Admixtures in Concrete…. how common is it?

A lot has been said and written about admixtures in concrete within the flooring industry over the past couple of decades, but does it accurately reflect what is occurring within the concrete industry? Does the concrete that you are installing on have one or more admixtures in it? And if so, is it the norm or an aberration?

One common admixture is a Water Reducing Admixture (WRA), and it is just as common to have a WRA in the concrete as it is to have cement in the concrete…….one should then assume that yes, the concrete that you are installing the floorcovering on does indeed have at least a WRA in it.

Admixtures are categorized according to function, and water reducing admixtures are a distinct category. One of the primary functions of a WRA is to reduce the water content for a concrete mixture around 5 to 10 percent. So, concrete containing a WRA needs less water to reach a required slump than concrete without a WRA. This means that the treated concrete can have a lower water-cement ratio, and concrete with a higher strength can be produced without increasing the amount of cement.

Why use a WRA?

Reducing the water cement ratio of concrete is one of the most important factors for making a high-quality durable concrete. Additionally, sometimes the cement content of the concrete may be reduced while still maintaining the original water cement ratio for reduction of costs and even a CO2 reduction calculation by using less cement per cubic yard of concrete. Water reducing admixtures also reduce segregation and improve the flowability of the concrete. Water is not added to the concrete for flowability, it is the admixture that does that. Therefore, they are commonly used for pumping applications as well. Look at most commercial concrete slab placements, and even residential concrete slab placements, and the concrete contractor is utilizing a pump for placement of concrete. It sure beats hauling it in wheelbarrows or even by crane. A boom pump is mounted on a truck and a line pump is a hose that attaches to a stationary pump (typically fitted on a trailer or similar for moving between jobsites). Adding water to the concrete to increase flowability will typically decrease the strength of the concrete, however, the admixture used for flowability does not have that same result.

Water-reducing admixtures typically fall into three groups: low-, medium- and high-range. These groups are based on the range of water reduction for the admixture and the water reduction percentage is relative to the original mix water required to obtain a given slump. More about slump later. Even though all water reducers have similarities, each specific group has an appropriate application for which it is best suited.

How does a WRA work?

When cement encounters water, dissimilar electrical charges at the surface of the cement particles attract one another, which results in what is known as flocculation, or grouping of particles. A large portion of the water is absorbed during this process. Water-reducing admixtures neutralize the surface charges on solid particles resulting in all surfaces to be of like charge. Particles with like charges will repel each other which reduces flocculation and allowing for better dispersion. In addition, the viscosity of the paste is reduced, resulting in greater slump. As a result of the better dispersion of the cement allowing for more complete hydration, all water-reducing admixtures increase the strength development.

Dosage

Water reducers are dosed by the weight of cement, referenced as fluid ounces per 100 pounds of cement (fl. oz./cwt). Most low- and mid-range water-reducing admixtures are dosed up front with the mix water at the concrete batch plant. High-range water-reducing admixtures are often added at the job site prior to concrete placement. It takes only a few ounces per 100 pounds of cement for the desired effectiveness of the WRA to be achieved. As an example, here is a typical 4,000 lb. mix design (commonly referred to as a six-sack mix since one sack of cement is 94 pounds):

Water comprises 6% by weight of the concrete in this mix design, which is a common average amount of water in a concrete mix design for slabs. Notice the seemingly small amount of the WRA….it takes only a few ounces to achieve the desired slump without adding additional water. In this case about ¼ pound for the entire 4,000+ lbs. of concrete.

Slump

When someone hears the term “slump,” the picture that comes to mind is of posture. Concrete slump is “concrete posture.” Technically, concrete slump is the measured indication in inches of the workability, or flow, of a mix. The “looser” the mix, the higher the slump, and the “tighter” the mix, the lower the slump. The concrete slump measurement process is defined by ASTM C143 – Standard Test Method for Slump of Hydraulic-Cement Concrete. On a jobsite, typically a concrete materials testing company is commissioned to pull samples of concrete and measure the slump along with conducting other various tests including making cylinders for the all-important strength tests. The concrete for the slump test is collected and placed into a cone. The cone is removed and placed upside down next to the pile of concrete. Using a straight edge or the tamping rod across the cone base, and with one end over the pile of concrete, the distance is measured from the bottom of the straight edge to the center of the slumped concrete. This measurement is recorded to the nearest ¼ inch. A 4-inch slump is the typical slump that is desired, although that can deviate +/- by an inch. In some cases, a slump beyond that may be requested.

 
The slump is important for the concrete contractor, as they are placing the concrete and workability and flowability of the concrete are key factors for a successful and time efficient concrete placement. Using a WRA to achieve the desired slump negates the need for adding water to the mix to attempt to achieve the same slump benefit.

What does this mean for the floorcovering installer?  

Well for one, it means that yes there is an admixture in that concrete, and one can easily check on a new construction site by asking the construction team for a copy of the concrete mix design. It may surprise you to see it is not the only admixture in the concrete, as there are many others that are quite common for achieving different characteristics. Secondly, it means that no extra water was added to the concrete mix beyond what was originally stated in the mix design, which is preferable since a lower water cement ratio means there is less water of convenience left in the concrete mix.
Certainly, not all concrete admixtures are the same, just like not all flooring products are not the same. Rest assured that the mix design was carefully chosen and specified by the architectural firm, precisely blended, and mixed into the concrete batch by the concrete contractor… and ready to turn into a beautiful floor by the installer!