Pavement recycling methods

Pavement recycling methods differ from the flexible to the rigid type of pavement. In flexible pavements, two basic methods are distinguished hot recycling (HR) and cold recycling (CR). Each one is further distinguished into in situ (or in-place) and in-plant (central plant) recycling. 

As for the rigid pavement, there is only one method of recycling, the cold in-plant recycling. 

In flexible and, to a certain extent, rigid pavements, cold planning (or milling) is considered as a recycling method. 

Hot recycling in flexible pavements consists of removing the existing asphalt layer to a determined depth and replacing it with hot asphalt resulting from the same reclaimed material, with the possible addition of new material, that is, mixed in situ or new/recycled asphalt mixed in a central plant. 

Rigid pavement recycling is the procedure of crushing and fragmentising the concrete slab and re-using the fragmented concrete as coarse aggregate to produce a new concrete mix or base/sub-base material (stabilised or non-stabilised material). 

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Cold recycling is a pavement recycling method in which no heating is required at any stage of work. The advantages and disadvantages of cold recycling are outlined in Table 18.4. 

This chapter discusses current research on the use of sulfur in recycled asphaltic concrete pavements. In addition, it describes the results of laboratory tests and theoretical predictions using the latest linear viscoelastic layered pavement analysis methods (15,16) to compare the performance of various sulfur-asphalt concrete pavements with conventional asphalt concrete pavements in a variety of climates. The relationship between pavement distress and performance used in the computer program was established at the AASHTO road test (17). Finally, the results of domestic field tests of sulfur-asphalt pavements are presented along with a discussion of future trends for the utilization of sulfur in the construction of highway pavement materials. 

The selection of recycling method depends on various factors such as pavement condition, road category, traffic volume, age of pavement, past interventions, availability of equipment/ machinery, expected pavement performance after recycling, cost and energy saving achieved. Selecting the most appropriate recycling method is not an easy task, since many parameters, which vary from one project to another, are involved. 

The milling machine is used in all recycling methods of flexible pavements, while the grinding machine is used for surface skid resistance restoration of rigid pavements. 

In addition to the pavement design methodologies covered in Chapter 12, the pavement design method developed by Merrill et al. (2004) specifically for pavements with cold recycled materials may also be used. 

Recycling. The methods proposed for the recycling of polyurethanes include pyrolysis, hydrolysis, and glycolysis. Regrind from polyurethane RIM elastomers is used as filler in some RIM as well as compression molding applications. The RIM chips are also used in combination with rubber chips in the construction of athletic fields, tennis courts, and pavement of working roads of golf courses. 

The test method measures the resistance to ravelling characteristics of cold asphalts with or without recycled asphalt pavement (reclaimed asphalt) by simulating an abrasion similar to early return to traffic. 

Today, there are also other factors that affect decisions as regards recycling rigid pavements. The main factors are as follows (a) preservation of natural deposits, (b) the lack of aggregate availability close to work site, (c) deposition of old concrete slabs, (d) need for cost reduction, (e) development of new methods and machinery for easy removal of reinforcement and (f) existence of great variety of additives for improving concrete strength and workability. 

Stroup-Gardiner M. 2011. Recycling and Reclamation of Asphalt Pavements Using In-Place Methods A Synthesis of Highway Practice. NCHRP Synthesis 421. Washington, DC Transport Research Board. 

The most impressive section of the book to my mind, which is not covered in nearly as much depth in other texts, is the treatment of bituminous materials... in particular their characteristics, appropriate methods to characterize their performance in the laboratory, and means of building and maintaining a road using these materials. In addition, the book provides a comprehensive coverage of the constituent layers of a road pavement, the material properties of these layers, and associated laboratory tests required to characterize these. The different types of methods appropriate to structural design are covered nicely, as are methods of recycling and maintenance. ... 

The use of local materials for pavement design has been assessed. Recycling of bituminous substances is one of the relevant solutions. This method has the advantage of reusing budding materials by mixing them with a proportion of refined ones to achieve the required performance (9). 

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