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Unbound materials collectively comprise a vast array of different materials, which may range in size from fine grains less than a millimetre in diameter up to stony material several centimetres in diameter.
Unbound materials are used in embankments, as fill materials, as capping, and to form the sub-base of paved areas.
All recycled and secondary aggregates (RSA) can potentially be used as the constituents of unbound materials used in construction, and often form 100% of such material.
The unbound material could be produced on site, using material from the construction site recycled in situ, or may be purchased from external suppliers.
The Quality Module provides further information on the standards governing use of RSA in unbound material, including the Specification for Highway Works Series 800, Design Manual for Roads and Bridges (DMRB) HD35/04, and relevant British and European Standards.
Factors that may need to be considered when choosing the constituents of unbound materials are grading, moisture content, moisture condition value, plasticity, hardness, sulfate and sulfides content (if the fill is to be placed close to concrete structures), organic matter content, pH, chloride ion content, and permeability. It is also necessary to know the constituents of any RSA used; hence careful control of the quality of material in stockpiles is necessary to avoid contamination. Generally however, the requirements are fairly easy to meet and do not normally preclude the use of RSA.