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.

The paper provides guidelines for the development of concrete mixtures that will function in an aggressive environment. Performance will be based upon durability and not high strength. In few cases will strength greater than 5000 psi (34.4 MPa) be of structural value. Some concrete has performed well in severe environments for more than 100 years while some newer concretes exposed to similar environments have deteriorated prematurely. Attention to basic concrete technology during the early years contributed to long-term durability. in the authors' opinions, emphasis on strength without regard to special needs for durability contributed to the current performance problems. Graphical means are suggested whereby the need for high performance concrete for durability can be identified by project type and environment. The requirements for various durability requirements are listed and summarized in a cross reference table that can aid in translating qualitative measures into quantitative terms.

Theodore Roosevelt Dam is a rubble-masonry dam, located on the Salt River, 76 miles northeast of Phoenix, AZ. The dam will be modified by adding a mass concrete gravity section to the downstream face of the dam. Over 350,000 yd 3 of mass concrete will be placed. A high-performance mass concrete mixture was developed that met conflicting low heat and strength development requirements. The mixture needed to meet thermal requirements of no more than 45 F total adiabatic temperature rise in 20 days, and less than 5 F adiabatic temperature rise after 20 days. In contract, the mixture needed to meet early-age compressive strength requirements of 1000 psi between 3 and 7 days and have sufficient paste to insure bond between the new concrete and the original masonry structure. The Bureau of Reclamation developed a concrete mixture with a 4-in. maximum-sized-aggregate (MSA), containing 270 lb of cementitious material per pubic yard that met design requirements. The cementitious material consisted of 80 percent cement and 20 percent fly ash. A low-heat, Type II cement was used, with a heat of hydration of 65 calories per gram at 7 days. The fly ash is an ASTM class F ash. The concrete has a water-to-cementitious materials ration of 0.53. The mixture is very workable, and reaches a compressive strength of 1100 lb/in.¦ in 7 days. It has a total adiabatic temperature rise of 43.4 F, with only 2 F temperature rise after 20 days.

Concrete is an essential component of the seal system planned for geologic repository under development for disposal of defense-generated radioactive wastes in the U.S. Performance requirements for concrete at this facility are unique: mass-concrete seals will be placed underground in a region where all the groundwaters are rich in chloride, and some also are highly concentrated in magnesium and sulfate ions. Sodium chloride in brines presents less of a problem than do other ions. In experiments simulating the worst-case of brine composition and availability, the nature and extent of deleterious chemical reactions were determined for materials being considered for use in mass concrete for a repository. Chemical degradation of cement pastes related to this concrete included loss of calcium and precipitation of magnesium compounds, and formation of other sulfate- and chloride-bearing phases. Calcium was lost first from calcium hydroxide and then from C-S-H. Strength loss is attributed principally to loss of these phases, and not to substitution of magnesium for calcium in hydration products.

To insure the tightness function of nuclear reactor containments, a special high-performance concrete (HPC) having a high silica fume content (30 kg/m 3) and a low cement content (270 kg/m 3) has been developed. The aim of this concrete formulation, which has a 28-day compressive strength of about 75 MPa and very good workability, is both to control the risk of cracking of the concrete in the structure and to reduce creep. This paper describes the feedback from experience acquired in the construction of the first HPC containment built in Civaux, France. The advantages and the difficulties encountered and overcome in the use of this material are presented, together with the results of tightness tests of the structure. The industrial mastery now achieved of this special HPC formulation also made it possible to take the performance of this concrete into account in the engineering of the work. This led to a new containment design, presented in this paper, combining HPC and very strong prestressing using 55 T 15 cables. This new design substantially improves the safety of nuclear reactors for severe accidents (core melting and hydrogen deflagration): the structure is guaranteed gas-tight up to an internal pressure of about 1 MPa.