Concrete tidal zone

As an example, a service-life calculation is given for Portland cement concrete with w/c 0.40 in marine environment (tidal zone), similar to the mix in the middle column of Table 11.4. The input values are given in Table 11.14. The value for was taken from a database. Using these input values in the DuraCrete model equations, the results are given in Table 11.15 for a mean service life (probability of failure 50%) and a reliable service Ufe (probability about 1%). 

The splash/tidal zone of bridges and walls represent cylindrical columns immersed in water. These structures involve entry in two dimensions. Chloride ions enter concrete by adsorption at the surface, which is given by an empirical equation. The effective chloride diffusion coefficient is derived from concrete permeability, water/cement ratio, and concrete resistivity. When concentration reaches a critical value in the vicinity of steel, corrosion begins. As shown in Fig. 12.6, a boundary layer exists adjacent to the concrete... 

Emirates (UAE) coast are exposed to high amounts of salinity (mainly chloride and sulfate). These two active ions are responsible for the corrosion of marine structures, especially at the tidal zones, and responsible for the corrosion of substructures on the coastal areas. Concrete protects reinforcement through its high alkalinity however, corrosion starts when this alkalinity breaks down due to penetration of acids, water, and oxygen into the proximity of the reinforcement. More research and studies should be conducted on corrosion in the UAE marine enviromnent and on methods of protection because there are many large projects and constructions both offshore and onshore along the coast line. 

If Portland cement is irsed as the binder, the dominant chemical reaction is the formation of ettringite, for which the aluminate and calcium ions are supplied by the cement paste and the sulfate ions are suppUed by the sea-water. In the first few years this results in an increase of compressive strength, as a result of pore filling. However, thereafter the formation of additional amounts of ettringite causes internal stresses and a strength loss (Sakoda et al, 1992). The concrete corrosion is most critical in and somewhat above the tidal zone, as in this region extensive salt crystalhzation is also involved in the corrosion process. 

Another geometry for laboratory specimens is a concrete cylinder with an embedded reinforcing bar known as a lollipop specimen. These specimens can be used to simulate marine piles, and frequently are partially immersed to represent the splash tidal zone area [9]. Concrete mixtures... 

The deterioration of concrete depends on the exposure conditions. Concrete not immersed but exposed to marine atmosphere will be subj ected to corrosion of reinforcement and frost action. Concrete in the tidal zone, however, will be exposed to the additional problems of chemical decomposition of hydrated products, mechanical erosion, and wetting and drying. Parts of the structure permanently immersed are less vulnerable to frost action and corrosion of the reinforcing steel. 

P. Gilbride, D.R. Morgan and T.W Bremner, Deterioration and rehabilitation of berth faces in tidal zones at the Port of Saint John , in V.M. Malhotra (ed.) Conorete in Marine Environment, AC I SP-109, American Concrete Institute, Farmington Hills, Ml, 1988, pp. 199-227. 

The author visited the Keys in 1989 and saw extensive delamination in the tidal and splash zones with severe corrosion of the rebar exposed when delaminated concrete was removed with a hammer. 

The problems found in the Florida Keys were first idenlified in the late 1980s. The Keys bridges were built in the late 1970s with FBECR. Routine inspection revealed localized corrosion in 1986 on V pier struts. Further corrosion was identified on a total of five bridges in subsequent inspections. The author visited the Keys in 1989 and saw extensive delamination in the tidal and splash zones with severe corrosion of the rebar exposed when delaminated concrete was removed with a hammer. 

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