Compaction degree is a core indicator for measuring the load-bearing capacity, durability, and stability of road surfaces. After the completion of compaction work, on-site testing is required to verify whether it meets the design standards (e.g., subgrade compaction degree ≥ 93%, asphalt surface layer ≥ 96%; specific standards shall refer to the project design specifications). Testing methods are mainly divided into two categories: destructive testing (directly measuring material density) and non-destructive testing (indirectly calculating density). Different methods vary in principles, accuracy, and applicable scenarios, and they are often used in combination in practical applications to ensure reliable results.

I. Destructive Testing Methods (Direct Methods): High Accuracy, Served as "Standard Reference Methods"

Destructive testing involves sampling from the compacted road surface, calculating the actual dry density through laboratory analysis, and then comparing it with the maximum dry density required by the design (the optimal compaction state density of the material obtained through standard compaction tests) to get the compaction degree (compaction degree = actual dry density / maximum dry density × 100%). Results of such methods are accurate, making them the industry-recognized "benchmark methods". However, they cause local damage to the road surface, which requires subsequent repair.

1. Ring Knife Method (Applicable to Fine-Grained Soil Subgrades/Bases, e.g., Clay, Silt)

• Core Principle: Use a "ring knife" (a metal ring with a volume usually of 200cm³ or 100cm³) of known volume to vertically cut into the road surface material, take out a complete soil sample, weigh the wet weight of the soil sample, dry it to a constant weight, calculate the dry density (dry density = dry soil mass / ring knife volume), and then compare it with the "maximum dry density" of the material to obtain the compaction degree.

• Operating Steps:

1. Select test points (avoid edges and joints, and arrange points randomly in accordance with specifications, e.g., no less than 3 points per 1000㎡).

2. Remove the surface loose soil, and press the ring knife vertically into the soil layer (with the help of a hammer or press to avoid disturbing the soil sample).

3. Dig out the ring knife, cut off the excess soil sample, seal it, and weigh it (to get the total mass of wet soil).

4. Put the soil sample into an oven (105℃±5℃) and dry it for 8-12 hours until the mass remains unchanged, then weigh it (to get the mass of dry soil).

5. Calculate the dry density and compaction degree, and determine whether they meet the standards.

• Advantages and Disadvantages: High accuracy (error ≤ 1%) and simple equipment (ring knife, oven, balance). However, it is only applicable to fine-grained soil and cannot test materials containing coarse aggregates (e.g., crushed stone bases, asphalt surface layers). Moreover, the sampling process is prone to affect results due to disturbance.

2. Sand Replacement Method (Applicable to Various Soil Subgrades and Bases, Especially Materials Containing Coarse Aggregates)

• Core Principle: Dig a test pit of a certain volume at the test point, collect all materials in the pit, weigh and dry them to calculate the actual dry density of the material. At the same time, use "standard sand" (dry sand with uniform particles and known density) to fill the test pit, and calculate the volume of the test pit based on the mass and standard density of the sand, finally deriving the compaction degree.

• Key Points:

1. The volume of the test pit should match the particle size of the material (e.g., the depth of the soil subgrade pit is 15-20cm, and the depth of the crushed stone base pit is not less than 3 times the maximum particle size of the aggregate).

2. The density of the standard sand should be calibrated in advance (ensure it is dry and has uniform particles, free of impurities). When filling, it should be poured slowly and the wall of the test pit should be tapped lightly to ensure the sand is fully compacted and reduce volume errors.

• Advantages and Disadvantages: Wide application range (applicable to fine-grained soil, coarse-grained soil, and crushed stone layers) and relatively high accuracy (error ≤ 2%). However, the operation is cumbersome (time-consuming for digging pits, collecting samples, and replacing sand), it has high requirements for the proficiency of inspectors, and it is not applicable to extremely soft soil or soil with excessively high water content (which is prone to collapse and cannot form a pit).

3. Core Drilling Method (Applicable to Asphalt Concrete Surface Layers and Cement Concrete Bases)

• Core Principle: Use a core drilling machine (equipped with a diamond drill bit) to drill a complete cylindrical core sample from the road surface (usually with a diameter of 100mm or 150mm and a height consistent with the thickness of the surface layer). Measure the volume and mass of the core sample, calculate the bulk volume density (a common indicator for asphalt surface layers), and then compare it with the "standard density" required by the design (e.g., Marshall test density, maximum theoretical density) to obtain the compaction degree.

• Operation and Judgment:

1. Sampling points should avoid joints, manhole covers, and repaired areas, and the core sample should be complete and free of cracks (if the core sample is loose and layered, it indicates insufficient compaction or construction defects).

2. Use a vernier caliper to measure the diameter and height of the core sample (accurate to 0.1mm) and calculate the volume.

3. Weigh the mass of the core sample (accurate to 0.1g) and calculate the bulk volume density (bulk volume density = core sample mass / core sample volume).

4. If the design requires "Marshall density", it is necessary to compare it with the Marshall test results of the same batch of asphalt mixture; if "maximum theoretical density" is required, it should be obtained through the vacuum method or calculation method.

• Advantages and Disadvantages: It directly reflects the compaction quality of the asphalt surface layer and can simultaneously observe whether the core sample has problems such as segregation and excessive voids. However, it damages the road surface (the core hole needs to be repaired with asphalt mixture), and the number of samples is limited (no less than 3 core samples per lane per 1km), so it needs to be supplemented with non-destructive testing.

