Grout 'cubes' vs. Concrete 'cylinders' 3

[techcrete]

The following scenario lead me to my eventual question:

A field technician is casting C-1107 hydraulic cement non-shrink grout specimens into 2X4 cylinder molds for compressive strength testing..(I have only used 2X4 molds for desktop pencil holders but I digress). The breaks are coming up extremely low. I am aware this is non-compliant with C-1107 & the designated C-109, The field tech's certification status is irrelevant to my forthcoming question. I believe I am in need of some engineering assistance please.

What is the correlation &/or significance of the specimen size, shape, & ratio for casting cubes (1:1) as opposed to cylinders (2:1)? Other than compositional differences between concrete and grouts why couldn't a 2X4 'grout' cylinder be accepted in place of a 2X2 cube? Any help would be greatly appreciated. Thank you in advance for your expertise.

 

[concretemasonry]

There is a significant difference in the aspect ratios of the two samples. If you look at the mode of failure after the tests, the differences may be obvious.

As an example, for masonry grout, the ASTM spec for testing requires a 2:1 ration because there was little correlation between cubes and cylinders, but the mold material (absorbent vs. impervious also had a large effect.

Dick

 

[techcrete]

Thanks for the reply Dick. Perhaps I'm slightly confused by your response. It is my understanding that there is indeed a significant difference in the aspect ratios between the two samples (cubes vs. cylinders), however this ratio is less significant for masonry grout where the mold material is more critical?

Nevertheless, I understand that cylinders & cubes will display different modes of failure, as they are two different shapes with different surface areas, but why couldn't the two be allowed as equivalent specimens for compressive strength testing? Will the two varying samples achieve similar compressive strengths (+/- 200 psi) if casting, cured, & broken the same? I guess my root question is what is the significance of the shapes? Cylinders as opposed to cubes. Thanks for you help in this matter.

 

[BigH]

For concrete - this question comes up quite often in my experience in Asia. British Stds - and hence many other countries - like to use cubes for concrete compression testing. Americans/Canadians like to use cylinders. As pointed out, there are different strengths as determined by cubes and by cylinders. Typically, one uses cube strength x 0.8 = cylinder strength although there is research that the actual correlation depends on the strength of the concrete. These correlations must be taken into account for concrete - and the relationship is not the same as 1:1 cylinder vs 2:1 cylinder (approx 0.87). I would surmise that a similarity exists for mortar. BTW - why is your technician casting in non-standard moulds?

 

[woofar]

It all comes down to flexural strength. When you crush a 2:1 cylinder you open up the possibility for a flexural failure rather a compressive failure.

As grout has a very low flexural strength it is much safer to crush a nice little cube.

HTH
Michael

 

[conceng]

The previous posts are all good points relative to aspect ratio, but I think they all miss the most important point. You are testing a NON-SHRINK grout. Typically non-shrink grouts undergo a certain amount of expansion that is restrained by the structural elements in order to offset the later shrinkage.

Non-shrink grout must be tested in brass or steel cube molds with a top plate at least 1/4" thick clamped on to provide restraint against the expansion of the non-shrink grout. If the grout is allowed to expand unrestrained, the compressive strength is much less. I'd have to look back at some testing we did for a contractor where the QA technician on the site was doing as you mentioned to give you a ratio. When tested properly, there were no problems.

Gregory A. Johnson, P.E.

http://www.midwesttestinglabs.com

 

[BigH]

Good point conceng - but again, why are they casting cylinders instead of cubes?

 

[conceng]

BigH,

I'd say it's probably ignorance and/or because it is cheap. A brass cube mold set is about $300. The cylinder molds are maybe $0.50 a piece at the high end.

Gregory A. Johnson, P.E.

http://www.midwesttestinglabs.com

 

[techcrete]

Thank you all for the helpful responses.

BigH, the technician is not an employee of mine nor of my company. I have not spoken with the technician so ultimately I do not know exactly why he was using 2X4 cylinder molds.

Woofar & conceng both make strong points! I believe that Woofars 'flexural concept' is applicable & partially addresses the ratio portion of my question but then raises the question why shouldn't we test concrete in a cube if we are Only testing for compressive strength?

Conceng, I believe your thoughts on the molding material (that concretemasonry briefly mentioned earlier) make great sense. If I understand correctly, the cube mold material being a rigid material (brass) restricts the non-shrink grout from expanding. Therefore a restrained shrinkage cube will possess a higher density & compressive strength than an expanded cube. Thanks.

Let's assume that all of the above are correct. If the 2:1 cylinder ratio is intended to allow an overall compressive strength taking into account flexural stresses as well, then why shouldn't a non-shrink grout be cast in a 2:1 rigid cylinder mold to achieve the overall compressive strength of the grout? Or.. given all of the above, it appears that concretes & grouts should equally be tested in equal ratio/cube form for compressive strength. We have designtated ASTMS to test for flexural strengths. No?

 

[concretemasonry]

Look at the failure modes of the materials in the way they are normally used.

Non-shrink grout is often used in a confined space and is relatively flat if you look at the ration of thickness to height, where you have a failure close to being a confined compression failure.

In most critical strength applications, concrete actually fails like the typical failure pattern of a 2:1 concrete cylinder. - Shear or diagonal tension.

All codes and standards are based on historical tests conducted using the appropriate ASTM testing procedures. If you do not test in accordance with the recognized requirements, you cannot relate to the proper codes and standards, no matter how you look for a "correction" factor.

Dick

 

[woofar]

Hi again,
There has been some very good points made in this thread
but here is another to consider.


There are 5 main forms of failure in a compression machine.
1 Flexural failure
2 Compressive failure
3 Shear failure
4 Edge failure
5 Cap failure

By observation,both during and after the crack, a good technician should know what he has got.

A well designed concrete mix should have enough flexural strength to take the first one out of play.

The last three are due to sloppy workmanship.

The type of failure encountered should be reported with the test result which allows everyone to decide whether they should pay much attention to the result.

HTH
Michael

 

[BigH]

I'd like to add point about cubes or cylinders. This is discussed in Neville's Properties of Concrete, 4th Edition in Chapter 12, Testing of hardened concrete. He describes correlations of cubes and cylinders . . . "It is difficult to say which typ eof specimen, cylinder or cube is 'better' but even in countries where cubes are the standad specimen, there seems to be a tendency, at least for research purposes, to use cylinders rather than cubes, . . . Cylinders are believed to give a greater uniformity of results for nominallly similar specimens because their failure is less affected by the end restraint of the specimen; their strengh is less influednced by the properties of the coarse aggregate used in the mix; and the stress distribution on horizontal planes in a cylinder is more uniform than on a specimen of square cross section." He also points out that for concrete, the cylinders are cast and tested in the same direction whereas for cubes the test is transverse.
With mortar, because of the lack of coarse aggregate, some of the reasons for cylinders is not applicable. Neville also discusses in detail the strength of cement - but doesn't touch on why cubes over cylinders although he indicates that the European Std (EN) requires the cubes to come from a 40x40x160 flexural beam. I suppose that the use of cubes by ASTM may just be a holdover to "tradition".

 

[techcrete]

Thank You all for your expert opinions. I have gained a wealth of knowledge from your comments. I look forward to contributing with you in the future.