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|Federal Highway Administration > Publications > Public Roads > Vol. 70 · No. 5 > Creep and Recovery|
Publication Number: FHWA-HRT-07-003
Creep and Recovery
by John A. D'Angelo, Raj Dongré, and Gerald Reinke
A new FHWA test offers a faster, cheaper method for ensuring adequate elastomeric polymer content in asphalt binders.
High traffic volumes and heavy loads exact a major toll on U.S. roadways and spur engineers and designers to develop and refine pavement materials that provide greater durability and can be used faster and more cost effectively. One method used by engineers to increase the durability and strength of asphalt pavements is through the use of polymer-modified binders, which help reduce rutting and cracking, among other benefits.
When a highway agency purchases binder, pavement experts typically conduct a series of tests to assess its quality and performance. One standard test involves assuring an adequate amount of elastomer (elastic polymer) in the binder, which influences the elasticity of the pavement and ultimately helps prevent rutting. In the past, the elastic recovery test was the accepted methodology for determining the presence of adequate elastomer content in a polymer-modified binder. The test evaluated the percentage of recoverable elongation measured after elongation using a conventional ductility test to determine the level of deformation without rupture.
State department of transportation (DOT) personnel and paving industry experts, however, were dissatisfied with the elastic recovery test for a number of reasons. First, the test is time consuming, potentially taking as long as 4 to 5 hours to complete. Second, although the American Association of State Highway and Transportation Officials (AASHTO) created a standard method for the elastic recovery test, many highway agencies use alternative testing protocols that produce differing elastic recovery values. According to Mike Anderson, director of research and laboratory services for the Asphalt Institute, "Because not all user agencies specify the same set of additional tests, asphalt producers supplying to multiple markets may be required to perform several different tests using different procedures on the same sample. The elimination of specification proliferation was one of the goals of the Strategic Highway Research Program [SHRP]."
In 2002, while addressing an entirely different problem, researchers at the Federal Highway Administration (FHWA) realized that a test known as the repeated creep and recovery test (RCRT) could not only streamline testing for the presence of elastomer in polymer-modified asphalt binders but also yield valuable information on the binder's likely performance. The RCRT method involves applying a specific load (pressure) to an asphalt binder specimen, removing the load, and then measuring the recovery response (how quickly the specimen regains or attempts to regain its original shape). Recent FHWA research indicates that the RCRT could provide a standardized protocol for testing polymer-modified binders, enabling pavement designers, highway agencies, and industry personnel across the United States to perform the test quickly and easily, using familiar equipment.
"If the test can meet the needs of the user agencies, then it should dramatically reduce the amount of additional testing required for modified asphalt binders," Anderson says.
Need for a New Test
The AASHTO SuperpaveTM specification M320, which is one of the most widely used asphalt binder specifications in the country, was developed several years ago based on the study of unmodified asphalt binders conducted under SHRP. As the use of polymer-modified asphalt binders continues to grow in the United States, many in the paving industry have questioned the applicability of the existing AASHTO M320 specification as it relates to modified binders. In addition, highway agencies are supplementing the existing Superpave binder specification with additional tests, including the elastic recovery, toughness and tenacity, and force ductility tests, to ensure that the binder includes the desired modifiers, such as elastomer.
A problem that arises from using these "Superpave Plus" (also known as SHRP+) tests is that in most cases they only indicate the presence of a particular modifier in the binder but do not determine performance. The results do not indicate the performance of the binder if any of the original parameters change, such as the type of base asphalt binder, or if additional additives are used. When using the SHRP+ tests, suppliers must follow processes that may increase costs with little or no increase in performance.
To address this problem, the Transportation Research Board's expert task group on binders challenged FHWA to provide the industry with a standardized test that identifies elastomeric polymer-modified asphalt binders in terms of their fundamental properties and performance. Such a test, the experts on the task group hoped, would eventually replace the current SHRP+ empirical tests and offer the added advantage of compatibility with future specification tests.
