When do concrete specimens have to be field cured? 1

[abusementpark]

I know that sometimes concrete cylinder specimens are field cured. Does ACI require this in some instances?

 

[Ron]

No. There is no good reason to field cure specimens as there is no correlation between the in-place conditions and the small specimens taken for compressive strength. While field curing is often done, it gives one a false sense of either acceptance or rejection criteria.

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.

 

[hokie66]

Site cured specimens are often specified to be tested to determine the appropriate time for tensioning prestressing strands.

 

[concretemasonry]

My thought is that field curing of cylinders is necessary but only enough to develop enough strength to handle the cylinders and get them to a lab without damage. I have seen far too many problems of cylinders being mis-handled, bumped, leaving a short set that becomes important to have an average to base a decision on. This practice can lead to a more accurate measurement of the concrete strength when delivered, although the in-place strength will depend on placement practices and actual curing of the elements.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[abusementpark]

Site cured specimens are often specified to be tested to determine the appropriate time for tensioning prestressing strands.

Why is it used for this application?

 

[Ron]

abp...while it is often used for this purpose, it can be misleading as noted above. It depends greatly on the ambient weather conditions at placement, the geometry of the members, the mix design and other variables. Most of the time it works because the conditions are not extreme enough to cause an issue and accuracy of the in-situ strength is not such a big deal.

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.

 

[hokie66]

I might agree with Ron, but the problem is who does the guessing. I think arguing against site cured cylinders for timing of tensioning goes against long established practice which already has the contractors' attention.

 

[Ron]

I agree with hokie66...it is a long established practice that the contractors understand.

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.

 

[abusementpark]

Thanks guys.

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.

 

[rapt]

abusementpark,

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.

 

[hokie66]

When you tell them to field cure the cylinders, they think they actually have to cure the slab as well...just another way of persuading contractors to do what is required for quality.