The sidewalk also has continuous reinforcing steel, the surface beneath has compacted crushed rock for drainage and stability, and the concrete was poured six inches thick.
Your driveway likely has only a single two-by-four running across the middle for expansion/contraction, may only be 3-4 inches thick, was poured on muddy soil over the top of a recently backfilled utilities trench, and rests on several feet of loose fill where it abuts the house.
Garage and basement floor slabs also crack at angle changes and intersections with walls, posts, and so forth. Many newer slabs have hidden control joints that prevent random cracking by creating a weak point just below the surface, allowing the cracking to follow a predetermined path. But in the end, concrete cracks because of movement, overloading, or lack of support.
Leaving cracks open in concrete allows water to enter, and allows undermining and freezing to occur, leading to further damage.
A perfect example of this is the walkway around the side of my garage. Hammered and lifted by a huge tree stump (recently cut down — yay!), undermined on one side, cracked and listing badly, with a downspout pouring water onto it, this pathetic example of shoddy maintenance is the poster child for "do as I say, and not as I do." But I digress.
Recent years have seen great improvements in the longevity of concrete patching materials. In years past I have complained in this space about patching materials that shrink, crack and pop out of position, leaving twice the amount of cracks to contend with at a later date. Newer self-priming pourable grouts and crack sealers have come on the market with much less shrinkage than previous materials. These squeeze tube products are very easy to apply, with the initial concrete cleaning being the most complex task involved. These are generally acceptable for cracks up to half an inch wide, and work well by all accounts.
Larger cracks will require surgery with a chisel. A larger patch will not stick if it is laid into a "V" groove. A hollowed-out hole with vertical walls is better, but ideally it should be backcut, or undercut, to provide a keyway.
In other words, the top of the crack will be narrower than the bottom. Yes, I realize, that's easy for me to say, isn't it? The minimum size of the crack should be ¾-inch wide-by-¾-inch deep, after chipping. After cleaning the crack, a bonding agent or primer needs to be used. The bonding agent helps the new concrete adhere to the old. A vinyl reinforced concrete patching compound should be used, allowing more flexibility.
With exposed-aggregate finish on the driveway, a larger patch will stand out visually. Adding pea gravel to the mix and exposing it with a spray of water before it has completely cured will help with the aesthetics, but could compromise the integrity somewhat. I wouldn't risk it unless you are highly experienced.
Latex and epoxy patching compounds will be more expensive, but will be stronger and have better adhesive properties. Ureas, injection resins, epoxy healers and other exotic materials are also available for crack patching, typically when water is an issue. The number and types of materials available on the market is staggering.
There is a patching material that is right for any need. Specialty suppliers will have the greatest selection and offer the best advice, but generic patching compounds are available widely.
Darrell Hay answers readers' questions. Call 206-464-8514 to record your question |
