Exposed Aggregate Concrete

Ordinary cement driveways and walkways are pretty “ho-hum.” They’re grayish-wish and look decent until the first stain. Even new they never match anything around the home and you would have to lay down a mat to prevent the odd oil drops from speckling the clean finish.

Exposed aggregate is concrete that is both colored and has a textured created by the addition of small stones, marble or shells. There are two school of thought when dealing with aggregate:

1. Compacted Aggregate: This method begins by applying a normal cement base either by cement mixer or ready-mix truck. The forms are put down where needed and reinforcement introduced and, much like a normal driveway, the cement is poured. Once the pad is smoothed and set pea gravel is poured on top from a bucket being careful to spread the gravel evenly across the whole area. The gravel is them pressed into the surface with a wooden float and, to set the stones in the concrete, it is compacted with a garden roller. When the wet sheen disappears the exposed rock is brushed and sprayed with water to expose the rocks. A mild muratic acid can be used after a day to reveal the stones.
2. Ready-Mixed Aggregate: With this method the stones are already mixed into the concrete and the pouring is done much like ordinary cement. Once the cement has set for a day a solution of mild muratic acid is applied to etch away the cement around the top layer of stones. This is then washed and let cure.

With both applications it is best to seal the finish with a clear-coat finish that can be bought at a home center. For best results and a long-lasting finish apply the clear-coat on a yearly basis.

Fortifying Cement Driveways

We’ve all seen it happen either to of us or to a neighbor or friend. The beautiful driveway with exposed aggregate stone that concrete workers poured the year before has a tiny diagonal crack running through it. It probably happened over the winter when the freeze-thaw cycle was at its peak and in another year the crack will widen. Then comes the patching and after six or seven years the $6,000 driveway begins to look like a minefield.

This is a crying shame because I have a cement sidewalk beside my farmhouse that’s over 75 years old and not a crack. This is mostly due to the fact that it was poured in a very thick pad but also that the ground below was flat and had no water problems. This is because as cement cures it gets harder – even after years.

Because of its very nature concrete is prone to cracking and much of this comes from shrinking after it has dried. In fact concrete shrinks 1/16th of an inch for every 10 feet feet of length. This shrinking creates a force that will find the weakest part of the surface, sometimes the one where the most weight is applied like under a vehicle. To prevent this grooves called “control lines” are cut into the surface with a diamond saw 1/4″ deep at 15 feet intervals.

Basic Concrete Driveway Guidelines:

• Compacted base: If the ground shifts the pad will crack
• Heavy-duty steel reinforcing mesh: This should be placed in the wet cement. This not only make the pad resist cracks but will prevent any tiny cracks from getting bigger.
• Thick Pad: 5″ minimum
• Control lines: 1/4″ every 15 feet
• Cement mix: 4,000 pounds per square with a curing compound.

After the driveway has cured you can apply a siloxane water repellent. Siloxane seals the pores of the concrete an repels oil and water to prevent staining. Unlike many consumer products a siloxane-based sealer will allow the cement to breathe. This is important because moisture enters concrete through the ground and you want it to be able to find a way out. A silicone dealer will not let this happen and, in a freeze-thaw cycle, will expand and crack the cement.

There’s no reason a cement driveway should not last 30-50 years.