Grout Motar Cubes not on Target 1

[03LabGrunt]

I have questions about Mortar Grout Cubes.
It seems extremely difficult to get consistant break strength. At times it seems to be no problem at all to make strength. Ive had several specimens that have actually gone down in strength at the second set of breaks. I have been sanding the sides that I intend to compress,(not the top or bottom) and I always use a guage and straight edge to ensure planeness. Any ideas?
The project manager and the on site engineer are baffled as well. Or at least it would appear so since their offers of solutions are at best mediocre.("Use the sander", "stop using the sander and go back to sanding by hand," "Are you sure its centered when compressing?"" etc. )I am compressing at a rate of 200 to 400lbs per second. Please any commentary would be helpful.
Thanks in advance.

 

[Rjeffery]

Ordinarily an inconsistent or variation in the compressive strength of mortar is found to be inconsistent batching or possibly retempering of the mortar prior to sampling. If the batching is being done "by the shovel full" then the variation is with the batching...

 

[03LabGrunt]

Shovel full? Can you give me an example?
Also, should the planeness of the the Grout Cubes really have such a signicant variation. By say a load difference of 10,000lbs?

 

[concretemasonry]

Are you testing mortar or grout? - Laboratory or field sampled? - For acceptance or for construction monitoring?

MORTAR -
The specification for masonry mortar is ASTM C270 and includes requirements for EITHER proportions OR for property specifications (not both!!!!). The ASTM C780 procedure specification is for "preconstruction" and " construction evaluation" and does not contain any reference to any required strength and is meant to verify the consistancy of materials and procedures. It specifically states that it is not representative of the actual strengths as everyone knows. Of course, the appendix of ASTM C270 clearly states the the engineer should use the weakest mortar possible to carry the loads, since other properties are equally or more important. Mortar actually has a minor effect on the compressive strength of a masonry prism, with the masonry units being the real controlling factor.

GROUT -
The specification for masonry grout (fine and coarse) is ASTM C476. The testing procedure is specified in ASTM C1019. ASTM C1019 requires samples to be constructed in a formed area that will permit the absorption of the excess water. Failure to not use an absprbtive form or a cardboard, wood or metal form will give highly variable and much lower compressive strtengths that the actual grout in place. The new ASTM C1019 (next year) will even include specifications for not reusing the forming materials.

Dick

 

[03LabGrunt]

I am testing Grout that has been put in Mortar cube molds.(Does that make sense?)
It is for tie backs in the slurry walls for a large project.
There is no absorption of any kind during the molding process. I took a look at the molds today and couldn't tell exactly what it was made of(covered in parafin from a previous job),but it was definately metal in construction.

Again ,how much does planeness of the compressed sides affect the overall load strength? I feel it should not make as much a difference as I specified earlier.(10,000 lbs)If the grout is any good it shouldnt be this finicky.

I need answers. The engineers I'm dealing seem to have no experience with these type of samples and their input(from the mundane to the ridiculous) appear to be grabbing at straws.

Lab-Grunt

 

[concretemasonry]

Whoever designed the project and is using the grout/mortar should have a specification or requirement for the material and testing procedure for the basis. Do you have a minimum required strength?(compressive, shear, tensile)

Any relatively wet cementitious material placed in a cube shaped non-absorptive mold may not be representative of the properties of it in place because of the materials surrounding it.

Excess or variable mositure can lead to poor and variable consolidation highly dpendant on the rate and amount of hydration of the cement. Due to the cubic shape and 1:1 aspect ratio you are only coming close to measuring the crushing strength of the material. The actual compressive, shear and tensile strengths are not measured.

Most concrete-based materials are tested in samples that have a aspect ratio (height/width) of 2:1 that provides a more meaningful measure of the properties. In these samples, the failure is actually a shear failure due to diagonal tension, but it is reported as a "compressive" strength. This is dependent on the strength of the concrete matrix and the consolidation of the material.

For concrete cylinders(2:1 ratio), you are measuring the strength of the concrete mix while cured under laboratory curing conditions. It is not the strength of the concrete in place, but a determination of the materials used are equivalent to the design mix.

For masonry grout, an absorptive mold (2:1 ratio) is used to simulate the the actual site absorption and consolidation that takes place initially. After that, laboratory curing is used to produce a measure of the grout compressive strength in place.

I suggest you find out what the requirements for your material are and what the specific curing and testing procedures are. I am not familiar with use of grout as tie backs for a slurry wall, but the engineer of record must have a requirement, basis and testing procedure.

 

[03LabGrunt]

Awesome, Thank you,

Min req strength is 5000 psi.(2x2 cubes.)

I wiil see about looking at the specs.

 

[labboss]

It sounds like you may be dealing with a non-shrink grout, or may be experiencing an expansive reaction in your samples from early age to later age breaks. Inconsitencies in results or an actual decrease in strength over time could be a sign of a possible expansive reaction. If this is the case, take a look at ASTM C 942 ( vol. 4.02). It requires that you restrain the samples in their molds in order to restrain the expansive reaction. Another issue that you may already be aware of is the size of the upper spherical bearing surface. The maximum diameter for a 2" cube sample should be 3", it should also be well oiled and move freely to properly seat on the specimen.

Good luck,

 

[dirtsqueezer]

Is there any coarse aggregate in your samples? You were talking about a slurry... If so, I would go to standard 4x8" concrete molds. The 2x2" molds you are using are more for stuff like base plate grout, with only sand as the aggregate, usually more like 10,000 psi.

 

[QCTechnician]

State highway projects use 2x2 molds. If the samples are casted correctly, there should be no "sanding" needed. Are you using capping compond or rubber caps?

 

[03LabGrunt]

No capping or rubber caps. The specimen is placed on the bearing block alone. I dont have the ASTM that quotes the use of the method in front of me. These sample s are cement and water only, no sand.
I still havent got access to the job specs for this grout. We are using it for tie backs.
Thank you for your response.

To add. For whatever reason the grout cubes have been breaking consistant now. The samples themselves are molded with greater care. I suspect whoever was making the grout was doing something wrong or using the wrong mix. There doesnt seem to be the loss of strength for the 7 day grout breaks as in previous sets.(early sets the 7 day was lower than the 3 or 4 day)Since no one is blaming the lab I believe they(at the site) discovered the problem and rectified it without telling anyone.

Thanks again,
Lab_Grunt03

 

[BigH]

Sometimes it is necessary to bring up what is obvious - when you test the cubes do you turn them 90 deg so that the "top" of the cast cube is on a side face? You don't break cubes with the cast top and bottom as the test top and bottom.