The short answer
Road aggregate is judged on grading, strength, particle shape, water absorption, soundness and, for bituminous work, stripping and polished stone value. Under MoRTH the strength limit tightens as you go up the pavement: aggregate impact value may be no more than 40 for GSB, 30 for wet mix macadam, 27 for dense bituminous macadam and 24 for bituminous concrete.
A quarry is approved as a source on the strength of a file of test results, and it stays approved as long as the results keep coming back inside the limits. These are the tests in that file and what each is really asking.
01The limits, layer by layer
| Test | GSB | WMM | DBM | BC |
|---|---|---|---|---|
| Aggregate impact value, max | 40 | 30 | 27 | 24 |
| Los Angeles abrasion, max | 40 | 35 | 30 | |
| Flakiness and elongation combined, max | 35% | 35% | 35% | |
| Water absorption, max | See note | See note | 2% | 2% |
| Passing 75 micron in coarse aggregate, max | 5% | 5% | ||
| Soundness loss, sodium / magnesium sulphate, max | 12% / 18% | 12% / 18% | 12% / 18% | |
| Retained coating after stripping test, min | 95% | 95% | ||
| Polished stone value, min | 55 | |||
| Plasticity index of fines, max | 6 | 6 |
Where both impact value and Los Angeles abrasion are listed, either may be used. For GSB, water absorption above 2 per cent calls for the wet impact value test. For WMM, it triggers the soundness test.
02Strength: impact value and Los Angeles abrasion
The impact test drops a 14 kg hammer fifteen times onto a small sample of 12.5 to 10 mm stone and measures how much is reduced to fines. The Los Angeles test tumbles a larger sample in a steel drum with steel balls and measures the same thing by wear. A lower number is tougher stone. Both describe the rock, and neither can be improved by anything a crusher does.
The limits step down from 40 to 24 because stress rises towards the surface. Rock that is sound enough for sub-base may be too weak for a wearing course, and a quarry is commonly approved for some layers and not others.
03Shape
The combined flakiness and elongation index measures how much of the sample is flat or long. It is the one test in the table that the plant influences as much as the rock does. The method and the remedies are in flakiness and elongation index.
04Water absorption and soundness
Absorption is the weight of water a dry stone takes up, as a percentage. Porous or weathered rock absorbs more. In a bituminous mix a thirsty aggregate soaks up binder that was meant to coat it. In any layer it suggests a rock that may break down with wetting and drying.
Soundness tests that directly. The sample is soaked in a sulphate solution and dried, five times over. Salt crystals growing in the pores imitate years of weathering, and the loss in weight is the result. It is slow, taking a week or more, so start it early when a new source is being assessed.
05Stripping and polishing
Two tests apply only to bituminous courses. The stripping test coats stone in bitumen and soaks it in water, and at least 95 per cent of the coating has to remain. Some rocks, quartz-rich ones especially, let go of bitumen when wet. That can usually be corrected with an anti-stripping agent or hydrated lime in the mix, so a poor result is a cost and not necessarily a rejection.
Polished stone value measures how slippery the stone becomes under tyres, and it applies to the wearing course alone. A minimum of 55 is asked for bituminous concrete. Some otherwise excellent hard rocks polish readily and fail it.
06Cleanliness and plasticity
Dust clinging to coarse aggregate stops bitumen bonding to the stone, which is why no more than 5 per cent may pass 75 micron in the bituminous courses. In the granular layers the concern is clay in the fines, measured as the plasticity index of the material passing 425 micron, with a ceiling of 6. Both are results a plant can fix: the first with washing or better screening, the second by keeping overburden out of the feed.
07What the plant controls and what the rock decides
| Result | Set mainly by | Can the plant fix it? |
|---|---|---|
| Grading | Screens and blending | Yes |
| Flakiness and elongation | Rock and crusher together | Partly |
| Dust coating, passing 75 micron | Screening and washing | Yes |
| Plasticity index | Clay in the feed | Yes, at the face |
| Impact value, Los Angeles abrasion | Rock | No |
| Water absorption, soundness | Rock and its weathering | Only by going deeper |
| Stripping | Rock mineralogy | No, though the mix can be treated |
| Polished stone value | Rock | No |
The bottom half of that table is the argument for testing a quarry before committing to it. If the rock fails on strength, no supplier and no machine will make it pass, and the only answers are a deeper bench or a different hill.
08Sampling decides the result
A sample scooped from the coarse skirt of a stockpile will fail a grading that the pile as a whole would pass. Take increments from several points and heights, combine them, and reduce the combined sample by quartering. Agree the sampling point with the consultant in advance, whether that is the conveyor, the stockpile or the truck, so that the two sides are testing the same thing.