Low Concrete Strength 1

[nicholi]

I have received the test results for a seven day break at 11.3 MPa. The design strength is 25 MPa. Based on the seven day break being 65% of max strength, the 28 day break should be 17.4 MPa.

The low concrete is for a canopy pad (with pedestal above) for a gas retail site.

I have proven theoretically that there is sufficient concrete strength to accept the canopy loads from above acting in compression on the pad below.

However, I don't feel satisfied and think I might have overlooked something and still want to tell the contractor to remove the concrete. (I know what the code says in terms of low strength concrete and acceptance.)

Any comments, opinions, ideas. Thanks in advance for the feedback.




I have researched the code stuff and know what their an

 

[PEinc]

Is there any special reason to break the contractor's balls? How much concrete would he have to remove and replace? Is it his fault the concrete came up low? Is this a one time occurance on this project. If the design using lower strength concrete figures, then maybe durability is the only question.

Is it possible the testing lab did not properly test the sample(s)? Can you wait 28 days and check the new tests?

 

[nicholi]

I should have given more info. Specifications of concrete slump was to be 80, at site it was 150. Air was to be 5%, and it came it at 7.2%. Also the concrete was poured on a day of 28 C.

The amount of concrete to be pulled out is four footings two at 1500 x 1500 and two at 1800 x 1800. Unfortunately with the client schedule, the GC cannot wait 28 days.

 

[PEinc]

Someone added water. Why didn't the job inspector (concrete technician) reject the load of concrete after the bad air and slump tests? If the inspector tested the concrete and still let the contractor pour the footings without objecting, the contractor may want to be paid to remove and replace the footings. If the contractor was told that the concrete did not meet specifications and then poured anyway, he should be responsible. If the contractor did his own concrete testing at the site, then he should be responsible.

The 4 footings are not very large and it would not be too expensive to replace them if it was the contractor's fault. If it was the inspector's fault and not the contractor's fault, it will be very expensive to replace the footings.

 

[sc]

Sounds like the concrete supplier is at fault. The contractor would be expected to carry the cost of remove and replace. The contractor would then have to chase the supplier.

I would also check the effect of the concrete strength on the hold down capacity of the bolts/column. As the reduced concrete strength will effect the bolt/column withdrawal strength.

regards
sc

 

[SirAl]

Were there admixtures applied to the mix? Also, are there sulfates in the soils that could require a higher strength for durability? There are many scenarios that could explain the low results. As a starting point, I would consider obtaining cores for compressive strength to verify the lab test results.

 

[Zambo]

SirAl has a good suggestion that cores should be carried out before breaking out anything. Is there any reason to suspect that the test cubes could be sub-standard? Perhaps compaction, curing or storage of the cubes was the problem.

28 deg. c shouldn't be a problem for the concrete, but clearly the slump is much more than specified and the concrete should have been rejected. I wonder if the truck driver didn't rotate the drum before giving a sample to the technician, perhaps just possible the sample wasn't representative.

You have a time related problem with your Client, what about using an impact testing hammer (Schmidt) to have a quick check on the results from the cubes.

Zambo

 

[jheidt2543]

If the specification requires a 28 day strength, how can you demand removal of the footings based on a 7 day break and a THEORETICAL 28 day strength? I've seen many low 7 day breaks where the 28 day break turned out OK.

I like the idea of coring the footings, one at each footing, and breaking them and taking the average of the four tests. Don't forget to apply size factors when analyzing the core breaks.

 

[Rjeffery]

All:

1) Unless specifically called out in the quality assurance section of the specification or delivered to the testing agency in writing, the concrete technician is NOT allowed to reject a load of concrete for any reason. They are only to report their test findings to the person in charge. ACI is my reference for this. If the testing was done by the contractor or the supplier then shame on the owner.

2) It has always been my experience that the first thing that is questioned is the qualification of the technician or the way they performed the procedures. The first thing that really should be done is to check an see if the material supplied is correct, then begin eliminating the physical thing that could have caused the failure of the test. did they add excess water? is the aggregate as specified, etc.?

