Acceptance of "Swiss Hammer" for concrete strength 2

[Hemifun]

I have a project where the 28 day concrete cylinders on 1 particular pour came back with substandard breaks(3278 psi for a 3750 psi mix). The contract documents require cores be obtained and tested and based on the cores the concrete either rejected or paid for at a reduced rate. I've already checked the calcs. and the structure will still be safe with the reduced strengths so we will not be requiring removal and replacement but are looking at a reduced payment. This ,of course, has the contractor furious and has proposed using a "swiss hammer" in lieu of cores. I initially rejected that proposal because I always thought that rebound testing was dubious at best and mainly an indicator of surface hardness rather than strength. Now, the contractor has raised the issue again and has cited ACI 1.6.5.2 which allows rebound hammers. What are your thoughts on this issue and the use of rebound hammers for strength evaluation??

 

[Swizzle]

I've always used the Swiss Hammer as an indicator of concrete strength. I would only expect the readings to be accurate to within 300-400 psi. There are too many variables, even in the testing procedure itself, for these results to be relied on. In place of cores,I have had good results with shooting Windsor probes. They are "non destructive" and usually less expensive than cores and you have results as soon as the probes are shot.

 

[SlideRuleEra]

IMHO, since your specification is clear (use cores), don't waive the spec based on what the Contractor "wants". This does not mean that you have to be "inflexible".

Require the Contractor to have the rebound hammer tests performed by a qualified, independent lab (at the Contractor's expense). Then require that the lab results be evaluated by an independent Engineer (also at the Contractor's expense) who then summits their signed /sealed finding and recommendations to you for your evaluation.

Make it absolutely clear that this is necessary if he wants you to "ignore" the contract specification. In any event, a reduced payment is "mandatory" since the Client did not get what they paid for (3750 psi concrete). However, don't just "pick" a dollar amount to reduce the payment. Calculate it in a way that is easy to explain to others (non-engineers). One simple way would be to pay for this portion of the work using the following "formula":

Actual Payment = Contract Payment x (3278 psi / 3750 psi)

I know that this does not make "technical" sense, but contract administration is different from engineering decision making. You can get yourself into a lot of "trouble" by making arbitrary decisions, even if they are based on sound technical principles.

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[JAE]

Technically, according to ACI chapter 5, the concrete may meet ACI specifications for acceptance. Your "low" test result was less than 500 psi off.

The following is from a previous post of mine from this last May:

[blue]The following is our in-house guideline that (I believe) follows ACI 318 Chapter 5:

Acceptance of Concrete
Per Chapter 5 of the ACI Building Code Requirements for Structural Concrete
(ACI 318)

A strength test is defined as the average strengths of TWO cylinders tested at 28 days.

A strength test is acceptable if BOTH of the following are met:
1. Every arithmetic average of any three consecutive tests (6 cylinders) equals or exceeds f'c.
2. No individual strength test (average of two cylinders) falls more than 500 psi. below the required strength, f'c.

If either of the above two requirements are not met, contractor should take steps to increase the average of future concrete tests.

If Item 2 above is not met, then the following should also be followed:

1. With the lower f'c derived from the strength tests above, the structural engineer should review the design of the affected portion of the structure to determine if the lower f'c is acceptable. If the lower f'c is acceptable, then no further efforts are required and the concrete can be accepted. Steps still should be taken by the contractor to increase the compressive strength of the concrete for future mixes.
2. If the lower f'c is found to be critical to the performance of the structure, then further testing should be performed using drilled core samples.

Core samples should be obtained – three at a time, for each strength test that falls below 500 psi. If the concrete in the structure will be dry under service conditions, cores shall be air dried for 7 days before the test and tested dry. If the concrete in the structure will be more than superficially wet under service conditions, cores shall be immersed in water for at least 40 hours and be tested wet.

The concrete can be accepted if BOTH of the following are met:
1. The average of the three cores is equal to or greater than 85% of f'c.
2. No single core is less than 75% of f'c.

If the cores don't meet the required levels the following are all options to consider:
1. Perform a load test of the structure in the area under consideration (per ACI Chapter 20)
2. Provide additional structural framing to strengthen the portion of the affected structure to meet the required load carrying capacity.
3. Accept the concrete if acceptable to the owner.
4. Reject the concrete and remove and replace the portion that is considered below strength.

[/blue]

 

[Hemifun]

Thanks for the replys. I guess I should clarify, the average of 4 cylinder breaks was 3278 psi with a low of 3090 psi and a high of 3450 psi.

 

[Dinosaur]

The device is a Schmidt Hammer. There is an ASTM on the use of Schmidt Hammer tests. However, I would stick to the contract language. If the contract says cores, then go with the cores. Get the PM to determine the appropriate follow-up tests, that's his job.

 

[BigH]

Hemifun:
The "test result" is based on the average of the 28-day individual cylinder strengths. It is not based on individual strengths. JAE gave the correct advice on ACI practice - however, your spec may differ from that but would have to be specific.
As for the Swiss (Schmidt) hammer - I would not use it for compressive strength test correlations:
1. See Nevilled and Brooks who explain this. (also Shetty and Gambhrir)
2. There is an ASTM and BS standard for this (few know how to carry out the test accordingly - it is not a single "blow"). But, these specs give you a "rebound number" - not a compressive strength correlation.
3. The so called compressive strength correlations that are "out there" are sometimes based on cube strengths and not cylinder strengths - so there is a correction to be invoked and it varies according to concrete strength.
4. The correlations are not based on your aggregate, cement and that but on someone else's. If you would consider the strength correlation it should have been developed for your mix design - not someone else's.
5. The hammer test gives an idea of the concrete's uniformity - not specific.
6. Cylinders in the lab do not measure the in situ strength - they are only a measure that the concrete used is consistent. Many forget this issue. Poor consolidation can reduce the strength. Confinement in your structure increases the strength over the unconfined compressive strength in the lab.
7. Use of cores - make sure you know the acceptance criteria.

 

[msucog]

i'd core it and be done with it. i recommend against using swiss hammer...besides, since all the concrete suppliers tend to automatically go to that instead of coring, that should tell you something (and i believe coring is prescribed to accompany the swiss hammer anyway if recall correctly so coring is required anyway). i personally would only use swiss hammer if it were something "nonstructural" like a simple sidewalk and it's more an exercise of providing some data that shows "something". however, i would first advise the client to do nothing rather than "rely" on something that has nothing to do with anything. again, core it--know what you've got in place--make a decision on real core data instead of wasting two days of everyone's time debating the suspect results (and the results will be debated by someone depending on which side of the fence they are on).

the above paragraph sort of shows the frustration i've experienced dealing with similar situations. core it...if it's low, take it out...if it's not, leave it in. it doesn't get much simpler than that. you might pre-think through what to do depending on what you're testing (column, slab, wall, etc).

 

[msucog]

oh, and i should clarify (the following should preclude what i just posted)...we typically suggest following the aci acceptance criteria too. if the breaks don't satisfy that or if the eor declines using that criteria, then core it.