Patent Application
20140116116
Workability, in the application of asphalt concrete mixtures, refers to a material characteristic determining the effort required to manipulate an un-compacted material with minimum loss of homogeneity. The term manipulate, as used herein, refers to operations such as mixing, handling, and placement at the construction site. A pavement mixture such as an asphalt concrete mixture that requires relatively less effort (lower force) has a higher workability than a mixture which requires relatively more effort (higher force) to be directed into the desired shape. A term related to workability used within the pavement industry is compactability, which relates to the materials behavior during the consolidation process of pavement construction. Workability is related to the mixing and placing processes, while compactability is related to the effort required to achieve consolidation in an unconfined condition during construction. Workability and compactability are interrelated temperature dependent characteristics for asphalt concrete mixtures. However, mixtures with the same workability may not have the same compactability. Construction temperatures for asphalt concrete mixtures are selected such that the material can be placed as needed and compacted to design densities in an unconfined condition.
Asphalt concrete mixtures, used primarily as pavement materials, consist of coarse and fine aggregate, and asphalt binder. Asphalt concrete pavement, refers to the bound layers of a flexible pavement structure in contrast to Portland cement concrete which is a rigid pavement structure. In many applications, asphalt concrete mixtures are placed at elevated temperatures and referred to as hot mix asphalt (HMA).
Warm-mix asphalt (WMA) is similar to HMA and usually includes additives such as zeolites, waxes, asphalt emulsions, or sometimes water in the asphalt binder. These additives act as coating, mixing, and lubrications aids enabling significantly lower mixing and compaction temperatures than HMA. These lower temperatures result in lower consumption of fuels, thus releasing less carbon dioxide, aerosols and vapors during construction. The lower temperatures realized by the use of these additives also improve pavement construction working conditions. An additional benefit to the WMA process is rapid opening of the pavement to traffic, which reduces the impact of the pavement construction or repair on the public's transportation needs. The use of these additives also extend the paving season into cold weather months and enables extended transport distances from the production plant to the construction site. The lower construction temperatures facilitated by the WMA processes contribute to a need to understand the asphalt concrete mixture rheological characteristics at lower temperatures. Understanding these characteristics prior to construction provides insight into the effectiveness of the WMA processes.
Current techniques for evaluating the rheological characteristics of the materials used in the WMA process rely on binder tests. These tests evaluate the binder viscosity, lubricity, or shearing properties to estimate the response of the material during construction. Traditionally, mixing and compaction temperatures for use in laboratory mix design were determined from binder viscosity measured at two temperatures (135° C. and 165° C.). With the increasing use of polymer modified asphalt mixes, the traditional binder viscosity based determinations of mixing and compaction temperatures result in overestimating the temperatures. Because of this limitation, researchers have sought alternative methods to determine mixing and compaction temperatures of polymer modified asphalt mixes. Examples of these alternatives include test methods based on zero-shear viscosity, high shear viscosity, shear flow viscosity, and the phase angle method. However, the asphalt concrete mixture coarse and fine aggregate structure contributes significantly to the workability and compactability of the material. These binder tests fail to account for aggregate contribution to workability and compactability of the mixture.
The economies of utilizing local materials for pavement construction make it highly desirable to make use of local mineral aggregate supplies. There is currently a lack of accepted test methods to measure mixture workability in the laboratory prior to construction and experience with the constituent materials is relied, upon for guidance. As material sources with known characteristics are consumed and new sources employed, experience with these new materials is limited. As a result, pavement test strips are often used prior to the roadway construction to determine the requirements to place and compact these new materials. These test strips facilitate determining the limits of workability and compactability of the material. These limits include temperatures such as where the material consolidates rather than flows and points where the material is more likely to flow than consolidate.
Other asphalt concrete mixture constituents such as reclaimed asphalt pavement (RAP) and Recycled Asphalt Shingles (RAS) provide economies to asphalt concrete mixtures by making use of the aggregate and the available bituminous material these materials contain. However, the use of these constituents in an asphalt concrete mixture can influence its workability and compactability during construction and thus a clear understanding of the impact on construction is important to the application of such recycled materials.
