Evaluates cracking resistance for concretes with compressive strengths between 60 and 110 MPa, including superplasticizers and/or condensed silica fume. Two types of concrete ring with 81 cm external diameter are tested and their shrinkage is measured over time. The first ring is cast around an aluminum ring, shrinkage-induced strain is measured, and the strains are subsequently transformed into stresses based on the theory of elasticity and knowledge of the elastic constants of aluminum. After some days, the ring breaks and the rupture stress by restrained deformation of the concrete is determined. A second concrete ring is cast, but without the internal metal ring. For this ring, measurement is made of the free shrinkage of the concrete. The value of the stresses and strains, in conjunction with the compressive and flexural strength, creep, and coefficient of hygrometric permeability (measured in other test specimens) are measured. Based upon available test data, the superplasticizer raised the mechanical strength but reduced the cracking strength of the concrete. The joint introduction of the superplasticizer, together with condensed silica fume, raised the mechanical strength of the concrete even further, but also increased its cracking resistance. To explain the test results, it is necessary to resort to the coefficients of hygrometric permeability and stress gradients, responsible for a reduction in the rupture stress of the concrete, which is higher in the first case than in the second.

Reports on high-strength lightweight and normal weight concretes. Sintered fly ash lightweight aggregates, crushed limestones, and two types of cement with different contents were investigated. All the concretes contained silica fume and a high-range water-reducing admixture. To obtain high specific strengths (i.e., ratio of strength to relative density), lightweight concretes were prepared with only lightweight particles (coarse and fine), reaching strengths higher than 60 MPa with density of about 1700 kg/m3. The results of physical (permeability, thermal conductivity, thermal diffusivity, and thermal expansion coefficient) and mechanical (compression, direct tension, direct shear, modulus of elasticity, bond strength, fracture energy, and compression softening behavior) tests, carried out on specimens cured for different ages at two curing conditions (20 C and 95 and 50 percent relative humidity, respectively), are reported and discussed.

Reports results of a study undertaken to develop high-strength lightweight concrete having compressive strength of about 700 MPa and a density of less than 2000 kg/m3. The materials used consisted of an expanded shale lightweight aggregate of Canadian origin, ASTM Type III cement, low-calcium fly ash, and condensed silica fume. A series of 7 concrete mixtures involving 14 concrete batches were made. The cement or cementitious material content of the mixtures ranged from 300 to 600 kg/m3. All mixtures were air entrained and superplasticized. A large number of test cylinders and prisms were cast for the determination of mechanical properties and drying shrinkage of concrete. From the results of this investigation, it is concluded that concrete with a compressive strength of about 70 MPa at 365 days and density of less than 2000 kg/m3 can be made incorporating supplementary cementing materials. The highest compressive strength achieved was 69.3 MPa at 365 days for a mixture with a cementitious material content of 600 kg/m3 of concrete; the highest flexural strength obtained was 8.7 MPa at 28 days.

When high-strength concretes are used in high-rise buildings, long-span bridges, and offshore structures, special attention must be given to the dimensional changes that occur in the concrete members. For design purposes, the length changes are usually considered to consist of instantaneous shortening, shrinkage, and creep. Instantaneous shortening depends on stress level, cross-sectional dimensions of the member, and modulus of elasticity of steel and concrete at the age when the load is applied. Shrinkage deformations generally depend on type and proportions of concrete materials, quantity of water in the mix, size of member, amount of reinforcement, and environmental conditions. Creep deformations depend on concrete stress, size of member, amount of reinforcement, creep properties of concrete at different ages, and environmental conditions. In recent years, questions have been raised about the validity of methods for calculating deformations in high-strength concrete members and the in-place properties of high-strength concrete members. These properties include compressive strength, modulus of elasticity, shrinkage, and creep. This paper reviews existing state-of-the-art technology concerning instantaneous shortening, shrinkage, and creep of high-strength concrete members.

Compressive strength and water penetration of three grades of high-strength concretes with cement contents ranging from 400 to 500 kg/m3 and a proprietary superplasticizer are reported. The control mixes were redesigned by adding a Class F-type fly ash at fly ash/cementitious ratios of 0.15 and 0.35. All concretes were designed for a similar workability. The strength development was monitored in three curing regimes. It is concluded that the superplasticized concrete developed a higher strength than that predicted from a reduction in the water/cement ratio. The curing conditions significantly influenced the strength development and the water penetration of the concretes. An optimum fly ash/cementitious ratio of 0.15 was found to be appropriate for the concretes; larger amounts of fly ash were found undesirable for higher strength development.