Cement Grout Requirements for Ground Anchors

[Obregon]

Hello everyone. I've just found this very interesting reply made by a fine fellow, on a threat posted on this topic a few years back: "In my experience, if the anchor passes its load test, the anchor is acceptable despite low grout test results."

Given that, in addition of the importance of the mechanical properties, say compressive strength, of cement-based grouts for the final shear capacity developed through the interface grout-soil/rock, this grout also plays a significant role in determining the longevity of the anchoring system, what are your thoughts on this very important subject? Thank you.

 

[Obregon]

Carlos R. Obregón-Mimbela

http://www.sueloanclado.com

 

[Obregon]

In my experience, you can prestress and have a 800 kN - design capacity ground anchor to pass a 100 % load test employing either a 28 MPa or a 18 MPa compressive strength grout, eventhough the former value has been called for by the designer. So, aren't specs important?

What it comes to my mind, is that there is much more the f'c of the grout being placed is telling us, and that a properly established criteria on the part of an experienced ground anchor designer, in terms of the required grout characteristics for a specific project and conditions, should be met in the field, particularly for permanent ground anchors.

 

[r13]

Cement grout gain strength over time.

 

[Obregon]

Thank you r13. Do you know how much time is needed for a 18 MPa compressive strength grout to reach the design target strength of 28 MPa?

 

[r13]

You can find the rate of concrete strength gain with age in many textbooks, or on the web. Here is one FYI. Don't trust it blindly, do a research yourself.

Link

 

[Obregon]

Thank you r13. Do you then think we can rely on these forecastings and save a few bucks by employing anchor grouts with strength values below the ones required by the design?

 

[r13]

I wouldn't rely on it for the new design, but may explore it on evaluation of existing setups for modification.

 

[Ingenuity]

Do you then think we can rely on these forecastings and save a few bucks by employing anchor grouts with strength values below the ones required by the design?

Penny-wise, pound-foolish.

Cement cost compared to the total cost of installed and tested ground anchors is peanuts. Do NOT intentionally use lower than specification grout strength. For your typical w/c ratios of cementitious grouts - with minimum bleed and good workability/flowability - strength will near always exceed the specification requirements.

 

[PEinc]

I agree with Ingenuity. Actually, grout strength is not usually a part of tieback and tiedown anchor and soil nail design. However, grout strength is an important design consideration for micropiles.

http://www.PeirceEngineering.com

 

[Obregon]

You´re right PEinc, although, as per FHWA-NHI-14-007, the minimum compressive strength for grout should be 1,500 pounds per square inch (psi) at 3 days, and 3,000 psi to 4,000 psi at 28 days.

 

[PEinc]

Of course, there should be specs for quality control of grout and its strength. However, if that strength is not met, there may still be reason to accept the anchors - especially if the particular anchors with low grout strength have been successfully load tested. Remember, by the time you test the grout (after 7 days, 28 days, etc.), it may not be possible or reasonable to replace or supplement an anchor. It is hard to determine the exact minimum grout strength required for an anchor. Therefore, the anchor proof or performance test (not grout strength) is usually the deciding factor on acceptance of the installed anchor. The standard grout mix of 5 to 5.5 gallons of water per 94 pound bag of Portland cement usually is sufficient to achieve 5000 psi compressive strength at 28 days.

http://www.PeirceEngineering.com