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In common with the rest of the construction industry, the paving and drainage trades are heavily dependent on the use of all sorts of aggregates. Vast quantities of rock are quarried for road-building stone, for cement production and for surfacing materials ranging from York Stone Flags to Pattern Imprinted Concretes and asphalts, river channels and estuaries are dredged for sands and gravels, clays are extracted for hoggin and for pipe and brick production. However, the industry has, like the rest of the world, had to face up to its environmental responsibilities, and so there is now a conscious effort to recycle and re-use aggregates and other materials if at all possible. Old concrete is crushed, graded and sold as selected fill; bituminous roads are planed down, heated up, re-mixed and relaid in one operation and the demand for reclaimed stone flags, setts and cobbles seems greater than ever. We probably won't ever see an end to quarrying, after all, humankind has been hacking and hewing away at the hills for thousands of years, but we can make best use of the resources we have, and ensure that we minimise the impact our modern society imposes on our natural environment. This page provides basic information on the aggregates most commonly used in the paving industry.

Ash Up until the mid-late 1970s, ash (aka 'clinker' or 'cinders') was routinely used as a bedding material for all sorts of pavings, but the demise of the coal-fired power stations, along with the concurrent reduction in other coal-powered industries such as textiles (up here in the north), steel-making, etc., saw the volume of available ash reduced and it has now been almost completely replaced by sands and grits. Being considerably less dense than sand or other quarry products, ash was a bedding material much-admired by the laying gangs, as it is easy to cart and to spread, and compacted well. It also had a couple of lesser-known uses; it was mixed with cement to make 'ashcrete' a lightweight cement-bound fill material suitable for non-load-bearing applications, and there were also pre-cast flags/slabs that known as 'cinder-backs' or 'ash-backs' that had an upper face of a dense, hard-wearing concrete atop a base of 'ashcrete'. As a boy in the 1960s, I recall seeing these 'cinder-backs' being manufactured at a small works in St Helens Junction, Lancashire, during a visit to the factory with my father. Nowadays, Pulverised Fly Ash (PFA) is a by-product of the modern power stations. The coal is crushed to a miniscule size, usually less than 1mm before being injected into the furnace and burnt . The resulting waste ash is used as a fill material and PFAs with a high silicon content can be used in specialist concretes as a partial replacement for cement. See also.... Cement (below) Sub-bases Page

Asphalt A black hydrocarbon that occurs naturally and can be manufactured as part of the oil distillation process. Chemically quite similar to bitumen. Asphalt is used in 2 main ways - as mastic asphalt for waterproofing, particularly on bridge decks, and as rolled asphalt (aka Hot Rolled Asphalt or HRA) for road and pavement surfacing. Asphalt is also used as a generic term in Europe and North America to include all asphalts and bitumen coated materials. See also.... Bitmac (below) Tarmac Page

Ballast As with a lot of terms used for aggregates, this one has several meanings. In some parts of the country, 'all-in ballast' refers to a mixture of gravel and/or small stone (<40mm) with sand, used to make concrete. Elsewhere, it can refer to the coarse granular stone (around 40-50mm in size) used to support railway lines or as a general 'fill' material of crushed rock, slag or gravel. See also.... Concrete (below) Concretes Page Gravel (below)

Bitumen/Bituminous Macadam Bitumen is a sticky black mixture of hydrocarbons occurring naturally or produced as part of the crude oil distillation process. It is soluble in carbon disulphide. It is widely used in the paving industry to coat aggregates to form Tar Macadam (Tarmac) or Bituminous Macadam (Bitmac). A sizable section of the construction industry is devoted solely to the chemistry, production and usage of bitumen products and as such, it is impossible to give comprehensive coverage of the subject on this site, although a general overview is given on the Tarmac page.

In its raw form, bitumen, or pitch as it is sometimes refererred to, was once the preferred joint sealant for sett paving, although this role has diminished as the area of sett work undertaken each year has reduced and as high-strength mortars have become more readily available. See also.... Asphalt (above) Tarmac Page Sett Paving F	Bitumen Jointed Setts
Cement Cement is a powder manufactured from limestone that is mixed with other aggregates, notably sands, gravels and stone, to produce mortars and concretes. The vast majority of cement used in the UK is Portland Cement, sometimes referred to as Ordinary Portland cement or OPC, although there are also specialist cements, such as Sulphate-Resistant Cement (SRC) which is often used for sub-surface works, and High-Alumina Cement (HAC).
It is generally sold in 50kg and 25Kg paper sacks, although the big commercial users, such as concrete batch plants and pre-cast manufacturers will often take delivery via powder tankers.	Cement being delivered by tanker
Cement is NOT the same as mortar or concrete, but is a component thereof. A pet bugbear of mine is the incorrect use of the word by the general public who use the term as a catch-all word for mortar, concrete and cement powder. See also.... Concrete (above) Mortars and Concretes Page
Clay Clay is primarily used as a source material for brick-making. Selected clays are extracted and shaped into bricks, either by hand or by machine, before being baked at high temperatures in a kiln. Different clays give different colours of brick, and will also determine other properties, such as hardness, frost-resistance, water-absorption etc. Second to brick-making, clay is used in the manufacture of drainage pipes and fittings where again, the clay is prepared and worked to shape or extruded into pipes before being fired in a kiln. Clay is also used a general fill material on earthworks projects and specialist clays, such as Bentonite, used in the drilling industry, may have commercial applications outside the construction industry. See also.... Clay Brick Paving Page Drainage Materials Page

