Лако-красочные материалы — производство

In general a coating in the context of concrete repair can be considered as a fluid applied to the surface which forms a continuous film. In so doing the coating must be capable of adhering effectively to the concrete substrate. The main purposes of using coatings on concrete, apart from reasons of appearance, are to control water absorption and the passage of vapour or to act as a barrier to protect it from aggressive chemicals.

The use of coatings on concrete has shown a marked increase in recent years in order to resist carbonation or to control the ingress of chlorides. These factors have been responsible for much of the reinforcement corrosion and surface spalling of concrete which is now evident.

Carbonation of concrete is the result of carbon dioxide gas diffusing into the concrete pores. Carbonation rates tend to be highest when the concrete is relatively dry since the pores contain little water to prevent entry of the gas but just sufficient to allow it to dissolve. Carbonic acid is formed which reacts with the free lime in concrete to form calcium carbonate and leads to a gradual fall in alkalinity from the surface inwards. Once the carbonation front reaches the steel reinforcement depassivation occurs and, in the presence of water and oxygen, corrosion can proceed.

Anti-carbonation coatings are thus designed to resist the diffusion of carbon dioxide and oxygen into the concrete. In addition they should allow the free passage of water vapour so that vapour pressure does not build up behind the film and cause loss of adhesion. The resistance of a coating to the diffusion of carbon dioxide is related to its diffusion resistance coefficient |xco^ a dimensionless parameter indicating how many more times resistant a coating is than static air. The product of |xCOz and the dry film thickness, s, gives an equivalent air layer thickness which it is normally accepted(5) should exceed 50 m for good anti-carbonation properties. Similarly the product of |xH^o and 5 should be less than about 4 m to avoid build up of water vapour pressure behind the coating. Examples of anti-carbonation coatings include acrylic emulsions which cure by drying, solvented chlorinated rubber systems and polyurethane resins which undergo a chemical cure by use of a catalyser. All may be supplied in pigmented form. However, if the concrete contains active cracks then the coating must be sufficiently thick and flexible to bridge the cracks and prevent carbonation near them. Examples include mastic asphalt on bridge decks and some high-build polyurethane formulations.

Chlorides penetrate concrete from the outside only when they are in solution in water. Even quite small amounts of chloride, e. g. 0.4% by weight of cement, can disrupt the oxide layer on the surface of steel reinforcement bars which normally inhibits corrosion. Thus coatings which resist water penetration will also serve to resist the ingress of chlorides but at the same time the concrete must be allowed to ‘breathe’. A number of coatings or sealers including epoxies, methacrylates, urethanes and chlorinated rubber are currently the subject of test programmes to assess resistance to chloride ingress. In addition water repellant pore liners such as silicones, siloxanes and silanes, which work by altering the surface tension in the pores, and pore blocking materials such as silicates and crystal growth materials are under study.

At present it is not possible to give advice based on broad generic type since performance tends to be product-specific.

In certain locations concrete structures may be subjected to particularly aggressive acidic environments. Examples include acid rain from release of sulphur dioxide into the atmosphere and culverts conveying water containing dissolved organic acids. In such circumstances coatings may need a fairly high degree of chemical resistance in addition to anti-carbonation properties. Epoxy paints or high-build epoxy/pitch coatings typify the type of material used to afford protection.

The efficiency of any coating depends on adequate adhesion to the substrate concrete and the normal rules for achieving this will apply. The surface must be clean of any oil or grease such as that used in mould release agents. Water jetting or steam cleaning may be necessary on old concrete, with the surface allowed to become reasonably dry prior to painting or spraying on liquid coatings. In many cases priming systems are recommended to promote adhesion of the coating system.