Non Destructive Concrete Testing of Old Foundations

[cl220]

I am looking for non destructive concrete testing methods to use on old foundations. I am testiing to check the integrity of the concrete. I have been researching Shmidt Test Hammer, Windor Probe, and UltraSonic Testing. Any preferences or suggestions would be helpful.

 

[CELinOttawa]

You can't. It is pretty much useless... Depending on the age, that is. In terms of foundations I consider a foundation to be "old" if it was batch poured, or any method pre ready-mix concrete truck delivery.

IF you have a "older", but still modern method built foundation, go ahead and use NDT. Remember that you have to adjust the value to the relative 28 day strength if you are going to rely on it for desing.

IF when you look at the foundation, or ideally review the history, and it is strip formwork poured or batch mixed concrete, your NDT tests are worse than useless. Even proper ASTM 42 cores are of dubious value, but the NDT is a complete waste of time. The problem is that old foundations will have a wildly varying strength depending on where you check.

I just did a job on a 1942 basement, creating new 4' long windows in the foundation to allow natural light into a new basement inlaw sweet. I approached the job as "minimum compressive strength only" on the existing concrete, considering 15MPa (~2000psi) and designed the two lintels as such. The owner then wanted me to look at the one existing 4' long window; The unreinforced concrete lintel had failed in shear, but caught on the heavy timber window frame. I advised we should through-fasten again to avoid having to rely on tension capacity of any post-fixed anchor. Due to Architectural concerns, the Owner wanted to try to chemical anchor. I told him I wanted three cores, and we would only rely on the concrete if the strengths were within a 50% range of one another. He looked surprised, but agreed. Here are the results, and my field report's discussion section:

Background/Information Received:

Laboratory Results:

Core 1 - 5 ft Vent - 14.3 MPa
Core 2 - South Window - 35.4 MPa
Core 3 - South Window 2 - 9.6 MPa


Discussion:

Site batched concretes frequently have highly variable compressive strengths, and it is typical for results of samples taken from disperate parts of the structure to show variations in strength. These variations are not normally so strong, and the samples have shown a high degree of large aggregate in the mix (as was visible on the surface during the wall restoration works).

These results lead us to believe that the concrete mix is sound and that the fundamental strenght is good, but that this strength is compromised by the inclusion of large aggregate which are likely to contain internal flaws, lines of weakness and voids.

We will not rely upon the concrete for tension loading of epoxy anchors.