Abstract:
The California Bearing Ratio test (CBR test) is a penetration test developed by the
California State Highway Department (Caltrans) in early 1928/29 for evaluating the
properties of construction materials. CBR testing is integrated in the design of flexible
pavements and since its development. The CBR test has been adopted internationally.
The South African road industry has implemented new CBR testing standards. The
adaptation of the new standards has created questionable CBR results versus the actual
material properties found in the field. Pavement design is based on the fact that the
minimum structural quality will be achieved for each layer of material in the road prism.
New construction techniques have allowed layers constructed previously in 150mm to be
constructed in 300mm layers (especially for fill materials in the subgrade). This has shown
a better structural quality as the top 150mm layer specification is achieved throughout the
300mm layer. This study, although not intensive, used a modified method of the CBR
determination to approximate the classified material field conditions of the constructed
layer. The modified method took into consideration a percentage (%) of the coarse
material with the size greater than 37,5mm to be crushed through 37,5mm sieve. The
modified method of the CBR indicated a substantial increase in the CBR values. The
modified testing procedure for a specific case study increased the classification CBR
value. These classified CBR values change from a minimum specified value of (G5 [CBR ≥
45% at 95% MDD]) with an average reading of 46.7 at 95% MDD to a substantial
improvement in the CBR value (G5) with an average reading of 60.8 at 95% MDD of the
material. Although the gradings in the modified method is finer than the standard test, it’s
still followed the same curve. The other properties had a minimal effect on changes made.
The Clegg Hammer Test was used to compare the strength of the constructed layer to the
CBRs from the two test methods; it was found that the modified CBR testing method
compared well with the Clegg hammer results. This confirms the importance of including to
a certain extend of the coarse material (> 37,5mm) in the CBR testing regime.
Keywords: CBR, Modified Test Method, Natural uncrushed material, Clegg
