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.

SP87 High-strength concretes are used frequently in applications requiring slender members to carry large loads or span long distances. Early applications of high-strength concrete emphasized its use to reduce column dimensions. More recently, high-strength concrete has been used to meet special project objectives such as large composite columns and stiffer structures. In turn, the use of high-strength concrete has prompted the applications of more stringent quality control requirements. This publication highlights the use, design implications, and research results of applications of high-strength concrete.

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.

Research at Cornell University over an eight, year period, on concrete with comprehensive strenght in the range from 6000 to 12,000psi 41-83MPa) has established a good basic for understanding the fundamental nature of the material and has also provided information on engineering properties such as moduls of elatisity, tensile strength, creep coeficient possion, ratio, rate of strength gain with age, and strain limit values. Some of these are reviewed briefly. The main purpose of the paper is to summarize more recent Cornell research dealing with the behavior of reinforced and prestressed concrete structural members, made using high strength concrete. Test have included axially-loaded members with varying amounts of spiral confinment steel, flexural critical beams with varying amounts of tensile and compressive reinforcement, and stirupps, reinforced concrete beams under sastained load of 3 years duration, shear critical reinforced concrete beams. It was found that while many provisions of the 1983 ACI code are applicable to high strength concrete materials and members certain code provisions must be reexamined, modified, or limited to insure structural saftey and servability.

The paper presents a comprehensive review of the material properties and structural behavior of high strength concrete. It is shown that in practice both early development of high strength and high final strength are desirable. Further, if such concretes are to be used economically, a high proportion of their strength needs to be utilised in design. Data are presented to show that by careful selection of the type of cement and design of mix proportions, strengths of 60 to 80 N/mm2 could be obtained with normal weight aggregates in 24 hrs. With light-weight aggregates, strengths of LO-25 N/mm2 in 12 hrs. and of 25-45 N/mm2 in 24 hrs. are reported. The paper then discusses the engineering properties of such concretes such as elasticity, shrinkage and creep. The implications on structural behavior, when high working stresses of 30 to 50% of the cube strength are used, are then discussed in terms of transmission length, prestress losses, short term structural behavior and longterm structural behavior. Particular emphasis is given to those aspects that need to be considered in design.