R. Christopher Williams

Effect of Reclaimed Asphalt Pavement (Proportion and Type) and Binder Grade on Asphalt Mixtures

Reclaimed asphalt pavement (RAP) has been used in the United States for more than 25 years because of the benefits in costs and environmental stewardship. The recent substantial increases in asphalt prices have led asphalt technologists to examine the increase in RAP use. The evaluation of the performance of the asphalt mixture containing RAP is therefore a priority for the asphalt materials community. This paper investigates the effect of RAP percentage and sources on the properties of asphalt mixtures. Ten asphalt mixtures, including two different RAP sources, three RAP content percentages (0%, 20%, 40%), and two different asphalt binders (PG 58-28 and PG 58-34) were investigated in this study. The complex dynamic modulus was performed on all mixtures at different temperatures and frequencies, and semicircular bend (SCB) fracture testing was performed for all mixtures at three low temperatures. Experimental results indicate that asphalt mixtures containing RAP have higher dynamic modulus values than the control mixtures containing no RAP. The stiffer asphalt binder was found to result in higher dynamic modulus values for both the control and the RAP-modified mixtures. Experimental data also show that the RAP source is not a significant factor for the dynamic modulus at low temperatures, although it significantly affects dynamic modulus values at high temperatures. In addition to test temperature, the RAP percentage was found to significantly affect the SCB fracture resistance of mixtures. However, for the dynamic modulus, values for the softer binder were higher than for the stiffer one at low temperatures. No significant statistical relationship between dynamic modulus and fracture energy was found.

Preliminary Dynamic Modulus Criteria of HMA for Field Rutting of Asphalt Pavements: Michigan’s Experience

This paper presents a comparative study of laboratory results of both dynamic modulus testing and field rutting performances of hot-mix asphalt (HMA) in the state of Michigan. Fourteen field-produced mixtures at various traffic levels and aggregate sizes were evaluated and compared to those of field rutting. These mixtures were collected from job sites and compacted with a Superpave gyratory compactor to imitate the common air void level used in Mich., which is 7%. Dynamic modulus E∗ was measured at temperatures ranging from −5 to 39.2°C and frequencies ranging from 0.1 to 25 Hz. The results show that the dynamic modulus values increased when the designed traffic level for HMA mixtures increased. The field rutting performance was evaluated based on theoretical pavement rutting life index. Two parameters, | E∗ | and | E∗ | /sin (φ) , were compared to the theoretical pavement rutting index. Based upon the preliminary study, it was found that E∗ was a suitable parameter in comparing the field and laboratory ...

Effect of Factors Affecting Fracture Energy of Asphalt Concrete at Low Temperature

ABSTRACT Low temperature cracking is considered one of the primary distress modes of asphalt pavements built in northern climates. The detrimental effects of low-temperature cracking of asphalt pavements have motivated new work in fracture testing of hot-mix asphalt (HMA). A comprehensive study was conducted to investigate the effect of various factors on the fracture resistance of asphalt mixtures at low temperatures. The Semi-Circular Bending (SCB) and Disc-Shaped Compact Tension Test (DCT) fracture tests were performed at three low temperatures to measure the fracture energy for 28 asphalt mixtures, which represent a combination of factors including binder type, binder modifier, aggregate type, asphalt content and air voids. In this study, the analysis from the experimental data indicates that fracture energy is strongly dependent upon temperature and significantly affected by type of aggregate and binder modifier. The results of the analysis show the significance of air voids in the SCB test, but no statistical difference was found for the DCT test data. The analysis also illustrates that richer mixtures, ones with more asphalt than the design optimum, are not necessarily more crack resistant.

Bio-Renewable Asphalt Modifiers and Asphalt Substitutes

The global asphalt market is to reach 118.4 million metric tons by 2015, according to a January 2011 report by Global Industry Analysis, Inc. The asphalt paving industry accounts for the largest end-use market segment of asphalt. With increasing growth in the developing markets of China, India, Brazil, and Eastern Europe, asphalt will be needed to construct their roadway infrastructure well into the next decade. The increased demand for asphalt, along with the need for improved asphalt materials/pavement performance, creates the opportunity for bio-renewable asphalt modifiers and/or asphalt substitutes.

