concrete compressive strength

huangyhg

concrete compressive strength??
i have a project that we are currently working on in the construction phase. several concrete compressive strength tests have come back well below our specified at 28 days. most of them achieved the desired strength at or near 56 days and a couple of them never achieved design strength. for example at 28 days the average strength was about 1800 psi - 2200 psi. specified was 3000 psi at 28 days.
my first question and probably the most important is what would be the proper way of handling such a situation? i have been asked if the concrete needs to be "ripped out". this seems a little harsh, are there any possible solutions?
thanks in advance.
andrew
find a job or post a job opening
the very low strengths of 1800-2200 psi are obviously far below specs. before tearing out, i would take a close look at the areas where the concrete was used. it could be it still may be strong enough to let it remain if you can identify the areas.
in place determination of the strength is possible, but the results are always difficult to interpret.
i would also take a close look at the preparation, handling and curing of the cylinders. it is really difficult to get strengths that low. since you have 56 day breaks, it seems to rule out freezing of the samples, but poor handling (contractor, driver, testing lab, etc.) could be a possibility. a green cylinder is easy to damage.
in the interim, i would suggest requiring making more cylinders and making both 7 and 14 day tests in addition to the 28 day. no one can object to early testing since there is a proven problem based on previous test results. any good ready-mix supplier should have some sort of time-strength history. in any case, it would put the supplier, contractor and laboratory on notice and give everyone the necessary warning of the problem. is the r/m plant certified by any association or qualified by any recognized group? is the testing lab certified?
have you taken a good look at he mix designs and the admixtures? it is always possible to get incompatible admixtures if specialized properties are desired. admixture companies are always will to jump in and help if only to protect their image.
cement is so cheap that a supplier usually has little to gain by cheating on the content, but there appears to be a problem somewhere in the supply/testing sequence.
in an earlier life, i was a concrete inspector and realize that the person making a cylinder can have a great effect (10 to 30%) on the strength. the same goes for handling, curing and sample preparation. a testing machine can be out of calibration or there could be a problem with the platten for applying an axial the load. - there are many possibilities, but usually a lab can/should spot these quickly.
in college we could get 50 to over 100% cylinder strength variations by using either sugar or rebar.
dick
i agree, consider the problem carefully before condemning.
if the concrete is exposed externally you will have to consider durability. i'm not sure, but i would think just because the concrete reaches strength at 56, or even 117 days doesn't mean it provides the same degree of protection to the rebar as designed. coatings can be applied to improve durability.
are you in a position to take a core or 2 to test actual insitu strength?
as concretemasonry has pointed out, there are a host of factors that can lead to low compressive strength test results.
depending on how the cylinders are handled from fabrication right through testing, results obtained, may not even provide a proper indication of the in-situ strength.
a few questions for you.
1. can you please post the mix design?  weights of material per cubic yards, including name of admix and dosage rate would suffice for now.
2. how has this mix performed in the past under similar conditions and around the same time of year? you should be ble to get this information from the ready mix supplier and or testing lab.
3. over what period of time have the results been low...a couple of days, weeks..?
4. were any specimens cast for 7-day results...and if so, were these results lower than expected?
5. were the cylinders fabricated by aci certified techs?
6. do you have any evidence that the initial curing of the cylinders was done in accordance with astm c 31? check section 9.2.1 and note 6 for editions of c31 prior to 2006. in the 2006 edition, check sec 10.1.2 and note 5
7. typically how long were the cylinders out in the field before they were picked up by the lab? you should be able to get this information from the lab. in cold weather, if they are left out too long without adequate protection/curing, the results will most likely be lower than expected...same thing can happen during hot weather.
there have been some other threads in the past that have dealt with low concrete strengths - and with some good answers by bigh and ron.  do an eng-tips search.
here's a short thread with a sequence of steps to take in accordance with chapter 5 of aci. you may be familiar with it but i thought it would be an appropriate link here:
thanks for the replies.
i have a call into the testing lab and concrete supplier.
i will get the mix info as soon as possible and post. thanks.
i too have concerns about durability and think coring the concrete would be a good idea.
cylinders are a measure of the strength of the concrete in the cylinders.  take cores and use swedish impact hammer to identify locations in the structure where strength seems less than specifications.  run calcs to see if the location can survive with the measured compressive strengths. ask for a extended warranty or a price reduction for the portion left in place with low strength. remove and replace where necessary.
if the strength was reached at 56 days, isn't that acceptable?  i know 28 days was speced, but the required compressive strength would have been reached.  
i've always wondered about the cylinder break results.  aren't these cylinders taken back to the lab and allowed to cure in water until it is time to break?  
if that is the case, how can that possibly reflect the actual curing conditions of the in-place concrete that is not kept consistently moist?  
it seems to me that the in-place strength should always be lower than the cylinders that have "ideal" curing conditions.
i used to work at a pre-caster that had a curing system that was able to adjust the temperature, as well as some other conditions, based on the information sent to it from wires embedded in the concrete beds.  this was just to ensure that it reached the required release strength.  once the pieces were stripped from the beds, the cylinders went in the water and the pieces went out in the yard.  it rained occasionally, but those pieces were certainly not kept constantly wet.
make sure you discuss this with your client.  if you approve the low strengths, you accept the liability if there's a problem down the road, such as severe cracking.  even if after all your research you approve the low concrete strength, make sure that you discuss the ramifications with your client and let that person go to the owner.  the owner, after all, is paying for 3000 psi and deserves to have a hand in the decision-making process along with your client.  
also check out aci 318 1.1.1, if applicable to your area.  going with less than the code minimum does not sound like a good idea.  i would make sure to do what tests you can, but if it needs to come out, so be it.  some one else's mistake is not worth losing your high-knee
i have read the posts and i have a couple of comments that may  help as we just went through this on four buildings where 25% the column strengths were less than the specified 5000 psi. one 56 day test result is not considered a test as two cylinders are required to constitute a test. therefore, it is up to the structural engineer of record if he wants to accept one 56 day test as an indication that the concrete has reached its design strength. we did not, as they were also low and we required testing in accordance with aci 318-02, section 5.6.5 - investigation of low strength test results, aci 214.4r-03 guide for obtaining cores and interpreting compressive strength results and astm c 42/c 42m-99 standard test method for obraining and testing drilled cores and sawed beams of concrete. the contractor & concrete supplier wanted to use a swiss hammer or winsor probe, but we rejected these due to the unrealibility of these test procedures to accurately determine the in place concrete strength. three 3 3/4" cores at each deficient column were taken, they were corrected for length as some of them did not meet the height of 2 times the diameter, and they were tested, and all but three cores failed to meet the
85% of f'c or 4250 psi required for cores by aci. the structural engineer of record ploted interaction curves of the column axial loads vs moments, showing the reduced concrete strength could be accepted. however; all the cost for coring including engineering time to determine acceptance was passed to the contractor/supplier, along with a extended 5 year warranty to repair or replace any of the colums that develop any future structural distress. while i think all this work was worthwhile from a life safety viewpoint, it should be pointed out that usually the cores i have tested in the past usually meet the 85% f'c. hope this helps, of course it is always best to resolve these issues as soon as they occur, but unfortunately that is not always the case.