The short answer
Wet mix macadam is crushed stone graded from 53 mm down to fines, with 0 to 5 per cent passing 75 micron, mixed with water in a plant at optimum moisture and compacted to 98 per cent of maximum dry density. The stone must show a Los Angeles abrasion value of 40 or less, or an impact value of 30 or less, and a combined flakiness and elongation index of no more than 35 per cent.
WMM is the layer that does most of the structural work in a flexible pavement and receives the least attention on many sites. It has one grading, a short list of limits and a method of laying that is easy to describe and easy to cut corners on.
01The grading
| IS sieve | Per cent passing |
|---|---|
| 53.00 mm | 100 |
| 45.00 mm | 95 - 100 |
| 22.40 mm | 60 - 80 |
| 11.20 mm | 40 - 60 |
| 4.75 mm | 25 - 40 |
| 2.36 mm | 15 - 30 |
| 600 micron | 8 - 22 |
| 75 micron | 0 - 5 |
Read down the column and the shape of the mix appears. Between a fifth and two-fifths of the material is coarser than 22.4 mm. Between a quarter and two-fifths is finer than 4.75 mm. Very little is dust. It is a dense grading: each size fills the gaps left by the size above, so that after rolling there is almost no room for a stone to move.
02The stone
| Test | Limit | Method |
|---|---|---|
| Los Angeles abrasion value | 40% maximum | IS 2386 Part 4 |
| or Aggregate impact value | 30% maximum | IS 2386 Part 4, or IS 5640 |
| Combined flakiness and elongation index | 35% maximum | IS 2386 Part 1 |
Either strength test may be used. Where water absorption exceeds 2 per cent, a soundness test under IS 2386 Part 5 is required. The plasticity index of the material finer than 425 micron must not exceed 6.
The shape limit is the one that separates WMM from sub-base. Flat and long particles bridge instead of packing, and they snap under a vibratory roller. How the index is measured and what a plant can do about it are in flakiness and elongation index.
03Blending the fractions
A plant does not produce WMM directly. It produces 40 mm, 20 mm, 10 mm and dust, and these are fed from separate bins into a pugmill in proportions found by sieve analysis. A common starting point is about a quarter 40 mm, a quarter 20 mm, a fifth 10 mm and the rest dust. That is a place to begin the trial, not a recipe. The fractions from each plant grade differently and the proportions have to be worked out for the stone in the bins.
04Moisture
The word wet in the name is exact. Water is added in the mixer to bring the material to its optimum moisture content, the moisture at which a given compactive effort produces the highest density. Too dry and the fines will not bind. Too wet and the roller pumps slurry to the surface and the layer shoves.
Optimum moisture is found by the heavy compaction test, IS 2720 Part 8. Because the test mould cannot take large stone, the fraction retained on 22.4 mm is replaced with material between 4.75 and 22.4 mm before testing. Allow for evaporation between the plant and the paver on a long haul or a hot afternoon.
05Laying and rolling
- Check the surface of the GSB for level, density and loose material. Dampen it lightly if it is dry.
- Spread the mix by paver finisher to the loose thickness that will compact to the design layer.
- Do not handle it more than necessary. Every extra tip and rake separates coarse from fine.
- Roll with a vibratory roller from the lower edge towards the upper, overlapping each pass, until density is reached.
- Finish the surface true to camber. Do not correct levels with a skim of fines.
- Keep traffic off, let the layer dry, and apply the prime coat before any bituminous course.
A single compacted layer must be at least 75 mm thick and may be up to 200 mm where the vibratory compaction and the engineer's approval allow it. Thicker bases are laid in two layers. Compaction is to at least 98 per cent of maximum dry density.
06Where WMM fails on site
- Segregation at the edges of the mat, where coarse stone rolls off the screed.
- Mix delivered below optimum moisture after a long haul.
- Rolling stopped at a pass count instead of at a measured density.
- Low spots made up with fines, which then ravel under the prime coat.
- Construction traffic run over the unsealed layer, loosening the surface.
Most of these are habits, not material faults. The comparison with the layer below is in GSB vs WMM, and the tonnage for a section in quantity per kilometre.