Re Concrete test hammer results

[Alex1264]

We are building a 40' by 60' steel building in Nebraska. The customer wanted a 3' foundation poured around the building with a 6" thick floor.
Once we had poured the footings the customer then asked for a strength test on the concrete.
The engineer came and used a test hammer on the foundations. His best result was 2000lbs psi.
The surface he was testing on was rough and he was unable to gain a smooth surface to test on. Will this affect the test results?
Also the foundations have only being down for 14 days. He told me that the concrete would not harden any more from this period.
Within the building is a pit that we have also placed. In this area he was able to gain a reading of 2500lbs psi due to the surface, again he told me that he could not get an accurate reading due to the brush finish on the base of the pit.
What is an acceptable level for strength?

 

[Ron]

Alex1264...The Rebound Hammer (Swiss Hammer/Schmidt Hammer)is not an appropriate test for compressive strength when used alone. The test is adequate to assess uniformity in the concrete, or to isolate areas of potentially bad or at least different strengths, but there are too many variables for it to be used as a definitive compressive strength test. When done it should be done in accordance with ASTM C805 "Standard Test Method for Rebound Number of Hardened Concrete". You will notice that the title does not say anything about compressive strength. In fact, the standard specifically states that "This test method is not intended as an alternative for strength determination of concrete."

The surface texture of the concrete, the type of aggregate, the water-cement ratio, the finishing techniques, the age of the concrete, the orientation of the hammer, and the operator are all variables that must be accommodated in order to evaluate the concrete. Correlation to cores taken from the structure should be done to validate the rebound number.

To properly evaluate the strength of your concrete, I would recommend extracting cores from both the slab and the footing areas. For the 6-inch thick slab area, the cores should be 2-1/2 to 3 inches in diameter, trying to maintain a length-to-diameter ratio of 2.0. Anything less than that requires an adjustment to the strength value. These cores should be taken and tested in accordance with ASTM C42 "Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete".

Most codes require a minimum compressive strength of 2500 psi at 28 days. Be sure the age of the concrete and the appropriate acceptance criteria are applied to the cores before the report is issued to the Building Official. If you have any questions about it, come back to the forum and we'll help if we can.

Good luck.
Ron

 

[Abedkayrouz]

Dear Alex,
Please be informed that the proper way to verify the concrete compressive strength is by extracting cores of 4inch diameter and that complies with ASTM C42,
Whereas for the strength as i understood from your message that the concrete is poured since 14 days. My recommendation is to wait to 28days and extract the cores since usually contract is based on 28 days compressive strength. Furthermore, concrete is considerable adequate when the average of 3 consecutive cores are not less than 85% from the designed strength f'c and that no core is less than 75% from f'c as per ACI 318.
Please refer to ASTM c42 for proper coring it is very important.
Should you need any further information or assistance please do not hesitate to contact me.
Abed El-Kader Kayrouz

 

[Polecat]

I agree with the others that cores are the best way to determine the strength.

Caution: You have to be sure that no pieces of wire mesh or other steel are trapped in the core. A piece of embedded wire will interfere with the core's natural tendency to make a conical break and result in a false reading. There is nothing you can do to make a concrete cylinder or core better than it really is, but there are dozens of things that you can do wrong in the process of sampling, handling and testing to make it worse than it really is.

I totally agree that a rebound hammer should not be relied upon as a stand alone, but rather as a comparative testing device. I have tested many a prestressed concrete pole for acceptance using a Swiss Hammer, but ONLY in cases where I could use the same hammer on another pole of a similar design which did have test cylinders previously taken and recorded. Then the rebound readings from the hammer will have some correlation that is based on a real test.

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