ENG2100 Construction Materials Lecture Notes PDF

ENG2100 - Construction Materials 3. Concrete Technology The use of burnt rocks containing calcium, such as limestone or gypsum, to provide inorganic bonding agents that, on the addition of water, allow the manufacture of a bound “concrete” mass has been documented over a period of nearly 10,000 yea...

ENG2100 - Construction Materials 3. Concrete Technology The use of burnt rocks containing calcium, such as limestone or gypsum, to provide inorganic bonding agents that, on the addition of water, allow the manufacture of a bound “concrete” mass has been documented over a period of nearly 10,000 years. For example, the ancient Egyptians, Mycenaean, and Minoan cultures all made extensive use of lime mortars. These, pre-Roman technologies, were based on heating of calcium carbonate bearing rocks, such as limestone or chalk, in a kiln to produce a form of calcium oxide, CaO, known as quick lime. As a powder this material reacts readily with water to create “slaked” lime or hydrated calcium hydroxide, Ca(OH)2. This can be used to bind sand, aggregates and other stony materials together in the form of a “mortar” that hardens by the reaction of the Ca(OH)2 with carbon dioxide, CO2, in the air to produce calcium carbonate, Figure 3.1. Figure 3.1 Basic reactions of “lime” mortar technology. This is generally a very slow process and works best at an ambient relative humidity (RH) of between 50 – 70%. Above 90% RH the absorption of CO2 and its subsequent diffusion through the outer hardened layer is extremely slow. As a result, these mortars cannot harden under water preventing the construction of large marine structures by cultures that relied on this material. Whilst the slow strength gain of lime mortars limits the rate of construction that can be achieved, perversely it means that such mortars often exhibit significant plasticity even after many years. This is a positive advantage when constructing masonry structures subject to earthquakes since it enables movement of the individual elements without necessarily inducing ultimate failure. 50BC - 500AD Great use of “Roman concrete” much of which contained finely ground reactive silica (pozzolana), in the form of a fine volcanic tuff taken from the sites of certain volcanoes, which in the presence of slaked lime reacted to give calcium silicates which would form a binder under water37. This was the first true “hydraulic cement” and provided the technological break-through to enable the construction of many large and durable structures, e.g., the Collosseum, the Pantheon dome, the Pont du Garde, Caligula’s Wharf, and Hadrian's Wall. Notable innovations included the use of specialist additives such as blood to entrain air, polymers from figs to waterproof the concrete, and the introduction of lightweight aggregates to reduce the self-weight of arch and dome constructions. 500AD – 1,100AD Saxon concrete, e.g. Reading Abbey (1121AD). 1756 John Smeaton made the first step to the production of Portland cement. His major advance was to add clay to limestone before heating. He used the resulting hydraulic cement in the construction of the Eddystone Lighthouse. 1796 James Parker patented 'Roman' cements - 30% lime, 25% silica, 30% carbonic acid. Used extensively by Telford and Brunel for Civil Engineering projects. 1824 Joseph Aspdin patents his process for the production of Portland cement. 37 We make use of this “pozzolanic reaction” today in cement blends of Portland cement and a variety of reactive silica materials such as Pulverised Fuel Ash and Micro-silica. 30  ENG2100 - Construction Materials 1845 Isaac Johnson clinkered 5:1, chalk:clay mixtures at 1400°C to give tricalcium silicate. 1854 William Wilkinson patents reinforced concrete, and it is used initially for the manufacture of boats and plant tubs! 1859 Main London sewage system manufactured using the 'new' cements. 1900 Asbestos-cement becomes commercially available. 1930 Fressynet patents prestressed concrete. 2002 Definition of Portland Cement38, 39 Portland cement clinker is a hydraulic material which shall consist of at least two- thirds by mass of calcium silicates (3CaO.SiO2 and 2CaO.SiO2), the remainder consisting of aluminium and iron containing clinker phases and other compounds. The ratio by mass (CaO)/(SiO2) shall be not less than 2.0. The content of magnesium oxide (MgO) shall not exceed 5.0 % by mass. 3.1 Concrete as a composite material Concrete is a multi-level composite material, Figure 3.2, consisting of a coarse aggregate bound together by a cement-based mortar that is itself a composite of fine aggregate held together by cement paste. The cement paste is a mixture of Portland cement and other cement-replacement materials with water. The size range of the particles within concrete covers the scale of centimetres to angstroms and a critical factor in controlling the properties of the concrete is the grading and proportions of its constituents. Figure 3.2 Composite nature of concrete and scale of constituent parts. (1 mm = 1 x 10-3 metres, 1 µ = 