how much does MOT compact ?

sm_jamieson coughed up some electrons that declared:

I don't know about MOT 1, but I can tell you for a fact that reclaimed washed railway ballast compacts from 8" as shovelled and raked out to 6" of solid mass under several runs with a standard single-person vibroplate compacting machine. I learnt this little fact after 3 of us busted a gut moving the damn stuff to help a mate make his new crossover and drive.

Said ballast consisted of rock lumps typically about 1.5" across, no fines and not much below 1". If that's similar to type 1 MOT, then I would expect similar compaction ratios.

HTH

Tim

That depends how you are compacting it, and what with.

I assume that you are using crushed limestone with a relative density of around 2.7 t/m3. Loose, it is about 1.3 t/m3. Compacted systematically with mechanical plant at the material's optimum moisture content, and it can go as high as 2.1 t/m3. Depending on how you compact it, and how meticulous you are, it could be anywhere between 1.6 and 2.0 t/m3 (tonnes per cubic metre).

If you wanted me to take a stab at a rough nominal thickness to start with, I would suggest 140mm for a final thickness of 100mm. But that is just a very rough guess. All depends on the method of compaction.

The two materials couldn't be more different.

Railway ballast is a single size material, typically 2" to 1" - the maximum size being no more than twice the minimum size is what defines a single size material. It is usually made of granite but can be made of other strong rocks.

MOT Type 1 is a fully graded material, with everything from 3" (75mm) down to dust, and all intermediate sizes present. It must be made of crushed limestone.

With MOT Type 1, the small particles fill the gaps between the larger particles, reducing the overall void ratio (the ratio of empty spaces, or voids, to the total volume) to 20% or less. Railway ballast has about twice the percentage of voids.

Bruce coughed up some electrons that declared:

Interesting. Presumably with MOT 1, there's less risk of the soil base working its way into the hardcore, resulting in the lab and hardcore sinking?

Cheers

Tim

From builders merchant, "MOT roadstone" it is. A whacker plate I will use. Simon.

Agreed, Runninn 30 ton trucks over it is way better than using a whacker ;-)

i'd guess at 30% if well smashed down.

I've got the spec somewhere but MOT type one is just a strength and size assortment, quite a loose one at that, plus the material needs to be angular, not round. It does not specify limestone and many quarry waste "crusher runs" will fit the spec.

I know this because I had to find and inert (pH 7 or lower) sub base for a car park on a sensitive acid heath and it's actually quite difficult to find an alternative to limestone scalpings for type 1, but they are available. In fact a granite type one is available but this too has a pH

In my searching I very nearly decided to have some local pebbles (rejects) crushed to mix my own type one.

The very fact that the spec is the correct assortment for no voids suggests it shouldn't compact much.

AJH

It doesn't pour without airgaps tho.

AND if you dont mix it thoroughly, you find all the larger lumps end up on top..or at one end...;-(

You're welcome.

As a result it is hard to work with a shovel. I found it harder work levelling the MOT than digging out the clay it replaced. If I remember correctly it didn't compact much, possibly an inch or so in a 6" base. It hasn't compacted any more in the two years it has been down.

Put a nominal 125m and whack down - it will go down about an inch, depending on moisture content, the wetter it is, the tighter it packs down, although it does make a bit of a mess, it makes for a better job, but saying that, under a floor slab, it isn't really that important - under a drive, it is.

Only up to a point. If it gets too wet, you cannot compact it.

It should be moist, but never wet.

Oh. So what happens if it has been raining ? I can't imagine real groundworkers waiting for it to dry out ! Simon.

I think Bruce means if it gets waterlogged. The chances of this happening are extremely slim considering it's mostly stone, but even if it does, when it dries out it's akin to a weak concrete.

That depends how heavy the rain is.

Just because you cannot imagine it doesn't mean that it doesn't happen.

You're welcome.

Bruce coughed up some electrons that declared:

On a tangential, but related note, which type of hardcore is best under a concrete house floor (just floor, not structural raft, though it might carry the weight of some brick internal walls, non load bearing)?

Thanks,

Tim

It cannot be beaten, because apart from spreading the loads into the ground like other granular materials, it has significant strength of its own.

Bruce coughed up some electrons that declared:

Cheers Bruce. that's the answer in advance to another bit of work...

I just had an image of lots of burly blokes sitting on deck chairs and anxiously glancing at the sky ! Simon.