Relationships Between Laboratory Measured Characteristics of HMA and Field Compactability

Abstract

Compactability of HMA mixtures is often used to describe how easy or difficult a mixture is to compact on a roadway. Several asphalt researchers have proposed laboratory measured parameters of mixtures and/or their components as indicators of HMA compactability and/or resistance to permanent deformation. However, most of these measured characteristics have not been validated with actual field performance. The first part of this study includes a comparison between the laboratory compactability parameters Compaction Energy Index (CEI), number of gyrations to reach 92% of Gmm (N@92%Gmm), Slope, Locking Point and Bailey Method ratios. The data used for this stage came from Superpave mixtures placed on the NCAT Test Track in the first two cycles (quality control samples). It was found that CEI, N@92%Gmm, Slope, Locking Point can be used to represent the applied energy to reach a level of compaction in the SGC. The second part of this study includes the determination of a field compactability indicator based on rolling operation (Accumulated Compaction Pressure – ACP) and correlation between this indicator and laboratory parameters. When all the combined data were used to correlate ACP and lab compactability parameters, the values of simple linear correlation (R-value) were always near zero. The results showed that t/NMAS and temperature significantly affected the applied compactive effort to reach the post-construction density level. The third part of this project includes compaction of specimens using the SGC at to meet the 8% air voids at thicknesses equal to those in the field. A multiple regression analysis showed that eighty two percent of the variability in the ACP can be explained by four predictors: PCSI, FAc ratio, lift temperature and number of gyrations to reach the post construction density level at lift thickness (N@field-density). The last part of this study involved density testing during the rolling operation. The purpose of this part was to determine the field compaction energy required to produce the same level of density as samples compacted in the laboratory and correlate that energy with laboratory compaction parameters. A multiple regression analysis provided a model with ACP@92%Gmm as the response, while ninety two percent of the variability in the response can be explained by the interaction temperature*thickness, % passing No 200 sieve, actual PG grade, slope, locking point/Slope ratio, FAc ratio and PCSI square.