Evaluation Guidance   Materials   Applications     Acknowledgements  

[ Material Description ] - [ Stabilized Base ] - [ Flowable Fill ]

FGD SCRUBBER MATERIALUser Guideline


Flowable Fill

Note: The use of flue gas desulfurization (FGD) material as flowable fill is an emerging application area. Published research and case studies on this subject are limited. As such, the use of FGD for this application is not well documented, and any use of FGD as a flowable fill should be considered somewhat experimental. Publication of such uses and laboratory research would aid in the understanding of FGD’s performance as a flowable fill.

INTRODUCTION

FGD material (both wet and dry) are being researched as a replacement for fly ash in flowable fill material. The cementitious reactions that occur with FGD material are well-suited for flowable fill applications.(1;2) Low unit weight and sufficient shear strength make FGD flowable fill a suitable alternative to commonly used compacted earth backfills.(3;2) Research conducted at Ohio State University (OSU) investigated the use of FGD material in flowable fill, the results of these studies make up the majority of this user guideline.

Flowable fill is used as a self-leveling, self-compacting backfill material. Typical flowable fill mixtures include filler material, cementitious material, and can contain mineral admixtures. Filler material usually consists of fine aggregate such as sand, but some flowable fill mixes may contain equal portions of coarse and fine aggregates.(4)

The use of flowable fill as a highway construction material is becoming more widespread throughout the United States. Most state transportation agencies have used flowable fill mainly as a trench backfill for storm drainage and utility lines on street and highway projects. Other applications for flowable fill include filling behind retaining walls, building excavations, underground storage tanks, abandoned sewers and utility lines, and slab jacking.

Flowable fill is considered a controlled low strength material (CLSM) according to ACI 116R as long as the compressive strength is less than 8270 kPa (1200 lb/in2).(5;6;7) In high strength CLSM applications, the strength of flowable fill mixes can range from 1380 to 8270 kPa (200 to 1200 lb/in2), depending on the design requirements. The desired range of compressive strength in flowable fill mixtures depends on whether the hardened material is designed to be removed in the future. The ultimate strength for excavatable flowable fill should not exceed 1035 kPa (150 lb/in2) or jack hammers may be required for removal.(4) For flowable fill mixes used in higher bearing capacity applications, such as structural fill or temporary support of traffic loads, higher compressive strengths can be designed.

ENGINEERING PROPERTIES

Engineering properties of FGD flowable fill mixes that are of interest include compressive strength, flowability, stability, bearing capacity, modulus of subgrade reaction, lateral pressure, time of set, bleeding and shrinkage, unit weight, and hydraulic conductivity. Unfortunately all of these properties have not yet been investigated and published in the literature.

A wet and dry FGD flowable fill testing program conducted at Ohio State University investigated unconfined compressive strength (UCS), flowability, unit weight, penetration resistance, and FGD flowable fill mix design. The tests were conducted on a wet fixated scrubber sludge and dry FGD generated with a lime sorbent(2;3).

Compressive Strength: Typical desired value for 28-day unconfined compressive strength of flowable fill ranges from 172 to 410 kPa (25 to 60 psi). The lower limit of this range corresponds to the strength needed to support construction vehicles while the upper limit strength is still excavatable. Test results showed that strength increased with curing time and that strength was directly related to water content.(3) The 28 day strength of wet fixated FGD scrubber sludge, with an additional 6 percent cement, was 1041 kPa (150 psi), while the addition of 6 percent lime produced a 28 day strength of 379 kPa (55 psi).(2) Dry FGD flowable fill unconfined compressive strength vs. time for varying water contents is shown in Figure 3.


