Micropile Grout Compressive Strength

[Aspensnow]

Recently received grout compressive strengths back with 28 day data all over the place. Design was for 4500 psi and breaks are from 3500-6500psi. I have both flow test and mud balance data as well that do not align with test results. Additionally, multiple days where grout compressive strengths are higher for 14 days than 28 days. Average strength was 4950psi and median 4900psi.

Should the client be concerned with the outliers?

P.S. This is Type I/L grout with a water reducer to thicken grout.

 

[TigerGuy]

Tell us more about these micropiles. Does their load carrying ability come exclusively from the soil to grout interaction? Are they end bearing into a stronger layer of soil/bedrock? Are the loads all compressive, or are there some tensile loads? What about steel within the element (size, strength, etc)? How long are the individual piles?

3,500 psi for a design strength of 4,500 psi would be concerning, but there are numerous factors that may eliminate that concern.

 

[LittleWheels]

If your 28 day strengths are lower than your 14 day strengths for the same batch of grout, that suggests significant issues with sampling, curing and testing. Do any of these aspects raise red flags to you after talking with and/or watching the sampler and testing lab?

 

[Aspensnow]

Tell us more about these micropiles. Does their load carrying ability come exclusively from the soil to grout interaction? Are they end bearing into a stronger layer of soil/bedrock? Are the loads all compressive, or are there some tensile loads? What about steel within the element (size, strength, etc)? How long are the individual piles?

3,500 psi for a design strength of 4,500 psi would be concerning, but there are numerous factors that may eliminate that concern.

The micropiles are 55ft hollowbar with casing in the top 15ft. They rely mostly on the grout ground bond for compressive strength with some minimal tensile strength. There are also some lateral components hence the casing in the top 15 ft. The 10% of the piles have been proof tested and passed. I would say these piles are conservative in the first place given the size of the inner steel compared to the design load (84 kips, T76 bar, 7 inch casing).

 

[Aspensnow]

If your 28 day strengths are lower than your 14 day strengths for the same batch of grout, that suggests significant issues with sampling, curing and testing. Do any of these aspects raise red flags to you after talking with and/or watching the sampler and testing lab?

I did not watch the samplers in the lab, but this is my thought exactly. I have samples broke at a 6100 psi average (3 cubes) and FHWA broke there 28 day cube at 3500 psi. To me this has to be a handling issue as all samples were broken lower (except 1) than the 3rd party testing agency. I have no way of proving that this is the case as testing agencies handled the cubes after 2 days of field cure.

 

[LittleWheels]

I would further investigate the differences between your 14 day cubes vs. 28 day cubes, be it handling, curing, the exact test procedures or whatever. I have several times found differences in how cubes are prepared and tested affecting results. Testing cubes dry or soaked is a significant difference, despite that aspect being defined in the codes I am familiar with (not US standards).

 

[PEinc]

Most inconsistent grout strength issues I see are with the sampling, varying mold sizes and shapes, storage on site, transportation, and test specimen prep. When I have low grout tests, I check the micropile design for the lower strength. Often I design for a lower grout strength but still call for 5000 psi. I rarely have confidence that the grout testing process is done correctly.

 

[Aspensnow]

If your 28 day strengths are lower than your 14 day strengths for the same batch of grout, that suggests significant issues with sampling, curing and testing. Do any of these aspects raise red flags to you after talking with and/or watching the sampler and testing lab?

Yes after further investigation, I believe the results indicate some sort of mis-curing of the cubes. Additionally, we were using a water reducer which means we should have followed ASTM C942 and not C109, which deals with confining the cubes during curing and breaking to represent the in-place grout. I believe the only way to show the grout in-place is adequate is to pull new cubes and follow the correct curing practices for ASTM C942.

 

[Aspensnow]

Most inconsistent grout strength issues I see are with the sampling, varying mold sizes and shapes, storage on site, transportation, and test specimen prep. When I have low grout tests, I check the micropile design for the lower strength. Often I design for a lower grout strength but still call for 5000 psi. I rarely have confidence that the grout testing process is done correctly.

As a contractor we typically try to mix for the "thickest" grout possible to not have to worry about variability of grout breaks. Unfortunately, I am finding it difficult to prove to FHWA that it is a testing issue and not representative of the material installed. Additionally, I think the special provision that was written followed your same logic and everyone knows the 6,000 psi is not a requirement of the design.