2.8 Effects of Aggregate Distribution on Effectiveness of Dust Suppressants
Other research studies clearly indicate that the characteristics or structure of the road surface material has a significant influence on how well the road will hold up under traffic and environmental conditions (Paterson, 1987). In view of the fact that road aggregate material varies with different characteristics from one borrow pit to another and from region to region, it is paramount that the influence, if any, of road aggregate characteristics on the effectiveness of suppressants be thoroughly investigated. Furthermore, research of this type can help generate a catalogue of suppressants for use on various base and surface material characteristics that unpaved road managers can use in decision making regarding the use of dust suppressants. This section of the report describes work done to investigate the two different sources of aggregate in Larimer County, Colo., and their effect, if any, on the effectiveness of different types of dust suppressants.
2.8.1 Test Sections
The method of construction of the test sections, materials used, and the location of the test sections are described in this section. Larimer County Roads and Bridges Department, partners and local sponsors of this research project, constructed the test sections using their personnel and equipment.
The county operates two road aggregate mining sites: the Horten Pit and the Strang Pit. These two sites supply the county with all of its road base material needs. In all, eight test sections, each 1/2-mile long, were evaluated. Surface material for four of the test sections was supplied from the Horten Pit and the other four test sections received surface material from the Strang Pit.
The test sections evaluated were all located in Larimer County, Colo. They were part of two stretches of unpaved roads located in the Weaverly area of the county. One stretch is County Road (CR) 11 which hosted six of the eight test sections and the other stretch is CR 68 which hosted the other two test sections. CR 11 and CR 68 serve a rural community of crop and livestock farmers. They also provide access to four nearby lakes used for recreation by boaters and fishermen. Two of the lakes are private clubs where boating and camping occur. The other two are open to the general public and used mostly for fishing.
The climate in this region is semiarid with average annual precipitation in the test site area of about 14 inches per year. The general soil/aggregate characteristics in this location are glacial till with some silty clay, sand, and gravel. The average daily high temperature during the test period: June - October is approximately 85F with an average relative humidity of about 25 percent. Figure 2.6 shows portion of the map of Larimer County with the location of the test site highlighted.
Figure 2.6 Larimer, County, Colorads - Road Map and Location of Test Sections
2.8.2 Materials
Larimer County operates two medium-size gravel mining sites from which all the road aggregate needs of the county are acquired. The two gravel borrow pits are called the Strang Pit and the Horton Pit. The Strang Pit is located in the Poudre River basin and thus produces material that appears cleaner with less fines. The Horton Pit is a non-river-basin-type quarry. Reddish weathered rock formations abound in the area, and the resulting aggregate appears less clean, with more clayey-type fines. There is a distinct color difference between the aggregate sources in that the Horton Pit gravel is reddish in color while the Strang Pit gravel is grayish in color.
The characteristics of each aggregate source were evaluated by performing a sieve analysis on material samples obtained from the two borrow pits. Other engineering property tests such as Atterberg limit, Los Angles abrasion, soundness, and specific gravity were not performed. Although these tests aid in providing a comprehensive description of the road surface material, the aggregate size distribution via sieve analysis testing turns out to provide a very representative characteristic value widely accepted for describing road aggregate material. Sieve analysis is quick, easy, and less expensive to perform and the results can readily be interpreted to arrive at a classification for the aggregate material using the AASHO Classification System or the Uniform Soil Classification System (USCS). The engineering properties of the Strang Pit gravel were reported in earlier research done at Colorado State University to study the effectiveness of road dust suppressants (Addo and Sanders, 1995).
The aggregate size distribution of the Horton Pit gravel and Strang Pit gravel are shown in Table 2.1/Figure 2.7 and Table 2.2/Figure 2.8, respectively. Figure 2.9 compares the two distribution size curves on the same axis. The quantity of material passing the No. 40 (0.425 mm) sieve is generally referred to as the fines fraction and is directly related to the amount of dust emission from unpaved road surface (Wood, 1960).
