By Mark Buncher, Ph.D., P.E. and Harold Von Quintus, P.E.
Polymer modified asphalt (PMA) binders have been used in North America for many years to improve hot mix asphalt (HMA) pavement and overlay performance and to increase overall pavement life. A new report by the Asphalt Institute, Calibration Factors for Polymer-Modified Asphalts Using M-E Based Design Methods (ER-235), provides guidance to the thickness designer who is using the new AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG) and wants to incorporate calibration factors that are specific to PMA mixtures. These calibration factors better reflect the improved performance that can be expected with the use of PMA.
Before discussing the MEPDG and calibration factors for PMA, it is beneficial to review a related study that was completed and reported on in 2005 by the Asphalt Institute as IS-215 and ER-215, Quantification of the Effects of Polymer Modified Asphalt for Reducing Pavement Distress.
2005 ER-215 Study
This study analyzed field performance data from 84 different sites to compare observed distresses of pavement test sections containing polymer modified binders to those containing conventional unmodified binders. These sites were from a variety of climates, traffic volumes and pavement cross-sections across North America, and included many Long-Term Pavement Performance (LTPP) sites. Figure 1 shows the location of some of these sites, illustrating the wide range of climates the data represents.
Each of the sites had a PMA section as well as a companion unmodified (neat binder) section that contained all the same features (other than binder). For each pair of PMA and unmodified sections, already collected distress data was compared for three performance categories: rutting, thermal (transverse) cracking and fatigue (alligator) cracking.
Direct Comparison of Performance
A direct comparison of the amount of distress measured for PMA sections (horizontal axis) plotted against the amount of distress measured on the for rutting, Figure 3 for thermal cracking and comparable Figure 4 for fatigue cracking. It is distress was less on the PMA section than on the companion, unmodified section the easy to see that the overwhelming majority of data falls above and to the left of the line of equality (black line), meaning the amount of majority of times.unmodified section (vertical axis) is shown in Figure 2
While the reduction of rutting and thermal cracking was expected for PMA modified sections versus unmodified sections, the reduction of fatigue cracking might be a surprise since agencies do not typically cite fatigue life as a reason for using PMA.
The last part of the ER-215 report used mechanistic empirical (M-E) distress prediction models for rutting and load-related fatigue cracking to quantify the improvement in performance life. Damage indices were computed (DI = n/Nf) for like pavement conditions and compared to actual field distress measurements for both PMA and unmodified sections to obtain different “expected service lives.” Increases in service life generally ranged from 2 to 10 years, depending on soil, traffic, climate, drainage and existing pavement conditions. More specifics can be found in IS-215 or ER-215.
Mechanistic Empirical Pavement Design Guide
The new AASHTO Mechanistic Empirical Pavement Design Guide (MEPDG), version 1.0, has recently been released by the National Cooperative Highway Research program (NCHRP). The MEPDG represents a major change in the way a pavement design is performed. MEPDG predicts multiple performance indictors and provides a direct tie between materials, structural design, construction, climate and traffic.
The use of the M-E-based methods makes it possible to optimize the design and to more fully ensure that specific distress types will be limited. This means that the design and analysis procedure calculates pavement responses (stresses, strains, deflections) and uses those mechanical responses to compute damage over time. The procedure empirically relates a pavement response parameter to the cumulative damage of observed pavement distresses. An important part of this method is the calibration between the cumulative damage and observed distress.
The MEPDG has distress-specific calibration factors that were determined under NCHRP Project 1-40D (NCHRP, 2006). These calibration factors are used in conjunction with the prediction models for rutting, transverse cracking and fatigue cracking that are built into the MEPDG. These calibration factors were determined using a very large database of test sections across the country from a wide variety of conditions (foundation soil type, traffic, climate, thicknesses, age, etc.). These test sections were mostly built with neat HMA mixtures.
Using these “default” calibration factors, the MEPDG and its prediction models assume that changes in dynamic modulus between neat and PMA mixtures will be sufficient to account for changes in distress predictions. This is not the case, however. Using the calibration factors based primarily on neat mixtures will overestimate the amount of distresses that will occur if PMA is utilized, and will subsequently underestimate the design life of PMA sections.
ER-235, Calibration Factors for Polymer-Modified Asphalts Using M-E Based Design Methods, presents calibration factors for rutting, transverse cracking and fatigue cracking of PMA mixtures. First it describes the mathematical relationships (models) used to predict each of these three distress types on flexible pavements and HMA overlays. The report then documents the determination of PMA-specific calibration factors to be used with each of these three prediction models in the MEPDG. These PMA calibration factors were determined relative to the calibration factors for the neat HMA mixtures, and only those sections with adequate data for the PMA and the neat HMA sections were used.
A more detailed description of the calibration factors determined from this study is beyond the scope of this article. The findings from ER-235 are being implemented in the FHWA education efforts around the MEPDG. The sponsors of the study and FHWA have ensured that both NHI courses that teach the MEPDG include ER-235 as part of the students’ packet of course materials.
A huge implementation effort for MEPDG remains to be done by the states and our industry. A major part of this implementation is to ensure that the most accurate performance predictions are calculated by utilizing the most appropriate calibration factors for a given set of conditions. This will allow the designer to get that “extra credit” for using premium materials such as PMA.
|AI’s ER-235: Calibration Factors for Polymer-Modified Asphalts Using M-E Based Design Method
This 32-page Engineering Report presents calibration factors for rutting, fatigue cracking, and transverse cracking specific to polymer-modified asphalt (PMA) mixtures for use with the new AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG). The report provides the analysis and methodology for a pavement thickness design engineer using the new MEPDG to adjust calibration factors of asphalt mixtures to reflect the improved performance expected when using PMA mixtures.
The primary author of this report was Harold Von Quintus of Applied Research Associates. The project was funded through the Asphalt Institute by several Affiliate Member Companies of the Asphalt Institute, the Federal Highway Administration, and the Association of Modified Asphalt Producers. This was a follow-up effort to an earlier study performed by Von Quintus to compare the performance of neat HMA mixtures to that of companion sections built with one or more layers of PMA mixtures. This earlier study on performance was published in 2005 by AI as IS-215 and ER-215, Quantifying the Effects of PMA for Reducing Pavement Distress.
To obtain a copy of ER-235, visit AI’s website, www.asphaltinstitute.org.
|Mark Buncher is the Director of Engineering for the Asphalt Institute. Harold Von Quintus is Principal Engineer for Applied Research Associates, Inc.