Лако-красочные материалы — производство

The use of reinforced concrete as a construction material has expanded enormously since the 1960s, not least for road and bridge structures and for marine applications. In both situations there is the possibility of ingress of chloride ions, in the first instance from the use of de-icing salts and in the second from the effects of sea spray. The presence of chlorides in slightly moist concrete creates an electrolyte in which small galvanic currents can flow. Because of variations in conditions along the length of a reinforcement bar, electrical potential differences occur and currents flow from anode to cathode. Material is dissolved at the anodes resulting in either general or local corrosion of the reinforcement bars. The former manifests itself as red rust which is expansive so that cracking and
then spalling of the cover concrete takes place (see Fig. 6.1). Localised corrosion in the form of black rust occurs at discrete sites and causes a much greater loss in reinforcement cross-sectional area. However, the chemical conversion is less expansive so that there is little disruption to the concrete and as a consequence it is more difficult to detect.

The consequences of the liberal use of de-icing salts on unprotected concrete bridge decks in North America has been quite dramatic. General corrosion has occurred on many decks within the first 5-10 years necessitating expensive remedial repairs. Research in the United States in the early 1970s(17) concluded that organic coatings, particularly epoxies, could be used to protect steel reinforcement bars in the concrete of bridge decks and buildings from rapid corrosion. During 1973 the first highway bridge to use epoxy coated reinforcement was constructed in Pennsylvania and from 1978 electrostatic epoxy-powder coated reinforcement (EECR) became a standard construction material in the USA and Canada(18).

In the UK various techniques have been considered to make the reinforcement more resistant to corrosion. These include galvanised reinforcement and the use of stainless steel. Epoxy coated bars complying with the relevant ASTM standard(19) were first brought into the UK in the early 1980s. However, the wide variation in coating thickness revealed by microscopy was regarded as unacceptable for Europe(20). To meet the European requirements a process capable of producing a uniform thickness of coating, in the range 150-250 microns, has been developed. The product is known as fusion bonded epoxy coated rebar (FBECR).

The process involves three basic stages:

(1) Surface preparation. The surface of the rebar is blasted to a surface cleanliness at least as good as Swedish Standard ASa 2.5. A surface texture of around 70 pm depth is aimed for. This is typical of steel surface preparation requirements for other structural bonding applications (see Chapter 6).

(2) Heating. The rebars are then heated using an induction heater. This does not contaminate the already clean bar surface and allows a constant coil voltage combined with a constant bar speed to produce very uniform surface temperatures.

(3) Coating. Traditionally coating has been done by electrostatic spraying rather than by dipping in a fluid bed of epoxy. The simple spraying of charged particles is limited in terms of the uniformity of coating which can be achieved on a deformed cross-section rebar. A technique known as tribostatic charging, whereby the particles of powder coating are charged by friction, offers advantages when coating rebar.

Europe’s first rebar coating facility was opened in Cardiff in 1987.

In comparison with North America where use in road bridge decks predominates, applications in the UK have been concentrated towards marine and water retaining structures. There are still some reservations about more widespread use based on the results of research by the Transport and Road Research Laboratory(21). These include reductions in bond performance for cold twisted bars, the effect of fatigue on the integrity of the coating, especially at deformations in the bar, and concern as to how defective areas can be repaired. The ability of epoxy coated bars to retain their integrity over long periods of time in alkaline environments is also questioned. Research at the Building Research Establishment(22) has shown that FBECR provides a significant reduction in the rate of deterioration of reinforced concrete containing chlorides. However, the use of these coatings does not provide total protection since corrosion may be initiated at breaks in the film. Nevertheless epoxy coating has been preferred to other methods for providing added protection to concrete bridges at the design stage(23).