Staying settled

Unpaved Roads Article July 05, 2011
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Gravel loss, primarily in the form of dust, is a common problem on Wyoming’s gravel roads.

The loss of the fine materials that compose dust leads to degraded road surfaces as well as environmental problems. When the binding effect is lost, surface distresses develop on the road. The loss of fine materials also allows water to permeate the surface more readily.

Environmentally, the dust entering the air increases the risk of violating federal air-quality standards. Dust is considered a particulate matter made up of particles that are 10 micrometers (microns) or less, denoted as PM-10.

Reclaimed asphalt pavement (RAP) is being evaluated by Wyoming T2/LTAP for the use of the Wyoming Department of Transportation (WYDOT) and local agencies. Two characteristics are evaluated: dust loss and road serviceability. The main objective of this story is to determine the effect of using RAP on dust loss in gravel roads.

Make it 8

Test sections were constructed in two Wyoming counties. Test sections on Schoonover Road in Johnson County carry in excess of 500 vehicles per day, mainly trucks. Also test sections on Atlas and Pry roads in Laramie County see mostly light trucks. Construction was administered by Johnson and Laramie counties and monitored by the Wyoming T2/LTAP Center. Road samples were taken by the T2/LTAP center and tested by WYDOT’s Materials Testing Laboratory.


Testing locations

The test sections in Laramie County consisted of five segments. The resulting test sections were as follows:

* A0 - Atlas Road: 100% gravel over 0.7 mile;
* A1 - Atlas Road: 1½-in. RAP depth over 0.7 mile;
* A2 - Atlas Road: 2½-in. RAP depth over 0.6 mile;
* P0 - Pry Road: 100% gravel over 1.2 miles; and
* P1 - Pry Road: 1½-in. RAP depth over 0.8 mile.

The Johnson County test sections consisted of three segments. For the blend, RAP and virgin aggregate were mixed (50/50) in a pugmill. A vibratory roller compacted the sections. Fourteen days after construction, dust abatement in the form of calcium chloride (CaCl) was placed on the roadway surface. The resulting test sections were as follows:

* S0 - Schoonover Road: 100% gravel with CaCl over 0.5 mile;
* S1 - Schoonover Road: RAP and gravel blend with CaCl over 0.5 mile; and
* S2 - Schoonover Road: RAP and gravel blend without CaCl over 0.5 mile. Taking a collection

Data collection involved dust measurements, moisture content, condition surveys, aggregate sampling and traffic data.

Dust Collection

A device known as the Colorado State University (CSU) Dustometer was used to monitor vehicular-generated dust from all test sections. The device consists primarily of the following: a fabricated metal box designed to hold a 10- x 8-in. glass microfiber filter paper, mounted to the bumper of a pickup truck behind the driver’s side rear tire; an electric power generator; a high-volume vacuum pump; and a flexible plastic tube connecting the suction pump to the filter box. The fabricated filter box has a 12- x 12-in. opening that is covered with a 200 ?m mesh sieve facing the tire. The filter paper is supported near the bottom of the fabricated box by a mesh sieve screen. The test vehicle was a 2001 ½-ton Chevy Suburban with a tire pressure of 50 psi. A vacuum provided the necessary suction to the Dustometer. Figure 2 shows the Wyoming T2/LTAP setup. The dust data was collected following a standard procedure to ensure uniformity.


Figure 2

Moisture Sampling

Moisture samples were collected from the top 1-in. layer of road surface. Every time dust measurements were collected, gravel samples were obtained to determine the moisture of the surface. These moisture contents were added to the database. In general, dust measurements were not collected when high moisture contents were present.

Condition Survey

Roadway surface evaluations were conducted based on distress surveys developed by the U.S. Army Corps of Engineers. The conditions of all test sections were collected and summarized in the database. The Unsurfaced Road Condition Index (URCI) scale is as follows:

* URCI from 100 to 85–Excellent;
* URCI from 70 to 84–Very Good;
* URCI from 55 to 69–Good;
* URCI from 40 to 54–Fair;
* URCI from 25 to 39–Poor;
* URCI from 10 to 24–Very Poor; and
* URCI from 0 to 9–Failed.

Traffic Data

Traffic data was collected using road tubes and counters installed on the test sections. Data was collected by both Johnson and Laramie counties, as well as by the Wyoming T2/LTAP. The focus of the data was directed toward speed, volume and vehicle classification.

Measuring dust

Material samples were gathered at each respective test site. Materials tests included: gradation, fractured faces (FF), plastic limit (PL), liquid limit (LL), hydrometer, asphalt content (AC) by chemical extraction with screened gradation, R-value and cohesion value (CV). Table 1 shows a condensed version of the traffic and material data for the test sections. Figure 3 contains gradations of the materials used on the test sections.


Figure 3

A comprehensive statistical analysis was performed on the collected data. Contrasts were made on the basis of dust and URCI. All eight sections in the study were included in this analysis. An alpha value of 0.1 was chosen due to the inherent variability of unpaved roads.

A contrast analysis was performed on the basis of dust. The dependent variable used was the log of the average dust weights. The log function was chosen to improve the necessary assumptions of linearity, equal variance and normality needed to trust the statistical results. The independent variables included the averages of age, moisture content and the wind factor.

Three contrasts were made using the following parameters: Laramie County vs. Johnson County; sections with no RAP vs. sections with RAP and sections with no CaCl vs. sections with CaCl. The Laramie County test sections had 288% more dust than the Johnson County test sections. Sections without RAP exhibited 147% more dust than sections utilizing RAP. Finally, sections without CaCl had 354% the amount of dust loss of sections with CaCl. These results are due to the fact that the test sections in Laramie County exhibited more material passing the No. 200 sieve than the Johnson County test sections. Also, the roadways in Johnson County were compacted using a drum roller. The percentage of material passing the No. 200 sieve is less for RAP than for gravel aggregate. Finally, the use of CaCl significantly reduced the amount of dust loss.

A contrast analysis was performed on the basis of the URCI. The dependent variable used was URCI, with dust, age, moisture content and wind factor being the independent variables. The same three contrasts were made as before. The URCI of the Laramie County test sections had better URCI scores compared with the Johnson County test sections by 527%. The volume of traffic on the Johnson County test sections was much heavier than the traffic in Laramie County.

Sections without RAP had better URCI scores compared with sections with RAP, most likely due to the RAP sections having more loose aggregate. Sections without CaCl had better URCI scores than sections with CaCl. This is mainly due to the presence of rutting in the CaCl sections.

RAP as a wrap

Eight RAP and gravel control test sections were built and monitored in Laramie and Johnson counties. Dust and surface condition data were collected on all test sections. This data was summarized and statistically analyzed. From the test sections in both Wyoming counties, it can be seen that using RAP in the road surface can significantly reduce dust loss but increase the amount of loose aggregate. Traffic volumes, especially trucks, play an important role in the roadway serviceability. Employing CaCl on the roadway will reduce dust loss but increase rutting.

The Wyoming T2/LTAP has thoroughly investigated the use of RAP in gravel roads in two Wyoming counties. From this investigation, the use of RAP is an effective tool for reducing dust loss. It also is recommended to incorporate RAP into gravel roads and use CaCl to further reduce dust loss. Attention should be directed to the initial gradations of the materials being used in order to help reduce dust loss prior to construction. Traffic volumes should be attended as there are trade-offs between using RAP and CaCl in gravel roads.

About the author: 
Koch and Zumwalt are both graduate students with the Wyoming T2/LTAP Center, Laramie, Wyo. Ksaibati is the director of the Wyoming T2/LTAP Center.
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