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.

Higher strength concrete which is defined to be that with uniaxial, 12000 psi compressive strength in the range of 6000 psi

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.

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.

High-strength concrete (8000 pounds per square inch or above) requires that a high level of structural integrity be achieved because of the demanding applications for which it is generally selected. natural limitations of As the inher-approached, the product are close control of materials production and placement is increasingly important. Statistical methods to provide such control are outlined in this paper.

Test results of a field experiment are presented where a 90 MPa (13 000 psi) silica fume concrete was used in the construction of an experimental column of a 26-storey highrise building. This concrete used a set-retarding agent in addition to a superplasticizer, had a water/cementitious ratio of 0.25 and was delivered at a slump of 250 mm (10 inches) after 45 minutes of travel. Maximum temperature was reached about 30 hours after mixing and was about 45°C (113°F) higher than the initial temperature of the fresh concrete. The thermal gradient inside the column was never greater than 20"C/m (21"F/ft) and no thermal stress problems were noted. Expressions of the modulus of rupture and modulus of elasticity, as a function of the compressive strength, are proposed. The 91 days shrinkage of this very high strength silica fume concrete was similar to that of plain concrete having a W/C of 0.40. In one concrete batch, due to a superplasticizer overdosage that resulted in an 18-hour set retardation, entrapped air macropores of 1.0 um size were created and caused a 10 MPa (1 450 psi) strength reduction at 91 days.