Compaction reduces the volume of air in HMA. Therefore, the characteristic of concern is the volume of air within the compacted pavement. This volume is typically quantified as a percentage of air voids by volume and expressed as “percent air voids”. Percent air voids is calculated by comparing a test specimen’s bulk density with its theoretical maximum density (TMD) and assuming the difference is due to air. Once TMD is known, portable devices can be used to measure HMA density in-place. The terms “percent air voids” and “density” are often used interchangeably. Although this is not wrong, since density is used to calculate percent air voids, the fundamental parameter of concern is always percent air voids.
Percent air voids is typically calculated by using AASHTO T 269, ASTM D 3203 or an equivalent procedure (AASHTO, 2000). These procedures all use laboratory-determined bulk specific gravity and theoretical maximum specific gravity in the following equation:
|where:||Gmm||=||theoretical maximum specific gravity of the particular HMA in question|
|Gmb||=||bulk specific gravity of the HMA in question|
These procedures require a small pavement core (usually 100 – 150 mm (4 – 6 inches) in diameter), which is extracted from the compacted HMA (see Figure 1 and 2). This type of air voids testing is generally considered the most accurate but is also the most time consuming and expensive.
Since core extraction is time consuming and expensive, air voids are often measured indirectly using a portable density-measuring device such as a nuclear density gauge (see Figure 3) or electrical density gauge (see Figure 4).
Each contracting agency usually specifies the compaction measurement methods and equipment to be used on contracts under their jurisdiction. Most agencies stipulate some sort of extracted core density testing and usually allow testing by nuclear gauge. Electric density gauges are relatively new on the market (in the last five years). Accurate calibration of these devices is essential for their proper use.
Although percent air voids is the HMA characteristic of interest, measurements are usually reported as a measured density in relation to a reference density. This is done by reporting density as a (1) percent of TMD (sometimes called Rice density), (2) percent of a laboratory density or (3) percent of a control strip density (a control strip is a short pavement strip that is compacted to the desired value under close scrutiny then used as the compaction standard for a particular job).
In sum, percent air voids is the critical HMA characteristic with which compaction is concerned. It can be measured using pavement cores or portable nuclear or electric gauges; measurement specifications vary from one contracting agency to the next. Percent air voids is usually reported as a density in one of three forms: (1) percent TMD, (2) percent of laboratory density or (3) percent of control strip density. Regardless of the measurement device or reporting method, the key characteristic is percent air voids.