Rebound Hammer Test

[xkybb]

Hello everyone!

I involved in a marine foundation project where rock socketed steel H-piles were constructed (see attached).

The piles are roughly 20m deep sitting on the bedrock, each pile consisted of a permanent steel casing of 610mm dia. inside placed with a steel h-section.

Grout of 50MPa was poured by tremie method (where the grout pipe remain at the pile base from start to finish) to fill the void between the steel H section and the permanent steel casing.

Grout cubes were sampled from the 1st batch of grout mix for compressive strength test (it took 25 batch to complete 1 pile).

However, the Contractor later found that the 28 day strength of the cubes were just 45MPa.

Instead of using a rotary drilling rig to drill a full depth verification core from the pile for compressive strength test, the Contractor claimed that conducting rebound hammer test on the top surface of the pile is sufficient to verify the strength as

1. Top surface of the grout is the weakest because it was the first batch of grout poured contacting with and displacing the soil debris inside the pile base;
2. The grout is only designed to transfer loading from the steel H piles to the rock socket (casing stop at the rockhead and grout in contact with the 5m rock socket), and grout of 45MPa was checked to be sufficient in providing the bond stress;
3. Coring on the pile is difficult which may damage the H-pile if the drilling was not properly control. Also, backfilling of the core hole with new grout may cause undesirable crack as the new grout will shrink and expansion again.

Is the Contractor correct?

 

[dik]

I'd check to see if 45MPa was OK and have the contractor get his own engineer to do the same. From a load point of view, the steel H section will be taking 20x (just a number from thin air) more load than the concrete.

Dik

 

[jayrod12]

I would also bet that after another 6 months it will have gain enough additional strength that you'll be close enough to 50MPa to not care anymore.

As dik mentioned, check to see if 50 MPa is really required. Somehow with a steel casing and a steel H pile I doubt the concrete is really doing much besides providing confinement for the H pile.

 

[dik]

Jrod... I wouldn't normally be concerned, except from a contractual and liability sense...

Dik

 

[jayrod12]

Agreed, but if it can be determined that the 50 MPa spec was not required structurally that's easy to fix with a SI or PCN.

 

[dik]

raises the question, 'why it was spec'd...'

Dik

 

[jgailla]

Rebound hammer testing will not accurately assess the strength of the concrete and should never be used for verification. Rebound hammer testing can be of some use in comparing different areas of concrete.
The method of verification for low strength samples (normally cores) should be in the specs.

 

[dik]

although not intended... I've used a rebound hammer to locate areas of dry-rot (brown rot). Works well.

Dik

 

[xkybb]

thanks for both of your answer.

But is the Contractor right about point 1 & 3?

1. Top surface of the grout is the weakest because it was the first batch of grout poured contacting with and displacing the soil debris inside the pile base;

3. Coring on the pile is difficult which may damage the H-pile if the drilling was not properly control. Also, backfilling of the core hole with new grout may cause undesirable crack as the new grout will shrink and expansion again.

 

[Ron]

Agree with jgailla...pile might be fine, but rebound hammer is not the way to assess concrete strength.