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FOUNDRY SANDUser Guideline


Flowable Fill

INTRODUCTION

Ferrous spent foundry sand can be used as substitute for natural sand (fine aggregate) in flowable fill.(1;2) Natural sand is a major component of most flowable fill mixes. Foundry sand can be used as a replacement for natural fine aggregate because foundry sand consists primarily (greater than 80 percent) of fine uniform silica sand.(3)

Flowable fill or controlled low strength material (CLSM) is generally composed of a mixture of sand, fly ash from coal-fired power plants, a small amount of cement, water, and admixtures. Flowable fill is defined by the ACI Committee 229(4) as a cementitious material that is in a flowable (self-leveling) state at the time of placement and that has a specified compressive strength of 1400 kPa (200 lb/in2) or less at 28 days. The unconfined compressive strength (UCS) is typically required to be at least 300 kPa (44 psi). This strength makes it possible for the material to be removed should future excavation be necessary. The applications of flowable fill are numerous and include restoration of utility cuts, backfilling structures, filling abandoned wells, filling voids under existing pavements, and pipe embedments. (See references 5, 6, 7, 2, and 8)

The three key characteristics of flowable fill are strength, flow, and setting time.(3) These characteristics are adjusted by varying the relative proportions of the main constituents of the flowable fill (sand, cement or fly ash, and water). While recycled foundry sand is found to be a suitable substitution for natural sand, the bentonite content can vary widely in foundry sands using clay as a binder, and can lead to variable performance.(3)

The specifications in most jurisdictions for flowable fill materials require that aggregates satisfy ASTM C33.(9) While spent foundry sand may not satisfy the gradation requirements of ASTM C33 for fine aggregates, the uniform, spherical nature of the particles produces a relatively free-flowing mixture.

PERFORMANCE RECORD

Known states that have used spent foundry sand in flowable fill include New York, Pennsylvania, Ohio, Wisconsin, Tennessee, and Indiana.(10;11) Pennsylvania has reported successful use of foundry sand as a sand substitute in flowable fill, as well as Ohio where a field demonstration showed performance on par with conventional sand flowable fills.(10) However, before to 1994, Illinois tried spent foundry sand and considered such use unsuitable due to poor performance and economics.(11)

MATERIAL PROCESSING REQUIREMENTS

Crushing and Screening

Crushing spent foundry sand may be necessary to reduce the size of oversize core butts or uncollapsed molds. The spent foundry sand can also be screened and oversize material from molds and cores that have not completely collapsed should be removed.

Quality Control

For spent foundry sand to be suitable as a replacement for fine aggregate in flowable fill, the sand should be free of objectionable material such as wood, garbage, and metal that can be introduced at the foundry. In addition, the sand should be free of thick coatings of burnt carbon, binders, and mold additives that could inhibit cement hydration.

Storage and Blending

Stockpiles of sufficient size should be accumulated and blended so that a consistent gradation can be achieved before transferring the material to ready-mix concrete plants/flowable fill producers.(12) When aggregates must satisfy the requirements of ASTM C33, the spent foundry sand should be blended with natural or other suitable fine aggregate materials to meet gradation requirements. The presence of organics (from some binder systems such as bentonite clay) may exceed ASTM C33 criteria and therefore should be closely monitored.

ENGINEERING PROPERTIES

Some of the engineering properties of spent foundry sand that are of particular interest when foundry sand is used in flowable fill applications include particle shape, gradation, strength characteristics, soundness, deleterious substances, and corrosivity.

Unit Weight: The unit weight of the slurry material is in the range of 1570 to 2115 kg/m3 (98 lb/ft3 to 132 lb/ft3).(13;14)

Particle Shape: The grain size distribution of spent foundry sand is more uniform and somewhat finer than conventional concrete sand.(15) The fineness of spent foundry sand contributes to good suspension, limiting segregation of flowable fill. The spherical shape of spent foundry sand particles contributes to good flow characteristics.

Gradation: Spent foundry sand may not satisfy the ASTM C33 gradation requirement; therefore, it may need to be blended with natural sand or other suitable fine aggregate materials to meet the requirements.

