cement for concrete

[drifter]

I need to lay an approx 3m x 1.5m x 100mm concrete base (ie about 450 litres).

I'll be buying a 0.6 c.m bag of ballast - delivered ex builders merchant.

What I'm unclear about is, how much cement is needed? I've never managed to get an ideal concrete - often goes a bit 'sandy' & tends to get a light green coat of lichen or moss after a few years. I can't get that glazed weather resitant look, and wonder if the cause is too little cement. Would like to do better this time.

A ratio of 1 cement:3 ballast seems to be the standard recommendation for maximum strength.

But is that by weight or volume? & How many 25Kg bags of cement should I expect to use?

All help very much appreciated.

[Michael McNeil]

I am not quite sure what this bag is.

That glazing comes from overworking the finish. Too much tamping brings the fat to the surface leaving the sand and gravel behind. A 6 or 7 to 1 mix is plenty. 3 shovels of gravel, 3 or 4 of coarse sand and 1 of cement. The builders suppliers should tell you what quantities you need of all the materials.

Shove it all in, tamp it down and let it go off for a few hours then work the finish with a steel trowel and a little water. Most people try working it when it's too wet. on the other hand you don't want to let it get away from you.