Temperature and Shear Susceptibility of a Nonpetroleum Binder as a Pavement Material

Most bituminous binders used for pavement materials are derived from fossil fuels, specifically crude petroleum. Nowadays, technical and economic prospects exist in using biorenewable resources to produce biobinders. Biobinders can be used in three ways to decrease the demand for crude petroleum–derived bituminous binders: direct alternative binder (100% replacement), bitumen extender (25% to 75% bitumen replacement), and bitumen modifier (<10% bitumen replacement). Applicability of developing biobinders from oakwood-based bio-oils to be used as a direct alternative has been investigated through studying the rheological properties. Temperature and shear rate (rate of loading) of biobinders and modified biobinders play major roles in changing the viscosity of bio-oils. The rheological properties of oakwood bio-oils have been investigated and compared with those of bitumen binders, to study the applicability of producing biobinders. Temperature and shear susceptibilities have been studied through measuring the viscosity of the bio-oils. Results reveal that the relationship between the viscosity of bio-oils and temperature and shear rates are log linear–like bitumen binders. In addition, temperature is the main contributor to the viscosity of the bio-oils in comparison with shear rate. Important is that the viscosity temperature susceptibility values for the bio-oils in comparison with bitumen blends indicate that bio-oils are more susceptible to temperature. Moreover, the addition of polymer modifiers leads to a change in temperature ranges of the bio-oils. In conclusion, the rheological properties of oakwood bio-oils are similar to and comparable with bitumen binders, and they represent a viable renewable alternative to petroleum-derived asphalt binders.

A Practical Dynamic Modulus Testing Protocol

The dynamic modulus test is widely accepted by pavement agencies as the critical parameter for the recently proposed mechanistic empirical design procedure and the candidate of the simple performance test to accompany the Superpave volumetric mix design process. However, the specified dynamic modulus test procedure is time-consuming and costly. State pavement agencies are seeking a more practical test protocol. This paper presents a method for identifying a practical dynamic modulus testing procedure. The currently well-adopted method of calculating the dynamic method is discussed and compared to the more fundamental dynamic modulus calculation method by using actual experimental data from two different asphalt mixtures. It was found that the NCHRP report proposed method produces higher modulus values, but the difference is less than 10 %, as indicates that the simple peak to peak method can be used in the calculation without compromising accuracy. A comprehensive dynamic modulus test, which incorporates strain level, test temperature, and frequency, was performed on one asphalt mixture. Experimental data were analyzed with t-test at the 95 % level of confidence. The analysis results show no statistical difference between the dynamic modulus for the two studied strain levels and no permanent damage was found on tested specimens for all three test temperatures. Comparison of the master curves built by different temperature and frequency combinations illustrates redundancy in test temperature and frequency. A more practical dynamic modulus test procedure is proposed based upon the evaluation. This research shows that three test temperatures, 4.4°C, 21.1°C, and 37.8°C, and six frequencies, 25, 10, 5, 1, 0.5 and 0.1 Hz, plus one additional frequency of 0.01 Hz at 37.8°C are adequate to build a smooth master curve to satisfactorily characterize asphalt mixtures.

Effects of Using Accurate Climatic Conditions for Mechanistic-Empirical Pavement Design

The implementation of the new mechanistic-empirical design guide has led to the need for more accurate inputs. One of the most important inputs that affect the performance of the pavement materials is the climatic data. The importance of accurate climatic data is investigated in this case study. Climatic files available with the design guide and ones developed based on historical information for counties in the state of Iowa through the Iowa Environmental Mesonet were used. The climatic files that were interpolated from the data available within the design guide predicted higher rutting (both total and asphalt cement layer) for the northern part of the state, lower thermal cracking, and lower international roughness index compared to the files developed in this study. Statistical analysis showed that the results calculated using both groups are different.

