Concrete Strength Factors

The compressive strength of concrete is expressed in Megapascals (MPa) or pounds per square inch (psi) at an age of 28 days. Since the strength is constantly increasing, the reference strength chosen by industry is 28 days.

Typical strengths are in the range of 25 – 35 MPa (3,600 – 5,000 psi). On my bridge projects, the deck concrete is usually “high performance concrete” with a 28-day strength of 50 MPa (7,250 psi).

There are three primary factors in concrete strength:

  1. Water-cement ratio
  2. Workability
  3. Admixtures

Water-Cement Ratio

Concrete’s strength derives from the chemical reaction between portland cement and water (called “hydration”). As soon as these two components are mixed, you have concrete. Without aggregate (gravel) this is usually called “grout” but the chemical reaction is complete. Aggregate basically fills up space, although it does contribute strength of its own (if the aggregate is weaker than the concrete around it, it is reducing the strength).

This is the most important consideration, the starting point when thinking about concrete strength. A higher water-cement ratio means lower strength. You would never use a concrete mixture in which there is excess portland cement, so you can safely assume that the portland cement has been fully “used up” and that adding water simply fills the space (thus reducing the strength).

Workability

If you’ve ever been on a big concrete pour you would know that workability is a major consideration. This is the big trade-off in concrete mix design. A lower water-cement ratio gives you greater strength, but turns the concrete into hard, unworkable mixture (more like clay). And if the concrete is not mixed well or contains voids in its final position, you might have a catastrophic failure. I’ve seen concrete forms removed to reveal large, unconsolidated masses of aggregates and voids (open spaces) even with good workability concrete. If the concrete needs to be more than a foot or so deep inside the forms, I would make sure the water-cement ratio is at least 0.45.

Workability is measured by the slump test. The higher the slump, the more workable it is.

Admixtures

If you’ve got the water-cement ratio vs. workability trade-off figured out, you’ve almost won the battle. But usually there is a need for an admixture or two and several admixtures have an effect on the strength.

  • Water reducing admixtures increase strength, but you would normally use them to reduce the water you need to produce the same strength. Usually they will reduce water content requirements by 5 – 10%.
  • Air entraining admixtures reduce strength because they fill more space with non-strength producing material (i.e. air). This occurs in the form of tiny bubbles. They are used in concrete that requires freezing/thawing resistance as well as resistance to chemicals like sulfate or alkali.
  • Superplasticizers increase strength but they are normally used to increase the workability while keeping the other strength properties the same.
  • Retarding admixtures decrease strength because they delay the setting time of concrete. As mentioned above, concrete is measured on 28-day compressive strength therefore delaying strength gain will result in lower strength. I’ve used these on my projects when the job site was far away from a concrete plant (a highway bridge).
  • Accelerating admixtures increase strength. Because concrete continually increases in strength throughout its lifetime, an acceleration is effectively a strength gain.
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