Concrete Concrete is a mixture of sand, gravel and cement that sets hard within a few hours of being mixed with water. The proportions of the fine aggregate (sand), coarse aggregate (gravel), cement and water are varied to give concretes of different strengths and workabilities. The amount of water used to mix a given volume of concrete is determined by the Water:Cement ratio (W/C ratio) which works on the principle that a a smaller quantity of water (lower W/C ratio) results in a stronger concrete (assuming proper compaction). Concrete is graded according to its characteristic strength after 28 days of curing, with a Class 7.5 mix being a relatively weak concrete for fill purposes or for bedding of kerbs, while a C40 mix is a relatively strong mix suitable for cast in-situ work and load-bearing applications. The table from BS882 giving mix parameters for various grades of concrete is given on a separate page.
Most concrete is delivered by special wagons from 'Batching Plants' which mass produce concrete of a guaranteed quality on demand. Most of the larger, national companies belong to the Quality Scheme for Ready Mixed Concrete (QSRMC) to guarantee their product meets or exceeds the requirements of the specifications. On larger projects, batching plants may be especially set up solely to provide concrete for that project.	Concrete Wagon
For smaller quantities, concrete may be mixed on site in small mechanical mixers that can churn out 0.25-1.0 m³ at a time, or for very small quantities, it may be mixed by hand. See also.... Mortars and Concretes page Concrete Pavements page Concrete Block Paving page Concrete Flags/Slabs page
DTp1 This is the most commonly recommended material for constructing a sub-base. The full title is Granular sub-base material to Type 1 of the Department of Transport Specification for Highway Works (Clause 803). It is usually a crushed rock, typically limestone, granite or gritstone, although it may be slag or some other inert hard material. See also.... Sub-bases page Rock Types for Road-building (below)

Glass (recycled) Large quantities of glass waste or cullet is produced each year in Britain and Ireland and there have been several initiatives to develop uses for recycled glass as an aggregate for the construction industry in the past decade. With the greater focus on recycling within the EU since the beginning of the new millennium, we expect the markets and opportunities for recycled glass to expand in the coming years. Some of the more successful current uses for recycled glass include... Tumbled glass as a decorative aggregate for surface dressing and/or mulches As a laying course material for flexible block paving, when crushed/tumbled to sand grain sized particles from Day Aggregates As a decorative coarse aggregate in concrete flags/slabs and block pavers in Charcon's "Malvern" Range of products As fine aggregate in concrete blocks from Conway Concrete Products Ltd. As a filler and aggregate in "Glasphalt", an asphaltic base course product from RMC PDF Documents on other websites... RMC documentation Transport Research Laboratory report As a filter medium in specialist drainage schemes

Gravel A granular material which can be of almost any rock type. It is usually between 60mm and 2mm in size which may be rounded, if from a marine or fluvial source, or angular if a quarried and crushed product. Gravels are sold in mixed sizes, e.g. 20-5mm or closely graded to a specific size, such as 10mm.	10mm rounded gravel	10mm angular gravel
Gravel has many uses in the construction industry, including pipe-bedding, coarse aggregate for concrete and as a surface dressing. See also.... Gravels page Concrete (above)

