Concrete is a mixture of cement, aggregates and water together with any other admixtures which may be
added to modify the placing and curing processes or the ultimate physical properties. Initially, when mixed concrete is aplasticmaterial, which takes the shape of the mould or formwork. When hardened it may be a dense, load-bearing material or a lightweight, ther- mally insulating material, depending largely on the aggregates used. It may be reinforced or prestressed by the incorporation of steel.
Most concrete is crushed and recycled at the end of its useful life, frequently as hard core for new construc- tion work. However, a growth in the use of recycled aggregates for new concrete can be anticipated, as this will have a significant environmental gain in reducing the demand on new aggregate extraction.
AGGREGATES FOR CONCRETE
Aggregates form a major component of concretes, typ- ically approximately 80% by weight in cured mass concrete. Aggregate properties, including crushing strength, size, grading and shape, have significant effects on the physical properties of the concrete mixes and hardened concrete. Additionally, the appearance of visual concrete can be influenced by aggregate colour and surface treatments. The standard BS EN 12620: 2002 specifies the appropriate properties including materials, size, grading and shape.
Aggregates for concrete are normally classified as lightweight, dense or high-density. Standard dense aggregates are classified by size as fine (i.e. sand) or coarse (i.e. gravel). Additionally, steel or polypropyl- ene fibres or gas bubbles may be incorporated into the mix for specialist purposes.
Dense aggregates Source and shape
Dense aggregates are quarried from pits and from the seabed. In the south-east of England, most land-based sources are gravels, typically flint, whereas further north and west, both gravels and a variety of crushed quarried rocks are available. Marine aggregates which account for 18% of production in England and Wales may require washing to remove deleterious matter such as salts, silt and organic debris. The total chloride con- tent should be monitored to ensure that it is within the limits to BS 8500: 2006 for reinforced or unreinforced concrete as appropriate. This may be achieved by using well-drained unwashed marine sand in conjunction with land-based coarse aggregates.
L I M E , C E M E N T A N D C O N C R E T E 7 1 The shape of aggregates can significantly affect the
properties of the mix and cured concrete. Generally, rounded aggregates require a lower water content to achieve a given mix workability, compared to the equivalent mix using angular aggregates. However, cement paste ultimately bonds more strongly to angu- lar aggregates with rough surfaces than to the smoother gravels, so a higher crushing strength can be achieved with crushed rocks as aggregate. Excessive proportions of long and flaky coarse aggregate should be avoided, as they can reduce the durability of concrete.
Aggregate size
For most purposes the maximum size of aggregate should be as large as possible consistent with ease of placement within formwork and around any steel reinforcement. Typically, 20 mm aggregate is used for most construction work, although 40 mm aggregate is appropriate for mass concrete, and a maximum of 10 mm for thin sections. The use of the largest possible aggregate reduces the quantity of sand and therefore cement required in the mix, thus controlling shrink- age and minimising cost. Large aggregates have a low surface area/volume ratio, and therefore produce mixes with greater workability for a given water/cement ratio, or allow water/cement ratios to be reduced for the same workability, thus producing a higher crushing-strength concrete.
Grading
To obtain consistent quality in concrete production, it is necessary to ensure that both coarse and fine aggre- gates are well graded. A typical continuously graded coarse aggregate will contain a good distribution of sizes, such that the voids between the largest stones are filled by successively smaller particles down to the size of the sand. Similarly, a well-graded sand will have a range of particle sizes, but with a limit on the proportion of fine clay or silt, because too high a content of fines (of size less than 0.063 mm) would increase the water and cement requirement for the mix. Usually a maximum of 3% fines is consid- ered non-harmful. This overall grading of aggregates ensures that all void spaces are filled with the minimum proportion of fine material and expensive cement pow- der. In certain circumstances, coarse aggregate may be graded as single-sizedor gap graded. The former is used for controlled blending indesigned mixeswhilst the latter is used particularly for exposed aggregate finishes on visual concrete. Sands are classified into three categories according to the proportion passing through a 0.500 mm sieve: coarse C (5–45%), medium
Fig. 3.5 Riffle box
M (30–70%) and fine F (55–100%). Only the coarse and medium categories of sands should be used for heavy-duty concrete floor finishes.
Sampling and sieve analysis
To determine the grading of a sample of coarse or fine aggregate, a representative sample has to be subjected to a sieve analysis. Normally, at least ten samples would be taken from various parts of the stock pile, and these would be reduced down to a representative sample using ariffle box, which successively divides the sam- ple by two until the required test volume is obtained (Fig. 3.5).
