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COAL FLY ASHUser Guideline


Flowable Fill

INTRODUCTION

Coal fly ash can be used as a component in the production of flowable fill (also called controlled low strength material or CLSM), which is used as a self-leveling, self-compacting backfill material in lieu of compacted earth or granular fill. 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.(1) Fly ash has been used as a replacement of partial replacement for all three constituents in flowable fill.

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).(2;3;4) 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 of the project in question. The desired range of compressive strength in flowable fill mixtures depends on whether the hardened material may have 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.(1) 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.

Since flowable fill is normally a comparatively low-strength material, there are no strict quality requirements for fly ash used in flowable fill mixtures. Fly ash is well suited for use in flowable fill mixtures. The fine particle size (nonplastic silt) and spherical particle shape enhances mix flowability. The pozzolanic or cementitious properties of fly ash allow for lower cement content than would normally be required to achieve equivalent strengths.

The unit weight of standard flowable fill is similar to that of well-compacted soil in the range of 1850 to 2300 kg/m3 (115 to 145 lb/ft3). With the addition of lightweight aggregate, such as crumb rubber, flowable fill can have a relatively low unit weight 1170 to 1570 kg/m3 (73 to 98 lb/ft3) and be considered a lightweight fill.(5) Flowable fill is placed as a liquid and therefore exerts fluid pressure on structures which needs to be considered during design. Tanks, pipes, and cables need to be secured to resist the buoyant force of uncured flowable fill.(4)

There are two basic types of flowable fill mixes that contain fly ash: high fly ash content mixes and low fly ash content mixes. The high fly ash content mixes typically contain nearly all fly ash, with a small percentage of Portland cement and enough water to make the mix flowable. Low fly ash content mixes typically contain a high percentage of fine aggregate or filler material (usually sand), a low percentage of fly ash and Portland cement, and enough water to also make the mix flowable.(6;7) Class F fly ash is well suited for use in high fly ash content mixes, but can also be used in low fly ash content mixes. Class C fly ash is almost always used only in low fly ash content flowable fill mixes because of the cementitious properties of Class C fly ash.(8) There are also flowable fill mix designs in which both Class F and Class C fly ash are used in varying proportions.(8)

MATERIAL PROCESSING REQUIREMENTS

Sources Control

Fly ash used in flowable fill does not have to meet strict specification requirements, such as ASTM C618 for use in concrete.(7;9) A high-quality source of ash is not required and fly ash with high LOI or carbon content is suitable.(7) High carbon content can be a concern with concretes containing air-entrainment where entraining admixtures are more susceptible to absorption. Since CLSM does not often have requirements for air content, carbon content does not affect the properties.(4) Dry or conditioned fly ash as well as reclaimed ash from settling ponds may also be suitable for flowable fill. No special processing is necessary prior to use.

Moisture Control

Pozzolanic-type fly ash can be introduced into flowable fill mixes in either a dry or moistened condition. Self-cementing fly ash should be introduced into flowable fill mixes in a dry condition to avoid presetting.

ENGINEERING PROPERTIES

Engineering properties of 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, density, and hydraulic conductivity. The properties of fly ash that are the most influential to the performance of flowable fill mixtures are the spherical particle shape and pozzolanic activity with Portland cement.

Compressive Strength: Strength development in flowable fill mixtures is directly related to cement content and water content, particularly when Class F fly ash is used. Most high fly ash content mixes only require from 3 to 5 percent of the cementitious material be Portland cement to develop 28-day compressive strengths in the 345 to 1000 kPa (50 to 150 lb/in2) range.(7) For low fly ash content mixes with Class C fly ash, the fly ash contributes to the strength development and can also be a complete replacement for Portland cement. Ultimate strengths may gradually increase well beyond the 28-day strength, perhaps even beyond 90 days, especially in high fly ash content mixes. As the water content is increased to produce a more flowable mix, compressive strength development decreases.(6;7)

Flowability: Flowability or fluidity is a measure of how well a mixture will flow when being placed. Mixes with higher water content are more flowable.(7) Flowability can vary from stiff to fluid depending on the job 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.(10;11;12) Flowability ranges associated with the standard concrete slump cone (ASTM C143) generally vary from 150 mm (6 in) to 200 mm (8 in).(10) Admixtures (such as water reducing agents) are not normally used in flowable fill. For high fly ash content mixes, the slump ranges can be expected to be at least 25 to 50 mm (1 to 2 in) higher than low fly ash content mixes at comparable moisture contents.

