Why Aren't Concrete Spacers Required In Foundations?

  • #1
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Main Question or Discussion Point

A discussion arose tonight about why spacers aren't used in the foundations in houses, as opposed to sidewalks where there are gaps every few feet to allow for thermal expansion. Some possible answers were that the foundation would be more constant temperature, or that the concrete was somehow different and able to withstand the stress. My best guess was that a sidewalk is effectively endless in the long direction, so it has nowhere to expand to. A house foundation, even if a 100 foot square, could expand in both directions. I was pretty happy with these answers, but then I remembered parking lots. While parking lots are usually blacktop, I'm pretty sure I've seen some large concrete ones.

Secondly, there was some uncertainty about how the cinder-block walls could withstand the expansion, particularly at the corners.
 

Answers and Replies

  • #2
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First can I correct some terminology?
Spacers are small blocks attached to the ouside of reinforcement cages to prevent the reinforcement touching the formwork used to hold the concrete while it is poured/placed, before it sets. This ensures a layer of concrete surrounding the outside of the reinforcement cage. This layer is known as the cover. Its function is to protect the reinforcement from the corroding (rusting) effects of exposure to air and water.

The correct term for what you are referring to is joints. Joints can be provided to counter the effects of both expansion and contraction as well as other movements induced by groundwater or earth movements, earth tremors etc.

The provision is partly a matter of engineering judgement partly code requirements and partly design fashion.

Parking, airport and motorway slabs tend to be much larger than foundation slabs (even though they might not look that way on the ground).
Also being laid out to the environment on the top they are subject to curling and warping effects that do not affect buried slabs which have a much more even environment and smaller temperature differentials and ranges.

This also applies to elements such as walls, connected to these slabs so the walls will also expand/contract more evenly.

Hope this helps

go well
 

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