Geotechnical engineers are generally responsible for determining the pavement thickness needed for a 20-year structural design life based on traffic loading, soil conditions, and local climate and jurisdictional factors. The pavement sections most often consist of asphaltic concrete or Portland cement concrete overlying a roadway base aggregate. This aggregate is called by different names in different area, but essentially consists of a durable, crushed angular gravel mixed with sand and some fines. In general, the rock is crushed to less than ¾ of an inch. In some cases, other elements such as chemically treated soil (lime or cement), geogrid, geotextile fabric, etc. are added to the design. The actual mix design of the pavement surface is generally performed by others, as is the mix selection. However, geotechnical labs often assist in developing mix designs.