International Concrete Abstracts Portal

Presents an evaluation of the effects of temperature and type of superplasticizer--high-range water-reducing admixture) (HWRA)--on there rate of chloride-ion diffusion through hydrated superplasticized ordinary portland cement (OPC) pastes and opc + fly ash (PFA) pastes. OPC pastes and OPC/PFA pastes with a water-cementitious materials ratio of 0.31 containing a HRWRA were prepared in the laboratory and cured at 21, 30, and 45 C for different lengths of time, so as to obtain approximately identical compressive strengths. Five different types of HRWRA were used in the study. At the end of the curing period, the specimens were placed in diffusion cells maintained at 30 C and the amount of chloride passing through the pastes was measured. Measurements of total porosity and pore-size distribution were made using helium pycnometry and mercury-intrusion porosimetry. The variations of chloride diffusion coefficients arising from changes in the temperature of curing and type of HRWRA are presented and discussed.

SP-139 The International Symposium on "How to Produce Durable Concrete in Hot Climates," sponsored by Committee 201 on Durability of Concrete, was held at the ACI Fall Convention in San Juan, Puerto Rico in October 1992. Altogether, ten papers were presented at the two sessions of the symposium. Approximately seventy-five persons were in attendance at each session. The symposium was noted for its international flavor and the variety of topics presented.

Concrete quality and strength are strongly affected by the curing procedures used in the initial days after concrete is cast. The relative advantage of any particular method and the strength-gain relations must be understood to assess concrete strength and quality adequately. In the present study, the strength gain with age of concrete up to 1 year with different compressive strengths and under different initial and subsequent curing conditions in warm and high-humidity climates was determined. The initial curing techniques evaluated were those most widely used in practice and were intended to represent actual, imperfect construction practice. The subsequent curing conditions were artificially modeled to simulate dry and rainy climates. Of the curing methods evaluated, ponding was found to be the most effective, followed by intermittent sprinkling, unsealed plastic covers, and curing compound. An initial period of three days can develop adequate concrete strength in Puerto Rico's climate; this has a good influence on concrete strength gain with age. Slabs that were kept indoors, and had no contact with water, showed in all cases a decrease in strength under saturated conditions at an age of 1 year, while those maintained outdoors with a rainy climate showed a continuous gain in strength with increasing slope at all times and developing strengths higher than the 28-day strength.

Concreting of thick sections involves large volume placements for cases such as foundation rafts and beams of exceptional dimensions, which frequently occur in highrise construction in Singapore. The heat of hydration generated in using concrete of structural grade is much higher than that associated with mass concreting of dams. For any thick section, The temperature differential between the warmer interior and the cooler surfaces gives rise to different thermal strains which may be sufficiently high to result in cracking. In hot climates, it also leads to very high peak temperatures (often above 70 C). Typical case histories of such placements in tropical climates are presented, including the mixtures used and the overall dimensions of the members. For two of the cases, the measured temperature histories are compared with those from numerical simulation using finite elements. The requirements of concrete mix design and precautions to be considered in relation to the planning and execution of large placements as well as the use of insulation to control temperature differentials are discussed.

The best time to place quality concrete is during cold weather, as long as the concrete is prevented from freezing. Why is it so hard to place quality concrete during hot climate conditions? The culprits are concrete temperature, air temperature, humidity, and wind velocity. There are secrets that can drastically improve the present quality of concrete placed in hot climates. This paper discusses how to cope with hot weather conditions and still produce high-quality concrete. Concrete in hot climates is affected by water demand, rapid setting times, and the resulting ultimate strength reduction. Understanding these detrimental effects and how to overcome them can result in high-quality, durable concrete. Many of these effects can be overcome by using the proper chemical or mineral admixture, but using techniques slightly different than the usual. There may be times when an accelerating admixture, or insulation, may be used effectively even in hot climates. Relatively high concrete temperatures may be appropriate to obtaining durable concretes.