SCHOOL OF HOUSING,BUILDING AND PIANINGBUILDING TECHNOLOGY

RET 565

WORK OF WORK OF ROAD ROAD

COMPACTIONCOMPACTION

Discussion the basic principles of field compaction. Can climatic conditions influence compaction in road construction?

Introduction

Compaction increases the density of a material by expelling air from the voids in the material and, thereby,

bringing the particles into more intimate contact with each other. Compaction is the cheapest and simplest method

for improving the shearing resistance of soil and minimizing future settlements. Therefore, soils in

embankments and subgrades in cuttings are usually compacted using special compacting equipment, such as

rollers, vibrators or tampers. The result of compaction work depends primarily on the moisture content of the soil,

the type of the soil, the compaction equipment used and the energy applied.

Compaction increases the density of a material. You can use rollers or other devices to expel air from the spaces in

the material and force the particles closer together, making it denser.

Increasing density gives soils or asphalt:

•greater strength for supporting heavier loads•reduced settlement over the life of the road

•less permeability to water and air•resistance to rutting

Types of Compaction in road Construction

We have two types of compaction for road construction

• compaction for soil

•compaction for asphalt

Objective of compaction for new road

Nature doesn’t always provide the ideal foundation for the construction of roads: natural rock. In the majority of cases, the ground is unstable, offering insufficient bearing capacity. Earthmoving operations are therefore usually the first step in the construction of a new road. When the course has been defined and set out, the ground underlying the new road needs to be prepared.

How to achieve proper compaction

In constructing roads you must compact each layer: subbase, base and surface. Depending on the type of road you are constructing, one, two or all three of these layers may be composed of the natural soils of the area. In order to compact them properly you must understand the two major factors which influence soil compaction: soil type and moisture content. Soil type dictates the relative number of passes required, the thickness of compaction layers, the type of compaction equipment, and how moisture affects compaction. And you must use different procedures and equipment to compact each soil type and still others for asphalt.

Compaction requirements

Compaction requirements are commonly specified using an end-product specification. Compaction tests on the particular soil are carried out in the laboratory, and the results are used to define the required density. During construction, the densities obtained in the field are determined and compared to that required. The most common methods applied for measuring field densities are the sand cone and the nuclear density gauge. It is normally recommended that the top of an embankment, as well as the upper 500mm of the subgrade in cuttings are compacted to minimum 95 per cent of the maximum dry density obtained in the modified proctor test.

Dynamic compaction is highly effective

Dynamic compaction of soils by means of vibrating or oscillating compaction equipment has proved to be particularly effective in earthmoving operations. Compaction is generated by the combined action of the vibrating or oscillating roller drum and the weight of the machine itself. Applying a dynamic load achieves significantly higher compaction effects than applying just the weight force.

Compaction of soil

Equipment use for soil Compaction

The choice of compaction equipment is wide. Most equipment is available in several sizes (deadweights). Compactors may be self-propelled or pulled by a tractor. The most common type of compactor is the self-propelled, smooth, steel wheeled roller. The slow-moving roller acts on the soil with ‘static’ loads. It can be used for compacting all types of soil, that is, gravel and sand as well as clay. The steel-wheeled roller may be equipped with devices for vibrating the wheels.

The vibration imposes pulsating stresses in the soil. This reduces the friction between the soil particles and results in a highly effective and deep compaction, particularly in gravel and sand. If the vibration is turned off, the roller can be used as a traditional, static roller. This makes the vibratory steel-wheeled roller.

Other types of compactor include the pneumatic-tyred roller and the sheepsfoot roller. A neumatic-tyred roller consists of rubber-tyred wheels mounted on an articulated frame, which provides a uniform load on each wheel. A sheepsfoot roller is made of a steel drum to which protruding, tamping feet have been attached. For compaction of backfill in narrow trenches and excavations for structures, a wide variety of hand-operated mechanical tampers and plate type vibrators are available.

Asphalt Compaction

Compaction is the final procedure in road construction, its goal being to produce a surface texture of high quality. What aspects need to be considered in asphalt compaction? When compacting an asphalt road, what impact will the road’s location or ambient temperatures have on the compacting operation?