Aggregate gradings are determined by passing the representative sample through a set of standard sieves (BS EN 12620 + A1 : 2008). Aggregate size is speci- fied by the lower (d) and upper (D) sieve sizes. For coarse aggregates the sieve sizes are 63, 31.5, 16, 8, 4, 2 and 1 mm and for fine aggregates 4, 2, 1, 0.250 and 0.063 mm. Coarse aggregates are usually defined as having a minimum size (d) of 2 mm, while fine aggre- gates often have a maximum size (D) of 4 mm. The sieve analysis is determined by assessing the cumula- tive percentage passing through each sieve size. This is plotted against the sieve size and compared to the limits as illustrated for a typical coarse aggregate (Fig. 3.6).
Aggregates for concreting are normally batched from stockpiles of 20 mm coarse aggregate and con- creting sand in the required proportions to ensure consistency, althoughall-in aggregate, which contains
Fig. 3.6 Grading of coarse aggregates
both fine and coarse aggregates, is also available as a less well controlled, cheaper alternative, where a lower grade of concrete is acceptable. Where exceptionally high control on the mix is required, single-size aggre- gates may be batched to the customer’s specification.
The batching of aggregates should normally be done by weight, as free surface moisture, particularly in sand, can causebulking, which is an increase in volume by up to 40% (Fig. 3.7). Accurate batching must take into account the water content in the aggregates in the cal- culations of both the required weight of aggregates and the quantity of water to be added to the mix.
Impurities within aggregates
Where a high-quality exposed concrete finish is required, the aggregate should be free of iron pyrites, which causes spalling and rust staining of the surface.
Alkali–silica reaction (ASR) can occur when active sil- ica, present in certain aggregates, reacts with the alkalis within Portland cement causing cracking.
Recycled aggregates
The proportion and nature of constituent materials in recycled aggregate for concrete must be determined
Fig. 3.7 Bulking of sands in relation to moisture content
according to pr EN 933-11 (pending) and declared according to BS EN 12620: 2002 Amendment A1: 2008 as percentage limits by mass.
Constituent categories of recycled coarse aggregate:
Rc Concrete, concrete products, mortar, concrete masonry units
Ru Unbound aggregate, natural stone, hydrauli- cally bound aggregate
Rb Clay masonry units (bricks and tiles), calcium silicate bricks, aerated non-floating concrete Ra Bituminous materials
Rg Glass
FL Floating material in volume
X Other material (clay, soil, gypsum plaster, metals, wood, plastic, rubber)
Quantities of deleterious material within recycled aggregates must be declared and carefully controlled, to prevent adverse effects on the quality of the concrete.
Current research is evaluating concrete made from a mixture of recycled aggregates, china clay aggre- gate waste from Cornwall (‘tip sand’ and ‘stent’ rock), together with a high proportion of pulverised fuel ash (PFA) replacing the Portland cement clinker. The mix- ture has approximately 30% less embodied energy than standard concrete.
High-density aggregates
Where radiation shielding is required, high-density aggregates such as barytes (barium sulphate), mag- netite (iron ore), lead or steel shot are used. Hardened concrete densities between 3000 and 5000 kg/m3, dou- ble that for normal concrete, can be achieved.
L I M E , C E M E N T A N D C O N C R E T E 7 3
Lightweight aggregates
Natural stone aggregate concretes typically have den- sities within the range 2200–2500 kg/m3, but where densities below 2000 kg/m3 are required, then an appropriate lightweight concrete must be used.
Lightweight concretes in construction exhibit the following properties in comparison with dense con- crete:
they have enhanced thermal insulation but reduced compressive strength;
they have increased high-frequency sound absorp- tion but reduced sound insulation;
they have enhanced fire resistance over most dense aggregate concretes (e.g. granite spalls);
they are easier to cut, chase, nail, plaster and render than dense concrete;
the reduced self-weight of the structure offers economies of construction;
the lower formwork pressures enable the casting of higher lifts.
The three general categories of lightweight concrete are lightweight aggregate concrete, aerated concrete and no-fines concrete (Fig. 3.8).
Fig. 3.8 Lightweight concretes
Many of the lightweight aggregate materials are pro- duced from by-products of other industrial processes or directly from naturally occurring minerals. The key exception is expanded polystyrene, which has the high- est insulation properties, but is expensive due to its manufacture from petrochemical products.
Pulverised fuel ash
Pulverised fuel ash, or fly ash, is the residue from coal- fired electricity-generating stations. The fine fly ash powder is moistened, pelleted and sintered to produce a uniform lightweight PFA aggregate, which can be used in load-bearing applications.
