Part 4 of our blog series “Why Sealant Joints Fail,” published by Karen Warseck, AIA and President of Building Diagnostics Associates, focuses on Substrate Failures.
Every material in which a sealant may be applied will have individual sets of precautions. Outlined below are various substrates and tips for avoiding substrate failures.
Masonry and Concrete
Both of these materials are extremely porous and special care must be exercised when selecting a compatible sealant. Due to their porous nature, some sealants may bleed into them causing unsightly discoloration. Acid cure sealants can etch the surface of limestone or marble. Concrete must be sufficiently cured and dry before sealant is applied. Additionally, if the stress on the sealant is stronger than the tensile strength of the concrete, the thin sharp edges of the concrete joint will spall. One way to minimize spalling is to chamfer the edges of the joint and the sealant bead must be kept below the bottom of the chamfer or tearing may occur.
Aluminum
Aluminum coatings contain mill contaminants, oils, graphite, and carbon residues and oxides that act as release agents for elastomeric sealants. Some baked on coatings will adversely impact adhesion. Adhesion testing to determine suitability should be performed by the sealant manufacture for each installation because variations in fabrication make each batch of coating unique.
Galvanized Steel
This material has a history of poor sealant adhesion. The surface will gradually erode based on the sacrificial nature of the galvanizing which impacts the long term performance of an elastomeric seal. Galvanized steel normally requires an alternate form of joint seal.
Wood
Unpainted wood does not make a good substrate for any sealant. It will absorb moisture and adhesion problems will eventually result.
Wood is best painted before sealing, the paint must be compatible with the sealant and the paint must be firmly adhered to the wood.
Redwood, teak and other woods naturally resistant to water have sealant issues as they contain natural oils that may affect the adhesion. Pine and other softwoods contain natural resins that may bleed out under the sealant causing adhesion failure.
Surface Sealers and Coatings
Both of these may prove unfriendly to joint sealants. Their main purpose is to provide a surface that will repel dirt, paint and/or water. Unfortunately, these compounds often end up repelling sealants. Visually, many of these coatings are clear and difficult to detect. If a masonry or concrete surface is suspected of having been treated with one of these compounds, an adhesion test should be performed prior to sealant application.
Some sealant manufacturers recognize the widespread use of such compounds and have pretested them to determine compatibility with sealants. However, surface compounds undergo frequent and drastic formulation changes and can vary widely from batch to batch.
Whenever possible, sealers and coatings should be applied after the sealant is cured, as it is extremely difficult to apply such materials on to the surface without contaminating the joints, no matter how careful the application.
Form Release Agents
If applied according to the manufacturers recommendations, usually do not affect sealant adhesion. However, too thick of an application may leave a brittle film that can flake off creating loss of bond in that area.
Petroleum based agents may be incompatible with the sealant.
Testing should be done on the actual materials used in construction to be sure that there will be no detrimental effects on the bond.
Testing
Follow the time honored advice “Better safe than sorry.” Sealant joints are only a small part of a building’s exterior but they are asked to perform a very large function. They are not a component where chances should be taken, and their specification and application must never be treated lightly.
Our recommendation to clients is to have their sealant supplier conduct accelerated aging tests to determine if there might be bleeding of the sealant carrier into the stone, and the sealant supplier should also recommend a primer that won’t bleed into the stone. The customer should require a sealant warranty for bond to stone and also a warranty against staining from the sealant supplier.
