In general, the structural member using high-strength concrete is accompanied by high brittleness, which may result in the unexpected dangerous failure. For economy and safety, high-strength concrete may be used for compressive members (vertical members) and low-strength concrete for flexural members (horizontal members). ACI 318-89 recommends that when the specified compressive strength of concrete in the column is greater than 1.4 times that specified for the floor system, the column concrete shall extend 600 mm into the slab from column face to avoid unexpected failure. The structural behavior of beam-column joints with two different compressive strengths of concrete for the beams and the columns has not been investigated adequately. ACI-ASCE Committee 352 recommends that for joints that are part of the primary system for resisting seismic lateral loads, the sum of nominal moment strengths of the column sections above and below the joint ( M c), calculated using the axial load, which gives the minimum column moment strength, should not be less than 1.4 times the sum of the nominal strengths of the beam sections at the joint ( M b). Thus, those recommended values should be examined before high-strength concrete can be used with confidence and convenience in structural members. The results showed that the ACI 318-89 extension distance of 600 mm is safe at least for members up to 300 mm in total depth, and the 2h (h is overall depth of the beam) extension distance was found to be safe also for members under flexural loading with a column-to-beam flexural strength ratio of 1.8.

Describes the development of a new type of high-performance concrete incorporating large volumes of ASTM Class F fly ash. Briefly, this concrete incorporates about 56 percent fly ash by weight of cement, and has a water-to-cementitious materials ratio of about 0.32. The portland cement and fly ash contents are of the order of 155 and 215 kg/m 3 of concrete, respectively. The flow slumps are achieved by the use of large dosages of superplasticizers. Because of the low cement content, the temperature rise in this concrete is low, and this concrete is ideally suited for concrete structures where excessive temperature rise is a concern. Also, the high-volume fly ash concrete has all the attributes of a high-performance concrete. It has excellent mechanical properties and demonstrates superior resistance to freezing and thawing cycling, chloride-ion penetration, sulfate attack, carbonation, and marine environment. Also, it has low permeability, and shows excellent performance in reducing potential expansion due to alkali-aggregate reaction.

The troll GBS platform is the world's largest concrete offshore concrete platform. The platform is designed for an operational lifetime of 70 years and will be installed in the North Sea during 1995. To improve the buoyancy of the platform during tow-out to the field, a concrete mixture with reduced density has been developed, providing a characteristic 28-day cube compressive strength of at least 75 MPa and an in situ density of 2250 kg/m 3. The weight reduction has been obtained by partly replacing the natural coarse aggregates by high-quality lightweight aggregates. The concrete is denoted as modified normal density (MND) concrete. The modification was expected to reduce both compressive strength, Young's E-modulus, and material ductility to some extent. A comprehensive testing program comprising laboratory tests and full-scale tests has been performed to investigate and to document all relevant concrete properties related to mechanical, durability, and constructibility performance of the concrete. A secondary purpose of the investigations has been to evaluate the possibility of retaining the mechanical properties of the original normal density concrete by replacing the remaining coarse granite aggregate with a more rigid quartz-diorite aggregate. The laboratory investigations included the determination of the following concrete properties: fresh concrete properties, compressive strength development, compressive strength at sustained load, compressive E-modulus, tensile strength and E-modulus, stress-strain in compression, fatigue, fracture energy and characteristic length, shrinkage, creep, water intrusion, and alkali-silica reactivity.

Results of an experimental study on the structural behavior of exterior beam-column joints made of high-strength concrete and subjected to large reversal loads are presented. Variables examined were the joint shear stress and the ratio of transverse reinforcement. Based on the experimental results, it was shown that properly designed and detailed high-strength reinforced beam-column joints display ductile hysteretic behavior.

Two Canadian aggregates, a reactive siliceous limestone and nonreactive crushed granite, were evaluated for their potential alkali reactivity (AAR) in high-performance concrete. The concretes were proportioned to have high strength and cement content greater than 400 kg/m 3. Concrete mixes were made using a silica fume blended cement and a cementitious system in which 25 percent of a CSA Type 20 low-alkali cement was replaced by ASTM Class F fly ash. Also, control mixes were made with a CSA Type 10 high-alkali cement. The susceptibility to AAR of these concrete mixes was evaluated by casting concrete prisms and subjecting them to various accelerated storage conditions in the laboratory. For comparison purposes, mortar bars were also made, and tested according to the ASTM P 214 (1990) accelerated mortar bar test procedure. The AAR concrete prism tests performed in this study have shown that none of the concrete prisms made with silica fume blended cement and low-alkali cement incorporating fly ash showed significant expansion after 18 to 24 months of testing either in 1N NaOH or in exposure conditions of 38 C and relative humidity greater than 95 percent. The accelerated mortar bar test results, however, suggest that long-term testing may be needed to evaluate the effectiveness of blended cements in reducing expansion due to AAR, especially for highly reactive aggregates.