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.

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.

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.

Reinforced concrete masonry structures can be effectively used in corrosive environments provided that the design is based upon a rational assessment of the exposure condition. An investigation of wall that had 6000 g of muriatic acid and 11,000 g of sodium hypochlorite stored along its exterior face indicated accelerated deterioration of the wall due to inadequate design and no protection afforded to the wall when the building's usage was changed from general warehouse to chemical storage. Poor construction practices also contributed to the distressed condition. The investigation utilized electrical, visual, and chemical means of assessing the structures's condition. The primary tool was a copper-copper sulfate (Cu-CuSO4) half cell conforming to ASTM C 876. The resulting equipotential contour map provided valuable information regarding the wall's corrosion potential. Visual observations of exposed, corroded reinforcing steel confirmed the half-cell readings. Chemical analysis of block, mortar, and grout samples extracted from the wall revealed high but inconsistent water-soluble chloride ion contents.

The service conditions for concrete construction in the coastal areas of the Arabian Gulf are considered to be those of one of the most aggressive environments in the world. Deterioration of hardened cement paste due to salt attack is one of the leading reasons for poor performance of concrete structures in this region. Calcium, magnesium, sodium salts of sulfates, chlorides, and carbonates extensively contaminate the ground, groundwater, and the aggregates. In such an environment, structures built with concrete which can be rated as good in temperate climatic conditions can hardly last for a decade or two. Field and laboratory studies are in progress at King Fahd University of Petroleum and Minerals at Dhahran, Saudi Arabia, to formulate preventive measures. As a part of this endeavor, the performance of in-service concrete structures is monitored. This paper details the investigations carried out to evaluate the performance of these concrete structures. Data developed in this investigation show that the aggressive service environment is the major cause for concrete deterioration, as such appropriate mix design techniques and construction practices are to be adopted for the production of a very dense and impermeable concrete.