Concrete Basics: The Role of Water in the Mix

Despite its simple chemical formula H₂0, water plays a complex role in making concrete. Water chemically binds together sand, crushed stone or gravel and portland cement, and makes it possible to work the mix into the concrete slabs, columns and other forms used in construction.

Getting the right concrete mix is a matter of properly balancing the amounts of water and dry ingredients. Adding an extra 10 pounds of water to a cubic yard of concrete increases its "slump" by one inch, and decreases its strength by 150 pounds per square inch (psi).

Slump, Strength and Workability

Slump is measured by placing a fresh concrete sample into a cone-shaped container with open ends. When the container is full, any excess concrete is scraped from the top and the container is slid off. Without the cone's support, the freestanding concrete slumps or sags, and the amount of slump is measured.

Strength and slump are inversely related; the greater the concrete's strength, the lower its slump.

The key ingredient in concrete workability is water. But water is also the element that combines with cement in a process called hydration, and the amount of water needed for hydration is less than that needed for workability.

For ready-mix concrete, most specifications allow water to be added to the truckload's mix only once, and only to meet the slump's spec. If the entire load can't be placed at the same time, the only way to make the remaining concrete more workable is by adding water and running the mixer's drum.

Beware Too Much Water or Too Little

Workers unfamiliar with concrete basics may ask the ready-mix truck driver to add water to the mix to reduce the muscle power needed to place the concrete in its forms. But skilled cement finishers know the downside of adding water.

Excess water makes concrete more porous, leaving voids as the water not needed for hydration slowly evaporates. Worse yet, excess water also causes shrinkage cracks which appear as the concrete cures.

Porosity can weaken concrete durability because salts and carbon dioxide can more easily penetrate the concrete and cause other chemical reactions, including reduced alkalinity. Alkalinity protects concrete reinforcing steel rods from rusting.

If a mix of concrete has more water than is needed for workability, it may not meet the strength specified, thus reducing its load carrying capacity. Similarly, too little water weakens the chemical bonding between the sand, aggregates and cement during hydration.

Whether the concrete is used for a parking structure, a high-rise building, or a slab-on grade, compromising its strength can be very costly, not only in terms of testing and rework but also in potential lawsuits. With concrete one of our specialties, we keep the mix right from start to finish.