Sand, gravel and crushed rock aggregates for construction purposes

Edited by M. R. Smith and L. Collis


In 1985, the Geological Society published Aggregates as the first volume in its Engineering Geology Special Publication series. It met with immediate acclaim, being awarded the Brewis Trophy by SAGA in 1986.

“If your work involves the use of aggregates, buy this book and read no further; this volume will be an essential and valuable reference that you will use for many years.” (Canadian Geotechnical Journal 1988)

In 1989, the working party whose work had resulted in the publication of Aggregates was reconvened to revise, update and extend their report. Each chapter was reviewed by independent referees. The second and greatly improved edition, published in 1993 and reprinted in 1998, represented the distillation of a vast body of knowledge and experience held not only by the members of the working party, but also by many international experts, scientists and engineers who contributed as reviewers, referees and corresponding authors.

Owing to continued demand for this unique reference book, a group of aggregate specialists was convened in 1999 in order to review thoroughly and update Aggregates for this third edition.

Outline of contents: Introduction; Occurrences; Field investigations; Extraction; Processing; Classification; Testing; Aggregates for concrete; Aggregates for mortar; Unbound aggregates; Bituminous bound aggregates; Rail ballast; Filter media; Appendix: Aggregate properties; Glossary; Index.

Working Party Members and/or third edition Reviewers: Mr L. Collis (formerly Sandberg); Professor P. G. Fookes (Chairman; consulting engineering geologist), Mr R. A. Fox (formerly RMC Aggregates (UK) Ltd), Professor G. P. Hammersley (formerly Laing Technology Group, now BRE), Mr P. M. Harris (formerly BGS), Dr I. E. Higginbottom (formerly Wimpey Environmental Ltd), Mr J. Lay (RMC Aggregates (UK) Ltd), Dr G. Lees (formerly University of Birmingham), Mr D. I. Roberts (Land and Mineral Resource consultants), Mr A. R. Roeder (formerly British Cement Association), Dr I. Sims (Secretary; formerly Sandberg, now STATS Limited), Dr M. R. Smith (formerly Imperial College, now the Institute of Quarrying), Dr R. G. Thurrell (formerly BGS), Dr G. West (formerly TRL).

  1. Page 1

    Aggregates are defined here as particles of rock which, when brought together in a bound or unbound condition, form part or the whole of an engineering or building structure.

    Natural sand, gravel and crushed rock aggregates are fundamental to the man-made environment and represent a large proportion of the materials used in the construction industry. Re-use of aggregates has become a more common practice and the substitution of natural aggregates by artificial aggregates made from waste products of other industries is a small part of the industry.

    Production of aggregates for civil engineering and building construction is one of the world’s major industries. Quarrying is the largest industry in terms of tonnage of output (but not value) in the United Kingdom and even in 1985, production was two and a half times that of coal. With the closure of so many deep coal mines in 1999 this figure was nearly seven times larger.

    Consumption of aggregates has more than doubled over forty years from 100 million tonnes in 1959 to between 200 and 300 million tonnes per annum throughout the last decade (see Fig. 1.1). Sand and gravel production in 1959 was 67% of the total with crushed rock providing the balance of 33%. By 1998 this situation had changed significantly with crushed rock production increasing substantially to 132 million tonnes (60%) and sand and gravel only to 86 million tonnes (40%).

    In 1989, growth in demand was strong and a compound growth rate of 3% was forecast

  2. Page 5

    This section is intended primarily as an introduction and guide for non-geologists engaged in the aggregates industry. It is intended to be useful in understanding the more straightforward day-to-day geological situations that arise in the development and management of workings. It provides a basis for recognizing the more complex and intractable problems for which specialist geological advice is likely to be needed.

    A main contribution of geologists to the study of aggregates is the recognition that rock material owes its properties to its origin, its mineral composition and to the geological processes that have affected it through time. Knowledge of the qualities that determine the suitability of a rock (or concentration of rock fragments) for use as aggregate enables the prospecting geologist to make an informed search for new deposits, recognizing and defining the clear, though not always simple, relationships that exist between the composition, texture, grain-size, fabric and state of weathering of a rock and its likely performance as an aggregate in an engineering structure or other application.

