Better Roads

July 2014

Better Roads Digital Magazine

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Page 13 of 31

HighwayContractor 12 July 2014 Better Roads A system that allows detection of thermal segrega- tion in the asphalt mat – in real time – and re- cords a thermal profi le is being used now by some state Departments of Transportation to eliminate inconsis- tent densities because freshly paved asphalt pavement did not compact properly after becoming too cool and the low temperatures contributed to these low densities. There are many factors that infl uence the choice of compac- tion equipment selected for a project. Sometimes it is based on the contractor's previous experience and other times by the type of soil, method specifi cation or available equipment. How well a machine will conform to the hauling and spreading operation are also considerations, and climatic and traction conditions are also important. There is no single compactor that will do all things in all applications. Each type has a defi nite material and operat- ing range on which it is most economical. In many cases, there are applications where machines of different sizes and types can achieve the compaction target; but choosing the machine that is most suitable will complete the work most economically and effi ciently due to reduced passes, reduced fuel use and less working time. Vibratory Compactors Vibratory compactors work on the principle of particle rearrangement to decrease voids and increase density and load bearing strength. They come in two types: smooth drum and padfoot drum. For increased versatility, smooth drum compactors can be equipped with optional padfoot shell kits, which allows the use of smooth drum rollers in padfoot applications, albeit with limited performance. Smooth drum vibratory compactors generate three com- pactive forces: static pressure, impact and vibration. Padfoot drum machines generate the same forces, plus they also generate manipulative force. Vibratory compactors provide uniform compaction throughout the lift. Density is achieved from the forces generated by the vibrating drum hitting the ground. Compaction results are a function of the frequency and amplitude of the blows, as well as the force of the blows and the time period during which the blows are applied. The frequency/time relationship accounts for slower working speeds on vibratory compactors. Working speed is important because it dictates how long a particular part of the fi ll will be compacted. For vibratory compactors, a Photo courtesy of Minnesota Department of Transportation (Editor's note: This excerpt first appeared in The Cat Paving Products Guide to Soil Compaction. The book is available on and through participating Cat dealers. Also available is the "Cat Paving Products Guide to Asphalt Compaction.") Choosing the Right Machine for Proper Soil Compaction Experience, type of soil, method specification and available equipment are all part of making this decision.

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