Comparing the newly implemented CBR test method with actual field construction :South Africa

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