    Rock is natural material that forms the crust of the parth. Some rocks are relatively soft, that is to say, weak and easily deformable. Others are hard, strong and durable. Rock so defined includes the ‘soil’ of engineers (that is, all unconsolidated deposits overlying bedrock). However, it does not cover the soils of pedologists (the earthy materials forming the ground in which land plants can grow). Rocks may be examined in cliffs and quarries at the surface and in mines and

  3. Page 41

    The location of aggregate materials through the appraisal of geological and geotechnical information and the eventual selection of sites from which to extract them, requires the collation of data from many sources through desk studies, followed by field reconnaissances and the evaluation of prospects to the necessary level of geological assurance. After the identification of a possibly useful deposit, further, more specific investigations of its physical and mechanical characteristics may confirm that it is potentially useful as aggregate, i.e. it is a resource (McKelvey 1972), and that certain parts of it may be capable of being worked at a profit in prevailing market conditions and so may be regarded as a reserve (§3.7.2).

    Whilst the main objective of this section is to outline methods for investigating and reporting on possible sources of aggregate, some attention is also given to operational, commercial, environmental and planning matters which, along with the geological factors, have to be considered by the engineer or geologist during the assessment of reserves for a quarrying prospect.

    Thus, previously unreleased records of mining and quarrying operations, former aggregate workings or of investigations for them or for civil engineering works such as major road schemes, may be relevant, along with land use or soil surveys for agricultural purposes, old topographic maps and even local oral tradition from well sinkers, farmers and the local population (Chaplow 1975).

    Although the assembly of published and unpublished data will draw primarily on the collation of available previous geological and geotechnical

  4. Page 73

    The subject of mineral extraction involves a number of highly technical and economically sensitive issues ranging from overburden and waste removal and disposal, through blast design and methods of loading and transporting ore to selection and scheduling of equipment and environmental protection. Many of these issues are more usually associated with the engineering of surface mines for coal and metals and, as such, are the subject of detailed studies and publications (IMM 1983; Hartman 1987; Kennedy 1990; Shaw & Pavlovic 1991).

    In the UK, the term ‘mine’ is defined by law (Mines and Quarries Act 1954) as any mineral extraction operation that takes place underground. Surface extraction is ‘quarrying’ although metalliferous surface mines are often referred to as ‘open-pits’ and surface coal mines as ‘open-cast’. At the present time, the lower cost of surface mining dictates that almost all aggregates are produced by quarrying. The methods and equipment employed in quarrying rock or excavating sands and gravels are similar to those used in surface mining operations and, in recent times, often approach the scale, capacity and output of large metalliferous mines.

    This chapter identifies some of the more important aspects of mineral extraction relevant to aggregate production but is necessarily a simplified and condensed review of the methods and equipment used in the quarrying industry.

    The methods and equipment employed to extract aggregates depend primarily on the type of deposit or source rock being worked. The selection of particular techniques and machines

  5. Page 107

    The purpose of the aggregate processing plant is to prepare the rock or mineral in a form suitable for its use as aggregate, commonly defined in terms of particle size and size distribution, particle shape and mechanical properties, e.g. compressive strength. As a result, the process plant usually contains only the unit processes of crushing and grinding (comminution) and sizing together with materials handling and transportation equipment such as conveyors and feeders.

    The use of water and wet processing techniques facilitates the sizing of fine particles (classification) and the dispersal and subsequent rejection of finely sized mineral particles, e.g. clays. Consequently, the process plant may frequently also contain pumping and slurry handling equipment and unit processes of solid-liquid separation for final dewatering of the aggregate products and even waste products.

    Mineral separation processes are occasionally employed to reject material of undesirable physical or chemical properties. In this respect the process of sizing is commonly used adventitiously or deliberately to separate a particular mineral fraction of the aggregate as will be discussed below.