1 x 10-6 metres, 1 Å = 1 x 10-10 metres) The proportions of the constituent phases (aggregate, cement, and water) are commonly expressed in one of three ways: i) Mass (kg) of material required to produce a unit volume (m3) of compacted concrete, e.g., cement = 360 kg/m3, aggregate = 1900 kg/m3, water = 140 kg/m3. ii) Ratios by weight: e.g. (total or free) Water/Cement ratio, and Aggregate/Cement ratio. (The aggregate typically being defined by type, maximum size, and grading.) iii) Nominal proportions by weight of cement:sand:aggregate, e.g.1:2:4, 1:1.5:3. Figure 3.3. shows the typical volume occupied by the constituents within a normal concrete mix. The aggregate represents be far the greatest volume of the concrete, ≈ 75-80 %, and is 38 BS12:1996 Specification of Portland Cement. (Now withdrawn) 39 BS EN 197-1:2000, Composition, specifications, and conformity criteria for common cements 31  ENG2100 - Construction Materials coated and bound together by the cement paste binder. All concretes also contain a quantity of pores (and other voids) that are the result of both air, trapped into the fresh concrete during mixing, and excess free water added to make the fresh concrete sufficiently workable that it can be easily placed and properly compacted into the formwork. The volume, type and continuity of such pores has a significant effect on the physical and mechanical properties of the fresh and hardened concrete and influence the durability of any structures manufactured from it by controlling the permeability and porosity. a). Binder: Portland Cement Water Cement Paste b). Aggregate: Natural vs Man-made (hard and inert) c). Pores: Water filled (capillary and gel) Air voids (micro and macro) Figure 3.3 Volume contribution of main constituents of Portland cement concrete. 3.1.1 Role of the Constituents (a) Portland Cement i) Reacts with water to create cement paste that hydrates over time to create inorganic binder, Figure 3.4. ii) Portland Cement is both expensive relative to the other constituents of concrete and variable in its composition and properties. iii) Can be supplemented with cement replacement materials, e.g., GGBS, PFA etc. iv) Type of cement affects rate of hardening and durability. (b) Water i) Fresh state - quantity affects “workability” and “cohesion” etc. ii) Hydration - exothermic chemical reaction. Imparts setting and hardening. iii) Hardened state - causes dimensional changes, e.g., shrinkage and creep. (c) Cement paste i) Fresh state - thixotropic material assisting cohesion and flow under vibration. ii) Hardened state - hard, durable inorganic binder. iii) Dimensionally unstable – controls shrinkage and creep behaviour. (d) Aggregate i) Cheap, inert, and generally hard filler material. ii) The grading, shape, texture, type, and quality of the aggregate particles will all affect the elastic properties of fresh and hardened concrete. (e) Porosity i) Fresh state - affects workability and cohesion. ii) Hardened state – influences strength, stiffness and density etc but more importantly affects the frost resistance of wet concrete, e.g., freeze-thaw cycling. iii) Air voids (mm), air/water-filled capillary pores (microns), gel pores (sub-micron) depending on the degree of hydration of the cement paste, Figure 3.5. (f) Admixtures Small quantities, typically less than 1% by weight of cement, used to modify the properties of fresh and hardened concrete. 32  ENG2100 - Construction Materials i). Instant of mixing ii). 7 days water unhydrated cement calcium hydroxide calcium silicate hydrate iii). 28 days Figure 3.4 Hydration of Portland cement over time in presence of water. 7 ml 33 ml Capillary Water Capillary 24 ml 60 ml Water Gel Water Water 12 ml Gel Water 31 ml Hydration 62 ml Products Hydration 40 ml Products Portland Cement 20 ml Portland Cement 0% 50% 100% Figure 3.5 Change in constituent volume with extent of cement hydration 3.1.2 Properties required The concrete must have satisfactory properties at two stages. (1) Fresh state (i). Cohesive - Ease of placing. (ii). Workable - Ease of compaction. These control the suitability of the fresh concrete mix for transporting, placing and compacting. (2) Hardened state (i). Strength - Compression, Tension, Fatigue. (ii). Dimensional stability - Elastic modulus, Creep, Shrinkage, - Expansion coefficient. (iii). Durability - Weathering, Chemical attack 33  ENG2100 - Construction Materials 3.1.3 Concreting timescales and life-cycle. The timescales that are relevant to concrete structures range from hours to centuries starting with the extraction of suitable aggregates and the resources required for the manufacture of Portland cement. Once the concrete mix has been properly designed for the application of interest then the process starts at the instant of mixing the cement and water with the aggregate, Figure 3.6. Over a period of 2-10 hours (depending on the temperature) the mix is referred to as “fresh” concrete and in this state can be compacted by vibration due to the thixotropic properties of cement paste. After some initial hydration of the