Freezing Concrete

[AUCE98]

Has anyone ever had an instance where concrete has frozen to reinforcement during concrete placement, or know how to determine if the bond capacity has been altered?

Thanks,


AUCE98

 

[samdamon]

AUCE-

If the concrete froze as you mentioned, bond may be the least of your problems. Unless you are making Portland Popsicles or something. Absolulely no offense intended, but this question drew a chuckle!

As you might be aware, customary cold weather guidelines provided by the ACI (See ACI 306R) require that freshly placed concrete be protected and kept warm enough to hydrate properly.

Do you know how cold the mix got and how long after placement it froze?

regards

 

[AUCE98]

SD,

As the concrete was being placed into a large foundation, reinforcement in the top mat was being coated by concrete passing through the reinforcement and freezing. Due to the size of the foundation it was one or two concrete trucks later before the top mat was covered. My concern is that the reinforcement that was coated by the concrete has diminished bond strength. The contractor did not believe that the reinforcement needed to be heated prior to placement, which I agree is common.

 

[Ron]

AUCE98....I agree with SD....least of your worries at this point. Your description implies that weather was so cold as to preclude placement without additional measures such as ACI 306. Your concrete might be damaged more than just in its bond strength to the rebar.

Assuming the frozen concrete got covered and remained frozen until the heat of hydration of the covering concrete could thaw it, you likely will get hydration and bond, but with a weaker concrete as a result. That is what will affect the bond...you have weaker concrete around the rebar.

 

[LPPE]

I believe its in the ACI Code of Standard Practice that says the reinforcing steel should not be below freezing, (nor coated with form oil...), just like the ground should not be frozen. If the concrete on the top mat froze to the reinforcing steel, then rather quickly thawed from the heat and mixed with the fresh concrete, I dont think it would be too much of a problem if the frozen bits mixed with the fresh and the fresh concrete brought the rebar up to temp. Otherwise, cold weather concreting tips says "Concrete must not freeze for 2 to 3 days, or before it reaches a strength of 500psi". If it froze right out of the chute to the bar, thats definately under 2 to 3 days.

 

[AUCE98]

I agree that with the amount of heat generated by the curing concrete that the concrete surrounding the reinforcement may have been melted, however there is no way to determine this, and the on-site material testing agency has deemed the foundation not in compliance of ACI 306. Would you, with confidence accept the liability of this work, knowing that the material inspection agency has gone on record saying that it is non-compliant due to the concrete freezing to reinforcement. The overall strenght of the concrete can be determined with the 28-day strength, however how do I verify the bond strenght of the reinforcement.

 

[jheidt2543]

I would suggest the following:

1. Document the condition with a letter to all parties and pictures if available. Verify air and concrete delivery tempertures, get copies of the concrete delivery tickets and mix designs.

2. I tend to agree with LPPE that it MAY be OK but, before making the decision to reject the concrete and/or the foundation, I would take some samples and have them tested both for concrete strength (standard core test) and a rebar pull out test. You should be able to identify a location where a test can be made without affecting the foundation's use should the test prove OK. The cost of this should be paid by the Contractor if the concrete is shown to be defective.

 

[Ron]

AUCE98....in response to your last point....NO.

You can't verify the bond strength nor the "microcosmic-strength" of the concrete around the rebar.

 

[AUCE98]

Thank you all for your advice.

Regards,

AUCE98

 

[DRC1]

1. The rebar acts as a heat sink and if it is cold will chill the surrounding concrete. If the bars were cold, the ground and or forms were also cold. In cold wweather, it is common to put out propane heaters in or near the forms to blow warm air into the forms for a couple of hours before the pour. (remember, you are only trying to warm the surfaces, not grill lunch)
2. If the concrete freezes at inital set, you will loose bond. After a day or so, it can usually stand cold weather, although three days is recomended.At this point strength gain will be retarded, but will continue once the warm weather returns.
3. The cylinders do not give you the insitu strength of the concrete. It only tells you the strength of the concrete under ideal curing conditions. To check field strength beams are cast and field cured and broken or the concrete is tested insitu.
4. I don't know if it would answer questions about concrete quality or not, but if you wanted to test insitu bod strength, I would locate straight rebar (vertical perfered), the smaller the better. Saw on each side of the rebar and chip away the concrete. Use a self leveling grout to pour a pad. Get a center hole jack rated for about twice the expected bond strength. Cut the bar slightly longer than the jack and weld a plate to the bar.
Fix a dial to the top of the bar, attached to an independent support. Jack in increments and record movement of the dial gauge. Point of bond failure should be evident.

 

[eric1037]

AUCE98:

Why would you want to take responsibility for something that the independant materials consultant says does not comply with specifications? It is clearly the responsibility of the contractor to provide a foundation built according to plans and specs. Therefore, they should demolish and replace the foundation at their own cost. I sure as heck wouldn't get involved.

 

[PT999]

Petrographic testing of cores may be what you need done.
This test is somewhat pricey, but definitive.

 

[66engineer]

Whoaaa....what a mess.

Whenever I become a part of problems like this, I consider several things before I jump to a solution/resolution, and I have not seen all of these discussed yet. As I am sure you know, there are MANY ways to solve a problem...

1. Who "dropped the ball" on this, i.e., who will pay for repair?? Contrator, engr., testing firm, etc.?
2. What are the specifics of the foundation? What size, pile supported, what is on it, etc.?
3. If it doesn't work as designed, what will happen? What are the risks of failure?
4. What specifics were recorded by the testing firm during placement? Do you know the ambient and concrete temps?
5. What safety factors or redundancies were included in the design? If you have time, core the cylinders at some time in the future, not exceeding 28 days, and see what the strength is. You can also back calculate safety factors based on actual strength, if it gets close. I agree with the others, frozen bonds to the rebar are the least of your potential problems. Let me know how this works out. Remember, the engineer's job is to not only read and repeat codes, it is to solve problems safely and cost effectivley.