Foamed blastfurnace slag
Blastfurnace slag is a by-product from the steel indus- try. Molten slag is subjected to jets of water, steam and compressed air to produce a pumice-like material. The foamed slag is crushed and graded to produce aggre- gate, which can be used in load-bearing applications.
Where rounded pelletised expanded slag is required the material is further processed within a rotating drum.
Expanded clay and shale
Certain naturally occurring clay materials are pel- letised, then heated in a furnace. This causes the evolution of gases which expand and aerate the interior, leaving a hardened surface crust. These lightweight aggregates may be used for load-bearing applications.
Expanded perlite
Perlite is a naturally occurring glassy volcanic rock, which, when heated almost to its melting point, evolves steam to produce a cellular material of low density.
Concrete made with expanded perlite has good ther- mal insulation properties but low compressive strength and high drying shrinkage.
Exfoliated vermiculite
Vermiculite is a naturally occurring mineral, composed of thin layers like mica. When heated rapidly the layers separate, expanding the material by up to 30 times, producing a very lightweight aggregate. Exfoliated vermiculite concrete has excellent thermal insulation properties but low compressive strength and very high drying shrinkage.
Expanded polystyrene
Expanded polystyrene beads offer the highest level thermal insulation, but with little compressive strength. Polystyrene bead aggregate cement (PBAC) is frequently used as the core insulating material within precast concrete units.
Aerated concrete
Aerated concrete (aircrete) is manufactured using foaming agents or aluminium powder as previously outlined in the section on foaming agents. Densities in the range 400–1600 kg/m2give compressive strengths ranging from 0.5 to 20 MPa. Drying shrinkages for the lowest-density materials are high (0.3%), but thermal conductivity can be as low as 0.1 W/m K offering excel- lent thermal insulation properties. Factory-autoclaved aerated concrete blocks have greatly reduced drying shrinkages and enhanced compressive strength over site-cured concrete. Aerated concrete is generally frost resistant but should be rendered externally to pre- vent excessive water absorption. The material is easily worked on site as it can be cut and nailed.
No-fines concrete
No-fines concrete is manufactured from single-sized aggregate (usually between 10 and 20 mm) and cement paste. Either dense or lightweight aggregates may be used, but care has to be taken in placing the mix to ensure that the aggregate remains coated with the cement paste. The material should not be vibrated.
Drying shrinkage is low, as essentially the aggregate is stacked up within the formwork, leaving void spaces;
these increase the thermal-insulation properties of the material in comparison with the equivalent dense con- crete. The rough surface of the cured concrete forms an excellent key for rendering or plastering which is necessary to prevent rain, air or sound penetration.
Dense aggregate no-fines concrete may be used for load-bearing applications.
Fibres
Either steel or polypropylene fibres may be incorpo- rated into concrete, as an alternative to secondary reinforcement, particularly in heavily trafficked floor slabs. The fibres reduce the shrinkage and potential cracking that may occur during the initial setting and give good abrasion and spalling resistance to the cured concrete. The low-modulus polypropylene fibres, which do not pose a corrosion risk after carbon- ation of the concrete, enhance the energy-absorbing characteristics of the concrete giving better impact resistance. Steel fibres increase flexural strength as well as impact resistance but are more expensive. Alter- natively, stainless steel fibres may be used where rust spots on the surface would be unacceptable. Typically,
polypropylene fibres are added at the rate of 0.2% by weight (0.5% by volume) and steel at the rate of 3–4%
by weight. Both polypropylene and steel fibre concretes can be pumped. Steel fibres to BS EN 14889-1: 2006 may be straight or deformed cold-drawn wire, alter- natively sheet fibres. Polymer fibres to BS EN 14889-2:
2006 may be thick or thin monofilaments or fibril- lated. (Glass-fibre reinforced cement is described in Chapter 11.)
Ultra-high-performance concrete
Ultra-high-performance concrete (UHPC) has six to eight times the compressive strength of traditional concrete. It is produced from a mixture of Portland cement, crushed quartz, sand, silica fume, superplasti- ciser, fibres and water with no aggregates larger than a few millimetres. Wollastonite (calcium silicate) filler may also be included in the mix. The fibres most frequently used are either high-strength steel for maxi- mum strength or polypropylene (PP) of approximately 12 mm in length for lower load applications. The con- crete can be cast into traditional moulds by gravity or pumped or even injection cast under pressure.
When cast into traditional moulds, the material is self-levelling, so only slight external vibration of the formwork may be required to ensure complete fill- ing. The material is designed for use without steel reinforcement bars.