The goal of asphalt compaction is to create a pavement that offers maximum evenness and stability, while simultaneously increasing both traffic safety and riding comfort. Compaction aims at producing a road surface of maximum regularity and skid resistance, and at creating a permanent bond between the various asphalt layers. Compaction by rollers results in an extremely tight interlock between the individual particles of the mineral aggregate, which enables a high degree of stability, excellent resistance to wear, and permanent evenness to be achieved. Rollers reduce the void content in the asphalt, making it more resistant to loads and more durable.

Equipment types

Three types of self-propelled compactors are currently used for asphalt compaction: 1.steel-wheeled static rollers2.pneumatic-tired rollers3.vibratory steel-wheel rollers

Time for Asphalt Compaction

For the contractor, time is the most important factor. This is how long it takes for the asphalt mixture to cool from laydown

temperature to a minimum compaction temperature, usually 175° F. Below this temperature, further attempts to compact

the asphalt mixture normally will not be effective and may fracture the aggregate in the mix, decreasing pavement

density and frustrating the purpose of compacting.

Influences on Asphalt CompactionCompaction is influenced by some factors, some related to environment, some determined by mix and structure design and some under construction and agency control during construction.Factors effecting compaction:1.Environment factor• Temperature•Ground temperature•Air temperature•Wind speed•Solar flux2.Mix property factor•Aggregate•Gradation•Size•Shape•Fractured faces•Volume•Asphalt binder•Chemical properties•Physical properties

3. Construction factor• Rollers• Type• Number• Speed and timing• Number of passes • Foundation support

Climatic conditions influence compaction in road construction

Base temperature —the temperature of the layer on which you lay the new layer—is more important than air temperature in determining time available for compaction because the layer cools downward more rapidly than upward.All other factors being equal, however, warmer air temperature slows the material's cooling rate, allowing more compaction time.

An asphalt layer will cool more quickly in high winds than when it's relatively calm. Wind has more affect on the surface of the layer than its interior. A strong wind can cause the surface to cool so rapidly that a crust forms. You must break this crust down with the roller before you can begin compacting.

A mix will cool slower on a sunny day than on a cloudy day (other factors remaining constant). Sunshine probably affects base temperature more than mix temperature

 

Steps in compacting asphalt surfacing

Asphalt pavements are usually compacted in three steps: breakdown rolling just behind the paving machine, intermediate rolling, and finish rolling which smooths out surface ruts and other deformities.

Rolling procedures

Rollers should move at a moderate but uniform speed with the drive wheel closest to the paver. Going too fast during breakdown rolling can harm the surface. Rollers should be in good mechanical condition and able to reverse direction without backlash.

Test Strip

•Test strip sets rolling pattern•Verifies that target density can be achieved•Straight run part of project•Nuclear density gauge checks progress while mat is still hot

Balanced Compactor Vibration

•Forces transmitted evenly when in balance•Mat accepts vibratory forces•Compaction maximized•Smooth machine operation

Compaction Issues

Impact Marks

•Amplitude too high•Vibrating cool mat -roll closer to the paver•Finish rolling too cool-move closer to intermediate roller•Finish roller too light•Drum straddling the joint•Vibrating on cool mat

Tire Marks

•Pneumatic roller in breakdown position on thick, hot lift•Ballast or tire pressure too high•Tires too narrow•Rolling too hot•Finish rolling too cool

Tire Pick-up

•Non-stick emulsions and soap solutions help prevent pick-up•Hand-held sprayers or on-board waterspray•Apply to clean tires before rolling begins•Discontinue after tires heat up•Re-apply if tires cool

Stopped Straight

•Straight stop creates bulge•Bulge may not roll out•Stop at 30-45 degree angle•Turn toward center of mat

Stopped on Mat

•Never park a roller on a hot mat•Deep mark won’t roll out•Park back on compacted mat or on shoulder

Summary

Compaction is the simplest, most economical method of improving the load-carrying capability of roads. When done during construction it costs very little per ton of pavement material and can significantly reduce future maintenance costs. Compaction also gives a road greater strength for supporting heavier loads, reduces settlement over its lifetime, and reduces its permeability to water and air and its susceptibility rutting.

References

•Road Engineering for Development, By Richard Robinson, Bent Thagesen.•Wisconsin Transportation Bulletin, Compaction Improves Pavement Performance, Transportation Information Center, UW-Madison, Department of Engineering Professional Development, ^ 432 N. Lake Street, Madison, Wl 53706.•Best/Worst Construction Practices from CATERPILLAR.•http://www.wirtgen-group.com/en/technologien/strassen-neubau/Einstieg_Strassen_Neubau.html