An example of an attempt to obtain information on the workability of asphalt concrete mixtures is disclosed in U.S. Patent Application 2010/0011841 A1 Mogawer, et al. The disclosed device utilizes a bucket with a mixing paddle. As the bucket is rotated, the mixing paddle engages the material sample. The torque required to resist the turning of the mixing paddle is utilized to obtain a measurement of the workability. However, during the shearing process the paddles engaging the material create an open pocket where the paddle exists. This creates significant error in the measurement because there is no test material interacting with the mixing paddle. Another disadvantage of this device is that it requires a large amount of material which is problematic for laboratory testing. Also, as noted previously, workability is a temperature dependent characteristic. The large material sample, open container, and long test time make it difficult to keep the specimen at a uniform temperature throughout the test. The resulting temperature non-uniformities further impact the results. The torque measuring bucket mixer type device does not adequately account for the aggregate skeletal structure that develops during pavement construction as the asphalt mixture consolidates.
Other mix based workability devices include the Nynas device, the University of New Hampshire Device, and Gyratory Shear force measurement. Table 10 of the National Cooperative Highway Research Program Report 691 “Mix Design Practices for Warm Mix Asphalt” list the advantages and disadvantages of current workability measurement techniques in tabular form. This report found that these devices only differentiate workability of WMA from HMA at temperatures much lower than those associated with WMA applications.
ASTM D6704 Standard Test Method for Determining the Workability of Asphalt Cold Mix Patching Material describes a method and apparatus in which the material is evaluated at temperatures below freezing making the method unsuitable for use with WMA or HMA applications.
Quality control measures employed for assessing the conformity of asphalt concrete mixture properties as produced to design requirements often rely on the volumetric properties of material samples. These measures assume that changes in the mixture constituents reflected in the volumetric property will reflect the performance characteristics of the material samples. To gain additional conformance measures, strength tests may be utilized to examine the structural properties of the plant produced material. However, these tests require tedious specimen preparation techniques and can take many hours to complete. While these performance tests provide useful quality assurance information for the material, they do not provide information that can be used in a timely manner to make component adjustments during production. In addition to assessing workability, the methodology described in this document provides a simple and quick measure that reflects changes in the mixture constituents providing valuable information that can be applied quickly for quality control management. The signal reflects changes to aggregate structure as well as binder properties and compaction aid additive levels.
The methods and apparatus disclosed herein provide means for characterizing materials and mixtures of materials, such as asphalt concrete mixtures, in the laboratory prior to construction and thus can be used to predict appropriate construction temperatures without requiring construction of test strips. Because the disclosed methods include all mixture components in the testing sequence, it is sensitive to material changes in aggregate and binder during production making the technique applicable as a quality control measure. Many pavement engineering and design laboratories already own the equipment required to perform this method making the technique a useful and novel tool to the pavement industry. While this description focuses on asphalt pavement mixes, those skilled in the art will recognize the application to other materials.
Asphalt concrete is a composite material composed of randomly oriented aggregates held together by an asphalt adhesive. At low temperatures and/or fast loading rates, the composite behaves elastically where at high temperatures and/or slow loading rates the composite exhibits viscoelastic behavior. The behavior is thus a function of testing temperature and loading rate as well as the relative proportions of the individual constituents. This temperature/loading rate behavior can be leveraged with a time-temperature transform function to predict behavior at different temperatures from testing a different loading rates as well as predicting behavior at different loading rates from testing at different temperatures. The methods include prediction of material characteristics by testing at multiple loading rates or at multiple temperatures.
A preferred and exemplary embodiment of the method of the present invention provides a simple laboratory test for evaluating asphalt concrete pavement mixture compactability and workability. The results can be used to predict material rheological characteristics at various temperatures aiding the selection of appropriate production and construction temperatures. This test can be performed on a modified Superpave Gyratory Compactor (SGC) or other suitable loading frame meeting the requirements outlined in the apparatus section.
As used herein, the term, compactability, refers to and generally means a rheological characteristic of an un-compacted asphalt pavement mixture describing how much effort is required to achieve consolidation in an unconfined condition. For example, the effort required to achieve consolidation influences when the material can be properly consolidated without flowing and thus impacts construction rolling techniques. Compactability is a temperature dependent characteristic for asphalt pavement mixtures.
