Masonry Magazine October 2004 Page. 20
STONE MATERIALS
Preservation Technology and Training (NCPPT) of the National Park Service. This study involves three distinct tasks:
1) Development of a methodology to image and describe degradation features of stone and mortar, application of the methodology in creation of a database containing detailed descriptions of the wall and archived stone specimens, and making that database publicly available through digital media and Internet access.
2) Detailed petrologic studies of the archival and selected wall micro-core specimens to characterize them with respect to their texture and mineralogy.
3) Correlating mineralogical and microstructural features to stone performance, and performance comparisons of similar stones from the same producer that have been used in building construction.
The project consists of creating an archive of images of both the actual stone in the wall and all the archive specimens, which have been stored indoors. The database organizes the images and stone descriptions while providing easy access to the data. This database will be dynamically updated as new data become available (i.e., microscopic investigation of the stone texture and mineralogy, photographs of buildings constructed with these particular stones, and any other research related to the stones).
Approximately two-thirds of the stones in the wall have an archival companion specimen that has been imaged and added to the database. The database allows users to search for the various stones by state or country, by stone type or simply by requesting a specific stone number. It is also easy to browse around the wall just by clicking on the wall's picture. As a result, the user gets a composed display of all the available database information along with two images, one of the indoor, archived specimen and one of the specific stone in the wall. The imaging of the individual stones in the wall was completed in June 2000, and the database is accessible through the Internet at http://stonewall.nist.gov.
18 Masonry
October 2004
Increasing Bond Strength with Polymer-modified Stone Veneer Mortar
"Common sense is the knack of seeing things as they are, and doing things as they ought to be done." -Harriet Beecher Stowe
Common sense tells us that any methods used to enhance the bond between stone veneer units, stone veneer mortar and the substrate are beneficial. When it comes to stone veneer mortar, the important fundamental properties are workability, water retention and bond strength. Using a moisture retention aid, such as lime or cellulose ether, can enhance the stone veneer mortar's mechanical bond by retaining moisture and thereby allowing more paste to be drawn into void spaces and pores within the substrate surface.
When designed properly, stone veneer mortar (SVM) can provide an excellent bond. However, various job site factors can adversely affect the bonding ability of ordinary stone veneer mortar. Factors affecting bond can include, but are not limited to, improper or inadequate substrate preparation, material incompatibilities, presence of bond-breakers, weather conditions and application methods.
Incorporating a polymer into the dry, pre-blended SVM can significantly enhance the bond between the SVM to both substrate and the stone veneer unit. A polymer-modified SVM is less sensitive to substrate compatibility issues than a SVM without polymer.
Polymer-modified SVM can be achieved in several ways. These include adding to mixtures of portland cement, lime and sand:
* a job site-added liquid acrylic,
* a job site-added liquid vinyl acetate ethylene,
* a liquid styrene butadiene rubber (SBR) latex as a replacement for all or part of the mix water,
* a redispersible vinyl acetate ethylene (VAC/E) powder added to the dry pre-blended mix or other different base polymers in combination with VAC, or
* a redispersible acrylic powder added to the dry pre-blended mix.
Redispersible polymers are often misunderstood. The confusion stems primarily from the word "redispersible" in that it is often thought that redispersible powders will redisperse and migrate out of the hardened mortar or concrete with each exposure to moisture. Redispersible powders are manufactured by spray-drying polymer dispersions. Prior to spray-drying a water-soluble colloid typically is added to the aqueous dispersion to aid in redispersion. During the spray drying process an anti-caking agent is added to prevent agglomeration. The solid polymer particles merge into larger polymer particle conglomerates. When these larger particle conglomerates come in contact with water, they redisperse and adjacent polymer particles combine to form a dense film. Once the film is formed, the polymer particles no longer are able to redisperse with further contact with water.
With the growing popularity of stone veneer wall systems, it makes sense to move toward this new technology to enhance the properties of stone veneer mortar.
Mark R. Lukkarila Technical Services Manager, SPEC MIX, Inc.
www.masoncontractors.org