Figure 3. Unconfined compression strength of dry FGD flowable fill vs. time at varying water contents.(3)

Flowability: Flowability is a measure of how well a mixture will flow when being placed. As the amount of water in an FGD flowable fill mix increases, flowability increases but unconfined compressive strength is shown to decrease. Flowability can vary from stiff to fluid depending on the project requirements. Flowability can be measured using a standard concrete slump cone, a flow cone, or a modified flow test using an open ended 75 mm (3 in) diameter by 150 mm (6 in) high cylinder.(8;9;10) Flowability ranges associated with the standard concrete slump cone (ASTM C143) generally vary from 150 to 200 mm (6 to 8 in).(8) For placement of flowable fill, a flowability range tested in accordance to ASTM 6103(11) of 180 to 210 mm (7 to 8 in) is recommended.(3) The results of flowability tests on wet FGD scrubber mixes were between 150 and 300 mm (6 to 12 in).(2) Flowability test results on dry FGD material and water mixes are presented in Table 6-8 along with dry unit weights. As shown in Table 10, flowability is increased with increased water content, but increased water content reduces strength and increases set time. The addition of fine aggregate may improve flowability at lower water contents and not adversely affect strength or set time.

Table 10. Flowability test and dry unit weight of varying dry FGD samples.(3)

Water
Content (%)

Flow mm (in.)

Dry Unit Weight kN/m3 (pcf)

65.0
72.5
77.0

152 (6)
203 (8)
330 (13)

8.96 (57)
8.49 (54)
8.96 (57)

Penetration Resistance: Long term penetration resistance characteristics of FGD material used as a flowable fill are comparable to conventional cement-based flowable fill. Short term penetration resistance characteristics of dry FGD material and water mixes showed penetration resistance values measured with ASTM D6024(12) less than 689 kPa (100 psi) after 24 hours and less than 1375 kPa (200 psi) after 144 hours (6 days).(3) Early penetration resistance can be increased with the addition of cement, lime, or admixture. In general, larger proportions of cement, lime, or admixture in the FGD flowable fill mixes cause flowable fill to harden faster, although higher longer term strength should be expected.(3)

DESIGN CONSIDERATIONS

Mix Design

Flowable fill mixtures traditionally are proportioned by trial and error. Most specifications for flowable fill provide quantities of constituents that produce an acceptable product, although some specifications are performance-based (usually based on a maximum compressive strength) and leave the proportioning up to the material supplier. ACI provides guidance for the proportioning of flowable fill mixtures.(6)

Mix designs for FGD flowable fill can be a simple as FGD and water or can contain cement, lime, and/or admixtures in varying amounts to achieve desired properties. Dry FGD and water mixes, at water contents between 65 and 77 percent, showed acceptable flowability and long term strength for flowable fill, but short term penetration resistance may be too low for some construction projects. Mixes containing dry FGD material, water, cement and/or lime (6 to 10 percent), and admixtures (1.3 to 5.9 percent) showed excellent flowability and long term strength along with achieving a penetration resistance of 2750 kPa (400 psi) in one to two days.(13)

Wet fixated FGD scrubber sludge, with a fly ash to filter cake ratio of 1.25:1 with an additional 5 percent lime, were used in flowable fill mix designs. The wet fixated FGD flowable fill mix designs were at a water content of 82.5 to 84 percent, and had additional cement or lime added at 6 percent of the dry unit weight of the FGD material. Although these mixes had good flowability, the short term (24 hr) penetration resistance for the 6 percent lime mix may be too low for some applications and the 6 percent cement mix developed too much long term strength for excavatable flowable fill.(2) Therefore, wet fixated FGD mixes may require admixtures to reduce initial set time and a reduction in cement to reduce long term strength. Long term strength, even beyond 28 days, may need to be investigated if future excavation is a required.