cement particles the mass of concrete stiffens (“sets”) and becomes “hardened” concrete that develops strength over subsequent days, weeks and years provided it is properly cured and protected. Extraction Manufacture Storing Batching Mixing  Fresh - Handling concrete - Placing - Compaction - Setting  Hardened - Curing concrete - Protection - Loading - Weathering (Hours) (Days) (Years) Figure 3.6 Concrete timescales. Well-designed and constructed concrete structures that are properly maintained can be expected to provide centuries of useful service. However, this tends to be the exception as changes in the urban environment tend to occur over a much faster timescale resulting in the need to remove or upgrade existing concrete assets. A simplified life-cycle of modern Portland cement-based concrete is shown in Figure 3.7. Unlike metals concrete has only limited potential for recycling which emphasises the need for good design that encourages long-term durability and facilitates reuse. Operation of Structure Inspection, Maintenance, ⊕ In situ Concrete Structure Repair ⊕ ⊕ End of Life Option Reuse ⊕ Precast Concrete components ⊕ ⊕ Refuse Recycle Production of fresh concrete ⊖ ⊕ ⊕ ⊕ ⊕ Aggregate Recovered CEM type I – V Landfill extraction & steel cements manufacture ⊕ ⊕ compounds of calcium, ⊕ Portland cement Cement replacement aluminium, and silicon in earths crust ⊕ manufacture material, GGBS, PFA Figure 3.7 Simplified life-cycle of Portland cement-based concrete structures. [Note: ⊕ indicates that the step results in an increase in the embodied energy (and carbon “footprint”) of the concrete, whilst ⊝ indicates that the embodied energy has been lost.] 34  ENG2100 - Construction Materials 3.1.4 Portland Cement (a) Manufacture of Portland cement BS EN 197-139 states: “Portland cement clinker is made by sintering a precisely specified mixture of raw materials (raw meal, paste or slurry) containing elements, usually expressed as oxides, CaO, SiO2, Al2O3, Fe2O3 and small quantities of other materials. The raw meal, paste or slurry is finely divided, intimately mixed and therefore homogeneous.” The major compounds required to produce Portland cement are about 4/5 calcium carbonate (CaCO3) and silica (SiO2) with smaller quantities of alumina (Al2O3) and ferric oxide (Fe2O3) being necessary for the clinkering process. The calcium carbonate is obtained from limestone, or chalk, while the silica, alumina and ferric oxide are obtained from clay or shale. These raw materials are mixed in the correct proportions, heated to drive off water and CO2 (calcining at 800°C) and then fired at 1300-1450°C in a rotary kiln until the material sinters (partly melts) and fuses into balls up to 25mm diameter known as clinker. The clinker is cooled (600-500°C) and ground to a fine powder with the addition of about 3-6% gypsum to control the setting time. The resulting product is known as Portland cement and is a fine, grey, odourless powder when dry and must be stored carefully to avoid contact with water. (b) Chemical composition of Portland cement There are two common ways of describing the chemical composition of Portland cements, i). Oxide composition, Table 3.1, and ii). Compound composition40, Table 3.2. The compound composition is probably more important for the prediction of the physical properties of cement since small differences in oxide composition can result in significant differences in compound composition and properties of the cement. Cement chemists use their own shorthand for describing the four main compounds are known as C3S, C2S, C3A and C4AF, the correct chemical designation being shown in Table 3.2. C3S forms the main early age strength generating compound (first 4 weeks) with C2S contributing additional calcium silicate hydrates at later ages. C3A would cause “flash set” unless prevented by the addition of gypsum and it also reacts with sulphates in ground water causing a potential durability problem. C4AF is dark in colour due to the iron but contributes little to the cementing process. An understanding of the role that the various compounds play in the hydration of different types of Portland cement is helpful in explaining the changes in colour, strength and rate of hardening which occur in concreting on site Percent by weight Oxide Average Range Lime (CaO) 63 59 - 67 Silica (SiO2) 22 17 - 25 Alumina (Al2O3) 7 3-9 Iron (Fe2O3) 3 0.5 - 6 Magnesia (MgO) 2 0.1 - 4 Sulphur Trioxide (SO3) 3 2-3 Alkais (Na2O, K2O) 0.6 0.4 - 0.8 Table 3.1 Typical oxide composition of Portland cement. Name of compound Oxide composition Abbreviation Rate of reaction with water Tricalcium silicate 3.CaO.SiO2 C3S Medium Dicalcium silicate 2.CaO.Si02 C2S Slow Tricalcium aluminate 3.CaO. Al2O3 C3A Fast Tetracalcium aluminoferrite 4.CaO.Al2O3.Fe2O3 C4AF Slow Table 3.2 Principal compounds in Portland cement. 40 The oxide and compound composition are linked by the Bogue equations. 