The results of the sieve analysis indicate that 10.6 percent of the Strang Pit material passed the No. 40 sieve. This is nearly the same as the 9.6 percent passing the No. 40 sieve reported in the suppressant effectiveness study (Addo and Sanders, 1995). The fines were determined to be non-plastic with no cohesion. An AASHO soil classification of A-1-a was assigned and poorly graded (GP) gravel was assigned under the USCS. The Horton Pit material had a 17.2 percent passing the No. 40 sieve, correlating very well with visual inspection of the material sample.
Various recommended aggregate mixes to achieve the best surface-wearing course performance has been published Horwell (1993); Woods, (1960). Table 2.3 compares the Horton and Strang materials with suggested aggregate mix as provided by Horwell (1993). Analysis of Table 2.3 data indicate that both Horton and Strang materials do not have enough fines to meet the 25 percent minimum suggested for passing No. 40 sieve. The lack of sufficient fines is more profound with the Strang Pit material than with the Horton Pit gravel.
Without sufficient fines the large-size aggregates cannot be bound into a tight matrix, and therefore aggregate pullout is easier. This results in rapid road surface degradation in the form of raveling and washboarding as vehicular activity increases. The lack of fines which serve as a binder for the coarser aggregate also means more a of driving hazard to vehicles passing each other as loose aggregate picked up by the tires of the vehicle are thrown around, breaking windshields and inflicting possible injuries. Insufficient fines also imply reduction in the total surface area available for ions in the dust suppressant, especially chloride compounds, to attach themselves to. As stated in Compendium 12 (1980), the greater the surface area the more moisture can be attracted to keep the road surface wet.
The chemical dust suppressants evaluated in this research include: magnesium chloride (MgCl2), lignin, and a 50/50 blend of MgCl2 and lignin.
Magnesium chloride (32 percent MgCl2 in solution) is a concentrated brine that draws moisture out of the air to the keep the road surface damp to control dust. It is known to sink into the road surface and create a tight, hard, compact, surface that resists abrasion.
Lignin is a by-product of the paper pulp industry, usually containing 50 percent solids in concentrated water solutions. Dust palliation is achieved by gluing and bonding the soil particles together.
MgCl2 and lignin is a blend of 50 percent of MgCl2 and lignin by volume mixed together. The resulting suppressant exhibits the dust suppressing properties of both chemicals.
Table 2.1 Results of Sieve Analysis - Horton Pit Gravel
| Sieve No. | Particle Size (mm) | Weight Retained between Sieves | Percent Passing | |
|---|---|---|---|---|
| grams | % | |||
| 5/8" | 16.00 | 89.7 | ||
| 3/8" | 9.50 | 78.1 | ||
| 1/4" | 6.70 | 67.6 | ||
| #4 | 4.75 | 58.8 | ||
| #10 | 2.00 | 39.1 | ||
| #20 | 0.85 | 25.1 | ||
| #40 | 0.43 | 17.2 | ||
| 881.6 | 17.2 | |||
| Total Wt | 5133.2 | |||
Figure 2.7 Aggregate Size Distribution - Horton Pit Gravel
Table 2.2 Results of Sieve Analysis - Strang Pit Gravel
| Sieve No. | Particle Size (mm) | Weight Retained between Sieves | Percent Passing | |
|---|---|---|---|---|
| grams | % | |||
| 5/8" | 16.00 | 81.8 | ||
| 3/8" | 9.50 | 66.9 | ||
| 1/4" | 6.70 | 59.2 | ||
| #4 | 4.75 | 53.8 | ||
| #10 | 2.00 | 38.7 | ||
| #20 | 0.85 | 21.2 | ||
| #40 | 0.43 | 10.6 | ||
| 321.8 | 10.6 | |||
| Total Wt | 3032.5 | |||
Figure 2.8 Aggregate Size Distribution - Strang Pit Gravel
Figure 2.9 Aggregate Size Distribution - Horton vs. Strang
Table 2.3 Suggested Aggregate Mix Comparison
| Designation | Percent Passing Sieve | ||
|---|---|---|---|
| Suggested Surface Course (Horwell, 1993) | Horton Gravel | Strang Gravel | |
| 5/8" | 75-100 | 90 | 82 |
| 3/8" | 65-100 | 78 | 67 |
| #4 | 55-85 | 59 | 54 |
| #10 | 40-70 | 39 | 39 |
| #40 | 25-45 | 17 | 11 |
2.8.3 Construction
The construction of the test sections followed the procedure recommended in most road construction literature and that of the dust suppressant suppliers. Important construction steps include:
- road surface scarification to remove all corrugations and potholes,
- addition of new aggregate material if required,
- adequate grading and smoothing of the road surface,
- application of dust suppressant in quantities sufficient for effective dust control,
- proper road finishing procedures that include the formation of surface crown, and
- optimum compaction of the road surface and proper drainage (Rural Transportation Fact Sheet, 1984).
