Thornton Exposed Aggregate Concrete

Second week of October. Thirty-eight degrees at seven in the morning, no sun on the back yard until ten, and a truck of aggregate mix idling in the street. That's the morning that tests whether a crew knows this finish, because a slab that hasn't set won't wash, and a slab that sets while everybody waits for the sun won't wash either.

Exposed aggregate is the one decorative surface where the weather on pour day writes the outcome. No sanding it back, no second pass, no fixing it Tuesday. The stone comes out clean and even across the whole slab or it doesn't, and the difference is a window of an hour or two that opens on the weather's schedule rather than ours.

Why the wash window runs the whole day

Here's the mechanic. We place and screed the slab like any other pour, then a surface retarder goes on top, which keeps the paste in the very top layer soft while the concrete underneath sets normally. When the slab below is firm enough to lay boards on, we come back with water and stiff brooms, or a pressure washer at a wide fan and a long distance, and take the softened paste off to reveal the stone.

Too early and you wash stone right out of the surface and leave pockets. Too late and the paste has gone hard through the retarder, so you fight it with pressure that scars everything around it. Warm dry June afternoon, the window might be forty-five minutes. Cool overcast October morning, it may not open until three. We build the crew schedule around that, not the other way around.

Picking stone that still looks right in February

Stone selection is the other half. Rounded river gravel in warm browns and tans reads soft and stays easy underfoot, which is why it lands on most patios. Crushed granite gives sharper contrast and more bite, and that's what we lean toward on a north-facing walk that holds ice for days. Smaller stone in a tight blend hides needles and dirt better than a big showy gravel, which matters around Quail Valley and Hunters Glen where the trees have grown in.

Color at 5,351 feet does something people don't expect. Natural stone holds its color essentially forever. It's the sealer over it that ambers or clouds. So if a washed drive nearby looks yellowed, that's the coating talking, not the rock. Strip it, recoat it, and the stone is what it always was.

Season, then. Mid-May through the end of September is when we place this finish, and June and September are the two best months for it. October pours happen with an eye on the overnight lows and a later wash than anybody wants. Winter is where we say no more often than on any other finish, because a wash needs liquid water on a slab that can't freeze that night, and blankets don't buy you a wash window. We'll pour a winter slab with a broom finish and wash a different one for you in spring.

A washed slab through the Thornton year

What goes into a washed pour from us

• Compacted road base in lifts, since a decorative surface still rides on Adams County dirt
• Four thousand PSI air-entrained mix with the blend you picked, ordered rather than substituted
• Number four bar tied on chairs at roughly two feet on center
• Surface retarder applied evenly, with edges timed separately since they set first
• Wash with brooms and water, working uphill, runoff contained instead of sent into your beds
• Control joints cut the same day, spaced to the panel size, and hand detail at every edge
• Sealer once the slab is dry, plus a written date for the next coat