International Concrete Abstracts Portal

The paper presents a review of some of the Australian code developments in the design of concrete structures. Topics such as shear design of beams, deflection control, punching shear strength of slabs and shear strength of walls are covered.

During its lifetime, a bridge deck slab is subjected to a very large number of wheels of different magnitudes. By contrast, the laboratory investigation of the fatigue resistance of a bridge deck slab is usually conducted under wheel loads of constant magnitude. No current method seems to be available for establishing the equivalence between the actual wheel population on a bridge and fatigue test loads. The design codes are also not explicit in this respect. Taking the statistics of wheel loads from a Japanese survey, an upper-bound data-base is prepared for expected wheel population in Canada. There is close correspondence between the maximum wheel loads observed in Japan and Canada. The data-base is likely to be applicable to the USA as well. A fairly simple mathematical model has been developed to determine the number of passes of one wheel load so that the damage induced by it is equivalent to the damage caused by a given number of passes of another load of known magnitude. The model, which doesn not necessarily assume a linear relationship between accumulated damage and the number of passes, can be predetermined wheel load statistics. The method can also be used to formulate specific fatigue load requirements for bridge deck slabs. Two sets of tests on full-scale models of both cast-in-place and precast deck steel-free slabs have been described briefly. It is concluded that both these slabs have more fatigue strength than required to sustain the projected population of wheel loads.

The service performance of repaired structures depends mainly on the mechanical properties of the substrate and repair materials and the mechanical behavior of the interface between them. However, in most studies in the literature, on ly bond strength is used to evaluate the repair and the deformation behavior of the interface is usually neglected. In this study, three slabs were cast using conventional concrete as a substrate. The substrate surface was roughened with jackhammers. A layer of 50-60 mm repair materials were cast by shotcreting or by hand. The test specimens were cored or cut to the required size. Direct tensile and compression tests were performed to evaluate the bond strength and mechanical behavior of the interface between substrate and repair materials. The test results indicate that the bond strenght was affected by the mix proportions and independent on the casting method and the inclusion of steel fibers. However the casting methods had a strong influence on the mechanical behavior of the interface between substrate and repair material.

The objective of this paper is to characterize the quasi-static and fatigue response of concrete subjected to biaxial stresses in the t-C-T region, where the principal tensile stress is larger in magnitude than the principal compressive stress. An experimental investigaion of material behavior is conducted. The failure of concrete in the stated biaxial region is shown to be a local phenomenon under both quasi-static and fatigue loading, wherin the specimen fails owing to a single crack. The crack propagaion is studied using the principles of fracture mechanics. It is observed that crack growth in constant amplitude fatigue loading is a two-phase processL a deceleration phase followed by an acceleration stage. The quasi-static load envelope is shown to predict the crack length at fatigue failure. A fracture-based fatigue failure criterion is proposed, wherein the fatigue failure can be predicted using the critical mode I stress intesity factor obtained from the quasi-static response. A mnaterial maodel for the damage evolution during fatique loading of concrete in terms of crack propagation is proposed. The model parameters obtained from uniaxial fatigue tests are shown to be sufficient for predicting the biaxial fatigue response.

Use of high strength composites for repair and rehabilitation of bridges and parking decks is steadily increasing. Since these structures are subjected to fatigue loading, the performance of strengthened beams under this type of loading needs to be evaluated. An analytical procedure that incorporates cyclic creet of concrete and degradation of flexural stiffness is presented. The method is verified by computing cycle dependent deflections and comparing them with experimental results. The results presented in this paper also provide a summary of an experimental investigation, in which reinforced concrete beams were strengthened with glass fabrics (sheets) and subjected to fatigue loading. The comparison shows that the analytical model provides reasonalbly accurate prediction of deflections, for both reinfoced beams and reinforced concrete beams reinforced with composites. Although Glass fiber composites were used for the evaluation, the model is alos applicable to other types of fibers.