Keep your concrete pliable with the right Aggregate
by Gail Elber

Any concrete mix is an attempt to balance cost, placeability and setting time while maintaining the necessary qualities of strength and durability. Ready-mix producers use different ingredients from region to region depending on what’s available and what kind of weather the concrete will experience before and after curing.

Coarse aggregate makes up more than 40 percent of a typical concrete mix. The type of aggregate that is available varies in different areas. In the northern parts of North America, round gravel occurs naturally, the result of glacial action during the Ice Ages. Other aggregates available are crushed rock, which is angular in form, and recycled concrete.

So what is the best aggregate to use for stamping applications? To answer this question requires an understanding of aggregate’s role in the mix.

A tale of two mixes
There’s no such thing as “the best mix” for decorative applications. “There’s not necessarily one right way, but there are upper and lower limits for each ingredient, and if you work within them, you’ll get better results,” says Bruce Chattin, executive director of the Washington Aggregate and Concrete Association. Examining two mixes from decorative concrete contractors in different parts of North America shows how great regional and personal differences can be.

Tom Ralston, of Tom Ralston Concrete in Santa Cruz, California, starts with this mix for one cubic yard of concrete:

Cement: 564 lb (6 sacks)
Sand: 1300-1500 lb
Aggregate: 1600-1800 lb
Water: 312 lb (39 gal)
Water reducer: 18 oz
Air-entraining admixture to produce 1 percent air (freezing weather is rare in Santa Cruz, which is on the coast south of San Francisco)

Ralston specifies that the mixture be poured at a 3- to 4-inch slump. Rather than trying to predict the weather, he orders the truckload to arrive 30 to 45 minutes before the pour, then doses it on the spot with Fritz-Pak admixtures to accelerate or retard setting as needed.

In Cincinnati, Ohio, Bob Ware, president of the Decorative Concrete Store, favors a different mix appropriate to his freeze-thaw climate:

Cement: 564 lb
Sand: 1330 lb
No. 8 gravel (pea gravel): 770 lb
No. 57 gravel (a crushed stone that can be as large as 11⁄2 in.): 1130 lb
Water: 230 lb
Water reducer: 21 oz
Air-entraining admixture to produce 6 percent air content

Though Ware also pours at a 4-in slump, his mix contains a lot of water reducer to make the water content unusually low in comparison with Ralston’s.

With these general mix proportions in mind, let’s look at how the size and shape of aggregate affect the properties of the wet mix and the finished product.