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The commercial utilization of high-strength concrete with 60 to 120 Mpa compressive strength is a recent phenomenon; therefore, long-term field experience with regard to durability in corrosive environments is not available. In this paper, a critical review of the factors necessary to obtain high strength and high durability is presented. Typically, the concrete mixtures contain high cement content, low water content, and several admixtures, such as a superplasticizer, a pozzolan, and at times an air-entraining agent. When properly placed, consolidated, and cured, such mixtures should have low permeability and high durability to corrosive environments. However, there is some concern that microcracking in the aggregate-cement paste transition zone, possibly due to a variety of causes, may impair the impermeability and durability. The results of a recent investigation are discussed, which show that the aggregate type can play an important role in controlling the strength of the transition zone and, therefore, the degree of potential microcracking of concrete in service.

An investigation was conducted to assess the structural integrity of a 17-year-old precast prestressed concrete conveyor bridge used to transport sodium chloride rock salt from a storage building to an outside stockpile area. The stockpile, depending on storage requirements, quite often buried most of the structure and/or subjected it to sodium chloride dust. The investigation revealed that the structure had performed remarkably well, considering the small concrete cover used to protect the reinforcing elements and the inadequate consideration of structural cracking induced by unanticipated loading from stockpiled salt. The concrete strength of the single tee members was estimated to be 7000 psi (48 MPa), with cover to the stirrups varying from virtually 0 to 1 1/2 in. (0 to 38 mm) and cover to the prestressing strands varying from 3/4 to 2 in. (19 to 51 mm). It was observed that aggressive prestressing strand corrosion causing pitting and some brittle wire failures occurred locally at flexural crack locations in single tee column members with little corrosion activity immediately adjacent to the cracks, even after 17 years of aggressive chloride exposure. This observation seems to conflict with the prevailing theory of the role of cracking on corrosion--that cracks perpendicular to steel reinforcement should result in limited early localized corrosion but, with time, chloride ions penetrate even uncracked concrete and initiate widespread corrosion.

Concrete will be immune to the effects of freezing and thawing if: 1) it is not in an environment where freezing and thawing take place, i.e., where freezable water may be present in the concrete; 2) there are no pores in the concrete large enough to hold freezable water when freezing takes place (i.e., no capillary cavities); 3) during freezing of freezable water, the pores containing freezable water are never more than 91 percent filled, i.e., not critically saturated; 4) during freezing of freezable water, the pores containing freezable water are more than 91 percent full and the paste has an air-void system with an air bubble located not more than 0.2 mm (0.008 in.) from anywhere (L ó 0.2 mm), sound aggregate, and moderate maturity. Sound aggregate is aggregate that does not contain significant amounts of accessible capillary pore space that is likely to be critically saturated when freezing occurs. The way to establish that such is the case is to subject properly air-entrained, properly mature concrete, made with the aggregate in question, to an appropriate laboratory freeze-thaw test, such as ASTM C 666, Procedure A. Moderate maturity means that the original mixing water-filled space has been reduced by cement hydration so that the remaining capillary porosity that can hold freezable water is a small enough fractional volume of the paste so that the expansion of the water on freezing can be accommodated by the air-void system.

Long service life of concrete depends on correct choice and use of materials. Problems such as ASR (alkali silica reaction) and the prospect of sulfate attack and corrosion need early and proper identification and attention. Resistant materials must be selected and properly used to insure control of these adverse conditions. Low alkali cement or sulfate-resisting cement must be used as appropriate in these situations. Other requirements often overlooked are those essential to prevent or minimize thermal cracking of massive structural concrete, as in power plants, bridge piers, foundation elements, and thick linings of large tunnels. The ordinary concrete in municipal use, especially in new subdivisions, is often short of durability and exhibits much cracking, due to failure to follow the most fundamental rules of good practice, especially freezing weather protection, enough cement, control of slump, ample provision of joints, and curing. Sidewalks and driveways are too often disfigured and disappointing. Curing is often neglected. Specifications for the work must cite the requirements in complete detail and be followed explicitly when the work is done.

Twenty-seven high-performance concrete mixes (with 28-day strengths in the 80 to 100 MPa range) were prepared to evaluate the deicer salt scaling resistance of such concretes after various periods of curing. Three water-cement ratios (0:30, 0:26, and 0:23) were used, and for each water-cement ratio a minimum of three mixes were made with different air-void systems: one with a spacing factor of approximately 200 æ, one with a slightly higher value, and one without any air entrainment. Canadian Type 30 cement with an addition of 6 percent silica fume was used for all mixes. The coarse aggregate was a 14 mm minimum size, crushed, very dense, dolomitic limestone. The curing period varied between 1 and 28 days. A total of 54 specimens (2 for each test condition) were submitted to 150 daily cycles in accordance with ASTM Standard C 672, using sodium chloride as a deicer. Weight loss was measured to evaluate the deterioration of the concrete surfaces. The scaling resistance was found to be extremely good in all cases, irrespective of the length of curing, water-cement ratio, or spacing factor value, weight losses after 150 cycles being always lower than 0.50 kg/mý. No correlation was found between the scaling resistance and the spacing factor or the length of curing. Loss of mass was generally concentrated around a few aggregate particles. These results indicate clearly that it is possible to prepare high-performance concretes with very good deicer salt scaling resistance without using any air entrainment.