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Crushing Question

While reading a material specification for crushed aggregate, the text mentions both "crushed stone" and "crushed gravel" are acceptable. These materials sound very similar, but they are mentioned individually. I assume that this means that they are not the same.

  • What is the difference?
  • How can I tell the difference if I see them on-site?

Example

This specification from Michigan is an example:

Aggregates shall consist of clean, sound, durable particles of crushed stone, crushed gravel, or crushed slag...

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    $\begingroup$ I would have guessed the distinction is similar to the one shown here, where gravel is stone transported by rivers and therefore usually of somewhat rounded shape. That being said, if the gravel itself is ground, then I don't know what the difference would be with regular stone. $\endgroup$ – Wasabi Feb 9 '16 at 2:27
  • $\begingroup$ Also, is this merely a curiosity or are these different aggregates treated differently? $\endgroup$ – Wasabi Feb 9 '16 at 2:31
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    $\begingroup$ @Wasabi I haven't had an issue yet (that I know of), but with all things related to specifications, there is bound to be an issue that comes up eventually. $\endgroup$ – hazzey Feb 9 '16 at 3:24
  • $\begingroup$ I understand, just thought that if they were treated differently somewhere, that might give a hint to the distinction. $\endgroup$ – Wasabi Feb 9 '16 at 18:52
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    $\begingroup$ @Air The Michigan spec was not the one that I read it in. It was just the first public one that I found. Mine is propriety. That may mean that there is a common "ancestor" specification where the wording came from. $\endgroup$ – hazzey Apr 29 '16 at 0:08
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It's of importance to note that historically, "stone", "gravel", and many other related terms have great regional variability. It is always advised to make certain what is meant by such terms when they are being used.

Although the term "gravel" does have a specific engineering definition (aggregate of a certain size range), in design specifications the terms "crushed stone" and "crushed gravel" have come to have a slightly different and generally agreed upon meaning. Using these terms together is a way of specifying several things at once, including the source of the material, and its size, as well as its shape characteristics.

In this context, "stone" is rock that is sourced- usually quarried- from some parent rock (such as granite, limestone, and dolomite). Stone has generally not been naturally created by weathering. On the other hand, "gravel" is rock fragments sourced from an existing deposit of weathered rock, often from rivers and streams, but also gravel pits. As such, gravel tends to be more rounded in shape.

The modifier crushed specifies two things at once. First, that the aggregate be angular in shape and not rounded. Second, that the aggregate consist of a variety of sizes. When required, these very general terms are often fleshed out a bit in the design specification by requiring the aggregate meet some kind of gradation curve, or a durability test (such as the LA Abrasion Test).

When these terms are listed such as in a design specification like the one quoted above, the specification is allowing for just about any material source, so long as the material meets the size and shape requirements.

If the material is high quality and meets specification, telling the difference between the two on site should be difficult. Look for smaller rounded pieces in the stockpile that were missed by the crusher to identify a gravel source. Rounded pieces from a quarried stone should be very rare.

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Stone is a general term and there is no real size limit on that and more like everyday term. However, gravel is an engineering term and in geotechnical engineering depending on the governing body it is generally between 2 and 40mm. USCS, AASHTO and a couple of others have their own sizes which are fairly close to this range.

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Completely changed answer, taking the first comments into account

Looking at the dictionary definitions for gravel, stone and slag gives a few clues: (from www.dictionary.com)

gravel: "small stones and pebbles, or a mixture of these with sand."

stone: "a rock or particular piece or kind of rock, as a boulder or piece of agate." or "a piece of rock quarried and worked into a specific size and shape for a particular purpose" or "a small piece of rock, as a pebble."

slag: Also called cinder. the more or less completely fused and vitrified matter separated during the reduction of a metal from its ore.

"Stone" and "gravel" are descriptive of the size / particle distribution or even the source of a material, while "slag" is a type of material, irrespective of particle size or even shape. I think he intended to specify where the material would be sourced i.e. "rock" (from a quarry), "gravel" (from a gravel pit) and "slag" (waste product from heavy industry).

The author then chose to specify "crushed" materials because he probably needs the properties of a crushed material (ragged edges).

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  • $\begingroup$ You seem to be considering "crushed gravel" as a blend of coarse and fine aggregate. I don't believe that is correct in general, but in the particular text referenced by @hazzey, it states that "the fine aggregate shall be produced by crushing stone, gravel or slag that meet the requirements for wear and soundness specified for coarse aggregate." That seems to imply that gravel is considered an appropriate coarse aggregate which can itself be crushed to produce fines. $\endgroup$ – Wasabi Apr 28 '16 at 20:26
  • $\begingroup$ Can you provide any citations? $\endgroup$ – Air Apr 28 '16 at 23:54
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I think crushed gravel means crushed bricks and roof tiles (tennis court). Crushed stone could be crushed natural shingle, for example from a river.

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    $\begingroup$ Any evidence or just your random thoughts? If just thoughts, do you have any experience or qualifications which back up your thoughts? This currently reads as a non-engineer who doesn't know engineering terminology and is just guessing; hence it is getting downvotes. $\endgroup$ – AndyT Feb 17 '16 at 11:07

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