36 AGGREGATES degraded and waste material that is not required as part of the aggregate In sand and gravel workings, the source material is excavated in either dry or wet pit working In marine environments, the process is based on suction and dredging using two techniques In the first, the dredger is anchored and a pit is created in the seabed; production continues as consolidated materials fall into the excavation In contrast, trail dredging is performed by a moving vessel, which excavates the deposit by cutting trenches in the seabed Extracted crushed rock, sand, and gravel are then prepared as aggregates through the use of jaw, gyratory, impact, and cone crushers The type of crusher is selected according to the individual sizes of the feed material Grading by screening is an adjunct to comminution and is also necessary in the production and preparation of the finished aggregate in cases where the particle-size distribution of the aggregate is important The product is also washed and cleaned The process of cleaning often uses density separation, with weak porous rock types of low density being removed from the more satisfactory gravel materials Classification The classification of aggregates has changed significantly over the years but has always suffered from the need to satisfy many different interests Most commonly aggregates are divided into natural and artificial and, if natural, into crushed rock, sand, and gravel If the aggregate is a sand or gravel, it is further subdivided according to whether it is crushed, partly crushed, or uncrushed It may then be important to state whether the material was derived from the land or from marine sources Once produced, the aggregate is identified by its particle size, particle shape, particle surface texture, colour, the presence of impurities (such as dust, silt, or clay), and the presence of surface coatings or encrustations on the individual particles Detailed petrographic examination is employed so that specific rock names can be included in the description This also helps in the recognition of potentially deleterious substances However, the diversity of rock names means that considerable simplification is required before this classification can be used to describe aggregates Following recognition of the main category of rock from the field data, more specific names can be applied according to texture and mineral composition Because aggregates are used for particular purposes, they are sometimes grouped according to their potential use This means that they may be incorrectly named from a geological point of view The most obvious example of this is where limestone is referred to as ‘marble’ In 1913 a list of petrographically determined rock types was assembled, with the rocks being arranged in Trade Groups This was thought to help the classification of road stone in particular It was presumed that each Trade Group was composed of rocks with common properties However, the range of properties in any one Group is so large as to make a nonsense of any expectation that the members of the Group will perform similarly, either in tests or in service The Trade Groups were therefore replaced by a petrological group classification However, even rocks within a single petrographic group can vary substantially in their properties For example, the basalt group includes rocks that are not basalt, such as andesite, epidiorite, lamprophyre, and spilite Hence a wide range of properties are to be expected from among these diverse lithologies In the first place a classification describes the nature of the aggregate in a broad sense: quarried rock, sand, or gravel; crushed or otherwise Second, the physical characteristics of the material are considered Third, the petrography of the possibly diverse materials present must be established This may require the examination of large and numerous samples While it may be reasonable to describe as ‘granite’ the aggregate produced from a quarry in a mass of granite, that aggregate will inevitably contain a wide range of lithologies, including hydrothermally altered and weathered rocks Whether a rock is geologically a granite, a granodiorite, or an adamellite may be less significant for the description of the aggregate than the recognition of the presence of strain within the quartz, alteration of the feldspar, or the presence of shear zones or veins Aggregate Grading Aggregate grading is determined by sieve analyses Material passing through the mm sieve is termed fine aggregate, while coarse aggregate is wholly retained on this sieve (Figure 1) The fine aggregate is often divided into three (formerly four) subsets – coarse, medium, and fine – which fall within specified and partly overlapping particle-size envelopes The size range is sometimes recorded as the ratio of the sieve sizes at which 60% passes and at which 10% passes The shapes of the particles greatly affect the masses falling in given size ranges For example, an aggregate with a high proportion of elongate grains of a given grain size would be coarser than an aggregate with flaky particles This can affect the properties of materials made using the aggregate for, say, concrete, road materials, and filter design Commonly materials needed for particular purposes have standard