35  ENG2100 - Construction Materials (c) Hydration of Portland cement Hydration is the process by which Portland cement becomes a firm, hardened mass after the addition of water. During hydration, heat is evolved and temperatures in mass concrete may exceed 60°C. The reaction products are a mixture of calcium silicate and calcium aluminate hydrates in fibrous, flattened solid and hollow, branched, and straight forms, typically 0.5 microns to 2 microns long and less than 2 microns across, Figure 3.8. These are the primary “glue” responsible for binding the mass of the cement paste together. Also included are thin hexagonal crystals of calcium hydroxide which help buffer the pH of the water in the capillary pores of the hardened cement paste at around pH=12.5. The hydrated cement is relatively insoluble but can suffer leaching of the calcium hydroxide if exposed to pure water. The gel porosity of hydrated cement is about 28% with a surface area of 200,000 m2/kg. The volume of pores is equivalent to 3 molecules of water bound all over the gel surface. The water held in the structure will be either ‘free’ (evaporable) or chemically combined. It is important that good curing enables sufficient water to remain in the surface layers of the concrete to allow hydration to continue for as long as possible if the potential strength and durability of the hardened mass are to be achieved. However, cement grains only partly hydrate and hence there will be unhydrated cement available in concrete for many years. This reserve is known to assist in the “autogenous healing” of cracks in concrete structures. The heat evolution that occurs during the hydration process reflects the exothermic nature of the various reactions that are initiated when water is mixed with Portland cement, Figure 3.9. Before water is added the surface area of Initial C-S-H, 1 micron thick colloidal gel the cement 350-450m2/kg. coat which forms a selectively permeable membrane After about 200 minutes membrane Infilling and consolidation giving a ruptures to give 0.5 - 2 micron C-S-H surface area  200,000 m2/kg. secondary growths Figure 3.8 Stages in the early hydration of Portland cement. Figure 3.9 Typical rate of heat output from hydrating Portland cement. 36  ENG2100 - Construction Materials Region A shows that on mixing there is a peak in heat output that results from the initial wetting of the cement powder which has a surface area of order 350-450m2/kg. This initial peak lasts only a few minutes before a low constant relatively dormant plateau B is reached which may last for two or three hours on a cool day. This period is crucial to civil engineers to allow the concrete to be placed while it is still workable but is significantly reduced in hot weather. At point C the cement is virtually at initial set41 and suctions develop as the C3S reacts to form tricalcium disilicate hydrate (C3S2H3 or C-S-H gel) and calcium hydroxide (CH) with more rapid heat evolution up to point C. The production of the C-S-H gel is the main strength producing reaction in the first few days and can be represented; 2C3S + 6H  C3S2H3 + 3CH Tricalcium silicate + Water  C-S-H gel + Calcium hydroxide Additional strength is generated by the more slowly reacting C2S; 2C2S + 4H  C3S2H3 + CH Dicalcium silicate + Water  C-S-H gel + Calcium Hydroxide 3.1.5 Composite (blended) cements The properties of Portland cement are strongly dependent on both the composition and fineness (m2 of surface area /per kg) of the powder. Varying these two parameters enabled the production of Rapid Hardening, Sulphate Resisting and Low Heat Portland cements that were the main cement types employed in the UK prior to the introduction of BS EN 197. However, there is a limit to what can be achieved simply by manipulating the basic properties of Portland cement and as a result there has been an increasing interest in, and use of, cement replacement materials to create so-called “blended” or composite cements. These replace part of the Portland cement in concrete mixes by alternative (and often cheaper) materials with different cementing actions. Typical materials are: (1) Pulverised Fuel Ash42 (PFA) PFA, also known as fly ash is the solid material extracted by electrostatic and mechanical means from flue gases of furnaces fired with pulverized bituminous coal. PFA is essentially a pozzolanic material and as such needs a source of calcium hydroxide before silicate hydrates can be formed. It is commonly blended in proportions between 5 – 50% PFA. (2) Ground, Granulated Blastfurnace Slag (GGBS), Blastfurnace slag is the clinker produced in in the reduction of iron ore in a blastfurnace and is composed of calcium, magnesium and alumino-silicates. The nature of the slag depends on the way in which it is cooled the best cementing properties being achieved when it is rapidly quenched to produce granules of a shattered vitreous (amorphous) structure known as Granulated Blastfurnace Slag. The fine powder resulting from drying and grinding this material is GGBS. Unlike PFA, GGBS is semi-cementitious and is capable of setting on its own. However, it is usual to blend 10-90% GGBS with Portland cement which releases both hydroxide and sulphate ions which accelerate the strength gain of the GGBS. Analysis GGBS (wt%) PFA (wt%) SiO2 36 48 Al2O3 10 26 CaO 40 2.5 MgO 8 1.5 Fe2O3 0.5 9 Relative density 2.9 2.0 - 2.3 Table 3.3 Typical composition of GGBS and PFA. 41 The term initial set implies a change from a semi-fluid to a relatively stiff state after which it is not possible to compact the concrete without damage. This should be > 60 minutes for cement strength classes up to 42.5 and > 45 minutes for cement strength classes of 52.5 or greater. 