Confining Force on Objects Embedded in Cured Concrete 4

[RobertP4321]

Hello,

How would one go about approaching an estimation of forces exerted by freshly cured (say 3 months) concrete on embedded objects. In particular I am interested in the forces on a vertical post embedded in a horizontal slab. As the concrete cures and shrinks, it should put quite a bit of pressure on the post. I'm at a loss finding how to go about doing the calculation.

Thank you for your consideration.

 

[1503-44]

Never really thought too much about that, but I envision it like this ...

When concrete shrinks, it makes microcracks because it is in tension. That's why minimum shrinkage reinforcement is added to concrete. Concrete around a post will want to pull away from the post surface. It will do so up until the shrinkage reaches its tension bonding stress to the post. If there is shrinkage reinforcement near the post, as the Concrete tries to shrink, it is more or less held in place as the shrinkage stress is transferred to the steel as a tension load in the bars. The bars will stretch slightly as they hold the Concrete together, not allowing the microcracks to advance. So the Concrete wants to pull away from the post and may try to, but the steel tends to hold it in place. Once the concrete to post bond stress reaches its maximum, the Concrete may release the post and pull away from the post, but only by the amount of stretching that the bars experience at that level of bond stress. With rebar in the Concrete, or not, the post could experience a maximum surface tension equal to the Concrete bonding stress, so some blistering of a soft material post might occur. After, if max bond stress is reached, the Concrete may disbond from the post and let it go, if concrete shrinkage continues. It may be beneficial to sleeve the post if concrete bond tension might damage the post's surface, if it was a soft material; soft wood for example.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[TigerGuy]

If a vertical post is poured into the middle of a slab, doesn't the slab contract against the post as the concrete cures and shrinks? Isn't that why the diamond block around each column in a large commercial building is poured after the main slab placement?

 

[BridgeSmith]

Concrete would shrink away from the post.

 

[TigerGuy]

In a single pour situation? A round penetration through a panel that is shrinking in length and width would expand?

Or is the shrinkage kept very local within the slab?

 

[Denial]

Unless there is something VERY strange going on, a void in a body that has contracted uniformly will have contracted to the same extent as the body itself.

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^{Engineering mathematician / analyst.[ ] See my profile for more details.}

 

[BridgeSmith]

The concrete along the edges of the hole is shrinking, as well. Depending on the overall size of the slab, it may or may not be enough to expand the hole, but even if it didn't do anything to mitigate the overall shrinkage, the contraction of the hole around the post is certainly not enough to damage the post. According to what I've read, the high end of shrinkage rate for concrete is around 1/2" in 100 feet, so the contraction around, say an 8" diameter post is at most about .00333". I seriously doubt that would have any measurable effect on the strength of the post. It might crush the fibers in the outer 0.2% of the post cross-section. Of course, unless the post had a very low moisture content when the concrete was poured, it will shrink far more than the concrete.

 

[1503-44]

Shrinkage of curing dehydrating concrete is not thermal expansion and contraction and how an object is restrained (soil friction under slab, or edge restraints on a plate) causes tension.

https://ncma.org/resource/crack-control-in-concrete-masonry-walls/

https://www.researchgate.net/public...ed_modeling_of_shrinkage_porosity_in_castings

http://mwengineering.net/wp-content/uploads/2241r_07.pdf

https://www.journeymanhq.com/9621/why-does-concrete-crack-when-drying/

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[dik]

There's a real funny anecdote to that. Over 50 years back, on one of my projects, the contractor wanted to know is he should remove a strip of plywood joint material. I told him the concrete would shrink away and he would be able to remove it by hand. If the slab's still around, the plywood likely is.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BridgeSmith]

I told him the concrete would shrink away and he would be able to remove it by hand. If the slab's still around, the plywood likely is.

It probably did shrink, but if the plywood wasn't oiled well, the plywood is well-bonded to the concrete.