A research program has been promoted in order to study the compositions and properties of High Performance Concretes to be produced in France. Concretes with a characteristic compressive strength of 50 to 60 MPa (7 000 to 8 500 psi) on cylinders tests may be fabricated every where in France at a reasonnable cost, by selecting aggregates, cement and superplasticizers. The Brittle behaviour of such concretes under loads action needs a reevalua-tion and a possible adaptation of the codes of design. The other characteristics of this material are significantly improved : reduction of creep and shrinkage, increase of compacity. The gain to be expected from a generalized use of such concretes results from short time characteristic, by decreasing the member sections and improving the rate of utilization of shuttering, and from long time characteristic by increasing durability. However a complementary program is now in progress in order to clear some uncertain-ties and to design an experimental structure.

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.

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.

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.

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.