Many people think of high performance concrete in terms of compressive strength. However, strength is not the only or even the most important design criterion. The concrete must be easy to mix, transport, place, and consolidate without separating. It must be durable in the environment it’s exposed to, whether that’s freezing and thawing, seawater, abrasion, impact, or some combination of these.
High performance is more than strength
Dimensional stability is important in some applications. Suppose you are placing a concrete topping on a precast concrete member. Or you are repairing a concrete structure by removing deteriorated concrete and replacing it with new concrete. In either case, you need to make sure that the new concrete doesn’t shrink too much. Otherwise, the new concrete won’t bond with the old and they won’t work together. However good the new concrete is, it isn’t high performance concrete if it won’t work with the existing concrete.
In prestressed- and posttensioned concrete, it’s important that you don’t lose too much of the prestress to creep. Limiting creep also matters in high-rise buildings, where the self weight of the building may cause the concrete in the lower floors to compress too much.
Designing concrete to do everything you need it to is as much art as science. Often you have to make trade-offs between one desirable thing and another.
For example, suppose the structural engineer for a high-rise building specifies high strength, high modulus of elasticity, low shrinkage, and low creep. However, the contractor needs to be able to pump the concrete to the top floor and consolidate it easily. He’d prefer self-consolidating concrete if possible.
High workability usually means a high paste content, but the paste is the component of the concrete that shrinks and creeps. It often has a lower modulus of elasticity than the aggregate.
The aggregate suspension method of mixture proportioning (ACI 211.6) provides a way to satisfy everyone. This method allows the engineer to fine-tune the proportions of the various aggregate size fractions to optimize the particle-size grading for high performance concrete.
ACI 211.6 is more complex than the more traditional ACI 211.1 that most of us learned in engineering school. The basic steps are as follows.
- Select the maximum size of aggregate in accordance with ACI 318.
- Select the aggregate combination that fits the 0.45 power curve, or use a lower exponent for greater workability.
- Calculate the voids content and the shape-angularity factor.
- Calculate the volume of paste + air.
- Select the maximum w/cm.
- Select the desired air content.
- Select the w/p and admixture dosages for workability.
- Calculate the volumes and masses of the individual constituents.
- Evaluate trial mixtures and adjust as necessary.
Beton has developed software that allows us to proportion our trial batches. For high performance concrete, it’s often necessary to verify the performance using a battery of tests to cover all the performance criteria. We can help you get the performance you need.