In the process of conducting research to identify the effect of changes in strain and stress on polymer- modified asphalts, FHWA researchers realized that the RCRT showed promise as a replacement for the elastic recovery test. In addition to characterizing the fundamental properties of the binder, the RCRT is easy to use and provides detailed information on how the binder will react under different loadings. Neat binders will flow when loaded and exhibit no recovery or rebound. Polymer-modified binders will deflect and then have some recovery after loading. The RCRT accurately measures this response under different loading conditions.
Refining the Test
Creep and recovery testing has been a basic material test for polymers and mixtures for many years. Researchers can run the tests using multiple creep and recovery steps and multiple stress levels.
Researchers working on behalf of the National Cooperative Highway Research Program (NCHRP) Project 9-10 initially developed the RCRT in 2001. In that project, researchers performed the RCRT at only one creep stress level—0.04 pound per square inch, psi (300 Pascal, Pa)—and repeated the test through 100 creep and recovery cycles.
A later study, "Exponential Shear in Asphalt," published in the Journal of Applied Asphalt Binder Technology, used a sliding plate rheometer (a tool for determining the flow properties of solids by measuring stress and strain over time) and showed that a single stress level did not completely account for the stress dependency of polymer-modified binders.
In the current study, FHWA researchers further developed the RCRT by raising the stress levels. Specifically, they tested a variety of polymer-modified asphalt binders at a range of stress levels from 0.0036 psi (25 Pa) to 3.713 psi (25,600 Pa) and compared the results with data collected from the elastic recovery test.
Although most pavement experts who conduct performance-grade testing are familiar with using a dynamic shear rheometer, the RCRT method is not yet well known, so FHWA developed a standard practice for conducting the tests in a format compatible for eventual adoption by AASHTO. FHWA also prepared specification criteria that facilitate identifying elastomeric polymer-modified asphalt binders.
First, the researchers aged the asphalt binder specimens by heating them in a rolling thin film oven, according to the AASHTO T240 or the American Society for Testing and Materials' (ASTM) testing method ASTM D2872. Then they tested samples of the aged asphalt using the dynamic shear rheometer, according to the temperature controls described in the AASHTO T315 (ASTM D7175-05) test method. They used 25-millimeter (0.98-inch) parallel plate geometry with a 1‑millimeter (0.04-inch) gap setting. All the data were collected at the high performance-graded (PG) temperatures. For example, the researchers collected RCRT data on a PG 64-28 binder at 64 degrees Celsius, °C (147 degrees Fahrenheit, °F).
They also tested specially formulated binders with different amounts of flux oil, with and without polymer. Cross-linked (reacted with sulphur to create a network of polymer molecules) and uncross-linked (no network created using sulfur) binders also were studied, along with elastomeric binders extracted from field cores taken from actual pavements.
As applied to asphalt binders, the RCRT typically involves a 1-second creep period, where researchers apply a specific load to the specimen, followed by a 9-second recovery period, after which they remove the load and measure the recovery response. For this study, the researchers performed 10 creep and recovery cycles at each of 11 stress levels.
Next, the researchers tested the samples for creep at two stress levels, followed by recovery at each level. This method enabled the researchers to evaluate a modified binder's ability to maintain elastic response at two distinct stress levels while being subjected to 10 cycles of stress and recovery. Percent recovery obtained at the higher stress level can indicate the presence of an asphalt modifier with elastic properties.
The researchers ran the tests at the selected temperature using a constant stress of 1 second followed by a zero-stress recovery lasting 9 seconds. The researchers ran the tests at two stress levels: 0.0145 psi (100 Pa) and 0.4641 psi (3,200 Pa). Ten cycles, run at each stress level, yielded a total of 20 cycles.
The researchers ensured that the required full stress for each creep cycle was achieved within 0.05 second from the start of the creep cycle. They recorded the stress and strain every 0.1 second for the total length of the test on a running time, such that in addition to other data points, the data points at 1 second and 10 seconds were explicitly recorded. There were no rest periods between creep and recovery cycles or changes in stress level. The total time required to complete the two-step creep and recovery was 200 seconds.
