Shrinkage cracks? photo 1

[demayeng]

Hi,
This photo shows a repair in a carpark slab on ground. Apparently a water hydrant line failed under this point so the builder saw cut a hole, fixed the pipe and did this repair.

You can see there are radiating cracks out from the hole. I am assuming this is due to a release of shrinkage tension in the mesh when the hole was cut out of it, however would like your expert opinions!

I don't have details of the original leak and symptoms.

 

[Ron]

Those are shrinkage cracks, but they were there before the sawcut for repair. Since it is on grade, I wouldn't worry about them.

 

[demayeng]

Thanks Ron. They actually happened after the sawcut. You're right though, normally they wouldn't be enough to worry about. In this case however, the developer is unhappy that he now has to live with an ugly repair that was caused by a builder stuff-up. We designed the slab and working for the developer we have to report on the cause.

 

[Ron]

demayeng...There's a good chance the cracks were not wide enough to be easily visible. The orientation of the cracks does not lead me to consider that they occurred after the cut. For example, if they occurred after the cut, they would more likely propagate from the corners of the cuts (a re-entrant corner condition).

In any case, stuff happens in construction, so the owner will have to accept that a repair was necessary and there's not much that can be done, short of fully removing the slab from joint-to-joint.

 

[demayeng]

They definitely happened after the cut. There is a crack radiating from each re-entrant corner, plus more part way along each side of the square. Each crack is widest at the repair and taper off to zero about 3-4m away from the repair. Plus they are the only cracks of this nature on the 300 square meter fully jointed carpark.
My job is just to give a cause of the cracks - I assume the developer is looking for something to sting the builder with. I believe the cracks are an inevitable result of having to cut a hole in the slab (mesh is cut so can no longer hold the shrinkage tension) and not the builder's fault. The only way of fixing is to replace the whole panel between joints.

 

[Delchi]

demayeng, Is it possible that there are expansive soils at this location? If so, it is very possible that the cracks could be from heaving below the patch, caused over time as the water from the broken hydrant slowly seeped into the surrounding clay soils. This would explan the crack pattern and the sequence of events, and would fit perfectly with a water-related event.

Ron, this would also fit your hypothesis that the cracks were there before the cut, probably radiating ouward from the original pentration of the hydrant through the slab.

 

[demayeng]

Good suggestion Delchi - it's all sand underneath, not reactive at all. One possibility is that a void formed under the slab as the hydrant pipe sucked sand into it. We get voids a lot around leaking stormwater pipes in the sandy sites around here.. I just thought the cracks looked a lot like shrinkage.

 

[Delchi]

As a side note: the presense of expansive soils could explain the cracking regardless of whether the cracks were present (and invisible) before the patch, or formed afterward.

 

[Delchi]

Sorry about that last post--I was writing as you were

Is it possible that the contractor lifted the soils upwards as the hydrant was yanked out of the ground, causing the slab to rise during the demo?

 

[demayeng]

Possible, but unlikely. It was just a pipe repair I'm pretty sure, so no yanking involved.

 

[Delchi]

Well, you got me, unless they overcompacted the sandy soil during backfill.

 

[apsix]

I know of a slab on grade blowing out due to a fire main failure.
Is it possible that the hydrant line failure was catastophic and caused uplift, or was it just a slow leak?

 

[demayeng]

Thanks. yeah it's possible I guess. I'll have to ask the developer some more questions. One of the cracks appears to originate at the end of one of the sawcuts though.

 

[hokie66]

I think they are definitely drying shrinkage cracks, and agree with demayeng that they probably formed after the sawcutting. Reentrant corners in slabs on ground almost inevitably crack due to the stress risers created, so we typically add some perimeter and corner bars to control the cracks. As there were no bars at these unplanned corners, the cracks did what they wanted.

 

[Ron]

demayeng...spray some water on other slab areas and watch as it dries. I'd be surprised if these are the only cracks in 300 m^2.

I agree if the developer does not like the look of the repair, then joint-to-joint replacement is the option.

There's no evidence of anything other than shrinkage being the mechanism. If heaving or uplift, there would be faulting at the cracks. The only thing that puzzles me is that there is only one true reentrant corner crack showing..perhaps I just can't see them in the photo.

 

[Teguci]

Shrinkage cracking doesn't fully make sense. The cracks are radiating from a patch. For shrinkage to be the mechanism then the patch must have caused localized shrinkage of the existing slab. Perhaps the patch was left open for a long time which allowed both top and bottom surfaces of the slab to dry faster? Because of settlement the bottom of the slab can dry faster? The patch pulled the existing slab in?

This is a puzzle.

 

[hokie66]

I will try to solve Teguci's puzzle.

When the hole was sawn in the slab, the entire slab was in tension due to the ground restraining the soffit of the slab. If we assume that the tension stress across a given section is uniform, and then we remove part of the section, the force taken by this removed part has to redistribute into the remaining slab. The cut edge adjacent to the removed part thus tries to take the force, resulting in cracking of the concrete until the force redistributes to the steel.

While cracks often propagate from reeentrant corners because of the stress riser effect, they can start at other places also.

 

[paddingtongreen]

I agree on shrinkage.
The concrete shrinks during curing, the minimum, "shrinkage reinforcing" makes sure that many fine cracks appear instead of fewer, wider cracks. When the piece is removed, the restraint provided by the rebar is gone and the local cracks open up, especially at the corners where the release is in both directions.

When making repairs or modifications to slabs where appearance is important, I usually specify core boring the corners first and then cutting the straight edges. It's not perfect but it helps.

Timing has a lot to do with the outcome of a rain dance.

 

[demayeng]

Thanks for all the replies to this. I agree with the last couple of posts.
I found out that the repair happened 3 days after pouring of the slab. I'm not sure how the location of the water leak was evident however - the parties involved are still not talking to each other!

 

[paddingtongreen]

"- the parties involved are still not talking to each other!"

LOL

Michael.
Timing has a lot to do with the outcome of a rain dance.