42 The use of GGBS and PFA in Concrete. Concrete Society. Technical Report No. 40. (1991) 37  ENG2100 - Construction Materials (3) Microsilica43 (Silica fume) A by-product resulting from the reduction of high purity quartz with coal in submerged electric arc furnaces in the manufacture of silicon and ferro silicon alloys. This material has a relative density of 2.2 (PC = 3.1) and has a very small particle size44 (0.1 micron). It is characterised by a high proportion of amorphous silicon (> 90% SiO2) which makes it an excellent pozzolanic material when used to replace Portland cement at levels between 5- 10% by weight. The ultra-fine microsilica particles provide nucleation sites for Ca(OH)2 and CSH hydrates. Its high reactivity means that 1 kg of silica can replace 3-4 kg of cement. (a) Advantages of cement blends. PFA and GGBS additions (a) Due to the lower density of the replacement, the volume of fine powder increases, leading to increased cohesiveness. There is less bleeding in PFA concrete, but GGBS can increase bleed at higher volumes. (b) Increased workability for the same water content and increased mobility for a given slump. (Due to particle shape). (c) Up to 70% cement replacement gives reduced heat of hydration and less restrained thermal stress. However, reduced creep may offset such effects and the tensile strain capacity could be less. (d) Improved sulphate resistance. (e) Decreased chloride ion diffusion. Hence reduced cover for a given strength, e.g., >100mm cover for C60 OPC concrete but only 40mm cover for > 40% replacement of PFA or >60% GGBS for C40 concrete. (f) Reduced alkali-silica attack. (g) Economy. Microsilica additions (a) High strength concrete  150 MPa (b) Quasi-thixotropic, cohesive, minimum bleed concretes. Low slump but flow under vibration. (c) Reduced alkali-silica reaction. (d) Reduced permeability. Possibly reduced chloride penetration compared to Portland cement and increased frost resistance. (e) Good sulphate resistance. (b) Problems with cement blends PFA and GGBS additions (a) Increased settings times (1-4 hours longer). (b) Longer curing times required to limit carbonation although final carbonation depths are similar. (c) More care required in winter concreting due to reduced heat generation. (d) Slower rate of strength gain although higher final strengths achieved if the curing is continued for long enough. 43 Concrete Society. Tech. Rep. 41. Microsilica in Concrete. 1993. 44 Effectively 1 grain cement = 50,000 - 100,000 grains of microsilica. Surface area  20,000 m2/kg. Due to this it is transported either as powder, pellitised or 50:50 water slurries. 38  ENG2100 - Construction Materials Microsilica additions (a) Mixing, handling and dispersing problems require more knowledge and care. (b) Higher price than cement. (c) Increased tendency to develop plastic shrinkage cracks, due to reduced bleed. (d) Some worries about strength regression at very high strengths. (e) Questionable resistance to Cl- diffusion. Increased depth of cover to the steel reinforcement is required compared to PFA and GGBS blends. 3.1.6 Composition and Specification of Common Cements BS EN 197-145 defines, and gives the specifications of, 27 distinct common cement products and their constituents, Table 3.4. They are grouped into five main cement types as follows: CEM I Portland cement; CEM II Portland-composite cement; CEM III Blastfurnace cement; CEM IV Pozzolanic cement; CEM V Composite cement. The definition of each cement includes the proportions in which the constituents are to be combined to produce these distinct products in a range of six strength classes. The definition also includes requirements the constituents have to meet and the mechanical, physical and chemical, including, where appropriate, heat of hydration requirements, of the 27 products and strength classes. EN 197-1 also states the conformity criteria and the related rules. Necessary durability requirements are also given. It is clear from looking at the various cements covered by this Standard that they all contain at least some fraction of Portland cement clinker within their composition but that the majority are essentially cement blends. As a result, BS EN 197 redefines cement as: ….. a hydraulic binder46, i.e., a finely ground inorganic material which, when mixed with water, forms a paste which sets and hardens by means of hydration reactions and processes and which, after hardening, retains its strength and stability even under water. Cement conforming to EN 197-1, termed CEM cement, shall, when appropriately batched and mixed with aggregate and water, be capable of producing concrete or mortar which retains its workability for a sufficient time and shall after defined periods attain specified strength levels and possess long-term volume stability. Apart from Portland cement clinker (K) the main constituents allowed in a CEM cement are: Granulated blastfurnace slag (S) Natural Pozzolana (P) Natural calcined Pozzolana (Q) Siliceous fly ash (V) Calcareous fly ash (W) Burnt shale (T) Limestone (L, LL) Silica fume (D) 45 BS EN 197-1:2000, Cement – Part 1: Composition, specifications and conformity criteria for common cements. 