Making a 4" hole in my foundation , will cutting the rebar cause the foundation to crack ?
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Well you have not provided information even for a real guess. We would need to know what part of the foundation, if a stem wall how tall, thick, number if stories and where the hole is to be drilled. For example on a 2’ wall 10” below the top on a single story it would not cause a problem because the top piece of rebar would be above and the lower pieces would be below. If you hit a verticals it is no big deal as that is holding the horizontal pice up and really is not doing much. However if you hit the top horizontal rod this can cause problems with the J bolts that hold the bottom plate. – Ed Beal May 03 '20 at 03:58
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Cutting the re-bar so you can complete your 4 inch hole could cause your foundation to crack. I suspect that you already know that or else you would not be asking this question.
The problem here is that there is no way to provide any answer regarding assurance that cutting the re-bar would not be harmful to your foundation. There are just to many unknown variables to be able to make any prediction. Some of these unknowns are:
- Are there other re-bars in the foundation
- What part of the foundation, wall or footing
- Type of construction of the foundation, cement blocks, poured concrete, stone or bricks
- How thick the foundation is
- Where the hole is located relative to edges, top, bottom or corners in the foundation
- Hole location above or below grade
- Past history of building structure foundation
- Temperate zone location
- Seismic zone conditions
Michael Karas
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If you are talking about a typical foundation for a house on even grade with a crawlspace (no basement) and no retained wall then disregard the other answer here. You're probably not going to ruin anything or cause a massive issue.
If you drill though one piece of reinforcement in your foundation wall then it won't cause cracking, though existing cracking, even if not yet visible to the eye, may widen some. It will expose the cut ends of the reinforcement to the air and therefore it will not be protected from rust anymore. Over longer time frames, this can become an issue.
Generally for this type of foundation the reinforcing is simply there to keep the concrete together as a single unit as the concrete cures and micro-cracks. This is called tension and shrinkage (T&S) reinforcement.
The T&S reinforcement does give the foundation an inherent baseline strength that is often considered in design in windy or seismic areas. For example, at shear wall holdowns the forces must often be resolved over some distance of the foundation and often that uses some of the extra capacity of the T&S reinforcement.
I assume that you need to drill this hole through the foundation as the only realistic solution to your situation. Your best option is to not hit reinforcing when you drill the foundation. For commercial construction the wall would be x-rayed or otherwise scanned in order to determine the location of reinforcing and locate a place free of it to drill the hole. You can make some educated assumptions to help locate your hole vertically, as the top piece of reinforcing is likely about 3" down from the top of the wall and the next piece 12", 15" or 18" below that (15", 18" or 21" from the top). Locating the hole horizontally to miss the vertical reinforcing is more of a guessing game. Likely it is 15" oc or 18" oc (the maximum allowed) and the layout probably starts about 3" from the outside face of one of the adjacent corners (but which one of the two is a best guess).
General Information About Reinforcing and Concrete Cracking
Reinforcing in concrete foundations is located in both the vertical and horizontal orientations.
Reinforcing is NOT to prevent cracking, it is what provides the tension strength of the concrete/reinforcing composite (concrete (usually) provides the compression strength). Concrete as very little strength in tension when not cracked and absolutely none when cracked. For design it is assumed that the concrete is cracked and the reinforcing is taking all the tension forces.
Concrete shrinks when it cures (dries) and that alone can cause cracking. Other times it cracks under load. Cracking of concrete in itself is not always, or even often, a design issue or failure, as long as the cracking is within certain limits.
While it may not be a strength issue it can still be an serviceability issue if the crack allows water to get to the reinforcing which can then oxidize (rust), and then later become a strength issue. The minimal cover of concrete over the reinforcing is 1-1/2" to properly protect it.
Sometimes cracks are visible and often cracks are not visible to the naked eye.
