International Concrete Abstracts Portal

Experimental investigations on the behavior of high strength concrete under uniaxial and biaxial short-term compressive stresses were conducted using thin square plate specimens. Strength, stress-strain relationship, mode of failure, and failure mechanism are discussed. Results confirm that a main cause of the increase in strength, stiffness, and proportional limit of concrete under biaxial compression is the confinement of internal microcracking preventing the development of a progressive failure mechanism. In addition, it was found that as the aggregate stiffness approaches that of the mortar, both the proportional limit and the discontinuity point of the concrete increase due to the reduction of stress concentrations. The observed failure mode for high strength concrete can be explained in terms of the limiting tensile strain criterion.

The use of high strength concrete is attractive in precast prestressed concrete structures and in earthquake resistant structures for which a reduction in mass is of paramount importance. Yet applications of high strength concrete are hindered by its relative brittleness. Such a drawback can be overcome by addition of fibers. The present study describes the main effects of fiber reinforcement on the compressive stress-strain properties of high strength fiber reinforced mortar and concrete. The influence of various fiber reinforcing parameters such as volume fraction of fibers, aspect ratio, and type of fibers is illustrated. Trade-offs to achieve ductility while maintaining high strength are explained.

The basic philosophy of the current ACI Code for confining concrete in earthquake design is that the increase of the strength of the core of the column due to confinement must offset the loss of strength due to spalling of the unconfined cover. The equatians given in the code are based on the assumption that when a reinforced concrete column is subjected to uniaxial load the maximum capacity of the confined core is reached when the unconfined cover starts spalling. It is not clear whether this assumption is applicable for high strength concrete. The strains at which the cover concrete and confined concrete -will reach their maximum capacities will depend on their respective stress-strain curves. In this paper, based on several sets of experimental data, analytical expressions are proposed for the stress-strain curves of confined and unconfined high-strength concrete. Using these analytical expressions, moment-curvature relationships are predicted. The predicted curves were compared with the experimental data of columns subjected to reversed lateral loading. Rased on the satisfactory comparison for normal strength concrete columns, the theoretical model is then applied to high‘ strength concrete.

High strength concrete is used in increasing volume in the construction of structural components. While much research has been done on reinforced concrete corbels, experimental data on the behavior of corbels using high strength concrete remain scarce. The ACI Special Provisions for Brackets and Corbels is based primarily on experimental results of corbels with concrete strength less than 6000 psi (41.4 MPa). The purpose of this study is to check the applicability of the ACI Code and the truss analogy theory proposed recently by Hagberg to reinforced concrete corbels with concrete strengths greater than 6000 psi (41.4 MPa). A total of eight corbels, divided into four series with concrete strength ranging from about 6000 psi (41.7 MPa) to 12,800 psi (82.7 MPa) were studied in the Rutgers Civil Engineering Laboratory. The corbels (shear span to dept ratio, a/d = 0.39) were loaded monotonically to failure and magnitudes of the strains in the primary steel, stirrups and cage steel were recorded along with the vertical load. Analysis of results indicated that the ACI Code Provisions are conservative. The truss analogy model predicts values which are safe and less conservative than the ACI Code. The degree of conservatism of the ACI Code found in the case of these tests will not necessarily be found in tests with larger a/d ratios and/or tests in which outward horizontal loads are applied to the specimens in addition to the vertical loads.

Twelve reinforced concrete beams with stirrups were tested to determine their diagonal cracking strengths and ultimate shear capacities. At a constant shear span/depth ratio of 3.6, the stirrup shear strength was equal to 50, 100, or 150 psi (0.34, 0.69, or 1.03 MPa). Within each group the nominal concrete compressive strength varied from 3000 to 12,000 psi (21 to 83 MPa) in otherwise identical specimens. The ACI shear design method was found to be very conservative. A new equations presented to more accurately predict the ultimate shear capacity.