The three-meter straightedge method is a commonly used field and laboratory technique for evaluating the surface flatness of pavements, concrete slabs, and other construction surfaces. It involves placing a rigid straightedge of known length (typically 3 meters) on the test surface and measuring the gap between the straightedge and the surface at various points using a wedge gauge or feeler gauge. By recording the maximum deviation values, engineers can assess the overall smoothness and uniformity of the surface. This method is simple, cost-effective, and widely adopted in quality control standards for road construction and building works, helping ensure compliance with specified flatness requirements.

1. Scope of Application

1.1 This method is applicable for measuring the maximum gap (δ) between the road surface and a 3-meter straightedge reference plane, in order to evaluate the flatness of the pavement surface.

1.2 This method is suitable for testing the surface evenness of each layer of subgrade and pavement after compaction and forming.

2. Instruments and Materials Requirements

(1) 3-meter straightedge:

The measuring reference straightedge shall be 3 m in length. The reference surface shall be straight and rigid, made of hard wood, aluminum alloy, or steel.

(2) Maximum gap measuring tools:

① Wedge feeler gauge:

A triangular wedge-shaped gauge made of hard wood or metal, equipped with a handle. The ratio of length to height shall not be less than 10. The width shall not exceed 15 mm. The edge shall be marked with height graduations, with a resolution not greater than 0.5 mm.

② Depth gauge:

A metal depth-measuring device equipped with a handle. The diameter of the measuring rod tip shall not be less than 10 mm, with a resolution not greater than 0.5 mm.

(3) Other tools:

Measuring tape or steel ruler, etc.

3. Method and Procedures

3.1 Preparation

(1) Selection of test method:

When evaluating the construction quality of asphalt pavement, a single straightedge test shall be used, and the test location shall be at the joint area. In other cases, a continuous 10-straightedge method is generally adopted.

(2) Selection of test location:

Unless otherwise required, the test position shall be located at the wheel track on one side of the traffic lane (0.8–1.0 m from the lane line). For existing roads with formed rutting, the center of the rut shall be used as the test position.

(3) The surface at the test location shall be cleaned to remove loose stones, debris, and other foreign materials.

3.2 Test Procedure

(1) Place the 3-meter straightedge longitudinally along the road surface at the selected test position.

(2) Visually inspect the gap between the bottom of the straightedge and the road surface to determine the location of the maximum gap.

(3) Insert the wedge-shaped feeler gauge with height markings into the gap to measure the maximum gap height; alternatively, use a depth gauge at the point of maximum gap to measure the depth from the top surface of the straightedge to the road surface. The maximum gap height is obtained by subtracting the straightedge height from the measured depth. The result is expressed in millimeters (mm) and recorded to the nearest 0.5 mm.

4. Data Processing

For a single-straightedge test, the pavement flatness shall be represented by the maximum gap (δₘ) between the 3-meter straightedge and the road surface.

For a continuous 10-straightedge test, the maximum gap (δₘ) measured at each test position shall be evaluated against the specified acceptance criteria. The compliance rate shall be calculated, and the average value of the ten maximum gaps shall also be determined and reported.

5. Test Report

The test report shall include the following information:

5.1 Test location details, including chainage (stationing), test method, and other relevant information.

5.2 Maximum gap (δₘ) measured between the 3-meter straightedge and the road surface.

5.3 For a continuous 10-straightedge test, the report shall also include:

The average value of the ten maximum gaps;

The number of non-compliant measurements;

The compliance rate (pass rate).

Explanation of the Provisions

According to the current standards for quality inspection and evaluation of highway engineering, pavement flatness is generally assessed using technical indicators such as the maximum gap (δ), standard deviation (σ), and the International Roughness Index (IRI). This test method specifies the procedure for measuring the maximum gap. It is widely used for evaluating the surface flatness of various layers of subgrade and pavement after compaction, particularly for construction quality control, due to its simplicity, practicality, and ease of implementation.

In this revision, the graduation requirements for the wedge feeler gauge and depth gauge have been updated in accordance with current industry practices and the accuracy requirements for pavement flatness measurement. These adjustments ensure that the measuring instruments are better suited to modern testing applications and provide sufficient measurement precision.