International Concrete Abstracts Portal

SP-118 This Special Publication of 13 papers presents advances in fracture mechanics involving characterization, resistance measurements, computation tools, and material toughness. The document is divided into two sections. One section deals with the application of fracture mechanics to cementitious materials. The other section covers the application of fracture mechanics to concrete structures.

Results of notched beam, direct tension, splitting tension, compression, shear beam, and flexural tests on plain mortar and on mortar reinforced with different volume fractions of short acrylic fibers are reported. An indirect J-integral technique is employed to determine the tension-softening curve and thus the tensile strength, the fracture energy, and the critical crack opening from the notched beam test results. As the volume fraction of fibers is increased, the strength in shear and flexure, the fracture energy, and the critical crack opening all increase, the tensile strength remains essentially constant, and the compressive strength shows some reduction. The characteristic length lch is used as a material property to characterize the post-peak tensile behavior. The shear and flexural strengths are related to the normalized dimension d/lch, and good agreement between the experimental results and theoretical predictions of decreasing strength with increasing d/lch is found.

Conventional concrete and mortar are both major construction materials because of their advantages in durability, economy, and comparably good mechanical properties. However, brittleness and low tensile strength are weak constitutions of these materials. Therefore, they provide less resistance to the propagation of cracks. Fibers can resist against the propagation of cracks due to the contribution of traction, resulting from the fibers-matrix bond mechanism, on the crack face. Some exact mathematical formulations to express thestress intensity factor and the crack opening displacement are proposed in this research to interpret the fracture behavior of fiber reinforced cementitious composites. Using these formulations, two fracture criteria can be performed to evaluate the tendency of crack propagation of this composite material. These two criteria are stress intensity factor and crack tip opening displacement. To achieve a more reasonable solution, the couple effect between the crack opening displacement and the fiber bridging traction is also considered. From the numerical results shown in this study, it is concluded that the fiber reinforced concrete provides higher resistance against the propagation of cracks than ordinary plain concrete, and one can clearly understand the resistance ability of fibers for the fracture behavior of concrete.

Many fracture mechanics models have been proposed in recent years to account for the nonlinear behavior of concrete around the crack tip region. These well-known models are the fictitious crack model (FCM) by Hillerborg, the crack band model (CBM) by Bazant, and the two-parameter fracture model (TPFM) by Jenq and Shah, etc. To model the fracture process zone or microcracked zone, these models often assumed the linear or bilinear stress-displacement relationship to simplify the analysis since actual relationships were not available due to difficulties in conducting direct tension tests. To avoid tedious numerical computation and the need of stress-displacement relationship, TPFM was proposed based on the simple LEFM concept. The model was quite accurate when applied to the notched beam test. All these models presented some degree of satisfaction when comparing with some experimental data. Since more direct tension tests with complete postpeak stress-displacement relationships have been successfully conducted in recent years, the need of assuming the stress-displacement relationship or using the indirect notched beam test is no longer necessary. An evaluation of the FCM using the observed stress-displacement relationships versus the assumed one seems to be an interesting task to verify the validity of the model. For TPFM, the proposed two unique fracture parameters should be verified for specimen size independence. A series of experiments were conducted on two types of test specimens (notched beam and compact tension) with different geometries. The results indicate that the parameters recommended in TPFM seem to be unique only for the notched beam specimen. The same two parameters were found to be tenfold larger for the compact tension specimen. For FCM, the predicted load-CMOD and load-deflection curves using the observed stress-displacement relationship are in better agreement with experimental data than those determined from the assumed linear relationship. Although theoretically both predicted load-CMOD and load-deflection curves should have the same order of accuracy, in this study, they were found to be substantially different.

Focuses on the formulation of a self-consistent model for a compressed concrete containing randomly distributed flat microcracks. A general formulation of the constitutive law for such material is obtained, finding the overall mechanical response to be strongly nonlinear in the region near the maximum in the stress-strain curve.