46 Hydraulic hardening of CEM cement is primarily due to the hydration of calcium silicates but other chemical compounds may also participate in the hardening process, e.g. aluminates. The sum of the proportions of reactive calcium oxide (CaO) and reactive silicon dioxide (SiO 2) in CEM cement shall be at least 50 % by mass when the proportions are determined in accordance with EN 196-2. 39  ENG2100 - Construction Materials Table 3.4 The main products in the family of common cements provided by EN 197. 40  ENG2100 - Construction Materials Minor additional constituents are specially selected, inorganic natural mineral materials, inorganic mineral materials which because of their particle size distribution, improve the physical properties of the cement (such as workability or water retention). They can be inert or have slightly hydraulic, latent hydraulic or Pozzolanic properties. However, no requirements are set for them in this respect. Calcium sulphate is added to the other constituents of cement during its manufacture to control the setting behaviour. Calcium sulfate can be gypsum (calcium sulfate dihydrate), hemihydrate, or anhydrite (anhydrous calcium sulfate) or any mixture of them. Additives are constituents that are added to improve the manufacture, or the properties, of the cement. The total quantity of additives shall not exceed 1,0 % by mass of the cement (except for pigments). The quantity of organic additives on a dry basis shall not exceed 0,5 % by mass of the cement. These additives shall not promote corrosion of the reinforcement, or impair the properties of the cement, or of the concrete or mortar made from the cement. 3.1.7 Aggregates Aggregates occupy about three-quarters of the volume of concrete, and they therefore have a large effect on both the economics of concrete production, and the properties of the fresh and hardened material, e.g., workability, strength, and durability. At the reconnaissance and planning stage of any major construction project involving the use of large quantities of concrete several aggregate sources must be tested to ensure strength, durability, and non- reactivity with the cement. Awards of contracts may depend on locating a cheap and good local source of available material. Most of aggregates for use in the production of new concrete come from natural sources although the latest BS EN Standards admit the use of recycled aggregates from a range of sources. Typically, the UK construction industry consumes 125 - 160 x 106 tonnes/annum in concrete manufacture. Tonnes/annum Land - sand + gravel 68-80 x 106 Marine - sand + gravel 17-22 x 106 Crushed rock 19-25 x 106 Recycled concrete 16-22 x 106 Costs at the point of production are £5-10/tonne but when delivered could be 50-100% higher due to transportation costs. Average truck journeys are 20-30 miles, but aggregates are transported longer distances by rail and ship. The main supply limitations are typically due to planners, environmentalists, and transportation costs. Aggregates for concrete are covered by the general requirements of BS EN 12620: 2002, Aggregates for Concrete. This standard covers aggregates with an oven-dried particle density greater than 2,000 kg/m3 for all concrete47, including concrete conforming to BS EN 206 and concrete used in roads and other pavements and for use in precast concrete products. It sets out48: a) general requirements and terminology, b) geometrical properties, c) physical & mechanical requirements, d) durability (thermal and weathering) characteristics, and e) chemical requirements. A detailed explanation of the requirements for aggregates is provided in Appendix A. 47Lightweight aggregates for concrete are covered by BS EN 13055-1. 48The necessity for testing and declaring all properties specified in this clause shall be limited according to the particular application at end use or origin of the aggregate. 