CBR Test

[danielneivaamorim]

In C.B.R. test you have two important readings (2.54 and 5.08).

To obtain the CBR value we should do the calculations based on the reading for 2.54 right?
But if the value obtained for 5.08 is higher than the value calculated for 2.54 we should repeat the test isn't it?

I constantly have higher values in the 5.08 readings than 2.54.
What could be the problem?
The compaction?
Is it completely wrong to do the calculations with the 5.08 value?

Btw, I'm working with the SAATC methods from Austral Africa.

(I hope I've explained myself in a way you could understand)

 

[fergie1133]

Yes, The ASTM Method D1883 is similar in that you calculate the CBR for both and compare. I also have the same issue (getting higher CBR for 0.2 in vs. 0.1 in) but I am often doing the test on soil with fine gravel and I find that causes issues. I don't know what kind of soil you are usually working with but I find I only get higher CBR at 0.1 in when I have a Silt or a Clay sample with little or no fine gravel.

I find it helpful to -

1. During the final compaction lift try to make sure there are not fine gravel particles directly under the area where the piston will be penetrating the sample.

2. Graph the results and do all corrections to the CBR value(for concavity etc.) before you do the comparison.

3. Instead of rerunning the entire test I flip the sample over and do the penetration on the opposite side. Usually I get lower overall CBR values on the flipped side but the CBR at 0.1 in is still lower than the CBR at 0.2 in.