned eventually.The advantage of present method is requiring a simple displacementcontrollable testing machine to perform a bending test only, which is easy torealize comparing with uniaxial tensile test. Two different criterions are usedduring the reversed analysis, which are cracking strength criterion andfracture toughness criterion. The basic ways to solve the fracture parametersof these two criterions are similar, but the conditions of crack propagation ofthem are not the same. This paper describes the calculating principles anddeducing procedures of these two criterions in detail and compares thedifferences of them. After the windows-based program has been designed withthe two criterions as kernel, only a few of basic parameters of specimen arerequired to calculate the corresponding stress-crack opening relationship fromthe load-crack mouth opening test curve.As applications of the calculating program, the influences of sand ratio,aggregate volume content and aggregate size on the stress-crack widthrelationship, tensile strength and fracture energy of concrete are evaluated.The program is also used to evaluate the size dependence of bendingparameters such as flexural strength, critical crack length and critical crackmouth opening of concrete. Based on the results, the following conclusion hadobtained: (1) The multi-linear function is accurate enough to reflect thecharacteristics of the stress-crack opening relationship of concrete, and thestress-crack opening relationship is significantly influenced by aggregate andthe cement matrix strength. (2) Cracking strength is almost not influenced bythe aggregate, but by the cement matrix strength. The higher the cementmatrix strength, the higher the cracking strength. (3) The tensile strength,flexural strength and fracture energy are all significantly influenced by thecement matrix strength. (4) The bending strength, the critical crack length andthe critical crack mouth opening of concrete beams are size dependence. Theinfluence of stress-crack opening relationship on the above materialparameters is significant.