My Crack Is Bigger Than Your Crack
Dear Builder’s Engineer,
We are building a new home in Indiana. The slab was recently poured and we have approximately eight cracks over a 1,500-square-foot pad. One crack is possibly a bit over 1/8th of an inch wide and the builder agrees to fix this one with the injection of an epoxy filler. All of the cracks are very long (more than 6 feet), but the remaining cracks are smaller in width, under 1/8th of an inch, but appear bigger than a “hairline.”
What is your opinion on the number of cracks? The largest number of cracks we have seen in any home other than ours is two. Will these cracks continue to pull apart over time, requiring vinyl or carpet replacement or making it difficult to sell the home?
P.S. Our builder seems to think that these cracks will cause no harm in the future.
Jeff and Tricia Veach
Jeff and Tricia,
Just when I thought I’d addressed everything a person wanted to know about concrete slabs on grade, you remind me that I have not. Thank you for your questions, they are good ones.
By the way, if you missed any of my previous columns, I am quite pleased to announce that the first 61 — completely updated and expanded with many new sketches and explanations — are now available in a professionally bound volume entitled “Cracks, Sags, and Dimwits — Lessons to Build On.” It is available in paperback or downloadable.
Now to the issues at hand. As I’ve said before, concrete is at its largest volume the moment it hits the forms. As it cures, water is lost to evaporation and hydration, causing shrinkage. With shrinkage comes cracking. Always. Every time. The trick is not stopping the cracks (you can’t) but rather managing them. Management is usually done via control joints, i.e. shallow sawcuts or tooled lines in the green concrete that provide a nice, neat place for cracking to occur. Since control joints are not mentioned in your e-mail, I’ll assume none was installed.
What is my opinion of the number of cracks? My opinion is that eight random cracks over 1,500 square feet is probably too few, and only two is way too few. What? Yes, here’s why: I specify control joints at no greater spacing than 10 feet in each perpendicular direction, and less if the contractor and owner don’t object. Assuming your 1,500 square feet are arranged in a 30’ x 50’ rectangle, you should have a minimum of four control joints across the long dimension and two across the short. This would result in 15, 10’ x 10’ square sections. If you counted each side of each square as a likely cracking place, that would be 22, 10-foot long cracks.
What about the width of cracks? If the slab did all its cracking in one or two places, I’d expect those cracks to be quite wide — maybe 1/4-inch or more. However, if there were control joints spaced no greater than 10 feet apart, in an ideal world, the cracking would distribute among those joints somewhat evenly such that none would be too large. Concrete almost never behaves ideally, so some cracks will be wider than others, and some cracks may stray from control joints. I would not get too nervous about a 1/8th-inch wide shrinkage crack.
Non-shrinkage cracks. What about cracks not caused by shrinkage? If settlement causes the cracking, now we’re in a whole different ballgame. Most of this article assumes that subgrade (the soil below the slab) is firm and strong. If so, cracks will be caused by shrinkage. If not, cracks can be caused by both shrinkage and vertical movement, aka settlement. In this case, the repair needs to be structural — probably underpinning (my book addresses this in detail).
Repair of cracks. If there is a crack and the contractor says he can repair it with epoxy, be warned, that repair is mostly cosmetic. If the crack was caused by soil settlement, epoxy is like putting a band aid on a broken bone. It won’t do much. Truly fixing a settlement problem requires serious structural measures. Epoxy will fill the crack until enough movement accumulates again to reopen it, or start cracking anew elsewhere.
Will cracks continue to pull apart? The answer is that it depends. If the cracks are due to settlement, then, yes, they will continue until a repair is made, or until all the settlement has played out of the soil below. How long is that? If the soils are clayey, not in your lifetime. If the soils are sandy, the settlement may stabilize after a few years, say two or three.
If the cracks are due to shrinkage only, they will continue until the concrete loses most of its water to hydration (curing). For all practical purposes, this takes less than a year, about 75% occurring within the first month — if the weather isn’t too cold. So installing carpet or vinyl flooring after a month should be fine. I might wait two or three months to put down tile, however, particularly if the slab was poured in the winter when curing takes longer.
To summarize, if I were you, I’d be happy to have a few more cracks than your neighbor. Hopefully, that way they’ll be narrower. And cracks, whether in a concrete slab or on someone’s anatomy, are better narrower.
Tim Garrison of ConstructionCalc.com, is a professional engineer, author, and software producer for the building industry. Check out his new book, “Cracks, Sags, and Dimwits,” available at www.lulu.com.
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