Swiss Hammer in Iraq
Swiss Hammer in Iraq
(OP)
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.





RE: Swiss Hammer in Iraq
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.
RE: Swiss Hammer in Iraq
RE: Swiss Hammer in Iraq
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.
RE: Swiss Hammer in Iraq
RE: Swiss Hammer in Iraq
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
www.con-cure.com
RE: Swiss Hammer in Iraq
RE: Swiss Hammer in Iraq
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
www.con-cure.com
RE: Swiss Hammer in Iraq
Fred Croen, RSM
Engius, LLC
Boston, MA
www: engius.com
RE: Swiss Hammer in Iraq