CC2 Phase V Test Results and Analysis
Rut Depth Evaluation
A 16-foot (4.88 m) long straightedge was used for rut depth measurements. In each test item, the rut depth measurements, and profile measurements, were made at two different longitudinal positions located at one-third and two-thirds the distance into 9 the test item. These locations were designated as NW and NE for the rubblized test items and SW and SE for the non-rubblized test items (N and S stand for north side and south side of the longitudinal centerline respectively). Figures 10 through 12 show the straightedge rut depth measurements for test items MRC, MRG, and MRS, respectively.
Except at the MRS-SW location, all of the test items showed similar rut depths during the first 5082 passes (55,000-lbs wheel load, 4-wheel landing gear configuration). In particular, there was no discernible difference between the performance of the rubblized and non-rubblized pavements. It was also observed visually that the surface deflections of the rubblized pavements under load were negligible and the response of the rubblized pavements appeared to be very similar to that of the non-rubblized pavements. Instrumentation was not installed in the pavements to measure surface deflections so this observation cannot be verified to any degree of accuracy. But the surface deflection of a flexible pavement under load can be readily observed visually, and without any magnifying aid. The surface deflection of a rigid pavement cannot be observed visually. It was therefore decided that the load should be increased to the largest extent practically allowed by the test vehicle loading system and tires to increase the possibility of inducing significant distress in the rubblized pavements. From 5083 passes to the end of trafficking, six-wheel triple-dual-tandem loading at 65,000 lbs (29.5 tonnes) wheel load was applied to the rubblized pavement and four-wheel dual-tandem loading at 65,000 lbs (29.5 tonnes) was applied to the nonrubblized pavements. The six- and four-wheel configurations at increased loading both had the same dual and tandem spacings of 54 and 57 inches (137.2 and 144.8 mm). At the MRS-SW location (non-rubblized), a test pit (5-feet by 4-feet) was opened in the concrete slab (for subgrade evaluation) prior to the placement of the HMA overlay. The concrete that was used to re-fill the test pit was severely broken up and a depression formed at this location during the placement and compaction of the HMA overlay. This severely weak area caused significant local accumulation of rutting during the early trafficking period.
After approximately 10,000 passes in MRC, 13,000 passes in MRG, and 15,000 passes in MRS, significant upheaval in the HMA layer at the longitudinal joints just outside the traffic path was observed in the rubblized test items. After this number of passes, the rut depth measurements are exaggerated because the straightedge was resting on top of the upheavals outside the traffic path. More accurate rut-depth measurements have been computed from the surface profile measurements.
Maximum rut depths from the transverse profiles at the end of trafficking were 4 inches (10 cm) on MRC-N, 2.5 inches (6.4 cm) on MRG-N, and 2 inches (5.1 cm) on MRS-N. Significant structural upheaval was also observed outside the wheel track on MRC-N and MRC-N, but neither the straightedge measurement nor the transverse profile measurements can separate the contributions of the underlying structural response and the asphalt upheaval movement. Measurements of the transverse profiles of the structural layer interfaces are currently being analyzed from trench data. These measurements are expected to give a more definitive estimate of the true structural response of the rubblized pavement structures.
The NE end of MRC was the first area of the rubblized pavements to show signs of failure. This failure was not representative of the structural performance of the test item as a whole because one of the pre-overlay test pits (for subgrade evaluation) was located where the pavement failed. A weakened support system resulted because the replaced subbase aggregate material could not be compacted to the same density as in the original construction. A depression in the pavement surface was observed at this location after about 400 load repetitions. The depression migrated longitudinally towards the east until it was about 15 feet (4.6 m) long, but the structure continued to support the full traffic load until it appeared to be in danger of suffering complete structural collapse at 11,814 passes. The weakened area did not migrate back into the west half of the test item and the declared structural life of MRC-NW of 14,256 passes is believed to be a true representation of the structural performance of the test item. Also, MRC-NW did not appear to be in danger of complete structural collapse as had MRC-NE. Trafficking in MRG and MRS was terminated after 25,608 passes. From visual inspection at the end of trafficking, MRG-N appeared to be suffering from structural upheaval outside the wheel track but MRS-N did not. More definitive estimates of the structural condition of the test items will be possible after analysis of the posttraffic trench data.
One of the rubblized pavements was observed to definitely suffer structural failure. Another of the rubblized pavements was probably suffering severe structural deterioration at the end of trafficking but retained sufficient structural capacity to support the applied load. The third rubblized pavement did not appear to be suffering severe structural deterioration at the end of trafficking despite having accumulated significant levels of rutting and asphalt shear flow. None of the nonrubblized pavements suffered significant structural deterioration or significant levels of rutting. Nor was any reflection cracking evident at the surface of the non-rubblized pavements, but this is to be expected because the tests were performed indoors during warm weather.
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