Placement of Old Concrete

[Everynameistaken]

Hi All,

We have a project where the contractor elected to pour concrete well past the time allowed for in the specifications.

My understanding is the concrete was starting to get a little difficult to place (after 2 hours ) and the contractor added some water and then continued to place the concrete finishing almost 3.5-4 hrs after the water and cement were mixed together.

There is an issue of why this was done and who gave the authorization to place concrete out of spec, but

My question to everyone out there is what is the implication on the performance of concrete in this situation?

Is it simply a lower strength and some risk for long term durability or is there more risk to the concrte, ie. very low shear or compressive strengths, could ares become cumbly? Can it be assessed visually to see if there are any very bad signs that would require removal?

This application does not have high flexural, compression or shear demand, to the concrete is not under great stress but want to know if there are other potential issues I am not thinking of?

 

[HTURKAK]

The best time for placing the concrete after mixing is 30 minutes and acc. to cement type , weather conditions the setting time may extent to 1.5 hours.

After almost 3.5-4 hrs , the setting time already completed ,adding water and mixing will kill the concrete . The strength will catastrophically reduce.

I remember a case , the concrete was poured after 3 hrs and after a few weeks , when the forms and scaffolding removed , all the bridge deck collapsed and the contractor was fired and no more at the nominated contractor list of the highway authority.
...

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[Tomfh]

It has an impact on concrete strength, durability, water tightness, shrinkage, etc.

It’s one of those things they do to varying degrees, and often get away with. A lot of concrete trucks get a little extra water if the mix is going off too quick.

Whether it’s acceptable in this instance is up to you, based on the actual concrete properties, and your requirements.

 

[lexpatrie]

It would probably show up in the compression test cylinders as low strength and be unacceptable. That's the risk. If this is a continuous footing under a wall the strength may be excusable but I'd be concerned with freeze-thaw resistance in my typical practice areas. Permeability may be another durability concern if there are sulfate soils (Las Vegas). It may be more porous and that may affect durability in other ways as well.

 

[TLHS]

For low strength applications where you're less concerned about the compressive strength the problem is that the surface finish and durability can be affected. You're more likely to end up with basically any of the common surface defects, it's going to finish worse because it is partially set when they're floating, troweling and finishing, corners are more likely to chip and you're more likely to get a powdery finish. In the long term you're more likely to get things like erosion of the cement matrix.

You see some of that workability related stuff at two hours even with a retarding agent. At four hours it seems like it would be impossible