    Comminution is an energy intensive and relatively expensive process whose use must be minimized and the agglomeration of fine particles to create larger sizes is rarely if ever economically justified.

    Therefore, as discussed in Chapter 3, in the case of sand and gravel deposits it is important to determine the relative proportions of the ‘sand’ and ‘gravel’ sizes.

    All plants should be provided with sufficient monitoring instrumentation, e.g. mass flow meters, sampling points and control

  6. Page 145

    The description and, in particular, the classification of aggregates in a manner appropriate to their use in the construction industry has long posed problems, not only of a scientific nature but also from practical and commercial points of view.

    Naturally occurring rock materials can be classified in a variety of ways, the method chosen depending on the nature of the rock and the use for which the classification is required. Age, colour, fossil content, grain size, mineralogy, mode of formation and compressive strength are but some of the many approaches that have been used. The most common method is that developed from the classical geological approach, which is based essentially on the mode of formation. Hence natural rock material is divided into three main classes: igneous, sedimentary and metamorphic. These groups are then subdivided, principally on the basis of their mineralogy and texture.

    The numerous subdivisions possible in this fundamental geological system inevitably results in a nomenclature which is too cumbersome for general use in the construction industry. As a consequence, various schemes have been developed to simplify the classification of aggregates, some intended for general use, others to meet specific purposes.

    Some level of petrographic examination is necessary for virtually all classification schemes and a detailed petrological description can be helpful in assessing the performance of an aggregate and in detecting potentially deleterious substances.

    This chapter reviews current classification schemes for natural aggregates and discusses their development. A recommended approach for classification is presented and procedures for the

  7. Page 167

    This chapter considers the tests and procedures used to describe or evaluate the physical, mechanical and chemical characteristics of aggregates, for the purposes of (a) prediction of the likely ‘in service’ behaviour of the material (b) comparison between competing materials (c) specification compliance or (d) quality control. Individual limits are not discussed here but are considered in the appropriate Chapter. Sub-base materials (Chapter 10) sometimes have to use tests in BS 1377, but this section confines itself to BS 812 and the relevant standards from other countries.

    The first step is the collection of samples. Statistically, a sample can be defined as an individual or group of individuals drawn from a large or infinite population, Information obtained from samples is only as representative of the material as the samples on which they are performed. If observations reveal little variation and there has been no bias in collecting, then a small sample or small number of samples may be highly representative of a population. If the variation is large then more and/or larger samples will be required Representative sampling, however, is perhaps the most difficult of the control operations to perform satisfactorily. Sampling, as with all types of test, introduces sources of variation and error, so that judgements of materials based on infrequent random tests are fraught with difficulties. In this connection, see also the remarks on sampling in relation to classification given in Chapter 6.A random sample is one in which each potential observation has an

  8. Page 199

    Concrete may be defined as a mixture of water, cement or binder, and aggregate, where the water and cement or binder form the paste and the aggregate forms the inert filler. In absolute volume terms the aggregate amounts to 60–80% of the volume of concrete and is, therefore, the major constituent. The aggregate type and volume influences the properties of concrete, its mix proportions and its economy.

    The desirable and undesirable properties of aggregates for concrete have been thoroughly reviewed in a number of texts on concrete technology (Orchard 1976; American Society for Testing and Materials 1978; Murdock, Brook & Dewar 1997; Fookes 1980; Neville 1995). In practice, difficulties are frequently encountered in translating these properties into specification requirements for aggregates, or in assessing aggregate test results to determine compliance or otherwise with already specified parameters. This chapter considers the principal properties of concrete aggregates with the aim of assisting in the selection of appropriate specification requirements.

    The essential requirement of an aggregate for concrete is that it remains stable within the concrete and in the particular environment throughout the design life of the concrete. The characteristics of the aggregate must not affect adversely the performance or cost of the concrete in either the fresh or hardened state.

