Swiss Hammer in Iraq

[WayneElliott]

I hope someone out there can help me. I'm working as QA for the Corps of Engineers in Iraq at a site where a contractor is building concrete structures. Many of the compressive tests (they use cubes, not cylinders) failed in the lab and the contractor now wants to use a Swiss hammer test to prove the concrete is acceptable. The specs only mention core tests as a solution for low breaks, but the contractor says a Swiss hammer is acceptable because we reference ASTM Standards in our spec and the Swiss Hammer is accepted by that ASTM standard. Searching other posts (thread590-75263 in particular), I'm not sure how accurate the Swiss hammer results will be. Two questions - Are Swiss Hammer results acceptable by ASTM and are we required to accept the test? Thanks very much to anyone that can help me.

 

[BigH]

Are you sure this is the Swiss Hammer? or is it the Schmidt Hammer? Assuming the latter, in my opinion, it is only for a qualitative assessment of the consistency of the concrete. There are many factors affecting this test result. Improper trowelling, being on a large aggregate, etc. Be careful in use. I have seen too many engineers test "once" and say - well, that's it. But, what is done is to do 10 to 12 tests (depending on the standard) and take the average. Toss out the ones that are <6 or >6 of the average and reaverage. This is your Schmidt Rebound Number. Correlations were developed in Germany years ago and these are the ones typically used. You should, to get a proper assessment, correlate the test to your own concrete mix(es). There are a couple of Indian authors (Shetty and Gambhir) who have good discussions of this in their books.
Secondly - you might be using cubes for the first time. We have had other threads discussing this - but the cube strength is not equiavalent to a cylinder strength. Typically, a factor of 0.8 is used (namely, Cube strength times 0.8 is the equivalent cylinder strength). However, some research indicates for strengths greater than 25 or so, 0.9 is more correct. UK has a full range of the values and I believe that one person put these in a previous thread.
Coring is likely a better solution for the strength of the concrete - you take three cores. If average of the three are >0.85 the required strength and no single core is less than 0.75 times, then the concrete would pass. Neville has a paper about the shortcomings of this as well. You might find some other tests - pull-out bolts that are embedded when you pour, or shooting a projectile into the concrete a better indication. In any event, too, remember that the strength of the cubes - or cylinders - is not a measure of the absolute strength of the concrete in situ but a measure more on the consistency of the batching of the concrete.
I know you are aware of this, but, some points you might well review.

 

[WayneElliott]

My questions remain - what does ASTM say about Swiss (or Schmidt) Hammers, and do you think we're required to accept the test results.

 

[BigH]

There is a specification on the Rebound Hammer in ASTM - no question about it. ASTM gives the method to carry out the test and to report the results - they do not tell or guide you on how you interpret the result from the point of view that you have taken. Given the test result, it is up to you to apply your engineering judgment on the test value and to see if you believe it enough to accept the concrete based on it.

I am going to quote Shetty for you:

"Schmidt's rebound hammer developed in 1948 is one of the commonly adapted equipments for mearsuring the surface hardness. . .

"Each hammer varies considerable in performance and needs clibration for use on conrete made with the aggregates from specific source. The test can be conducted horizontally, vertically . . . or at intermediate angles. At each angle the rebound number will be different for the same concrete and will require separate calibration or correction chart. . . .

"Limitation: Although, rebound hammer provides a quick inexpensive means of checking the uniformity of concrete, it has serious limitations and these must be recognized. The results are affected by:

a. Smoothness of surface under test
b. Size, shape and rigidity of the specimen.
c. age of specimen.
d. surface and internal moisture condition of the concrete.
e. type of coarse aggregate.
f. type of cement.
g. type of mould.
h. carbonation of concrete surface.

"Rebound Number and Strength of Concrete
Investigations have shown that there is a general correlation between compressive strength aof concret and rrebound number; however,there is a wide degree of disagreement among various research workers regarding the accuracy of estimation of strength from rebound readings. The variation of strength of a properly calibrated hammer may lie between plus/minue 15% and plus/minus 20%.

" . . . 1965 and 1967 . . . international survey . . . spoke agasint the use of Schmidt rebound hammer in acceptance testing. The consensus was that, "the Schmidt rebound hammer is useful to very useful in checking uniformity of concrete and comparing one concrete against another but it can only be used a s rough indication of concrete strength in absolute terms."

Given this and my previous post, the call, mon ami, is yours. and good luck.

 

[WayneElliott]

Sorry I didn't check back sooner. Thank you for an excellent answer. I've gotten a copy of the ASTM standard you mention and will make sure the contractor follows it to the letter, then use some judgement on the results to determine whether to drop this issue or not. Thanks again.

 

[JohnGn]

Wayne

Have you considered using maturity testing as an additional means of estimating in-place strength? It, too, is an ASTM standard (C1074) and has been around for 15-20 years. Not many people know about it yet, but it works well. We've done more than 5000 in-place tests with very good results. Many of my clients have dramatically reduced the number of cubes (cylinders) they take because they find the maturity test more accurate in terms of the in-place. And, the in-place is typically curing faster than the cylinders so they get done faster.

The schmidt hammer, and other tests like it (windsor probe, break-off test, etc) are all post-mortem tests, typically used when you have a problem. Since most of the time the problem us usually the cylinders, it makes sense to try to reduce the reliance on such a highly variable sample. That way, you can prevent problems before they develop, or at the very least, spot a problem very early on (usually within just a few hours) so it can be corrected sooner.

Hope this helps you in the future.


John Gnaedinger, Pres.
Con-Cure Corporation
St. Louis MO

http://www.con-cure.com

 

[BigH]

John - I agree the maturity test should be considered at the start of a job. There was a nice little half page on it in Roads and Bridges Magazine back about 5 years ago - but Wayne's problem seems to be the "post mortem" type where the contractor has low break values and wants to save his derriere.

 

[JohnGn]

BigH:

There have been quite a few articles on maturity testing in some of the trade journals very recently. One, in Concrete International, was entitled "Maturity Testing is the Future". Couldn't agree more. My reply to Wayne's post was in the hopes of helping him prevent the need for post-mortems, or at the very least allow other non-destructive tests like the swiss hammer or windsor probe to be used much earlier before the situation gets out of hand (such as when they build on top of bad concrete).
Glad you agree with maturity testing being useful--spread the word, will you?

John

John Gnaedinger, Pres.
Con-Cure Corporation
St. Louis MO

http://www.con-cure.com

 

[concreteguru]

John has the right idea and anytime you can avoid a 'post mortem' by doing things right the first time, you are miles ahead. NIST and OSHA funded a great deal of research on Maturity and their findings were that Maturity Testing of concrete represented the most accurate, non-destructive test method to accurately estimate the in-situ strength of concrete. It can be used to time P-T stressing, when to cut joints, when to strip forms, when to load a structure, and when to remove re-shores. It is also a valuable QC tool to measure the uniformity of your concrete as delivered to your project or to evaluate the effectiveness of cold weather concrete measures. Thanks JohnGn for the valuable post.

Fred Croen, RSM
Engius, LLC
Boston, MA
www: engius.com

 

[concreteguru]

A swiss hammer is quite useful as a paper weight on my desk. It is a useful tool to measure uniformity of strength on a single concrete member. The test is subjective at best and almost any result you want to get, you can get. That makes it the least expensive, all time favorite of the contractor that wants to avoid more expense, more reliable test means.