Grit
Another granular material that can be thought of as a transition stage between a coarse sand and small pebbles. Generally 2-6mm in size, grit has limited uses in the construction industry on its own, other than as a surface dressing. However, over recent years with the development in block paving specifications, it has become a viable alternative bedding material for permeable paving and other forms of elemental paving used in areas of high water ingress. See also.... Permeable Paving page	6mm grit
Hardcore A term that has unfortunately been usurped by the porn industry and which provides a constant source of amusement to me when I see the web logs for just how many saddoes have surfed over to this site in search of hardcore, only to find a pile of old bricks and broken concrete. :~) Generally, hardcore is any hard, inert material that can be used as a fill, although it is generally thought of as demolition waste and rubble. In these environmentally-friendly times when we have a progressive tax regime imposed on the dumping of waste materials, more and more 'hardcore' is being crushed and processed to form suitable fill material for all sorts of construction projects. See also.... Sub-bases page
Hoggin A well-graded gravel mixed with natural clay as a binder and used as a surfacing for low-speed roads and footpaths. Common in the south-east of the country, but rarely seen any further north than Birmingham. See also.... Gravels page
Planings When a bituminous or asphaltic road surface is renewed, the old surface is often removed by a beast of a machine known as a Cold Milling Machine, or, more familiarly, a Planer, which mills and reduces the existing surface to small chunks, usually 15mm or less in size. In some cases, these planings, as they are called, are recycled by reheating an mixing with a fresh binder to create a new bituminous or asphalt-bound surfacing material, but quite often they are used as a poor quality fill, often ending-up on farm tracks and the like.	Planing Machine in operation
They are not considered to be a suitable fill or sub-base material for most construction projects because asphalt/bitumen is a perishable product and as such, will degrade over time, which could lead to problems of settlement of worse should they be used in any load-bearing capacity.

Sand There seems to be more confusion regarding the different types of sand than any of the other aggregates. Sand is composed of grains of rock, in the size range 0.06mm to 2.0mm, mostly quartz, (aka silica or SiO2) although there are often other minerals mixed in, especially clays and iron, which gives some sands their distinctive reddish colouring.

They occur as natural drift deposits, especially in marine and/or fluvial environments, although some of the coarser grit sands are produced as a by-product of quarrying and rock-crushing. A uniform sand consist of grains of mainly one size range, be it coarse[C], medium[M] or fine[F] (see table), whereas a graded sand contains a mixture of grain sizes. This runs counter to everything I learned as a student of geology, where a graded sand is one where the grains are predominantly of one size, ie, what civil engineers call a uniform sand.

Grading of Fine Aggregate Percentage by mass passing B.S sieve Sieve Size Overall Limits Additional Limits for Grading C M F 10.00 mm 100 - - - 5.00 mm 89 -100 - - - 2.36 mm 60 -100 60 - 100 65 - 100 80 -100 1.18 mm 30 -100 30 - 90 45 - 100 70 -100 600 µm 15 -100 15 - 54 25 - 80 55 -100 300 µm 5 -70 5 - 40 5 - 48 5 - 70 150 µm 0 - 15 - - - Sand grading limits - from BS 882

Building /Soft sand

Grit/Sharp sand

For general on-site construction purposes, there are two main working categories of sand, namely building sand (aka soft sand) which is primarily used in the preparation of mortars, and grit sand (aka sharp sand), which is used as a bedding material for paving. Naturally there are intermediate sands and applications thereof, such as jointing sand, but these are the two most regularly encountered categories.

Building sand tends to be a fine sand, often with a clay content of 10% or more while a grit sand will have a low clay content and a greater proportion of larger grain sizes. For laying course sands, the usual specification followed is that given in BS7533:Part 3, for a sharp sand with not more than 3% clays or silts by weight. For Paving Flags/Slabs, a sand complying with grade C or M of BS 882 is recommended. The table opposite gives grading limits for sands suitable for bedding and for jointing conventional (NOT permeable) block paving. Note that the bedding sand is considerably coarser than the jointing sand.

Laying course and jointing sand for conventional block paving

As can be seen from the base of the above table, just to complicate things further, there is a further categorisation of sands based on the percentage by mass that passes through the finest-mesh, namely the 63 µm sieve. In the table above, the different pavement categories having different values for the maximum recommended fines content.

Block pavements are assigned to one of these five zones (aka "categories") based on thier typical usage and extensive research over a number of years has shown that the sands with a lower proportion of the finer particles (zone 1 or zone 2) are more reliable as bedding sands for heavy applications, and that natural sands with rounded grains are less prone to settlement and fluidity than the angular, crushed-rock sands.

Rounded grains pack together tightly, giving a stable bedding layer, whereas angular grains have been found to have their 'rough edges' broken off under loading, thereby increasing the proportion of very fine particles in the sand which can be carried by percolating water, giving rise to a 'lubricating' effect which, in turn leads to separation of the sand into layers of differing grain size, possibly causing problems in heavily trafficked projects. However, don't be daunted by all this soil mechanics micro-detail: for private driveways, patios and other low-traffic projects, a standard grit sand as sold by the local Builders' Merchants will more than likely be perfectly adequate as a bedding material. It is only for adoptable roads and high-traffic/heavy-loading applications that such in-depth consideration of the bedding layer sand is required. Finally, two specialist sands; jointing sand, as mentioned above, is used to fill the joints of block paving and is specially selected to have a very low clay content and have grain sizes that will generate a high degree of friction, thereby giving stability and resistance to loading in a block pavement. Silver sand is a sand with a very low iron content, and is much favoured in the manufacture of high-quality optical glass. Its silver (sort of) colour is a result of it being almost pure silica (SiO2) and there being virtually no contamination with other minerals. It has limited uses in the construction industry. See also.... Mortars and Concretes page Screeding tips page

