Different Types of Compaction Equipments
Effective compaction of material is an essential part of construction engineering operations concerned with foundations, construction and maintenance of the roads, airports, dams etc. The equipment required for these work range from the smallest vibrating rammers to the largest rollers of vibrating type.
Quality characteristics such as strength, wearing resistance, evenness and imPermeability of pavement depends on the degree of compaction of various layers of the pavement and it’s sub-grade supporting it.
The better the compaction, the better will be the strength, density and bearing capacity of the individual layers and with that the lasting quality of the embankment or road construction.
For explaining this, we take an example of road, the complete road, from base to wearing course must be compacted to higher specifications than the compaction value of the axle weights of the vehicles using them. If this is not achieved, the vehicles will in a very short period break up the surface because of the further reduction of the voids as the heavier vehicles continue to compact the material.
Rollers are used for compaction since long olden days. Over a thousand years ago the Chinese used huge cylindrical shaped stone rollers for road works, and the world famous builders of Rome used towed stone rollers. The self-propelled road rollers, powered by steam engines were built in England in nineteenth century.
As the time passed, road roller design continuously improved. Since the manufacture of the first diesel-engine-powered road roller in 1929, the development of the compaction equipments has taken place rapidly.
What is Compaction ?
Compaction is the process whereby material particles are constrained to pack more closely together through a reduction of air void content, generally by mechanical means.
Compaction can also be defined as the process of densifying or increasing the unit weight of a soil mass through the application of static or dynamic force, with the resulting expulsion of air and, in some cases, moisture. Static forces are produced by a roller or compactor which densifies soil primarily by its heavy weight, whereas dynamic-forces utilise combination of weight and energy to produce a vibratory or tamping effect on the soil.
Compactors utilise both static and dynamic forces to achieve the required soil density. Primary functions of compaction is to produce the reasonable surface finish in accordance to the layer position within the whole road or embankment construction. Compaction is measured in terms of dry density.
The amount of compaction applied can be controlled by the choice of compaction equipment, the thickness being compacted, the speed of the compaction equipment and the number of passes of compaction equipment.
Types Of Compactors- (Rollers).
Today’s compaction equipment are represented by many large, highly responsive, and versatile self-propelled units. These compactors and rollers can be divided into following major classes :
- Static Smooth Wheeled Rollers.
- Sheep Foot or Pad Foot Rollers.
- Pneumatic Tyred Rollers
- Vibratory Rollers.
Static Smooth Wheeled Rollers.
These are rollers used with or without ballast and may be 3 wheeled or tandem type (two rolls of equal width). These are the conventional rollers used for almost all types of rolling. It is not effective on uniformly graded sand, gravel or silt and on over moist cohesive soil due to poor traction.
Diesel powered rollers with modern technology of hydrostatic transmission rollers have replaced the traditional steam rollers which were first introduced 100 years ago. The dead weight rollers (static rollers are sometimes called by this name) rely on weight alone to compact the material over which they pass. The units of 8 to 10 tonnes imparting pressure of 20 to 40 kg per linear cm are generally used. For light works, lighter rollers of weight upto 1 ton are also available.
Sheep Foot or Pad Foot Rollers.
These may be self driven or tractor driven and are suitable on cohesive soils specially when water content is on the higher side. The inherent dynamic beat developed by these tamping pads or feet in many cases produce a high degree of compaction at a faster rate.
These are used for compaction both the earthwork and bituminous road construction. Because of the oscillating axle layout they produce more even compaction across the rolling width than a wide steel wheel smooth rollers, which sometimes bridge the material leaving uncompacted areas of fill.
On bituminous wear courses, it can eliminate surface cracking and material crushing caused by steel rolls – the tyres fold and knead the material to produce an almost total sealed finish. These rollers are available in weighing range from 6 to 30 tonnes with 500 to 4000 kg per tyre and tyre pressure from 2 to 8 kg/sq.cm.
Different types and sizes of vibratory rollers to suit specific conditions are available like :
- Rollers with large vibrating drum infront with 2 steel or pneumatic tyred rear driving wheels. Pneumatic tyres offer better traction on gradients and in over moist soil. These may be articulated or non articulated type.
- Roller with tandem smooth wheels, either one wheel vibrating and driven or both wheel vibrating and driven.
- Single roll pedestrian vibrators rollers weighing nearly 400 kg are used for compacting small areas such as pavements, foundations etc.
- Towed vibrating rollers weighing 5000 to 11000 kg. are aged for mass earthworks, base constructions embankments, rockfill dams etc.
Vibrations are produced by rotating eccentric weights within the rolls at high speed. These roll thus hitting the material to be compacted at high frequency. The vibration reduces the friction between the particles of material which are brought closer together as air voids are eliminated. Thus binding the material into denser state. Further. compaction results from the dynamic forces applied by vibrating roll onto the material being compacted.
Vibrating rollers are very effective on free draining type soil and granular base course. These are not effective on uniform graded sand, gravel or silt. Rollers with high amplitude are preferable for soil and granular base course. At high amplitude vibrations the effective pressure on the compacting materials is around twice the static weight of the roller.
Tampers and Vibrating Plates.
Tampers with small vibrating feet are used to tamp footings, trench bottoms, position posts, lay paving slabs etc. These can also be used on small road repair jobs, footpath construction and compacting trench bottoms for pipe laying.
Operational weights fall in range upto approximately 80 kg for tampers and as much as 300 kg for vibrating plates. Although sizes larger than about 120 kg are seldom used. The tampers can deliver front 800 to 4500 blow/min and are generally powered by engines of 2.5 to 12.5 H.P.