In all, eight test sections were constructed for evaluation in this project. Refer to Table 2.4 for the test section matrix. Virgin or fresh aggregate material was used for the construction of the test sections. As previously mentioned, four of the test sections were constructed using the Horton Pit material and the other four test sections were constructed using the Strang Pit material.
Table 2.4 Test Section Matrix
| Strang Pit Gravel | Horton Pit Gravel | ||
|---|---|---|---|
| Test Sec. No. | Test Section Type | Test Sec. No. | Test Section Type |
| T-1 | Untreated | T-7 | Untreated |
| T-2 | MgCl2 treated | T-8 | MgCl2 treated |
| T-3 | Lignin treated | T-10 | Lignin treated |
| T-4 | MgCl2/Lignin blend treated | T-9 | MgCl2/Lignin blend treated |
Preconstruction of the test sections consisted primarily of blade dressing the road shoulders and ditchlines as well as reclaiming aggregate pullout using a motor grader. The construction of all the test sections followed the same construction procedure. Each test section surface was watered down and scarified to a depth below the deepest pothole or approximately 6 inches. Trucks brought in new aggregate which was spread on the surface to augment the existing scarified material. The new material averaged approximately 4 inches in thickness. The following provides additional details on the construction of the test sections.
Untreated Test Sections
For the control untreated test sections, water was added while the grader windrowed the loose material from one side of the road to the other to achieve a good mixture. The road surface was shaped to a crown, leveled, and compacted in the presence of more water to form a firm wearing course.
Treated Test Sections
Material for each test section was treated with sufficient water to achieve an optimum water content while the grader worked the material. Each selected dust suppressant was sprayed on the loose material by the supply truck at the supplier-recommended application rate. The grader windrowed the mixture until the suppressant was well mixed in with the aggregate material. This suppressant application technique is referred to as the mixed-in-place method. The road surface was leveled, shaped to crown, rolled and compacted to a firm wearing course using a rubber tire type pneumatic compactor. The dust suppressant application rates are:
- MgCl2 (32% solution): 1/2 gal/yd2
- Lignin (50% solids): 1/2 gal/yd2
- MgCl2 and lignin blend (50/50 by volume): 1/2 gal/yd2
The test sections were opened to traffic immediately after construction.
2.8.4 Measurements
This research was intended to be a continuation of the "Relative Effectiveness of Road Dust Suppressants" studies started at Colorado State University (Addo and Sanders, 1995 and Sanders et. al., 1997). The emphasis of this study is to assess the effect that road surface material characteristics as described by aggregate size characteristics might have on the effectiveness of some commonly used dust suppressants (soil chemical admixtures).
Various laboratory studies have been reported (Palmer et al., 1995; DeCastro et. al., 1996 and etc.). All of them investigated strength and density modification of unpaved road soils due to chemical additives. The results of these laboratory studies have provided valuable quantitative physical and chemical measurements worth investigating further through more research.
This research study also will attempt to validate some of the laboratory findings under field conditions where the true effectiveness of unpaved road soil admixtures in improving overall road surface performance can be measured. Field measurements in unpaved road studies can be difficult because all the elemental factors (rain, snow, wind, traffic, construction procedure, and etc.) are in play. The lack of a standardize protocol, procedures, and equipment for performing field assessment adds to the challenge.
