Strength gain by concrete in cold weather 2

[gp238]

I have a reinforced concrete basement wall for a new home poured by a contractor at Amherst, NY on December 14th. The high and low temperatures on that day were 32 and 20 degree. The high & low tempreatures on the next day were 40 and 24. The water-cement ratio of concrete was 0.54 and type III cement was used with super-plastisizer and 5-8 % entrapped air. After 7 days I tested the concrete strength of the wall by a Swiss rebound hammer. I tested at least at 20 different points and the correlated compressive strength was 1000-1200 psi. Also, when I tapped the surface of the wall by a small hammer it left a dent on the wall. When I contacted the contractor about the problem, he informed me that the concrete supplier has tested one cylinder from the batch and the 7-day strength is 2000 psi. In my opinion , the tested cylinder is not cured at the site and thus does not reflect the in-situ strength of the concrete wall. I am very worried about the basement wall and hesitant to let the builder start framing. Could you or someone please advice me.
Thanks!
gp238

 

[Ron]

gp238...a couple of observations...

1. Was the concrete protected from freezing in its first 48 hours? If not, you might have significant damage to the matrix as it was setting up, with the result being lower strength.
2. The cylinder strength does not reflect the in-place strength condition. The purpose of cylinder testing is to check the design mix validity, not to check the in-place concrete strength.
3. The Swiss hammer is not reliable for this sort of testing either. You should have cores taken from the wall and tested to determine the in-place strength.

 

[gp238]

Ron, Thanks for your response!

Insulated blankets were placed on top of the wall but they were removed after 24 hours along with the form work. Also, the blankents only covered top half of the wall.
For core test don't we have to wait for 28 days?

 

[Ron]

No, you do not have to wait for 28 days. Go ahead at about 14 days or beyond. The strength gain (or lack of ) will be evident.

 

[gp238]

Ron,
Today I tested the Concrete wall (12 days old) by rebound hammer and the correlated strength is around 1500 psi. Since the bottom part of the wall was not protected from freezing at all and the top part was covered by a blanket only for 24 hours, I fear that concrete has been damaged due to freezing. Thus, it will not attain the required 3000 psi in 28 days. Also, it is my understanding that concrete with type III cement gains the '28-day strength of type I' in 7 days. The concrete supplier has tested a cylinder after 7 days and it's strength is only 2000 psi. It seems that the mix was also weak.

I will be meeting with my builder, the concrete contractor, and the concrete supplier on Wednesday morning to discuss and decide what to do. Could you please advice me what should I ask them to do? I don't want to get involve with lawyer. If the wall is heated and the tempreatures around it is kept at 50 to 60 degrees for next few days, would that help to gain strength quickly?
I do understand that it will not attain 3000 psi on 28th day. However, 2500 psi would be fine because I have 4 tons of steel rebar in it.
Thanks again for your help.

 

[BigH]

gp238 - Ron gave some good advice - put the Schmidt hammer away and keep it there. It is okay for determining general consistency of the concrete but not for real strength determinations. I had a long discussion about this in another thread - I think it was the concrete testing forum where I even quoted Shetty on the suitability of the rebound hammer. Secondly, using it, did you do 10 to 12 points in the same 1ftsq? If not, then you weren't following BS or ASTM standards. You do not do one rebound at a location, then walk away to another location. You do 10 to 12 (depending on which standards you are using) rebounds in a 12 by 12 square. Take average, toss out those more or less than 6 from average and take average with what is left. Also, to use it right, you need to calibrate it to the concrete at hand, not use published correlations developed in Germany in 1960s.

Do as Ron says - take a few cores (three of them at least) from the same spot to form a set. Then test them.

I want to again reinforce Ron (as he has done for me in the past) - cylinders (or cubes) taken for control testing do NOT measure strength of concrete in place - but give only that the concrete is consistent to the mix design.

 

[Ron]

BigH...thanks for the reinforcement!

gp238...I realize you do not want a lawyer involved, but you are quickly headed down that path and it is not something you want to tackle on your own. You can bet that the contractor has much more experience in weaseling out of issues than you have of pinning him down.

All that you've done thus far leads only to conjecture and guesswork. The Swiss Hammer is not intended for definitive values....it is just as BigH says.

Get the cores, do your homework, compile all your data, then stand firm. Your house is being constructed on a compromised foundation. Don't accept that. Just because you have 4 tons of rebar in there, doesn't mean it's structurally sound. The concrete and rebar work together.

 

[gp238]

Ron & BigH,
Thanks for your valuable advice. I will have the contract do the core test. If the 28th day strength comes out to be around 2200 - 2500 psi, is in that sufficient for a two story residential house with 5 to 6 feet backfill sandy clay against the basement wall?

 

[fuchsjr]

gp238

You may want to do the rebound hammer at the core locations prior to taking the cores. That way, when the cores are tested, you can get some sort of correlation between the rebound value and the actual strength for the cores.

Next, since you are in New York, you need to check the NYS International Residential Code or International Building Code for the minimum permitted structural concrete. In the IRC, there is a table that tells you what you should have.

As for taking cores, the more cores the better. You need to arrive at a level of confidence that is acceptable for basing your determination on what the strength of the concrete is. What this means, is that if three cores are taken, you should not use the average or even necessarily the lowest core strength. Those assumptions will not stand up in court. See ACI 228.1, an example is provided in section 3.5 and table 3.3.. If you do not follow these, the homeowner could hire an expert that is aware of these specifications to testify in court. If the concrete doesn't work, the owner is entitled to something, money or a credit; they paid for a minimum concrete strength.

Good luck.

