International Concrete Abstracts Portal

Fracture behavior of epoxy and polyester polymer concrete (PC) systems are investigated in Mode I fracture using single-edge notched beams with varying notch depths. The beams were loaded in four-point bending. Influence of polymer content on the fracture behavior of epoxy PC andpolyester PC at room temperature was studied using uniform Ottawa 20-30 sand. The polymer content was varied between 10 and 18 percent of the total weight of the composite. The flexural strength of the polymer concrete systems increase with increase in polymer content while the flexural modulus goes through a maximum. The critical stress intensity factor KIC was determined by two methods, including a method based on crack mouth opening displacement. At the same polymer content, the epoxy PC has a higher fracture toughness than polyester PC. The KIC for epoxy PC and polyester PC increases with increase in polymer content and PC flexural strength. The critical stress intensity factor of PC is represented in terms of polymer content and polymer strength. Numerical tests based on random sampling and stratified sampling procedures were performed to substantiate the experimentally observed fracture toughness values of polymer concrete.

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.

A fracture mechanics approach is presented. In this approach, the complete material behavior during tensile fracture is described in a way suitable for the analysis of failure of structures. Two examples are given of practical applications, in which the results can be compared with code specifications. The first is the cracking strength of a beam. It is demonstrated that the formal flexural stress that causes cracking decreases as the depth of the beam increases. The second example is the shear strength of a beam without shear reinforcement. The theoretical results show a good agreement with test results. There seem to be reasons to revise the rules in the ACI Building Code regarding the influence of beam depth, of span-to-depth ratio, and of the amount of longitudinal reinforcement on the shear strength. The tensile toughness of concrete, expressed as fracture energy, proves to be an important material property, which ought to be taken into account.