International Concrete Abstracts Portal

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.

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.

Paper acquaints those interested in concrete durability with the scope and duration of a new long-term field and laboratory testing program which began in 1989 and will continue through 2004. It has been commissioned by the Reinforced Concrete Research Council (RCRC) of the American Society of Civil Engineers, and is designed to compare the effects of warm and cold seawater environments on the durability of reinforced and prestressed concrete elements made using concrete materials and additives which have become available over the past 15 years. It is a follow-up study to those conducted by the U.S. Army Corps of Engineers, and guided by the RCRC, during the period 1950 through 1976.