15412Chemical Admixtures

Selection of type, brand, and dosage of chemical admixtures should be based on performance relative to other materials being considered or selected for use in the project. Significant increases in compressive strength, control of rate of hardening, accelerated strength gain, unproved workability, and durability are contributions that can be expected from the chemical admixtures. Reliable performance on previous work should be considered during the selection process.

Superplasticizer plays the key role in the development of high performance for HPC. Specifications for superplasticizer are covered in ASTM Types F and G. Matching the admixture to the cement, both in type and dosage, is important. The slump loss characteristics of a superplasticizer will determine whether it should be added at the plant, at the site, or a combination of both.

Use of a superplasticizer in HPC may result in different effects according to specific conditions: to increase strength by keeping the same slump with reference concrete by reducing the w/b ratio or to increase slump by keeping the same w/b ratio with reference concrete. The method of addition should distribute the admixture throughout the concrete. Adequate mixing is critical to uniform performance. Supervision is important for the successful use of a superplasticizer. The use of superplasticizers is discussed further in ACI SP-68.

Combinations of superplasticizer with normal-setting water reducers or retarders have become common to achieve optimum performance at low cost. Improvements in strength gain and control of setting times and workability are possible with optimized combinations.

When using a combination of admixtures, they should be dispensed individually in a manner approved by the manufacturer. Air-entraining admixtures should, if used, be dispensed separately from water-reducing admixtures.

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