Precast Concrete Aggregates
Precast Concrete Aggregates share requirements with those for Portland Cement Concrete (PCC). These properties, which are presented below, are important because the aggregate represents 70% to 85% by weight of PCC. Precast concrete aggregates at our locations will also include a number of crushed aggregates. Crushed aggregates can help form a strong matrix within the thinner profiles that are common in precast applications.
Although some variation in aggregate properties is expected, a few characteristics that should be considered when selecting aggregate for precast concrete include:
- size and grading
- particle shape and surface texture
Size and Grading
The maximum size of an aggregate designates the smallest sieve size through which 100 percent of the material will pass. Grading of an aggregate is determined by sieve analysis. Maximum size and grading are controlled by specifications that prescribe the distribution of particle sizes to be used for a particular aggregate material for mixtures. The distribution of the particle sizes help determine the stability and density of the mixture.
Some aggregates contain foreign or deleterious substances that make them undesirable for concrete mixtures. (Example: clay lumps, shale, organic material, etc.) The sand-equivalent test, is a method of determining the relative proportion of detrimental fine dust or clay-like materials in the portion of aggregate passing the No. 4 (4.75 mm) sieve.
Aggregates for precast concrete should be durable. They should not deteriorate or disintegrate under the action of weather. Items for consideration under weathering action are freezing, thawing, variations in moisture content, and temperature changes. The soundness test is an indication of the resistance to weathering of fine and coarse aggregates.
Particle Shape (Flat and Elongated Particles)
Particle shape changes the workability of the mix as well as the effort necessary to obtain the required density and molded shape. Particle shape also has an effect on the strength of the precast concrete mixes. Irregular or angular particles tend to interlock when compacted and resist displacement.
All of the Precast Concrete Aggregates produced at our facilities are monitored through our continuous quality control program, which includes daily, weekly, and annual testing. This testing is performed by our in-house testing facilities, independent laboratories, and finally by Governmental Agencies such as CalTrans. If you need any test data, please contact the location you are interested in.
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