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Types of Compaction
Roadway workers use four types of compaction techniques on soil and asphalt materials: vibration, impact, kneading and pressure. These methods utilize two major types of consolidation forces: static and vibratory.
Static force techniques, such as kneading and pressure, apply the dead weight of the machine itself downward on the soil surface. The only way to vary the compaction force is to add or subtract weight to the machine. The effects of this type of compaction method apply to the upper soil layers.
Workers apply vibration forces with an engine-driven device together with the machine's weight to deliver a rapid series of blows to the surface. This technique affects deeper layers as well as the uppermost layers. The vibration moves through the particles and shakes them together, squeezing out excess water and void spaces in the soil.
Compaction Techniques and Moisture
Soil consolidation, or compaction, is the process of physically increasing the weight per unit volume of the road bed soil. There are several ways to achieve this desired result. A drum roller can consolidate the soil by providing its own weight to press down the upper layers of the soil bed, and a vibrating roller shakes the soil particles together more densely as it rolls along. A sheep's foot roller contributes a kneading action.
Proper soil consolidation depends on the appropriate moisture content of the soil. Acting as a lubricant, moisture slides the particles together. However the water content must be at a proper balance to ensure compaction. Too much water fills voids in the soil and weakens the load-bearing capacity. If the soil is too dry, it resists compaction by assuming a dusty composition. Transportation engineers use the Proctor or the Modified Proctor Test to determine the maximum density achievable for the materials used.
Compaction Machines
Frequency and amplitude are two important factors in determining the type of force produced by a compaction machine. The frequency is the speed of rotation of the off-angle axle that makes a roller vibrate; the amplitude refers to the maximum movement of a vibrating roller in one direction. These parameters vary according to the weight of the roller and the consistency of the soil as progressive consolidation proceeds.
Rammers and Vibratory Plates
Rammers can be operated by one person and deliver a high amplitude impact force. The frequency ranges from 500 to 750 blows per minute. A small diesel powered engine runs a large piston with two sets of springs. Inclined at an angle, it travels forward under its own power as the machine jumps along. There are three types of compactions covered by rammers: vibration, impact and kneading.
Vibrator plates work well on granular soils and asphalt. They deliver low amplitude and high frequency blows and vibratory plates of different weights to increase the consolidation effort. They work well with asphalt, as they use a water tank and a sprinkler system to keep asphalt from sticking to the plate.
Rollers
Rollers include several varieties of compaction equipment such as walk-behind and ride-on equipment with smooth drums, padded drums and rubber-tired models. Workers find that the walk-behind equipment is smooth and easy to steer as a one-person operation. It compacts the soil and asphalt by vibration. A ride-on is a static-wheel roller used for asphalt surface sealing and finishing work on the asphalt bed.