Does anyone know the formulation for a concrete mixture to make precast objects such as a 24" square pad or perhaps a small catch basin? I assume large aggregate is not used... would you use pea gravel instead?

Better to just use smaller crushed stone...aggregate should be less round and more angular so there is mechanical bond between the stone and the concrete. Other than that, assuming you'll be pouring the concrete in normal weather, there is nothing special involved in the mix. Don't forget some steel reinforcing though...doesn't have to be much for such small items, but it should be there.

So.. a standard 1:2:3 mix is all there is to it? No high strength stuff or admixtures? If so then great!!!

High early strength concrete is used in commercial precast work, primarily so they can re-use the forms quickly. If you have the time to wait a week before removing your stuff from the forms, then no problem. If you need to recycle the forms quickly, then high early strength concrete will probably be necessary, but I don't know the exact chemistry of that mix.

I once worked for a company that manufactured pressed concrete products, and if I remember correctly, they used 8000-psi concrete and high-early-strength mixtures, and they were able to take products out of the forms almost immediately, partly because the products were not just poured but also pressed.

Thanks for your suggestions Richard. I'm very pleased with the end results.

It is always a good idea to add fiber to the mix, a small handfull will strengthen the rnd product immensly.Any redimix outfit or concrete contractors supply has this in premeasured bags of 1 or 1.5 pounds, the dosage rate for one cubic yard.
I ran a prefab shop for a few years , septic tanks to pavers, and we used it in most everything. Cuts cracking loss to almost nothing.

quote:

Originally posted by K Ivey:
It is always a good idea to add fiber to the mix, a small handfull will strengthen the rnd product immensly.

Thanks K Ivey for that tip. I will look into that on the next piece that I have to cast.

Mike C.