Dry is fruitful

Taking care of aggregate one of the keys to WMA foam

By Ala R. Abbas, Ph.D., Ayman W. Ali and Munir Nazzal; Contributing Authors / April 28, 2015
APA specimens before loading
APA specimens before loading

Hot-mix asphalt (HMA) is the most commonly used material for asphalt-paving applications.

It is produced by drying the aggregates prior to mixing with the heated asphalt binder. The temperature at which this material is produced generally ranges from 300°F to 325°F for unmodified asphalt binders, and even higher temperatures are used for modified asphalt binders. The use of such temperatures ensures the aggregate is completely dry and thoroughly coated with a thin film of asphalt binder. It also ensures the mix is workable and compactable to an acceptable density in the field, resulting in a mixture that is durable and capable of withstanding repeated loading from traffic.

In recent years, a new group of technologies commonly referred to as warm-mix asphalt (WMA) has been introduced in the U.S. and allows for the production of asphalt mixtures at temperatures 30°F to 100°F lower than what is commonly used for traditional HMA. The viscosity of the asphalt binder in WMA mixtures is reduced either by incorporating organic or chemical additives or by introducing cool water into the heated asphalt binder under controlled temperature and pressure conditions to produce a foamed asphalt binder.

WMA prepared using foamed asphalt binders, commonly referred to as foamed WMA, has gained increased attention from the asphalt-paving industry in Ohio, since it does not require the use of costly additives. Other advantages to the asphalt-paving industry include: reduced energy consumption due to lower production temperatures; increased hauling distance since WMAs are able to retain their temperatures for a longer period of time; improved conditions for construction workers due to lower odor, fume and emission levels; and improved compactability and the ability to reach the desired density with fewer roller passes.

In spite of the many benefits gained from using foamed WMA, several concerns have been raised regarding the performance of this material, because of the reduced production temperature and its impact on aggregate drying and asphalt binder aging. Main concerns include an increased propensity for moisture-induced damage, since water is used during the production process and aggregates are heated to lower temperatures and therefore may not dry thoroughly before being mixed with the asphalt binder. In addition, foamed WMA might show increased susceptibility to permanent deformation (or rutting) since the asphalt binder may not harden as much at lower production temperatures and because the aggregates may absorb less asphalt binder. Other considerations in using foamed WMA include concerns about insufficient coating of coarse aggregates and the inapplicability of HMA-mix-design procedures to the production of foamed WMA mixtures.

Testing the limits

This study was conducted to evaluate the performance of foamed WMA and determine its limitations. A comprehensive laboratory testing plan was implemented to characterize the behavior of foamed WMA with regard to permanent deformation (or rutting), moisture-induced damage (or durability), fatigue cracking and low-temperature cracking and compare it to that of traditional HMA mixtures (Figure 1). As can be noticed from this figure, several tests were included in the experimental testing program. The asphalt pavement analyzer (APA), dynamic modulus (E*) and flow number (FN) tests were used to evaluate the rutting performance of foamed WMA and HMA mixtures. Figure 2 shows pictures taken for the APA specimens before and after running the test. The susceptibility of foamed WMA and HMA mixtures to moisture-induced damage was characterized using the AASHTO T 283, dynamic modulus ratio and wet APA tests. The indirect tensile strength (ITS) setup utilized in the AASHTO T 283 test is presented in Figure 3. The fatigue cracking and the low-temperature cracking characteristics of both mixtures were evaluated using the dissipated creep strain energy (DCSE) and low-temperature indirect tensile strength tests, respectively.

Four material combinations were selected for the laboratory evaluation: two surface mixtures prepared using PG 70-22 asphalt binder and limestone or crushed gravel, and two intermediate mixtures prepared using limestone and PG 64-28 or PG 70-22 asphalt binders. These material combinations were selected to facilitate the determination of the effect of the mix type, aggregate type and asphalt binder type on the performance of the asphalt mixtures. With the exception of the intermediate mixture prepared using limestone and PG 70-22, all mixtures met the Ohio Department of Transportation (ODOT) Construction and Material Specifications (C&MS) for Item 442. Although ODOT requires using PG 64-28 for Superpave intermediate mixes, a nonstandard intermediate mixture was included in the study to allow for determining the effect of the asphalt binder type on the mix performance.

A laboratory-scale asphalt binder foaming device was used in the production of the foamed WMA mixtures (Figure 4). This device consists of an asphalt binder tank, a water tank, an air tank, an asphalt pump, heating components, a foaming nozzle, air- and water-pressure regulators and a control panel. The asphalt binder is heated to the mixing temperature to ensure that it is easily circulated through the foaming device. Within the foaming nozzle, the heated asphalt binder is mixed with small molecules of cold pressurized water. Upon mixing, the cold water will vaporize to form steam, which in turn foams and expands the asphalt binder and eventually reduces its viscosity. The maximum foaming water content currently permitted by ODOT during the production of foamed WMA is 1.8% of the total weight of the asphalt binder. Once the foaming parameters (i.e., air and water pressures, asphalt foaming temperature and foaming water content) have been selected and the foaming device has been calibrated, the foamed asphalt binder is discharged from the foaming nozzle into a mixing bowl that contains the aggregates, which had been preheated to the WMA mixing temperature. It is noted that the current ODOT specifications for foamed WMA allow for using a compaction temperature 30°F lower than that of the corresponding HMA. However, ODOT does not control the mixing temperature of the foamed WMA. It is up to the contractor to determine the appropriate mixing temperature for this material.