Collecting the Data
The researchers found that the RCRT took approximately the same amount of time as the current Superpave dynamic shear rheometer test. They recorded the creep and strain measurements for the 100 Pa and 3,200 Pa stress levels for each of the 10 cycles, noting the following:
Then, for each of the 10 cycles, they calculated the percent recovery at both creep stress levels. Using the response at 100 Pa, they could evaluate a binder at a low stress level, where it will exhibit its greatest elastic response. By increasing the stress level to 3,200 Pa, they could evaluate the binder's response at a severe stress, which would help determine how well the binder would maintain its structure and elastic response under heavy traffic loading. To achieve these results, they needed to run the test at 100 Pa before conducting the test at 3,200 Pa.
The researchers found that when they conducted the test at 100 Pa and 3,200 Pa, consecutively, they could measure the elastic response of the binder directly. Once it became clear that the data at 100 Pa and 3,200 Pa could be used to identify elastomeric polymer modifiers, the researchers created a separate database to house data at those stress levels for an array of both unmodified and modified binders. The database included data on several asphalt polymer systems and elastic recovery data for a subset of the binders.
In the database, they logged the following information: the sample identification, PG grade and test temperature rounded to the nearest 0.1 °C (0.18 °F), the percent recovery at 100 Pa, the percent recovery at 3,200 Pa, and the difference between the two percentage recovery values.
This table summarizes the percent recovery data derived from the RCRT when performed on a variety of unmodified and polymer-modified asphalt binders. The unmodified binders show little or no recovery at either the 100 Pa or 3,200 Pa stress levels, as indicated by the low values for mean percent recovery at both 100 Pa and 3,200 Pa. Most plastomeric binders, such as those modified with ethylene vinyl acetate (EVA) and linear low-density polyethylene (LLDPE), exhibit little recovery at the 3,200 Pa stress level, or show very significant drops in the amount of recovery from the 100 Pa to the 3,200 Pa level, leading FHWA researchers to recommend a minimum requirement of 15 percent recovery at 3,200 Pa with no greater than a 75 percent drop in recovery between the 100 Pa and 3,200 Pa level for discriminating elastomeric binders. Source: FHWA.
Analyzing the Results
Based on the typical RCRT results obtained for unmodified and polymer-modified asphalt binders, the researchers determined that most elastomeric styrene-butadiene (SB) or styrene-butadiene-styrene (SBS)-based binders exhibit recovery greater than 15 percent at the 100 Pa or 3,200 Pa stress levels. In fact, the unmodified binders showed little or no recovery at either of those stress levels. Most plastomeric binders, such as ethylene vinyl acetate and linear low-density polyethylene-modified asphalt binders, will exhibit greater than 15 percent recovery at 100 Pa but exhibit little recovery at the 3,200 Pa stress level. This observation led the researchers to recommend a minimum requirement of 15 percent recovery at 3,200 Pa at the binder grade temperature as a preliminary value for determining if the binder has suitable elastomeric properties, which can indicate improved performance.
The researchers selected 3,200 Pa as the high stress level based on evaluation of many binders with known polymer structures and compatibility with the base asphalt. For example, the researchers compared the responses of two polymer-modified binders. The first was a highly networked (cross-linked polymer molecules) resin Elvaloy, and the other was a noncross-linked SBS. The researchers normalized the recovered strain on the test by dividing by the applied stress to determine the compliance value (average strain divided by creep stress). Both binders exhibited uniform behavior at the lower stress levels. As the stress level increased, the increased slippage of the polymer chains in the noncross-linked SBS became evident by an increase in the compliance value. A polymer-modified binder with good interaction between the binder and entanglement of the polymer chains typically would show uniform behavior beyond the 3,200 Pa stress level. Binders with less structure would show increases in compliance at stress levels below that value. This increase in compliance was the basis for selecting 3,200 Pa as the high stress level for running the RCRT.
The research also revealed some exceptions to these observations. Some plastomer-modified asphalt binders showed more than 15 percent recovery at the higher stress level. However, a closer examination revealed that these binders also demonstrated a precipitous drop in percent recovery between 100 Pa and 3,200 Pa. Based on this result, the researchers modified their recommendation of a 15 percent minimum recovery at the 3,200 Pa stress level to permit a maximum drop of 70 percent recovery between 100 Pa and 3,200 Pa. The researchers concluded that the combination of the two requirements will enhance the capability of the RCRT to identify elastomer-modified binders of suitable quality.
