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In Japan, a new super-workable concrete, which has higher flowability and filling capacity, has attracted attention as being effective in rationalization of concrete execution. It can be applied for simplifying placing work while securing high quality of reinforced concrete structures. Especially in case of heavily reinforced structures, it is highly applicable because of its excellent filling capacity or lower consolidation effort. For several years, the authors have studied improvements of workability of some special concretes, such as anti-washout underwater concrete, expansive grouting concrete for inverted placement, and ultra high-strength in-site concrete, and have consequently succeeded in developing super-workable concrete, suitable for rapid placing or perfect filling without consolidation. The authors also have established a new evaluating method for segregation resistance of mortar and aggregate, that is useful to design mix proportion, or keep high quality of super-workable concrete in site. Recently, opportunities to apply super-workable concrete to several actual structures with difficult construction conditions have arisen. One is the LNG (liquefied nitrogen gas) in-ground storage tank, which has much complicated reinforcement at the junction of base mat and side wall, another is a tall, thin reinforced concrete wall, which must be placed from upper point, 6 to 8 m in height. This paper describes the basic properties of super-workable concrete, the new method of quality control, and a summary of applications to reinforced concrete structures mentioned.

SP-140 Many recent innovations in advanced concrete materials technology have made it possible to produce concrete with exceptional performance characteristics. Recognizing the need to encourage the development of such high performance concrete technology and to expedite its transfer into practice, the ACI Technical Activities Committee formed a Subcommittee on High Performance Concrete (THPC) in 1992. High performance concrete is defined by THPC as concrete which meets special performance and uniformity requirements that cannot always be achieved routinely by using only conventional materials and normal mixing, placing, and curing practices. The requirements may involve enhancements of placement and compaction without segregation, long-term mechanical properties, early-age strength, toughness, volume stability, or service life in severe environments.The Symposium on High Performance Concrete in Severe Environments held at the ACI Fall Convention in Minneapolis, Minnesota on November 9, 1993, is the first formal activity organized by THPC. Co-sponsored by RILEM, the symposium emphasizes field applications. This volume contains 14 papers, of which 13 have been scheduled for presentation at the symposium.

Underwater repairs to, and rehabilitation of, existing reinforced concrete velocity caps of the circulating water intake structure at St. Lucie Powerplant, Fort Pierce, Florida were made utilizing high-performance in a marine environment. Use of this repair technique avoided the necessity of constructing a cofferdam for repair work in the dry, and thus minimized interruption to plan operation, and resulted in considerable savings. Mix proportions for the high-performance concrete included cement, fly ash, silica fume, and antiwashout admixtures as well as high-range water-reducing and set-retarding admixtures. The mix proportions were tested extensively in the laboratory and field conditions to optimize the slump and the initial setting time of concrete while assuring early compressive strength requirements for conformance with the specified requirements. Large scale mock-up tests, utilizing both tremie and pumping methods, were conducted to simulate under water placement in the surf zone and to select the actual concrete placing method, rate of placement, and to identify surface preparation and protection requirements. Construction procedures for the new reinforced concrete slabs involving approximately 3000 yd 3 precast and tremie concrete utilizing a barge-mounted concrete batch plant; quality control and post-placement inspection measures are also discussed.

A super-workable concrete, which has excellent deformability and resistance to segregation and can be filled into heavily reinforced formwork without vibrators, was developed. This new type of concrete is made not only with the general materials for concrete such as ordinary portland cement, aggregates, water, and air-entraining water-reducing agent, but also with blast-furnace slag, fly ash, superplasticizer, and a newly developed viscosity agent. When the slump of this super-workable concrete is tested, diameter of the flow is more than 60 cm. Since the super-workable concrete has excellent durability as well as superior filling ability, it should be a proper concrete for projects under severe conditions. The super-workable concrete was employed in the construction work of a 20-story building. It was placed in the center-core from the basement to the third floor. The building was designed as a hybrid structure, in which the reinforced concrete core was surrounded by the steel structures. The specified design strength of the concrete was 480 kgf/cm 3 (47.1MPa). The greatest nominal diameter of deformed bars was 51 mm, and they were very congested. The super-workable concrete was produced in ready mixed concrete plants near the construction site, and 1500 m of the super-workable concrete was placed successfully. Through this project it was confirmed that the super-workable concrete can be supplied from general ready mixed concrete plants with practical care of quality control in materials.

Three types of High Performance Concrete (HPC) for highway applications were investigated: Very Early Strength (VES), High Early Strength (HES) and Very High Strength (VHS). Two of the objectives of the research were to measure the chloride permeability of these concretes and explore an alternate method using AC impedance. Many of the concretes had coulomb values of 4000 and higher, placing them in the "high permeability" category as specified by AASHTO T 277 - Rapid Chloride Permeability Test (RCPT). Coulomb values were also found to decrease with concrete age and with increased silica fume content. Coulomb values were found not to vary significantly with dosage of calcium nitrite used as accelerator, up to 6 gal/yd 3 (29.7 l/m 3). The AC impedance test results (ohms) were found to correlate well with the RCPT results (coulombs) and were sufficiently accurate to place the concretes in the proper chloride permeability category. The advantages of the AC impedance test are that it is faster and less expensive than the RCPT and it avoids the potential heating problem sometimes encountered in the RCPT. AC impedance was found to increase with concrete age and with increased silica fume content and decrease with increased calcium nitrite dosage.