International Concrete Abstracts Portal

Concrete structures are being increasingly utilized for a wide variety of applications in the marine environment. As the structures become more sophisticated (e.g., prestressed); and as they are located in areas of more severe exposure (e.g., ice, open sea, etc.), subjected to dynamic cyclic and impact loads, their performance requirements have become increasingly severe and critical. A great deal of relevant research has been carried on in recent years as an outgrowth of the extensive use of concrete platforms in the North Sea and the Netherlands Delta Plan. A summary of these research programs furnishes a useful starting point. In addition, there are a number of proprietary programs from which the results are not yet publicly available. Important problems still remain. These can be divided into five categories: (a) relating to internal response of the structural ele-ments, (b) relating to the environmental conditions and forces under which the structure must serve, relating to new materials and configurations, (d) relating to construction practices, including repairs, and (e) relating to new uses in the ocean. Concrete is destined to play an increasingly important role in man's expansion into the oceans. A strong and viable research program is a necessary ingredient of this evolving technology, in order to ensure optimal performance.

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.

Cylindrical reinforced concrete specimens, 102 mm. in diameter by 457 mm. and containing 0-4% Ca(N02)2 by weight of cement were partially submerged in sea water. A single 356 mm. length of no. 4 reinforcing steel was symmetrically positioned along the central axis of each specimen with an electrical lead penetrating the top surface. The corrosion state of the embedded steel was characterized by periodic electrochemical potential measurements, and it was considered that the onset of significant corrosion corresponded to a noble-to-active potential shift. Corrosion exposure of some specimens was terminated subsequent to potential becoming active, and these specimens were cracked open and the reinforcing steel examined. It was determined that the time for potential of the reinforcing steel to become active lengthened with increasing Ca(N02)2. Possible reasons for the effectiveness of Ca(N02)2, in mitigating reinforcing steel corrosion are presented, and significance of the present results with regard to serviceability of reinforced concrete in corrosive applications is discussed.

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.