Scalpings Scalpings are, correctly, the off-cuts or shards created by dressing stone for building work, ie, a quarry waste. They are not as common as they once were, as we now use machinery to dress stone and the waste is re-processed into, say, a select fill, a Type 1 sub-base material, or, as mentioned above, a crushed-rock sand, but the term persists in some parts of the country. The main use of scalpings nowadays is as a medium quality fill or sub-base material used in the construction of access paths, car parks, driveways or other light-use applications where a DTp1 is unavailable or uneconomic. As there is no hard and fast specification laid down as to what qualifies as 'scalpings', almost any quarry waste or road planings can be, and are passed off as suitable for driveway construction. For the extra couple of quid per tonne it costs for a Type 1 material, we prefer not to spoil the ship for a ha'porth of tar, as the saying goes. :~) See also.... Sub-bases page

Shale Yet another term that refers to more than one type of aggregate. Shale is, geologically, a laminated claystone or siltstone that readily cleaves along its bedding planes. However, in some circles, shale refers to colliery waste, which may or may not be a true shale geologically. It is often 'burnt', either by self-combustion or by industrial process, which renders it inert. It is rarely used as a sub-base material, as it tends to be far too soft, but is used as a surface dressing, especially the 'burnt red' type. See also.... Gravels page

Shingle Another non-specific term, usually referring to gravel, particularly a mixed gravel containing pebbles of various sizes, grits and sands. May be used in the preparation of concretes. See also.... Gravels page Concretes & Mortars page

Slag A waste product once common but, with the reduction in steel-making capacity in Britain over the past 30 years, is now rarely seen, other than when re-working an existing pavement. Slag is an inert fill material of varying size (although some slags may be graded to form selected fills or even Type 1 sub-base material) and is usually very dense, because of the high iron content. See also.... Sub-bases page

Slate There are two main types of slate used for paving purposes, and both are as surface coverings, rather than as sub-bases or fill materials. Slate is a metamorphic rock, formed when a mudstone, siltstone or other fine-grained agrillaceous sedimentary rock is subjected to supernormal temperatures and pressures. The subsequent change in nature gives slate its characteristic tendency to cleave along parallel planes. This fissile nature and low strength precludes it from being used as a sub-base material, and its tendency to crush to dust under load give it a fairly limited role as a fill material. The most common use of slate is as flagstones, either rectangular or irregular in shape, and generally 20-40mm thick. These are laid in much the same manner as pre-cast concrete or yorkstone flags are laid, on a cementitious bed with mortared joints. They are invariably much more expensive than pcc flags/slabs, and, depending on the type of slate, can be even pricier than yorkstone. The primary reason for choosing slate flagstones as a paving is for their colour, usually a blue, purple or green, and/or for their texture.
Slate mulch of mixed colours	Over the past few years, a new craze for slate has swept the nation and is a real triumph of marketing. Slate mulches are, in fact, slate waste, the offcuts, the trimmings, the unusable dross left over from the roofing slate industries of North Wales and other parts of the country. 15 years ago, the quarries could hardly give the stuff away. And so it sat there, in huge mounds of waste, slowly weathering and supporting absolutely no plant life in its barren state, for year after year until the trendy garden designers latched upon it. Since then, demand has rocketed and it sells for as much as 10 quid for a 40Kg bag - that's 250 quid per tonne, for what is, to all intents and purposes, quarry waste!
See also.... Gravels page Stone Paving page

Stone Stone is by far the most widely used aggregate. It is used in its crushed form as a fill or a sub-base material; it can be dressed into paving units, such as setts or flagstones; it can also be dressed into walling blocks, known as ashlar; it can be pulverised and turned into a sand or processed into a cement. For road construction purposes, there are 11 groups of rocks: Artificials Basalts Flints Gabbros Granites Gritstones Hornfels Limestones Porphyrys Quartzites Schists These rocks are subjected to a number of tests, such as crushing strength and polishing value, to determine their usefulness as a road-building material. See also.... Sub-bases page

Topsoil The upper layer of soil, usually containing a mixture of clays, silts, sands, grits and humus (organic material). Topsoils vary throughout the country from sandy soils through loams to clay soils. It supports a massive range of flora and fauna and is often placed as a top dressing to earthworks in order to support grasses, trees or other plantings.

Comparative densities of various aggregates

More data about the densities of various construction materials and aggregates are available on the SI Metric website

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