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FOUNDRY SANDUser Guideline


Asphalt Concrete

INTRODUCTION

In the United States, asphalt concrete is used to cover over 2 million miles of roadway, accounting for over 94 percent of all pavements.(1) Foundry sand has been used successfully as a partial replacement for the fine aggregate in hot mix asphalt pavements (HMA).(1;2;3;4) Some general material characteristics must be met when using foundry sand in asphalt concrete pavements. These user guidelines will summarize these characteristics and suggest useful considerations for design and construction of asphalt concrete pavements using spent foundry sand.

PERFORMANCE RECORD

Recycled foundry sand has successfully been used in HMA in Pennsylvania, Michigan, and Tennessee.(1) Pennsylvania DOT allows the use of 8 to 10 percent foundry sand in asphalt mixtures.(1) One asphalt producer in Michigan consistently supplies HMA with 10 to 20 percent recycled foundry sand to replace conventional aggregate and meets Michigan DOT specifications.(1) In Tennessee, HMA with 10 percent foundry sand had been reported to compact better and outperform HMA containing washed river sand.(1) Foundry sand has also been used by a hot mix producer in Ontario, Canada since 1994 in both foundation and surface HMA layers.(1)

Superpave performance tests in Wisconsin found a potential for positive performance in using recycled foundry sand. In particular, the stability of mixes with recycled foundry sand can be higher than HMA with conventional sand; moisture resistance was higher than mixes with conventional sand; and some mixes demonstrated increased resistance to rutting, while others did not.(5)

MATERIAL PROCESSING REQUIREMENTS

Crushing and Screening

Crushing and screening of spent foundry sand may be necessary to reduce the size of any oversize core butts or uncollapsed molds prior to use as aggregate. This is readily accomplished using conventional aggregate processing equipment (closed loop crushing and screening process equipped with magnetic separating capabilities). Manufacturing consistency (primarily gradation) is important for HMA production. Variations in sand composition that effect gradation between foundries require that spent foundry sands be examined and evaluated on a source-specific basis.

Quality Control

Sands with fines contents (passing #200) larger than 10 percent may need to be processed to abide by AASHTO M 29. Angularity of the foundry sand can be measured by the fine aggregate angularity test in accordance with AASHTO T 33.(6)

For spent foundry sand to be suitable as a partial replacement for natural fine aggregates in asphalt pavements, it should be free of objectionable materials such as wood, garbage, and metal, which can be introduced at the foundry. Spent foundry sand should also be free of thick coatings of burnt carbon, binders, and mold additives. These constituents can inhibit adhesion of the asphalt cement binder to the foundry sand. Clay clumps can be removed by screening and/or washing, while iron and rubbish can be removed with magnets and/or hand separation.(6)

The clay content and organic-based additives should be quantified and limited when producing an asphalt mix containing spent foundry sand. For many foundry sands, the sand equivalent test (ASTM D2419) is not applicable.(1) Instead, the methylene blue test (AFS 2211-00-S) is a better method for determining the clay content. The loss of ignition test (AASHTO T 267) is a good method for quantifying the fraction of organic-based additives.(1)

Storage and Blending

Stockpiles of sufficient size should be accumulated so that product uniformity can be achieved. This may necessitate the accumulation of a substantial quantity of spent foundry sand in a central site at a specific foundry or group of foundries before transferring the material to hot mix producers.

To satisfy the gradation requirements for HMA fine aggregates (AASHTO M 29),(7) the spent foundry sand must be blended with natural sand at the hot mix plant.

ENGINEERING PROPERTIES

Some of the properties of spent foundry sand that are of particular interest when used in asphalt paving applications include particle shape, gradation, angularity, absorption, clay content, durability, and plasticity. With the exception of gradation, clean, processed foundry sands can generally satisfy the physical requirements for HMA fine aggregate (AASHTO M 29).

Particle Distribution: The grain size distribution of spent foundry sand is very uniform, with approximately 85 to 95 percent of the material between 0.6 mm and 0.15 mm (No. 30 and No. 100) sieve sizes.

