What is Hot Mix Asphalt?
Asphalt is the most widely-used paving material on the nation’s highways and roadways, covering more than 94% of pavements, which translates into more than 2 million miles. Asphalt paving is also used in many other construction applications with lower load-bearing requirements, such as parking lots, bike paths and tennis courts. Asphalt, known technically as “asphalt concrete” or “bituminous concrete,” is produced in both fixed and portable asphalt plants located in or near most communities. Hot mix asphalt (HMA) is a mixture of coarse and fine aggregates and asphalt binder that is heated, placed and compacted. HMA is the most common asphalt production technique, although asphalt contractors are exploring the use of warm mix asphalt to reduce energy consumption. Most HMA pavements consist of two layers, the lower layer providing support for the top layer, which is known as the surface course.
How is Foundry Sand used in Hot Mix Asphalt?
Foundry sand has successfully been used as a fine aggregate substitute in HMA for many years on both public and private sector HMA projects. HMA producers can often save money by sourcing foundry sands instead of virgin sands. Up to 30% of the total aggregate mix can be replaced with foundry sand. To meet state DOT asphalt specifications that are generally for high volume roadways, mix designs in the 10-15% range are more common. Extensive research has been performed to show that foundry sands will meet Federal Superpave requirements. Ferrous foundry slag is also used as coarse aggregate in HMA.
How does Hot Mix Asphalt perform using Foundry Sand?
In general, tests on mixtures of HMA containing foundry sand have performed as well or better than mixtures made with natural sands. Some foundry sands have shown improved moisture resistance and as well as increased rutting resistance. Pennsylvania DOT allows the use of 8-10 percent foundry sand in HMA. Michigan and Tennessee DOT's have also allowed foundry sand to be substituted at 10%. Mix designs for smoother surfaces such as tennis courts have successfully incorporated up to 30% foundry sand aggregate. ASTM is developing a standard for the use of foundry sand in HMA.
What are the technical issues associated with Foundry Sand in Hot Mix Asphalt?
Standard asphalt design mixes can be used to design HMA containing foundry sand. Foundry sand is generally finer than conventional bank run sands and may have a higher fines content. The fines content will generally be the limiting characteristic as to what percentage of the fine aggregate can be replaced with foundry sand. Asphalt producers should run test batches with the local foundry sand source to adjust the mix design. Clay-bonded foundry sands (green sands) may be more sensitive to moisture. There is some evidence to suggest that some foundry sands increase the potential for stripping. HMA producers should consider the Immersion Marshall test to evaluate the stripping potential of a foundry sand mixture, and incorporate an anti-stripping agent such as lime if needed. One type of foundry sand (sodium silicate binder) has been found to degrade the overall asphalt performance, but most foundry sands are unaffected by this issue.
Are there any specific QA/QC issues that suppliers or end users need to be aware of?
Foundry sand used for HMA must be clean of extraneous material and screened to grain size. 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, and tested for specific gravity in accordance with ASTM D2726 and in-place density in accordance with ASTM D2950.
Are there any environmental issues associated with the use of Foundry Sand in Hot Mix Asphalt?
Extensive research studies have been performed on foundry sand in HMA, including a large-scale study funded by the U.S. Department of Energy. None have shown any potential for environmental damage from the addition of ferrous or aluminum foundry sand to HMA. A number of studies have shown that ferrous and aluminum foundry sands are safe substitutes for virgin sands in construction applications. The US EPA promotes the use of ferrous and aluminum foundry sands in pavement structures. The specific environmental issues are therefore similar to those involving natural sands, such as dust control.