"Over-compaction" refers to a phenomenon in compaction operations where the pressure or vibratory energy exerted by a road roller on the material exceeds its suitable bearing range, leading to damage to the material's structure or performance. This situation is often caused by improper operation, incorrect parameter settings, or unfamiliarity with material characteristics, and it requires special attention, especially in asphalt pavement construction.

Harms of Over-Compaction to Different Compacted Materials

1. Harms to Asphalt Mixtures

Asphalt mixtures are composed of aggregates, asphalt binder, and air. Over-compaction disrupts the balance of their internal structure:

• Aggregate Crushing: Excessive pressure or vibration causes the edges and corners of hard aggregates to break, reducing the interlocking force between particles and lowering the strength and stability of the pavement.

• Asphalt Film Stripping: Excessive kneading force damages the asphalt film on the aggregate surface, reducing the pavement's resistance to water damage and making it prone to early diseases such as potholes and peeling.

• Pavement Shoving and Bumping: Over-compaction at high temperatures increases the fluidity of the asphalt binder. The mixture accumulates in front of the roller, forming shoving, which becomes permanent bumps after cooling, affecting flatness and driving safety.

• Surface Bleeding: Over-compaction "squeezes" the asphalt inside the mixture to the surface, forming an oil film that reduces the pavement's friction coefficient and increases the risk of skidding in rainy weather.

2. Harms to Subgrade Soil

Over-compaction of subgrade soil (especially cohesive soil) changes its physical structure:

• Soil Shear Failure: When the shear strength of the soil is exceeded, soil particles undergo shear displacement, forming "springy soil" or "rubber soil" that rebounds when rolled over by vehicles.

• Uneven Moisture Distribution: Over-compaction squeezes water out of the pressure area, leading to local over-saturation or under-saturation, reducing the overall stability of the subgrade and making it prone to settlement or uneven deformation.

• False Compaction: The surface compaction degree meets the standard, but the deep layer may experience "false compaction" due to shear failure, which is prone to settlement during long-term use.

3. Harms to Crushed Stone / Graded Crushed Stone

Crushed stone materials rely on particle interlocking to form strength. Over-compaction damages their skeleton structure:

• Particle Crushing: Excessive static pressure or high-amplitude vibration crushes crushed stone particles, changing the gradation and reducing internal friction and bearing capacity.

• Segregation: Over-compaction concentrates fine materials (stone chips, dust) on the surface while coarse aggregates sink, resulting in uneven material composition and affecting the integrity of the structural layer.

Summary

The essence of over-compaction is a mismatch between compaction energy and material characteristics. It not only fails to improve compaction quality but also causes targeted damage to different materials. During construction, it is necessary to reasonably select the road roller type, adjust vibration parameters (frequency, amplitude) and rolling passes based on factors such as material type, thickness, and moisture content. The optimal compaction plan should be determined through on-site tests to avoid over-compaction.