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Weird concrete slab surface pattern

Weird concrete slab surface pattern

Weird concrete slab surface pattern

I found this weird pattern on an existing concrete slab (exaggerated by some black-ish growth on the higher points), I wonder if anybody can tell me what it is - I have never come across it before.

The slab was constructed in the UK about 10 years ago. It is approx 2m thick, and was built using separate rebar rods (not mesh), with a nominal cover of 50mm. The surface does not appear to be cracked, however the surface profile 'bulges' slightly in the gaps between the (I presume) rebar positions. It's as if the rebar was pressed into the surface, and then not levelled., however the reinforcement cannot have been installed like this.

Otherwise, the concrete looks fit and healthy.

My guess is that the construction team walked on the rebar cage immediately after the concrete was poured, leaving a slight void under the rebar. The concrete surface was then levelled, and in the next few hours the concrete slumped into the voids creating a slight depression in the concrete surface where the rebar is located?

RE: Weird concrete slab surface pattern

You are essentially correct. In a thick slab like that, even with good vibration compaction, the concrete will continue to settle for a while. If it is screeded without revibration, patterns like you are witnessing can result. It is common to see cracks following the bars due to this plastic settlement cracking, but perhaps the finishers closed the cracks.

RE: Weird concrete slab surface pattern

It's the opposite way round though - the concrete is higher between the bars, not lower.

RE: Weird concrete slab surface pattern

Is it possible that with the thickness of the slab, that 'air' or bleed water worked itself up to the U/S of the rebar and that the rebar 'settled' into this 'void' while it was still plastic?


RE: Weird concrete slab surface pattern

no doubt workmanship. That said, I don't really know. Maybe they pulled the reinforcing up after the initial pour (i.e., as if there were no chairs below the reinforcement)?

Who knows. . .


ípapß gordo ainÆt no madre flaca!

RE: Weird concrete slab surface pattern

Agree with hokie66....they walked in the mat from the top, screeded and didn't revibrate.

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