Bases and Subbases
What is a Base or Subbase?
In a pavement structure, the granular base and subbase are compacted aggregate layers between the asphalt concrete or the Portland cement concrete pavement layers and the underlying subgrade material. The base is the load-bearing layer immediately below the pavement layers, and serves to provide strength and support to the overlying pavement. The subbase is below the base and serves as the foundation for the overall pavement structure, transmitting traffic loads to the subgrade while providing drainage and frost protection. These layers are formed by spreading the aggregate in thin layers and then compacting it with heavy equipment until the material forms a dense layer of interlocking aggregate. To perform well, base and subbase aggregates must be strong and durable, and must meet very specific gradation requirements.
Traditionally, aggregates for base and subbase applications were composed of sand, gravel, crushed rock, quarry stone and other virgin mineral materials that provided the necessary strength and durability. These materials have to be quarried, sized, and transported to the job site, all of which requires considerable energy. Although natural aggregates are still the primary materials used in base and subbase applications, aggregate shortages are beginning to be felt in some parts of the country, especially in urban markets. Aggregate shortages mean that trucks have to drive farther to obtain aggregate, increasing transportation costs and air pollution.
How is Foundry Sand used in Base and Subbase?
The base and subbase layers of a pavement structure are responsible for transmitting the traffic loads from the surface layers to the underlying subgrade material. The make up of the base and subbase layers varies depending on the anticipated loads and the pavement type (concrete or hot mix asphalt). Foundry sand is used as an aggregate for subbase layers under flexible and rigid pavements. It is typically placed above the subgrade to improve the structural integrity of the pavement. Foundry sand has also been shown to be an excellent working platform for heavy machinery during construction. Foundry sand can be placed with conventional construction equipment and requires no specialized construction practices.
How does Base and Subbase perform using Foundry Sand?
Foundry sand has been used as base and subbase in many local construction projects throughout the U.S. Larger scale highway projects are common in Wisconsin and Ohio. One of the advantages of using foundry sand in base and subbase is its consistency. Most highway projects are built over many miles of naturally varying subgrades, which can affect the long-term pavement performance. The use of foundry sand provides a more consistent substructure for the paving course. Some foundry sands have also proved to resist freezing, meaning that it still handles well in cold temperatures. In winter months, that allows for the construction season to be extended, saving money for project owners and taxpayers.
What are the technical issues associated with Foundry Sand in Base and Subbase?
Similar to embankments and fill applications, the major technical issues are related to the presence or absence of bentonite clay binder. It is important to know which type of sand is being used, and what the fines content is if the sand is to be used where frost susceptibility or permeability are important issues. Foundry sands should be tested for physical properties such as plasticity index and CBR or resilient modulus. As long as required CBR ratios are maintained, there should be no limits on foundry sand in this application. The moisture content of foundry sand is also important during the construction phase. The presence of the bentonite clay tends to increase the optimum water content of foundry green sands compared to regular sand, so additional water may be needed to keep the sand at theoptimum water content to maintain high density.
Are there any specific QA/QC issues that suppliers and/or end users need to be aware of?
The major QA/QC issues related to foundry sand are in meeting the gradation requirements for base and subbase aggregates. First, chunks of sand and binder, as well as metal pieces need to be screened out. In addition, transportation agencies have limits on the amount of fines (passing the #200 or 0.075 mm mesh), which would include any bentonite clay. A typical limit is around 5% of the total material. Suppliers and end users need to be aware of the presence of binder materials that will increase the fines content. As noted earlier, water content needs to be kept at or below optimum.
Are there any environmental issues associated with the use of Foundry Sand in Base and Subbase?
The US EPA has determined that ferrous and aluminum foundry sands are non-hazardous and can be used in geotechnical applications such as road base and subbase layers. However, the environmental requirements for foundry sand vary from state to state. Some states have limitations on the placement of foundry sand in unbound applications; others require testing (TCLP, SPLP, etc.) prior to placement. Foundry sand suppliers should be able to provide evidence that foundry sand is approved for this use in the state in which the project will take place.