3) look then to the procedures. Was the sample consolidated properly? Were the cylinders picked up too soon or too late? was the transportation method proper i.e. padded? were the cylinders lab cured or field cured? Was there a problem with the lab cure method?

The list goes on and on.

4) If after using Occam's very sharp knife to cull the possable causes them look at the cement. the last low break I had was because the fineness of the grind of the cement supplied to the plant was not fine enough. this showed up over several days on many other projects for plants that used one supplier for their type I cement.

5) If you core, use the ASTM (or other recognised) standards for taking, preping and testing. that eliminates any claim on that end.

6) Finally, Windsor Probes and Shmidt Hammer testing is only good for testing the surface toughness. I have found that if rounded (bank run) quartz gravel is used (which is a 7 on the Mohs hardness scale) The concrete appears strong but because the cement matrix does not adhear well to the surface of the gravel the concrete is weak in compression. Therefore cores are the only way to go.

OK, I'll get off my soapbox now...

 

[Ron]

There are many reasons for lower than anticipated concrete strength at 7 days. I agree with those who have said it is inappropriate to remove the concrete at this point. The "official" test does not occur until 28 days. Even coring at this point would have to be extrapolated beyond the norm.

The slump is an issue, but might not be the only cause for lower early-age strength. You will only be able to tell this by review of the mix design and further testing.

As Rjeffery stated, don't waste your time with Windsor probe and Schmidt hammer testing. They are not valid without core correlation anyway.

Don't jump off the deep end until you know more. To require the contractor to remove the concrete at this point would be precipitous and might lead to a claim by the contractor. Just tell him that it can stay until after the 28-day results are in, just that he'll have to remove it if it doesn't meet the specifications. He'll figure out what to do from there!

 

[PEinc]

The big problem here is, as usual lately, the project schedule. Nicholi says the schedule won't let them wait the full 28 days to check the concrete strength. If they wait 28 days and the strength is still no good, they've lost several weeks of the schedule and may have to rebuild the footings. The risk of waiting could be worse than the cost of replacing. The question with replacing the footings now is who pays.

 

[RDK]

As far as I am concerned the ONLY reason to take concrete cylinders is to determine if it is necessary to take core samples to determine if the concrete is acceptable. If the cylinders break at or above the specified strength then core samples are not necessary. If they are low then core samples are required.

Take core samples and see what the in situ strength is. These do not have to be at 28 days since there is a good and well-known correlation between strength at any time and 28 day strength. These curves are in every basic concrete textbook I have ever seen.

There appears to be some confusion in this thread between quality control and quality assurance. Quality control is in using the right mix design, admixtures and procedures to produce quality concrete. This is the contractor’s responsibility. Quality assurance is a monitoring of the contractor’s quality control to ensure that the right procedures are being followed and that the end result is acceptable. Quality assurance is quality control of the quality control.

If the concrete strength is low then it’s the contractor’s responsibility to replace. Any lost time due to the need to replace the concrete is the contractor’s responsibility. The site technician’s only role is quality assurance. Every specification I have seen makes the responsibility for the final product the contractor’s responsibility.

The technician could be faulted for not speaking up about the high air entrainment and high slump in the first place. However I do not believe that this makes the owner responsible for the final result.

If the contractor is not willing to remove and replace the defective work then put it up to him to come up with a better solution. Since load capacity is not an issue, have the contractor suggest some additional measures that may provide additional durability. Do not get side tracked with the argument that if 17 MPa is enough the contractor has supplied sufficient strength and is off the hook. Remember two things. First you paid for 25Mpa. If the contractor bought a big truck when he could ride in a smaller truck, would this be acceptable to him? Secondly concrete strength is also a proxy measure of other factors that maybe important especially durability.

Also make it clear to the contractor that any delays in resolving this issue will be his and his alone. In similar situations I will issue an order to the contractor requiring hat he remove and replace the concrete and that any time that this takes and expense is at his cost. Then he can suggest alternatives but every discussion and written correspondence repeats this point.



Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[Zambo]

RDK, yes good points except as mentioned earlier admixtures can affect the relationship between strength at any time and 28 day strength. A temperature of 28 deg. c has been mentioned,, possibly a plasticer/retarder has been used which was not specified. I realise that this should not happen but these are small footings and possibly a mix has been sent from another project placed on the same day.