Structural components in ultra-high-performance concrete may, after setting, be subjected to steam treat- ment for 48 hours at 90◦C. This enhances durability and mechanical properties, eliminates shrinkage and reduces creep. The material does not spall under fire test conditions.
The enhanced compressive and flexural strengths of ductile fibre-reinforced ultra-high-performance con- crete enable lighter and thinner sections to be used for structural components such as shell roofs and bridges, creating an enhanced sleek aesthetic. A high-quality durable surface is produced from appropriate moulds (e.g. steel) coated with proprietary release agent.
TRANSLUCENT CONCRETE
By embedding parallel fibre-optic threads into fine concrete, the material is made translucent without any appreciable loss of compressive strength. Translu- cent concrete can be manufactured as blocks or panels provided that the fibres run transversely from face to face. If one face is illuminated, any shadow cast
L I M E , C E M E N T A N D C O N C R E T E 7 5 onto the bright side is clearly visible on the other face,
whilst the colour of the transmitted light is unchanged.
The material has many potential applications includ- ing walls, floor surfaces and illuminated pavements.
Recent claims suggest that up to 80% light transmis- sion is possible.
INSULATING CONCRETE FORMWORK (ICF) Polystyrene
Large, hollow, interlocking polystyrene system blocks fit together to create permanent insulating formwork, which is then filled with in situ concrete to produce a monolithic concrete structure. A range of units is available giving a central core of 140–300 mm con- crete and total insulation thicknesses between 100 and 300 mm according to the structural and ther- mal requirements. The two faces of the insulation are connected by a matrix of polystyrene links which become embedded into the concrete. The units, typi- cally 250 mm high, are tongued and grooved to ensure correct location, and horizontal steel reinforcement may be incorporated if required for additional struc- tural strength. Special blocks are available for lintels, wall ends, curved walls and fire walls. A pumpable grade of concrete (high slump) will fill the void space by gravity flow without the need for mechanical vibra- tion. Some temporary support for the formwork is required during construction to ensure accurate align- ment. Internal and external finishes may be applied directly to the polystyrene which is keyed for plaster or lightweight render. Alternatively masonry, timber or other claddings may be used externally and dry linings (e.g. plasterboard) may be attached to the inner leaf with appropriate adhesives.
Lightweight concrete
Concrete shuttering blocks are manufactured in normal, lightweight or wood-chip concrete to Stan- dards BS EN 15435: 2008 and BS EN 15498: 2008, respectively. Systems are available with and without additional thermal insulation for use as internal, exter- nal and partition walls when filled with concrete.
Some systems have lateral interlocking (e.g. by tongue and groove) and may be laid with or without mor- tar according to manufacturers’ specifications. Hollow insulated blocks, 300 mm wide, manufactured from 80% woodchip, when constructed and filled with con- crete, can give a wallU-value of 0.27 W/m2K.
POLYMER CONCRETE
The incorporation of pre-polymers into concrete mixes, the pre-polymers then polymerising as the concrete sets and hardens, can reduce the penetra- tion of water and carbon dioxide into cured concrete.
Typical polymers include styrene–butadiene rubber and polyester–styrene. Epoxy resin and acrylic-latex modified mortars are used for repairing damaged and spalled concrete because of their enhanced adhe- sive properties. Similarly, polymer-modified mortars are used for the cosmetic filling of blowholes and blemishes in visual concrete. Resin-bound concrete construction products include street furniture, dec- orative elements and window sills. The materials are covered by the standard BS EN 15564: 2008.
WATER FOR CONCRETE
The general rule is that if water is of a quality suitable for drinking, then it is satisfactory for making concrete.
The standards BS EN 1008: 2002 and pr BS ISO 12439:
2009 give the limits on impurities including sulphates.
CONCRETE MIXES
Concrete mixes are designed to produce concrete with the specified properties at the most economical price.
The most important properties are usually strength and durability, although thermal and acoustic insula- tion, the effect of fire and appearance in visual concrete may also be critical.
In determining the composition of a concrete mix, consideration is given to the workability or ease of placement and compaction of the fluid mix and to the properties required in the hardened concrete. The key factor which affects both these properties is the free- water content of the mix after any water is absorbed into the aggregates. This quantity is defined by the water/cement ratio.
Water/cement ratio
water/cement ratio=weight of free water weight of cement
The free water in a mix is the quantity remaining after the aggregates have absorbed water to the sat- urated surface-drycondition. The free water is used to hydrate the cement and make the mix workable.
With low water/cement ratios below 0.4, some of the