The standard formula for concrete typically contains Portland cement and water along with aggregate. The more types of materials that are added to a concrete mixture, the more challenging it is to control.
A Typical Composition
Concrete is mixed in a variety of compositions to meet specific requirements and standards. However, the typical composition based on its volume is between 7% and 15% cement, along with the inclusion of 14% to 21% water, and finally 60% to 80% of aggregate. This is the range of values that are acceptable to creating a concrete mixture. However, it is the specific combination of components that provides its greatest strength.
It has long been known that there is a direct correlation between the strength of the concrete, and its water-to-cement ratio. This means that the concrete has greater strength when mixed with less water. Minimal amounts of water reduce the pores of the paste in the concrete.
There are issues with pouring concrete that is extremely dry. It often makes it very challenging to place, due to its stiff properties. It has long been noted that reducing the amount of water used in the mixture can increase durability and strength, but does not reduce the potential of shrinkage.
Importance of Aggregate Size
Generally speaking, 1 ½” aggregate is the largest size used in concrete placement. However, the size of the aggregate should never be more than one-third of the thickness of the poured slab. Usually, contractors will pour concrete at three-quarter inch or slightly less when stamping the surface for a quality finished product.
A Proper Mix
Creating a proper mix that is easy to place requires just the right amount of water. Adjustments to the water consistency of the proper finishing mix are usually made on-site. Adding water to the mixture at a rate of 10 pounds for every cubic yard (approximately 1.2 gallons) will increase the slump 1” to make the placement easier to finish.
There are various admixtures available to maximize the efficiency of the curing concrete. The most significant admixture is air-entrained materials that help minimize reaction to freeze/thaw cycles, along with de-icing salt. This helps prevent spalling on the concrete surface, which can eventually lead to a complete breakdown.
Contractors often use water reducers as an effective admixture. Often recognized as a super-plasticizer, these water reducers are highly effective at maintaining a certain slump using less water. Additionally, accelerator and retarder admixtures assist with the pouring process when the contractor is dealing with hot or cold temperatures.
Pollozan SCM (Supplementary Cementitious Material) admixtures including slag cement, fly ash, and metakaolin are used as an effective replacement for traditional concrete. This is because they have extremely tiny particle sizes that help reduce the concrete permeability.
With the right amount of cement, water, aggregate and sometimes additional admixtures, the contractor can pour concrete to meet any type of specification under nearly any condition.