Concrete Cylinders Initial Curing 2

[Jameschen6]

As a consultant we cast and test the concrete cylinders. recently cylinders from a high rise construction site have some low 28-day strength. from time to time I can see the supplier casting their cylinder too onsite. Even though the situation has not be improved. about 30% of the specimen still have low strength (up to 10% lower). We follow CSA standard strictly for this sensative project. One thing comes to my mind is the initial curing. We leave the specimen in a elavated and air conditioning trailer. Is the shaking of the trailer resulted by the people walk, shutting the door and other a negative impact factor to the initial curing? I suspect. Any comments to my thought? Thanks

 

[TehMightyEngineer]

You shouldn't have them in an air conditioned trailer. Cure them in the same conditions as the actual slab.

I doubt the vibration is that significant in your strengths.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[JAE]

Check out this reference:

https://www.nrmca.org/aboutconcrete/cips/34p.pdf

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[Agent666]

I thought the samples are supposed to be submerged in water after the initial cure, storing them in an air conditioned trailer is likely to suck moisture out of the cylinders affecting the water available for the cement reaction.

Keep in mind it may be other things, do the results from site agree with similar cylinders taken from the batching plant (are they low too?) as this could indicate a mix issue?

 

[darthsoilsguy2]

It is a tough situation. I've been there. If the spectre of doubt is raised about this, it doesn't matter if it is actually affecting it. You are bound by the test method to have them stored in a controlled environment or report otherwise which opens pandoras box... As the testing company, you put the burden of providing that controlled on-site environment on the contractor since they are in control of the site and they have interest in the results. The contractor provides this which meets the ASTM (or CSA) in explicit language on temperature... but the "disturbance" is anecdotal. All you want to do as the testing company is provide unambiguous results that are fully defended so you aren't considered part of the problem or have to fork in free effort for investigative troubleshooting. No good decision... if you choose to not put them in the trailer, the concrete company will say the future cylinders didn't have initial curing conditions... After fighting about initial curing too many times, the testing firm i worked with took the position of including initial curing in our big projects and we bought 3 or 4 climate controlled on-site curing boxes and we rented those tanks in our proposals to the Owner. We still required in our proposal to owners that the contractor provide power and a location away from traffic. our lab mgr at the time made the purchase but i think they were these ones or at least look the closest to this.

https://www.atlanticsupply.com/prod...te/curing-tank-heaters-and-circulating-pumps/

BTW agent666, the OP is talking about only the initial 24 hr cure (or at least that's what i think the OP is saying... if these things spend 28 days in a trailer all-bets-are-off)

 

[Ron]

Curing temperature has a lot of affect on strength gain. Just a few degrees below the 70-76 degree standard curing temperature will cause a reduction in strength gain (ultimate strength will likely be unaffected as long as temperatures are returned to normal).

Air conditioned storage in the first 48 hours will likely retard strength gain for two reasons...temperature reduction and loss of humidity as Agent666 noted.

Protect the cylinders from damage, vibration and moisture loss during the first 24 hours then store in moisture room or submerged for the remainder of the curing cycle. Initial curing should be neither too hot nor too cool.

 

[epoxybot]

To put a finer point on Ron & Agent666' comments, a 10 degrees C (18 degress F) change + or - can double or half a chemical reaction.

 

[Ingenuity]

...hot off the press...ACI's monthly magazine "Concrete International" has this article in the August 2018 edition:

Download copy available here: Link. Free for ACI members.

 

[mikeofBelAir]

Not quite enough information....
"An air-conditioned trailer"...ASTM allows the temp for the initial cure to range for 60 to 80 degrees F. Is the trailer cooled below 60 degrees F? My guess is probably not. So the "low temp" of the initial cure is probably not an issue.

Vibration from walking etc---depends--how much vibration? Again, not enough information. Are they sitting directly on the floor, or on some sort of pads? Might be interesting to cast a couple of extra cylinders and use some foam padding on them and see if there is a difference....and how much of a difference at 28 days.

I doubt that there would be significant loss of moisture during the initial cure if the cylinders are properly capped. Are they?..or are there no caps and the cyls are left open to the atmosphere?

 

[TXStructural]

This is a hot topic in ACI 301 (as far as who provides what for initial curing). ASTM C31 is the spec you need to use to guide how you cure. There is a difference between field-cured and standard condition-cured cylinders.
As far as vibration goes, people walking around in the trailer should not be sufficient to disrupt the curing. The problem we see frequently is cylinders bouncing around in the back of a pickup, which IS a problem. I agree that temperatures have a much greater influence on the development of strength. Pull some cores and compare to the cylinder strengths to see if the cylinders correlate well to the in-situ concrete.

 

[Ron]

TXstructural....cylinders are not meant to represent the in place concrete...they are meant to represent the mix design. There can be a significant difference.