In the past 20 years or so, surface barriers have also become increasingly used to prevent indirect deterioration due to reinforcement corrosion induced by the ingress of chloride ions or by carbonation. Due to its extensive occurrence in many concrete structures around the world, reinforcement corrosion is an area of major concern. This is particularly so in the Gulf region due to its very severe environment, notably in the coastal areas, and to the heavy contamination of soil and groundwater with aggressive salts.

As with any other construction material, excellent initial properties must be maintained over an acceptable service period for cost-effectiveness. This places a high demand upon external coatings used in the Gulf area as climatic factors responsible for degradation, such as temperature and solar radiation, are extreme. Consequently, emphasis is given here to the durability of surface coating systems.

Coating Options

Some order can be brought to the many proprietary products available for coating concrete by classification according to generic type (Figure 1).

A more detailed overview could be obtained with complementary classifications based upon:

Selection

Cross-referencing the elements of these classifications to specific applications could then provide a framework for assisting selection and specification.

This complexity reflects the versatility of raw materials and the consequent wide variety of coatings and surface treatments available for use on concrete. Given the variety of applications referred to earlier, it also has several important consequences:

It must also be noted that generic characteristics (for example, ''epoxies bond well and have good chemical resistance'') can encompass significant variations, depending on the detailed formulation of a product. Furthermore, significant variation between laboratory-based assessment and in-service performance can result from elements of workmanship, application and cure conditions, surface characteristics of the substrate and particular exposure environments. Assessment carried out under laboratory conditions of 20 deg C/60 per cent relative humidity, for example, may have little relevance for materials used in the more extreme Gulf climate.

This snapshot of some of the difficulties which can attend the selection of coatings is not intended to present an unduly negative picture. Rather, it seeks to emphasise the importance of considering all factors and options that are relevant to a given application.

Nevertheless, at present there is no code of practice in this area equal in detail to that published in the UK in 1973 for the protection of iron and steel. Consequently, it is useful to consider the stages that might be followed in selecting a coating for concrete (Figure 2).

The chart is not intended to be rigid or prescriptive but rather to provide a checklist of items that need careful consideration, regardless of the nature of a given application. In many cases, familiarity with certain products and applications eases selection; in others, more extensive evaluation or specialist advice is required. Figure 2 underlines several important points:

Application

It has to be accepted that external coatings will gradually deteriorate and that maintenance is inevitable. Premature failure, within only several years or less, however, is unacceptable, leading to unplanned expenditure and, possibly, costly litigation.

Early failure, due to inappropriate or poor products, should be eliminated by careful selection and specification, following the broad guidelines given in Figure 2. It then remains essential that the coating is applied correctly. Detailed requirements are published widely and should also be available in supplier's literature.

In broad terms, selection then becomes important once again - in this case, that of suitable sub-contractors, operatives and supervisors. Informed main contractors can contribute to the likelihood of the work being carried out correctly by using criteria other than lowest cost, such as the demonstrable experience of applicators, the quality of submitted method statements and, in some cases, pre-contract or pre-qualification demonstrations.

Durability

For a coating to provide an acceptable protective or aesthetic function, it must be formulated for satisfactory application and a certain level of performance immediately after curing. However, for a coating to be cost-effective, its initial properties must remain at an acceptable level for as long as possible under service conditions - at least to the accepted maintenance period. When maintenance is needed, simple reapplication with minimal surface preparation is economically desirable. Ideally, this would occur when the general appearance, rather than adhesion or any protective function, has deteriorated.

All organic polymers have limited durability (although with very wide limits) when subjected to external exposure. In the absence of mechanical effects, factors that cause degradation are sunlight, moisture, oxygen, and, usually to a lesser extent, ozone and atmospheric pollutants. Although the general effects of these factors on many polymers are well known, the detailed mechanisms of degradation are usually extremely complex.

Synergistic effects (for oxygen/light) and the effect of temperature on the rate of degradation add to this complexity. For certain applications, polymer-modified cementitious coatings are very attractive as sunlight and oxygen have very little, if any, effect on their performance over long periods.

The high demands imposed by the Gulf climate are such that this region can be very useful for the accelerated assessment of materials used in more temperate locations.

For a coating applied to concrete, internal stresses (resulting from temperature cycling in service or from shrinkage during drying/curing) and alkaline hydrolysis can also contribute to degradation.

Durability is complicated by the fact that different properties frequently deteriorate at significantly different rates. For example, loss of a barrier property is not necessarily accompanied by a change in appearance such as loss of gloss or discoloration, or by a mechanical defect such as cracking. Similarly, loss of adhesion can occur while the coating film itself is virtually unchanged.

These considerations have several important implications:

Real structures can undoubtedly provide a good ''test bed'' for assessing the performance of materials, but difficulties include: access for retrieval of specimens; matching the specimens that can be obtained to the requirements of a test method; cost; and the priority of shorter-term objectives. Where these difficulties can be overcome, extremely valuable information can be obtained.

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