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COAL FLY ASH User Guideline

Flowable Fill

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 self-leveling, self-compacting backfill material in lieu of compacted earth or granular fill. Flowable fill includes mixtures of Portland cement and filler material and can contain mineral admixtures, such as fly ash. Filler material usually consists of fine aggregate (in most cases, sand), but some flowable fill mixes may contain approximately equal portions of coarse and fine aggregates.(1) Fly ash has also been used as filler material.

The desired range of compressive strength in flowable fill mixtures depends largely on whether or not the hardened material may have to be excavated and removed at some future time. If removability is necessary, the ultimate strength of flowable fill should not exceed 1,035 kPa (150 lb/in2) or jack hammers may be required for removal.(1) For flowable fill mixes used for higher bearing capacity applications, such as structural fill or temporary support of traffic loads, a greater range of compressive strength mixtures can be designed.

Flowable fill is considered a controlled low strength material by ACI as long as its compressive strength is less than 8270 kPa (1,200 lb/in2).(2) In higher strength applications, the strength of flowable fill mixes can range from 1380 to 8270 kPa (200 to 1,200 lb/in2), depending on the design requirements of the project in question.

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.(3)

There are no specific requirements for the types of fly ash that may be used in flowable fill mixtures. "Low lime" or 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. "High lime" or Class C fly ash, because it is usually self-cementing, is almost always used only in low fly ash content flowable fill mixes.(4) There is also a flowable fill product in which both Class F and Class C fly ash are used in varying mix proportions.(4)


The use of flowable fill as a highway construction material is becoming more widespread throughout the United States. Data received from questionnaires sent by the Pennsylvania

Department of Transportation (PennDOT) in 1991(5) and the Transportation Research Board (TRB) in 1992(6) indicated that approximately 30 states had some experience with the use of flowable fill, and at least 24 states have a specification for flowable fill.(6)

Most state transportation agencies have used flowable fill mainly as a trench backfill for storm drainage and utility lines on street and highway projects. Flowable fill has also been used to backfill abutments and retaining walls, fill abandoned pipelines and utility vaults, fill cavities and settled areas, and help to convert abandoned bridges into culverts. The most frequent use of flowable fill is reported in the states of Minnesota, Maryland, Michigan, Iowa, and Indiana.(5)

Although most states have somewhat limited experience to date with flowable fill, nearly all states that have used the material have thus far indicated satisfactory performance with little or no problems. Several states have noted that metal or plastic pipes tend to float unless anchored, and some states have reported some resistance to the use of the material by contractors or engineers.(6)

Since flowable fill is normally a comparatively low-strength material, there are no strict quality requirements for fly ash used in flowable fill or CLSM mixtures. Fly ash is well suited for use in flowable fill mixtures. Its fine particle sizing (nonplastic silt) and spherical particle shape enhances mix flowability. Its relatively low dry unit weight (usually in the 890 to 1300 kg/m3 (55 to 80 lb/ft3) range) assists in producing a relatively lightweight fill, and its pozzolanic or cementitious properties provide for lower cement requirements than would normally be required to achieve equivalent strengths.


Sources Control

Fly ash used in flowable fill mixes does not have to meet strict specification requirements, such as ASTM C618 for use in concrete.(7) A high-quality source of ash is not required. In addition to dry or conditioned fly ash, reclaimed ash from settling ponds may also be suitable for use in flowable fill. No special processing of fly ash or pond ash is necessary prior to its use in flowable fill mixtures.

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 of this material.


The engineering properties of fly ash that are the most influential in its performance in flowable fill mixtures are its spherical particle shape and pozzolanic activity with Portland cement.

Some of the engineering properties of flowable fill mixes containing fly ash that are of particular interest when fly ash is used as a principal component in flowable fill mixes include compressive strength, flowability, stability, bearing capacity, modulus of subgrade reaction, lateral pressure, time of set, bleeding and shrinkage, density, and permeability.

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 Portland cement by dry weight of fly ash or ponded ash to develop 28-day compressive strengths in the 517 to 1034 kPa (75 to 150 lb/in2) range. For low fly ash content mixes, Class C 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 will probably be somewhat lowered.(3)

Flowability: Flowability or fluidity is a measure of how well a mixture will flow when being placed. The higher the water content, the more flowable the mix. The consistency of flowable fill is probably best monitored by determining the flowability of the material. Flowability can vary from stiff to fluid depending on the job requirements. Flowability can be measured using a standard concrete slump cone,(8) a flow cone,(9) or a modified flow test using an open ended 75 mm (3 in) diameter by 150 mm (6 in) high cylinder.(16) Flowability ranges associated with the standard concrete slump cone (ASTM C143) generally vary from 150 mm (6 in) to 200 mm (8 in).(8) Admixtures (such as water reducing agents) are not normally used in flowable fill.

