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.