ENVIRONMENTAL CONSIDERATIONS

Although not specifically from leachate studies on FGD flowable fill, FGD material leachate show pH typically exceeds 11.0, and some sources can exceeded the Resource Conservation and Recovery Act limit of 12.5 for toxic waste, although this high leachate pH is expected to decrease over time.(14;15) The high pH of FGD grout has been used beneficially to neutralize acid mine drainage.(16;17) The low hydraulic conductivity of flowable fills in general reduces the rate at which water permeates and trace elements leach from flowable fill. Trace element concentrations in FGD leachate are generally low.(14)

UNRESOLVED ISSUES

Further research is needed on both the mechanical and environmental aspects of FGD flowable fill. Due to the variety of FGD systems, acceptable reuse from one FGD product and source does not imply universal acceptability. Research is needed on FGD flowable fill with respect to: stability (friction angle), bearing capacity (CBR), corrosivity, resilient modulus, modulus of subgrade reaction, lateral pressure development, bleeding and shrinkage, and hydraulic conductivity. In addition, research is needed in the interaction between admixture, cement, and the properties of FGD flowable fill.

REFERENCES

A searchable version of the references used in this section is available here.
A searchable bibliography of FGD scrubber material literature is available here.

  1. NETL National Energy Technology Laboratory. Commercial use of coal utilization by-products and technology trends. Department of Energy Office of Fossil Energy; Washington, DC: 2003.
  2. Butalia TS, Wolfe WE, Zana B, Lee JW. Flowable fill using flue gas desulfurization material. Journal of ASTM International 2004;1(9):1-12.
  3. Butalia TS, Wolfe WE, Lee JW. Evaluation of a dry FGD material as a flowable fill. Fuel 2001;80:845-50.
  4. Smith A. Controlled low-strength material. Concrete Construction 1991:389-98.
  5. ACI Committee 116. Cement and concrete terminology. Report nr 116R-00, American Concrete Institute (ACI), Detroit, Michigan: 2000.
  6. ACI Committee 229. Controlled low strength materials (CLSM). Report nr 229R-99, American Concrete Institute (ACI), Detroit, Michigan:1999.
  7. Ramme BW, Tharaniyil M. Coal combustion products utilization handbook. We Energies; Milwaukee, WI: 2004.
  8. ASTM C143/C143M-05a standard test method for slump of hydraulic-cement concrete. In: Annual book of ASTM standards. American Society for Testing and Materials; West Conshohocken, Pennsylvania: 2005.
  9. ASTM C939-02 standard test method for flow of grout for preplaced-aggregate concrete (flow cone method. In: Annual book of ASTM standards. American Society for Testing and Materials; West Conshohocken, Pennsylvania: 2002.
  10. Balsamo NJ. Slurry backfills – useful and versatile. Public Works, April 1987;118:58-60.
  11. ASTM D6103-04 standard test method for flow consistency of controlled low strength material (CLSM). In: Annual book of ASTM standards. ASTM; West Conshohocken, Pennsylvania: 2007.
  12. ASTM D6024-02 standard test method for ball drop on controlled low strength material (CLSM) to determine suitability for load application. In: Annual book of ASTM standards. American Society for Testing and Materials; West Conshohocken, Pennsylvania: 2002.
  13. Lee JW, Butalia TS, Wolfe WE. Potential use of FGD as a flowable-fill. In: 1999 international ash utilization symposium, Center for applied energy research. University of Kentucky; 1999.
  14. Kost DA, Bigham JM, Stehouwer RC, Beeghly JH, Fowler R, Traina SJ, Wolfe WE, Dick WA. Chemical and physical properties of dry flue gas desulfurization products. Journal of Environmental Quality 2005;34:676.
  15. Cheng C-, Tu W, Zand B, Butalia TS, Wolfe WE, Walker H. Beneficial reuse of FGD material in the construction of low permeability liners: Impacts on inorganic water quality constituents. Journal of Environmental Engineering 2007;133(5):523-31.
  16. Stuart BJ, Novak G, Payne H, Togni CS. Use of flue gas desulfurization by-product for mine sealing and abatement of acid mine drainage. In: 1999 international ash utilization symposium, Center for applied energy research. University of Kentucky; 1999.
  17. Electric Power Research Institute (EPRI). Flue gas desulfurization by-products: Composition, storage, use, and health and environmental information. EPRI, Inc; Palo Alto, CA: 1999.

[ Material Description ] - [ Stabilized Base ] - [ Flowable Fill ]

Last Update 7/28/08