Sand and stone are most of what is in the truck
Cement and water start the chemistry, but sand and stone are what fill out the volume of a concrete mix. Fine aggregate is sand. Coarse aggregate is stone, and the size of that stone is not picked at random, it is chosen against the job the concrete is going into, the forms it has to fill and, on our jobs, the equipment it has to travel through to get there.
Cement paste is what binds the aggregate together and gives the mix its strength, but paste alone is not what you are pouring. A load that was mostly paste and no stone would cost far more and shrink far more as it cured. Aggregate is what makes concrete an affordable, dimensionally stable building material instead of an expensive one that cracks itself apart.
Top stone size has to fit the line it is riding through
On a pumped load, the aggregate has to make the trip through hose with room to spare. Our line has a fixed inside diameter, and stone that runs large against that size is how a load bridges the bore, where two or three pieces catch each other across the opening and the rest of the load stacks up behind them. That is one of the more common ways a line packs.
The two line sizes we run are not interchangeable on this point.
- 2.5 inch line is the less forgiving of the two about top stone size
- 3 inch line has more room inside it for a larger stone
- Neither size is a reason to change what you are pouring, only a reason to say which line is coming when the aggregate gets chosen
A full spread of sizes travels better than a gap in the middle
The biggest stone in the load is only half the picture. A mix carrying a full range of sizes, coarse down through fine, moves more predictably than one with a gap somewhere in that spread, whether it is going down a chute or through a line. Grading is a mix design decision at the plant, and telling them we run 2.5 inch and 3 inch line lets them choose it against the real hose instead of against a chute that will never see this job.