Semisequicentennial Transportation Conference Proceedings
May 1996, Iowa State University, Ames, Iowa

Bentonite Treatment for Fugitive Dust Control

K.L. Bergeson and S.G. Brocka

Geotechnical and Materials Division,
Department of Civil and Construction Engineering,
482B Town Engineering Building,
Iowa State University,
Ames, Iowa 50011.

This research project was directed at laboratory and field evaluation of sodium montmorillonite clay (bentonite) as a dust palliative for limestone surfaced secondary roads. It was postulated that the electrically charged surfaces (negative) of the clay particles could interact with the charged surfaces (positive) of the limestone and act as a bonding agent to agglomerate fine (- #200) particulates, and also to bond the fine particulates to larger (+ #200) limestone particles. Test roads were constructed in Dallas, Adair, and Tama counties in Iowa using bentonite treatment levels (by weight of aggregate) ranging from 0.5 to 9.0 percent. Quantitative and qualitative periodic evaluations of the roads were conducted with respect to dust generation, crust development, roughness, and braking characteristics. As the bentonite treatment level increased, dust generation decreased. About a 70 percent reduction can be achieved at nine percent bentonite treatment. Wheel path crust development is improved. Braking distance and braking handling characteristics under wet surface conditions do not appear to be adversely affected up to the nine percent treatment level. The bentonite appears to be functioning as a bonding agent to bind small particulates to larger particles and is acting to agglomerate fine particles of limestone. This bonding capability appears recoverable over a wide range of environmental conditions. The bentonite appears to be able to interact with new applications of limestone maintenance material and maintains a dust reduction capability.


The Highway Division of the Iowa Department of Transportation (Iowa DOT) has acted to address "fugitive dust" under research projects HR-151 (1) and HR-194 (2). This work utilized a number of different palliatives and proprietary products through laboratory screening and demonstration test sections. A common problem encountered is that many additives are good palliatives but are not cost effective. Past work (1,2,3) also indicated that for a dust palliative to be effective, it is necessary that the fine particulates be flocculated, aggregated, or somehow physically bound to themselves or to larger particulates in order to prevent them from becoming airborne under traffic.

Other laboratory research (4) indicated that bentonite treatment might have the potential for functioning as an effective dust palliative and a stabilizing agent for limestone surfaced secondary roads.

DALLAS AND ADAIR COUNTY TEST ROADS

Bentonite is commonly used as a drilling fluid because of its thixotropic properties. It becomes very slippery and sticky when mixed with water. One of the primary concerns on the field projects was the influence of bentonite treatment on braking and handling characteristics and general driving safety. Initial demonstration projects were constructed in Dallas and Adair Counties under Iowa DOT project HR-297 (5) utilizing low levels of bentonite treatment.

The Dallas county road ( ~ 75 vpd) was constructed utilizing test section bentonite treatment levels of 0.5, 1.0, and 1.5 percent (by weight of aggregate). A bentonite slurry was spray applied to loose surfacing materials using conventional spray equipment. Fugitive dust testing was conducted over a two-year period, using high volume air sampling equipment. Test results indicated an overall average dust reduction of about 19 percent for the 1.5 percent treatment section. Braking tests indicated no apparent adverse effects on braking or handling characteristics. Scanning electron microscopy indicated particle to particle bonding and agglomeration of the fines was being accomplished. The Adair County road (80 vpd) was constructed using bentonite treatments up to 3.0 percent. A dry bentonite application method was used for construction. The 1.5 percent treatment section exhibited an average reduction of about 17 percent. The 3.0 treatment reduced dust about 41 percent over the one-year test period. Braking and handling characteristics were not affected. The complete test data is presented in the final report for HR-297 (5).

TAMA COUNTY TEST ROAD

Based on the results and experience of the Dallas and Adair County test roads, a project was initiated in Tama County which incorporated 3.0, 5.0, 7.0 and 9.0 percent bentonite treatment levels. Construction was by the dry application method used in Adair County and proceeded rapidly (it was completed in one day under traffic).

Balling up or agglomeration of the bentonite was observed beginning with the five percent bentonite treatment section during the wet mixing process. This balling effect became more pronounced (but not drastic) as the treatment level increased and was most apparent in the nine percent section. Based on field observations and review of the lab data, the following changes in procedure were recommended for future construction: 1) utilize a soda ash solution concentration (by weight of water) equal to 1/10 of the bentonite treatment percentage and 2) alter the construction wet mixing procedure by saturating the dry bentonite mixed surfacing material with soda ash solution prior to the first wet mixing pass of the patrol. The fact that the seven percent and nine percent treated sections did not get the bentonite as well dispersed throughout the material may have reduced its effectiveness.

