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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 fine aggregate in hot mix asphalt pavements.(1,2,3) Satisfactory performance has been obtained from hot mix pavements incorporating up to 15 percent clean, spent foundry sand.
Hot mix asphalt pavements with more than 15 percent of clean spent foundry sand content (blended with natural sand) are 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, and accelerated pavement deterioration. The problem can be mitigated by using antistripping additives.
Spent sands from nonferrous foundries and foundry baghouse dust can contain a high concentration of heavy metals that could preclude their use as an aggregate in pavement construction.
The commercial use of spent foundry sand in the United States is extremely limited. There are no documented field uses of foundry sand in asphalt paving mixes. In an American Foundrymen’s Society study of asphalt concrete properties (using 10 percent foundry sand) compared with control mixes (without foundry sand), the results indicated little difference in Marshall design properties (e.g., voids, voids in mineral aggregate, stability, flow, and unit weight).(4) A more recent study was undertaken at Purdue University with samples containing up to 30 percent foundry sand. Increasing foundry sand blends above 15 percent lowered the unit weight, increased the air voids, decreased the flow and stability of the mixes, and reduced the indirect tensile strength (after immersion in a hot water bath), which is indicative of samples that are susceptible to stripping problems.(4)
MATERIAL PROCESSING REQUIREMENTS
Crushing and Screening
It may be necessary to crush and screen the spent foundry sand 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 separator, as necessary).
It is also important that consistency (primarily gradation) be maintained for hot mix asphalt production. Variations between foundries require that spent foundry sands be examined and evaluated on a source-specific basis.
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.
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 hot mix asphalt fine aggregates (AASHTO M29),(5) the spent foundry sand must be blended with natural sand at the hot mix plant.
Some of the properties of spent foundry sand that are of particular interest when foundry sand is used in asphalt paving applications include particle shape, gradation, durability, and plasticity. With the exception of gradation, clean, processed foundry sands can generally satisfy the physical requirements for hot mix asphalt fine aggregate (AASHTO M29).
Particle Shape: 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. The grains are generally rounded to subangular in shape.
Gradation: The gradation tends to fall within the limits for a poorly graded fine sand that has relatively uniform size (passing 0.3 mm and retained 0.15 mm), with fines content (less than 0.075 mm (No. 200 sieve)) ranging from 5 to 15 percent.
Durability: Spent foundry sands display good durability characteristics with resistance to weathering.(6,7)
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 levels comply with AASHTO requirements for fine aggregates.
Stripping is one of the more critical properties that should be assessed when foundry sand is incorporated into an asphalt mix.
Stripping: 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.
Asphalt mixes containing foundry sand can be designed using standard asphalt mix design methods (Marshall, Hveem).
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 T283-85,(8) which compares the tensile strength ratio of wet and dry specimens; T182-84,(9) T195-67,(10) or the Immersion Marshall test following the MTO LS-283(11) procedure, which compares the retained Marshall stability and visual appearance of Marshall briquettes before and after immersion in a heated water bath. Stripping resistance can be enhanced by adding hydrated lime or commercially available antistripping additives.
Conventional AASHTO pavement design methods are appropriate for asphalt paving incorporating spent foundry sand as fine aggregate.
Material Handling and Storage
The same general methods and equipment used to handle conventional aggregates are applicable for foundry sand.
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.
The same field testing procedures used for conventional hot mix asphalt mixes should be used for mixes containing foundry sand. Mixes should be sampled in accordance with AASHTO T168,(14) and tested for specific gravity in accordance with ASTM D2726(15) and in-place density in accordance with ASTM D2950.(16)
There is a need to establish standard methods of assessing the suitability of foundry sand for hot mix asphalt use. The Immersion Marshall test appears to be appropriate for assessing stripping potential.
Additional performance data are required to determine the maximum amount of foundry sand that can be incorporated in hot mix asphalt without deleterious effects.
There is a need to define the potential environmental problems associated with phenol discharges from foundry sand stockpiles, and to determine appropriate treatment strategies, if necessary.