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Publication Number: FHWA-RD-97-148

User Guidelines for Waste and Byproduct Materials in Pavement Construction


Application Description


Flowable fill refers to a cementitious slurry consisting of a mixture of fine aggregate or filler, water, and cementitious material(s), which is used primarily as a backfill in lieu of compacted earth. This mixture is capable of filling all voids in irregular excavations and hard to reach places (such as under and around pipes), is self-leveling, and hardens in a matter of a few hours without the need for compaction in layers. Flowable fill is sometimes referred to as controlled density fill (CDF), controlled low strength material (CLSM), lean concrete slurry, and unshrinkable fill.

Flowable fill is defined by the American Concrete Institute (ACI) as a self-compacting cementitious material that is in a flowable state at placement and has a compressive strength of 8.3 MPa (1,200 lb/in2) or less at 28 days. Most current applications for flowable fill involve unconfined compressive strengths of 2.1 MPa (300 lb/in3) or less.

Flowable fill materials are primarily used in below grade applications such as utility trenches, where low strength and ease of placement are required. Flowable fill is typically placed using conventional ready-mix concrete trucks. In many cases, these materials are designed so that they are comparable in strength to the surrounding soil after hardening, making excavation at a later time possible.



Fine aggregates or fillers (usually sand) are often used in flowable fill mixtures that are produced at ready-mix plants, especially higher strength CLSM mixtures. Portland cement and/or supplementary cementitious materials and water are essential ingredients in all flowable fill mixtures, since it is the hydration of these cementitious materials that enables the flowable fill mixture to harden and develop strength.

Fine Aggregate or Filler

Fine aggregate or filler material provides the solids to develop compressive strength, as well as load-carrying capability. For purposes of this discussion, fine aggregates are materials with particles in a size range of 4.75 mm (No. 4 sieve) to 0.075 mm (No. 200 sieve), and filler refers to those materials with a size range of less than 0.075 mm (No. 200 sieve). The properties of fine aggregate or filler material that are most relevant to its use in flowable fill are its gradation and unit weight. The composite material must be sufficiently finely graded to enhance the flowability of the mix, but may also be granular enough to be able to drain some of the excess water from the mix prior to initial hardening.

Sand is the most commonly used flowable fill material, although other materials (such as coal bottom ash, fly ash, spent foundry sand, quarry fines, and baghouse dust) have also been used. Depending on the unit weight of the flowable fill material, a cubic yard of flowable fill may contain between 680 and 1400 kg (1500 and 3000 pounds) of fine aggregate or filler material.

Cementitious Materials

It is possible to use a variety of cementitious materials to produce a suitable cementitious slurry with desirable compressive strength and flow properties. These materials can be divided into three general categories. They include Portland cement, pozzolanic materials, and self-cementing materials.

Portland cements are hydraulic cements that set and harden by reacting with water, through hydration, to form a solidified mass. The amount of Portland cement in a flowable fill mix, together with the water and the quantity of Portland cement added, determines the ultimate strength of the mixture. At lower cement contents (in the 3 to 5 percent by weight range), the 28-day unconfined compressive strength of a flowable fill mixture is typically in the 0.5 to 1.0 MPa (75 to 150 lb/in2) range.

Pozzolanic materials are materials composed of amorphous siliceous or siliceous and aluminous material in a finely divided (powdery) form (similar in size to Portland cement particles) that will, in the presence of water, react with an activator (typically calcium hydroxide and alkalis) to form compounds possessing cementitious properties. Descriptions of various kinds of pozzolans and their specifications are provided in ASTM C618. Self-cementing materials are materials that react with water to form hydration products without any activator.

Coal fly ash is frequently used in flowable fill mixtures as a cementitious material and because its fine, spherical shaped particles greatly improve the fluidity or flowability of the mix. Fly ash generated during the combustion of bituminous coals exhibits pozzolanic properties and reacts with calcium oxide to form a cement paste. Fly ash generated during subbituminous coal combustion exhibits self-cementing properties and is sometimes used instead of Portland cement in areas where it is readily available.


The amount of water in a flowable fill mix has a direct effect on the flowability and strength development of the mixture. Sufficient water must be added to lubricate the solids in the mixture in order to achieve the desired degree of flowability, which is frequently related to the slump of the mix. At a given cement content, an increase in the water content usually results in a slight decrease in the compressive strength development of the mix over time. Water requirements for mixture fluidity depend on the surface properties of the solids in the mixture. A range of 250 to 400 liters per cubic meter (50 to 80 gallons per cubic yard) will satisfy most material combinations.



Fine Aggregate or Filler

Proper size and grading are needed for a fine aggregate or filler material to effectively contribute to the flowability of a flowable fill mixture. The most commonly used filled materials are sand and coal fly ash (pozzolanic), with the choice of material usually decided by availability and comparative cost. The unit weight or specific gravity of the fine aggregate or filler material determines to a great extent the unit weight or specific gravity of the resultant flowable fill. The following is a listing and brief comments on some of the more important properties of fine aggregate or filler material used in flowable fill mixtures.