Gradation: The gradation tends to fall within the limits of a poorly graded fine sand that has relatively uniform size (passing 0.3 mm and retained 0.15 mm), with fines content ranging from 5 to 15 percent. Spent foundry sands have also exhibited a range of P-200 between a minimum of 0.9 percent and a maximum of 16.5 percent.(8) This range is caused by the different amounts of bentonite clay used by foundries.

Angularity: Natural sand has generally been limited to 15 percent by mass of the aggregate blend to minimize round smooth particles that are easier to compact under traffic loads. Recent HMA mixtures have focused on the angularity of fine aggregate. Waste foundry sands have shown a range in angularity between 39.4 and 48 percent which is in the range of low to medium traffic level volumes (minimum 40 percent) to high traffic level volumes (minimum 45 percent).(8) The high angularity of the material appears to be the result of rough surface texture and irregular rounded shapes. The surface texture is the result of burnt additives strongly adhering to the surface.(8)

Absorption: Foundry sand has an absorption between 0.73 and 6.20 percent.(1) In general, a higher percentage of fines contributes to higher absorption percentages which increases both quality and cost.(8)

Durability: Spent foundry sands display good durability characteristics with resistance to weathering.(9)

Plasticity: Spent foundry sand generated by foundries using green sand molding systems, in which bentonite clay and sea coal are added to the casting, should be examined to ensure that plasticity values are below a plasticity index of 4, the maximum allowed by AASHTO M 29. A study of Indiana foundry sand showed that eight out of 10 green sands tested had PI values below 4, yet others have found PIs ranging from non-plastic to as high as 12 percent.(2;10;11) Therefore, the plasticity index of spent foundry sand should be determined before using the sand in an asphalt mix.

Stripping: Stripping is one of the more critical properties that should be assessed when foundry sand is incorporated into an asphalt mix. Spent foundry sand is composed primarily of silica sand, coated with a thin film of burnt carbon, residual binder (bentonite, sea coal, resins), and dust. The hydrophilic nature of the (primarily silica) foundry sand, however, can result in stripping of the asphalt cement coating surrounding the aggregate grains, with resulting loss of fine aggregate and accelerated pavement deterioration. This problem can be mitigated by limiting the content of spent foundry sand in the mix to 15 percent of the total mass of aggregate or using an antistripping additive. Hydrated lime has been used to treat the aggregate against moisture damage.(8)

DESIGN CONSIDERATIONS

Mix Design

Asphalt mixes containing foundry sand can be designed using standard asphalt mix design methods. As expressed previously, the amount of foundry sand used in an asphalt mixture depends largely on the amount of fines in the foundry sand. Studies have shown that foundry sand can be used to replace between 8 and 25 percent of the fine aggregate content in asphalt mixes.(1) Depending on the characteristics of the foundry sand, as much as 19 percent by weight can be used to replace virgin fine aggregates in asphalt mixture. The acceptable amount of foundry sand however can be much lower when the fines content is high or the active clay is higher. It is recommended that foundry sand be considered as another source of aggregates that can be mixed to produce good performance asphalt mixes.(8) HMA pavements with more than the recommended amount of spent foundry sand (blended with natural sand) can be susceptible to moisture damage due to the hydrophilic nature of the (primarily silica) foundry sand, resulting in stripping of the asphalt cement coating surrounding the aggregate grains, loss of fine aggregate, mixture brittleness, and a consequent premature cracking.(2)

The potential for stripping of asphalt mixes containing spent foundry sand should be assessed in the laboratory as part of the overall mix design. Several tests are available, with the most common including: AASHTO T 283(12) which compares the tensile strength ratio of wet and dry specimens; AASTHO T 182,(13) AASHTO T 195,(14) or the Immersion Marshall test following the MTO LS-283(15) procedure, which compares the retained Marshall stability and visual appearance of Marshall briquettes before and after immersion in a heated water bath. The problem of stripping can also be mitigated by adding hydrated lime or commercially available anti-stripping additives.

The optimum asphalt content for HMA mixtures containing various types of foundry sands rages between 5 and 6.2 percent.(8) This is comparable to the asphalt content of mixes not containing foundry sand. As expected, the addition of foundry sand does not significantly change the optimum asphalt content of mixtures.(8;16)

Structural Design

Conventional AASHTO pavement design methods are appropriate for asphalt paving incorporating spent foundry sand as fine aggregate.

