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The ultimate test of the durability of concrete is its performance under the exposure conditions in which it is to serve. Although laboratory tests yield valuable indications of probable durability, the potential disrupting influences in nature are so numerous and variable that actual field exposures are highly desirable to assess the durability of concrete when exposed to natural weathering. The exposure station located at Treat Island in Cobscook Bay near Eastport, Maine, has been in use by the U. S. Army Corps of Engineers since 1936. Its location makes it ideal for exposing concrete and concreting materials to severe natural weathering. Its effect is to provide a natural field laboratory where no size limitation is placed on the exposed specimens. The specimens are installed at mean-tide elevation and the alternating conditions of immersion of the specimens in sea water, then exposure to cold air, provide numerous cycles of freezing-and-thawing of the concrete during the winter. The effect of the relatively cool summers is to lessen, in general, autogenous healing and chemical reactions in the concrete. There are currently 36 active research programs in progress at Treat Island involving the exposure of some 1700 concrete specimens. The annual testing and continuous monitoring of these programs yield valuable data on the durability and performance of concrete and concreting materials.

This paper explains some of the current programs and future plans for the Treat Island Exposure Station. During the past few years, the U. S. Army Corps of Engineers has been very interested in research and technology transfer, and as such determined to do everything possible to avoid duplication of other research programs. To coordinate all of this research requires knowledge of work being performed by government agencies and private qrganizations around the world. Because the Corps already has the Treat Island facilities, many specimens from other research organizations have been incorporated into the programs, consequently reducing the cost for these other organizations in maintaining a facility of their own. Future plans are to include more specimens. Another area requiring much work is to correlate the Treat Island results with in situ concrete structures. In the past, many of the results reported could be visually determined by inspection; in the future, a greater in-depth analysis will need to be developed so that the contribution of all of the parameters including concrete constituent properties to deterioration may be analyzed.

Subsequent to the investigation of the correlation between laboratory accelerated freezing and thawing and weathering at Treat Island, Maine, reported in 1953 (1) there have been some developments especially in the field of sulfate resistance of concrete that serve to further elucidate the findings then reported. There have also been many contributions to improve understanding of the causes and nature of the many different kinds of chemical reactions that can and do occur between aggregates of all kinds and the surrounding cement paste, so long as the exposure of the concrete is such as to keep it moist--as is the case at Treat Island, Maine. This paper suggests that there is a complex series of interactions among the constituents of cements, aggregates, and seawater not previously appreciated. Sulfate susceptibility of blended cements is becoming better understood and the use of certain blended cements promises to provide a useful alternative to moderately or highly sulfate-resisting portland cements. It also now is clear that no aggregate particle in portland-cement concrete long stored in an environment of moistness and moisture movement can be regarded as completely inert. What is becoming clear is that aggregates differ greatly in the nature, degree, and consequences of their chemical activity.

A study was begun in 1950 to determine the effects of severe natural weathering to stressed, reinforced concrete beams of various compositions and degrees of stress. The objectives of the study were to obtain information on the long-term weathering of air-entrained and non-air-entrained concrete beams containing steels of different compositions, types of deformation, and different levels of stress. The beams were placed on the beach at the natural weathering exposure station on the south side of Treat Island, Cobscock Bay, Eastport, and Lubec in Maine. The beams were subjected to twice daily tidal cycles and, during the winter months, to cycles of freezing and thawing. The beams were inspected annually during the exposure period and evaluated by a team of inspectors rating the degree of deterioration. Maximum crack widths were measured each year from 1956 to 1975 when the exposure period was concluded. Thirteen of the 82 beams that were originally placed remained in 1975 and 11 were returned to the laboratory for testing. The results of the exposure study, the laboratory investigation, and other findings are discussed in this paper.

Twenty post-tensioned I beams, eight feet long, were placed in the tidal zone at Treat Island, Maine in 1961 to determine the behavior of various types of end anchorage protection. In 1973 and 1974, eight beams were removed to the Vicksburg Waterways Experiment Station for testing and detailed examination. The behavior of all components of the eight beams are reported on, as well as the behavior of the twelve remaining beams still in the tidal zone. The continuing study has revealed some unexpected behavior of concrete and epoxy. It has shown that in eleven years, the prestressing steel was not structurally damaged by exposure to severe freezing and thawing sea-water environment and that the same seems to be indicated for the beams exposed seventeen years. The best type of end anchor protection appears to be a flush or pocket type.