Frozen Concrete Slab

[KootK]

According to our materials consultant, a large concrete floor slab that was poured last week partially froze before it had a chance to cure properly. Apparently the top 1.5 inch or so has been frozen. There are numerous concerns that we have here:

1) With the expaned ice trapped in the concrete, will there be voids, even after the ice melts?

2) With the obvious stratification that will be present in the slab, will we be able to justify treating the full slab depth as composite, as we assumed in design.

If anybody has any experience in this arena, please let me know your thoughts. What other things should we be concerned about? Any ideas for a solution?

Thanks,

Adam

 

[jike]

Solution = jackhammer!

How thick is the slab? If it is only 4" or 5", I would tear it out.

Is it reinforced with bar or mesh?

If this is a thick slab and has bar reionforcing, you might consider removing down to solid concrete and then scarifying that surface and then pouring a bonded topping.

 

[KootK]

Thanks for the suggestions jike. For what it's worth, it's a 9" lab reinforced with rebar.

 

[hokie66]

I agree with jike. Frozen concrete cannot be accepted.

 

[cvg]

before you get too excited, have your "materials consultant" make a recommendation based on an appropriate investigation of the slab. You may want to take some cores to the lab for petrographic examination and for compression testing etc. before you rip it out.

 

[civilperson]

If a set was obtained prior to freezing, then probably no damage is inherent in the slab. After thaw and continuation of the cure process, the strength may achieve the desired F'c. I had a PC Cement concrete street placement freeze in February and by end of April was up to 5000 psi. Be patient and then cut some cores to prove what you have.

 

[msquared48]

1. BIG voids...

2. Breakout and repour the slab when the weather is not freezing.

Mike McCann
MMC Engineering

 

[DRC1]

If the slab froze with in the first 24 hours, it is probably no good. freezing under 500 psi allows the frost action to destroy the bond. 24 to 72 hours its a maybe, after 72 you are probably okay, assuming the slab was protected. However, even if the slab is okay, it will gain strength only above 32 deg F and if the temperature is close to that, only very slowly. If you are pouring slabs in cold weather, recomend well thought out winter protection and use of contiuosly recording thermocouples.

 

[msucog]

i wouldn't get crazy jack hammering yet. take cores (lots of them), drag a chain (to check for surface delamination), have a materials person take a look at the slab. the contractor should be entirely on the hook for a longer period of time to warranty the slab if it is decided to leave it in place. they are responsible for curing and protecting the concrete so they should be on the hook to replace it if needed, give a credit to the owner for "damage" due to their negligence, and/or warranty the slab for an extended period of time to see if the longer term performance goes to crap. if it truly froze, it could really be ugly. however, if it set sufficiently prior to freezing, it could be okay (may not be though). perhaps it turns out that only the upper 1-2" needs to be taken off and replaced at the contractor's expense. this might be a good application to use a rebound hammer to supplement the coring.

out of curiousity, what was the lowest air temp and lowest wind chill and how soon after placement did these temps occur? was there an admix used? is the slab "on grade" or elevated? why the heck didn't the contractor follow the appropriate steps to protect the slab per aci?

 

[LobstaEata]

If I recall the behavior of concrete materials correctly, I beleive if your slab has been placed last week, then temps in Alberta remained above 20 F until Friday, so if the placement was prior to Tuesday, then freezing damage of the entire slab may not be at issue. The internal heat of hydration would have kept most of the slab warm enough to prevent freezing, however if the top surface was left unprotected, then your materials guy may be correct about damage to the top 1.5 inches. The top surface is where bleed water wants to migrate and the higher paste content due to consolidation is likely weaker than the deeper region of the placement.

Definitely take the advice of cvg and msucog, before insisting on ripping out the entire slab. Find out the level of damage for sure. If you don't have good evidence that the level of damage requires complete replacement, then it could get messy with the contractor, if it does not turn out to be so. He'd probably hit the engineer and owner with delay claims if you're wrong.

 

[dik]

I've encountered structural flat slabs that have been frozen. It depends when this occurs, at what point in the hydration process. If the concrete has not started to set up, then it can likely be frozen and thawed without any detrimental effect. Your material guy should be able to offer an opinion. Can do a core and examine the concrete gel microscopically to determine what if any damage has occurred.

The simplest solution is to tear it out...

Dik

 

[DonPhillips]

I seem to recall a NDT test called a Windsor Probe that can test the surface strength of the concrete in place. It should tell you if the frozen portion is below the minimum compressive strength.


Don Phillips

http://worthingtonengineering.com

 

[msucog]

windsor probe to supplement coring. i'd be sure to have the contractor pay for the testing that the owner/architect/engineer chooses so that they don't soon forget to follow construction standards.

if the slab itself did not freeze, you may have damage due to that up layer freezing during the finishing process.

 

[concretemasonry]

Adam -

How did the consultant determine the depth and if the concrete was frozen. He may have the information to proceed.

If it is fresh concrete a windsor hammer would be meaningless regarding the future condition.

Cores sliced and run through a microscopic exam would be a very good method to determine any destruction or disturbance that will not heal. The location of the cores can be important depending on the curing, protection and exposure.

A good testing lab frequently works with other in the analysis and it frequently done here on projects or by concrete suppliers.

Dick

 

[msucog]

you can use windsor probe at regular intervals across the slab to provide a relatively quick and "cheap" general assessment of the slab. then, as necessary to obtain "real" data with the probe, you take cores at the high, low, mid result locations for testing and to get a good look at what the concrete looks like (as well as correlating the probe results). also spot some cores "randomly". the windsor probe wouldn't necessarily tell how the slab will perform long term but could provide indication as to the extent of damage (at the surface). in other words, instead of taking cores at a 1000 locations, you could cores say 50 locations and windsor probe at a much closer intervals to provide a relative comparison of where the damage may be.

this may not be the ideal way of evaluating the overall condition, anticipated long term performance, etc of the slab but it is definitely a good, practical place to start in my opinion. if the damage is there and near the top, it should be evidenced by what i describe above. for that matter, you could probably split cores in to layers and perform testing on those layers to help identify where/how deep the damage occurred, if at all.