Reinforcement Rippling

[SFlynn]

As a civil Engineer I have cast several large internal and external bays, however recently a concreting concreter has told me about the 'reinforcement rippling' effect.
See attached photos showing the rippling effect in the finished slab, we were casting 'Hit and Miss' as this was a 'miss' they were screening off two fixed points level should not of been an issue. The deflection of the finished slab would appear uniform with the mesh.

Can anyone please give me some advise as to why this has happened, whether this 'reinforcement rippling' effect is true, and if the concrete has been laid correctly. Concrete spec - C35 Air entrained.

 

[TehMightyEngineer]

Here you go: [URL unfurl="true"]http://www.concrete.org.uk/fingertips-nuggets.asp?cmd=display&id=645[/url]

Was any heavy equipment operated near the slab after it was finished? The slab could have been unintentionally vibrated after it was finished.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[chicopee]

That does seem like the outline of rebar, so was there enough concrete cover over the rebar? or perhaps the slump was not there or too much settling while the concrete was being vibrated while it was poured.

 

[jike]

This effect is normally called "ghosting". Did they "walk the mesh" into the concrete after it was poured? It usually happens with mesh and not conventional reinforcing. I think this may have something to do with spacing of wires and the flexibility of the mesh.

The primary cause of ghosting is pushing reinforcing material into the concrete after the forms are filled, which causes a pumping action directly under the reinforcing. This pumping action disrupts and changes the cement paste, which in turn results in ghosting.

Another cause of ghosting is to hang the reinforcing in the forms but pour the concrete through it. If the stream of concrete is split by the reinforcing but does not get a chance to remix below it, the aggregate gets pushed aside but the paste and cream flow back together. This causes a paste/mortar matrix change directly below the reinforcing that will cure slightly differently, resulting in ghosting.

A third way ghosting occurs is when reinforcing already buried in fresh concrete is moved or vibrated. The movement causes localized segregation, which in turn causes ghosting.

I know of no way to fix it......