The Silica Fume Association is aware of instances in which silica-fume concrete prepared in a laboratory has failed to produce the expected hardened concrete properties, whether the property is compressive strength or low permeability. This problem is particularly common in laboratories having small, and often less efficient, concrete mixers.

Silica fume is a very fine powder — the particles are approximately 1⁄100 the diameter of portland cement grains. When used to produce high-performance concrete, silica fume is typically 4-15% of the cement weight. The exact addition rate depends upon the specific performance characteristics to be improved. Compared to the other ingredients in concrete, the amount of silica fume used is small. For the silica fume to be effective, there are two issues that must be addressed:

■ First, the agglomerations that make up the densified silica fume must be broken down.

■ Second, the silica fume must be distributed uniformly throughout the concrete.

When making concrete in the laboratory, the key to both of these issues is batching the silica fume at the appropriate time and then mixing the concrete adequately.

 

ASTM C192, “Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory,” paragraph 7.1.2 recommends: “Mix the concrete, after all, ingredients are in the mixer, for 3 min. followed by a 3-min. rest, followed by a 2-min. final mixing.” Unfortunately, these recommended mixing times are simply not long enough to break down the agglomerations and to disperse the silica fume.

  1. Place 75% of the water in the mixer.
  2. Add coarse aggregate.
  3. Add silica fume slowly into the revolving mixer.
  4. Mix 1-1/2 minutes.
  5. Add cement and fly ash or slag cement, if being used, slowly into the revolving mixer.
  6. Mix 1-1/2 minutes.
  7. Add fine aggregate.
  8. Wash-in all ingredients using the remaining 25% of water Finnish by mixing as follows:
  9. Mix 5 minutes.
  10. The rest 3 minutes.
  11. Mix 5 minutes.

(time may be extended by the user based on equipment and performance results.)