The flow cone test (ASTM C939) is a standard procedure for determining the flow rate of grout. A desirable rate of flow for most applications of flowable fill is a time of 30 to 45 seconds through a standard flow cone.(12)

The modified flow test conducted according to ASTM D6103 involves filling a 75 mm (3 in) diameter by a 150 mm (6 in) cylinder mold with flowable fill, emptying the contents of the cylinder on a flat surface, and measuring the diameter of the flowable fill. This test is best suited to mixtures that contain primarily fine aggregates (low fly ash content mixtures). For good flowability, the diameter of the spread material should be at least 200 mm (8 in).(12;13)

Stability: For low fly ash content flowable fill materials, designated as Type 1 CLSM by the Ohio DOT, triaxial tests indicate a drained friction angle of 28° and a cohesion of 33 kPa (685 lb/ft2) for 7 day strength. For high fly ash content flowable fill materials, designated as Type 3 CLSM by the Ohio DOT, triaxial tests indicate a drained friction angle of 33° and a cohesion of 34 kPa (705 lb/ft2) for 7 day strength.(14)

Bearing Capacity: The unconfined compressive strength of flowable fill increases with time; therefore, the bearing capacity increases with time. Small scale penetration tests to measure ultimate bearing capacity tests on Ohio DOT fly ash flowable fill showed an almost 6 fold increase in capacity from 2 hours to 2 days, from 900 kPa (19,000 lb/ft2) to 5250 kPa (110,000 lb/ft2).(14) With an unconfined compressive strength of 685 kPa (14,400 lb/ft2) flowable fill has two to three times the capacity of most well-compacted granular soil fill materials.(4)

California Bearing Ratio (CBR) is a measure of the bearing strength of subgrade. Flowable fill has shown CBR ranging from 40 to 90 percent.(15;16) CBR testing of typical 690 kPa (100 lb/in2) flowable fill resulted in a CBR value of 50 within 24 hours of placement. As the compressive strength of the flowable fill material increases, the CBR can be expected to increase.

Modulus of Subgrade Reaction: The modulus of subgrade reaction (k), used for the design of rigid pavement systems, is usually in the range of 8.2 to 49.2 MPa/m (50 to 300 lb/in3) for most soils and 82 MPa/m (500 lb/in3) for a good granular subbase material. For flowable fill, k is usually 820 MPa/m (5000 lb/in3) or higher, meaning flowable fill is superior to any earthen backfill.(17)

Lateral Pressure: Because of lateral fluid pressure at the time of placement, flowable fill installations at depths in excess of 1.8 m (6 ft) are normally placed in separate lifts, with each lift not exceeding 1.2 to 1.5 m (4 to 5 ft).(18) Once flowable fill has hardened, the lateral pressure is reduced.

Time of Set: For most flowable fill mixes, especially those with high fly ash content, an increase in the cement content or a decrease in the water content, or both, should result in a reduction in hardening time. Typical high fly ash content flowable fill mixes (containing 5 percent cement) harden sufficiently to support the weight of an average person in about 3 to 4 hours, depending on the temperature and humidity. Within 24 hours, construction equipment can operate on the surface without damage. Some low fly ash content flowable fill mixes, especially those containing self-cementing fly ashes, have harden sufficiently to allow street patching within 1 to 2 hours following placement.(7)

A setting time of 15 hours for mixes of fly ash and crushed sand, regardless of cement content, have been observed with test method ASTM D6024, which takes into account field parameters. As the cement is the main component that dominates the setting time for flowable fill mixes, reduced setting time is expected as the cement content increases. However, when fly ash in large quantities (500 kg/m3) is used, the fly ash contributes to the setting process by shortening the setting time, and the change in the cement content is less dominant.(19;20)

Bleeding and Shrinkage: High fly ash content flowable fill mixes with relatively high water contents (250 mm (10 in) slump) tend to bleed water prior to initial set. Evaporation of the bleed water often results in shrinkage of approximately one percent of flowable fill depth. The shrinkage may occur laterally as well as vertically. No additional shrinkage or long-term settlement of flowable fill occurs once the material has reached an initial set.(7) Low fly ash content mixes, because of their high fine aggregate content and ability to more readily drain water through the flowable fill, tend to exhibit less bleeding and shrinkage than high fly ash content mixes.

