How do you determine the appropriate amount of concrete core samples?

[LakeNormanTiger]

All,
I am working for a concractor on a restoration of an existing 2 story building. The building has a foot print of 50ft x 300ft. The building has a concrete floor system for the 2nd floor. The restoration project requires some extensive modifications to the existing truss system that supports the roof. The building has high ceilings, and due to the loads of the steel components being used in the renovation, power equipment will be needed to hoist and hold the material into place. Using some simple beam analysis, using the EOR's Live design loads, I beleive that the girders and beams will support a piece of equipment.

One area of concern that I do have is in regards to the strength of the concrete floor itself. During my initial analysis I assumed a compression strength of 3000 psi. I then used the steps in ACI-318 (Chp. 22) to determine a unreinforced nomial shear and moment strength. These nomial values are setting the limits on the piece of equipment that can be used. To confirm my assumption, I have elected to hire a testing company to come out and take some cores of the 2nd floor. Hopefully this will atleast confirm my assumption, or better yet, allow my equipment load limits to increase. Which leads me to my question. How many core samples should I have the testing company take? Is there also a random sampling procedure that I should follow. I know the more samples the better the analysis, but wanted to know if there was either a certain ACI/ASTM procedure that I should follow, or a good rule of thumb, (ie, one core for every 1000 sqft or something like that).

Thank you for any advice or information that you can give.

LNT

 

[Ron]

Considering a 4-inch thick floor slab, each 81 square feet represents one cubic yard of concrete. Each transit mixer truck generally delivers 10 cubic yards of material, or 810 square feet of area. If you cored enough to represent each truck, then you would need 19 cores. That's probably a bit much, since you only need to verify an assumption.

Given that, I would consider 6 to 9 cores, and possibly some consistency verification using either a swiss hammer or Windsor probe. This way you minimize the holes and maximize the relevant data.

ASTM C42 is the procedure for coring. Evaluation can be done under ACI guidelines.

 

[BigH]

To add to Ron's comments, when taking cores to confirm that "low" strength concrete can be accepted, one would take three cores and then follow the ASTM (or other similar procedures) to get a "strength" result. While the results of several "single" cores can give an indication, you could translate the "three" core system to perhaps get a better handle on the slab's design strength - i.e., the average of three cores must be more than 85% of the specified strength - no single core less than 75%. Of course, given that the slab is several years old, you will have additional strength. I think that what you are looking for is whether the slab is 3ksi, 3.5 ksi or 4ksi compressive strength. Have you checked the original design dwgs (perhaps with the city engineering dept) to see what was specified for the project?