Strength Characteristics: Some organic binder materials can interfere with cement hydration, which produces a low strength development that in most cases is more desirable with flowable fill to permit excavation at a later date. Flowable fill incorporating spent foundry sand aggregates readily satisfies specified limited strength criteria.(3) The addition of fly ash tends to reduce the strength, reduce bleed, and increase setting time of the flowable fill.(16) Bentonite has the effect of reducing the amount of fly ash and increasing the amount of Portland cement necessary to meet strength requirements.(16)

The unconfined compressive strength (UCS) is a function of the water-to-cement ratio as well as the proportion of foundry sand in the mixture.(1;14) Figure 3 shows 7, 28, and 91-day UCS results for varying proportions of foundry sand and fly ash. For low strength mixtures, the UCS is affected by the cement content more strongly than the water-cement ratio (W/C).(17) A large drop in UCS occurs between a W/C of 4 to 6.5, whereas for W/C > 6.5, the UCS is typically in the range of 0.3 to 1.0 MPa (44 lb/in2 to 145 lb/in2).(3)

Figure 7-3
Figure 3. Compressive strength versus age for fly ash and spent foundry sand flowable fill.(14)

An empirical relationship was determined between water-cement ratio and 28-day UCS based on two dozen tests using differing types of foundry sand and Class F fly ash: (1)

Figure 7-3

where Sc is the 28-day unconfined compressive strength in kPa and W/C is the water-cement ratio. Although long-term strength gain can be estimated by the 28-day strength, the increase in 90-day strength is approximately 20 to 30 percent greater than the 28-day strength.(1;3;18)

Flow Behavior: A typical target flow for flowable fill is 230 mm ± 5 mm (9 in ± 0.2 in).(3) Mixtures with foundry sand require considerably more water than mixtures prepared with river sand or base silica.(1) Grain size and bentonite content primarily affect water demand. Finer average grain sizes demand less water, while higher bentonite contents increase water demand.(18) At a bentonite content between 10 and 12 percent, much greater water requirements are necessary as the impedance to flow caused by the bentonite dominates.(3) Fly ash concentration was found to have little affect on flowability apart from a decrease in water demand at low concentrations.(1)

Soundness: The performance of spent foundry sands in soundness tests depends on the amount of clay binder materials present in the spent foundry sand, the amount of agglomeration of the fines, and the coating on the individual particles. The greater amount of clay binder or agglomeration, or the thicker the coatings, the higher the soundness loss. Regardless, spent foundry sands generally exhibit favorable performance in soundness testing, with soundness losses less than 10 percent (indicative of durable aggregate).(15)

Deleterious Substances: Poorly managed spent foundry sand can contain objectionable materials such as wood, garbage, metal, carbon, and dust as well as large chunks of sand. For use in flowable fill, spent foundry sand must be managed to ensure that the sand is clean and processed to the proper sand size. Because both spent foundry sand and fly ash contain porous carbon that uptakes water, to maximize the use of foundry sand, fly ash should be used in lesser amounts, which therefore uses less water and cement.(19) Organic content interferes with hydration of the cement and subsequent strength. The organic content of aggregate can be measured by a color test.(20)

Corrosivity: Depending on the binder and type of metal cast, the pH of spent foundry sand can vary from approximately 4 to 8:(21) therefore, some foundry sand can be corrosive to metals.(22) Others have indicated that flowable fill mixes containing spent foundry sand are noncorrosive because of the absence of chlorides and high pH (11.1 to 12.3).(1)

Lateral Earth Pressures: For retaining wall applications of flowable fill, the coefficient of lateral earth pressure immediately after backfilling can be assumed to be 0.8 - 1.0. The earth pressure decreases very rapidly as the fill gains strength. After 12 hrs, the coefficient of lateral earth pressure can be assumed to be zero.(23)

Hydraulic conductivity: The average hydraulic conductivity of foundry sand flowable fill is between 7.2x10-5 and 2.6x10-6 cm/s.(1;14;13) Lower hydraulic conductivity occurs with higher cementitious content. Higher fly ash mixtures exhibit a decreased permeability likely due to densification of material microstructure from pozzolanic reaction of the fly ash (Table 6).

Table 6. Average permeability of slurry mixes as a function of relative proportions of foundry sand and fly ash.(13)

Foundry Sand (%) Fly Ash (%) Hydraulic conductivity (x10-6 cm /s)
30 70 4.8
50 50 6.5
70 30 12.6
85 15 64.9

DESIGN CONSIDERATIONS

Mix Design

Flowable fill mixes are usually designed on the basis of compressive strength, generally after 28 days of ambient temperature curing, but sometimes on the basis of longer term (90 days or more) strength. Mixes are designed to have high fluidity during placement (typical slump of 150 mm to 200 mm (6 to 8 inches)) and to develop limited strength (typically between 340 kPa and 1400 kPa (50 and 200 lb/in2)), which is sufficient to support traffic without settling, yet can be readily excavated.(24)

Many jurisdictions specify the use of fine aggregates conforming to ASTM C33 in flowable fill, which generally precludes using spent foundry sand unless it is blended with natural sand or other suitable materials.

For bentonite contents greater than 6 percent, no fly ash is necessary because the bentonite will be sufficient to prevent segregation. For bentonite contents less than 6 percent, some fly ash is necessary. The mixes in Table 7 can be used as starting point for mix design.

