I conducted CBR tests on unbounded aggregates. As you can see the CBR test results in the attachment, the curves concave up and there is a relatively enormous shift between two test results. Considering that two curves are parallel, it, somehow, can be concluded that two samples has comparable properties, but the presence of the shift make the interpretation of the results difficult.
Based on New Zealand and British standard these curves should not be corrected, however some references in the literature recommended correction of these type of curves (I attached the PDF file).
Question A- If I correct the curves (as explained in the PDF file) or shift the sample-2 the results for two replicated samples are comparable. Under this conditions, do I need to correct the CBR test results?
Question B- If I correct the curve and the resultant curve does not meet 2.5 mm penetration how can I interpret the CBR test results?
Question C- If I should not correct the curves what would be the CBR test results in my case?
As per AASHTO T-193 if the curves start concave upwards they should be corrected by extending the linear portion of the curve until it intersects the x-axis (penetration) and use the intersection as the new origin of the axis.
Dear Ebrahim Sangsefidi ,
I suggest you to stick to the code you follow for your research. There are obvious ambiguities between the codes.
Thank you.
Regards,
Shiva
Dear Ebrahim Sangsefidi,
To the best of my knowledge, I would suggest you to question the way your results will be used. In which condition your unbonded aggregate will be used ?
From what you gave, I suggest that your samples were not completely compacted before the CBR test, though the PDF file shows penetration curves that have the same pattern as yours. Can you control that ?
If you only want to compare your value with other material or to use it in a compacted layer, I suggest you to consider only the steep part of your curve.
Either, pay attention to the initial fabric of the sample to understand what you are measuring. Especially, if you want to compare two (or more) identical samples, your initiation of the penetration test must be very precise and controlled.
Concerning the question B : maybe make another test with higher range of penetration. I don't have any other ideas, unfortunately.
Kind regards,
Nathanaël
As per AASHTO T-193 if the curves start concave upwards they should be corrected by extending the linear portion of the curve until it intersects the x-axis (penetration) and use the intersection as the new origin of the axis.
The zero-point of the load-penetration curves must be adjusted. According to ASTM D-1883 and AASHTO T-130, the linear portion of the concave curve which has the steepest slope must be extended back to the zero-base line (i.e., x-axis). The intersection point at the x-axis is the new zero-penetration point which is used to establish a new corrected CBR curve.
Regards!
I also agree with Thomas Makedon and Alaa Shaban comments and suggestion.
The graph shown in above picture with no zero correction is not right and need to do the zero correction as stated above two persons.
A: Yes you need to do zero correction.
B: now your 2.5mm or 5.00 penetration will be shifted because of zero correction but still these will be considered the 2.5mm/5.00mm penetration values.
When the CBR test is performed and the piston enters the ground there is an accommodation of particles that were not compacted on the surface creating that concavity in the load-penetration curve. The correction is made by drawing a line tangent to the point of concavity and moving the origin to the point where the line intersects with the x-axis (penetration) I hope I explained it briefly, if you need more details ask for them without problems
regards
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