International Concrete Abstracts Portal

The ultimate test of concrete durability to natural weathering is how it performs in the environment in which it is to serve. Laboratory testing yields valuable indications of service life and durability. However, the potential disrupting influences in nature are so numerous and variable that actual field exposures are highly desirable to assess the durability of concrete exposed to natural weathering The U.S. Army Corps of Engineers, through the Waterways Experiment Station, Structures Laboratory, maintains a natural weathering exposure station. It is located on Treat Island in Cobscook Bay near Eastport, Maine. This station has been in use since 1936 and is an ideal location for exposure tests, providing twice-daily tide reversals and severe winters. The average tidal range is about 18 ft (5.4 m) with a maximum of 28 ft (8.5 m) and a minimum of 13 ft (4 m). In the winter, the combined effect of air and water temperatures creates a condition at meantide where specimens are repeatedly thawed and frozen. There have been 23 completed investigations and many of these have been previously reported. There are currently 40 active investigations. Four of these investigations are briefly discussed in this paper.

The No. 1 ore dock at Great Lakes Steel Division's Zug Island facility was originally constructed in 1909. Damage caused by freeze-thaw cycling, abrasion wear, severe impact loadings, and reinforcing steel corrosion resulted in a need for repair and rehabilitation. Multiple Dynamics Corporation conducted extensive condition surveys and testing to develop repair strategies for this structure. The remaining service life was then predicted to assist in economic planning. This case history provides an excellent example of concrete performance in an aggressive environment.

During the 12 years since construction of the bridge, cracking and spalling have developed in the concrete superstructure, predominantly on the underside of the bridge deck in the area of expansion and construction joints. The evidence indicates the deterioration was initiated by leakage of expansion and construction joints, and that poor performance should be attributed to design and construction practices whose effectiveness falls short of the environmental demands. Moisture, deicing salts, and debris that spill through the joints had deteriorated concrete at an accelerated rate and penetrated to the reinforcing steel. The concrete breakdown caused by corrosion of reinforcing steel, as well as from freezing and thawing action, and the expansion resulting from alkali-aggregate reaction damaged the bearing areas of cantilever spans and adjacent parts of suspended slabs, and was a cause for concern for the bridge's structural integrity. The paper addresses the main factors involved in the initiation phase of the corrosion mechanism: carbonation, chloride diffusion, and water penetration into concrete. The selected materials and methods are discussed, as well as importance of compatibility of materials for durable repairs. The paper outlines a need to integrate knowledge and understanding of the mechanism of deterioration with concrete design, materials, and methods of repairs.

A characteristic example of reinforced concrete structural damage in an urban environment after 25 years' service is the east end of a stadium in Zagreb, Yugoslavia, for 11,000 spectators. This paper presents research works that served as a basis for the design of repairs to prolong the structure's service life. The damage is classified by types. The basic causes of the damage are explained with a detailed description of the influence of carbon dioxide from the air on the concrete. The repair design is described. The basic principle in repairing the upper and lower surface of the stand was that the materials and construction methods must be compatible with the existing concrete and also meet durability criteria. The repair design prescribes conditions for the materials, construction methods, and durability criteria. The paper presents preliminary investigations to select the optimum composition of a mortar that complies with the criteria required by the design. The influence of two polymer dispersions based on acryl and latex, as well as the influence of silica fume added to the mortar, are investigated. To repair the stand slab, the selected mortar applied was the cement mortar modified by added silica fume and superplasticizer to obtain a dense and compact composition and increased chemical resistance. The proposed solution for the lower surface was shotcrete improved by special admixtures. In designing the overlay, care was exercised that the additional load should not require strengthening of the stand structure. Acceptance of the repair work performed is outlined.

Chloride-induced corrosion is a problem common to steel reinforced concrete exposed to chloride ions. A severe case is the use of reinforced concrete in seawater. The high-chloride concentration in salt water, the geometry of concrete piles, and the moisture differential between concrete above and below the water line are all factors that complicate the problem. The corrosion resistance of steel reinforced concrete is a function of the concrete cover of the steel, concrete permeability, surface chloride concentration, and ambient temperature. In this paper, the authors present diffusion curves for chloride ingress into concrete piles. The diffusion coefficients are based on extensive laboratory and field studies. They also discuss the usefulness of this model, based on Fick's law of diffusion. By estimating the chloride ion concentration at the steel reinforcement after a given amount of time, the lifetime of the structure can be predicted. In addition to concrete quality, concrete admixtures affect the corrosion of steel in concrete. Two concrete admixtures are discussed--calcium nitrite and microsilica. As demonstrated in other publications, both of these additives delay the onset of corrosion. It has also been shown that calcium nitrite affects the rate of corrosion upon initiation. The appropriate dosage of each admixture can be determined using the chloride diffusion curves. Examples are described in the paper.