Andrew Braham

Characterizing Compactability of High RAP and Warm Mix Asphalt Mixtures in the Superpave Gyratory Compactor

Compactability of asphalt concrete mixtures is critical for successful long-term performance in the field. Laboratory samples are necessary for volumetric analysis and performance testing during the design process, so the ability to quantify compactability during specimen production in the lab would be highly beneficial. While there has been significant work in defining compaction characteristics of asphalt concrete, most of this work has revolved around traditional Hot Mix Asphalt (HMA), and little work has been done with Warm Mix Asphalt (WMA) or high percentage Recycled Asphalt Pavement (RAP) mixtures. This research examined HMA without RAP (HMA Virgin), HMA with 35 % RAP (HMA RAP), and two WMA technologies with 35 % RAP (WMA 1 and WMA 2). Four compactability metrics were evaluated, including number of gyrations to 92 % density (N92), Construction Densification Index (CDI), the Construction Force Index (CFI), and the newly introduced Normalized Shear Index (NSI). This research found that the WMA technologies improved the compactability of asphalt concrete, as did the addition of RAP. In general, the WMA 1 showed better compactability than WMA 2, but this could be partially attributed to a softer binder grade, higher asphalt binder content, and the potential for a tender mix, as indicated by the normalized shear curve. The HMA RAP mixture also had a higher asphalt binder content compared to HMA Virgin, which could have contributed to the improved compactability. The NSI metric consistently showed the lowest Coefficient of Variation (COV) values and has the potential to distinguish tender mixtures.

Capturing mixed-mode cracking of asphalt concrete using the Arcan test

Reflective cracking of asphalt concrete has been shown to be a combination of both opening (Mode I) and sliding (Mode II) cracking. The Arcan test configuration was developed at Southeast University to test Mode I, Mode II and three levels of mixed mode. This research used two asphalt concrete mixtures with the same asphalt cement but with two gradations: AC-20 and AC-10; four loading rates: 0.1, 0.5, 1.0 and 10 mm/min; and three testing temperatures: − 10, 0, +10°C, to explore the use of the Arcan test. Overall, it was found that the Arcan test is able to capture mixed-mode characteristics of asphalt concrete, but these characteristics can change significantly over a relatively small range of temperatures. Although Mode II fracture energy was highest at − 10°C, with a steadily declining fracture energy as the level of Mode I increased, there were no clear trends at 0 or +10°C.

Exploration of a Performance Test for Emulsion Treated Asphalt Surfaces

Asphalt emulsions are a key material used for pavement preservation. Over time, asphalt concrete pavements become oxidized, which can lead to cracking and other surface deterioration. The addition of pavement maintenance treatments, such as scrub seals, chip seals, or fog seals, can rejuvenate the pavement surface as the asphalt emulsion penetrates the oxidized layer of pavement. However, most existing test methods for asphalt emulsions are empirical in nature and do not directly address field performance. With the aim of improving material characterization and testing so as to better capture field properties, this research explored using a bending beam rheometer (BBR) to measure the stiffness and rate of change of the stiffness (or m-value) of asphalt concrete mixture beams treated with asphalt emulsions. There were three components of this study. First, procedures for fabricating BBR beams from field asphalt concrete samples were developed, as the top portion of pavement is often brittle after field aging and oxidation. It was determined that beams could be successfully fabricated with reasonable geometric variability. Second, asphalt concrete BBR specimens compacted and fabricated in the laboratory were sawn and tested, with and without asphalt emulsion, to determine whether the addition of emulsion could be detected. Third, the same emulsion applied to the laboratory fabricated specimens was applied to field mixes to determine the influence of asphalt emulsion on candidate materials for pavement preservation. Overall, beam fabrication was repeatable, and coefficient of variation values for test results were lower for the laboratory compacted plant mix (10 % to 25 %) than for the field mixtures (9 % to 57 %). Emulsion addition increased the m-value and decreased the stiffness of all pavements, which indicates rejuvenation of the asphalt concrete. BBR mixture beams appear to be able to capture the effect of adding emulsion to both lab produced and field specimens, but more types of emulsions, a more comprehensive conditioning regime, and more asphalt concrete mixtures should be examined in order to determine the effectiveness of the measured performance properties of asphalt concrete with the addition of asphalt emulsion.