41  ENG2100 - Construction Materials 3.1.8 Admixtures An admixture may be defined as any material, other than water, aggregate or Portland cement, which is added to the concrete during mixing to modify one or more properties of the fresh or hardened concrete. Admixture must be evaluated by trial mixes using similar cements and aggregates to the proposed mix. About 40% of all concrete manufactured in the UK contains admixtures and they are used for a wide variety of purposes: a). Workability aids/water reducing admixtures When water is first added to cement the individual grains of cement tend to come together to form agglomerations (“flocs”) of considerable size, Figure 3.10. These increase the apparent particle size and stop the uniform of cement grains within the available free water severely reducing the workability. Workability aids deflocculate the suspension of cement particles by giving the cement grains a negative charge so that they repel each other and become more uniformly distributed. Additionally, the cement particles are surrounded by a layer of oriented water molecules that prevent the grains from getting too close to each other. Figure 3.10 Deflocculation of cement particles. There are two main classes of workability aids, i.e., plasticisers and superplasticisers. i). Plasticisers: have three main uses; increase strength at constant workability meet the specified strength at lower cement content increase the workability at constant mix proportions There are two main types of plasticiser commonly in use; Ligno-sulphonates (which both retard set and also entrain air), and Hydroxylated carboxylic acid (which retards set but does not entrain air). These are typically added at concentrations of 0.1-0.2% of active material by weight of cement and result in about 10% less free water being required to achieve a given level of workability. If the cement content is kept constant, then the resulting decrease in the free water/cement ratio produces a strength increase of 10-20% at 28 days. ii). Superplasticisers: have five main uses; reduced or zero vibration with rapid placing very rapid pumping self compacting tremmie concrete uniform and compact surfaces high early strength There are two main types of superplasticiser commonly in use; Melamine and Napthalene formaldehyde resins. These are typically added at concentrations of 1-5% by weight of cement into medium slump mixes just before placing49. These can change a 50-75mm slump concrete mix into one with a collapse slump with so-called flowing, self compacting properties. They can also be used to produce very high strength concrete (>100N/mm2) at low w/c ratios (0.28) but which still exhibits high slump in the fresh state. However, to achieve this the mix design will need to be modified, e.g., 400-450Kg/m3 of sub 300 micron particles and a slump of 75mm. 49 The fluidising effect only lasts about one hour so rapid placement is essential. 42  ENG2100 - Construction Materials b). Set modifiers i). Set retarders: have several uses; hot weather concreting, to allow required handling time to avoid 'cold' joints in slip-forming where the fresh concrete has to be transported long distances to enable exposed aggregate finishes by brushing off paste. There are several types of set retarder available including hydroxylated carboxylic acid, sugar50, borax, and lignins. These are typically added at concentrations of 0.1-0.3% by mass of cement and can retard the setting time by up to 50% or up to 24 hours. ii). Set accelerators: are typically used to increase the speed of hardening where urgent repair works are required, and the concrete must be put back into service as quickly as possible. Common set accelerators include, Sodium carbonate, and Aluminium chloride. Figure 3.11 Impact of retarders and accelerators on the early hydration process. c). Accelerated hardening Accelerated hardening is employed for precast concrete and in concrete. There are several methods for achieving accelerated hardening; the addition of 1-2% by weight cement of Calcium Formate can give the 7-day strength in 3 days or, if heated, the 3-day strength in 1 day; the direct application of heat curing can also increase strength gain. d). Air entrainment Air entrainment is used in the construction of highway and airfield pavements, bridges, marine works, lightweight concrete. There are a range of additives which can help entrain air into the concrete including; animal and vegetable fats, wood resins, and alkali sulphates. These are typically added at 0.1-0.2% by weight of cement and increase the cohesion mix, reduce any tendency to bleeding and segregation, and greatly improve the frost resistance of the hardened concrete. Figure 3.12 Air entrainment. 50 3kg/m3 kills set completely! 43  ENG2100 - Construction Materials 3.2 Properties of Fresh Concrete A batch of concrete that can still be properly mixed and placed as a viscous and cohesive semi-fluid mass, and which will respond to vibration by increasing its rate of flow, is said to be in the FRESH state51. At normal temperatures encountered in the UK this stage typically lasts one to three hours from the point that the constituents are initially mixed but exceptionally could be a few minutes or up to six hours. An important property that affects the proper handling, placing and compaction of fresh concrete is its WORKABILITY. This property of fresh concrete reflects the ease with which air entrapped in the mass can be removed and is a function of the useful internal work necessary to overcome friction between individual particles in the mix in order to achieve full compaction*. There is no single measurement technique currently available that will quantitatively measure workability in terms of the above definition and instead descriptive terms are generally used. Thus, a concrete mix that requires a large amount of external work by people, or by vibration with a poker, to achieve full compaction is said to have “Low” workability. Conversely a “High” workability mix requires little, or no, extra work or vibration to become fully compacted. 