Some SB and SBS binders also showed less than 15 percent recovery after being tested at 3,200 Pa. A closer examination revealed that these modified binders did not have an adequate amount of polymer or were not specifically formulated for SHRP+ specifications, such as elastic recovery. This happens when a manufacturer formulates a modified binder merely to meet a specific PG level, which is accomplished using a small amount of modifier. If the same binders were formulated to meet PG and SHRP+ elastic recovery requirements, they would have shown more than 15 percent recovery at 3,200 Pa.
Evaluation of the data from the RCRT also indicated that the test could provide some information about how well the polymer blended with the base asphalt binder and how well the polymer established a networked structure within the binder. For example, the researchers looked at a highly cross-linked binder—the PG 70-28 binder—in a compatible base asphalt, as indicated by the high percent recovery at both 100 Pa and 3,200 Pa and only a 32 percent reduction in percent recovery between the stress levels. For contrast, they studied a noncross-linked SBS 70-28 modified binder in a compatible base, which showed a greater loss in percent recovery between 100 Pa and 3,200 Pa testing. Another sample involved a small amount of SBS-modified 70-22 mixed into a binder that is likely not very compatible. The percent recovery at both levels was low, and at the higher stress level was even below 15 percent. The test shows how there is insufficient polymer to provide a good interaction between the molecules to produce increased recovery. Typically, the higher the amount of recovery, the more rut- and crack-resistant the binder will be.
The table shows that most elastomeric styrene-butadiene (SB), styrene-butadiene-rubber (SBR), or styrene-butadiene-styrene (SBS)-based binders exhibit recovery above 15 percent at the 100 Pa or 3,200 Pa stress levels. Both the 100 Pa and 3,200 Pa numbers are noticeably higher in the table on this page than they are in the table on page 27, showing that the polymer-modified binders indeed are better designed than unmodified binders to recover from stresses, such as traffic loading. Source: FHWA.
RCRT Versus Elastic Recovery
The researchers also tested several asphalt binders using the elastic recovery test, according to appropriate protocols. Comparing the results from the multiple-stress RCRT trials with data generated using the elastic recovery test, the researchers found a positive correlation for many of the binders, confirming the similarities between the two tests and earlier observations about the general relationship between the elastic recovery test and the RCRT percent recovery. In particular, the results confirmed the validity of the 15 percent minimum requirement for the RCRT percent recovery.
One of the key findings of the study is that some binders, which appear to be quite elastic based on the elastic recovery test, demonstrate substantial loss of elasticity between the 100 Pa and 3,200 Pa stress levels.
An important distinction between the elastic recovery test and the RCRT is that the former is an empirical test conducted using several different protocols, whereas the latter produces a fundamental material property, conducted using a standard testing protocol. Unlike the elastic recovery test, which is typically conducted at 25°C (77°F), the RCRT is conducted at the PG temperature. Consequently, in addition to indicating the presence of an elastomeric polymer in a modified binder, the RCRT also provides information on the binder's relative performance at service conditions. Binders with higher percentages of polymer or those that provide a better network in the base binder typically exhibit less rutting and cracking. The amount of recovery is a direct indication of the polymer response to loading. The RCRT also indicates the compatibility between the polymer and the base asphalt and the amount of networking or structure in the binder.
Results From Field Cores
According to Gaylon Baumgardner, executive vice president with Paragon Technical Services, Inc. (a full-service testing and development company for the specialty asphalts and asphalt paving industries), RCRT is not only applicable to laboratory prepared materials but can be extended to field cores as well. "The RCRT is a simple test that provides a suitable quality-control tool for hot-mix asphalt mixtures," he says. "It allows testing of volumetric specimens in a minimal time period with relatively low cost of equipment and testing."
