The performance of structural lightweight concrete in a marine environment is reviewed beginning with the construction of concrete ships in World War I. Major laboratory programs, utilizing different methods of evaluating the durability characteristics of structural lightweight concretes are described. Physical properties that influence the weathering characteristics of structural lightweight concrete, that differ significantly from corresponding properties of normal weight concretes are reported. Long term field exposure of lightweight concrete structures, including a 60 year old ship and a 25 year old bridge deck are reported. Criteria for the construction of durable lightweight concrete structures exposed to marine conditions are recommended.

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.

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.

Although concrete in tropical marine environment is never exposed to freezing and thawing, high temperature and humidity accelerate corrosion of steel and deterioration of concrete. There are a number of other factors which make it difficult to obtain a durable concrete in tropical areas. These factors include quality of aggregates and workmanship, special design requirements for easy-to-build structures and problems related to the remoteness of many jobsites. Corrosion mechanisms are mentioned briefly, repair-methods are explained in detail, beginning with the removal of deteriorated concrete, the restoration of reinforcing steel, replacement of concrete with cement-bound mixtures or epoxide mortars to adequate surface protection. Epoxy mortar placemnt under water as a repair method for concrete piles is described in detail,using a fluid mix of a high-density epoxy compound, which is poured into suitable, recoverable metal forms and which has been in use for 5 years at the Maraven refinery in Punta Cardon,on the Paraguand peninsula, Venezuela.

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.