 

[BigH]

fuchsjr - what do you mean that you should not use average or even necessarily the lowest core strength? Canadian Stds, Indian Standards, etc. use the following for acceptance: Average of three cores forming a set > 85% of specified strength and no single core < 75%. Neville has a very nice paper (I have it somewhere) on the "idea" of cores to represent strengths. Cores do not have to exceed or equal the required strengths. Note that there is a minimum core diameter to be used in most specs.

 

[Ron]

gp238...I wouldn't bother with the Swiss Hammer and trying to correlate with cores from surface measurements. That's not the appropriate way to correlate the Swiss Hammer and will give you one more piece of confusing and confounding data.

I would observe the coring and observe the core compressive strength testing. Follow the cores all the way through the process and make sure those removed from the wall are the ones tested.

 

[gp238]

Thanks everyone,
The rebound hammer I have is correlated at a testing lab. Before testing my basement wall I used the hammer to test basement walls of two adjacent homes and the results were between 3200 to 3500 psi as it should be.
Certainly we will do the core test ASAP.
I am a civil engineering and do know that when type III cement is used the 7th-day test should give the required 28-day strength of concrete with type I cement. The 7th-day strength test done by the concrete supplier has yielded a 2000 psi strength.
As a Civil Engineer, I see two reasons for this problem:
1) The concrete mix provided by the supplier was weak.
2) The Concrete contractor failed to provide adequate cold weather protection to the wall as required by ACI, IRC & NY Building Codes (at least 48 hours of protection against freezing).

 

[fuchsjr]

BiGH

The difference in this case is not a design or the testing of cylinders, it is the evaluation of concrete in place. ACI 228.1 is entitled as "In-Place Methods for Determination of Strength of Concrete." Some cylinder sitting in a curing box is not representative of a core taken from a building that has not been properly protected from harsh elements. This ACI standard provides a methoology of determining a concrete strength that will stand up in a court of law; I have used it before with lawyers. The other party's legal representation may not be aware of it. It is a useful club for whatever party you are representing. If you are representing the homewoner, it insures with only a confidence level of 75%, that some random core taken from anywhere in the foundation that you can state with a confidence level of 75% that it will not be lower than. Taking the average or the lowest core strength may not result in a defensible postion in court.

I had one case where the cores taken after months were in the 1000 to 1300 psi range. This was in similar conditions. Freezing temperatures, a muddy site, poor consolidation, no cold weather protection, etc. The concrete at the top of the wall was so poor, that you could pick it apart with a pen tip.

I needed a strength that would stand up in court, and ACI 228.1 provides a methodology to provide such a basis.

For the problem at hand, who is picking where the cores will be taken? The owner's engineer? The contractor? You know from above, where the potential weakest area may be, low on the wall. Is there any visible layering of the concrete due to load placements and poor consolidation between layers? Almost every residence has some, unless the mix was a soup. In which case, it will be weakened by a poor water cement ratio. If there is visible layering, and I am representing the homeowner, than I want the cores near those layers, they'll be weaker.

Also, you have to worry about poor strength in regards to exposed concrete. Will it be more susceptible to freeze-thaw losses and spalling?

As a homeowner, do you want the maintenance headaches associated with the life-cycle costs of poor concrete? Not me. The list goes on and on.

 

[fuchsjr]

Lastly, sometimes it does not matter to the homeowner that the concrete foundation is structurally adequate. In their mind, they paid for a 3000 psi of 3500 psi mix. That is what is shown on the drawings. Why should they settle for an average strength of 2000 psi? They paid for the higher strength, that is what they were expecting. As the design engineer, would you want to defend the actions of a poor contractor to cover his/her butt? Not me. Provide the facts to the homeowner, let them evaluate the potential legal costs, and then proceed from that point. It gets tiring trying to defend the many fly-by-night contractors that are in for the quick killing and are not overly concerned with quality.

Just my two-cents.

 

[gp238]

Fuchsjr,
Thanks for your staright comments. I am the home owner and I meet today with the concrete contractor and the concrete supplier to discuss the wall problem. We tested the wall (14 day old) today with rebound hammer and the strength was 1500-1800 psi. We decided to have a core test done from samples taken on 28th day. The concrete contractor says that he will "take care of the problem" if the wall does not have the required strength (as per ACI code for core testing, 85% of 3000psi).
I have also talked to my lawyer and he wants me to make sure that the core testing is done properly.

 

[BigH]

fuchsjr - I wouldn't mind a copy of the ACI Code you are talking about. You seem to indicate that ACI code is 85% of the 3000 psi strength. This seems along the line of what I indicated earlier by the Canadian and Indian Codes for core strengths. My earliest post very clearly pointed out that cubes/cylinders taken and cured in bath/steam room are not indicative of the concrete strength in situ.

You are quite right in that there is a differnce in the strength of the concrete (from a structural/technical point of view) and the contractor's stipulated requirement to produce what he has said he will produce. This is why, even if the concrete is technically okay, you would go for a reduced price (hopefully not the drastic step of knocking the wall down - if technically okay).

You might be interested in Neville's paper. If so, get me your EM and I will forward it.
and thanks for the good discussion!

 

[fuchsjr]

BigH,

The latest copy of ACI 228.1 that I have is 1995 with a total of 41 pages. The copy I had originally referred to was 20 plus. The 1995 version has the calculation in chapter 6, table 6.2; rather than the 3.5 and table 3.3 that I referred to in an earlier post.

If you want, let me know how to get a copy to you, or you can purchase the latest version at ACI.

Have a Happy New Year!

 

[fuchsjr]

gb238,

You may find the following of interest:

http://www.nrmca.org/aboutconcrete/cips/CIP 10p.pdf

http://www.cement.org/tech/faq_rebound_hammers.asp

 

[BigH]

fuchsjr - for now just table 6.2. see thread274-59980
for a great 2005 (I'm already there).

 

[Ron]

BigH...give me a contact and I'll get you the 2000 version of that ACI document.