Density: The density of standard flowable fill is similar to that of well-compacted soil in the range of 1850 to 2300 kg/m3 (115 to 145 lb/ft3), with the material being heaviest when first placed. High fly ash content flowable fill mixes are usually lighter than low fly ash content fills and can have densities as low as 1450 kg/m3 (90 lb/ft3). With the addition of lightweight aggregate, such as crumb rubber, flowable fill can have a relatively low density 1170 to 1570 kg/m3 (73 to 98 lb/ft3) and be considered a lightweight fill.(5) Densities as low as 400 kg/m3 (25 lb/ft3) have been achieved in mixes by using foaming agents.(21)

Hydraulic Conductivity: Hydraulic conductivity of high fly ash content flowable fill mixtures decrease with increasing cement content and are in the range of 10-6 to 10-7 cm/sec.(22;23) The hydraulic conductivity of low fly ash content flowable fill mixtures is greater than that of high fly ash content mixtures, and typically are in the 10-4 to 10-6 cm/sec range.(12) In general, hydraulic conductivity increases as the slump increases.(24) hydraulic conductivity may be reduced by adding bentonite to the mixture.(23)

DESIGN CONSIDERATIONS

Mix Design

Flowable fill mixtures traditionally have been 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 mix proportioning of flowable fill mixtures.(3)

High fly ash content flowable fill mixes are proportioned on the basis of the percentage of Portland cement (usually Type I cement) per dry weight of fly ash. A 5 percent Portland cement mix is fairly typical with a 95 percent dry weight proportion of fly ash, although in some areas self-cementing fly ash accounts for 100 percent of the cementitious material.(7) The amount of water added to the mix is a variable that is determined by the desired degree of fluidity or flowability in the mix and depends on the surface characteristics of the solids in the mixture. For most material combinations, 250 to 400 liters of water per cubic meter of flowable fill is sufficient (50 to 80 gallons per cubic yard of fill).(7) When conditioned fly ash is used, the amount of water in the fly ash must be included with the amount of added water in the mix to determine the moisture content.(6)

A broader range of mix designs exist for low fly ash content mixes.(7) Since fly ash is not the principal component in these mixes, the cement content is not based on a percentage of the dry weight of the fly ash in the mix, but as a percentage of the filler material and fly ash. Because of the rapid setting nature of Class C fly ash it is used in lesser amounts in low fly ash mixes. Flowable fill mixes are designed to develop a desired range of compressive strength. In the case of trench backfilling, a specified maximum ultimate strength in the range 690 kPa to 1035 kPa (100 to 150 lb/in2) may be the basis for design.(6) Unconfined compressive strength testing in accordance with ASTM D 4832-02 is recommended.(25)

Structural Design

Structural design procedures for flowable fill materials are no different than geotechnical design procedures used for conventional earth backfill materials. The procedures are based on using the unit weight and shear strength of the flowable fill to calculate the bearing capacity and lateral pressure of the material under given site conditions.

CONSTRUCTION PROCEDURES

Material Handling and Storage

If fly ash is to be added in a dry form (usually in low fly ash content mixes) the fly ash should be stored in a silo or pneumatic tanker. Fly ash (usually Class F fly ash) in a conditioned form in high fly ash content mixes can be stockpiled. If fly ash is stockpiled for an extended period in dry or windy weather conditions, the stockpile may need to be periodically moistened to prevent dusting.

Mixing and Placing

Flowable fills can be batched and mixed in pugmills, turbine mixers and central-mix concrete plants. High fly ash content flowable fill mixes have been mixed in rotary-mix concrete trucks or in mobile-mix vehicles. Batching and mixing in individual mobile-mix vehicles is usually done only when small quantities of flowable fill are required at a particular location. Under such circumstances, attaining a uniform distribution of cement throughout the mix may be difficult.

Central-mix concrete plants work especially well with low fly ash content mixes, in which a high percentage of sand is used. The flowable fill mix is batched as a regular concrete mix without any coarse aggregate. Pugmills are well suited for mixes prepared with ponded or conditioned ash. A second feed bin can be added to a pugmill if sand (or other filler) is used.

