Concrete strength of vibrating equipment foundation

[ammuc]

During the construction of a pump foundation, for which dynamic analysis was carried out, the construction contractor came back with a concrete compresssion strength test result of 18 MPa at 56 days. The specified was 35 MPa.
What would be the implications of this? Cracking? Is it right to assume that the dynamic analysis results will not be affected by the compressive strength of concrete, because dynamic analysis is releted to the mass of the foundation, not its strength.

 

[dik]

I'd stay with the 35MPa... or maybe reduce it to 30MPa... harder and more durable. It's not a strength issue.

Dik

 

[amec2004]

It all depends on the flexural characteristics of you dynamic equipment foundation.
In other words, it depends on if it’s a rigid block type or flexural table-top type foundation.

If it’s a rigid block type foundation, you are OK and the impact of reduced concrete modulus Ec caused by reduced fc' is minimum.

If it’s a flexural table-top type foundation, you are in big trouble as it might change the foundation’s dynamic characteristics such as natural frequency etc significantly, and you have to plug-in the new static modulus of elasticity as per ACI 318-11 8.5.1 Ec=57000*SQRT( fc' ), or use revised dynamic modulus of elasticity as per ACI 351.3R-R2011 4.4.1.2 and re-check pretty much everything.

From ACI 351.3R-R2011 Foundations for Dynamic Equipment Page 45

4.4.1.2 Dynamic modulus of elasticity—The dynamic modulus of elasticity is stiffer than the static modulus, although not in any simple form. [red]Established relationships suggest that the ratio of dynamic to static modulus can vary from 1.1 to 1.6, with significant variation with age and strength [/red]. In practice, engineers treat this strain-rate effect differently. In some firms, engineers perform calculations using the higher dynamic modulus while other firms and engineers consider the difference unimportant and use the static modulus from ACI 318. [red]The distinction is more important for elevated tabletop-type foundations where the frame action of the structure is stiffer if a dynamic modulus is used. The difference can also be important in compressor foundations where the stiffness of the machine frame must be evaluated against the stiffness of the concrete structure. For simple, block-type foundations, the concrete modulus of elasticity has no real effect on the design. [/red]

If the low strength concrete applies to you concrete pile as well, it will also affect you as it impacts the foundation’s damping coefficient C and spring stiffness K. You may need to re-run your dynamic analysis with the revised K and C and re-check everything.

anchor bolt design per ACI 318-11 crane beam design

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[ammuc]

Amec2004,
The foundation type as used in DYNA 5 was PILE FOUNDATION WITH RIGID CAP which I assume behaves in a similar manner to a rigid block type foundation. Is that correct?
Thank you.

 

[amec2004]

It should be OK if that low strenth concrete doesn't apply to pile

anchor bolt design per ACI 318-11 crane beam design

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[ammuc]

The specified compressive strength for piles was 25MPa. This was achived by the contractor.
But the strength in question is for the pile cap. The specified compressive strength for pile cap is 35 MPa and only 18MPa was obtained even at 56 days.

 

[amec2004]

Normally for block type cap the shear, flexural, and pile punching shear check etc are not a problem.

You may look into that if you have a thin mat, majority of the forces for about check are from static load, not dynamic load for centrifugal pump case.

Most of the time we design the dynamic foundation with static part ratio less than 0.5 so the cap concrete lower strength should not be a problem as long as the dynamic part is not affected.

anchor bolt design per ACI 318-11 crane beam design

http://www.civilbay.com

 

[dik]

How was the f'c determined... by cylinders or by coring? and any explanation for the low strength?

Dik

 

[amec2004]

>>How was the f'c determined

When the contractor poured the concrete on field they made a couple cubes on the spot and sent these cubes to designated labs

After 3 days ? one of the cube got crushed and get fc-3

After 7 days ? one of the cube got crushed and get fc-7
.....
After 28 days ? one of the cube got crushed and get fc-28

I don't know the exact days and just put some number there, pls check the code yourdelf.

If a certain percentage of the 28-day strength fc-28 is less than the design strength, I will request the nondestructive tests to be performed on site

There are several nondestructive tests (NDT) that can be used to evaluate the strength of hardened concrete. Two of the most common NDT methods are the Schmidt rebound hammer and the Windsor probe, both of which measure the surface hardness of concrete as a quick indicator of relative compressive strength.

If the NDT also looks not good and my design re-check fails at the lower strength, the construction contractor will ask the commercial concrete provider to pay for all the cost. Sometimes you have to tear down and re-build if it's a nuclear reactor or turbine foundation ...

anchor bolt design per ACI 318-11 crane beam design

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