For high fly ash content flowable fill 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.(16) The modified flow test 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 spilled 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).(16)

Stability: For low fly ash content flowable fill materials, triaxial strength tests have indicated friction angles of 20° for mixes containing fine sand and up to 30° for mixes containing concrete sand. Cohesion measured from triaxial testing has been found to vary with the compressive strength. Mixes with a 344 kPa (50 lb/in2) compressive strength have exhibited 120 kPa (2,500 lb/ft2) cohesion, while mixes with a 690 kPa (100 lb/in2) compressive strength have exhibited 200 kPa (4,200 lb/ft2) cohesion.(16)

Bearing Capacity: The allowable bearing capacity of hardened flowable fill has been shown to vary directly with compressive strength and friction angle. For example, the allowable bearing capacity for flowable fill with compressive strength of 690 kPa (100 lb/in2) may range from 78 metric tons/m2 (8 tons/ft2) at a 20° friction angle to 156 metric tons/m2 (16 tons/ft2) at a 30° friction angle.(16) This is approximately two to four times the bearing strength of most well- compacted granular soil fill materials.

California Bearing Ratio (CBR) is also a measure of the in-place bearing strength of a subgrade material compared with that of standard crushed stone. Previous testing has exhibited CBR values ranging from 40 to 90 percent.(10,11) 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 value 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 N/cm3 (50 to 300 lb/in3) for most soils and 82 N/cm2 (500 lb/in3) for a good granular subbase material. The k value for flowable fill is usually 820 N/cm2 (5,000 lb/in3) or higher, meaning it is superior to any earthen backfill it would replace.(12)

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).(5) Theoretically, once flowable fill placed against a retaining wall or abutment has hardened, the lateral fluid pressure exerted during placement and initial curing should be significantly reduced. Limited load cell instrumentation of flowable fill abutment backfills has shown that flowable fills exert lateral pressure similar to that of granular materials.(13)

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 apparent damage. Some low fly ash content flowable fill mixes, especially those containing self-cementing fly ashes, have reportedly hardened sufficiently to allow street patching within 1 to 2 hours following placement.

Bleeding and Shrinkage: High fly ash content flowable fill mixes with relatively high water contents tend to release some bleed water prior to initial set. Evaporation of the bleed water often results in a shrinkage of approximately one percent (1/8 in per ft) 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. 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 or shrinkage than high fly ash content mixes.

Density: High fly ash content flowable fill mixes are usually lighter than compacted natural soils. Typical wet density values may range from 1460 to 1945 kg/m3 (90 to 120 lb/ft3), with the material being heaviest when first placed. Low fly ash content flowable fill mixes may have wet density values ranging from 1785 to 2190 kg/m3 (110 to 135 lb/ft3).(14) Significant decreases in density (as low as 325 kg/m3 (20 lb/ft3)) have been achieved in high fly ash content flowable fill mixes by the use of foaming agents in proprietary mixtures for purposes of load reduction.

Permeability: Permeability values for high fly ash content flowable fill mixtures have been found to decrease with increasing cement content and are generally in the range of 10-6 to 10-7 cm/sec.(15) Although few data are available regarding the permeability of low fly ash content flowable fill mixtures, the permeability of such mixtures is greater than that of high fly ash content mixtures, apparently in the 10-4 to 10-6 cm/sec range.(16)


Mix Design

Proportioning of flowable fill mixtures has been developed to a great extent by trial and error. Most specifications for flowable fill provide a recipe of ingredients that will 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 CLSM mixtures.(2)

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. 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. 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.(3)

Because low fly ash content mixes contain an additional ingredient (sand or filler), there is a broader range of mix designs, compared with high fly ash content mixes. 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 on a percentage of the filler material and fly ash. If Class C fly ash were used, it would be used in lesser amounts than Class F fly ash because of its rapid setting characteristics.

Flowable fill mixes should be designed to develop a desired range of compressive strength. In the case of trench backfilling, a specified maximum ultimate strength (often in the 690 kPa to 1035 kPa (100 to 150 lb/in2) range) may be the basis for design. Unconfined compressive strength testing is recommended to be performed on cylindrical test specimens (usually 75 mm (3 in) diameter by 150 mm (6 in) height) cured in sealed plastic bags at ambient temperature for 7, 14, 28, 56 and 90 days.(3)

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.


Material Handling and Storage

If the fly ash to be used in a flowable fill mixture is to be mixed in a dry form (usually in low fly ash content mixes), the fly ash must be stored in a silo or pneumatic tanker until it is ready for use. Fly ash (usually Class F fly ash) that is to be used in a conditioned form in high fly ash content mixes can be stockpiled until it is ready to be used. 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 unwanted dusting.

Mixing and Placing

Flowable fill materials 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 ready-mix concrete trucks or in mobile-mix vehicles. Batching and mixing in individual mobile-mix vehicles is usually done only where small quantities of flowable fill are required at a particular location. Under such circumstances, it may be difficult to attain a uniform distribution of cement throughout the mix.

