Q&A / 

How To Repair Concrete Driveway Cracks

DEAR TIM: My concrete driveway has eight large slabs. The driveway slopes down from my garage to the street. The straight-line gaps between the slabs has enlarged, some cracks are as wide as 2 inches now. I patch them and they open up again. Is there a guaranteed way to repair these cracks so I don't have to do it every other year? What's happening and how would you fix the cracks? Ed S., Steator, IL

DEAR ED: Concrete cracks of any type are the bane of many a homeowner. They can be in flat slabs, retaining walls, foundation walls and steps. Concrete has fantastic compressive strength, but usually only ten percent of that in tension when you try to pull or stretch it apart. This lack of tensile strength explains, for the most part, why concrete cracks.

Let me say right away that it's impossible to completely cover this topic in this tiny column. Entire books have been written about concrete crack repair. I'm just going to try to focus on your situation and give you an interesting option.

This crack is now 2 inches wide and opens up every year. It can be filled and made to look presentable. Photo Credit: Ed Sedlacko

First, your slabs look to be in great shape other than the gaps between the slabs. This makes the repair job much easier to do so that you get results that are aesthetically pleasing. It's important that the repair not detract from the overall look of your driveway, but still be functional.

If you try to fill the cracks with a hard material like more concrete that contains small aggregate like pea gravel or very coarse sand, it will eventually crack as well. The gaps you have are far too wide for most institutional and commercial caulks designed for filling concrete cracks.

Doing a specialized and exhaustive search on the Internet, you may discover some product used by airports, state departments of transportation or other owners of massive amounts of concrete pavement that will fill those massive cracks. But the look of the repair may not be in your best interest.

As crazy as this sounds, I'm going to suggest you repair the cracks with wood. I know this may sound quite nonsensical, but wood and concrete have been used for years together. It was a fad in the 1950's and 1960's to install redwood slats in-between concrete slabs and sidewalks in modernistic homes that were popular in that time period. I've personally inspected houses with this combination and the redwood used had lasted for well over 25 years.

Because of the natural water repellents that are in redwood, it would last for decades, even though it was exposed to moisture in a slab on the ground. Using some new water repellents and drainage aids, I believe you can get strips of wood to last at least thirty or forty years, if not more.

The first thing you need to do is clean out the gaps between the slabs to the thickness of the slabs. Scrape off any old patching tar with a stiff putty knife and remove residual tar or asphalt compounds with mineral spirits and gentle scrubbing with a wire brush.

Dig a 4-inch wide trench along the sides of the slabs from the crack or gap closest to the house all the way down to the street or sidewalk. This trench should be a deep as the slabs are thick.

Fill the trenches on either side of the driveway with small rounded pea gravel. This gravel is the size of marbles or actual peas that you eat. Do the same for the gaps between the concrete slabs, but stop filling the gaps about 2 inches from the top of the slabs.

Purchase some redwood or other fine-grained lumber like teak that's naturally resistant to rot. Cut the strips of wood to fit each of the gaps making sure the shape of the strips is wider at the top than at the bottom. This tapered shape will allow you to tap the pieces of wood into the gaps between the slabs with little effort.

Each strip should be cut so that it's 1 and three-quarter inches from top to bottom. This dimension allows the wood to be recessed below the top of the slabs about one-quarter inch.

Before you install the wood, completely coat all the sides, edges and ends with three coats of a high-quality synthetic resin water repellent. It's very important that the ends are well coated. In fact, I would dip the end of each piece of wood into the can of the sealant for at least two minutes allowing the water repellent to be drawn deeply into the end grain.

Once the wood strips are in place, fill any small gaps between the wood and the concrete slabs with medium sand. The sand will be natural looking and it will finish off the job nicely.

The pea gravel under the wood strips and on the sides of the driveway acts as an interconnected series of drainage channels that will remove any standing water from around the wood. This will significantly extend the life of the wood strips.

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