• Measuring and compensating for moisture is a critical part of the quality control process.

    Credit: Braun Intertec

    Measuring and compensating for moisture is a critical part of the quality control process.

{Question} Why should I care about the moisture condition of my aggregates when I’m manufacturing concrete? I design my mixture with aggregates in the saturated surface-dry (SSD) condition.

{Answer} The moisture condition of aggregates in concrete is very important. We are all familiar with Abrams Law, which states that, all other things being equal, compressive strength of concrete is dependent on the ratio of mass of water to cementitious material. But what isn’t understood as well is that the water-cement ratio controls the pore volume and pore sizes in the concrete, thereby determining not only the strength but also durability.

In the laboratory, and when designing concrete mixtures, we really talk of only two moisture conditions. The first, oven dry, means exactly what it sounds like. The aggregates, which contain pore spaces of their own, have those pore space filled only with air.

The second, saturated surface dry condition, is defined as the condition in which all of the pore space within the aggregate is full of water, but no water is present on the surface. Both of these are laboratory definitions. In practice, aggregates in stockpiles, plants, and in the mixture are rarely, if ever, at these moisture conditions.

The absorption of the aggregate indicates the quantity of water which will be absorbed into the pore structure. Most commonly, aggregates will have a moisture content that is either below or above this absorption limit. When batching, it is of critical importance to account for this water. Obviously, if the aggregates are in a surface wet condition, meaning the moisture content is greater than the absorption, that surface water increases the water-to-cement ratio and affects strength and durability.

Potentially equally damaging is a condition when the aggregates are only partially saturated, with the moisture content below their absorption value. While this will potentially increase strengths and even durability if not accounted for, placing these dry aggregates into the wet concrete mixture will allow the aggregates to take up water from the paste, filling their pore spaces but decreasing the amount of water available to allow the particles to slip by one another during pumping, placing, and finishing.

Even if the concrete is tested at the plant and the workability is acceptable, dry aggregates can increase the risk of water being added at the point of placement. Since the absorption of water from the paste by aggregates that are drier than the SSD condition takes time, they can cause slump loss in transit.

So what can you do about this situation? Know the moisture content of the aggregate being used to produce concrete. Modern batch control systems, if given the absorption and moisture content, will either hold back or add additional water to compensate for that water which is carried on the surface of the aggregates or that will be absorbed into the pore structure of dry aggregates. This results is concrete that is much more predictable, that will finish properly, and meet the requirements of the mix designer and the finished project.

Measuring and compensating for moisture in the aggregates is a critical part of any quality control process. If the means and method are not available to measure the moisture content of the aggregates, at least estimating them by reviewing the stock pile is better than ignoring this input in the batch computer.

Contributed by Alf Gardiner, principal engineer with Braun Intertec, agardiner@braunintertec.com, www.braunintertec.com; and Kevin MacDonald, Beton Consulting Engineers, kmacdonald@betonconsultingeng.com, www.betonconsultingeng.com.