Table 24-14 lists standard test methods used to evaluate fine aggregate or filler materials for use in flowable fill.

Table 24-14. Fine aggregate or filler material test procedures.

Property Test Method Reference
Gradation Particle Size Analysis of Soils ASTM D422
Unit Weight Unit Weight and Voids in Aggregate ASTM D29
Specific Gravity of Soils ASTM D854

Cementitious Materials

The type of cementitious materials used in the flowable fill mix design will play a major role in determining the final compressive strength and flowability of the mix. Some of the more important properties of cementitious materials used in a flowable fill mix include:

Table 24-15 provides a list of standard laboratory tests that are presently used to evaluate the mix design or expected performance of cementitious materials for use in flowable fill mixtures.

Table 24-15. Cementitious materials test procedures.

Property Test Method Reference
General Specifications Portland Cement ASTM C150
Blended Hydraulic Cement ASTM C595
Expansive Hydraulic Cement ASTM C845
Pozzolan Use as a Mineral Admixture ASTM C618
Fineness Fineness of Hydraulic Cement by the 150 mm (No. 100) and 75 mm (No. 200) Sieves ASTM C184/
Fineness of Hydraulic Cement and Raw Materials by the 300 mm (No. 50), 150 mm (No. 100) and 75 mm (No. 200) Sieves by Wet Methods ASTM C786
Fineness of Hydraulic Cement by the 45 mm (No. 325) Sieve ASTM C430/
Fineness of Portland Cement by Air Permeability Apparatus ASTM C204/
Fineness of Portland Cement by the Turbidimeter ASTM C115/
Setting Time Time of Setting of Hydraulic Cement by Vicat Needle ASTM C191/
Time of Setting of Hydraulic Cement by Gillmore Needles ASTM C266/
Time of Setting of Hydraulic Cement Mortar by Modified Vicat Needle ASTM C807
Compressive Strength Compressive Strength of Hydraulic Cement Mortars (Using 2-in. or 50 mm Cube Specimens) ASTM C109/
Compressive Strength of Hydraulic Cement Mortars (Using Portions of Prisms Broken in Flexure) ASTM C349
Specific Gravity Density of Hydraulic Cement ASTM C188/

As noted previously, coal fly ash can be used in several ways in flowable fill, depending on the type of fly ash and the amount that is used. Pozzolanic fly ash, when used in relatively small quantities (15 percent or less by weight of total mix) can enhance strength development and improve the flowability of the mix. When used in large quantities (up to 94 percent by weight of total mix), pozzolanic fly ash also acts as the filler in the mix. Self-cementing fly ash can be used in relatively small quantities (15 percent or less by weight of total mix) to increase strength development or reduce hardening time in combination with Portland cement, or to simply function as a substitute or complete replacement for Portland cement. In either case, the most important properties of fly ash (or other pozzolans) used in flowable fill mixes include:

Table 24-16 lists standard test methods used to evaluate pozzolans, including coal fly ash, for its suitability in flowable fill applications.

Table 24-16. Pozzolan test procedures.

Property Test Method Reference
Fineness Sampling and Testing Fly Ash or Natural Pozzolans for Use as a Mineral Admixture in Portland Cement Concrete ASTM C311
Pozzolanic Activity Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concrete ASTM C618



Flowable fill mixtures possess the characteristics of a high-quality compacted earth backfill after hardening, while resembling a very workable, lean concrete mix when produced, transported, and placed. The most important physical characteristics of flowable fill mixtures are its strength development, flowability, hardening time, bleeding and shrinkage, unit weight, bearing capacity, shear strength, and corrosion resistance.

A list of the standard test methods usually used to evaluate flowable fill materials is given in Table 24-17.

Table 24-17. Flowable fill test procedures.

Property Test Method References
Strength Development Unconfined Compressive Strength of Cohesive Soil ASTM D2166
Unconfined Compressive Strength Index of Chemical-Grouted Soils ASTM D4219
Flowability Slump of Portland Cement Concrete ASTM C143
Flow of Grout for Preplaced-Aggregate (Flow Cone Method) ASTM C939
Hardening Time Time of Setting of Concrete Mixtures by Penetration Resistance ASTM C403
Bleeding and Shrinkage Change in Height at Early Ages of Cylindrical Specimens from Cementitious Mixtures ASTM C827
Unit Weight Unit Weight, Yield, and Air Content of Concrete ASTM C138
Bearing Strength California Bearing Ratio (CBR) of Laboratory-Compactive Soils ASTM D1883
Shear Strength Unconsolidated Undrained Compressive Strength of Cohesive Soils in Triaxial Compression ASTM D2850
Direct Shear Test of Soils Under Consolidated Drained Conditions ASTM D3080
Corrosion Resistance pH of Soil for Use in Corrosion Testing ASTM G51
Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method ASTM G57
Optimum SO3 in Portland Cement ASTM C563



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

Portland Cement Association. Cementitious Grouts and Grouting. Skokie, Illinois, 1990.