Will Caruthers

One of the characteristics of portland cement, the bonding agent in masonry mortars and concrete, is that it shrinks because of internal desiccation caused by hydration (autogenous) or drying (evaporation). In mixtures restrained by internal reinforcing frameworks or rough base courses (underlayment) the strains induced by shrinkage and restraint can exceed the material’s tensile strength and result in cracking. For this reason, concrete slabs and structures generally contain saw joints (cuts) at intervals specified by building codes. However, cracking is not always confined to or prevented by these joints, and over time this cracking process causes unsightly and functionality undermining deterioration. As an example of the cost of correcting concrete deterioration problems, the County of Santa Barbara in California estimated in the county’s 2012-2013 Road Maintenance Annual Plan that unfunded projects currently on the county’s books to repair concrete hardscape (dividers, supports, etc.) along county roadways totaled $38 million in this one region alone.^­­[1] Reducing cracking due to shrinkage can reduce costs for government bodies and private entities and prolong the life of cementitious structures. Shrinkage reducing admixtures (SRAs), chemical compounds that work in a variety of ways to mitigate shrinkage related cracking, are used to help accomplish this goal.

In partnership with the University of Akron (UA), Tennessee Tech University (TTU) has identified and developed a number of compounds for use as SRAs that may perform better than compounds currently on the market. In the interest of determining the performance of these compounds in construction grade concrete relative to similar concrete treated with commercially available SRAs and similar untreated concrete, a prototype scale field test is being designed, constructed, and monitored. This test will record both the longitudinal shrinkage of and internal strain forces within concrete poured in a non-reinforced slab of a minimal thickness representative of normal interior and exterior applications. The slabs will be poured both over a base course of compacted sand and/or gravel (in accordance with code building practices) to simulate an exterior application and over a compacted sand and/or gravel base course overlaid by a 6 mil (mil = 1/1000 inch) polyethylene vapor barrier of the kind mandated by International Residential Code (IRC) R506.2.3 to simulate an interior concrete application. The exterior application test slabs will be left exposed to the elements, while the interior application test slabs will be protected from wind, rain, and direct sunlight. In addition to the data retrieved directly from the slabs, rainfall, temperature, humidity, wind speed, and other pertinent environmental data will be collected at the test site and presented with the comparative findings. At present, the test is expected to run no less than six months from the time of construction.

[1] United States. County of Santa Barbara California. Santa Barbara County Public Works – Transportation Division. 2012 – 2013 Road Maintenance Annual Plan – RdMAP. Ed. Eunice Lee. County of Santa Barbara. Web. 20 June 2014, see <http://www.countyofsb.org/uploadedFiles/pwd/Roads/2012%20RdMap_v4_cs4.pdf>.