Passing concrete cylinders but failing cores
Passing concrete cylinders but failing cores
(OP)
I have high strength concrete cylinders that broke fine at 56 days yet cores taken of the same concrete came in below 85%, as low as 56% of specified strength. ASTM procedures were followed across the board during coring. Temperature has been mild since placement (within a range of 65 to 80 degrees F). Can anyone think of a reason for this?





RE: Passing concrete cylinders but failing cores
The concrete cylinders are intended to reflect the properties of the mix design as delivered to the jobsite. They are fabricated, cured and tested under standardized procedured and conditions.
The in place concrete can be significantly different in strength than the test cylinders.
You say that they came up to strength in 56 days. If you had to wait 56 days to achieve the design strength, you already have lower strength concrete than was desired.
Was there any water added to the mix after the cylinders were taken? Was the concrete pumped? What was the frequency of sampling/testing? You probably only took one sample for every 50 or 100 cubic yards of concrete. That's a lot of concrete that you didn't sample that has gone into the building. What were the weather conditions at time of placement? Hot? Cold? Humid? Dry? Was there any curing done for the in-place concrete? (not just spraying some curing compound on the surface, but actual moist curing)
Give us a little more info to work with here.
RE: Passing concrete cylinders but failing cores
You point out a potential reason in questioning the frequency of the sampling. Any thing else you can think of relative characteristics o high strength concrete?
Thank you for your response Ron.
RE: Passing concrete cylinders but failing cores
You can do some correlation testing using cores and Windsor probe to check the in-place strength. You might also consider using low-frequency ultrasonics (pulse-echo) to check the consistency of the strength, since I assume you have access to both sides of the wall.
RE: Passing concrete cylinders but failing cores
RE: Passing concrete cylinders but failing cores
RE: Passing concrete cylinders but failing cores
Mix proportions per cubic yard
Cement 375 lbs
Water (batched) 30.1 gal
Fine Aggregate 1160 lbs
Coarse Aggregate #57 1210 lbs
Coarse Aggregate #8 645 lbs
Admixture (BASF/Glenium 7500 - High Range) 85 fl oz
Slag 525 ibs
Silica Fume 50 lbs
Chloride Ion 0.11% by wt. of cement
RE: Passing concrete cylinders but failing cores
RE: Passing concrete cylinders but failing cores
RE: Passing concrete cylinders but failing cores
I would not use this little amount of water in a mix, since EVERYTHING would have to be perfect for proper field hydration.
Have a petrographic examination done on a core and see what the ratio of unhydrated cement is. I think you'll find an issue with that.
Further, your slag cement to portland cement ratio is very high. We generally do not like to see more than 50 percent of the cementitious material as slag cement. When you have that much slag cement, it will retard the strength gain significantly, particularly in the field as compared to the lab.
RE: Passing concrete cylinders but failing cores
RE: Passing concrete cylinders but failing cores
Cylinders are not always perfect, but they do represent a measure of the properties of a mix under more controlled conditions.
The technician does have some control despite the procedures required (loop-holes). When rodding the the rod can be slightly extended. It can be drawn out when not truely vertical. Timing is also a factor. These are not tricks, but common errors. For a true comparison, the same technician should do all cylinders.
Job site is not always perfect, but it is reasonably so.
Transportation is not always perfect because not everyone straps cylinder to a pallet so they cannot tip. Fortunately, the handing mistakes are usually obvious by the results to exclude them.
When we had a concrete technology class we made cylinders and unknown to us, the professor allowed the usual tricks. Some were inserting a 10" long rebar into a cylinder (a 10" long straighten coat hanger makes a big difference). Sugar and salt were the common forms od pranks and sabatoge. Fortunately, the professor knew them all and used his background to show what could occur in the real world.
Cylinders are certainly a good measure of the mix properties if you have a good technician and lab.
Cores only measure the conditions the mix was subjected to after batching.
Dick
RE: Passing concrete cylinders but failing cores
I'd also get the slag supplier involved. I think the % slag used is really pushing the limits, but that also makes it very fun and interesting. Please let us know what you find out.
RE: Passing concrete cylinders but failing cores
That mix in itself is also a bit odd. Seattle has mixes that achieves that strength on a consistant basis, but we don't use Silica Fume, way too variable.
Regarding the sampling, have you thought of batch plant inspection? Also, make sure that the cores taken are also from the aproximate location the cylinders were taken.
Other than that, do a petro on both cores and cylinders. Also, with that much slag, was the column contained a few extra days within its form, or did they 'pour and pull', stripping the form the next day? Remember that slag will reduce the initial hydration, so leaving on a few extra days will give it that bump needed.
And one last thing, gotta ask, did the tech break the cores according to ASTM regardin rate of load?
RE: Passing concrete cylinders but failing cores
RE: Passing concrete cylinders but failing cores
RE: Passing concrete cylinders but failing cores
typically (95% of time from my experience) the cores break higher than the cylinders (keeping in mind you're looking at two different strengths and are not necessarily appropriate to compare directly to each other--only compare "apples to apples" thought process)
also, what are you coring? where did you core? how many cores versus how much concrete placed versus how many sets of cylinders?
and i don't want to sound like i have all the answers, but cylinders and cores are rather "no brainers"...given things can happen but for my company and from all my experience, it usually boils down to the contractors actions in the field. sometimes it's the mix and sometimes the difference is somewhat attributed to simply the randomness of sampling and/or difference in curing. very very seldom do i ever see that the cylinders and/or cores were or even could be the problem.
i'd focus on what the contractor may have done after the concrete left the truck...but still take at look at all aspects of the mix and testing results as well.
RE: Passing concrete cylinders but failing cores
Petrographic analysis will let you know the specifics of what is is you've really tested, and a worthwhile investment.
Then you can start to figure out why the apples are oranges!