Most underground sewerage and storm drain concrete structures which require the most immediate attention and rehabilitation are 10 to 100 years old. The quality of construction of materials used then, whether brick and mortar, cement, block and mortar, paving stones, and other materials, are always questionable. The past mentality of "out of sight out of mind" combined with the lowest price gets the job in municipal contracts, in addition to simple wear and tear, population growth, and more abundant chemicals in the sewer and storm infrastructure has created havoc to collection systems. Politicians have always had a taunting task of spending money to build a new park, or recreation area, improving streets, etc versus spending money on underground, out of sight infrastructure. Failing underground infrastructure is contaminating our drinking water via clean water infiltrating into the sewer, or via sewer surcharges into our drinking water sources. Although, clearly defining bond strength, of resurfacing materials and /or protective linings and coatings is interesting, some of us for decades have been via trial and error have found working solutions for rehabilitation of underground infrastructures. Its only of late ( 5 to 10 years) have more scientific studies and testing been conducted to quantify and sometimes justify coatings on wet, compromised structures. All underground concrete or masonry structures experience high hydrostatic pressures, seismic conditions, freeze thaw cycles, ground wet and dry cycles, chemical attack, and other predatory infrastructure conditions. Protective lining and coating of these structures on questionable existing construction materials is risky but doable. Resurfacing is Not the only method to reduce pinholes in the new coatings. Application of high strength (relative to the host structures) resurfacers and coatings is important, however, the structures are always wet, especially if they are underground, in outfalls or near creeks or near water sheds. If structures have been exposed to hydrogen sulfide, then the concrete and masonry structure is permeated with low ph moisture and gases throughout the concrete matrix snd sometimes into the adjacent ground. During changing water tables the acidic water can and does penetrate and chemically attack your new resurfacer and coatings. The only way to remove these contaminants in the concrete is to completely replace them, which is not financially feasible. After successfully lining over 150,000 underground compromised structures, we feel we can confidently offer an opinion on the matter. Having to perform adhesion testing or moisture testing inside active sewer systems creates more cost and does not necessarily prove your coating materials will last. A true long term performance warranty from BOTH coating manufacturers AND installer should be expected by owners and weed out many. One trend that has been also unforeseen, is that as we reline sewer lines, rehabilitate more underground structures such as manholes, lift stations, and pump stations, we are reducing infiltration of ground water, which in turn make the sewer more septic, thus more corrosive. Therefore, those structures that are lined with just cementitious mortars, will corrode again to low ph environments.
Thank you for your question and comment Mario.
Understanding the general differences between tensile, shear, and peel stress is important to best answer your question regarding thick-film elastomeric coatings.
Tensile stress is a loading that tends to produce stretching of a coating material by the application of axially directed pulling forces. The standard for testing direct tensile strength adhesion is ASTM D4541 (ASTM D7234 – Concrete). As you pointed out, elastomerics—even when scored—can demonstrate excellent direct tensile adhesion results.
Shear stress is caused when a force is applied to produce a sliding failure of the coating material along a plane that is in parallel to the direction of the applied force. Common test methods for shear adhesion are ASTM D3359 – “Standard Test Methods for Measuring Adhesion by Tape Test” and ASTM D6677 – “Standard Test Method for Evaluating Adhesion by Knife.” The latter is probably more practical for thick-film elastomeric coatings, although the results can be subjective.
And finally, peel stress or peel strength which can be explained by the amount of force required to remove the coating from the substrate. I am not aware of a formal method to evaluate peel strength of elastomeric coatings. However, we have used a technique inspired by ASTM D1876 “T-Peel” test to assess the in situ peel strengths of thick film elastomers. This technique involves cutting the elastomer with a razor knife down to substrate into 1”x 6” strips. Next a clamp is affixed to the top unbounded flap of the elastomer strip. A fish scale is attached to the clamp and the coating is slowly pulled at a 90 degree angle (perpendicular) to the surface. The corresponding reading is interpreted as pounds per linear inch (PLI). A good elastomer coating might exhibit around 20-30 PLI peel strength when using this technique.