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Excellent advice!
I have a concreting question myself... I am putting a pad down, again the standard 4 inches. It's going where there is an old but knackered cobble path, which leaves me 2 choices for sub base, rip up the cobbles and fill with scalpings (will probably take a lot as the cobbles are prob 6 inhces deep in places) or just lay straight on top of the cobbles (which have been down for 10o years or so)...
Thoughts?
Sam_w-
If they've gone nowhere in 100 years they're not about to, go straight over the top! What you could do is remove a couple of lines of cobbles where the shuttering will go and go 3" deep over the remaining cobbles with your concrete. Unless you are storing elephants, 3" of concrete over a solid base would be plenty.
ColinTheBald
For speed of mixing: water first (enough to turn cement into a slurry), half bag of cement, 12 BIG shovels of ballast, add more water as necessary. The bottom of each ton bag will be stonier and need less water. To put it in context, my record is 13 tons mixed in a day. 5 bags cement per ton bag, this should be slightly too much, but saves having to run off for more.
Ensure shuttering is level and measure diagonals to square it up, 4x2 timber is ideal. Start at one end and tamp down to the shuttering with another lump of 4x2 until it spills over the far end, making sure your 4x2 is in contact with the concrete all the way across. If your shuttering is right, your concrete will be.
Pro tip: dig the shuttering in half an inch deep to give a concrete depth of 3 1/2 inch. This is ample and will ensure you don't run out of ballast. Having a bit of ballast left over is less of a pain in the arse than running out!