CONSTRUCTION PROCEDURES

Material Handling and Storage

The same methods and equipment used for conventional HMA pavement are applicable to pavements containing spent foundry sand. If the foundry sand is dry (less than 5 percent moisture), the sand can be metered directly into a pugmill (batch plants only) or through a recycled asphalt feed (drum plants) where the sand can be further dried, if necessary, by the already heated conventional aggregates.(19)

Foundry sand, which is usually obtained in a dry form, can be stored in covered structures to preserve this condition and reduce energy required for drying. Special measures may be required to control the leachate (containing phenols) from open stockpiles (including temporary stockpiles).(12) The use of an impervious pad (to collect surface moisture or precipitation passing through the stockpile) and subsequent filtration (using an activated carbon filter) of the leachate has proven to be effective (but potentially expensive) in limiting the phenol concentration of the discharge.(6,7)

Mixing, Placing, and Compacting

The same methods and equipment used for conventional hot mix asphalt pavement are applicable to pavements containing spent foundry sand. If it is dry (less than 5 percent moisture), spent foundry sand can be metered directly into a pugmill (batch plants only) or through a recycled asphalt feed (drum plants) where it can be further dried, if necessary, by the already heated conventional aggregates.(13)

The presence of bentonite and organic binder materials can increase the time required for drying and can increase the load on the hot mix plant dust collection system (baghouse). Any coal and organic binders that are present are usually combusted in the process.

The same methods and equipment used for placing and compacting conventional pavements are applicable for pavements incorporating foundry sand.

Quality Control

The same field testing procedures used for conventional HMA mixes should be used for mixes containing foundry sand. Mixes should be sampled in accordance with AASHTO T 168,(20) and tested for specific gravity in accordance with ASTM D2726(21) and in-place density in accordance with ASTM D2950.(22)

PROPOSED USER GUIDELINES

Although recycled foundry sand can be successfully incorporated into asphalt designs, large variability can exist between sands. Each sand be treated as a unique source of aggregate.(16) Furthermore, if the sand contains sodium silicate as a binder, the sand should be rejected. Foundry sand containing bentonite can be washed to reduce the fine content that effects performance.(16)

The following steps are suggested for asphalt contractors to introduce foundry sand in HMA mixtures in accordance with the Superpave level 1 (volumetric) design method.(8)

STEP 1: Select gradation and conventional materials that are known to perform well based on past experience.

STEP 2: Obtain a representative sample of the foundry sand and conduct duplicate measures of angularity, absorption, sieve analysis dry and wet, and clay content.

STEP 3: Use an optimization technique to determine optimum amount of sand to be used. The technique should include the gradation of all the materials that are going to be used, including the foundry sand. In addition, constraints for the graduation and the physical properties of fine blend or components should be included. The optimization technique suggested includes gradation limits, cost constraints, minimum angularity requirements, maximum absorption requirements, and maximum clay content requirements.

STEP 4: After obtaining the optimal amounts of foundry sand and aggregates to meet the gradation and the physical property requirements, mix design procedure should be carried out. Select 3 or 4 asphalt contents based on previous tests to mix with the aggregates. Select compaction levels that meet the traffic volume and the local air temperature range following the Superpave volumetric mixture design procedure.

STEP 5: Following the mixing and compaction of the HMA mixtures containing foundry sand, measurements of the samples volumetric properties should be completed. Based on the volumetric results, an optimal asphalt content should be determined to achieve 4 percent air voids. If the volumetric properties failed to meet the Superpave mix design requirements, reexamination of the gradation and the optimization process should be performed. Return to step 1 for selecting new gradation and/or materials.

STEP 6: Hot mix asphalt mixtures at the optimum asphalt content should be mixed and compacted to achieve 7 percent air voids following the AASHTO T 283(12) test procedure. The samples should be examined for moisture susceptibility. If the samples failed, an anti-stripping additive, such as hydrated lime, should be added to the mix, and reexamination should take place.

