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.

On April 13, 1992, the engineer of the Merchandise Mart, one of Chicago's downtown buildings, reported flooding of the building basement. A few hours later flooding was found to be related to an eddy observed at the Chicago River. The flooding was occurring through a system of service tunnels built at the beginning of the century and abandoned in the late 1940s. The failure of the tunnel was caused by wood pilings installed at the end of 1991 to protect the bridge abutment in the Chicago River. The flooding of these tunnels affected more than 100 downtown Chicago businesses, which had to be evacuated for several weeks. The repair of the tunnel was conducted in two stages using high-performance concrete (HPC). First, an interim plug was placed using a high-performance, underwater concrete. The severe environment caused by the current in the tunnel required concrete to be highly fluid, have anti-washout properties, set quickly, and gain strength rapidly Second, a permanent plug was placed using HPC concrete designed to reduce heat of hydration and minimize potential for thermal cracking. Actual temperature of the permanent plug was monitored by thermocouples and compared to a computer-generated model. The use of this system to predict performance of special concretes allowed the concrete supplier to start a new generation of high-performance concretes.

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.

A high performance concrete (HPC) mixture was developed in the laboratory and later used in a bridge construction project. The HPC mixture was designed based on 752 lb (341 kg) of cement with 0.33 water-cement ratio. The weight of the cement was partially replaced by fly ash (20 percent) and silica fume (8 percent). The concrete mixture incorporated 4.5 gal./yd 3 (22.3 L/m 3) of calcium nitrite corrosion inhibiting admixture. Other chemical admixtures included air-entraining agent, and/or standard and high range water-reducing/retarding admixtures. A wide range of field and laboratory tests were performed on fabricated concrete specimens, as well as on cores from field models and newly cast bridge members. The main tests included field and laboratory testing of permeability, and compressive strength. Results of tests on laboratory and field concrete were very close. The chloride permeability AASHTO T277) of the HPC was very low, ranging between 618 to 1055 coulombs. The compressive strength was high, ranging between 8600 to 10,670 psi (59 to 74 MPa). This study shows that laboratory produced HPC with multiple cementitious materials and chemical admixtures can be successfully implemented in construction without compromising its durability. It is also demonstrated that sacrificial concrete models cast and cured at the job site can provide accurate evaluation of the durability and performance of newly cast structures. The study also emphasizes the need to test the permeability as well as strength for more precise assessment of concrete durability.

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.