The quality of a completed concrete slab depends on the skill of individuals who place, finish, and test the material. As an aid to assure a high-quality finished product, the specifier or owner should consider requiring the use of qualified concrete contractors, testing laboratories, and concrete finishers who have had their proficiency and experience evaluated through an independent third-party certification program.
Concrete mixers that produce a volume of concrete requiring less than one bag of cement should not be used. For small quantities of concrete, packaged products meeting the block making machine are more convenient, and can be more accurately proportioned.
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Mixing time should be sufficient to produce uniform concrete with the required slump and air content. The on-site mixer with capacity less than 1 cubic yard (0.76 cubic meter) shall be stirred for not less than 3 minutes; ordinarily 15 seconds should be added for each additional cubic yard (0.76 cubic meter) of capacity or fraction thereof, unless a turbine mixer is used. A longer mixing time is required for concrete with a slump of less than 3 inches. (75 mm).
Equipment for volumetric batching and continuous mixing at the site is available. Concrete produced in this manner should comply with ASTM C 685.
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