Port Works Design Manual : Part 2

36viscosity allows it to penetrate into the concrete and provide a good mechanical key forsubsequent coats. It is also advisable to use multi-coat paint systems for concrete becausegreater film thickness will enable better overall protection and continuity of the application.Relative humidity should always be taken into account when coating materials are selected.For example, some moisture-cured polyurethane paints tend to blister if the relative humidityof the substrate surface is too high. Water-based coatings will not cure properly in theseconditions, because sufficient water evaporation cannot take place.Surface temperature is also a concern. Many paint systems, particularly those based onepoxy resins, will cure only very slowly at low surface temperatures. Polyurethane andvinyl coatings are among those that are not so sensitive and may be used at lowertemperatures.There are various generic types of protective coatings available in the market. The mostcommon types used in marine environment are epoxies and penetrants. Special attentionshould be paid to the following in choosing the appropriate type of coatings :• Barrier coatings such as epoxies are applicable to slab soffits and concreteelements in splash and tidal zones. The disadvantage is that they require ahigh standard of surface preparation to achieve the necessary bond strength.The choice of colour is rather limited, as most of the epoxy products are blackin colour and may not be appealing for application on exposed surfaces.• Penetrants such as silane require the substrate to be relatively dry in order thatthey may penetrate the concrete to sufficient depth and in sufficient quantity.It is advisable to apply penetrants on precast units in a well-controlledenvironment such as a sheltered precast yard.• Other generic types are also available such as acrylic and polyurethane. Acombination of coatings may be used depending on the recommendations of themanufacturers and the results of site trial.In general, the life of coatings is about 5 to 10 years. It is likely that re-coating at somestage is necessary as an ongoing maintenance commitment.Wrapping with special fabric assisted by resin or bituminous binders has also been proven tobe effective. If properly applied, the wrapping will provide an effective barrier to ingress of