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So Many Admixtures

William S. Phelan

How Do They Impact Structural and Other Key Properties?

Admixtures are used to modify and improve the properties of fresh and hardened concrete. The use of the proper admixtures can result in increased workability, cementitious efficiency (psi per pound of cement), and optimum setting time with mixes, even those with low water/cementitious ratios.

The setting time of concrete can be modified or extended for long distance transportation of concrete by using retarding admixtures or can be modified or accelerated to provide rapid set and/or high early strength concrete by using accelerating admixtures. Today, concrete mixes contain water reducing and/or high-range water reducing admixtures. The concrete mix designs for slump or slump flow (spread) and self-consolidating concrete (SCC) can be proportioned with the setting time for the agreed upon placement program and modified as the season changes. The goal in all cases is to minimize water of convenience.

Selection of Concrete Admixtures

Concrete admixtures should be selected per the specifications and should be used as recommended by the manufacturer or the project testing laboratory.

Set Retarding Admixtures

Set retarding admixtures help extend the working time of concrete by slowing down the hydration process of cement. Set retarding admixtures prolong the dormant period, allowing increased workability and slump life.

Silica Fume

The use of silica fume in concrete has increased significantly in recent years. The benefits are:

Admixture Benefits

The use of the cited admixtures results in maximum mix enhancements in both the plastic and hardened concrete. Plastic concrete benefits are:

  1. Slump or slump flow SCC of choice
  2. Rapid or extended set times and increased workability.
  3. Hardened concrete benefits are:
  4. Increased compressive and flexural strengths
  5. Lower shrinkage

Successful Sequencing

Key sequencing activities include:

The majority of concrete mixes have a water content higher then what is needed to hydrate the cement properly. Proper mix designs minimize this water of convenience and optimize the slump or slump flow with approved water reducing and/or high-range water reducing admixtures.

In general, high strength concrete for reinforced formed members has a negative water slump. The required slump or slump flow is entirely achieved with higher than normal high-range water reducing admixture dosages. This is undoubtedly true of high strength concrete, i.e., 8,000 psi to 16,000 psi.

Example:

10,000 psi at 28 or 56 days is required

The w/cm is 0.30

The water content is 250 lbs/cy (0 or negative slump)

The cementitious content is 833 lbs/cy

The mix is designed to be flowing (9 inches +/- 1 inch slump) or SCC

The high-range water reducing admixture provides the slump or spread.

Trowel finished floors and slabs are often placed with slumps of 5½ inches to 7 inches at the point of deposit. These mixes must have a water slump of 2 inches to 3 inches. Lower water slumps present finishing problems.

A negative water slump means that the water content alone would not allow the concrete to be properly mixed. All of the slump or spread is achieved by the use of the high-range water reducing admixture.

Summary

All concrete today contains admixtures to enhance the concrete performance in the plastic and hardened state. Proper air-entrained concrete is necessary to provide resistance to freeze-thaw cycles. Water reducing and high-range water reducing admixtures are used to enable the user to select the optimum slump or slump flow for their project. While maintaining a low water content, shrinkage is related directly to water content, paste content, and water/cement ratio.▪