Quantifying Timing of Return to Traffic for Asphalt Cement Based Full Depth Reclamation Mixtures in the Laboratory

Full depth reclamation (FDR) is a pavement structure rehabilitation technique that uses in-place material to build structural capacity of a roadway. By mixing together 8–12 in. of pavement structure with a binding agent, a higher structural capacity can be achieved. However, there is often a period of time prior to the binding agent fully curing where traffic is released to the FDR before a surface course is applied. In this research, five laboratory testing devices attempted to quantify how asphalt cement based FDR builds resistance to raveling during this traffic. Four in-house designed and built testing devices were compared to the existing cold in-place recycling raveling test. Factors explored during evaluation included curing time (0–48 h), binding agent (asphalt emulsion and asphalt foam), and curing condition (ambient temperature and 40°C). In general, all five testing devices showed a decrease in potential raveling with longer curing times using an asphalt emulsion binding agent at ambient curing temperatures. Asphalt emulsion FDR showed higher resistance to raveling than asphalt foam FDR at ambient curing temperatures, but curing at 40°C did not give conclusive evidence on resistance to raveling versus ambient curing temperatures. Finally, this testing was applied in the laboratory to give a general indication of the performance of each testing device; therefore, the testing devices need to be taken into the field to verify these initial laboratory findings and to begin building correlations between the lab test results and actual raveling susceptibility in the field.

Development of Fracture Resistance Curves for Asphalt Concrete

To date, a significant portion of research investigating the fracture characteristics of asphalt concrete has consisted of calculating a single number. This number includes values such as the stress intensity factor, fracture energy, or the J-integral. Unfortunately, by using only a single number, it can be confounding to differentiate between different types of asphalt concrete mixtures, especially at different testing temperatures. This research used a common fracture analysis technique, called resistance curves, or R-curves, to construct fracture resistance curves that include fracture characteristics of asphalt concrete at multiple testing temperatures. Sets of R-curves were collected at three testing temperatures and joined together to form a single R-curve, encompassing fracture characteristics across a temperature range, similar to the concept of constructing master curves collected for dynamic modulus testing. The technique was developed using data collected from the disk-shaped compact tension testing geometry. The effect of polymer modification type, air voids, aggregate type, and asphalt cement content were analyzed. Using R-curves instead of a single number allowed for a deeper understanding of the fracture characteristics of asphalt concrete. Unlike previous research, it was found that the effect and type of polymer modification can be better understood using R-curves, 4% air voids have a higher cracking resistance versus 7% air voids, and energy specific turning points were found that can influence the choice of asphalt concrete material components by local and federal agencies. Although this study is a preliminary analysis of the use of fracture R-curves for the analysis of the cracking resistance of asphalt concrete, it does identify the potential power of this method.

Correlating field performance to laboratory dynamic modulus from indirect tension and torsion bar

Dynamic modulus has several useful functions in flexible pavements, including stress/strain characterisation, rutting and cracking characterisation, an input into several analytical and numerical models, and a primary input into Pavement ME Design. While the traditional dynamic modulus test is run in the uniaxial configuration, this is not possible for field cores. Therefore, the indirect tension dynamic modulus (IDT |E*|) and torsion bar shear modulus (torsion bar |G*|) have been developed. However, there has been limited research looking at analysing the data from field cores for these two geometries, comparing modulus data from the two geometries, examining in-service ageing of dynamic modulus, and quantifying pavement conditions using dynamic modulus. This research examines 10 field sections in Arkansas, comprising of 4 “good” performing sections, 2 “medium” performing sections, and 4 “poor” performing sections in an attempt to address these four questions. First, this research found that using AASHTO T342 and AASHTO R62 can lead to irrational coefficients but provide rational results. Second, while the IDT |E*| and torsion bar |G*| values were similar at high modulus values, the IDT |E*| values began to increase as the modulus decreased compared to the torsion bar |G*| values, increasing to over a decade of difference. Third, a noticeable difference was observed between the modulus values of the bottom surface layer and top surface layer, with the bottom surface layer showing higher modulus values in all cases. While the upper surface layer showed higher oxidation, other weathering effects such as moisture and traffic appear to have overwhelmed the oxidation effect and pavement deterioration has reduced the integrity of the mix. Finally, both the IDT |E*| and torsion bar |G*| were not able to quantify a noticeable difference between poor and medium performing sections, and medium and good performing sections, but were able to quantify a difference between the poor and good behaving sections. Overall, the IDT |E*| and torsion bar |G*| tests were able to produce consistent master curves, correlate to each other, identify differences between surface course lifts, and quantify differences in field performance.

