Three different kinds of base material of varying base thicknesses were tested at the Washington State University Test Track on Ring #4 during the fall of 1968 and the spring of 1969. Twelve 18-foot test sections consisting of 4.5, 7.0, 9.5 and 12 inches of untreated crushed rock surfacing top course base; 2.0, 4.0, 6.0 and 8.0 inches of sand-asphalt base; and 0.0, 2.0, 3.5, and 5.0 inches of Class "F" asphalt concrete base, covered by a uniform 3.0-inch thick Class "B" asphalt concrete wearing course were tested during this period. The pavement structure was built on a clay-silt subgrade soil.
Instrumentation consisted of moisture tensiometers, strain gages, pressure cells, LVDT gages and thermocouples for measuring moisture, strain, stress, dynamic deflections and temperatures. Benkelman beam readings were taken.
The testing revealed that the fall failure modes were different from the spring failures. The fall failure pattern started from transverse cracks in the thin sections which developed into alligator cracking patterns. These cracks appeared after a period of cold weather and heavy rains followed by a warming trend. Thermal and mechanical loads in conjunction with adverse environmental conditions during construction and prior to and during the fall testing period were believed to be responsible for the early fall failures on the thin sections. The spring failures were very rapid and sudden and were due to environmental factors which led to highly saturated subgrade, thus resulting in poor bearing capacity. Punching shear was the failure mode. The thickest sections survived without cracks but developed severe rutting.
Comparison of the results with those obtained from Rings #3 and #4, show that they were similar in many respects. This indicates that the test track is capable of replicating results and is a reliable research instrument.
Equivalencies were developed for the different materials. On this basis the Class "F" asphalt concrete base was superior to the sand-asphalt and untreated crushed rock bases in that order.
Maximum values for static and dynamic deflections, strains and stresses for different times and temperatures were developed. The lateral position of the dual tires with respect to the gage severely affected the strain, stress and deflection values. Temperature also caused variations in the measurements.
Spring instrument readings for static and dynamic deflections, strain and stress show values as much as 2 to 4 times of those obtained in the fall. Spring subgrade conditions probably were responsible for these differences.
Ring #4 series operational time was half that of Ring #3 and about the same as Ring #2. Ring #4 sustained about a quarter of the wheel load applications of Ring #3. Construction and testing environmental conditions were inferior to those for Ring #3, although they were similar for Ring #2 and hence contributed to the lower test period. This points out that environmental factors are very important in pavement life.