Concrete Work 1: Exposed Aggregate Driveway

My wife and I recently put in an exposed aggregate driveway. We waited about 19 years, after we built our house, until we got our new driveway. Here are a few tips that I will share with you if you intend to put in a similar driveway.

Exposed aggregate concrete is batched with sand, cement and pea gravel, along with water and the necessary additives. There is no difference in how the concrete is poured but it is not troweled to a smooth finish. After the bullfloat is dragged across it to give it a level and straight surface a retardant is sprayed on to slow down the extreme top layer of the slab from hardening.

This is where the professional guys come in - it is all about timing. When to spray on the retardant or Exposal and when to wash off the cream of the concrete, or laitance to expose the aggregate. To wash too early is to wash the pea gravel away, to wash too late is to not be able to get the laitance off at all.

Some guys seed the top of the concrete with special stones after the placing of the concrete, then trowel them in carefully. In my area, we pour the slab with the pea gravel already in the primary mix. I was going to attempt to try this, being a true do-it-yourselfer, but I'm glad I hired a pro crew, who did an excellent job.

I prepared the surface of the driveway by removing some of the fill near the asphalt pavement at the street, to allow for 4" of concrete. The entire surface, after leveling, was compacted with a rented plate compactor. When the concrete truck backed up the driveway to get close enough to the garage door to spout from the truck, I noticed the tires hardly left any impression in the fill at all. It was a very hard compacted surface, which is very important for any concrete job.

I also drilled out the edge of my existing garage slab at the point of contact with the driveway to install #3 (3/8") rebar between the old and the new concrete. Since my driveway is downhill from the house, I wanted to be sure the driveway slab would not pull away down the slope leaving a gap at the garage door.

I put the rebar in at 12" centers and tied them all together with a long length of rebar. I formed the edges with a slight splay at the street to give a bit more room to turn out. I used 1x4 for the forms and at the splay I cut slots about 1/2" deep every inch apart, on the back side of the 1x4, so that the wood would bend easily.

At the other end of the driveway, the street end, I re-cut the existing pavement to give the guys a clean edge to finish the concrete to.

It worked well.

The finishers put in construction or control joints so that with the expansion and contraction of the concrete any cracks would be formed in the base of these joints and would be less conspicuous.

Below, Rick, the contractor, under the watchful eyes of the boss, is putting in another construction joint. The red flagging tape was to keep the wild deer in our area from walking across the newly poured concrete.

Almost finished. You can see the bull float being used here to smooth the surface of the concrete.

Rick sprays the Exposal or retardant on the wet surface to retard the setting up of the concrete to the top 1/8th inch of the surface.

Rick washes the laitance off carefully, while his helper is sweeping the worst of it off the slab. The young kid dug a hole in the front of our property, at the street, so that the excess cement paste would be trapped in the hole, rather than washing down the street. We washed the street off a while later, as well. Concrete is a messy business.

Here's a photo of our finished driveway.

Thanks, Rick and guys, for a job well done!

Dave