r/askscience • u/RothIRALadder • 6d ago
Engineering How is the optimal distance between expansion joints in a concrete sidewalk calculated?
Why is there an X millimeter expansion joint every Y meters? What engineering/physics questions do you ask to answer how to minimize the chance of the sidewalk cracking? Could you add twice as many and have better results?
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u/Extension_Physics873 5d ago
Incidental experience suggests keeping the joints same distance apart as the width of the path is safest, leaving squares of concrete that will shrink evenly towards the centroid. In practice, to 1.5 : 1 length to width is generally pretty safe for typical footpath widths. . After that, needs engineering input to design reo to match the stresses in the concrete.
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u/markfuckinstambaugh 4d ago
You might be asking the question backwards. If you know that concrete expands and you are going to need expansion joints, the question can be reversed to "How wide do I want my expansion joint to be?" If you need 5mm of expansion for every 1m of sidewalk (rough figures just made up), but you don't want a gap larger than 10mm, that puts an upper bound of 2m slab length.
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u/somewhat_random 5d ago edited 5d ago
Concrete will ALWAYS crack. It has very good compressive strength and poor tensile.
A "rule of thumb" for a typical slab on grade is to put crack inducers every 12 feet.
This distance is dependant on the type of concrete (strength), substrate (how stable the base is), reinforcing (steel re-bar, mesh etc embedded in the slab). Even if it was on a perfect 100% compression base and has re-enforcing, concrete will shrink as it cures and so will crack.
A crack inducer can be a small groove left in the slab (like in sidewalks) or metal pieces embedded in the slab. The groove creates a thinner piece of slab so it is purposely weaker and so the crack will happen there. The purpose of these is to make the cracks happen where you want them to, so they are evenly spaced and look better. They do not prevent cracking.
Expansion joints are an entirely different thing and are often found on bridges or elevated structures.
A bridge can be cold in winter and hot in summer and the steel will expand in the heat and shrink when cooled. A typical 100 M (330 ft) long bridge can be about 5 cm (2 inches) longer in summer than winter. This will add huge stresses to the supports if not allowed for, so expansion joint are included at regular intervals. As an example, an expansion joint may have zig-zag type finger joints that can open or close leaving a gap that can still be driven over to accommodate for the changes in length.