UNRESOLVED ISSUES

There is a need to establish standard methods of assessing the suitability of spent foundry sand for HMA use. The Immersion Marshall test appears to be appropriate for assessing stripping potential. Also, the influence of spent foundry sand on the moisture susceptibility of HMA should be studied further. The University of Wisconsin-Madison is currently developing better methods to quantify clay and organic-based additives to predict how foundry sands influence moisture damage.(1) There is also a need to further understand the potential for phenol discharges from foundry sand stockpiles, and to determine appropriate treatment strategies, if necessary.

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. Federal Highway Administration. Foundry sand facts for civil engineers. Federal Highway Administration (FHWA); 2004 Report nr FHWA-IF-04-004.
  2. Javed S, Lovell CW, Wood LE. Waste foundry sand in asphalt concrete. Transportation Research Record 1994(1437):27-34.
  3. Javed S, Lovell CW. Use of waste foundry sand in highway construction. Department of Civil Engineering, Purdue University; 1994 Report nr C-36-50N.
  4. 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.
  5. Delange K, Braham A, Bahia H, Widjaja M, Romero P, Harman T. Performance testing of hot mix asphalt produced with recycled foundry sand. In: Annual Meeting of the Transportation Research Board; 2001.
  6. Hughes C. Recycled foundry sand in the mix. Hot Mix Asphalt Technology 2002:29-38.
  7. AASHTO. Fine aggregate for bituminous paving mixtures, part I specifications. Washington, DC 20001: American Association of State Highway and Transportation Officials; 2003. Report nr M 029-03.
  8. Miller E, Bahia HU, Benson C, Khatri A, Braham A. Utilization of waste foundry sand in hot mix asphalt mixtures. American Foundry Society Transactions 2001;103(1):1393-407.
  9. 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.
  10. Abichou T, Benson C, Edil T. Beneficial reuse of foundry sands in construction of hydraulic barrier layers. Madison, WI: Dept. of Civil and Environmental Engineering, University of Wisconsin-Madison; 1998. Report nr Environmental Geotechnics Report 98-2.
  11. Goodhue MJ, Edil TB, Benson CH. Interaction of foundry sands with geosynthetics. J Geotech Geoenviron Eng 2001;127(4):353-62.
  12. AASHTO. Resistance of compacted bituminous mixtures to moisture induced damage, part II testing. Washington, DC 20001: American Association of State Highway and Transportation Officials; 2003. Report nr T283-03.
  13. AASHTO. Coating and stripping of bitumen-aggregate mixtures, part II testing. Washington, DC 20001: American Association of State Highway and Transportation Officials; 1986. Report nr T182-84.
  14. AASHTO. Determining degree of particle coating of bituminous-aggregate mixtures, part II testing. Washington, DC 20001: American Association of State Highway and Transportation Officials; 1986. Report nr T195-67.
  15. Ontario Ministry of Transportation. Resistance to stripping of asphaltic cement in bituminous mixture by immersion marshall - laboratory testing manual. Ontario Ministry of Transportation; 1995. Report nr LS 283.
  16. 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.
  17. Johnson CK. Phenols in foundry waste sand. Modern Casting 1981:273.
  18. Winkler E, Bol’shakov AA. Characterization of foundry sand waste. Chelsea Center for Recycling and Economic Development, University of Massachusetts; 2000 Report nr 31.
  19. D’Allesandro L, Haas R, Cockfield RW. Feasibility study on the environmental and economical beneficial use of waste foundry sand in the paving industry. University of Waterloo; 1990:Report for MRCO and the Canadian Foundry Group.
  20. AASHTO. Sampling bituminous paving mixtures, part II tests. Washington, DC 20001: American Association of State Highway and Transportation Officials; 2003. Report nr T168-03.
  21. ASTM D2726-05a Standard test method for bulk specific gravity and density of non-absorptive compacted bituminous mixtures. In: Annual book of ASTM standards. West Conshohocken, Pennsylvania: ASTM; 2005.
  22. ASTM D2950-05 Standard test method for density of bituminous concrete in place by nuclear methods. In: Annual book of ASTM standards. West Conshohocken, Pennsylvania: ASTM; 2005.

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Last Update 7/28/08