The use of new presses to carry out the CBR test (California Bearing Ratio) in Switzerland has shown that, for materials with a high CBR coefficient and identical compaction energy, the shape and dimensions of the hammer have a definite influence on the results obtained.
In addition, for these materials having a high CBR coefficient, the deformation of the dynamometric ring of the old (but still often used) CBR presses, which modifies slightly the penetration speed of the punch, also influences the results obtained significantly.
The research should clarify these different influences, notable through the use of systematic tests and a critical and comparative examination of foreign standards recognised internationally (ASTM, BSI, AFNOR, DIN).
Swiss standards on the subject need to be modified as a function of the research results.
The objective of the project is to specify, especially by using inter-laboratory comparison, certain points of the standard concerning CBR presses with a view to obtain similar results in all of the laboratories. In particular: the hammer shape and the verification of the penetration speed of the punch.
The purpose of the research presented in the final report is to highlight these various influences, in particular by systematic tests and by a critical and comparative examination of the foreign codes recognized at an international level (ASTM, BSI, AFNOR, DIN). Here are main conclusions of the project:
- The CBR obtained with the 75 mm diameter cylindrical hammer are always the higher values. Those obtained with the 51 mm diameter cylindrical hammer are almost every time weaker than these latter. The weakest values are those obtained with the sector shape hammer.
- The values determined with the 75 mm diameter cylindrical hammer are the closest to those obtained with the manual compaction using a 51 mm diameter cylindrical hammer (the reference procedure of the standards).
- The CBR value obtained with different punching speeds shows only a very slight increasing tendency, according to the punching speed. The number of tests carried out is however not sufficient to be able to clearly quantify this relation.
The Swiss standards on the matter have to be modified consequently, in relationship with the European standards.