IMPACT OF CONSTRUCTION VARIABILITY ON PAVEMENT PERFORMANCE

The long term performance of hot mixed asphalt (HMA) pavements is significantly impacted by the properties of the HMA mixture. Regardless of how well the mix design and structural design have been prepared, the properties of the materials delivered to the job site, such as gradation, binder content, and the in-place compaction will ultimately control the behavior of the pavement under the combined action of traffic and environment. The job mix formula allows for certain tolerances in the HMA construction, this research studied the effect of construction variability on performance if the delivered product goes outside the tolerances range. The first phase of the study evaluated the impact of construction variability on northern (Lockwood) and southern (Sloan) Nevada aggregate sources mixed with an unmodified AC-20 and AC-30, respectively. The second phase of the study evaluated the impact of construction variability on the same aggregate sources mixed with polymer modified binders PG64-28NV for the northern source and a PG76-22NV for the southern source. Mixtures were prepared for each source and tested for general strength using the resilient modulus (Phase I only), for rutting using the Asphalt Pavement Analyzer, for fatigue using the flexural beam fatigue and for thermal cracking using the Thermal Stress Restrained Specimen Test. Construction variability has a significant impact on pavement performance regardless of the aggregate source and binder type. However, some laboratory prepared mixtures may provide better performance than the optimum mixture but such mixtures may be impractical in the field. If the contractor violates the specification limits, then there is 80-88% chance that the pavement section will have lower performance than the optimum mix, therefore quality control is recommended to keep the mixes within the specification limits. The use of polymer modified binders has the tendency of masking the impact of variability on the performance of HMA mixtures. Therefore, any demerits should be based on the relative performance of neat binders and the use of polymer modified binders should only be considered as an additional assurance for better long term performance. The use of polymer modified binders significantly improved the performance of Nevada Department of Transportation's (NDOT's) HMA mixtures. The data generated in this research showed that such improvements are in the order of 300 to 3000% for the standard MM22 mix on both aggregate sources. This indicates that the additional cost of using polymer modified binders is easily off-set by the drastic gains in the performance of the HMA mixtures.