Central-mix concrete plants work especially well with low fly ash content mixes, in which a high percentage of sand is used. Essentially, the flowable fill mix is batched as a regular concrete mix without any coarse aggregate. Pugmills are well suited to the use of ponded or conditioned ash, although a second feed bin can be added 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 materials. 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.(5)

Flowable fill materials are most commonly transported to the site and discharged using ready-mix concrete trucks. However, flowable fill may also be placed by means of pumps, conveyors, chutes, boxes, buckets, tremie, or in any way that concrete can be placed. Flowable fill materials require 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 at any one time and also prevents excessive heat of hydration, especially if self-cementing fly ash is used.(5) Temperatures within the flowable fill in excess of 90° to 100°F (32° to 38°C) are normally considered excessive.(10)

When flowable fill is used to backfill pipe trenches, some lighter-weight pipes, such as corrugated metal pipes, will have to be tied down or in some way restrained to prevent them from floating as the flowable fill is being placed.

Flowable fill can be placed where water either flows or has accumulated and the flowable 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, it may be advisable to cover the exposed surfaces of flowable fills to minimize evaporation and the subsequent development of shrinkage cracking.

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 fill materials do develop some heat of hydration in place, especially those that contain self-cementing fly ash. Consequently, flowable fill can be placed at, or even possibly somewhat below, freezing temperature. However, heated water should be used and the excess bleed water at the flowable fill surface should have the opportunity to dissipate so that it does not freeze. 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.(6)


Although there are a wide variety of mix designs for flowable fill, some standardization of mix design methods is needed. The use of concrete admixtures (such as air-entraining agents) in flowable fill mixtures needs further investigation. More data are needed on long-term strength development of various flowable fill mix designs. More experience is needed in the setting time and rate of strength development of low fly ash content mixes containing self-cementing fly ash. Additional information is also needed on the engineering properties and performance of other by-products (foundry sand, quarry fines, etc.) as filler materials in low fly ash content mixes.


  1. Smith, Anne. "Controlled Low-Strength Material," Concrete Construction, May, 1991, pp. 389-398.

  2. American Concrete Institute. Controlled Low Strength Materials (CLSM). Report No. 229R-94, ACI Committee 229, Detroit, Michigan, July, 1994.

  3. Collins, Robert J. and Samuel S. Tyson. "Utilization of Coal Ash in Flowable Fill Applications," Proceedings of the Symposium on Recovery and Effective Reuse of Discarded Materials and By-Products for Construction of Highway Facilities, Federal Highway Administration, Denver, Colorado, October, 1993.

  4. Hennis, Kay W. and C.W. (Bill) Frishette. "A New Era in Control Density Fill," Proceedings of the Tenth International Ash Utilization Symposium, Electric Power Research Institute, Report No. TR-101774, Volume 2, Palo Alto, California, January, 1993.

  5. Newman, F. Barry, Luis F. Rojas-Gonzales, and David L. Knott. Current Practice in Design and Use of Flowable Backfills for Highway and Bridge Construction. GAI Consultants, Final Report, Research Project 90-12 for Pennsylvania Department of Transportation, Harrisburg, Pennsylvania, September, 1992.

  6. Collins, Robert J. and Stanley K. Ciesielski. Recycling and Use of Waste Materials and By-Products in Highway Construction. National Cooperative Highway Research Program Synthesis of Highway Practice No. 199, Transportation Research Board, Washington, DC, 1994.

  7. ASTM C618-92a. "Standard Specification for Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete," American Society for Testing and Materials, Annual Book of ASTM Standards, Volume 04.02, West Conshohocken, Pennsylvania, 1994.

  8. ASTM C143. "Standard Test Method for Slump of Portland Cement Concrete," American Society for Testing and Materials, Annual Book of ASTM Standards, Volume 04.02, West Conshohocken, Pennsylvania, 1994.

  9. ASTM C939, "Standard Test Method for Flow of Grout for Preplaced-Aggregate Concrete," American Society for Testing and Materials, Annual Book of ASTM Standards, Volume 04.02, West Conshohocken, Pennsylvania, 1994.

  10. American Stone-Mix, Inc. Physical Properties of FLO-ASH. Product Brochure, Towson, Maryland.

  11. Brewer & Associates. Load Transfer Comparisons Between Conventionally Backfilled Roadway Trenches and Those Backfilled with Controlled Low Strength Material -- Controlled Density Fill (CLSM-CDF). Prepared for the Cincinnati Gas & Electric Company, Cincinnati, Ohio, May, 1991.

  12. Krell, William C. "Flowable Fly Ash," Presented at the 68th Annual Meeting of the Transportation Research Board, Washington, DC, January, 1989.

  13. Newman, F. Barry, Anthony M. DiGioia, Jr., and Luis F. Rojas-Gonzalez. "CLSM Backfills for Bridge Abutments," Proceedings of the 11th International Symposium on Use and Management of Coal Combustion By-Products. Electric Power Research Institute, Report No. TR-104657, Volume 2, Palo Alto, California, January, 1995.

  14. K-Krete, Inc. K-Krete New Controlled Density Fill. Technical Information Brochure, Minneapolis, Minnesota.

  15. Glogowski, P. E., J. M. Kelly, and G. F. Brendel. Laboratory Testing of Fly Ash Slurry. Electric Power Research Institute, Report No. CS-5100, Palo Alto, California, December, 1988.

  16. Balsamo, Nina J. "Slurry Backfills Useful and Versatile," Public Works, April, 1987, pp. 58-60.

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