When do concrete specimens have to be field cured?
When do concrete specimens have to be field cured?
(OP)
I know that sometimes concrete cylinder specimens are field cured. Does ACI require this in some instances?
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When do concrete specimens have to be field cured?
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RE: When do concrete specimens have to be field cured?
Here are two examples:
Suppose the ambient site temperature is 60F. You cast 4 cylinders in the field for field curing, to represent two interior columns and two interior beams, each having 24"x24" cross sections. The concrete is delivered to the site at 70F.
The field cured specimens will reach equilibrium at 60 degrees in a few hours. The larger mass of the structural members will not reach equilibrium for several days because of the exothermic hydration reaction in the concrete.
The field cured specimens are tested and show lower than expected strength because of the lower curing temperature. Form release is delayed, causing extra cost.
Now the other example:
Ambient site temperature is 90F. Concrete delivery temperature is 75F. Field cured specimens reach equilibrium at somewhere between 75 and 90 degrees in a few hours. Structural members reach equilibrium temperature in a few days, but it is less than field cured specimen's temperature. FC specimens are tested and result shows higher than needed strength. Forms are released. Concrete in structure not as strong as FC specimens indicated....problems.
The specimens are to check the mix design, not the in-place concrete. While one might think it is a stretch to base the entire design on the mix design properties, it is more valid than trying to account for the numerous variables encountered for site conditions.
The design is done. The basis of the design is f'c. The mix design is to represent f'c and the specimens are to represent the mix design.
If you want to know the in-place variation in the concrete, then use a variety of methods...all of which must be correlated, with the exception of cores. Common methods are ultrasonic pulse-velocity, Lok-Test (pullout) testing, or cores. Methods such as the Swiss Hammer and Windsor probe have some minor validity in evaluating concrete, but in my opinion, are never valid for the acceptance or rejection of concrete based on their indications of compressive strength.
RE: When do concrete specimens have to be field cured?
RE: When do concrete specimens have to be field cured?
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
RE: When do concrete specimens have to be field cured?
Why is it used for this application?
RE: When do concrete specimens have to be field cured?
Given a reasonable mix design with a strength gain curve, reasonable placement control and reasonable weather conditions, you can guess whether it is OK to tension about as well as you can test for it using field-cured specimens.
RE: When do concrete specimens have to be field cured?
RE: When do concrete specimens have to be field cured?
One reason for doing the test of field-cured specimens is that it gives a definitive number upon which someone can rely, correctly or incorrectly. The "guessing" process increases the liability of the "guesser", so it is less likely to be palatable though perhaps equally or more accurate.
RE: When do concrete specimens have to be field cured?
So it sounds like there is really no code requirement for field curing of post-tensioned concrete, just a well-established practice that nobody often bothers to change or question.
RE: When do concrete specimens have to be field cured?
maybe many have questioned it and decided it is the best practice. codes do not define everything. Engineers are supposed to be logical peopel who can make sensible engineering decisions for themselves. Dont rely on codes to hold your hand for everything.
I have seen cases of non-field cured cylinders used for PT stressing decisions. the cost of the repairs for the burst anchors is not nice.As well, the danger to operators is not worth the risk.
Do field curing for PT cylinders as is logical.
RE: When do concrete specimens have to be field cured?