To me, gravel is a particular "grain size range" - 75mm to 4.35 mm (#4 sieve) Unified Soil Clasification. If a material was "pure" gravel - then only these sizes would be present. If there is sand, silt, etc., then - according to sticklers of ASTM, it would be given specific names that you could look up in ASTM specification for classifying soils; according to my old system (Burmeister, I think) we would say sandy gravel (for 20-35% sand and the rest gravel) or sandy gravel, trace silt (similar but with <10% silt).
An aggregate base would be a particle size grading range that is desired consisting of sand and gravel, mainly with a trace of silt - meeting a particular specification. it may also have requisite properties like PI < 6, maximum particle size of 37.5 mm, LA Abrasion less than 25, etc. Each state or Province have their own - in Ontario, an aggregate base would be called MTC Granular A for example. If the aggregate "base" would be used as a subbase, the range of particle sizes would be somewhat different and not as strict.
gravel is generally considered to be a naturally occuring aggregate which is probably processed to remove most of the finer material. Often with rounded or subrounded particles, generally not crushed rock. Pure gravel will be screened and washed and have very small percentages of sand or finer material. It drains well, but is not generally suitable for making your road base. An example would be "pea gravel", with all particles approximately pea size.
Agregate base course is generally made of crushed rock and well graded from small to large particles. It compacts well, is very stable, doesn't drain quite as well as gravel. It is probably suitable for road base and structural fills
Now we're talking, gravel v. "gravel." The ASTM definition of gravel is based on size alone, angularity is another descriptor. The lay descriptions of "gravel" typically are related to pea gravel or such.
The reason I'm asking is that we're a big multi-discipline company. We have master specifications with gravel and AB in them. I can't just remove the section on gravel becuase I don't know if some other discipline is using it for bedding or something. If I'm working in a different part of the country I can always find a local specification for AB, but not for gravel. So I don't know what to put in for gravel and I'm not sure that AB is an equivalent.
As cvg said gravel is a naturally ocurring material. Depending on the spec, grain size etc. the gravel may be acceptable as a subbase or roadbase material. If it does not meet the spec it may be possible to mix it with another material or even use it as cement stabilised material(CSM)
Aggregate Base Course is crushed rock, it will be run through the crusher to meet the required gradation specification.
Is "pea gravel" gravel? I think this a colloquial term, but to me it is 10mm clean (i.e single sized) crushed rock.
I do not agree that gravel is a naturally occurring material. Gravel is a SIZE of particle - this is what all classifications follow. In a vernacular sense, what Zambo says is correct - but for specifications, one should be very careful. As for the OP, for specifications where "gravel" is to be used, I would opine something like: "well graded natural or manufactured material in the gravel size . . ." - better yet, all specifications should clearly state materials with the appropriate gradation ranges that would be considered acceptable. I detest when I see specifications called for "common fill" or "earth fill" or "free-draining common fill". This all leads to confusion or conflict in the application of the specification. What a contractor will consider as "free draining common fill" might include significant quantities of silt and clay whereas I would say "No way." Be specific with gradation ranges - rather than incorporate vernacular terminology in a technical specification. And, I don't consider aggregate base has to be "crushed" although for most state highway projects it would be crushed rock or require that natural deposits have a crushed face or two. Many aggregate bases (and subbases) are naturally occurring bank-run deposits.
Yes...gravel is a distinctive size, just as "cobbles" are distinctive in size. Gravel is very much "gap graded". The intent of aggregate base course is to have a consistent gradation, from fines to the top size, so that you get a lower void ratio just from the gradation alone, then compaction reduces it even further.
A standard spec for gravel which actually does require rounded natural material but may allow some crushed:
Gravel: Material designated herein as gravel shall be composed entirely of particles that are either fully or partially rounded and water-worn. Crushed rock obtained by crushing rock which exceeds ASTM D-448 maximum gradation sizes may be combined provided it is uniformly distributed throughout and blended with the gravel. The quality and gradation requirements shall be as stated in this specification.
another spec for base materials which prefers crushed but allows natural:
Materials for use as aggregate base shall be classified in the order of preference as follows:
(A) Crushed Aggregate.
(B) Processed Natural Material.
(C) Processed Steel Slag.
(D) Decomposed Granite.
cvg...be very careful if you specify steel slag as a base material. Make sure that testing shows the material to be non-expansive. There's usually a lot of flux stone in steel slag (MgO, periclase), which converts to MgOH2(brucite) in the presence of moisture....the hydration causes great expansion and can damage pavements, abutments, retaining walls, structures, etc. Have first hand experience with the results (fortunately as the "finder" not the designer!)...not pretty!! A major international airport had to have a runway extension and taxiway replaced because of it. There are many other examples of problems with steel slag.
Ron
I have never specified steel slag in over 25 years, but this is a standard agency specification for a very large city and unfortunately this is what we have to use. The good thing is that there is no local supply of slag, so I'm not even sure why it is in the specification...
From now on, I will exclude it as well as decomposed granite. In fact, I am generally only interested in seeing crushed rock on my projects.
cvg - at least the specification you quote has defined "gravel" with respect to its usage within the specification and also defines, I gather, the grain size. I prefer that all base course be either crushed rock or crushed "cobbles/boulders" - in Miquan China we had extensive deposits of cobbles and boulders (no infilling) that could easily have been crushed for concrete and base courses.
absolutely BigH, the standard specs as well as my own always give the required gradations, PI, percent fines etc for any type of AB, gravel, riprap or other rock product. And these are all generally materials that are purchased at local plants and trucked in. The plant operators already know they have to follow the spec.
I do not agree with the notion that "gravel" is "gap-graded" - may or may not be, just one option, just as "poorly- or well-graded" are options. Otherwise, within the context of a specification, gravel can be whatever you define it to be. Knowing that this is the nature of the OP, I think CVG's definition is just fine, even though it does not conform to the generally-accepted definition that is used by ASTM or the USC (unified soil classification).
f-d..you have a point about the gap grading. It is gapped in comparison to a graded aggregate base course, but you're right...gravel alone does not necessarily mean gap grading.
Am sorry to add more, but...
I once made a friendly jab to a fellow about his quarry not being able to donate their usual 20 tons of gravel to a local charity auction. He then responded that gravel comes from a river pit, and that they sold crushed rock. After that I have been careful in the maintaining the proper distinction between those two items.