Portable batch plants, such as those used for grouting, are often employed for on-site mixing of flowable fill. On-site mixing using self-cementing fly ash has been done successfully with slurry jet mixers. Dry ash is stored in large tanks on site and is pneumatically discharged through Y-shaped nozzles with metered amounts of water.(18)

Flowable fill materials are most commonly transported to the site and discharged using rotary-mix concrete trucks. However, flowable fill may also be placed by pumps, conveyors, chutes, boxes, buckets, tremie, or in any way that concrete can be placed. Flowable fill requires no compaction or vibration following placement.

For placement of relatively deep backfills behind abutments or retaining walls, several lifts or layers are recommended. This limits the amount of lateral pressure exerted by the flowable fill and also prevents excessive heat of hydration, especially if self-cementing fly ash is used.(18) When flowable fill is used to backfill pipe trenches, some lighter-weight pipes, such as corrugated metal pipes, will have to be restrained to prevent floating as the flowable fill is placed. Flowable fill can be placed in flowing or ponded water because the fill will displace the water, thus eliminating the need for pumping prior to placement.

There are normally no requirements for the curing of flowable fill, although during periods of hot weather, covering the exposed surfaces of flowable fills is advised to minimize evaporation and shrinkage cracking. Temperatures within the flowable fill in excess of 90° to 100°F (32° to 38°C) are considered excessive.

Quality Control

A quality assurance program is recommended to monitor the consistency, properties, and performance of flowable fill. As a minimum, such a program should consist of initial mix design testing, determination of key mix properties (such as strength development, flowability, setting time and density), and field testing of these properties, with flowability considered the most important quality control parameter to be monitored in the field prior to placement of the material.

Special Conditions

Flowable fills do develop heat when placed, especially mixes containing self-cementing fly ash. Consequently, flowable fill can be placed at, or even below, freezing temperature. However, heated water should be used and bleed water at the fill surface should be removed. Also, a protective layer should be placed above the top surface of the flowable fill to minimize or prevent freeze-thaw damage. Ice or frozen surface material should be removed before placing additional layers of either flowable fill or pavement material.(26)

ENVIRONMENTAL CONSIDERATIONS

Leaching of substance from fly ash flowable fill can occur due to the permeation of liquid through the fill.(27) Low hydraulic conductivity of flowable fill reduces the rate at which water permeates and trace elements leach from flowable fill. Flowable fill containing fly ash is typically designed to maximize fly ash content while at the same time meeting strength requirements. Larger concentrations of fly ash correlate to a larger potential to leach trace elements.(28) Extraction procedure toxicity test results on leachate samples from flowable fill indicate that the leachate is not hazardous.(29) In a separate study on high fly ash content mixes, leachate from fly ash flowable fill was below the enforcement standards of the Wisconsin Department of Natural Resources ground-water quality standards and met practically all of the drinking water standards.(27)

REFERENCES

A searchable version of the references used in this section is available here.
A searchable bibliography of coal fly ash literature is available here.