100 * The need for full compaction reflects the Poor compaction % Theoretical Strength observation that the compressive strength 80 (Low workability) of hardened concrete is strongly influenced by the presence of macro-voids, Figure 60 3.13. Therefore, it is important to compact the concrete as fully as possible if the full 40 potential strength of the concrete is to be obtained. This must be balanced against 20 the time and effort required to vibrate, or Vibration (thixotropy) otherwise work, the concrete to remove the Self-levelling concrete 0 entrapped air. 0 5 10 15 20 % Air Voids Figure 3.13 Effect of voids on strength. 3.2.1 Factors affecting workability Water content Workability generally increases with increase in water content. Cement content (a) For a given W/C ratio, increase in cement results in an increase in workability. (b) For a constant water content, an increase in cement decreases workability due to a general stiffening of the paste with decreasing W/C ratio. Aggregate shape, texture, maximum size, and grading The important factor is the surface area to be wetted. For a given water content a high specific surface area gives a low workability. Generally, for a given workability less water is required when: (i) the aggregate is more rounded, (ii) the aggregate has a smoother texture, (iii) the aggregate has a greater maximum size and coarser grading. 51BS EN 206 –1:2000 defines fresh concrete as “concrete which is fully mixed and still in a condition that is capable of being compacted by the chosen method”. 44  ENG2100 - Construction Materials Admixtures These are chemicals that are added in small quantities and affect workability by either entraining air or reducing surface tension or inter-particle friction in other ways. By 'entrained' air is meant air intentionally incorporated in the concrete by means of a suitable agent, usually organic chemicals such as vinsol resin. The air is uniformly distributed in the paste phase in the form of bubbles with diameters generally less than 0.1mm at about 0.25mm spacing. The total volume of the entrained air is typically about 2% to 8% of the concrete volume, but considerably greater in the cement paste. The effects on the fresh concrete are: (a) Reduction in mixing water (b) Increase in workability (c) Increase in cohesion (d) Reduction in bleeding and segregation (e) Reduction in unit weight The air bubbles acting as a fine aggregate of very low surface friction probably cause the improvement in workability. The mix behaves as if it is over sanded and hence the sand content should be reduced by 2% to 3% of the total aggregate. 3.2.2 Segregation, Cohesion and Bleeding The workability of a concrete mix is not always related to the ease with which it can be placed since the mix must also be sufficiently cohesive to resist segregation and bleeding. (a) Segregation This is the separation of different phases in the concrete during handling and placing. This may occur in transporting the concrete in dumper trucks, dropping from skips, pumping or during vibration. There are two common forms: (i) Dry segregation: Often occurs in dry lean mixes with the coarser particles separating out. The addition of water and fines can improve cohesion. (ii) Wet segregation: Grout (cement paste and fine sand) may separate to the top of the pour on compaction forming planes of weakness and dusting. This may be caused by an excessively wet mix or poor particle grading. Cohesion - is the property that enables concrete to resist segregation. It can be increased by raising the amount of fine particles ( 3 sec; degree of compactability  1.04 and < 1.46; flow diameter > 340 mm and  620 mm. For compliance purposes the consistence of fresh concrete may be specified either by reference to a consistence class, Table 3.5, or, in special cases, by a target value, see Table 3.6. 48  ENG2100 - Construction Materials Slump classes Vebe classes Compaction classes Flow classes Table 3.5 Consistence Classes for fresh concrete. Note that the classes for consistence are not directly related to each other in any fundamental manner or equivalence. Table 3.6 Target values and related tolerances. 3.2.4 Other Tests Air content (BS EN 12350-7:2000) The test relies on Boyle's Law, i.e., when concrete is compacted in A an increase in pressure at B will reduce the volume of entrained air. Calibrated to read air content directly. Gravimetric a Va = 1 −  where,  a = concrete density containing air and  equals theoretical air free density. Lack of knowledge of aggregate relative densities reduces accuracy. Volumetric Uses the volume of concrete before and after expulsion of air. 49