Although Schmidt hammer testing has been much maligned by Rjeffrey and Ron it is not quite as bad as suggested and does give a correlation between surface strength and compressive strength. These hammers are better at providing a result in the vertically downward direction and considering the size of these footings should not be disregarded immediately.

It is common that the contractor himself will take the test samples and make the cylinders. For a job of the scale discussed I guess that no one other than the contractors staff was involved and nicholi as an engineer for the designer is representing the client/owner. It is a failing in the system if a high slump test is ignored as it can indicate problems in the mix such as low cement content, excess water or incorrect fine aggregate/coarse aggregate ratio. This example indicates the hassle concerned by not rejecting concrete. Remember that if you reject a load it will probably end up on someone elses project and if you are seen to be a soft target - you get the reject loads.

As mentioned earlier there is good reason to check whether there are sulphates or other factors which could determine that durability/density is as important as compressive strength. If it can be shown that the concrete even with reduced compressive strength is acceptable for all reasons the Client will probably be pleased to have a solution. In the meantime the contractor must assure that this wil not happen again, perhaps change his supplier.

Zambo

 

[nicholi]

Thanks for all the feed back. Some of you have mentioned durability and sulphates. The concrete is type 50 (sulphate resistant). Does this have any impact?

 

[SirAl]

It is my experience that sulfate resistant concrete does develop strength gain at a slightly slower rate than that of normal Portlant cement concrete. Even with Type 50, a severe sulfate environment may necessitate a high compressive strength for durability considerations.

 

[Ron]

WOW! This is one of those threads that shows the differences in practice around the world!

Rick...I almost always agree with your great answers, but this time have to take exception to one of the comments. Coring prior to 28 days gives you an idea of in-situ strength. To attempt to correlate that to published curves of strength gain will not get you there.

Strength gain curves are done on mix classes under controlled conditions. The field conditions might be better or worse (usually worse). The purpose of field sampling and lab testing is to check the mix design as delivered. It has little to do with the in-place concrete. The design extrapolation is made based on a mix design performance, not field performance. That's what the "National Fudge Factors" are for.

Sulphate resistant concrete will have an inherently lower strength gain rate than common Type I portland cement. If there are any pozzolanic materials in the mix (GGBFS or fly ash), this strength gain will be retarded even more. Sometimes we have to look at 56-day strengths instead of 28-day strengths for these reasons.

I reiterate the point that if the options are laid out to the contractor, with the repercussion of removal if it doesn't achieve strength, he'll make HIS monetary/schedule decision rather quickly. Make it the contractor's risk management, not yours.

 

[nicholi]

To all, thanks for the info. To update you on what happened, the contractor cored the footings and had the cylinders broke. The hurdle was 80% at 14 days. Now the CSA allows the concrete to be deemed acceptable if f'c is within 85% at the 28 day break, etc, etc. Therefore, design f'c = 25 MPa, so at 85% f'c becomes 21.25 MPa, thus 80% of 21.25 = 17 Mpa. The four core tests were 17.2, 14.1, 18.3 & 18.7. The one footing with 14.1 was retested at 17 days at 15.6 MPa. This footing will be removed and is to be replaced. Note that all the concrete for the footings was from the same truck and this footing was #2 of 4 poured.

 

[Laser28]

Almost 7 months after the fact, I have to contribute this: CSA statese very clearly that the reason for casting and testing concrete cylinders is to determine the quality of the concrete AS-DELIVERED. The cylinders are NOT indicative of the concrete in-situ because of the differences in placing, finishing and curing.

Further, if the test cylinders indicate low strength, removal is the last thing to do. The first considerations are structural and for durability. What are the requirements for strenght from the designer, and will the concrete be durable enough. Secondly, concrete will generally gain more strength if curing is reinitiated (albeit at a slower rate) so the contractor should have been required to restart moist curing for at least a month to get as much out of the concrete as possible. Thirdly, as you quote CSA, I assume that you are from the Great White North, and that the concrete specified (25 MPa) does not meet CSA requirements for DURABILITY!!

Read CSA BEFORE you have a problem, not after.