In article , Autolycus
writes

"fred" wrote in message ...

My limited experience of site mixed concrete has been of a huge truck
with sealed component compartments, a sealed auger at the back and
little for the operator to screw up, a lever controls the strength of
the mix
but other than that it looked pretty foolproof.

The truck that my cowboys used was a 6-wheeled Volvo (that dripped oil)
with ballast in a hopper almost the length of the wagon body. A
conveyor under the hopper loaded ballast, under operator control, into a
drum mixer mounted on the back of the truck, and powered by hydraulic
motors driven by a pump on the lorry's main engine. There was a water
tank, with its outlet to the mixer again manually controlled by the
operator (the water tank, incidentally filled up on my metered water).
I didn't see exactly what the lad did when he clambered into a
compartment almost at the back of the lorry, but it appeared that he was
emptying bags (or, from the strength of the ensuing mix, probably bag)
of cement into another hopper, and thence into the mixer drum. This may
(I'm not certain) have been the only method of controlling the quantity
of cement used. The operator claimed the mixer was 1/3 cubic metre
capacity, and based his claim for extra payment on the assumption that
he had completely filled it for each mix.

The brilliance of the design of the whole shebang is illustrated by the
fact that the lorry had to be parked with its back wheeels on two lumps
of timber to get the mixer high enough to tip into a standard builders'
barrow.

Thanks for the extra info, looks like there's potentially a lot to screw up in
that method. Think I'll carry on recommending the concept but if you aren't
around to beat me to it I'll be warning of the dangers of open mix over the
mix in the sealed auger.
--
fred
Plusnet - I hope you like vanilla