Concrete is no longer produced as a rational mixture of just cement, stone and water. It has matured to a mixture which includes both chemical and mineral admixtures. A mixture of admixtures is used to obtain special concretes that are characterized by very high flow, good cohesion and self-leveling properties, that can be place underwater without the aid of tremies, that expand and induce compressive stresses or reduce shrinkage, and can be placed and cured at sub-zero temperatures. The use of a mixture of admixtures I a single mix Specific incompatibilities between certain admixtures are cited and the manner of resolving such problems discussed.

The Second CANMET/ACI International Conference was held in Brazil in 1999 and showcased information on emerging high-performance concrete in Brazil and other South American countries. Over 100 papers were submitted from all over the world and were reviewed in accordance with ACI policy. Forty-five were accepted for publication in this volume. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP186

A 12.9 km bridge was recently constructed across the Northumberland Strait in Eastern Canada, connecting Prince Edward Island (Canada's smallest province and major tourist area) with the mainland. The bridge is a precast, post-tensioned segmental box grinder structure. The bridge is believed to be the longest highway structure over seasonally frozen water. Durability concerns which had to be addressed in the design and construction of the bridge included abrasion of ice on the piers in late winter and early spring, corrosion of reinforcement in a marine environment, alkali-aggregate reactivity, sulphate attack, freezing and thawing resistance of concrete in a saturated condition, salt scaling and control of thermal cracking. The design life of the bridge was 100 years, a first of this magnitude in Canada. The paper discusses some of the unique features of the bridge and the selection of concrete mixture proportions to meet the durability requirements of bridge components.

For high strength concrete, the use of superplasticizer and high cement leads to a very low W/C. In this concretes, part of cement used cannot complete its hydration due to the unavailable space to locate the reaction products. In this case, the addition of limestone filter can modify the packing of cement grains and it increases the hydration degree of portland clinker leading to the same strength level. In Europe, a vast experience has been developed with limestone blended cements. However, the use of limestone filler is a recent practice in South American countries. In this paper, the effect of a limestone filler in the production of high strength concrete was studied using 0, 9.3, and 18.1 percent of replacement by mass of clinker. Concretes were design to achieve 50 MPa compressive strength at 28 days (450 kg/m3 of cement content and a water-to-cementitious material ratio of .34). The mechanical properties, including compressive strength, split tensile strength and elasticity modulus were evaluated at 1,3,7,28 and 150 days. The results show that limestone filler increases concrete strength at very early ages due to enhance the clinker grains hydration. It did not affect significantly the mechanical strength of concrete containing 9.3 percent of replacement while for concretes with 18.1 percent of filler, a relative strength reduction was observed after seven days. At all ages, limestone filler replacement improves clinker efficiency but, it decreases with hydration progress.

The chlorides from the sea and the marine breeze are the main source of corrosion in marine environments. Their penetration into concrete occurs through capillary absorption, diffusion or a mix of both. A chloride threshold for producing reinforcement corrosion can be predicted through mathematical models but no reliable results may be obtained if the action of environmental agents as the RH, temperature, winds, rains, and drying periods are not well known. Although these limitations are recognized in several works, there are few field data in the literature to support, according to different exposure conditions, the form of the chloride penetration profiles. This work presents the form of chloride profiles from different exposure conditions. It discusses their behavior and justifies the results according to the presence of the environmental conditions. Some of the results indicate that the environments with chloride saturation produce profiles with a well defined concentration gradient, while those with strong periods of rains, drying, winds, as well as strong variations of RH and temperature show a two-zone profile. In the last case, one zone is close to the concrete nucleus where the supposed reinforcement is positioned and that stays dampened due to the high chloride concentration, and the other one is close to the concrete surface, in which continuous wetting and drying cycles take place. It was also found that, under this research trial, the chloride penetration mechanism did not change with the micro-climate but nucleus concentration changed with the distance from the sea and concrete quality.