A wide range of products and systems are available and may be classified as pore liners, pore blockers and coatings according to their function, the materials used and method of application. Whilst these systems are generally regarded as beneficial, lack of characterisation of the materials application procedures used makes quantitative comparison between the different products difficult. Similarly, there is little detailed understanding of the mechanisms of how they work or how they may be optimised.
The concrete surface is saturated with organo-silicon compounds such as silanes and siloxanes. In the presence of moisture, these react with the hydrated cement matrix forming a discontinuous film on the surfaces of the pores and capillaries within the concrete, maintaining an open and `breathable’ pore structure. The hydrophobic nature of this chemically modified surface layer reduces moisture ingress and provides good protection against, e.g. chloride ion penetration (Basheer, 1998). Depth of penetration achieved in a given time depends on such factors as pore size distribution of the mortar matrix, water content and the composition and reactivity of the compound being impregnated (Sosoro, 1998; Basheer, 1998). Whilst formulations of silanes/siloxanes are popular, there is some concern regarding their toxicity and this has instigated investigations into more environmentally friendly alternatives (Chamberlain, 2005). One such solution is water-based and after application is said to precipitate hydrophobic crystals that adhere to the pore walls of the concrete. Although details of the material used are vague the treatment is said to conform to Highways Agency Standard (BD 43/03, 2003).
These materials essentially harden the surface of the concrete and reduce its porosity by partially or totally filling the pores and capillaries of the cement matrix. Silicates and silicofluorides, which react with any free lime, are available for this purpose although the polymers used for polymer impregnation described in Section 10.4 may be used. Compared to pore liners, there is little penetration of the concrete surface, (Basheer, 1997).
The aim here is to produce a continuous protective layer on the surface with a typical thickness of 0.1±5.0mm depending on the application. Coatings function by sealing the surface of the concrete against water absorption, whilst allowing transmission of water vapour (Dietricht, 1998) and ingress of carbon dioxide and chloride ions. They should also be effective in crack bridging. A range of systems are available with epoxy and polyurethane performing better than acrylic, chlorinated rubber and polymer emulsion coatings (Almussallam, 2003) although noticeable variations in performance of the same type but from differ- ent sources was found. Some results (Ibrahim, 1999) suggest that a combination of silane treatment plus an acrylic top coat was very effective in reducing carbonation and chloride ion ingress. Presumably in this case the silane acts a coupling agent between the polymer and concrete.
Guidance on the use of these systems is given in two Standards (BS EN 1504-1: 1998 and BS EN 1062-1: 1997) with the approach being described by Hurley (2004).