Spent Foundry Sand in Hot Mix Asphalt

Application Overview

Asphalt concrete, also known as bituminous concrete, consists of a mixture of aggregates bound together by bituminous asphalt cement.  The aggregates in this mixture must meet ASTM specifications.  The most important properties of the aggregate are gradation, shape and density.  These properties have the greatest effect on the characteristics of asphalt concrete.

Asphalt concrete is the most popular paving material used on our highways and roadways in the United States, with both fixed and portable asphalt plants available in or near most communities. Over 94% of all pavements in the U.S. are covered with asphalt. This translates to over 2,030,000 miles. The most prevalent type of asphalt paving material is hot mixed asphalt (HMA). This consists of a combination of plant-dried coarse and fine aggregates.  The aggregates are coated with hot asphalt cement, which acts as a binder.

The use of foundry sands as fine aggregates in hot mix asphalt is well documented but not widely practiced. The durability of foundry sand as a fine aggregate substitute used in the production of HMA is one of the major determinants of how long asphalt pavement can remain in service. In the mid 1980s, the Federal Highway Administration (FHWA) undertook a program to assess the suitability of multiple aggregate sources. This evaluation program, known as Superpave, has been widely accepted in state DOT asphalt specifications. Foundry sands are finer than conventional asphalt sands, but typically will meet Superpave requirements when properly designed (using Superpave, Marshall, or Hveem techniques).

Fine aggregate substitute rates for foundry green sands will average roughly 10-15% in HMA mix designs, but can be as high as 30% if the clay and organics have been removed. One binder type, however, (sodium silicate) appears to degrade the HMA performance

Market Assessment

Using foundry sand as a partial replacement of fine aggregates in asphalt concrete represents a large market that could use all of the foundry sand produced in the United States.  Foundry sand can typically replace between 12 and 15 percent of fine aggregates in asphalt.  This percentage can be increased if the foundry sand is processed and fines are removed.

Commercially sustainable markets will hinge on the price and availability of foundry sands in comparison to local construction sands. Use of foundry sands can be cost effective for both the foundries and the HMA industry. Highway agencies and contractors could switch to the recycled material when it is geographically and economically competitive. The foundry sand must be clean of clay, dust and other deleterious materials, as well as metals that can be removed manually or mechanically, making sure to meet environmental requirements.

Aggregate Requirements

In order to meet American Association of State Highway and Transportation Officials (AASHTO) M29 gradation specifications, only between 5-10% of the fine aggregate may pass through the No. 200 sieve. To avoid a higher percentage, foundry sands which tend to be fine may be blended with coarser natural sands. Cleanliness is tested to measure for clay and organic portions, which are detrimental to aggregate binder adhesion.  The Methylene blue test (AFS 2211-00-S) is a good measure of the clay content, while the loss of ignition test is a good measure of the amount of organics contained in the sand (AASHTO T 267-86). Nearly all of the foundry sands meet the soundness specification (AASHTO T104). They also typically fall into the specified 40-45% aggregate angularity test (AASHTO TP33). Finally, foundry sand is generally non-plastic and has low absorption, but has been linked to stripping. Moisture damage is highly dependent upon the clay and organic content, and the University of Wisconsin-Madison is currently developing better testing to determine how clay and organic content influence moisture damage.

Other Opportunities in Asphalt Production