As used herein, the term, workability, refers to and generally means a rheological characteristic determining the effort required to manipulate an un-compacted asphalt pavement mixture with minimum loss of homogeneity. As used herein, the term manipulate refers to operations such as mixing, handling, and placement at the construction site. A mixture that requires relatively less effort (lower force) has a higher workability than a material which requires relatively more effort (higher force) to be directed into the desired shape. Workability is a temperature dependent characteristic for asphalt pavement mixtures.
A pavement material specimen, conditioned at a pre-described temperature, is confined to a cylindrical steel mold while a consolidation pressure is applied at controlled travel rates (rate of strain). The resulting stress versus strain response of the material is used to determine an index value representing the relative workability of the material at the test temperature. Materials are typically tested at more than one temperature and different loading rates may be employed over the testing range.
The workability value obtained in this test provides a relative index ranking the behavior of the loose material related to the effort required to place and compact the material during construction. The index can indicate the relative temperature performance of materials and may be applicable for predicting appropriate production and construction temperatures. Lower production and construction temperatures result in energy savings, lower emissions which reduce workers exposure to harmful vapors, and also contribute to the reduction in the release of pollutants to the environment.
Results of materials tested at multiple temperatures utilized to predict lower construction temperatures can be applied to extend haul or transport distances, expand ambient temperature limitations during placement (extend paving season), and determine construction requirements for proper placement without test strips to determine rolling requirements. The results are also applicable for evaluating the warm mix additive levels and the effectiveness of such additives.
The information extracted during the test is also applicable to quality control procedures utilized to monitor production materials meet design requirements and consistency specifications. The signal may reflect changes to aggregate structure as well as binder quality, additive levels, and changes to other constituents, such as reclaimed asphalt pavement and recycled asphalt shingles.
As used herein, the term or name loading system refers to and means a system, apparatus or device that is generally capable of applying a force at a programmed rate of travel (velocity or strain rate) to a material specimen confined in a container or apparatus such as for example a cylindrical mold of known diameter, while simultaneously measuring and recording the applied force (or stress), height of the material specimen, and time index. A typical or exemplary loading system may include, for example, an electromechanical, electro-hydraulic, or electro-pneumatic instrument comprised of the following system components: reaction frame, drive system, loading platen, force indicating and recording system, position indicating and recording system, mold and mold end plates. Some models of SUPERPAVE Gyratory Compactor (SGC) modified to operate to the specified protocol have proven to be acceptable loading systems.
The loading system is capable of providing a programmable rate of travel of the compaction ram 12 at various rates. Systems with fixed rates of travel may also be applied. Some loading systems may require compensation techniques to account for frame compliance. Rate of travel may be adjusted in some applications to a constant rate of strain on the material sample as sample height changes.
The position measurement and recording system is generally capable of indicating the height of the specimen S during the test process to the nearest 0.1 mm.
Specimen Molds
Specimen molds such as mold 15 preferably conform to the requirements of ASTM D6925. Molds are secured to the reaction frame throughout the test sequence. Molds of various diameters and lengths may be applied.
Mold End Plates
Mold end plates or plate 16 preferably conform to the requirements of ASTM D6925.
The data acquisition system preferably records at a minimum rate of 10 data sets per second throughout the test. Each data set may include for example, at minimum, a time index (second), applied pressure (kPa), and specimen height (mm). NOTE-Applied force (N) along with specimen diameter (mm) used to calculate the applied N/mm2 applied is an acceptable alternative to applied pressure.
Thermometers
Preferred thermometers for the disclosed methods are armored, glass, or dial type thermometers with metal stems for determining the temperature of aggregates, asphalt binders, and asphalt mixtures between 10° C. and 232° C., with a minimum sensitivity of 3° C. or other suitable means for measuring, recording, or otherwise indicating the specimen temperature. Some loading systems are equipped with instrumentation to measure and record the specimen temperature throughout the test.
Balance
The balance preferably has a minimum capacity of 10,000 g with a sensitivity of 0.1 g. The balance preferably conforms to Specification D 4753 as a Class GP2 balance.
