Pavement Cores

A core is a cylinder of pavement drilled out with a water-cooled diamond bit. It is the most direct sample of what was actually built: the mix as compacted, the concrete as placed and cured, and the layer as thick as it really is. Cores are slower and more expensive than nondestructive tests and leave a hole to patch, so they are used where their accuracy matters: for acceptance of asphalt density, for calibrating nuclear and electrical gauges, for checking concrete thickness, and for resolving disputes about strength.

Asphalt Cores

Asphalt cores for density are taken under AASHTO R 67. The points that most affect results (Western Alliance for Quality Transportation Construction, 2025¹):

• Timing. The mat must be cool enough that the core isn't distorted when it is drilled and handled. On a hot day that can mean waiting until the next day, or cooling the spot with ice or dry ice.
• Location. Core locations are chosen randomly within each sublot (see Acceptance Plan Basics). Joint-density specifications add cores at a set offset from the longitudinal joint.
• Diameter. Cores are usually 100 or 150 mm (4 or 6 inches). The larger size is needed for coarse mixes and gives a less variable result.
• Separating layers. When the core contains more than one lift, the lift being accepted is separated with a saw at the layer interface, not pried apart.
• Handling. Cores are supported on a flat surface, kept out of the sun, and protected from being dropped or squeezed. A core that is cracked, distorted or damaged at the edge is replaced with another at a nearby random location, as the specification allows.

The core's bulk specific gravity is measured in the laboratory (see Bulk Specific Gravity) and compared with the theoretical maximum specific gravity to give percent compaction or in-place air voids (see Compaction Measurement and Reporting).

Cores are also used to correlate density gauges: the gauge reading at each core location is compared with the core result, and the average difference becomes a correction for that mix and gauge (see Nuclear Density Gauge).

Calculator

Gauge-to-core correction

ASTM D2950 · AASHTO T 355

Units

US customary SI (metric)

Paired sites

Site	Core, pcf	Gauge, pcf
1
2
3
4
5
6
7

Gauge reading to correct, pcf (optional)

CalculateExample values. Many agencies require more pairs; follow your specification.

Concrete Cores for Strength

When cylinder results are low, or no cylinders exist, cores are drilled to measure the in-place strength (AASHTO T 24, ASTM C42). In-place concrete has had a different curing history from standard-cured cylinders, so core strengths are usually lower and are judged against their own criteria (Texas DOT, 2022²).

• Size. The core diameter should be at least about 95 mm (3¾ inches) and at least twice, and preferably three times, the nominal maximum aggregate size.
• Length. The ideal length-to-diameter ratio is about 2. Shorter cores are tested and their strength corrected with factors given in the method; very short cores can't be tested for strength.
• Avoid reinforcement and joints, and drill perpendicular to the surface.
• Moisture conditioning. Core strength depends on moisture. After drilling, the surface water is wiped off, the cores are sealed in plastic bags or nonabsorbent containers, and they are kept sealed for a set period before testing so that moisture differences from drilling even out.

Acceptance of cores is set by the specification. Building codes (ACI 318) accept the concrete in an area represented by three cores if their average is at least 85 percent of the specified strength and no single core is below 75 percent (National Ready Mixed Concrete Association, 2014³). Highway agencies often use their own rules, so check the contract.

Concrete Cores for Thickness

Concrete pavement thickness is checked with cores measured to the nearest millimeter or so with a caliper device at several points around the core (AASHTO T 148, ASTM C174). The method takes a set of length measurements, at the center and at points spaced around a circle, and averages them. Many agencies reduce payment for a thickness deficiency beyond a tolerance and require removal beyond a larger one; cores for thickness are taken at random locations in each lot.

Filling the Hole

Core holes are cleaned, dried and filled promptly: asphalt holes with hot mix or an approved patching material compacted in lifts, concrete holes with a nonshrink grout or concrete bonded to the sides. An unfilled core hole collects water and becomes a pothole.

Standard Test Methods

• AASHTO R 67: Sampling asphalt mixtures after compaction (obtaining cores)
• AASHTO T 166: Bulk specific gravity of compacted asphalt mixtures
• AASHTO T 24, ASTM C42: Obtaining and testing drilled cores of concrete
• AASHTO T 148, ASTM C174: Measuring the length of drilled concrete cores

@misc{pavint-pavement-cores,
  author = {{Pavement Interactive}},
  title = {Pavement Cores},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/testing/sampling/pavement-cores/}},
  note = {Accessed [date]}
}