Table 7. Recommended mix design of flowable fills containing foundry sand.(3)

Mixture Description Water (kg/m3) Foundry Sand (kg/m3) Cement (kg/m3) Fly Ash (kg/m3)
No Fly Ash, BC = 6% - 10% 475 1300 50 0
No Fly Ash, BC = 10% - 13% 500 1225 45 0
Fly Ash, BC = 0% - 6% 475 1000 - 1250 40 150 - 400

Structural Design

Structural design procedures for cured flowable fill materials are no different than geotechnical design procedures 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

The same methods and equipment used to mix, transport, and place flowable fill made with conventional aggregates may be used for flowable fill incorporating spent foundry sand.

Material Handling and Storage

The same general methods and equipment used to handle conventional aggregates are applicable for foundry sand. Laboratory studies indicate that several organic compounds are present in spent foundry sand but leachable only at low concentrations. Studies on green sands have demonstrated lower organic compound leaching potential compared to chemically bonded systems.(25) With the presence of phenols in chemically bonded foundry sands, there is a possibility that leachate from stockpiles could result in phenol discharges.(21;15;26) Because of the high temperatures encountered during the molding process, residual organic compounds in spent foundry sands are found only in small quantities. Therefore, spent sand, after casting, typically does not contain organic contaminants above regulatory threshold levels, however, fresh casting mixtures and core sand that have not been in contact with hot metal may contain organic contaminants.(25)

Mixing, Placing, and Compacting

Flowable fill can be produced at a central concrete mixing plant in accordance with ASTM C94(9) and delivered by concrete truck mixers or using a mobile, volumetric mixer for small jobs. High fluidity (slump greater than 150 mm (6 in)) should be maintained to ensure that the flowable fill material entirely fills all voids beneath pavements and around structures and utilities.

Quality Control

Various standard field and laboratory tests for flowable fill mixes are given by AASHTO T027,(27) ASTM D6103-04 Standard Test Method for Flow Consistency of Controlled Low Strength Material (CLSM),(28) ASTM D6023-02 Standard Test Method for Unit Weight, Yield, Cement Content, and Air Content (Gravimetric) of Controlled Low Strength Material (CLSM).(29)

ENVIRONMENTAL

Leaching of metals from flowable fill is a long process due to the low permeability of the material. Some leaching will occur, however, through diffusion and the permeation of liquid through the mix if the permeability is high enough. The results of a study of leachates from flowable fill mixtures containing foundry sand and Class F fly ash are presented in Table 8. The leachate results of these CLSM materials were below the enforcement standards of the Wisconsin Department of Natural Resources ground-water quality standards and also met practically all the parameters of the drinking water standards.

Table 8. Leachate characteristics of fly ash with and without foundry sand.(14)

Parameter Concentration (mg/L) Drinking water standards (mg/L) GWQS
Enforcement standard (mg/L) Prevention action limit (mg/L)
Foundry Sand (%) 0 50 70 85      
Iron <0.01 <0.01 <0.01 <0.01 - 0.30 0.15
Barium 0.79 0.88 0.62 0.48 1.0 2.0 0.4
Magnesium <0.03 <0.03 <0.03 <0.03 - 0.05 0.025
Zinc <0.01 <0.01 <0.01 <0.01 - 5 2.5
Arsenic <0.001 <0.001 <0.001 <0.001 0.05 0.05 0.005
Chromium 0.036 0.018 0.023 0.021 0.05 0.1 0.01
Lead <0.001 <0.001 <0.001 <0.001 0.05 0.015 0.0015
Selenium 0.008 0.01 0.015 0.007 0.01 0.05 0.01
Cadmium <0.0002 <0.0002 <0.0002 <0.0002 0.01 0.005 0.0005
Mercury <0.0002 <0.0002 <0.0002 <0.0002 - 0.002 0.0002
Chloride <1.0 <1.0 1 1   250 125

UNRESOLVED ISSUES

Most existing specifications require that the fine aggregate for flowable fill satisfy ASTM C33. Since foundry sand does not meet the gradation requirements of this standard, there is a need to review gradation requirements and investigate the impact of alternative gradations to permit wider use of spent foundry sand for this application.

REFERENCES

A searchable version of the references used in this section is available here.
A searchable bibliography of foundry sand literature is available here.