Ovens
Two ovens are recommended for laboratory produced specimens. One oven is generally a forced draft oven capable of maintaining the temperature required, nominally 135° C., for short term aging. At least one additional oven is generally available for heating aggregates, asphalt binders, and equipment. This oven also has a range to a minimum of 204° C., thermostatically controlled to ±3° C.
Miscellaneous
Miscellaneous equipment may include: flat bottom metal pans for heating aggregates; scoops for batching aggregates; containers for heating asphalt binders; mixing spoons; trowels; spatulas; welders gloves for handling hot equipment; 150 mm paper disks; lubricants for moving parts; laboratory timers; and mechanical mixers.
Molds and mold end plates should be clean and free of damage prior to the start of each test.
Calibration of the following items is generally verified following the manufacturer's recommendations annually: applied force, indicated height, time recorder, mold and mold end plate dimensions, oven temperature, and thermometer.
Material samples may be tested at multiple temperatures to determine the workability index at each test temperature; therefore, multiple material specimens are typically utilized, but are not required. Two material specimens are typically tested at each testing temperature with the results averaged. A single specimen may be tested at multiple temperatures.
Preparation of Aggregates—appropriate aggregate fractions are weighed and combined to a desired specimen weight. For example, generally 4600 g of aggregate are required for aggregates with combined bulk specific gravities in the range of 2.55 to 2.70. For specimens of 4600 g mass, a final measured height will be approximately 130 mm. Other specimen diameters and mass may be tested. If the test utilizes molds other than 150 mm diameter, the weight is adjusted to achieve the desired specimen final height. Details of aggregate preparation may be found in any suitable mix design manual, such as the Asphalt Institute's MS-2. SUPERPAVE™ volumetric design specimens have been shown to be of appropriate mass.
Blended aggregate specimens and asphalt binder are placed in an oven and bring to the required mixing temperature. The mixing container and all necessary mixing implements are heated to the required temperature.
A preferred laboratory mixing temperature range is typically defined as the range of temperatures where the un-aged asphalt binder has a kinematic viscosity of 170±20 mm2/s (approximately 0.17±0.02 Pa-s for an asphalt binder density of 1.000 g/cm3) measured in accordance with Test Method D 4402. Modified asphalt binders, especially those produced with polymer additives, generally do not adhere to the viscosity ranges noted.
The heated mixing bowl is charged with the dry, heated aggregate and dry mix. A crater is formed in the heated aggregate blend and the required amount of asphalt binder is weighed into the aggregate blend. Mixing is begun immediately.
The asphalt binder and aggregate is mixed as quickly and thoroughly as possible to yield an asphalt mixture having a uniform distribution of asphalt binder. A mechanical mixer is preferable for the mixing process.
Laboratory prepared material specimens require relatively short term aging prior to testing.
After completing the mixing process, the loose mix is subjected to short-term conditioning according to AASHTO R30 Mixture Conditioning of Hot Mix Asphalt (HMA) for 2 h±5 min at the conditioning temperature±3° C., and then stirred after 60±5 min to maintain uniform conditioning.
Plant produced specimens may be sampled following procedures such as ASTM D979/D979M Standard Practice for Sampling Bituminous Paving Mixtures and ASTM D3665 Standard Practice for Random Sampling of Construction Materials.
For samples of HMA plant-produced mix, one of the following short-term aging conditions may be specified. For example, no conditioning, test immediately as produced once the specimen is at test temperature for a period of 15 minutes. Another conditioning that the user can demonstrate will replicate the design conditioning.
The material specimen is heated to the predetermined test temperature and held for 15 minutes. A temperature offset may be applied to help counter temperature loss during the test.
For construction temperature prediction models, test temperatures are typically selected to achieve workability values less than 150 kPa or greater than 170 kPa. Depending on material type, the temperature for a workability value less than or equal to 150 kPa may range from 60° C. (140° F.) for Warm Mix Asphalt, 71° C. (160° F.) Hot Mix Asphalt, to 80° C. (176° F.) for Polymer Modified materials. The temperature to achieve a workability value greater than or equal to 170 kPa may range from 104° C. (220° F.) for Warm Mix Asphalt, 120° C. (248° F.) Hot Mix Asphalt, to 150° C. (302° F.) for Polymer Modified materials.
