Establishment of a strong bond to the substrate is one of the most important prerequisites for successful performance of construction polymers in almost all applications. Failure to establish a strong bond during or disbonding soon after application can lead to failure in bonding applications in the placement of fresh (plastic) on old concrete, external reinforcement (steel plate bonding), bolt or dowel grouting, in crack repair and in many other uses. Coatings and other surfacings, weakly bonded to the substrate will fail to provide the expected protection, may show the typical failures of delaminating, peeling, blistering and cracking and may actually hide destructive actions under the surface, until massive failure occurs.
A very high percentage of warranty and other remedial work in construction polymer uses is necessitated by the fact that the substrate on which these products were placed was not properly evaluated and/or surface preparation was inadequate for the application.
The strength of the bond to the substrate (or the internal surface of a substrate hole) must be high enough to resist the forces the bonded member or object may exert. The bond and the bonding agent must never become the site where stresses can concentrate. Stresses must be transmitted by the bonding agent to the substrate where they are distributed and rendered harmless within the limits of the application. For a successful bonding application the strength of the substrate surface (as an indicator of its bulk strength) is equally as important as a clean surface, the absence of contaminants and the best profile that can be achieved.
Coatings and other surfacings in construction polymer applications are generally used to protect a substrate from mechanical abuse or chemical attack. In “on grade” concrete floor applications the forces of hydrostatic pressure cannot be resisted, when a weak bond was established and/or an unsuitable bonding agent (primer) was used. Design performance of all products can be expected only when a strong bond to the substrate was established during application and is maintained in use.
Bond strength in construction polymer applications is not only a function of substrate/surface strength, cleanliness, profile and absence of contaminants, but is also affected bonding agent compatibility, possible physical and chemical reactions and temperature.
The following tests are recommended to evaluate the surface condition of concrete and other substrates and the effectiveness of surface preparation.
The strength of the surface in direct tension may be determined by a pipe cap pull-off test (ACI 503 R-93) or with the aid of an Elcometer adhesion tester (ASTM D 4547).
On concrete the preferred method depends on the surface texture. Coarse aggregate appearing in the surface requires use of the pipe cap pull-off test. Surfaces with sufficient fine aggregate, steel and wood can be tested with the Elcometer adhesion tester.
For concrete, the pull-off strength should be at least 200 psi. One test should be carried out for every 200 square foot area. The test frequency should be increased in critical and in questionable areas. On masonry failure within the substrate surface at values above 50 psi generally indicates adequacy for bonding applications. The pull-off strength on steel surfaces is generally greater than 1000 psi. For wood surfaces, tearing failure in the wood grain indicates an adequate bond.
The presence of grease, wax or oils on a concrete surface may be detected by placing a small amount of muriatic acid on it. No reaction indicates that contaminants are present. Oils that may have penetrated the concrete may be detected by raising the temperature of a small area to about 150 F with a heat lamp.
Presence of oil is indicated if the area becomes “greasy” to the touch. The presence of bond breakers is more difficult to detect and requires use of the tensile pull-off test. Low bond strength and failure at the concrete/adhesive bond line indicate the presence of a bond breaker.
On steel and wood surfaces contamination can generally be detected by visual inspection. In questionable cases a pull-off test is recommended to establish the bondability of a surface.
Presence of Other Polymer Surfacings
Removal of existing polymer surfacings is generally recommended to achieve the desired result in a new bonding application. When the removal of such surfacings is not possible nor indicated, the potential for bonding should be determined with a pull-off test. Generally, surfacings with fillers are more likely to be useful substrates than clear coats.
Remove grease, wax and other contaminants by scrubbing with an industrial detergent or degreasing compound and follow with mechanical cleaning. Remove weak or deteriorated concrete down to sound concrete by bush-hammering, grit blasting, scarifying,shot-blasting, water blasting or other suitable mechanical means. Remove dirt, dust, laitance and curing compounds by sand or grit blasting.
Remove dirt dust, laitance, deteriorated surfaces and other contaminants by sand or grit blasting.
Remove dirt, grease and oil with suitable industrial grade cleaning and degreasing compounds.Remove paint, rust, and mill scale by sand- or grit blasting. Follow sand- or grit blasting with oil-free dry air blasting. The steel surfaces must be cleaned to “white metal” according to SSPC SP 5.
Remove all surface contaminants with sandpaper; follow cleaning procedure with air blasting using oil-free, dry air.
Roughen surface to be bonded with coarse sandpaper to remove all surface contaminants and create a surface for interlock with the bonding agent.
Existing Polymer Surfacings
Remove surface by sand or grit blasting or heavy sanding to expose filler in surfacing. Remove all clear coats.
The adequacy of any cleaning procedure should be confirmed by tensile bond adhesion tests. For concrete surfaces the recommended test frequency is indicated under “Surface Evaluation”. For other surfaces visual inspection generally suffices.
Holes may be cut by rotary percussion drilling followed by air blow-out with oil-free compressed air or preferably diamond core drilling with water flush. Holes prepared in a precasting operation should be cast undersized and drilled to fit later on. The optimum hole size is 1/4 inch larger than the bar diameter. Significantly larger annular spaces are less desirable and may require use of sand/bonding agent blends.