The FHWA researchers also studied the applicability of the RCRT method on several core samples obtained from a highway pavement. They extracted an asphalt binder, known to be SBS-modified, using the standard method of ASTM D5404 with toluene and ethanol as solvents. Then they recovered the binder using a rotary evaporator, which removes low-boiling chemicals from a mixture of compounds. To minimize oxidation, the researchers employed a nitrogen blanket throughout the extractions. The researchers next conducted the elastic recovery test and the RCRT on the extracted binder.
The results showed that the RCRT could be used on the extracted asphalt binders and provide results similar to those produced by the elastic recovery test. The researchers noted that the elastic recovery test yielded a desirable result of 68 percent because it was conducted at 25°C (77°F). But at the grade temperature, the 100 Pa recovery was lower than the other cores, and the recovery value at 3,200 Pa was less than 15 percent. The researchers concluded that the RCRT had identified a binder of questionable quality that the elastic recovery test did not (and could not) identify; but more research is necessary to validate this observation.
Next Step: Implementation
This research yielded a number of beneficial results for the paving community. First, the standard test protocol developed during the study appears to produce repeatable data on percent recovery that can help pavement experts identify elastomeric polymer-modified asphalt binders and provide insight into how effectively the polymers function in the binder system. Second, because the RCRT is performed at the PG high temperature, the method provides an added benefit of testing modified binders at service temperatures.
Because the RCRT uses equipment and procedures already familiar to the industry, no additional equipment purchases are necessary to perform the test. And, whereas the traditional elastic recovery test can take up to 4 or 5 hours and require 60 to 70 grams (2.12 to 2.47 ounces) of binder, the RCRT can be completed in about 10 minutes, using just 1 to 2 grams (0.04 to 0.07 ounces) of material—saving both time and money.
Based on the favorable results achieved through this study, AASHTO and ASTM have accepted the RCRT in specification format, and binder manufacturers and some State DOTs already are using the RCRT routinely in place of the traditional empirical SHRP+ tests.
Baumgardner views RCRT as a valuable tool for evaluating the performance of hot-mix asphalt mixtures and has purchased and begun using the proposed testing protocols in development efforts. His company already has presented RCRT in two recent research proposals. "Not only has [the company] begun using the RCRT," he says, "but also agencies such as the Louisiana Transportation Research Center have utilized the RCRT and submitted it as a method in recent research proposals."
This table compares percent recovery data generated through the RCRT and the elastic recovery test. The researchers determined that the RCRT produces data trends similar to those of the elastic recovery test. The correlation between the two tests led FHWA to select 15 percent as the minimum requirement for RCRT percent recovery. Source: Mathy Technology and Engineering Services, Inc.
According to Anderson from the Asphalt Institute, whether the RCRT ultimately supplants the current high-temperature specification parameter or simply replaces the common SHRP+ tests like the elastic recovery test, in either case the benefits are clear. "If [the RCRT] does a better job of characterizing the true high-temperature performance of the asphalt binder, it will eliminate the need for all the [SHRP+] tests currently used," Anderson says. "If it simply is used as a replacement for elastic recovery, it offers ease of use, good repeatability, and faster results."
John A. D'Angelo is an asphalt materials engineer with the Office of Pavement Technology at FHWA. He has been with FHWA for 29 years. For the past 16 years, he has been involved with developing and implementing the SHRP Superpave asphalt design system in the highway industry. He also is responsible for materials quality assurance, recycling, and pavement design. He has published numerous papers on materials testing and quality control.
Raj Dongré, Ph.D., is an FHWA consultant and owner of Dongré Laboratory Services Inc., in Fairfax, VA. For the past 13 years, he has been involved with refining various Superpave specifications and developing standards. He has published numerous papers on material testing and specification. Dr. Dongré received his B.S. in civil engineering from the Maharaja Sayajirao University of Baroda, India, and his M.S. and Ph.D. from The Pennsylvania State University.
Gerald Reinke is president of Mathy Technology and Engineering Services, Inc. in Onalaska, WI. Reinke has worked in the asphalt industry for 35 years, where he has conducted research on the properties of asphalt binders and their impacts on mixture performance.
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