Field Evaluation

With bentonite treatment, dust is reduced but is still being generated after treatment. It is not as dramatic as with chloride treatments, and the traveling public may not perceive a reduction. Because of this, field testing on the Tama County test road consisted of qualitative evaluations by a panel. Field testing consisted of qualitative evaluation of dust generation, crust development, roughness, and braking characteristics. Evaluations were conducted periodically and independently by a panel composed of personnel representing Marshall County, Tama County, the Iowa DOT and Iowa State University. An evaluation form was developed and used by all the panelists for their observations. The panel evaluated the amount of dust generation for each bentonite treated section and evaluated the crust development and roughness of each section. The dust generation was expressed as a percentage of the control, with the control having a value of 100 percent. The crust development and roughness were evaluated on a rating system from zero to five, with zero being the worst and five being the best rating. Figures 1 and 2 show the data in graphical form.

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The dust generation data indicated a relatively uniform standard deviation of plus or minus 11 percent for all test sections; therefore, the data shown in Figure 1 must be viewed with this in mind. From Figure 1, the three percent bentonite treatment results in a dust reduction of about 45 percent. This compares roughly with the 41 percent quantitative wheel path reduction observed for the same treatment level at the Adair County test road under the HR-297 project (5). At a nine percent treatment level, dust reduction is on the order of 70 percent. These qualitative data indicate that bentonite treatment is effective in reducing dust generation. It should be noted that the data for the seven and nine percent treatment may not reflect actual dust reduction capability due to inadequate dispersion of the bentonite during the construction process. With a standard deviation of 11 percent, differences between treated sections may be subject to question. Crust development observation data are shown graphically on Figure 2. These data indicate the three percent treated section had somewhat better crust development than the control. The five, seven, and nine percent sections appeared to be relatively even with values about two times better than the control. Good crust development would be expected to extend the life of the surfacing material. Again the standard deviation is about 1.5 and differences may be questionable. As Figure 2 indicates, there was no clear trend evident with respect to roughness, with all sections exhibiting a rating from three to four. Samples from the minus #200 sieve fraction from the control section and three and nine percent bentonite treatment sections were prepared for SEM micrographing. The bonding of the fines to the larger particulates, and agglomeration of the fines was evident.

A major concern at the start of this project was the influence of high levels of bentonite treatment on braking characteristics and safety. Tama County and ISU personnel were at the site and conducted braking tests under wet to very wet conditions independently, and at different times. There were no differences in braking distance, or handling characteristics during braking on any of the sections that they could discern.

Economic Considerations

Bentonite treatment construction cost breakdown data for materials and installation is summarized in Table 1. Equipment and labor costs were 1993 costs provided by the counties and assumes construction of 1.6 km (one mile) per day, which is conservative. Materials costs are F.O.B. costs to the Des Moines, Iowa, area.

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Cost of a 38 percent concentration calcium chloride treatment at an application rate of 0.95 L (0.25 gal) per square yard was approximately $1,600 per 1.6 km (one mile) in 1993. In Iowa, a minimum of two applications per summer are normally required for dust control. This yields a minimum cost of about $3,200 per 1.6 km (one mile) per year. The nine percent bentonite treatment cost is about $1,750 per 1.6 km (one mile) per year assuming the bentonite is effective for only one season. This is believed to be a conservative comparison since the longevity of dust reduction using bentonite has not yet been firmly established. Current data indicates that a single treatment acts to reduce dust over an extended time period (> 2 seasons).

CONCLUSIONS

The results of this research indicate that bentonite treatment of limestone surfaced secondary roads is a cost effective dust reduction treatment. Data from the Dallas, Adair, and Tama County test roads indicate the following:

Benefits

The use of bentonite as a dust reduction treatment appears to have the following benefits:

REFERENCES

  1. J.M. Hoover, K.L. Bergeson, D.E. Fox, C.K. Denny, and R.L. Handy. Surface Improvement and Dust Palliation of Unpaved Secondary Roads and Streets. Final Report, Iowa Dept. of Transportation Research Project HR-151, July 1973.
  2. J.M. Hoover, J.M. Pitt, M.T. Lustig, and D.E. Fox. Mission Oriented. Dust Control and Surface Improvements Processes for Unpaved Roads. Final Report, Iowa Dept. of Transportation Research Project HR-194, May 1981.
  3. R.L. Handy, J.M. Hoover, K.L. Bergeson, and D.E. Fox. Unpaved Roads as Sources for Fugitive Dust. Transportation Research News, Vol. 60, 1975, pp. 6-9.
  4. G. Oren, T. Demirel, and D. Boylan. Reducing Dusting of Limestone Fines. Final Report, Iowa Limestone Company, Des Moines, Iowa, January 1985.
  5. K.L. Bergeson and A.M. Wahbeh. Development of an Economic Dust Palliative for Limestone Surfaced Secondary Roads. Final Report, Iowa Dept. of Transportation Research Project, HR-297, ISU-ERI-Ames 90407, February 1990.

This research has been sponsored by the Highway Division of the Iowa DOT and the Iowa Highway Research Advisory Board under Iowa DOT project HR-297 and HR-351. The opinions, findings and conclusions expressed in this paper are those of the authors and not necessarily those of the Highway Division of the Iowa DOT. The cooperation and assistance of the Adair, Dallas, and Tama county engineers and their personnel is gratefully acknowledged. Special thanks to Gary Harris, research engineer for the Iowa DOT, for his assistance.

CTRE is an Iowa State University center, administered by the Institute for Transportation.

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