grout resistance not met in ground anchors

[joncle10]

hello I would like to have a feedback regarding the below

I have a diaphragm wall which I am stabilizing using ground anchors. the grout specified must have 27 Mpa at 7 days before tensioning the cables.
The contractor is getting 18 Mpa at 10 days so he is not meeting the designer requirements.
I would like to know how much is critical the fact that the grout capacity is not being met ?
A ground anchor test will be done. If the results are OK, should I still be worried regarding the above ?

Thanks in advance

 

[jayrod12]

Ask the designer. My bet is the answer would be no it is not ok. 18MPa at 10 days will likely never reach 27MPa in it's lifetime. Pre-mixed grout? Redi-mix? or site mixed?

 

[joncle10]

site mixed grout. note that the 27 Mpa should be met at 7 days

 

[joncle10]

but lets say that testing of the ground anchors using this grout with this capacity gave acceptable results, cant I admit that the ground anchors will be ok or not necessarily ?

 

[jayrod12]

I'm going to exercise my 5th amendment right on that question. That would be up to the ground anchor designer. It would depend on what you consider acceptable results compared to what they consider acceptable results. Yesterday would have been a great time to get them on board, because if they require removal and reinstallation, you've already lost a day guessing. In the end, it's their liability on the line.

 

[jonastp]

Hi,

That would be, in my opinion, the decision of the designer of the anchor. for ground anchor, it could be acceptable since the ''ground-to-grout'' friction could still be smaller than the ''grout-to-steel''.
And let's say that the test gives you good results, I would ask for testing on all anchor (no need to be a long test for all anchor, lets say reaching 1.5 times the design load in 5 steps, wait 30 minutes to check for creep, and unload in couple of steps, or to lock off load). the grout-water ration or the storing of the grout must be the reason for the low resistance.

Hope I could be of some helps, and do not hesitate to ask if you have questions, I design lots of anchor with a contractor.

Jonathan

 

[hokie66]

The low strength grout probably has insufficient cement. In other words, cheap. So as well as the strength issue, the protection against corrosion will be compromised.

 

[PEinc]

In my experience, if the anchor passes its load test, the anchor is acceptable despite low grout test results. On most jobs I see with grout testing for tieback anchors, the testing is done incorrectly, the samples are made incorrectly, the wrong grout molds are used (not in accordance with ASTM grout test procedures), the freshly made samples are left too long on the job site without proper curing, the samples were disturbed or mishandled prior to the grout fully setting, or the bearing ends of the grout cubes were not able to provide full, level bearing during the test. Test results can vary greatly due to the shape of the grout samples (cubes vs. cylinders), the size of the samples, and the material from which the sample molds are made (steel, aluminum, plastic, etc.). Also, by the time that grout samples are made, cured, and tested, the tieback anchors represented by the test samples are already installed, possibly tested, and the contractor is on to the next lower tieback level. A better way to check grout is to use a mud balance to check the grout density when the grout is mixed, just before or during placement of the grout.

http://www.PeirceEngineering.com

 

[joncle10]

Noted. Thanks for all. I have already sent it to the designer for him to check. I will ask the contractor to use a higher grade of cement to achieve the results. Water cement ratio is fixed at 0.5

 

[jayrod12]

It might be he just needs to adhere to the 0.5 W/CM ratio.