Rebound Hammer Calculations

[concreteworld]

Situation:
Cylinders for columns failed to achieve strength at 28 days. Contractor wants to run rebound tests to ascertain strength of the columns.

I have made some calculations for Schmidt hammer test. As per ASTM C805 and ACI 228.1, correlation curves have to be established for each type of mix design for which the tests are intended. I followed the below procedure;

1. Took 3 cylinders at 7 days for the same class of concrete poured in the structure (60 Mpa flowable); Marked 15 points on each cylinder 50mm and 120 degrees apart. Ensured 15% of load is applied on the cylinder in the compression machine. Then 15 rebound readings were taken. Average was calculated. Similar procedure was followed for 3 cylinders @ 28 days.
2. Later the cylinders were tested in compression and recorded the strength.
3. Plotted Rebound Numbers Vs Compressive Strength for each cylinder.
4. A correlation curve was established.
5. Site readings were taken on the structure to compare with the correlation curve to establish the strength of the concrete in the structure.

Kindly let me know the correctness of the above procedure. Some people say that the correlation curve is on the Schmidt hammer and the strength shall be calculated from the curve, but I believe that curve is not applicable for every concrete grade and may not represent the actual strength.

 

[concreteworld]

see also calculations in the attachment...

 

[JedClampett]

I'd be very careful about accepting concrete based on rebound hammer tests. Unless I understood why it's failing the 28 day tests, (too much fly ash, bad test cylinders, etc.), I'm going to be very hard to convince. Are there spare cylinders taken?

 

[Ron]

I agree with JC. In fact, ASTM C805 has a specific warning against acceptance or rejection based on the rebound hammer correlation...

5.5 This test method is not intended as the basis for
acceptance or rejection of concrete because of the inherent
uncertainty in the estimated strength.

As JC noted, find out the reason for the failure. Take cores for acceptance/rejection as necessary. Depending on the mix design, strength gain might just be delayed. Check all parameters before resorting to Voodoo science.

 

[dik]

Subject to JC's and Ron's cautions regarding impact tests...

Your methodology appears to be correct. Having directly calibrated your 'hammer' to the concrete strength, your data appears to be pretty uniform... not much 'scatter', so there is a degree of reliability.

A couple of caveats... columns loaded in compression are about the worst place for low concrete strengths... affects not only compression capacity, but also Ec values (a tad).

First step I'd look at would be to see how critical the loading is... I'm not big on taking cores from columns... if you do, make sure you locate all rebar and try to miss it! You have to do this even if you use hammer tests. What are you prepared to accept... and determine this before hand.

Dik

 

[BigH]

Suggest you do a search of the site (rebound hammer). This has been discussed in detail several times before. Cheers