BaseBase material, or more accurately, "Rock Base," is a very frequently used mixture of rock and sand, which is able to attain a high rate of compaction. It is very common to place a section of base under new asphalt. The base section typically will be anywhere from 4" to 9" thick or, in extraordinary circumstances, may be even thicker. There are several purposes behind the utilization of base material; usually, it is used as a barrier between the native soil and the new asphalt. The reason for this is that many areas contain what is called "expansive soil ". This term means exactly what it says; the native soil has a high degree of expansion and contraction, and if asphalt were to be placed directly on this native material, it would crack and fall apart in a very short period of time. The more expansive the soil conditions, the thicker the base section that will be required. Another reason for the utilization of base material is that it provides a strong underpinning to the new asphalt being placed. Heavy traffic areas will destroy new asphalt if it is not on a strong material already. Often, even if the native soil is not expansive, it is not able to reach a high rate of compaction, or it may have a tendency to shift. Base, on the other hand, allows a certain amount of movement from the native soil beneath it while still maintaining its integrity to support the asphalt above it. Finally, there are different qualities of base material that can be used, including: Crushed Aggregate Base (CAB), which is crushed rock and usually the most expensive material; Class II Base, which can be a combination of crushed rock and sand and is the most typical type of material used; and Crushed Miscellaneous Base (CMB), which can be a mixture of crushed rock, crushed concrete and/or crushed asphalt material. Such material is often referred to as Recycled Base. There are a few other types of material which you may encounter on a very limited basis, including: Cement Treated Base (CTB) which is rarely utilized and, as the name implies, mixes cement in with the base for an application that requires even higher compaction rates than normal; and Slag, which is typically a by-product of foundry operations. Slag is occasionally used in lieu of base because it is less expensive and may be used in large open areas, such as trucking facilities, when customers do not wish to spend large sums of money to pave their parking areas, or wish to do so at a later time. The large disadvantage of slag is that it is nearly impossible to compact. The material is placed, graded, and rolled a few times to ensure good placement, however, due to the virtue of the material, there will never be substantial compaction. |