Form of material

The adhesive should be a two-part epoxy comprising resin and hardener components. The resin will normally be based on the diglycidyl ether of ‘bisphenol A’ or ‘bisphenol F’ or a blend of the two. The hardener, or curing agent, will normally be from the polyamine group, since these tend to result in adhesives with better resistance to moisture than have the polyamides and are likely to cause less concern over creep performance under sustained load than the polysulphides. Other additives, such as coupling agents, diluents, flexibilizers, plasticizers, toughening agents, surfactants and inert fillers, may also be incorporated into the formulation to improve the application or performance characteristics of the adhesive. In the case of inert fillers, these may be supplied as an alternative third component for inclusion at the time of mixing.

If a filler is used it should be a non-conductive material and may be treated with a coupling agent appropriate to both filler and resin type. The filler should be highly moisture resistant, should withstand temperatures up to 120 °С without degradation, and should have a maximum particle size of 0.1 mm.

The toxicity of the chemicals used in both the adhesive and primer should be low enough to enable safe use in a normal workshop environment and on a construction site. They must satisfy the intent and requirements of the Health and Safety at Work Act, and if special ventilation is necessary the requirement should be specified in detail.

The adhesive should be supplied in liquid form and should mix readily to a smooth, paste-like consistency of initial viscosity between 20 and 150 Pa at a shear rate of 10“1 s, and have a yield stress of at least 20 Pa at 20 °С suitable for spreading both on vertical and horizontal surfaces(2). The resin and hardener should be of dissimilar colour to aid thorough mixing. The mixed material should be free of lumps, and the components, including filler, should not settle out or separate during the pot life of the adhesive.

Due regard should be paid to the practical difficulties of achieving a high standard of substrate surface pretreatment in repair and strengthening works on site. The stability of the adherend/adhesive interface is probably the most important factor in the durability of bonded joints.

Suitable primers of the corrosion inhibiting type should be specified for use with this adhesive, gritblasted steel adherends and hardened concrete.

Particular regard should be paid to the highly alkaline nature of concrete and any possible adverse effect on bond strength, especially in the long-term. For new construction, the amount of free moisture associated with pouring wet concrete on to the adhesive-coated steel should be noted. It is also likely that a joint configuration employing permeable adherends such as concrete will be inherently less durable than metal/metal joints.

Consideration should be given to the difference in elastic modulus and thermal characteristics of the steel, concrete and adhesive.

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