    For both technical and contractual reasons, these requirements have to be defined quantitatively. This involves the selection of relevant tests and assessment procedures and the specification of appropriate acceptance criteria.Hence the need for continued

  9. Page 225

    This chapter, which is concerned with aggregates for mortars, necessarily introduces some degree of overlap with Chapter 8. However, there are major differences between mortars and concretes, arising mainly from the manner of their use.

    The term ‘mortar’ is used in the building industry to denote a mixture of natural sand or other fine aggregate and some binding agent, used as a jointing or a surface plastering and rendering material (Fig. 9.1). In the United Kingdom the demand for building sand grew steadily up to 1973 to reach a peak of 23.6 Mt, representing some 18% of the total sand and gravel production (HMSO 1989) for the country. Since then it has fluctuated considerably, falling to 15.7Mt in 1981 but rising again to 21.89 Mt in 1988 (Fig. 9.2). It should be noted that the output figures for building sands are far in excess of figures for mortar production.Until about 30 years ago the choice of binders in the UK was limited to lime or cement or cement-lime mixes. Mortars made with lime alone as binder are no longer used for building except for some specialist applications in the repair of historic buildings. The choice of cementbinders has been widened to include masonry cementS(specially blended mixtures of Portland cement with finely divided mineral plasticizers and air-entraining agent). Increasingly, air-entrainment in mortars, not only to confer frost resistant properties but also to aid workability, is being used even with cement/lime/ sand mortars. The use

    Until about 30 years ago the choice of binders in the UK was limited to lime or cement or cement-lime mixes. Mortars made with lime alone as binder are no longer used for building except for some specialist applications in the repair of historic buildings. The choice of cement binders has been widened to include masonry cements (specially blended mixtures of Portland cement with finely divided mineral plasticizers and air-entraining agent).

    Increasingly, air-entrainment in mortars, not only to confer frost resistant properties but also to aid workability, is being used even with cement/lime/sand mortars.

  10. Page 249

    In highway and airfield pavements, aggregates are used in various types of unbound or bound materials (standard nomenclature for pavements as used in the United Kingdom is illustrated in Fig. 10.1). This chapter is concerned with both primary (naturally occurring) and secondary (artificial or recycled) aggregates which are not bound by cementitious or bituminous binders. Unbound layers are used in the UK mainly for sub-bases or capping, but elsewhere may be used for bases or, in the case of low volume roads, the whole structure. Cement bound aggregates are discussed in Chapter 8 and bitumen and tar bound aggregates are discussed in Chapter 11. Figure 10.2 shows bitumen macadam being laid over unbound sub-base.

    Unbound layers in pavement construction may fulfil some or all of the following functions:

    1. (a)

      a working platform for construction;

    2. (b)

      a structural layer (load spreading and resistance to rutting);

    3. (c)

      a replacement for frost-susceptible subgrade (if necessary);

    4. (d)

      a drainage layer.

    The main use in the UK is as sub-base for which (a) is probably the most important function. Guidance is given in Powell et al. (1984).

    The ability to spread load (high stiffness) and to resist rutting (low permanent deformation) is usually associated with closely graded materials whereas open gradings are thought necessary for good drainage. The apparent conflict between these requirements has received considerable attention over recent years (Roy 1981; Jones & Jones 1989). These and other aspects of the

  11. Page 265

    Both natural and artificial aggregates of coarse and fine sizes are employed in bituminous mixtures for highway and airfield pavements (Fig. 10.1) and in hydraulic and building appHcations (The Shell Bitumen Handbook 1990).

