An overview of Mix Designs
Concrete is essentially two components that are mixed together, aggregate and paste. Though other supplementary cementitious materials like fly ash, silica fume, or slag may be used, the paste is usually a mixture of Portland cement and water.
There are five basic different types of cement:
- Type 1 – Normal
- Type 2 – Moderate Sulfate Resistance
- Type 3 – High Early Strength
- Type 4 – Low Heat of Hydration
- Type 5 – High Sulfate Resistance.
The type of cement used will depend on the applications of the concrete and what kinds of specifications are being sought after in the concrete. When water is added to the cement, a chemical reaction known as “hydration” will occur. After water is added to the cement, numerous microscopic crystals begin to form. These crystals will eventually harden, and the strength of the concrete mix will continuously increase as long as moisture is present.
In terms of water quality for concrete, there is a rule of thumb that if the water is acceptable to drink, it is acceptable for concrete. Also, the amount of water used is an important aspect of the strength and durability of the concrete. While adding more water will increase workability, it will also drastically reduce the strength and durability with every gallon added. As a result, a relatively low water to cement ratio is more desirable for the overall quality of the hardened concrete. Workability issues can be addressed with proper proportioning, aggregate gradation, and chemical admixtures.
The other component, aggregate, can be split up into two groups: coarse and fine. The coarse aggregate, generally limestone, ranges between 3/8″ and 1-1/2″. The fine aggregate, generally sand or crushed stone, is anything less than .2″. Between the two types of aggregate, they comprise about 60%-75% of concrete’s volume and 70%-85% by mass. Gradation of aggregates is important to the strength of concrete. Well graded aggregates will reduce the amount of voids between each other, resulting in a much more durable product.
The X factor, air, is arguably the most important element in a concrete mix. Although air is weightless, it usually occupies 4-8% of the concrete volume. Not only will air increase the workability of the mix for the finisher, it will also increase freeze/thaw resistance of concrete to help in moderate and severe weather environments.
Although many precautions can be taken to control and reduce the chances of cracking (such as admixtures, jointing, etc.) -concrete WILL crack. Following basic concrete rules, calculating proper mix designs, and referencing reliable troubleshooting techniques can save anyone time, and more importantly, money in the end.