Pavement thickness is one of the most important indicators of road quality and structural performance. Accurate measurement is essential for quality control, construction acceptance, and long-term durability evaluation. Among the various testing methods, core drilling and excavation are widely used due to their reliability and direct measurement results. This article introduces technical requirements, methods and procedures, and data processing for instruments and materials, helping engineers and inspectors achieve more accurate and consistent results in the field.

1. Scope of Application

This method is applicable to the measurement of pavement structural layer thickness. The excavation method is suitable for testing the thickness of base layers or gravel roads. The core drilling method is suitable for measuring the thickness of asphalt surface layers, cement concrete pavement slabs, and base layers from which intact core samples can be obtained.

2. Equipment and Material Requirements

(1) Excavation tools: pickaxe, shovel, chisel, hammer, small trowel, and brush.

(2) Pavement core drilling machine: push-type or vehicle-mounted, equipped with a water cooling device. The standard diameter of the drill bit is φ100 mm. If the core sample is used only for thickness measurement and no other tests are required, a φ50 mm drill bit may also be used for asphalt surface layers and cement concrete slabs. When the base material may be damaged during sampling, a φ150 mm drill bit may be used; however, the drilling depth must reach the full thickness of the layer in all cases.

(3) Measuring tools: steel ruler, vernier caliper, with a graduation value not exceeding 1 mm.

(4) Others: ruler, enamel plate, cotton yarn, etc.

3. Methods and Procedures

3.1 Preparation

(1) The test location for excavation or core sampling shall be determined in accordance with T 0902 and relevant international practices such as AASHTO T 166 / ASTM D5361. For existing roads, areas with visible defects such as potholes, cracks, or joints shall be avoided to ensure representative results.

(2) At the selected test location, choose a flat surface of approximately 400 mm × 400 mm, and clean it thoroughly using a brush or appropriate cleaning tools to remove dust, debris, and loose particles.

3.2 Excavation Method for Thickness Measurement

(1) According to the hardness of the material, suitable tools such as a pickaxe, shovel, or chisel shall be used to excavate the pavement layer until reaching the bottom interface of the layer. The excavation area shall be minimized as much as possible while ensuring operational convenience, and the pit should be approximately circular. Excavated materials shall be collected and stored in an enamel tray.

(2) Clean the bottom of the pit using a brush to confirm that the interface of the underlying layer has been fully exposed.

(3) Place a straightedge across the top of the pit. Using a steel ruler, measure the vertical distance from the underside of the straightedge to the bottom of the pit at the center point. The measured value is recorded as the layer thickness t₁ (mm), with an accuracy of 1 mm.

3.3 Core Drilling Method for Thickness Measurement

(1) Core drilling shall be carried out in accordance with T 0903 and ASTM D3549 / EN 12697-36, using a pavement core drilling machine to obtain intact core samples. The drilling depth shall exceed the bottom of the tested layer.

(2) Remove the intact core sample and clearly identify the interface between layers.

(3) Using a steel ruler or vernier caliper, measure the height from the surface to the interface at four symmetrical points along a cross direction of the core circumference. The average of the four measurements shall be taken as the layer thickness t₁ (mm), with an accuracy of 1 mm.

3.4 Post-Test Cleaning

Clean all residual materials inside the pit. Any water remaining from drilling shall be removed using absorbent materials such as cotton yarn. After drying, the excavated area shall be restored using the same type of material and compacted to the original condition.

4. Data Processing

4.1 Thickness Deviation Calculation

The difference between measured thickness and design thickness shall be calculated using the following formula:

ΔTᵢ = T₁ᵢ − T₀ᵢ

Where:

T₁ᵢ — Measured thickness of pavement layer i (mm)

T₀ᵢ — Design thickness of pavement layer i (mm)

ΔTᵢ — Thickness deviation of pavement layer i (mm)

4.2 Statistical Evaluation

According to the provisions of this specification and international statistical evaluation methods (such as ISO 5725 and AASHTO R 80), the mean value, standard deviation, and representative thickness value of the test section shall be calculated.

5. Test Report

The test report shall include the following technical information:

(1) Field test location details (chainage/station number, pavement structure type, etc.), in accordance with EN 13108 and ASTM reporting requirements.

(2) Measured thickness values, design thickness values, and thickness deviations for each test location.

(3) Mean value, standard deviation, and representative value of pavement thickness for the test section.