2. Discussion the basic principles of field compaction. Can climatic conditions influence compaction in road construction?
3. 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 .
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6. 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.
7. 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.
8. 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.
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11. 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.
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13. 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.
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15. 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.
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17. 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.
18. 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.
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24. 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.