The composition of traffic using an unpaved road is a major contributing factor to the degradation of the unpaved road. Therefore understanding vehicle type, size and weight, traveling speed, and volume are essential in any unpaved road maintenance studies. CR-11 and CR-68, which include the test sections, serve a rural community of crop and livestock owners. The roads also provide access to four recreational lakes in the area. A field traffic observation survey carried out during the research study indicated that nearly two-thirds of the vehicles using the roads are pickup trucks. They range in sizes from 1/4-ton to the full 1-ton size. About 30 percent of the trucks pulled a boat or horse trailer. The other third of the traffic volume composed of cars and farming equipment such as tractors. The average daily traffic (ADT) volume for the test sections is 25. The value was obtained from the latest Larimer County traffic count records. Traffic counters were installed at strategic locations along the test sections during the research period May - September 2001 to validate the ADT of 25.
To quantitatively measure the effect of aggregate distribution on the effectiveness of dust suppressants, dust emission from each of the test sections was monitored under field conditions. The Colorado State University Dustometer, a dust-sampling device developed and used in earlier dust studies at Colorado State University (Addo and Sanders, 1995), was used to monitor vehicular-generated dust from each of the test sections for comparative analysis. The variables in this research study are: type of dust suppressants, construction procedure, field/environment conditions, and traffic volume/activity. Different road aggregate material is used for the construction of the two sets of test sections.
Dustometer: The Dustometer can be described as a moving dust sampler that provides a real-time quantitative dust emission measurement for a section of a road. Its dust measurements are precise, reproducible, and easily obtained (Sanders and Addo, 2000). It provides a uniform procedure for gathering and comparing data from many test sections. Many data points can be generated within the shortest possible time. The device consists primarily of the following: a fabricated metal box designed to hold a 10 x 8 in (25.4 x 20.3 cm) glass fiber paper, mounted to the bumper of a pickup truck behind the driver's side rear tire; an electric power generator; 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 (30.5x30.5 cm) opening that is covered with a 450 m mesh sieve that faces the tire. The 450 m screen prevents any non-dust particles from being drawn onto the filter paper during dust measurement. The filter paper is supported near the bottom of the fabricated box by a sieve mesh. Figure 2.10 illustrates the Dustometer.
Figure 2.10 Schematic Diagram of Colorado State University Dustometer Setup
To perform a typical dust measurement in this research, a 1/2-ton pickup truck fitted with the Dustometer was used. A test speed of 35 mph - the designated driving speed for the roads was used. As the truck is driven at the constant speed of 35 mph a portion of the dust generated is collected on a pre-weighed filter paper in the filter box mounted on the bumper of the truck. At the end of a test run, the filter paper laden with dust is gently removed and put into in a very thin plastic bag and stored to be weighed later in the laboratory. The filter box is refitted with a new pre-weighed filter paper and another test is run. The difference between the pre-test weight and the post-test weight is the weight of dust sampled.
Each test section is 1/2-mile long. A test run consisted of driving the test truck in the driving lane in both directions for a total of one mile of dust measurement. Three test runs were performed per test section on a given test day. The average of the three runs is a data point for a test section.
2.8.5 Results
The dust measurements from the two sets of test sections were taken from the periods June 2000 through September 2000 and then from May 2001 through July 2001. After the snow season and before the second period dust measurement, all the test sections received a periodic maintenance of blading, grading, and compaction without adding new road material or treatment. Essentially the second period dust measurement was a continuation of the first period dust measurement.
