Factors Affecting the Inter-Laboratory Reproducibility of the Bulk Specific Gravity Of

FACTORS AFFECTING THE INTER-LABORATORY REPRODUCIBILITY OF THE BULK SPECIFIC GRAVITY OF SAMPLES COMPACTED USING THE TEXAS GYRATORY COMPACTOR MacKean, C

Abstract

The Colorado Department of Transportation (CDOT) uses the inter-laboratory variation of bulk specific gravity results from compacted asphalt mix samples to measure the reproducibility of laboratory compactive efforts. Matched asphalt mixes tested in different laboratories had average differences of sample bulk specific gravities which were statistically significant during the 1993 paving season. This investigation identified and addressed some of the causes of these inter-laboratory differences in sample bulk specific gravities. The findings have implications for the Texas gyratory compactor and possibly for the SUPERPAVE gyratory compactor. When this investigation's recommendations were implemented in Colorado, inter-laboratory differences in average bulk specific gravity results dropped dramatically. Absolute differences between CDOT Region laboratories and the CDOT Central laboratory decreased from an average of 0.016 in 1993 to 0.002 in 1994. By taking the following steps, repeatable compaction results were achieved. Formal, scheduled equipment calibration verification was necessary for all laboratories testing asphalt mixes. Also, the loading of the mold and the compaction procedure had to be standardized to a very high degree. The loading and compaction methods must minimize segregation, have strict time limits, be achievable by novice testers, be easily taught, and have high operator acceptance to minimize intentional procedure modification.