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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
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. Spent sands from nonferrous foundries and foundry baghouse dust can contain high concentrations of heavy metals that may preclude their use in flowable fill applications.
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. It is defined by the ACI Committee 229(3) as a cementitious material that is in a flowable state at the time of placement and that has a specified compressive strength of 1400 kPa (200 lb/in2) or less at 28 days. This 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 in county roads, backfilling structures, filling abandoned wells, filling voids under existing pavements, and pipe embedments. (See references 4,5,6,7,8 and 9.)
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.
There has been limited reported use of spent foundry sand in flowable fills or cementitious applications. It is reportedly being used in flowable fill applications in the Buffalo, New York area.(10) Pennsylvania has reported successful use of foundry sand as a sand substitute in flowable fill. Illinois, however, has tried spent foundry sand and considered such use unsuitable due to poor performance or economics.(11)
MATERIAL PROCESSING REQUIREMENTS
Crushing and Screening
It may be necessary to crush the spent foundry sand 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) removed.
For spent foundry sand to be suitable as a replacement for fine aggregate in flowable fill, it should be free of objectionable material such as wood, garbage, and metal that can be introduced at the foundry. It should be free of foreign material and 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) Where it is specified that aggregates must satisfy the requirements of ASTM C33, the spent foundry sand must 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 must therefore be closely monitored.
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.
Particle Shape: The grain size distribution of spent foundry sand is more uniform and somewhat finer than conventional concrete sand.(13) 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. However, increased particle fineness and sphericity also result in lower strength bearing capacity (CBR) of the hardened flowable fill.(14)
Gradation: Spent foundry sand may not satisfy the ASTM C33 gradation requirement for concrete aggregate and, therefore, it may need to be blended with natural sand or other suitable fine aggregate materials to meet the requirements.
Strength Characteristics: Although some organic binder materials can interfere with cement hydration, low (rather than high) strength development is in most cases more desirable with flowable fill to permit excavation at a later date. It has been reported that the flowable fill incorporating spent foundry sand aggregates, fly ash, a small quantity of Portland cement, and water readily satisfies specified limited strength criteria.(15)
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).(13)
Deleterious Substances: Poorly managed spent foundry sand could 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. Foundry sand is often contaminated with organic material and can have an organic content of up to 12 percent.(16,17)
Corrosivity: Depending on the binder and type of metal cast, the pH of spent foundry sand can vary from approximately 4 to 8.(18) It has been reported that some spent foundry sand can be corrosive to metals.(19) Others have indicated that flowable fill mixes containing spent foundry sand are noncorrosive in nature because of the absence of chlorides and high pH values obtained (11.4 to 12.3).(1)
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. They 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.(20)
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.
Structural design procedures for 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.
The same methods and equipment used to mix, transpor, 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. Special measures may be required to control the early contact water leachate (containing phenols) from spent foundry sand stockpiles. The construction of an impervious pad (to collect surface moisture or precipitation passing through the stockpile) and subsequent filtration (through an activated carbon filter) of the leachate has reportedly been effective in limiting the phenol concentration of the discharge.(19)
Mixing, Placing, and Compacting
Flowable fill can be produced at a central concrete mixing plant in accordance with ASTM C94(21) and delivered by concrete truck mixers or using a mobile, volumetric mixer for small jobs. It is important that high fluidity (slump greater than 150 mm (6 in)) be maintained to ensure that the flowable fill material entirely fills all voids beneath pavements and around structures and utilities.
Various standard field and laboratory tests for flowable fill mixes are given by AASHTO T27,(22) ASTM Provisional Standard 28 - Provisional Test Method for Flow Consistency of Controlled Low Strength Materials; and ASTM Provisional Standard 29 - Provisional Test Method for Unit Weight, Yield, and Air Content (Gravimetric) of Controlled Low Strength Material.
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. There also is a need to develop standardized mix design methods for assessing the suitability of foundry sand in flowable fill as well as a need to assess the environmental suitability of spent foundry sand for flowable fill from ferrous and particularly nonferrous foundries.
TRT Terms: Waste products as road materials--Handbooks, manuals, etc, Pavements, Asphalt concrete--Design and construction--Handbooks, manuals, etc, Pavements, Concrete--Design and construction--Handbooks, manuals, etc, Pavements--Additives--Handbooks, manuals, etc, Fills (Earthwork)--Design and construction--Handbooks, manuals, etc, Roads--Base courses--Design and construction--Handbooks, manuals, etc, Wastes, Environmental impacts, Recycling