Place a mold and mold end plates into an oven at a predetermined temperature for a minimum of 45 min prior to the testing of the first specimen (during the time the mixture is in the conditioning process). Temperature offsets may be required to help compensate for temperature loss during placement of material into the mold. Molds with heating elements may also be employed. The settings on the loading system should be verified.
At the end of the conditioning period, the specimen is brought to the desired test temperature. The loose mix specimen, mold, and mold end plates are removed from the oven. A paper disk is placed on the bottom end plate inside the mold to aid separation of the specimen after testing. The specimen is placed into the mold in a single lift using a transfer bowl or other suitable device. Segregation of the mixture in the mold is preferably minimized. After the mixture has been completely loaded into the mold, the specimen may be rodded to provide uniform constituent distribution throughout, the top of the specimen is leveled and a second paper disk is placed onto the material specimen to avoid material adhering to the second mold end plate. If required, a second end plate is installed in the mold.
The prepared specimen and mold is placed into the loading frame and the test sequence initiated. The loading frame applies a force to the specimen at a controlled rate of travel of the loading platen until the desired final stress is achieved. Alternate rates and stress parameters may be used for different applications.
Record the time, applied force, and specimen height at least every 0.1 seconds.
At the completion of the test profile, measure and record the temperature of the specimen. For molding apparatus equipped with temperature measurement instrumentation, the specimen temperature is recorded throughout the test. The specimen is then removed from the mold.
If the loading system is a gyratory compactor, at the completion of the workability test profile, the material specimen may be compacted utilizing gyratory compaction. Specimens compacted in this manner have the advantage of further evaluation.
Calculate the Normal Stress at each index point n for the test sequence:
Calculate the % Volumetric Strain at each index point n for the test sequence:
Calculate the Dongre Workability Test (DWT) Value (slope of stress vs. volumetric strain curve between 550 kPa and 650 kPa):
Those skilled in the art will recognize that the slope of the stress-strain curve may be evaluated at other stress levels or at other rates for certain applications. Lower slope (less steep) reflects lower workability. A representative plot of stress vs. volumetric strain as determined by Equations 1-3 is shown in
For temperature workability predictions, a minimum of two workability values are required with one value preferably less than or equal to the lower construction workability target (DWTL) and the second value preferably greater than or equal to the higher workability target (DWTH). If the workability values do not meet these requirements, the material may be re-tested at temperatures which provide the desired values.
Calculate the temperature (TL) corresponding to the DWTL value from the data by linear regression.
Calculate the temperature (TH) corresponding to the DWTH value from the data by linear regression.
Asphalt concrete mixtures will typically exhibit a sigmoid function shape for the workability versus temperature curve. The linear prediction calculation assumes a linear estimate is representative of the material behavior over the region of interest. For data sets with widely diverse DWT values, this estimate may not be appropriate. For data sets containing more than two temperature values, alternate prediction regressions such as polynomial fits may provide more representative predictions.
The disclosed methods provide numerous advantages over existing techniques for assessing workability and compactability of asphalt concrete mixtures in that:
No specialized equipment or training beyond that needed for typical asphalt concrete design is required. The mixture is evaluated as a composite, not as individual components. Specimen mass does not require large material batches. Workability can be easily characterized at various temperatures and/or loading rates providing an assessment of construction behavior from laboratory tests.
The disclosed test methods are simple and results are available quickly making it useful for quality control applications. The test methods are highly repeatable and thus provide reliable data.
The temperature reduction effected by the use of warm-mix additives can be determined.
The methods are capable of detecting changes in asphalt concrete mix components: binder, aggregate, warm mix additives, polymer modifiers, and other constituents. The methods are capable of resolving differences in workability due to aggregate gradation, asphalt binder grade, warm mix additives, polymer modifiers, and other constituents. Field compaction temperatures and workabilities can be predicted for various warm-mix, hot-mix and polymer modified asphalt mixes by running the test at two or more temperatures or a two or more loading rates.
This application claims benefit of U.S. Provisional Patent Application No. 61/719,715 filed Oct. 29, 2012.
| Number | Date | Country | |
|---|---|---|---|
| 61719715 | Oct 2012 | US |
