International Concrete Abstracts Portal

SP-121 The Second International Symposium on the Utilization of High Strength Concrete was held in Berkeley, CA, May 1990. A substantial amount of research work and project construction with high strength concrete was completed since the last Symposium. Recent findings were presented and discussed.

Deals with the design of a concrete capable of increasing the airtightness of the primary containment of nuclear power stations. The general context of structures of this type and the types of damage commonly found in them (thermal cracking) are introduced. Then an ideal concrete is described and an attempt is made to approximate it by applying a rigorous formulation process. The result is a high-strength concrete having a low cement content (270 kg/m3), a 28-day strength of about 70 MPa, and a high workability through the use of silica fume and calcareous fillers. This concrete and a more conventional concrete are put through a series of characterization tests which makes it possible to conduct numerical simulations of the temperatures and restrained deformations in the containment. The reduction of the risk of thermal cracking is clearly demonstrated. Finally, all of these laboratory investigations are verified on a full-scale containment element, in which all the benefits of using this new type of high-performance concrete appear (temperature rise cut by 25 percent, near disappearance of cracking, tenfold reduction of airleaks). The advantages of such a concrete are not restricted to the nuclear context, but cover all applications for which a dense, crack-free concrete is desired.

Two test methods were used to investigate the frost resistance of high-strength concrete with and without air-entraining agents: a volume deterioration method (ASTM C 666) and a salt-scaling method (SwedishStandard SS137244) similar to ASTM C 672. In addition, low-temperature calorimetry was used to measure ice formation in concretes after a drying/resaturation treatment. For concretes with 0 and 10 percent silica fume contents and water-binder ratios from 0.40 to 0.25, the calorimetry results showed only very minor ice formation down to 20 C. The cement used was a high-strength type (Norwegian P30 4A). This result contrasts an earlier calorimeter result with ordinary portland cement, and indicates that the P30 4A cement produces a more finely divided capillary pore structure. The salt-scaling tests showed that the high-strength concrete with water-to-binder ratios less than about 0.37 exhibits acceptable resistance to salt-scaling, even without air entrainment. The ASTM C 666 test results showed relatively severe damage to concretes with water-to-binder ratios down to 0.28. No air-entrained concrete was tested with ASTM C 666. This result is in apparent conflict with the calorimetry results and suggests that the damage may be related not to ice formation but to thermal fatigue effects caused by differences that are too large between the thermal expansion coefficients of aggregates and binders.

Presents data at ages up to 1 year on the strength development characteristics of high-strength concrete ( > 80 MPa) incorporating blast furnace slag and/or silica fume or high volumes of ASTM Class F fly ash. Six concrete mixtures of various compositions were investigated in this study. Five of these mixtures had the same cementitious materials content of 485 kg/m3 of concrete, and the sixth mixture was typical of high-volume fly ash concrete incorporating a cement content of 150 kg/m3 of concrete and large volumes of fly ash. The concrete was obtained from a commercial ready-mixed concrete plant. For each mixture, three types of structural elements simulating a thick wall, a thin wall, and a thick column were fabricated for testing under field curing conditions. Cores, 100 x 200 mm in size, were drilled at ages up to 1 year for determining the in situ compressive strength of the various concrete elements. In addition, a number of 150 x 300 mm cylinders were cast from each mixture for long-term strength testing. The test results indicate that compressive strengths approaching 100 Mpa at 1 year can be achieved using a superplasticizer, with or without the use of supplementary cementing materials. The moist-cured test cylinders and the drilled cores from the various concrete elements indicate continued gain in strength of concrete at ages at least up to 365 days. The use of silica fume is generally required if high early-age strengths are to be achieved in structural elements. However, if high early-age strength is not a critical factor, then the high-volume fly ash concrete seems to be the most promising system.

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.