Flowable Fill

What is flowable fill?

Flowable fill is another name for Controlled Low Strength Material (CLSM), a concrete technology that is used instead of compacted soil in a variety of construction settings and utility work. Flowable fill is normally produced in conventional ready mix concrete plants, although some specialized volumetric mixers have been developed just for this application. Many CLSM mixes are designed to be excavatable, with compressive strengths ranging from 50 to 300 psi, considerably below standard concrete.   Other CLSM mixes have compressive strengths ranging up to 1200 psi and are used in structural fill applications, but one of the characteristics that distinguishes all CLSM mixes is that they must flow easily and require no additional placement handling.

Flowable fill has a number of advantages over conventional soil or granular fills in many types of construction applications.  CLSM is delivered and placed by ready mix concrete trucks, avoiding the need for other construction equipment on site.  Flowable fill is self-compacting and self-leveling, allowing it to fill many types of void spaces that are difficult to reach with conventional equipment. That provides advantages to the job owner and contractor both in terms of time on the job and worker safety.  Flowable fill cures quickly and is an ideal material to be used for utility trenching and road cuts where it is important to restore traffic as quickly as possible.  Some municipalities specify flowable fill in all routine utility and roadway maintenance work.

How is foundry sand used in flowable fill?

Flowable fill mixtures consist of sand, water, cement and sometimes fly ash. The fine aggregate component of CLSM mixes generally ranges from 40 to 50 percent of the mix. Foundry sand is used as a substitute for the virgin sand, typically in rations that range from 1:3 to 1:1 parts of foundry sand to virgin sand. Foundry sand has been shown to be an excellent fine aggregate for flowable fill production, often increasing the flowability of the mixture because the finer sand particles do not segregate as easily. Sometimes other industrial materials such as fly ash or ground steel slag are used as fine aggregates in CLSM mixtures. Because it has no cementitious properties, flowable fill mixes with foundry sand can be easier to proportion than those with other industrial materials.

In some mix designs, foundry sand can be used as a 1:1 replacement for virgin sand, entirely replacing the use of virgin aggregate. For ready mix concrete producers with local sources of foundry sand available, use of foundry sand in lieu of virgin sand can help them produce more sustainable flowable fill products, often at lower cost.

How does flowable fill perform using foundry sand?

Foundry sand is essentially a high quality natural sand, and so it makes very good fine aggregate for flowable fill. Foundry sand in flowable fill has been extensively tested in a number of state DOT-funded research studies, as well as two national studies sponsored, respectively, by the National Highway Cooperative Research Program (NCHRP) and the U.S. Department of Energy’s Metalcasting Industries of the Future program.   

Both laboratory testing and actual field use have shown that foundry sand can be used to make good quality flowable fill that meets American Concrete Institute (ACI) standards (ACI 229). Demonstration projects have been conducted in Ohio, Indiana, Pennsylvania, Texas and Wisconsin, as well as other states. The results for the Ohio tests were successful and led to the specification entitled "Flowable Fill Made with Spent Foundry Sand."

US EPA has a Comprehensive Procurement Guideline (CPG) requiring the use of ferrous foundry sands and/or coal fly ash in Federally funded flowable fill projects which have more than $10,000 in Federal dollars invested. (Note:  The CPG was developed prior to EPA’s comprehensive risk assessment, which also shows that aluminum foundry sands are safe for construction uses.)  An AASHTO draft specification for the use of foundry sand in CLSM was developed as part of a Department of Energy-funded research study. (Both of these references are accessible through the RESOURCE link below).

What Are the technical Issues associated with foundry sand in flowable fill?

Flowable fill made with clay-bonded foundry sands (“green” sands) typically has a lower compressive strength than the same mixture made with pure virgin sand. Since many flowable fills are designed with future excavation in mind, a lower compressive strength is often an advantage in mix design.

Studies have shown that flowable fill made with foundry sands often has superior flowability to CLSM made with conventional construction sands. Grain fineness is a key determinant for optimal flowability in flowable fill mix designs, along with water content. Because foundry sands are generally finer than conventional construction sands, less water is required to meet flowability targets.  When clay-bonded foundry sands are used in CLSM mixes, the need to add additional fines from fly ash or other materials is also reduced.

CLSM made with foundry “green” sands are sensitive to the water-cement ratios but this can be an advantage because the most expensive component of flowable fill is the cement.  Research has shown that foundry sands with bentonite contents above 6 percent are particularly good candidates for flowable fill so long as hydration of the bentonite is part of the mixing and placement process.

Are there any specific QA/QC issues that suppliers and/or end users need to be aware of?

Foundry sands to be used in flowable fills should be tested for gradation; absorption; angularity; and bulk specific gravity. As is the case with many other applications, foundry sands destined for use in flowable fill and other concrete products must be screened to be free of extraneous materials, including tramp metal, core butts, and general refuse. Core butts can be crushed and blended into the sand mixture so long as the material is returned to grain size.  

The amount of bentonite clay binder and the shape of the grains play a role in the properties of the fresh and cured flowable fill.  A higher percentage of passing 200 sieve size can be accommodated in flowable fill than in some other applications. Suppliers and end users need to confirm that the properties of the foundry sand remain consistent so that there is no change in properties of the flowable fill. Numerous studies have shown that foundry sand from individual sources will have consistent physical and chemical properties. If bleeding is a critical factor, admixtures or air entrainment may need to be added.

Are there any specific environmental issues associated with the use of foundry sand in flowable fill?

No. Extensive testing has been done both on the flowable fills and on the bleed waters from flowable fills made with foundry sands. US EPA has determined that properly screened sands from iron, steel and aluminum foundries are safe for use in construction projects. In fact, US EPA has a Comprehensive Procurement Guideline to encourage the use of ferrous foundry sands in flowable fill projects. Some restrictions may apply for other non-ferrous sands (other than aluminum) as some have been found to have metal leachate concentrations above RCRA standards.