Toe-to-toe with HMA

The laboratory test results revealed comparable rut-depth values in the APA (Figure 5), slightly lower E* values, slightly lower FN values, slightly lower ITS values in the AASHTO T 283 test and slightly lower DCSE values for the foamed WMA than the HMA mixtures. However, the difference was found to be statistically insignificant between the two mixtures. These results indicate the performance of foamed WMA mixtures is comparable to that of traditional HMA mixtures in terms of rutting, moisture-induced damage and fatigue cracking. As for low-temperature (thermal) cracking, the foamed WMA mixtures exhibited slightly lower ITS values at 14°F and comparable or slightly higher failure strain values than the HMA mixtures. Through statistical analysis, it was found that the effect of the mix type was significant on the low-temperature ITS values, but not on the failure strains. Since the HMA mixtures had significantly higher ITS values and comparable failure strains to the foamed WMA mixtures, the HMA mixtures are expected to have better resistance to thermal cracking.

The laboratory-testing program also included an evaluation of the effect of temperature reduction, foaming water content and aggregate moisture content on the performance of foamed WMA (Figure 6). As can be noticed from this figure, the foamed WMA mixtures were produced using three production temperatures (30°F, 50°F and 70°F—lower than the traditional HMA), three foaming water contents (1.8%, 2.2% and 2.6% by weight of the asphalt binder) and three aggregate moisture contents (0%, 1.5% and 3%). The APA test was utilized to evaluate the rutting resistance and the modified Lottman (AASHTO T 283) test was used to evaluate the moisture sensitivity of the asphalt mixtures.

The experimental test results revealed the performance of foamed WMA mixtures prepared using a 30°F temperature reduction, 1.8% foaming water content and fully dried aggregates was comparable to that of the traditional HMA mixtures. However, further reductions in the production temperature of the foamed WMA resulted in increased susceptibility to permanent deformation and moisture-induced damage. In addition, producing foamed WMA using moist aggregates resulted in inadequate aggregate coating, leading to concerns with regard to long-term durability. It also was found that increasing the foaming water content (up to 2.6% of the weight of the asphalt binder) during production of foamed WMA did not seem to have a negative effect on the rutting performance or moisture sensitivity of foamed WMA.

Their own device

As part of this study, a new device was designed and fabricated to evaluate the workability of foamed WMA and HMA mixtures (Figure 7). This device utilized the torque generated while stirring a mix to measure the workability. Each workability test was performed on mixtures heated to 302°F, and the test was terminated when the mixture’s temperature reached approximately 212°F. The new device had several advantages, including the ability to thoroughly mix the asphalt mixture using an improved mixing paddle design; the ability to obtain accurate temperature and torque measurements using an infrared thermometer and a stationary torque sensor; the ability to run the test at varying speeds ranging from 5 to 35 rpm using a motor and a speed-drive control unit; the ability to record test results to a personal computer; and improved safety features such as a specially designed safety cage and an emergency stop button.

Figure 8 presents the average torque values obtained at the high and low testing temperatures (302°F and 212°F, respectively). As can be noticed from this figure, the foamed WMA mixtures had lower torque readings than the corresponding HMA mixtures for both high and low testing temperatures. This difference in torque values can be attributed to the reduction in asphalt-binder absorption for the foamed WMA mixtures. Another factor that might have contributed to the reduction in the torque values for the foamed WMA mixtures is the presence of vapor pockets entrapped within the foamed asphalt binder that keep the binder slightly expanded and reduce its viscosity.

Foam is fingered

In summary, producing foamed WMA using fully dried aggregates and current ODOT specifications (i.e., 30°F temperature reduction and 1.8% foaming water content) resulted in relatively comparable performance to traditional HMA. However, reducing the production temperature of foamed WMA led to increased susceptibility to permanent deformation (rutting) and moisture-induced damage. 

Therefore, it is recommended to continue to use a reduction temperature of 30°F for the production of foamed WMA. In addition, increasing the foaming water content (up to 2.6% of the weight of the asphalt binder) during production of foamed WMA did not seem to have a negative effect on the rutting performance or moisture sensitivity of foamed WMA. Therefore, a higher foaming water content can be specified for the production of foamed WMA in Ohio.

Furthermore, producing foamed WMA using moist aggregates resulted in inadequate aggregate coating leading to concerns with regard to moisture-induced damage and long-term durability. Therefore, it is critical to use fully dried aggregates in the production of foamed WMA to ensure satisfactory mix performance. Given that foamed WMA is typically produced using lower production temperatures than conventional HMA, the aggregates may need to be dried for a longer period of time. Finally, since the performance of the foamed WMA was comparable to that of the HMA, no modifications are needed to the current mix-design process used by ODOT for foamed WMA mixtures. AT

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