Published: Sept. 26, 2020, 6:02 a.m.
Have you ever wondered about those long, straight cracks on concrete sidewalks or slabs? Well, wonder no more. Contraction, or control, joints in concrete have an important purpose.
Here’s a crash course on control joints, including why you need them, how to create them, and what to do if your concrete slab lacks the proper support.
About Control Joints in Concrete
Extreme temperature changes cause building materials to expand and contract, and concrete is no exception. This shifting can cause hairline cracks to form on the concrete’s surface.
In addition, concrete mix shrinks as its water content evaporates, and this can lead to structural and aesthetic issues if you don’t create ‘planned cracks.’ Cracks are inevitable, and most people want to have a say in their location, or have greater ‘control’ of the situation.
Creating control joints in concrete to take stress from the slab doesn’t just preserve the slab’s appearance; this also prolongs its life. Otherwise, elements such as stormwater can enter the cracks, enlarge them, erode underlying soil and weaken the structure.
How to Create Control Joints in Concrete
You can create control joints in concrete while pouring the concrete, or you can wait until the slab has cured just enough so it won’t chip when you cut into it.
Grooving tools made of bronze or heavy-gauge stainless steel are used for scoring freshly poured concrete, and cost just $25 to $35. Or you can saw cured concrete.
A good rule of thumb is to space control joints about two to three times, in feet, the size of the slab’s thickness, in inches. For instance, plan control joints every 12 to 18 feet for a 6-inch slab.
In addition to those intervals, you also need to cut control joints at a specific depth — about 25 percent of the slab’s depth. So, for the same 6-inch slab, the joints should be 1.5 inches deep.
What If There Are No Control Joints?
Your concrete slab should have a control joint to protect the slab’s appearance and prolong its life. But if it doesn’t, that could cause all kinds of problems.
That’s the situation Genee, of Kansas, is experiencing. The contractor enlarged a concrete patio with poor results, she says.
For this job, creating control joints in the concrete was, of course, necessary, to prevent hairline cracks. But because the contractor added to the patio, an expansion joint between the two slabs would have prevented sinking in that area.
The situation is now an unsightly mess, and she’s wondering what to do about it.
We’ll talk about that, and much more, on this episode of the Today’s Homeowner Podcast.
- [1:09] Which nails you need to attach new wood trim — and how to not split or dent the wood
- [5:54] Tips to remove old cigarette smells from once-smoky rooms
- [10:46] What to do when Hardie Board siding chips or cracks
- [12:20] Best New Product: Professional-Duty Waterproof Storage Container from Husky
- [13:22] How to remove rust stains from asphalt shingles
- [16:50] Simple Solution: White nail polish as a necessary part of every workshop…? Yes!
- [19:36] What to do when your cast-iron weathervane freezes
- [24:02] Question of the Week: “What is the best solution for an ugly concrete patio?”
Simple Solutions
Easy Tool Identifier — There aren’t many reasons to use nail polish in your workshop, but here’s one. Many tools come with markings that are virtually impossible to read. Wire strippers have little gauge numbers, squares have numbers and graduations that are hard to read, and sockets are some of the worst offenders.
Well, here’s a tip that eliminates having to strain to see vital information on your tools. We’re going to highlight these numbers and graduations with white nail polish!
Shake the nail polish to ensure it’s mixed well. Then simply brush it onto the tool. Don’t worry if it’s a little sloppy; most will be wiped off later.
Coat the entire area on and around the numbers. Wait a few minutes, then take a damp cloth and wipe away the excess nail polish, leaving the white polish in the recessed areas.
Easy Drip Irrigation — Here’s how to create a simple drip irrigation system out of an empty 2-liter soda bottle: First, drill some 1/16-inch-diameter holes through the sides and bottom of the bottle. These might seem like tiny holes, but you want the water to seep out slowly.
Next, dig a hole in the garden, near a shrub — or even a large potted plant — and bury the bottle, leaving just an inch or so of the top sticking out. Twist off the bottle’s cap and fill it with water using a hose or funnel and water jug. You can also add fertilizer to the water, if you’d like.
Then twist the cap back on and spread mulch over the area. Check the bottle in three or four days, and refill, if necessary. Question of the Week
Q: “What is the best solution for an ugly concrete patio? A concrete pad was poured when the house was built. We had this enlarged with poor results. The contractor never came back to score it [for a control joint] and now it has cracked all over and even sunk a bit at the cold joint.
We are considering staining in a marbled tone of brown since it would blend well with the cedar house. We also looked at an epoxy coating or Quikrete resurfacing. With it being in full sun, it seems like there are negatives to all options.”
A: The additional patio wasn’t properly poured or supported, and it’s caused quite a headache for this homeowner. One option is to frame a wood deck over the whole thing. Then you can install composite decking over that.
Fun fact: You actually can glue composite decking to a concrete slab, if it’s in good condition, and the best part is there’s low maintenance — no stripping, staining or painting — compared to a wood deck.
