International Concrete Abstracts Portal

Highway structures ranging in age from 4 to 12 years and constructed from concretes incorporating high-range water reducers were surveyed for extent of surface scaling. Core samples were obtained from selected areas. Samples were subjected to petrographic analysis and linear traverse measurements. Air-void systems were characterized using standard ASTM C 457 parameters, chord distribution analysis, "Philleo" factors, and modal chord lengths. Statistical models relating degree of surface scaling to various combinations of parameters were tested. A three-parameter linear model incorporating spacing factor, water-to-cement ratio, and alteration of air voids in near-surface regions was found to yield the highest level of correlation with observed scaling.

Concrete durability has become a matter of great practical significance. It is particularly critical to reinforced and prestressed structures where lack of concrete durability can lead to lack of structural stability and integrity through steel corrosion. The two significant factors affecting and controlling concrete durability are permeability and porosity. While nonadmixture low w/c concrete can be durable, this paper puts forward the thesis that it is superplasticizers that can insure long-term durability of concrete and concrete structures. For too long, water-reducing agents, plasticizers, and superplasticizers have been looked upon as workability/pumping agents with possible savings in cement and increases in compressive strength. It is suggested that this concept of these admixtures is misleading and ill-informed. While good workability is recognized as an essential component of placing and compacting, the more critical role of superplasticizers should be seen to reduce the porosity and sorptivity of concrete through water reduction. Paper presents test data on concrete and mortar mixes with blended cements and superplasticizers and having water-binder ratios of 0.35 to 0.40. The properties of these concretes are presented and discussed in terms of strength development, permeability, pore structure, carbonation, and microstructure. It is shown that superplasticizers should be seen as agents of concrete durability rather than as agents of concrete workability.

Steel is used widely in reinforced concrete for its structural properties and because the alkaline environment normally protects the steel from corrosion. However, this alkalinity does not protect steel in the presence of chloride ions. Furthermore, in environments subjected to freezing and thawing, durability can also be affected severely. The corrosion resistance of embedded metals can be improved by the use of concrete admixtures. Calcium nitrite improves corrosion resistance by promoting passivity of metals in concrete. Superplasticizers reduce chloride ingress by allowing the use of lower water-cement ratios. Microsilica (silica fume) substantially increases concrete resistivities as well as lowering permeability to chloride. In this paper it is demonstrated how various combinations of these admixtures improve the corrosion resistance of steel in concrete, while giving greatly improved strengths, necessary freeze-thaw resistance, and handling properties conducive to rapid placement and consolidation.

Atomic Energy of Canada Limited is undertaking a research and development program on cement-based grouts for possible use in sealing an underground nuclear fuel waste disposal vault. Silica fume and superplasticizer were added to a finely reground sulfate-resistant portland cement to produce a durable, low-permeability grout that would penetrate very fine fissures in granitic bedrock. The superplasticizer additive permits very low water-cement ratio grouts (w/c less than 0.6 by mass) that exhibit no segregation or bleed. The silica fume additive contributes to improved chemical stability and leach resistance of the grout. The developed grout has been injected into granitic rock at AECL's Underground Research Laboratory in Canada and at the NEA/OECD Stripa Facility in Sweden. No problems were encountered in the field trials in mixing, handling, or pumping of the grout. The injected grout produced only a very limited geochemical signature in the ground water and appears capable of penetrating microfissures in the granite with apertures of less than 20 æm.

The construction procedure of a nuclear containment concrete structure recently built required a highly workable concrete to facilitate placing around heavily congested steel embedments and reinforcements. Use of a superplasticizing admixture was considered essential. A test program was undertaken with several commercially available admixtures to study the properties of the fresh and hardened concrete made with low-heat cement (CSA: A5 Type 40). Under controlled conditions, four of these products met the construction requirements without adversely affecting the properties of the concrete.