16.10 CONCRETE

Concrete provides a substrate surface that changes during its cure and also during environmental exposure. The main difficulty with bonding concrete is due to the presence of moisture. This moisture can be retained in fresh concrete and may be present in old concrete due to the environment.

Concrete is a substrate whose surface characteristics are also likely to be affected by the environment in which it cures. The surface can be different depending on the temperature and humidity conditions during the cure of the concrete.

Once it is cured, concrete has several surface characteristics that are problems for bonding or sealing. The concrete surface is extremely alkaline and will destroy any hydrolysis-sensitive materials that are present at the interface. It often has a weak, porous surface layer that must be penetrated or removed before being bonded. Thus, sealers and primers are commonly used to moisture-proof and strengthen the concrete surface prior to bonding.

The substrate should be free from contaminants such as cement laitance, oils, waxes, greases, and curing compounds. While the old concrete does not need to be thoroughly dry, any standing water must be removed before the adhesive is applied.

Adhesives commonly used on concrete must be formulated so that they cure well in the presence of moisture. The adhesive should have a viscosity that allows it to penetrate the concrete for mechanical bonding. Epoxy and epoxy-polysulfide adhesives are especially effective adhesives for concrete. The starting formulations given in Table 16.16 are for an ambient cure...