calling BigH (Geotechnical) and core results 5

[boffintech]

On 29 Mar 05 12:50 BigH (Geotechnical) wrote, "Neville's paper was in Concrete International - about 2001 or 2002, I believe - I have a copy and will look it up tomorrow."

Did you ever find this? I've looked and can't find it.

Similar to the thread 592-119191 we are having a problem with 6000psi concrete on a particular job.

The commentary in ACI 318-02 states: R 5.6.5 A core obtained through the use of a water-cooled bit results in a moisture gradient between the exterior and interior of the core being created during drilling. This adversely affects the core’s compressive strength. The restriction on the commencement of core testing provides a minimum time for the moisture gradient to dissipate.

Don’t large masses of concrete duplicate this sort of moisture gradient, not by wetting the exterior but just due to its mass retaining heat/moisture on the interior? Say you have a concrete column 26” x 26” x 14’. Since mass traps heat and moisture and heat and moisture cures concrete, should one expect higher peak heat and moisture closer to the interior of the column and that this interior condition may translate into a higher compressive strength from a core sample from the interior of a column? Is a 26"x26"x14' column big enough to cause this gradient or is this legalistic hair splitting?

Cores were taken and they were 1500 psi higher than the our cylinders. Cylinders are running between 4500 and 5000 at 28 days and cores are running 6000 to 7000. I pointed out concreteguru's opinion that taking one long core and cutting it into three specimens does not best represent an average of the in-situ strength of the concrete member and that the objective would be better served by random sampling core locations and then averaging the results.

FYI, no initial curing at job site in very hot weather 95F+ daily and no one understands why low breaks. Go figure.

 

[Ron]

Boffintech...just to jump in a bit before BigH chimes in...

Your premise is correct, though the peak of the gradient and its duration will be lower for your example because of the column size. The hydration process is exothermic, but columns have a high surface area to mass ratio, so the heat buildup dissipates fairly soon. As for the moisture, a fair amount of it remains in place with drying occurring mostly at the surface. The equilibrium moisture content of most concretes will be in the 3 to 6 percent range.

The "wet core" condition is transient. Further, it approaches saturation at and near the surface during the coring process, but the moisture content within the core might be lower just adjacent to the outside, but greater on the inside. So you have a potential for high-low-high moisture gradient, which can affect the stress distribution inside the core, particularly if saturation occurs (now you actually have pore pressure buildup). That's the reason for not allowing testing within the first 48 hours after coring under ACI 318. As time goes on, the moisture condition of the core reaches equilibrium and testing can occur without undue influence of the water.

 

[boffintech]

Speaking of surface area and volume with regard to effects on cylinder strength, core strength, initial curing, and lab curing...

on a 6x12 cylinder
volume of cylinder = 339.29 in3
surface area of cylinder = 282.74 in2
ratio of vol to SA of 1.2

column 26"x26"x10'
volume of column = 81,120 in3
surface area of column = 13,156 in2
ratio of vol to SA of 6.1

First of all is that an accurate way to calculate the ratio between surface area an volume? And second what are the effects of having these differences in vol to surface area ratios between the two members?

 

[Ron]

boffintech...I was comparing your column to a mass concrete placement, for instance a mat foundation. Let's take a 4-foot thick mat. The surface area to mass ratio of this placement is 0.26 (square inches of surface divided by lbs of concrete for the thickness). For your column, look at the thickness of the column and look at a strip through the column of 1 foot high, 1 foot deep, and the width of the column. In this case, the surface area to mass ratio is 1.98 (you have three sides of this concrete exposed to air, where in your mat you only had one side)...about 8 times as much. That gives you much greater dissipation of the exothermic reaction.

A concrete cylinder has a surface area to mass ratio of about 10.5. That's one reason you don't want to make cylinders and leave them in the field to "represent" the in-place concrete...they don't. Even if you leave the molds on, there's no real relationship between "field cured" conditions and in-place conditions. If you want to know the strength in place, do some correlation between a nondestructive method (ultrasonic or pull-out test) and strength.

Initial field curing of cylinders is important. Extreme temperature changes in the first 48 hours or so can make a strength difference, even after having the lab curing process. Further, the lab curing must be done with good temperature control as just a few degrees on the low side will cause your breaks to be off (low). The moisture condition is maintained in the lab so that the concrete does not loose moisture...it isn't intended to add moisture to the concrete, just keep it from losing a bunch.

In your calculations, you have compared volume to surface area. What you need to compare for the thermal reaction is surface area to mass. If you invert your calculations so that you compare surface area to volume, you get a similar relationship as what I got (note that the cylinder is still quite a bit higher than the column) since the mass is related to the volume by the unit weight of the concrete.

The effect of having these variations is that the concrete in the cylinder does not reflect the concrete in place. The purpose of taking cylinder specimens under standardized procedures, curing them under standardized procedures, and testing them under standardized procedures is to see what the MIX DESIGN is capable of doing. That's all. The cylinders represent the mix design. If the mix design is capable of producing concrete of a certain strength, then by inference it is OK in place from a design perspective (that's f'c). As we both know, the contractor can screw up the in-place concrete pretty badly by adding water that didn't get included in the test cylinders, improper placement, improper finishing, improper curing, etc. The more the contractor screws with the concrete, the worse the cylinders will be off relative to the in-place condition.

Cores are still the best method of determining the in-place strength of concrete.

 

[JAE]

...and thank goodness for the [Φ] factors!

 

[JAE]

...sorry Ron, its Sunday night and I just finished my second glass of wine. I am always continually amazed at your depth of knowledge of concrete.

 

[Ron]

Hey JAE...where's my glass?!

Agree on the factors. We'd be in a lot of trouble without some our National Fudge Factors!

Thanks...I'm blessed with a good memory of things gone bad!

 

[henri2]

Thought provoking questions posed by Boffintech.... impecacable elucidation from Ron

 

[BigH]

I pass my hat to Ron and henri2 for knowing the finer details of concrete in much more detail than I. The article you want to find is:
Adam Neville, "Core Tests: Easy to Perform - Not Easy to Interpret"; Concrete International, November 2001. My copy is in the order of 1.05MB. Neville speaks of many of the factors related to the strength of cores.
He does note that one study (with a very wide scatter) suggests that dry cores may be in the order of 14% higher in strength than the strength of cores soaked in water. Assuming that your cores were "dry" when tested, it might be possible that the strengths are higher than your soaked laboratory specimens. Would suggest a read of the article.
Ron - appreciate a bump in my education every time you post!

 

[Ron]

BigH...that's very kind, but mostly I think we just swap subjects! I get the same when you play in the dirt!

 

[BigH]

Boffintech - did you find the reference?

 

[boffintech]

Yes, I got it. Thanks.