CRD-C130-01 Standard Recommended Practice for Estimating Scratch Hardness of Course Aggregate Particles   

Date: 
01-01-2001

This recommended practice describes a method of estimating the quantity of soft particles in coarse aggregates on the basis of scratch hardness. It is intended to be used to identify materials that are soft, particularly those which are so poorly bonded that the separate particles in the piece are easily detached from the mass. The test is not intended to identify other types of deleterious materials in aggregates, separate mention of which should be made in specifications. Neither is it intended to imply that certain types of aggregate such as limestone, whose mineral constituents may be relatively soft but well bonded together, will not perform quite adequately in concrete or other end products even though it may be possible to produce a groove in the particles by the scratch hardness apparatus.

This recommended practice is intended primarily for field use in estimating the quality of a deposit of coarse aggregate. It will be helpful in the laboratory, and when used with visual inspection will afford a fast, convenient means of determining the amount of soft particles in aggregate. The test may be useful for preliminary screening of aggregate sources, but should not be considered for specification enforcement. In case of question, the scratch test should be made on a freshly broken surface of the aggregate particle. If the particle contains more than one type of rock and is partly hard and partly soft, it should be classed as “soft” only if the soft portion is one third or more of the volume of the particle. Scratch hardness tests can be made on the exposed surface of a particle provided consideration is given to softening of the surface due to weathering. A particle with a thin, soft, and weathered surface and a hard core should normally be classed as “soft.”

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