In pavement compaction operations, "over-compaction" refers to a phenomenon where the number of compaction passes and compaction energy applied by the roller to the construction material exceed the material’s optimal compaction requirements. Essentially, it means the compaction operation goes beyond the physical and mechanical bearing limit of the material itself or the designed compaction standards. To determine whether over-compaction has occurred, a comprehensive assessment is required based on the material type (e.g., asphalt, stabilized soil, sand-gravel), designed compaction degree requirements, and on-site performance, rather than simply measuring the number of rolling passes.

I. What Is "Over-Compaction"? Core Judgment Standards

The core of over-compaction lies in the "imbalance between compaction effect and compaction cost/damage"—it is not the case that "the higher the compaction degree, the better." The judgment criteria for over-compaction vary by material, as detailed below:

II. Adverse Effects of Over-Compaction on Pavements

Over-compaction damages pavements in three dimensions: "structural stability, functionality, and durability." The damage manifestations vary by material, but ultimately, all will shorten the pavement service life and increase maintenance costs:

1. Effects on Asphalt Pavements (Most Intuitive and Harmful)

• Surface Function Failure: Over-compaction causes asphalt bleeding, leading to a sharp drop in the pavement friction coefficient (from the normal 0.6-0.8 to below 0.3). This makes vehicles highly prone to skidding and rear-end collisions on rainy days, especially posing significant risks on highways or steep sections.

• Permanent Deformation of Structural Layers: After being excessively squeezed, the internal aggregate skeleton of the asphalt mixture breaks, making it unable to bear vehicle loads. In the short term, ruts (sinking of more than 3-5mm at wheel tracks) and bulges (heaving at lane edges) will appear. In severe cases, milling and repaving are required; otherwise, further issues such as cracks and potholes will occur.

• Reduced Aging Resistance: The bleeding asphalt surface is directly exposed to ultraviolet rays and high temperatures, accelerating oxidation. Peeling and looseness will occur within 3-5 years, whereas the service life of a normal asphalt pavement can reach 8-10 years.

2. Effects on Stabilized Soil Subbases (Cement/Lime-Stabilized Soil)

Stabilized soil serves as the "load-bearing layer" of the pavement; over-compaction directly damages its load-bearing capacity, which in turn causes cracking of the surface layer:

• Increased Drying Shrinkage Cracks: Over-compaction forcibly compacts the internal pores of the stabilized soil, leading to rapid moisture evaporation. The soil shrinks due to water loss, producing a large number of transverse and longitudinal cracks (0.5-2mm in width, 1-3m in spacing). These cracks will reflect upward to the asphalt surface layer, forming "reflection cracks." After rainwater seeps in, it will also corrode the subbase, leading to pavement collapse.

• Strength Decline Instead of Improvement: The strength of stabilized soil relies on the chemical reaction between cement/lime and soil particles (forming cementitious substances). Over-compaction damages the structure of these cementitious substances, resulting in a compaction degree that meets standards but an unconfined compressive strength that decreases by 20%-30% (e.g., the designed strength is 3MPa, but the actual strength is only 2.1-2.4MPa). This makes it unable to support the load of the surface layer, and long-term use will cause the subbase to "harden and break."

3. Effects on Sand-Gravel/Graded Crushed Stone Cushions

The role of the sand-gravel cushion is to "drain water, level the surface, and distribute loads"; over-compaction disrupts its gradation balance:

• Loss of Drainage Function: Over-compaction causes fine materials (sand, stone powder) in the sand-gravel to be pressed into the gaps of coarse materials, blocking drainage channels. On rainy days, rainwater cannot penetrate into the ground, forming a "water accumulation layer" between the cushion and the subbase. Long-term immersion will soften the subbase, leading to pavement pumping (a "bubbling" phenomenon on the pavement when vehicles pass).

• Reduced Uniformity of Load-Bearing Capacity: After the loss of fine materials, coarse materials lose interlocking support. The surface of the rolled cushion will have "local loose areas," resulting in uneven thickness of the surface layer after paving. Subsequent compaction will also enter a vicious cycle of "local over-compaction and local under-compaction."

III. How to Avoid Over-Compaction? Key Control Measures

The essence of over-compaction is the "mismatch between compaction operations and material properties." To avoid it, measures must be taken from three aspects: "pre-construction calculation, in-process monitoring, and post-construction testing":

Pre-Construction: Determine the "Optimal Rolling Plan"

Based on the material type, thickness, and moisture content, conduct a "trial compaction test" to determine the optimal number of rolling passes, roller tonnage, and travel speed. For example:

• Asphalt pavements: Use a 6-8t double-drum roller for initial compaction (speed: 2-3km/h), an 18-22t pneumatic-tired roller for intermediate compaction (speed: 3-4km/h), and a 6-8t double-drum roller for final compaction (speed: 2-3km/h), with a total of 6-8 passes.

• Stabilized soil: Use a 12-15t vibratory roller (amplitude: 1.5-2mm) with 4-6 passes.

In-Process: Real-Time Observation of "Roller Tracks and Surface Condition"

During rolling, if the roller tracks change from "clearly visible" to "blurred with a shiny appearance" (for asphalt), cracks appear on the surface (for stabilized soil), or aggregates roll (for sand-gravel), immediately stop rolling to avoid further pressure application.

Post-Construction: Verification with Test Data

After compaction is completed, verify the effect through compaction degree testing (ring knife method, sand replacement method), flatness testing (3m straightedge or continuous flatness meter), and strength testing (unconfined compressive strength test for stabilized soil). If the compaction degree has met the design value (e.g., ≥96% for asphalt, ≥97% for stabilized soil), there is no need to continue rolling even if slight tracks are visible.

In conclusion, over-compaction is not a "trivial matter of excessive compaction" but a key issue that directly affects pavement structural safety and service life. It must be strictly avoided through scientific rolling plans and on-site management.