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.

Report presents the results of a multi-year laboratory exposure of more than 150 concrete samples to alternate immersion exposure in flowing sea water and flowing fresh water. Other exposure variables included loading, cracking, and electric currents. The validity of the controlled-exposure samples was determined by comparing the results with the results from selected samples removed from concrete structures throughout the United States. The results from a marine seawall are presented in this report and compared with previously reported results from marine masonry structures, highway bridges, and other structures.

"A collection of 24 papers form an international panel of experts on topics ranging from fundamental laboratory studies of concrete durability to case histories of concrete rehabilitation. The volume is arranged in three parts. Part 1: covers the more fundamental aspects and laboratory investigations. Topics include freeze-thaw resistance, durability of high strength concrete, corrosion of reinforcing steel, air voids in concrete, and effects of high range water-reducers. Part 2: covers field studies where concrete is exposed to natural conditions. Topics include carbonation of concrete, deicer scaling resistance of roller compacted concrete pavements, performance in marine environments, and microbiologically-induced deterioration. Part 3: covers case histories of the performance and rehabilitation of concrete structures in severe service environments. The types of structures include cooling tower shells, precast prestressed concrete conveyor bridge, heavy duty dock, elevated road way, and a masonry structure under corrosive exposure." Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP122

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.

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.