Table 2.5 shows the dust measurement in grams for each of the test sections evaluated. Test sections TS-1 through TS-4 are of the Strang gravel and TS-7 through TS-10 are constructed using the Horton pit gravel. In all, 13 data points were acquired during the field measurement periods. Figure 2.11 and 2.12 represents the results of the dust measurements of the Strang gravel test sections and Horton gravel test sections respectively. No clear dust measurement pattern was established. The untreated control tests for both the Strang and Horton gravels on average produced more dust than the treated test sections as expected. But it is interesting to note (Figure 2.11) that near the end of the dust measurements the MgCl2-treated Strang gravel test section produced equal or more dust than the untreated test section. This could be due to the fact that the MgCl2 at that stage of the treatment life (more than a year old) has lost all of its dust-control properties through leaching and downward migration of the ions. This observation supports the recommendation by DeCastro et. al., (1996) that chloride compound soil admixtures should be applied twice a year for effective dust control.
Comparing the Strang gravel to the Horton gravel, Figure 2.13 through Figure 2.16 shows the comparative dust measurements of the different test sections. With the exception of the MgCl2 test sections (Figure 2.11) all other test sections - untreated, MgCl2/lignin blend, and the lignin test sections, indicate consistently that more dust was produced by the Horton gravel test sections as compared to the Strang gravel test section.
Table 2.5 Dust Measurements
| Sampling Date | Strang Test Sections | Horton Test Sections | ||||||
|---|---|---|---|---|---|---|---|---|
| Untreated (T-1) | MgCl2 Treated (T-2) | Lignin Treated (T-3) | MgCl2 / Lignin (T-4) | Untreated (T-7) | MgCl2 Treated (T-8) | Lignin Treated (T-9) | MgCl2 / Lignin (T-10) | |
| 6/8/2000 | 3.12 | 1.16 | 0.90 | 0.70 | 5.96 | 1.20 | 0.61 | 1.07 |
| 7/3/2000 | 2.58 | 2.13 | 1.75 | 0.64 | 4.19 | 4.04 | 5.24 | 4.17 |
| 7/13/2000 | 4.46 | 1.49 | 1.01 | 1.13 | 4.69 | 1.47 | 2.13 | 3.41 |
| 7/27/2000 | 5.29 | 1.00 | 0.53 | 0.49 | 6.04 | 1.76 | 1.25 | 1.30 |
| 8/8/2000 | 4.38 | 1.23 | 0.53 | 0.64 | 5.77 | 1.03 | 1.89 | 2.30 |
| 8/21/2000 | 2.47 | 2.58 | 1.85 | 1.23 | 3.50 | 1.36 | 1.33 | 1.29 |
| 9/16/2000 | 4.40 | 1.42 | 1.45 | 1.82 | 4.78 | 2.32 | 3.41 | 4.31 |
| 10/8/2000 | 3.11 | 2.43 | 1.24 | 0.64 | 3.77 | 1.75 | 1.98 | 2.13 |
| 5/17/2001 | 2.60 | 1.64 | 0.98 | 0.75 | 4.45 | 1.22 | 1.98 | 1.49 |
| 5/31/2001 | 2.63 | 3.16 | 1.17 | 1.50 | 2.93 | 1.01 | 1.82 | 2.33 |
| 6/21/2001 | 2.83 | 2.72 | 1.11 | 1.69 | 5.33 | 1.90 | 3.05 | 3.87 |
| 6/28/2001 | 1.26 | 2.02 | 1.55 | 0.61 | 3.25 | 2.51 | 2.83 | 2.47 |
| 7/18/2001 | 2.70 | 2.62 | 1.17 | 1.17 | 6.07 | 3.04 | 4.14 | 4.17 |
Dust weight = average of three measurements
Length of run for each test section = 1.0 mile
Length of each test section = 1/2 mile
Passes are run on both driving lanes
Sampling speed = 35 mph
Figure 2.11 Strang Gravel Test Sections - Dust Data
Figure 2.12 Horton Gravel Test Sections - Dust Data
Figure 2.13 Untreated Test Sections - Dust Data Comparison
Figure 2.14 MgCl2 Test Sections - Dust Data Comparison
Figure 2.15 Lignin Test Sections - Dust Data Comparison
Figure 2.16 MgCl2 Lignin Blend Test Sections - Dust Data Comparison