Three simple ingredients can be blended and proportioned numerous ways to make concrete:
- aggregate
- cement
- water
In concrete, the single most significant influence on most or all of the properties is the amount of water used in the mix.
In concrete mix design, the ratio of the amount of water to the amount of cement used (both by weight) is called the water to cement ratio (w/c). These two ingredients are responsible for binding everything together.
The water to cement ratio largely determines the strength and durability of the concrete when it is cured properly. The w/c ratio refers to the ratio of the weights of water and cement used in the concrete mix. A w/c ratio of 0.4 means that for every 100 lbs of cement used in the concrete, 40 lbs of water is added.
Typical w/c ratios are as follows:
- Normal for ordinary concrete (sidewalks and driveways): 0.6 to 0.7
- Specified if a higher quality concrete is desired: 0.4
The practical range of the w/c ratio is from about 0.3 to over 0.8.
- A ratio of 0.3 is very stiff (unless superplasticizers are used).
- A ratio of 0.8 makes a wet and fairly weak concrete.
Typical compressive strengths when concrete is properly cured are:
- 0.4 w/c ratio –> 5600 psi
- 0.8 w/c ratio –> 2000 psi.
The simplest way to think about the w/c ratio is to think 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, the porosity, the shrinkage and the color.
The strength is reduced mostly because adding more water creates a diluted paste that is weaker. Think of it like over-diluting grape Kool-Aid. The more water you add, the weaker the Kool-Aid is.
Explained more technically, more water results in larger spacing of the cement particles. As the crystals grow, they are too far apart to knit together and form strong bonds.
Concrete with a higher w/c ratio is also more susceptible to cracking and shrinkage. Shrinkage leads to micro-cracks, which are zones of weakness. Once the fresh concrete is placed, excess water is squeezed out of the paste by the weight of the aggregate and the cement paste itself. When there is a large excess of water, that water bleeds out onto the surface. The micro channels and passages that were created inside the concrete to allow that water to flow become weak zones and micro-cracks.
Using a low w/c ratio is the usual way to achieve a high strength and high quality concrete, but it does not guarantee that the resulting concrete is always appropriate for concrete 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 mix design.