Masonry Magazine November 2012 Page. 40
PATCHING
Securing available moisture will offer another way to cure a repair that, otherwise, would present curing problems.
Latex modifiers
LATEX MODIFIERS are not simply the "milk" we add to mortar. Latex modifiers offer many different compounds with many different properties to the mortars that they enhance.
A few of the compounds commonly found and labeled as latex modifiers are carboxylated styrene butadiene latex emulsion and unique reactive acrylic additives. The bottom line is that latex modifiers improve mechanical performance, workability, resistance to the elements, and most important-adhesion.
Latex modifiers are not for every patch application, but they do have a place in the market. Currently, the cultured stone industry has mandated that all mortars shall be latex modified - a plus for the applicators in this industry.
Mortar
TYPICALLY, masonry mortar is not a good choice for patching many types of masonry. Sure, it will hold for a little while, but as a long-term patching material, the properties of mortar will not perform well. The addition of latex modifiers will help it to "stick" to the substrate, but it is recommended that, if you are considering using masonry mortar as a patch, do not use it for a structural patch or a decorative patch. The labor will not be worth the long-term results.
Concrete patching
CONCRETE PATCHING can be as simple as rubbing slurry over pock holes or parging concrete into a large chip. This type of patching is not the intent of this article, as the focus is on products formulated especially for specific concrete repairs. Sika manufacturers a single-component patching material that uses latex modifiers and, typically, has an early strength characteristic that makes workability and bond superior to standard mortars. These patching mortars can be used on pre-cast, some cast stone products and many terra cottas found in the industry. With the ability to handle above and below-grade applications, these products allow for many opportunities for a quick repair when least expected. Patching mortars can be water permeable, and it is suggested that the product manufacturer be contacted after curing for recommendations on whether to use a water repellant.
Preparation of all patches requires that the substrate be of sound material. Roughing up of the surface will increase surface bonding area and offer courser material to which the patch can bond.
Concrete patches in both new construction and historic restoration can have steel reinforcement. In the past, many anchors were found to be of common steel. Terra cotta failures are largely occurring, due to the steel support system of pipes and wires that exhibit corrosion and displacement. When setting anchors for any of the mentioned products, it is important that the anchors be set in a substrate that exhibits integrity, with the ability to hold the additional eccentric loading of the patch. Hot-dipped galvanizing and stainless steel anchors are the most commonly used products in construction.
When setting dowels for a patch, the contractor should consider using threaded rod with embedment equal to one-half the length of the exposed dowel. It also is recommended to place a one-inch bend at 45 degrees on the end of the exposed portion of the dowel, for better holding abilities of patching materials as well as increasing the length of the dowel in shallower patches. It is important to note that pilot holes drilled to hold the dowels must be no more than 1/16-inch greater in diameter of the dowel. Epoxies used to set the dowels must be chosen based on the substrate to which the dowels are attaching,
Latex modifiers typically are used when applying the Sika patch to the base stone, just prior to applying the patching material.
Bonstone
BONSTONE OFFERS a patch compound that has ultra violet stability and is intended for interior and exterior aesthetic repairs to limestone, sandstone, concrete and various masonry applications. The patching material is suitable for mitered corners, coarse grained stones, foot traffic on stairs and walkways, and some decorative applications.
Bonstone's patching material is water absorptive - a positive factor when patching a masonry restoration project in which residual moisture exists. An absorptive patch also will allow moisture to exit through the patch, which is a positive feature when repairing older structures.
Application of the patch material can be performed successfully below freezing, but there's no reason to patch anything under these conditions, since frozen moisture within the substrate can pose problems for the contractor. This product can be color matched for a more professional surface patch appearance. IMAS Steve Fechino is engineering and construction manager for Mortar Net USA, Ltd. You can contact him at sfechino@mortamet.com.