Characterising emulsion effects on aged asphalt concrete surfaces using Bending Beam Rheometer mixture beams

A function of asphalt emulsions is often to rejuvenate the surface of an aged asphalt concrete roadway. However, it is unclear as to the effectiveness of different types of asphalt emulsions, as most current testing is empirical and does not give an indication of field performance. Using asphalt concrete beams from the surface of a roadway in the Bending Beam Rheometer is a new approach that may give an indication of an asphalt emulsions ability to decrease the stiffness. Seven emulsions at three application rates on two roadways were examined to identify the effect of emulsion on the stiffness and m-value of asphalt concrete. The addition of asphalt emulsion generally decreased the stiffness of the aged pavement samples; however, results were fairly erratic and inconsistent compared to change in m-value. The addition of emulsion consistently increased the m-value. Requiring an aged asphalt m-value increase of 0.05–0.06 was suggested as an initial value for consideration within specifications for projects where rejuvenation is a first-order concern.

The Investigation of R-Curves of Asphalt Concrete

One of the primary distress mechanisms in asphalt concrete is cracking. Whether it is caused by fatigue, low temperature, or reflective mechanisms, the development of cracks in the pavement surface is undesirable. Currently, there are several performance tests that quantify cracking, including the four-point bending beam (fatigue), the Superpave Indirect Tension Test (low temperature), and the Texas Overlay Tester (reflective). Another type of test has gained traction in recent years in quantifying cracking of asphalt concrete: fracture testing. To date, most fracture testing has simply reported a single value, such as the stress intensity factor, the J-integral, or the fracture energy. However, when a single value is reported, valuable information about the crack initiation and crack propagation is lost. By using resistance curves, or R-curves, much of this information can be retained. This research introduces the concept of R-curves through two different methods: plotting the cumulative energy versus the crack mouth opening displacement (CMOD) and plotting the cumulative energy versus the crack length. Several laboratories are equipped to capture energy/CMOD data, but relatively few are able to capture energy/crack length data. Therefore, both methodologies are beneficial for discussion. The University of Arkansas has evaluated both of these methodologies at two test temperatures, two loading rates, and two test geometries on a single asphalt concrete mix. Not only has the fracture energy been calculated, but new information about the physical state of the material (linear elastic, inelastic, quasi-brittle, compliant, etc.), the development of the cumulative energy during testing over time, and the energy required to form the cohesive zone of asphalt concrete has been captured. With this preliminary data, the use of R-curves appears to have the capability of capturing new and vital data that furthers the understanding of how cracks develop and move through asphalt concrete.

R-curves characterisation analysis for asphalt concrete

Abstract Fracture tests, especially at lower testing temperatures, have become quite popular in quantifying low-temperature cracking. However, current fracture testing analysis methods often use a single number, such as fracture energy or fracture toughness, to quantify cracking resistance. These tests do not capture both the initiation and propagation of the crack. The Resistance Curve, or R-curve, is widely applied in many fields, such as metal, polymer and composites. The R-curve considers cracking resistance as a function of crack extension, which includes initiation and propagation. In this research, three asphalt concrete mixtures, including hot mix, hot mix with reclaimed asphalt pavement (RAP) and warm mix with RAP were tested at two temperatures, three levels of ageing and two levels of moisture condition by the Semi-Circular Bend fracture test. R-curves were constructed using the data from the fracture test, and digital images were utilised to capture the crack extension. In addition to capturing the traditional fracture energy, two new parameters were explored using the R-curves: the cohesive energy and the propagation parameter energy rate. It was found that cohesive energy was always in a narrow range (approximately 500–1000 J/m2) compared to the fracture energy range (approximately 500–1700 J/m2) over all combinations of ageing and moisture conditions, which indicates that the crack initiation may not be as sensitive to temperature, ageing and moisture as fracture energy. The results of energy rate indicated that moisture and short-term ageing impact the crack propagation by reducing the resistance of crack growth. These results proved that R-curves are a potentially useful tool to quantify the cracking resistance of asphalt concrete in both crack initiation and propagation.

Linking the Field and Lab Performance of Interstate Pavements

In 1999, the Arkansas State Highway and Transportation Department (AHTD) began an ambitious program to rehabilitate over 300 miles of Interstate in 5 years. As part of this rehabilitation program, approximately 270 miles of deteriorated concrete pavement was rubblized and overlayed in an attempt to provide a sustainable alternative to traditional removal and replacement of the pavement structure. Many of the pavements constructed during this program are exhibiting a severe level of cracking. Most of these severely cracked asphalt pavements are located west of Conway on Interstate 40; while the rubblized pavements east of Little Rock on I-40 and I-30, which were constructed at virtually the same time, exhibit much less cracking. In this research, two interstate sections are compared. One section was evaluated as “good” condition and the second “poor” condition. A full forensic analysis included a review of the job diaries, rutting and cracking development over the life of the pavement, mix design, mixture testing from cores, and binder testing from cores. Several properties are currently being evaluated and discussed with AHTD in order to prevent future premature deterioration of the state highway network.