Asphalt Core Sample: Drilling, Testing and What Cores Show
An asphalt core sample is a round plug of finished pavement cut out with a water-cooled diamond drill bit. Inspectors and contractors pull cores to measure how thick the asphalt really is, how dense it was compacted, and how the layers are bonded. For an owner or estimator, a core is the most direct way to see what was actually placed, and it ties straight back to tonnage. This guide covers how cores are drilled, what they are tested for, how to turn a core into a density or thickness check, and what to do with the hole afterward.
The short answer
- What it is: a cylinder of compacted asphalt drilled out of the mat, usually 4 or 6 inches across, through the full depth of the layer being checked.
- Why it is taken: to measure in-place density, verify thickness, check bond between lifts, calibrate density gauges or investigate a failure.
- Main lab test: bulk specific gravity (Gmb), which is compared with the mix's maximum specific gravity (Gmm) to get percent compaction.
- Who decides: on agency work the specification sets core size, how locations are picked and who drills; on private work it is whatever the contract says.
- The hole: it gets filled and compacted after coring, using a material the job allows.
How an asphalt core is drilled
Coring uses a drill rig mounted on a stand or trailer with a hollow, diamond-tipped bit. Water runs through the bit to cool it and carry away cuttings. The operator drills straight down through the layer being tested, then lifts the plug out with a thin retrieval rod or hook so it is not cracked or bent.
- Bit size: Washington State DOT's coring procedure gives 4-inch and 6-inch inside diameters as the usual choices and allows cores anywhere from 2 to 12 inches when needed. South Carolina DOT acceptance cores are 6 inches.
- Timing: cores for acceptance are drilled after rolling is complete. Fresh mat may still be warm, so state procedures from West Virginia and South Carolina describe cooling the spot with ice or water first so the core does not smear or deform.
- Separating lifts: when only the top layer is being tested, it is cut off the rest of the core with a wet masonry saw. SCDOT's method specifically says not to pry layers apart with a screwdriver, because that can damage the core and throw off the density result.
- Labeling: each core is marked with its location, lot and number so the result can be traced back to a spot on the road.
Where cores are taken is usually random, chosen by the inspector with a random-number method rather than by the paving crew, so the sample represents the whole lot.
What an asphalt core sample is tested for
| Check | How the core is used | Why it matters |
|---|---|---|
| In-place density | Bulk specific gravity of the core, divided by the mix's maximum specific gravity | Low density means high air voids and a shorter pavement life |
| Thickness | Measure the height of the core or the layer cut from it | Confirms the design depth, and with it the tons actually placed |
| Bond between lifts | See whether the layers stayed together when cut, or test shear strength in a lab | Poor bond lets the top lift slip and crack |
| Gauge correlation | Compare core density with nuclear or electrical gauge readings at the same spot | Lets quick gauge readings be trusted for the rest of the job |
| Forensic review | Look at mix, layers and damage on an existing road | Explains why a pavement rutted, stripped or cracked |
For background on the two lab values, see our posts on bulk specific gravity of asphalt and theoretical maximum specific gravity.
From core to percent compaction
The lab dries the core, weighs it in air, weighs it underwater, then weighs it again with the surface patted dry. Those three weights give the bulk specific gravity. In the standard method (AASHTO T 166) the formula is:
Gmb = dry mass ÷ (saturated surface-dry mass − mass in water)
Percent compaction = Gmb ÷ Gmm × 100, and air voids (%) = 100 − percent compaction
Worked example
The numbers below are made up to show the math, not taken from a real project.
- Dry mass 3,900 g, saturated surface-dry mass 3,920 g, mass in water 2,240 g
- Gmb = 3,900 ÷ (3,920 − 2,240) = 3,900 ÷ 1,680 = about 2.321
- If the day's maximum specific gravity is 2.480, compaction = 2.321 ÷ 2.480 × 100 = about 93.6 percent
- That leaves about 6.4 percent air voids in place
Whether 93.6 percent passes depends on the specification for that mix. Our air voids in asphalt post explains why the target sits where it does.
Using cores to check thickness and tons
Thickness is where a core speaks directly to tonnage. If the plan called for 2 inches and the cores average 1.75 inches, the job used noticeably less mix than was priced. If they average 2.5 inches, either more tons went down or the mix was spread over a smaller area than planned.
Tons placed ≈ area (ft²) × average core thickness (ft) × compacted density (lb/ft³) ÷ 2,000
You can turn the core's own Gmb into a density in pounds per cubic foot by multiplying it by 62.4, the weight of a cubic foot of water. In the example above, 2.321 × 62.4 = about 145 lb/ft³. Plug that, the area and the measured thickness into the formula, or let the asphalt tonnage calculator do it. For a quick depth check without drilling, see how to probe asphalt thickness.
Cores vs density gauges
Gauges are fast and leave no hole, so crews use them during rolling to see how compaction is going. Cores are slower and destructive, but they give a direct lab measurement, which is why many agencies use cores for pay and acceptance and use gauges for control. A common approach is to take a few cores at gauge test spots early in the job and correlate the two. Our posts on the nuclear density test and the non-nuclear density gauge cover the gauge side.
Coring a driveway or parking lot
- When it makes sense: a dispute over thickness, a large commercial lot, or a failing pavement you want to diagnose before choosing between an overlay and a rebuild.
- Who can do it: geotechnical and materials testing firms core asphalt and run the lab tests. Some paving contractors own a core drill for thickness checks.
- How many: enough to represent the area. Spread them out rather than grouping them in one corner, and avoid edges and patches unless those are what you are investigating.
- Mark locations: note where each core came from on a sketch, so a thin or loose result can be tied to one part of the lot.
Filling the core hole
Every core leaves a round hole that has to be closed. Dry out the water from drilling, coat the sides with tack if the patch material calls for it, then fill with hot mix or a patch product the specification allows and compact it in thin layers until it is flush. A hand tamper or the end of a small plate works for a 4 or 6 inch hole. Our asphalt compaction factor post explains why loose mix has to be mounded slightly above the surface before tamping.
Asphalt core sample FAQ
What is an asphalt core sample?
It is a cylinder of finished asphalt drilled out of the pavement with a water-cooled diamond bit, usually 4 or 6 inches across, and used to check thickness, density and layer bond.
How is asphalt core density tested?
The lab dries the core and weighs it dry, underwater and saturated surface-dry to get bulk specific gravity, then divides that by the mix maximum specific gravity to get percent compaction.
What size are asphalt cores?
Most agencies use 4-inch or 6-inch diameter cores. Washington State DOT allows sizes from 2 to 12 inches when a test needs them, and South Carolina DOT acceptance cores are 6 inches.
Can a core show how thick my driveway is?
Yes. Measuring the height of a core is the most direct way to confirm asphalt thickness. Take several cores spread across the area and average them rather than relying on one spot.
Why are cores cooled before drilling?
Fresh asphalt can still be warm and soft after rolling. Cooling the spot with ice or water keeps the core from smearing or deforming, which would make the density result wrong.
What happens to the hole after coring?
It is dried, filled with hot mix or a patch material the job allows, and compacted in thin layers until it is flush with the surrounding pavement.
The bottom line
An asphalt core sample is a drilled plug of the finished mat that shows how thick and how dense the pavement really is. Cores are cooled, cut cleanly, weighed for bulk specific gravity and compared with the mix's maximum specific gravity to get percent compaction. Average core thickness times area and density tells you how many tons went down, and the hole should be filled and compacted before the job is closed out.
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