Reporting Results from the Hamburg Wheel Tracking Device

As the list of states adopting the Hamburg wheel tracking device continues to grow, there is a need to evaluate how the results are used. AASHTO T-324 does not standardize the analysis and reporting of test results. Furthermore, the processing and the reporting of the results of manufacturers are not uniform. This inconsistency is partly the result of the variation of agency reporting requirements. Some requirements include only the midpoint rut depth; others include the average across the entire length of the wheel track. No guidance is given when the stripping infection point (SIP) is reported. To eliminate bias in reporting, statistical analysis was performed on more than 135 test runs on adjoining gyratory specimens. Measurement location was found to be a source of significant variation for rut depth in the Hamburg wheel tracking device. This variation was likely the result of the nonuniform wheel speed across the specimen, geometry of the specimen, and air void profile. Eliminating this source of bias when rutting results are reported is feasible although the feasibility depends on the average rut depth at the final pass. When the results of a test that has an average rut depth of less than 12 mm at the final pass is reported, the average of the measurements along a 6.4-in. path beginning at 0.5 in. from the specimen edge nearest the gear housing should be reported. When the average final-pass rut depth is greater than 12 mm, it is reasonable to report the average of the measurements just off the center of the rutting track along a 3.2-in. path beginning at 2.1 in. from the specimen edge nearest the gear housing. SIP values were also analyzed. It is reasonable to report the average of the SIP values across all locations while calculated values measured more than 2 in. from the specimens edge are discarded and for cases in which the ratio of the stripping slope to creep slope exceeds 2.0. Validation is needed for multiple machines.

Reconsideration of the fatigue tests for asphalt mixtures and binders containing high percentage RAP

Abstract When applying reclaimed asphalt technology in a flexible pavement project, most performance concerns are related to low temperature and fatigue cracking since the stiffness of the HMA mixture could dramatically increase through adding a high percentage of reclaimed asphalt pavement (RAP) material. The purpose of this study is to evaluate asphalt mixtures with high RAP contents, prepared using two RAP addition methods, for their performance based on fatigue-cracking resistance rather than relying on volumetric properties. Asphalt mixture samples were prepared with three RAP binder content replacement percentages (30, 40 and 50%) using two preparation methods: the as-is RAP gradation (traditional method) and the splitting of the RAP gradation into coarse and fine fractions (fractionated method). Asphalt mixture beam fatigue and binder fatigue time-sweep tests were performed. Beam fatigue samples also underwent freeze–thaw cycling for freeze–thaw damage evaluation. Rather than basing the performance based solely on S–Nf curves to illustrate the fatigue performance, the beam fatigue test data was analysed through a dissipated energy approach. Faster fatigue degradation was observed for the 40% RAP binder and beam mixture when subjected to repeated loading. From a morphology aspect, this can be explained by the binder’s phase separation and physical hardening effects.

The impact of fine aggregate characteristics on asphalt concrete pavement design life

The development of the Mechanistic–Empirical Pavement Design Guide (MEPDG) provides an opportunity to simulate the performance of pavements. This paper considers the impact of fine aggregate on the predicted performances of pavements by simulating the performance differences between pavement mixes prepared with different sources of fine aggregate with different gradations using the MEPDG. A natural and four manufactured sands from parent material consisting of dolomite, limestone, traprock (TR) and a glacial gravel (GG), and five gradations were utilised in this study. This resulted in 19 different sand/stone combinations being tested for dynamic modulus to enable level 1 analysis in the MEPDG. The results indicate that the fine aggregate angularity (FAA) test adequately ranks aggregates from the same source, but does not appropriately rank aggregates from different sources. TR and GG were identified as the best performers within the investigated aggregate sources and that the FAA, aggregate source and gradation are not significant in determining mixture performance.