Single-sized aggregate

In the UK, BS 63 [8.3] specifies requirements for single-sized aggregates (50, 40, 28, 20, 14,10, 6 and 3 mm) for general purposes and for surface dressing (although limestone is rarely used for surface dressing). The single sizes may be recombined to produce the particle size distribution required for a particular application. 

SMA differs from HRA, since the mortar fills the voids, whereas in HRA, it constitutes the base of the mixture to which coarse, almost single-sized aggregates are added. 

Since there is no separation of the hot aggregate before mixing, the number of cold aggregate fractions used should be as many as possible (single-sized aggregates are used) to ensure consistency of the produced mixture. This increases the number of cold bins required. 

The use of this type of batch plant is limited nowadays because of its high capital cost, its relatively high maintenance cost, the requirement of almost single-sized aggregate and its low output. 

The technique consists of the creation of a membrane approximately 3.5 mm thick, capable of absorbing the tensile stresses that developed. Thus, it is known as the stress-absorbing membrane (SAM) technique. On the membrane created, and immediately after spraying the modified bitumen, single-size aggregates of 8/10 or 10/16 mm are spread and slightly rolled with a pneumatic-tyred roller. The aggregates are applied hot for better adherence to the modified bitumen. 

On the reinforced bitumen membrane formed, single-size aggregates are spread and lightly compacted. The fibre-reinforced chipped surface may be exposed to traffic or covered soon by an asphalt overlay. The quantities required per type of material and the type of bitumen used are specified by the supplier/producer. 

I. Growth or Tumble Agglomeration (see Chapter 7) The mechanism of growth (see Section 7.1) of this method, the addition of single or aggregated particles to the outside of an agglomerate that increases in size, results in a typical structure with rather constant porosity. Porosity depends mostly on the size, distribution, shape, and surface... 

As many aggregate specifications require specific particle size distributions, processing plants generally screen the stone into single sized fractions with a size ratio of approximately 1.4 (e.g. 20 to 28 mm, 14 to 20 mm and 10 to 14 mm) and store them separately. The fractions are then blended, using sophisticated weighing-out equipment, to meet the size distributions specified by the customers. 

Single-sized generally refers to aggregate, which passed through a screen deck with an aperture size of 1.4 units and is retained on one with an aperture size of 1.0 units (e.g., passing 14 mm and retained on 10 mm). 

Asphalts are characterised by the particle size distribution of the aggregate mixture. Theoretically, there are unlimited types of asphalts, namely, from asphalts consisting only of almost single-sized coarse aggregates to mixtures consisting only of fine aggregates (sand). All types of asphalts used range between these two extreme cases. 

Aggregates should be single sized and crushed from appropriate hard rocks, free of dust or soil, complying to CEN EN 13043 (2004). 

The design aggregate spread rates for single seals with size aggregates 10 mm and larger and for any type of binder to be used are given in Table 15.10. 

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