What Is a Pavement Core?

A pavement core is a cylindrical sample that a core drill cuts out of a pavement, to measure the layers below the surface.

What a core shows

A survey of the surface sees cracks, patches and wear. It does not see how thick each layer is, how dense the mix is, or whether one lift is bonded to the next. A core gives that information at one point. AC 150/5320-6G says cores of an existing pavement give information about its structure. It asks for enough cores to evaluate the condition and to help find the extent and possible causes of distress.

Cores on a new asphalt pavement

FAA item P-401 uses cores to accept a new asphalt surface. For each sublot, one core is taken for mat density and one, centred over the longitudinal joint, for joint density. A mat core is not taken closer than one foot (30 cm) to a joint. The core is at least 5 inches (125 mm) wide and is cut under ASTM D5361.

Three results come from the cores:

• Density. The bulk specific gravity of the core is found under ASTM D2726. Percent compaction is that value divided by the theoretical maximum density of the sublot. Table 5 sets the lower limits as 92.8 percent for a surface course, 92.0 percent for a base course and 90.5 percent for a joint.
• Thickness. The same cores give the thickness of each lift. At no point may a lift be more than 1/4 inch (6 mm) thinner than shown. The average may not be below the indicated thickness.
• Bond. Each lift must be bonded to the layer below. If a core shows that it is not, more cores find the extent, and the unbonded area is milled out and replaced.

Air voids are not read from these cores. P-401 finds them on specimens that are compacted in the laboratory (ASTM D3203).

Cores on a new concrete pavement

Item P-501 takes one core per sublot, at least 4 inches (100 mm) wide, to check the thickness. The thickness is the average of caliper readings (ASTM C174), and each core is photographed. If the concrete is suspected to be poorly consolidated, referee cores are cut and their density is found under ASTM C642. The average must be at least 97 percent of the original mix density, and no core may be below 96 percent.

Core holes

The contractor fills each core hole within one day. For asphalt, the method must be acceptable to the Resident Project Representative (RPR). For concrete, the filler is a non-shrink grout.

Cores and a drone survey

A drone survey records the surface. AC 150/5320-6G names ASTM D5340 as the usual survey of visual condition and says that layer thickness comes from cores or ground penetrating radar. A crack map shows where distress is. A core can then show how deep it goes and what lies below it. Before milling, the same circular says to take cores to find the thickness and condition of the existing surface.