Q.: I've heard that pea-gravel concrete shrinks more than
concrete made with a larger maximum-size coarse aggregate. Why?
A.: Cement paste shrinks and most aggregates don't, so
increasing the aggregate content (reducing the paste content)
reduces shrinkage. To produce a workable concrete, a small
maximum-size aggregate such as pea gravel requires a higher sand
content than does a larger maximum-size aggregate. This
increases the aggregate surface area that has to be coated by
cement paste. If you left the cement paste content the same,
each aggregate particle would have a thinner layer of paste
surrounding it. The result would be a lower-slump concrete
that's difficult to place, consolidate, and finish. If you add
just water, you increase the water/cementitious materials ratio,
reducing strength and increasing shrinkage. When you add water
and cement to keep the w/cm the same, you don't reduce strength
but you still increase shrinkage because you're increasing the
amount of material that will shrink-the paste