Q: My concrete driveway is in pretty good shape except for the two-by-four wooden spacers that separate its segments. After 41 years, some of them have rotted out. All 260 feet of separators need refurbishing. I had pulled out some, and inserted strips of new redwood, but this is a poor solution because the fit is bad and the wood has a tendency to pop up.

I have considered filling sections with a flexible material but find that these are made to be used in small quantities. The amount of material I need would be expensive.

What I seem to need is a flexible filler material, preferably at low cost. Got any ideas? I really do not want to replace the entire driveway just because the wooden separators have rotted out.

A: We say replace the redwood and don’t worry about synthetic solutions.

In the past, redwood two-by-fours were commonly used as expansion joints to separate and decorate large sections of concrete-like patios and driveways. Breaking up a monolithic concrete slab into smaller geometric shapes can be aesthetically pleasing and, from a practical viewpoint, the smaller sections reduce the likelihood of the concrete cracking.

Concrete has a very low tensile strength–it doesn’t bend. The larger the slab, the more likely it is to crack. Dividing a large slab by using control joints guards against cracking. But, dividing the slab into smaller sections creates another problem. Each piece will move independently, depending on the moisture and movement of the soil beneath it. The wood that was initially installed–and lasted 41 years–acted as a shock absorber as the pressure between concrete sections waxed and waned.

We would think real hard and do a lot of homework before replacing the rotten wood with a synthetic mixture. Shrinkage, longevity and color retention would be our major concerns.

Since the original redwood lasted four decades, why not go this route again?

Use two-by-twos instead of two-by-fours. To get the wood to lie down flat, “nail” it in place with 12-inch-long, 3/8-inch carriage bolts. The threads on the bolts will help hold the boards in place, so you should try to get a fully threaded bolt.

Remove the rotten wood, then fill and compact the gaps between the concrete so that the new two-by-twos will fit snugly and are level with, or just slightly below, the concrete slabs.

Because today’s dimensional lumber is thinner than it was 40 years ago, you may have to buy rough lumber and rip it to size. If you don’t have a table saw, the lumberyard will do the sawing for you at a nominal price.

Drill holes every four feet and drive the carriage bolts through the two-by-twos into the ground with a 3-pound sledgehammer.

It’s important that you select straight-grain lumber to minimize cupping or warping. If the board naturally wants to cup, place the “arch” side of the board up and drive the bolts from the center to the outside edges. Finally, caulk the edge where the new wood meets the driveway with one of the flexible concrete fillers.

This will guard against water penetration between the joints.


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