In this paper, an attempt is made to determine the double-K fracture parameters Kini Ic and Kun Ic using three-point bending notched beams. First, based on the knowledge from extensive investigations which showed that the nonlinearity of P-CMOD curve is mainly associated with crack propagation, a linear asymptotic superposition assumption is proposed. Then, the critical effective crack length ac is analytically evaluated by inserting the secant compliance cs into the formula of LEFM. Furthermore, an analytical result of a fictitious crack with cohesive force in an infinite strip model was obtained. The double-K fracture parameters Kini Ic and Kun Ic as well the critical crack tip opening displacement CTODc were analytically determined. The experimental evidence showed that the double-K fracture parameters Kini Ic and Kun Ic are size-independent and can be considered as the fracture parameters to describe cracking initiation and unstable fracture in concrete structures. The testing method required to determine Kini Ic and Kun Ic is quite simple, without unloading and reloading procedures. So, for performing this test, a closed-loop testing system is not necessary.

This paper shows how the double-K fracture parameters Kini Ic and Kun Ic can be determined for concrete using CT-specimens and wedge splitting specimens. The experimental results collected from the fracture tests on the very large size CT-specimens and small size wedge splitting specimens carried out by many researchers are utilized to investigate the characters of the obtained double-K fracture parameters Kini Ic and Kun Ic. It was found that the double-K fracture parameters Kini Ic and Kun Ic determined from fracture tests on the large size CT-specimens are size-independent. And the values of Kini Ic and Kun Ic determined from small size wedge splitting specimens with same dimensions are independent of the relative preformed notch length a0=D. However, when the dimensions of small size wedge splitting specimens change from 150

Many attempts have been focused on the textile reinforced mortar structures. The advantages of textiles are mainly a very good load bearing capacity, excellent ductility, thin size and light weight of components, resistance to corrosion, and no magnetic disturbances. Such structures are expected to have wide application foregrounds. The transfer of forces from reinforcement to mortar is accomplished through a bond. Therefore, understanding and further improving bond properties of textiles in mortar are important. In the paper, bond characteristics of various textiles in mortar obtained from pull-out tests and numerical simulations are shown and compared. The textiles used include carbon, alkali resistant glass and aramid. They were plain and impregnated with epoxy resin. Prestressed and nonprestressed specimens were tested. The experimental results show that epoxy resin impregnating and prestressing can strongly enhance the bond strength. Numerical simulations show good agreement with the experimental results and significantly contribute to a better understanding of the bond characteristics of textiles in mortar.

The results of experimental investigations using laser speckle interferometry on small size three-point bending notched beams and using photoelastic coating and the strain gauges on very large size compact tension specimens of concrete are presented in detail. The investigations showed that there exists a stage of stable crack propagation before unstable fracture occurs. The results are in agreement with other researchers' investigations using moire interferometry, holographic interferometry, dye-impregnation method and microscope. Further detailed study shows that the three different states, i.e., crack initiation, stable crack propagation and unstable fracture can be distinguished in the fracture process in concrete structures. In order to predict the crack propagation during the fracture process in quasi-brittle materials a double-K criterion is proposed. The double-K criterion consists of two size-independent parameters. Both of them are expressed in terms of the stress intensity factors. One of them reflects the initial cracking toughness, denoted with Kinic, which can be directly evaluated by the initial cracking load, Pini, and the precast crack length, a0, using a formula of LEFM. The other one refers to the unstable fracture toughness, denoted with Kun c, which can be obtained inserting the maximum load, Pmax, and the effective crack length, ac, into the same formula of LEFM. The values of the two parameters, Kini Ic and Kun Ic, obtained from the small size three-point bending notched beams and the large size compact tension specimens show that Kini Ic and Kun Ic are size-independent. Evaluating with the KR-resistance curves obtained from the same test data, it is found that the proposed double-K criterion is equivalent to it in basic principle, but, the double-K criterion can be applied more easily than the KR-resistance curve. Finally, as a practical example, the application of the double-K criterion to the prediction of the crack propagation in a concrete dam is discussed.

The influence of the fracture process zone (FPZ) on the fracture properties is one of the hottest topics in the field of fracture mechanics for cementitious meaterials. Within the FPZ in front of a traction free crack. Cohesive forces are distributed in accordance with the softening stress-separation constitutive relation of the material. Therefore, further crack propagation necessitates energy dissipation, which is the work done by the cohesive forces. In this paper gƒ the local fracture energy charaxterizing the energy consumption due to the cohesive forces, is discussed. The computaional expression of gƒ in the FPZ can be obtained for any stage during the material fracture process regarding the variation of FPZ, whether in terms of its length or width. Gƒa, the average energy consumption along the carack extension region. hsa also been computed and discussed in this paper. Thee experimental results obtained from the wedge splitting tests on specimens with different initial notch ration are employed to investigate the property of the local fracture energy gƒ and the average value Gƒa over the crack extension length. These results can be used to indicate the influence of the FPZ. Additionally, changes in the length of the FPZ during the fracture process are also studied.