    The emphasis in this chapter is upon the use of bituminous materials in pavement construction. The same factors affecting mix density, strength, stiffness and adhesion, apply in hydraulic applications as in pavement construction. However, in the former, greater attention is paid to such properties as permeabiHty (which may be required to be high or low depending on the particular application), resistance to flow on steep slopes and, where relevant, to wave impact (Hills & McAughtry, 1986). Polishing resistance characteristics of aggregates clearly have no importance in hydraulic applications but, in building construction, mastic asphalts used in bridge decks and in decks and ramps of multi-storey car parks require a high skid resistance. Chief among the requirements of mastic asphalt for bridge decks, roofing and car parks is impermeability in order to protect the underlying concrete construction from water and frost attack and from the effect of de-icing salts and other chemicals (Mastic Asphalt Conferences 1989). The major factor in these applications is the mix design, involving use of a high bitumen content in accord with the high content of fine aggregate and filler in the aggregate grading.

    It should be noted that, at the time of going to press, several relevant developments were taking place with respect to the

  12. Page 285

    Railway track formations generally consist essentially of a layer of coarse aggregate, or ballast, in which the sleepers are embedded (see Fig. 12.1). The ballast may rest directly on the subgrade or, depending on the bearing capacity, on a layer of blanketing sand. The layer of ballast is intended to provide a free draining base which is stable enough to maintain the track aUgnment with the minimum of maintenance. The function of the blanketing sand is primarily to provide a filter to prevent contamination of the ballast by fine particles derived from ascending waters (see Fig. 12.2).

    The fundamental engineering is reasonably well understood. It concerns the transmission of the load from the base of the sleeper on to an area of closely packed ballast beneath. The sleeper bears on to the points of the angular ballast which are, in consequence, liable to fracture or abrade under load, a problem more severe with concrete than with wooden sleepers. If the layer of ballast is not close-packed and of sufficient depth there is a tendency for the sleepers to move up and down as the wheels of the traffic pass in succession, eventually causing the sub-grade to be disturbed.

    Consequently the ballast layer must be thick enough to hold the track in position and prevent traffic loads from distorting the subgrade, while the aggregate under the sleepers must be tough enough to resist the abrasion and degradation caused by the intermittent traffic loads. The main requirement, confirmed

  13. Page 291

    Although the total volume of aggregate used for filters is relatively small, filters nevertheless play important and diverse roles in many projects. Table 13.1 shows some of the main uses of filter aggregates and the first four parts of this chapter discuss some of the requirements and properties of these materials. The final three parts of the chapter describe the functions and specification of filter aggregates in their three main applications, namely water filtration, effluent treatment and as drainage filters for earthworks and other civil engineering structures. These will normally be subject to different design criteria and may also call for filter materials with distinct sets of physical and mechanical properties.

    Filter aggregates generally consist of sand, gravel or crushed rock. Manufactured aggregates are also occasionally used and these often include blastfurnace slags.

    Although filter materials for water and effluent treatment works are often used in relatively small quantities, the high quality of aggregate normally required is not always readily available from commercial production processing, which may be designed to yield a satisfactory general purpose aggregate at least cost. On the other hand, drainage layers in major civil engineering works, such as embankment dams, are usually designed to make the best use of the available natural materials with the minimum of processing.

  14. Page 299

    In this appendix some information on aggregate properties is given. First some properties of currently produced British aggregates, second some more generalized data on aggregate properties worldwide, and third some information on how weathering affects the properties of aggregates. It is strongly emphasized that all the information given in this Appendix must be regarded as providing only a general appreciation of aggregate properties and should not be relied upon for specific test values for particular rocks or sources.

    Table A1 gives some test results for 96 aggregates from quarries in Great Britain. The information is not exhaustive but comes solely from fact-sheets issued by the suppliers or from information provided to members of the Working Party by the suppliers. None of the data has been confirmed independently. Where the quarry is not named in the Table this omission is at the request of the supplier. Because aggregate properties are liable to vary, the test results in the Table should not be taken as an indication of present production for particular quarries; instead prospective users should seek up-to-date information from the suppliers. It should be borne in mind that suppliers will not usually work poor quality sources of aggregate, nor provide data on them.

    The rock descriptions are those of the suppliers, supplemented in some cases by reference to entries in directories of quarries (BGS 1984 and BACMI 1986). The classification in Table A1 is by the broad rock groups used in the

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