  1. Smith A. Controlled low-strength material. Concrete Construction 1991:389-98.
  2. ACI Committee 116. Cement and concrete terminology. Detroit, Michigan: American Concrete Institute (ACI); 2000. Report nr 116R-00.
  3. ACI Committee 229. Controlled low strength materials (CLSM). Detroit, Michigan: American Concrete Institute (ACI); 1999. Report nr 229R-99.
  4. Ramme BW, Tharaniyil M. Coal combustion products utilization handbook. Milwaukee, WI: We Energies; 2004.
  5. Pierce CE, Blackwell MC. Potential of scrap tire rubber as lightweight aggregate in flowable fill. Waste Management 2003;23(3):197-208.
  6. Collins RJ; Tyson SS. Utilization of coal ash in flowable fill applications. Symposium on recovery and effective reuse of discarded materials and by-products for construction of highway facilities, Federal Highway Administration; 1993.
  7. Federal Highway Administration (FHWA), American Coal Ash Association (ACAA). Fly ash facts for highway engineers. Federal Highway Administration (FHWA); 2003. Report nr FHWA-IF-03-019.
  8. Hennis KW; Frishette CW. A new era in control density fill. Tenth international ash utilization symposium, Palo Alto, California: Electric Power Research Institute; 1993.
  9. ASTM C618-05 standard specification for fly ash and raw or calcined natural pozzolan for use as mineral admixture in Portland cement concrete. In: Annual book of ASTM standards. ASTM; West Conshohocken, Pennsylvania: 2005.
  10. ASTM C143/C143M-05a standard test method for slump of hydraulic-cement concrete. In: Annual book of ASTM standards. ASTM, West Conshohocken, Pennsylvania: 2005.
  11. ASTM C939-02 standard test method for flow of grout for preplaced-aggregate concrete (flow cone method. In: Annual book of ASTM standards. ASTM, West Conshohocken, Pennsylvania: 2002.
  12. Balsamo NJ. Slurry backfills – useful and versatile. Public Works 1987 April;118:58-60.
  13. ASTM D6103-04 standard test method for flow consistency of controlled low strength material (CLSM). In: ASTM; Annual Book of ASTM Standards, ASTM, West Conshohocken, Pennsylvania: 2007.
  14. Masada T, Sargand SM. Construction of flexible pipe system using controlled low strength material - controlled density fill (CLSM-CDF). Columbus, OH: Ohio Department of Transportation; 2001 September. Report nr FHWA/OH-2001/08.
  15. American Stone-Mix I. Physical properties of FLO-ASH. 2000.
  16. Brewer & Associates. Load transfer comparisons between conventionally backfilled roadway trenches and those backfilled with controlled low strength material -- controlled density fill (CLSM-CDF). Cincinnati, Ohio: Cincinnati Gas & Electric Company; 1991.
  17. Krell, WC. Flowable fly ash. 68th annual meeting of the transportation research board; Washington, DC: Transportation Research Board; 1989.
  18. Newman FB, Rojas-Gonzales LF, Knott DL. Current practice in design and use of flowable backfills for highway and bridge construction. Harrisburg, Pennsylvania: Pennsylvania Department of Transportation; 1992. Report nr 90-12.
  19. Katz A, Kovler K. Utilization of industrial by-products for the production of controlled low strength materials (CLSM). Waste Management 2004;24(5):501-12.
  20. 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. ASTM, West Conshohocken, Pennsylvania; 2002.
  21. Chugh YP, Chaudhuri SK, Wilcox H, Sengupta S, Jennings J. Coal combustion residues management projects. Carterville, IL: Illinois Clean Coal Institute; 1998. Report nr 97-1/3.4A-2.
  22. Glogowski PE, Kelly JM, Brendel GF. Laboratory testing of fly ash slurry. Palo Alto, California: Electric Power Research Institute; 1988. Report nr CS-5100.
  23. Gabr MA, Bowders JJ. Controlled low-strength material using fly ash and AMD sludge. Journal of Hazardous Materials 2000 9/15;76(2-3):251-63.
  24. Doven AG, Pekrioglu A. Material properties of high volume fly ash cement paste structural fill. J Mater Civ Eng 2005;17:686-93.
  25. ASTM D4832-02 standard test method for preparation and testing of controlled low strength material (CLSM) test cylinders. In: Annual book of ASTM standards. ASTM, West Conshohocken, Pennsylvania: 2002.
  26. Collins RJ, Ciesielski SK. Recycling and use of waste materials and by-products in highway construction. National Cooperative Highway Research Program Synthesis of Highway Practice 199. Transportation Research Board; Washington, DC: 1994.
  27. Naik TR, Singh SS, Ramme BW. Performance and leaching assessment of flowable slurry. J Environ Eng 2001;127(4):359-68.
  28. Gaddam R, Inyang HI, Young DT, Umoh E. An economic feasibility model for ash use in flowable fill with integration of logistics and contaminant leaching factors. International Journal of Environment and Waste Management 2006;1(1):20-38.
  29. Türkel S. Long-term compressive strength and some other properties of controlled low strength materials made with pozzolanic cement and class C fly ash. Journal of Hazardous Materials 2006 9/1;137(1):261-6.

[ Material Description ] - [ Asphalt Concrete ] - [ Portland Cement Concrete ] - [ Embankment or Fill ] - [ Stabilized Base ] - [ Flowable Fill ]

Last Update 7/28/08