Through careful design and good construction practices, the protection provided by Portland cement concrete to embedded reinforcing steel can be ensured. It is not the sophistication of the structural design which determines the durability of a concrete member in a corrosive environment, but the detailing practices. The provision of adequate drainage and the method of removing damage water from the structure are particularly important.
Limits to Chloride Content in Concrete
The chloride ions content is expressed as a percent of weight of cement content of concrete. The following upper limits should be ensured :
- Prestressed Concrete 0.1%
- Reinforced Concrete 0.2%
Corrosion of prestressing steel is generally of greater concern than corrosion of conventional reinforcement because of the possibility that corrosion may cause a local reduction in cross section and failure of the steel.
Concrete, which is continuously submerged in seawater rarely exhibits corrosion-induced distress because there is insufficient oxygen present. Similarly, where concrete is continuously dry, such as the interior of a building, there is little risk of corrosion from chloride ions present in the hardened concrete . However, interior locations which are wetted occasionally, such as kitchens or laundry rooms, or buildings constructed with light weight aggregate which is subsequently sealed (e.g., with tiles) before the concrete dries out, are susceptible to corrosion damage. Whereas the designer has little control over the change in use of a building or the service environment, the chloride content of the mix ingredients can be controlled.
Water Cement Ratio
40 mm cover plus maximum of 0.40 water-cement ratio concrete is sufficient to protect embedded reinforcing steel against corrosion. Studies have shown that cracks less than about 0.3 mm wide have little influence on the corrosion of reinforcing steel. Other investigations have shown that there is no relationship between crack-width and corrosion. Consequently, it has been suggested that the control of surface crack widths in design codes is not the most rational approach.
Cracks
For the purposes of design, it is useful to differentiate between controlled and uncontrolled cracks. Controlled cracks are those which can be reasonably predicted from a knowledge of section geometry and loading. For cracking perpendicular to the main reinforcement, the necessary conditions for crack control are that there be sufficient steel so that it remains in the elastic state under all loading conditions, and that'at the time of cracking, the steel is bonded (i.e. cracking must occur after the concrete has set).
Examples of uncontrolled cracking are cracks resulting from plastic shrinkage, settlement. or an overload condition. Uncontrolled cracks are frequently wide and usually cause concern, particularly if they are active.
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