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Case histories of deteriorated Portland-cement concretes exposed to sea water, both in mild and cold climates, show that permeability is the most important characteristic determining the durability of concrete. Whether due to improper mix proportions, or poor concreting practice, or cracking of concrete, permeable concretes tend to deteriorate in marine environment. This is because the hydration products of portland cement are chemically unstable to certain aggressive components present in sea water. In this paper, the chemical reactions between the aggressive components of sea water and the constituents of hydrated portland cement are reviewed. The physical processes of deterioration associated with these chemical reactions are discussed. Also discussed are the fundamental anodic and cathodic reactions involving corrosion of reinforcing steel in concrete exposed to sea water. A summary of recent work on the effectiveness of various admixtures in reducing the permeability of hydrated portland cement is given.

For fully hydrated concrete of excellent mix proportions, the minimum void volume is about 10%. The largest portion of the void volume is located in the cement paste which, viewed by itself as a solid matrix, has a minimum void volume of 28%. The size of the voids in the hydrated cement paste are sub-microscopic, but water molecules can move about and permeate the paste. Hence, the best concretes are permeable to water; however, the quantity of permeated water may be extremely small. Most of the published work on the permeability of concrete was based on using freshwater in the experiment. This paper summarizes some of the past work and presents results from a few studies on concrete exposed to seawater. One important new finding is that concrete permeated by seawater shows a decreasing permeability rate and it appears that permeability eventually stops. It is postulated that the reason for the decreasing permeability rate i s the blocking of pore space by crystallization or precipitation of chemical products created by the inter-action of seawater and hydrated cement.

Five kinds of commercial Portland cements and one C3S sample were used for the study. The test prisms were made of mortar with a cement-sand ratio of 1 : 3 and a water-cement ratio chosen in order to give an ASTM flow of 110 +_ 5 %. After 28 days water curing, the test prisms were immersed in seawater, then, at fixed periods, up to 3 years, they were investigated by means of strength tests, chemical analysis, X-ray diffraction and scanning electron microanalysis. Results obtained show that SO3 and especially Cl diffuse rapidly in the cement mortars, but their penetration is soon slowed down by the formation of an almost impermeable Mg(OH)2 and/or aragonite layer on the mortar. It is postulated that this formation of a protective layer which occur with all the cements investigated is the main reason why immersed cement mortars are little attacked by seawater, even when the cement is C3A-rich whereas attack is greater at tide level where the Mg(OH)2 and/or aragonite layer is subject to cracking.

In a marine environment the durability of permeable concrete is a function of the chemical resistance of the hydrated cement paste to sea water. Portland cements with various amounts of C3A, blast-furnace slag cements, and pozzolan cements were investigated. The test specimens were stored both in laboratory and natural sea water conditions. The sequence of chemical reactions between the hydrated components and the aggressive ions dissolved in sea water was followed by scanning electron microscopy, electron probe microanalysis and X-ray diffraction. It is concluded that as a result of diffusion of C1- and SO42- ions, degradation of Ca(OH)2 and C-S-H occurs due to the substitution of Mg2+ for Ca2+ and formation of secondary products such as CaS04.2H20, C3A.CaC12.10H20, C3A.3CaS04.32H20, and CaSiO3.CaS04.CaC03.15H20.

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.