How to Test Hybrid Concrete: Carefully!

• Concrete 101 at Hyper-Speed
• A Pedestrian Perusal of the Hybrids
• Longevity Defined

In a 2011 global energy report, the World Wildlife Federation declared that the concrete industry produces eight percent of the world's annual carbon dioxide transmissions (primarily in the production process). Yet, production has quadrupled since 1970, and the WWF claims the industry can reduce its carbon footprint by 90 percent.

Concrete 101 at Hyper-Speed

Concrete is one part water, one part cement, and one part aggregate; mix 'em together and what do you have? Concrete. Forget about the cement and the aggregate- that's the solid stuff. Sit it does until water turns it to paste. Pour it, leave it. The slab forms, hardens, strengthens. Dry and strong = bring it on!

So it is, and so it will be as long as there's moisture content. But water vapors and their move from the concrete mix don’t happen at a predictable rate. From bottom to top, they evaporate when they're good and ready, and a number of factors can slow or speed the process, from drying methods to accidental over-troweling or even when the ingredients change.

A Pedestrian Perusal of the Hybrids

New visions of future concrete construction are being tried and tested, and their environmental impacts appear to be positive.

Calera proposes to convert flue gases to solid, stable minerals which form a type of cement comprised of metastable calcium and magnesium carbonate and bicarbonate minerals. These polymorphs eliminate the conventional calcination process.

Novacem proposes its version of "new" cement based on magnesium oxide, which has a negative carbon footprint (i.e. carbon dioxide is absorbed in production).

Brent Constantz, a biomineralization expert at Stanford University, suggests creating cement with carbon dioxide and water, in a similar process to the way corals build reefs in nature.

But will they work? The answer lies in the fundamental chemical reactions and moisture migration (http://www.wagnermeters.com/pdf/moisture_mitagation.pdf) which needs to occur if concrete is to form compressive strength and performance durability.

Concrete floor moisture test procedures have evolved with an advanced understanding of the moisture content processes of concrete formation. Relative humidity (RH) is now officially recognized by the American Society for Testing and Materials (http://www.astm.org/Standards/F2170.htm) as a determinant of internal slab moisture content as assessed by in situ RH probes.

Even currently, conventional concrete is increasingly designed with varying proportions of water, cement and aggregate to better suit the environments in which such slabs will exist. This change has led to testing like CaCl being disallowed for certain concrete applications because it was found to give ineffective results. In effect, concrete producers are already engaged in a conventional form of hybrid concrete.

Chemical admixtures are also more specialized than in the bygone past. As admix producers "green" their own products, the natural chemicals within cause a different type of chemical reaction when applied to existing concrete mixtures. Throw those variables into flux, and what do we have?

Longevity Defined

Concrete longevity may, ironically, be defined by the universal versatility of water, one of concrete's requisite ingredients. If other hybrid concrete ingredients change, we still need water to bond the coarse particles into a concrete paste. Eco-producers may include green admixtures to enhance compressive strength, but they must still mix with moisture content if a strong and durable concrete slab is to form. And that moisture content will still need to be monitored for equally durable flooring and finish applications.

In this context, a concrete floor moisture test still provides comparative data by which contractors can determine moisture content levels, especially in experimental concrete. Perhaps new, experimental concrete mixtures will result in equally-modified concrete moisture test methodology. As long as moisture content contributes to a strong, durable, eco-friendly concrete floor, accurate concrete floor moisture test methods will be necessary to determine when moisture contents are low enough to install floor coverings.

Presently, the most effective concrete moisture content measurement methods use in situ relative humidity probes, like the Rapid RH (www.rapidrh.com) placed at various locations inside the slab at 40 percent depth. This way, builders can truly assess slab moisture content levels rather than just seeing what is happening at the surface with conventional moisture meters and test methods. Combined with a working knowledge of the external environmental influences on concrete’s drying process, a better picture of new concrete types’ strength and durability will also begin to take shape.