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Control Joints and Concrete

Have you ever wondered while walking down a sidewalk why are there lines in the concrete? Or, a better question, what's the purpose of those lines? I thought for years it was for esthetic reasons, or so I could walk down the street and in my head recite the jingle, "step on a crack break your back, step on a line you'll be fine."

Today's blog features an exerpt from a report written by BFI for one of its clients explaining the purpose and function of control joints (those lines strategically placed in concrete, see attached photo). Hope this explanation is informative.

It is an accepted axiom of concrete technology that, a 100 foot long concrete slab can shrink between ½" and 1", and sometimes as much as 1½". When required, a commonly used restriction on concrete shrinkage is 0.04% maximum, and this translates to about ½" shortening per 100'. It is also typical that slabs-on-grade are usually restrained to some degree by the materials that support them. Thus, cracking in concrete slabs is not uncommon. Generally, such shrinkage manifests randomly with three pronged crack intersections, each between 1/32" and 1/16" wide, which are semiregularly spaced along the slab length. Effective control of objectionable random cracking is usually provided by some combination of limiting concrete shrinkage and the mandate of details which cause the cracking to occur at straight lines called control joints.

Control joints work by creating intentional planes of weakness through a slab which lead to preferential linear fracturing along an intended joint line. When shrinkage forces develop, they concentrate tensile stress at this reduced slab section or weakened plane, and the slab cracks neatly along such a joint, rather than in some random, visually and potentially dysfunctional and objectionable fashion. Thus, cracking is not eliminated but merely relocated and concealed within a joint. Another means of reducing tensile forces within slabs which can lead to cracking is to reduce conditions of restraint.

Consider a slab which is free to shrink along its length and width; that is, a slab that could shorten without friction, without pushing or pulling on the soil, and without pushing or pulling on adjacent work. If the slab could freely shorten by sliding along the material supporting it, no cracking would occur and its length would shorten slightly at its edges and joints toward the middle. However, if the ends of the slab were prevented from moving, if the friction between the slab bottom and the base restricted shortening, if the slab thickness is not uniform, but varies in section, leading to resistance to shortening, or if other construction interferes with the slab’s ability to shorten, then such restraint to shortening increases the frequency and width of random cracking. Timing of various aspects of construction influences the manifestation of cracking, as well.

- BFI Staff Writer

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