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.

The shear resistance of reinforced concrete beams without shear stirrups has been shown to be dependent on the size of beams. It was reported that as the beam depth increases, the shear resistance of the reinforced concrete beams decreases. Furthermore, the final failure mode of the reinforced concrete beams were found to be dependent on the strength as well as beam size. All other factors (i.e., maximum aggregate size, steel ratio, and proportion of specimen dimensions) being equal, large beams and early age beams (which have relatively low strength) were observed to fail in diagonal shear while small beams and matured beams failed in flexure. To explain the size effect on the shear resistance and final failure mode of reinforced concrete beams, a fracture mechanics approach was used in the present study. It was concluded that the effect of size on the final failure mode and shear resistance of reinforced concrete beams can be reasonably explained using the fracture mechanics concept.

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.

This symposium contribution gives a preliminary report on tests of the size effect in torsional failure of plain and longitudinally reinforced beams of reduced scale, made of microconcrete. The results confirm that there is a significant size effect, such that the nominal stress at failure decreases as the beam size increases. This is found for both plain and longitudinally reinforced beams. The results are consistent with the recently proposed Bazants size effect law. However, the scatter of the results and the scope and range limitations prevent it from concluding that the applicability of this law has been proven in general.

The stress-deformation relation now generally accepted for tensile fracture, i.e., with the descending branch described by means of a stress-displacement relation in a localized band, has been applied to the compressive stresses in a bent, reinforced beam. The displacement in this band is averaged over a length, which is proportional to the depth of the compression zone. The resulting average stress-strain relation, which is strongly size-dependent, is used for the analyses of the stresses in a rectangular beam section, and for the corresponding moment-curvature relationship. The results differ appreciably from those from conventional assumptions. The new approach shows a better agreement with test results than the conventional approach. Further test comparisons are, however, recommended. The new approach may form the basis of changed design assumptions, particularly for high-strength concrete.