International Concrete Abstracts Portal

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.

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.

Twenty roller-compacted concrete loads were cast at St. Constant near Montreal during the fall of 1987. Three types of cement (Canadian Types 10, 30, and 10SF), four different aggregate gradings, and three water-cement ratios (0:27, 0:33, and 0:35) were used to prepare the various mixes. Most of these mixes contained an air-entraining admixture. Approximately one-third of each concrete surface was moist-cured for 7 days, another third was covered with a white curing compound, and the remaining portion was not cured at all. Samples representative of all mixes and all curing conditions were taken from the pavement after 28 days and then tested for freeze-thaw durability (ASTM C 666) and deicer salt scaling resistance (ASTM C 672). The characteristics of the air-void system of all concretes were determined in accordance with ASTM C 457. With no exception, all samples withstood, without any significant deterioration, 300 cycles of freezing and thawing in water. However, the loss of mass after 50 cycles in the presence of a deicer salt solution ranged between 2 and 18 kg/mý (i.e., higher than the usual 1 kg/mý limit in all cases), even if most of the spacing factor values were below 250 æm. The best results (a weight loss of approximately 2 kg/mý after 50 cycles) were obtained for a mix containing Type 10 cement and no air-entraining admixture. In addition, this mix was not cured at all. Overwoking of the concrete surface during compaction is considered to be one of the possible explanations for the discrepancy between the results of the C 666 and the C 672 tests. It is also possible that the relationship between spacing factor and freeze-thaw durability does not apply to such concretes with a high permeability, numerous irregularly shaped compaction air voids, and large porous zones in the paste. This series of tests is the first phase of a 3-year research project on roller-compacted concrete pavements at Laval University, in collaboration with Canada Cement Lafarge. In the second and third years of this project, various ways to improve the scaling resistance (mostly by micro structural changes) will be studied.

Paper reports the results of part of a program to determine the extent and severity of carbonation in buildings in Canada. About 350 core samples drilled from 28 buildings in Toronto were tested by two procedures to determine the depth of carbonation. Tests were made on cast-in-place balconies and vertical components and on precast cladding. A proportion of the total sample was found to be susceptible to carbonation damage within a reasonable service life.

Long service life of concrete depends on correct choice and use of materials. Problems such as ASR (alkali silica reaction) and the prospect of sulfate attack and corrosion need early and proper identification and attention. Resistant materials must be selected and properly used to insure control of these adverse conditions. Low alkali cement or sulfate-resisting cement must be used as appropriate in these situations. Other requirements often overlooked are those essential to prevent or minimize thermal cracking of massive structural concrete, as in power plants, bridge piers, foundation elements, and thick linings of large tunnels. The ordinary concrete in municipal use, especially in new subdivisions, is often short of durability and exhibits much cracking, due to failure to follow the most fundamental rules of good practice, especially freezing weather protection, enough cement, control of slump, ample provision of joints, and curing. Sidewalks and driveways are too often disfigured and disappointing. Curing is often neglected. Specifications for the work must cite the requirements in complete detail and be followed explicitly when the work is done.