The Importance of Water-Cement Ratio

Question:
Why is the water-cement ratio of a countertop mix so important?

Answer:
Three simple ingredients can be blended and proportioned numerous ways to make concrete: aggregate, cement and water. In concrete, the single most significant influence on most or all of the properties of the mix is the amount of water used. In concrete mix design, the ratio of the amount of water to the amount of cement (both by weight) is called the water-cement ratio (w/c). These two ingredients are responsible for binding everything together.

The w/c ratio largely determines the strength and durability of the concrete after it has fully cured. The w/c ratio refers to the ratio of the weights of water and cement used in the concrete mix. For ordinary concrete slabs, such as sidewalks and driveways, a w/c ratio of 0.6 to 0.7 is considered normal (or 60 to 70 pounds of water for every 100 pounds of cement). A lower w/c ratio of 0.4 is generally specified if a higher-quality concrete is desired. The practical range of the w/c ratio is from about 0.3 to 0.8. A ratio of 0.3 is very stiff (unless superplasticizers are used), and a ratio of 0.8 makes a wet and fairly weak concrete. For reference, a 0.4 w/c ratio is generally expected to produce concrete with a compressive strength (or f’c) of about 5600 psi when it is properly cured. On the other hand, a ratio of 0.8 will make a weak concrete of only about 2000 psi.

The simplest way to understand the influence of w/c on concrete quality is to remember that the greater the amount of water in a concrete mix, the more dilute the cement paste will be. This not only affects the compressive strength, it also affects the tensile and flexural strengths, porosity, shrinkage, and color of the concrete. The more you increase the w/c ratio, (that is, the more water you add per a fixed amount of cement), the more you reduce the strength of the concrete because the diluted paste is weaker and more susceptible to cracking and shrinkage.

Shrinkage is a problem in countertops because it leads to microcracks. When fresh concrete containing too much water is placed, the weight of the aggregate and the cement paste squeezes out the excess water, which then bleeds out onto the surface. As the water flows through the concrete, it creates microchannels and passages that become zones of weakness.

Using a low w/c ratio is the usual way to achieve high-strength, high-quality concrete, but it does not guarantee that the resulting concrete is appropriate for countertops. Unless the aggregate gradation and proportion are balanced with the correct amount of cement paste, excessive shrinkage, cracking and curling can result. Good concrete results from good mix design, and a low w/c ratio is just one part of a good design.