  1. Bhat ST, Lovell CW. Design of flowable fill: Waste foundry sand as a fine aggregate. Transp Res Rec 1996(1546):70-8.
  2. Ambroise JA, Amoura, Pera J. Development of flowable high volume - fly ash mortars. In: 11th international symposium on the use and management of coal combustion by-products (CCBs). American Coal Ash Association; 1995.
  3. Dingrando JS, Edil TB, Benson CH. Beneficial reuse of foundry sands in controlled low strength material. Journal of ASTM International 2004;1(6):15-30.
  4. ACI Committee 229. Controlled low strength materials. Concrete International 1994;16(7):55-64.
  5. Adaska WS, Krell WC. Bibliography on controlled low-strength materials (CLSM), Concrete International 1992;14(10):42-3.
  6. Naik TR, Ramme BW, Kolbeck HJ. Filling abandoned underground facilities with CLSM fly ash slurry. Concrete International 1990;12(7):19-25.
  7. Larsen RL. Sound uses of CLSM in the environment. Concrete International 1990;12(7):26-9.
  8. Newman FB, Di Gioia AM, Rojas-Gonzalez LF. CLSM backfills for bridge abutments. 11th International Symposium on the use and Management of Coal Combustion by-Products (CCBs) 1995;2.
  9. ASTM C33-03 standard specification for concrete aggregates. In: Annual book of ASTM standards. West Conshohocken, Pennsylvania: ASTM; 2003.
  10. Smith E. A review of the literature on the beneficial use of spent foundry sand in flowable fill. The Pennsylvania State University: Dr. Paul J. Tikalsky; 1996.
  11. Collins RJ, Ciesielski SK. Recycling and use of waste materials and by-products in highway construction. Washington, DC: Transportation Research Board; 1994. Report nr National Cooperative Highway Research Program Synthesis of Highway Practice 199.
  12. Leidel DS, Novakowski M, Pohlman D, MacRunnels ZD, MacKay MH. External beneficial reuse of spent foundry sand. AFS Transactions 1994;102.
  13. Naik TR, Singh SS. Permeability of flowable slurry materials containing foundry sand and fly ash. J Geotech and Geoenvir Engrg 1997;123(5):446-452.
  14. Naik TR, Singh SS. Performance and leaching assessment of flowable slurry. J Environ Eng 2001;127(4):p359.
  15. Emery J, Canadian Foundry Association. Spent foundry sand - alternative uses study. Queen’s Printer for Ontario: Ontario Ministry of the Environment and Energy (MOEE); 1993.
  16. Tikalsky P, Gaffney M, Regan R. Properties of controlled low-strength material containing foundry sand. ACI Materials Journal 2000;97(6).
  17. Abichou T, Edil TB, Benson CH, Bahia H. Beneficial use of foundry by-products in highway construction. In: Geotechnical engineering for transportation projects: Proceedings of geo-trans 2004, jul 27-31 2004. Los Angeles, CA, United States: American Society of Civil Engineers, Reston, VA 20191-4400, United States; 2004. ID: 32; Compilation and indexing terms, Copyright 2006 Elsevier Inc. All rights reserved.
  18. Tikalsky P, Bahia H, Deng A, Snyder T. Excess foundry sand characterization and experimental investigation in controlled low-strength material and hot-mixing asphalt. U.S. Department of Energy; 2004. Report nr Contract No. DE-FC36-01ID13974.
  19. Javed S, Lovell CW. Use of waste foundry sand in highway construction. Department of Civil Engineering, Purdue University; 1994 July, 1994. Report nr C-36-50N.
  20. Federal Highway Administration. Foundry sand facts for civil engineers. Federal Highway Administration (FHWA); 2004 May 2004. Report nr FHWA-IF-04-004.
  21. Johnson CK. Phenols in foundry waste sand. Modern Casting 1981:273.
  22. Emery J. Mineral aggregate conservation - reuse and recycling. Queen’s Printer for Ontario: Ontario Ministry of Natural Resources (MNR); 1992 February, 1992.
  23. Lee K, Cho J, Salgado R, Lee I. Retaining wall model test with waste foundry sand mixture backfill, GTJODJ 2001;24(4):401-408.
  24. PCA. Cementitious grouts and grouting. Skokie, Illinois: Portland Cement Association; 1990.
  25. Winkler E, Bol’shakov AA. Characterization of foundry sand waste. Chelsea Center for Recycling and Economic Development, University of Massachusetts; 2000 October 2000. Report nr 31.
  26. Ham RK, Boyle WC, Engroff EC, Fero RL. Determining the presence of organic compounds in foundry waste leachates. Modern Casting 1989.
  27. AASHTO. Sieve analysis of fine and coarse aggregates, part II tests. Washington, DC 20001; Washington, DC 20001: American Association of State Highway and Transportation Officials; 2006. Report nr T027-06.
  28. ASTM D6103-04 standard test method for flow consistency of controlled low strength material (CLSM). In: West Conshohocken, Pennsylvania: ASTM; 2007.
  29. ASTM D6023-02 standard test method for unit weight, yield, cement content, and air content (gravimetric) of controlled low strength material (CLSM). In: West Conshohocken, Pennsylvania: ASTM; 2007.

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Last Update 7/28/08