Make your concrete job a (non) cracking success - August 14th 2012
Now that summer has finally arrived, I’ve heard from many homeowners over the past week who are now ready to move forward with their concrete projects, such as patios, driveways, and sidewalks.
A common question I hear from people planning projects involving concrete and whether they should use fibre-reinforced concrete, rebar, wire mesh, or a combination of these three items to increase the structural strength of the concrete slab.
Steel-reinforced concrete uses rebar set in a grid pattern to provide inner strength to concrete slabs and columns. The rebar is suspended in approximately the center of the concrete. When concrete begins to crack, the rebar acts to retain pieces together but does not limit the spread of the crack since the rebar can only provide structural strength within the concrete.
Fiber-reinforced concrete uses Fibermesh, a proprietary additive, to increase structural strength limit or prevent cracking. Small particles of synthetic or steel fiber are blend into the concrete mixture. These fibers uniformly mix throughout the concrete and align in no particular direction. When concrete settles, it shrinks or expands causing cracking, the crack has a limited distance over which it can spread before reaching a fiber, preventing the crack from growing
I am a firm believer in using fibre-reinforced concrete when placing concrete for a patio, deck, walkway or other similar items. If you use fibre-reinforced concrete you do NOT require wire mesh or rebar.
I do strongly suggest jointing. This prevents cracking the slabs by encouraging the cracks to occur in the joints as opposed to on the surface of the concrete. Control joints are normally spaced at intervals equal to the width of the pour. It is recommended, however, not to exceed 10 feet in any direction without a joint. The joint should be cut at least one-fourth the depth of the slab. A jointer tool or saw is used for this step. Place a straight-edge across the surface and run the jointer along the straight edge to create a nice straight line. As with the edger, hold the front up slightly when pushing forward. Control joints in large slabs can also be cut after the concrete cures, using a masonry blade in a circular saw or concrete saw. I prefer adding control joints after the concrete is cured using a circular or concrete saw.
Expansion joints must be cut in large slabs. A jointer held against a straight edge produces a nice, straight-line groove.
Even when an installer has followed all the proper guidelines and procedures, cracking can occur. While proper jointing procedures will control cracking in the vast majority of cases, even properly jointed concrete can crack in places other than the joints. No one can guarantee that concrete will not crack however. You must understand that cracks are usually not an indication of a serious problem in most cases. Most of these cracks are simply unplanned joints (in many cases unstable substrate under the slab is the cause) - they are no more a threat to long-term appearance and maintenance than the joints that were intentionally applied.