II. Non-Destructive Testing Methods (Indirect Methods): High Efficiency, Suitable for Large-Area General Surveys

Non-destructive testing does not damage the road surface. It indirectly infers the compaction degree by testing the physical properties of the material (e.g., density, wave velocity, resistivity). It is suitable for large-area rapid general surveys or as a supplementary verification for destructive testing. However, the results are easily affected by factors such as material water content and particle composition, and need to be "calibrated" with destructive testing results to ensure accuracy.

1. Nuclear Density Gauge Method (Widely Used for Subgrades, Bases, and Asphalt Surface Layers)

• Core Principle: Use gamma rays emitted by radioactive elements (e.g., Cesium-137, Americium-241) to penetrate the road surface material. By detecting the correlation between "transmitted ray intensity" and "material density" (the higher the density, the more the rays are absorbed, and the weaker the transmitted intensity), the compaction degree is directly displayed (calibration with standard blocks or sand replacement method results is required in advance).

• Two Testing Modes:

1. Transmission Method: Place the radiation source and detector on both sides of the road surface (e.g., road surface and pit bottom), suitable for testing deep density (e.g., 20-30cm depth of subgrade).

2. Reflection Method: Both the radiation source and detector are on the road surface, suitable for testing surface density (e.g., 5-10cm depth of asphalt surface layer), with more convenient operation.

• Precautions:

1. It must be operated by personnel holding a "Radioactive Operation Permit", and radiation protection regulations must be strictly followed (e.g., wearing a dosimeter, avoiding long-term close contact).

2. The instrument must be calibrated with a "standard density block" before testing, and re-calibration is required after every 50 test points or when the material type is changed.

3. The water content of the material will affect the test results (excessively high water content will enhance ray absorption), so the water content must be measured simultaneously and corrected.

• Advantages and Disadvantages: Fast testing speed (only 1-2 minutes per point) and continuous testing capability. However, the equipment cost is high (regular maintenance is required), it is greatly affected by water content, and it is not applicable to areas close to steel bars and pipelines (metals shield rays).

2. Ground Penetrating Radar (GPR) Method (Applicable to Asphalt Surface Layers and Cement Concrete Pavements, Capable of Testing Void Ratio and Compaction Uniformity)

• Core Principle: Transmit high-frequency electromagnetic waves (1-10GHz) to the road surface through an antenna. The electromagnetic waves will be reflected at the interfaces of materials with different densities (e.g., between surface layer and base, between voids and aggregates). After receiving the reflected waves, the material density and void ratio are inferred by analyzing parameters such as "wave velocity" and "amplitude" (the higher the density, the higher the wave velocity; the higher the void ratio, the stronger the reflected amplitude).

• Application Scenarios:

1. Conduct large-area scanning to check whether there is "segregation" in the asphalt surface layer (areas with concentrated aggregates have high density and fast reflected wave velocity; areas with concentrated asphalt have low density and slow wave velocity).

2. Assist in judging compaction uniformity and identify areas with insufficient local compaction (e.g., joints, edges).

• Advantages and Disadvantages: It can quickly obtain continuous cross-sectional data (capable of testing 1-2km per hour) and intuitively display the internal structure of the road surface. However, the equipment is expensive, the result interpretation requires professionals, and it is greatly affected by the flatness and water content of the road surface, so it needs to be verified in combination with the core drilling method.

3. Falling Weight Deflectometer (FWD): Indirectly Reflects Compaction Quality (Focusing on Load-Bearing Capacity)

• Core Principle: A heavy hammer (e.g., 50kN) is dropped freely to impact the road surface, generating an instantaneous load. The "deflection value" of the road surface (vertical deformation of the road surface under load) is measured — the higher the compaction degree, the greater the road surface stiffness, and the smaller the deflection value (the design deflection value shall be used as the judgment standard).

• Application Scenario: It is mainly used to test the overall load-bearing capacity of the road surface, but it can indirectly reflect the compaction quality (if the deflection value exceeds the standard, it may be due to insufficient compaction leading to insufficient stiffness). It is often used in combination with density testing.

• Advantages and Disadvantages: High testing accuracy and ability to simulate the effect of vehicle load. However, the equipment is heavy (needs to be towed by a trailer), the testing speed is slow (3-5 minutes per point), and it is not applicable to road sections where the base is not yet formed.

III. Judgment and Handling of Test Results

• Data Statistics Requirements: Arrange points randomly in accordance with specifications (e.g., no less than 3 points per 1000㎡ for subgrades, no less than 3 points per lane per 1km for asphalt surface layers). The compaction degree of a single test point must be ≥ the design value, and the pass rate must meet the requirements (e.g., the pass rate for municipal roads is required to be ≥ 95%, and for expressways ≥ 98%). Meanwhile, the compaction degree of unqualified points must not be lower than 96% of the design value (specific standards shall refer to project specifications).

• Handling of Unqualified Results: If the compaction degree of a test point fails to meet the standard, it is necessary to find out the reasons (e.g., insufficient excitation force of the roller, insufficient rolling times, excessive water content of the material) and take rework measures (e.g., supplementary rolling, milling and repaving). After rework, re-testing is required until the standard is met.

• Principle of Method Combination: For large-area general surveys, prioritize the "nuclear density gauge method + ground penetrating radar method" for rapid screening. For suspected unqualified areas, use the "sand replacement method (for subgrades)" or "core drilling method (for surface layers)" for accurate verification to ensure comprehensive and reliable test results.

Summary: Selection of Testing Methods for Different Scenarios

Through the reasonable combination of the above testing methods, it is possible to comprehensively and accurately judge whether the road compaction degree meets the design standards, providing a guarantee for the subsequent service performance and durability of the road surface.