Lecture 1 â Concrete
Lecture 1 – Concrete Concrete CONCRETE is made by mixing: cement, water, coarse & fine aggregates, admixtures (if required The aim is to mix these materials in measured amounts to make concrete that is easy to: transport, place, compact & finish and which will set, and harden, to give a strong and durable product. The amount of each material (i.e. cement, water and aggregates) affects the properties of fresh and hardened concrete
History • Ancient Egyptians used calcined (thermal decomposition) impure gypsum (CaSO 4 ). • The Greeks and Romans used calcined limestone † and later learned to add lime and water, sand and crushed stone or brick and broken tiles. This was the first concrete in history. († thermal decomposition of limestone CaCO3 CaO + CO2 at 848°C) • The Romans ground together lime and a volcanic ash to produce what became known as pozzolanic cement from the name of a village of Pozzuoli, near Vesuvius. • Patent for ‘Portland cement’ was taken out by Joseph Aspdin, a Leeds builder, in 1824. Cement was prepared by heating a mixture of finely-divided clay and hard limestone in a furnace until CO2 had been driven off. The name Portland cement, given originally due to the resemblance of the colour and quality of the set cement to Portland stone – a limestone quarried in Dorset – has remained throughout the world. (Hence ‘OPC’) = ordinary Portland cement
Manufacture • Dry Method – most common 1) Limestone (CaCO3) is crushed, usually in two progressively smaller crushers 2) Add iron ore or fly ash and ground, mixed and feed to a cement kiln. Heat to about 1500°C. 3) Elements unite to form a new substance called clinker, grey balls about the size of marbles. - manufacturers now import clinker to move CO2 emissions elsewhere 4) The cooled clinker is inter-ground with gypsum in a ball mill consisting of several compartments with progressively smaller steel ball – cooled faster = better quality. Gypsum = delays setting time 1) limestone ball mill + water = slurry • Wet Method – raw materials are ground with water before being fed into the kiln (pumped storage 1) Limestone (CaCO3) is crushed, usually in two progressively smaller crushers, then fed tanks) - lime content into a ball mill with the clay dispersed in water. There the comminution of the limestone adjusted (to the fineness of flour) is completed, and the resultant cement slurry is pumped into storage tanks. The slurry is a liquid of creamy consistence, with a water content of 2) slurry rotary kiln between 35 to 50%. The lime content of the slurry is adjusted by blending with slurries - water driven off + CO2 from different storage tanks. liberated = lime + 2) The slurry is passed into a rotary kiln where pulverised coal is blown in by an air blast silica + alumina = to temperatures from 1300 to 1500°C. CLINKER (hard ceramic3) At first water is driven off and CO2 is liberated: further on the material becomes liquid like balls) and lime, silica and alumina recombine to become ‘clinker’ – hard balls of ceramic-like 3) clinker cooled, material. The cooled clinker is inter-ground with gypsum in a ball mill consisting of interground with several compartments with progressively smaller steel balls gypsum in ball mill
Chemical Composition • Raw materials consists mainly of lime, silica, alumina and iron oxides • After processing, the four major constituents of cement are:
= Light colour, early strength = light colour, late strength = light colour, quickly sets = dark colour, little value (SiO2 = silica, Al2O3 = alumina, Fe2O3 = iron oxide)
Hydration of Cement • In the presence of water, the silicates and aluminates form products of hydration, which in time produce a firm and hard mass – the hardened cement paste •The C2S provides most of the ultimate long term strength and the C3S gives the high early strength •Since hydration starts at the surface of the cement particles, it is the total surface area of cement that represents the material available for hydration •The rate of hydration depends on the fineness of the cement particles, and for a rapid development of strength high fineness is necessary (320-380 m2/kg for GP to 450-650 m2/kg for HE) – HE = high early strength - finer particles = higher quality = more surface area to react where the water goes = in the hydration process, the water is ‘used’ in the chemical reaction – if there’s too much water it won’t set = doesn’t need evaporation
Types of Cement The cement powder, when mixed with water, forms a paste. This paste acts like glue and holds or bonds the aggregates together. There are 6 major types of cement sold in Australia: Type GP (General Purpose Portland cement) Type GB (General Purpose Blended Cement) Type HE (High Early Strength cement) Type LH (Low Heat cement) Type SR (Sulphate Resisting cement) Type SL (Shrinkage Limited cement) The most common types of cement are Type GPand Type GB. Blended cements contain Portland cement and more than 5% of either fly ash, ground slag, silica fume, or a combination of these (supplementary cementitious material)
Type GP = General Purpose Portland Cement Type GP is intended for use in most forms of concrete construction when there is no exposure to sulphates in the soil or ground water, and where special properties of other types – such as high early strength, low heat of hydration – are not required. May contain up to 5% of approved mineral additions – CaCO3, fly ash and slag. More economical and more uniform product. LESS THAN 5% MINERAL ADDITIONS - less environmental but faster strength gain Type GB – general purpose Blended cement Contains greater than 5% of slag, fly ash or both and/or up to 10% silica fume. Indeed, there is very little concrete used in general building construction which does not contain a proportion of fly ash and/or slag. - MORE THAN 5% MINERAL ADDITIONS. – more environmental but slower strength gain & CHEAPER Type HE – high early strength cement Is where the 3-day strength is of the same order as the 7-day strength of the GP cement. The increased rate of gain of strength is achieved by a higher C3S content and by finer grinding of the cement clinker. The use of HE cement is indicated where a rapid development is desired, e.g. early removal of formwork. The rapid strength development is usually accompanied by a higher rate of heat evolution, and should not be used in thick concrete sections or in mass construction Type LH – low heat cement was first produced for use in large gravity dams in the US and is usually used in large concrete pours to limit the rate of heat generation – Heat of Hydration= energy/heat released when cement and water react Type SR – sulfate resisting cement has a low C3A content (less than 5%). Type SR cement is intended primarily for use where resistance to ground waters containing sulfates in solution is required. AS3972 replaced the limit on C3A for sulfate resisting Portland cement by a performance test and a performance limit after tests have shown that limiting the C3A content was not appropriate. - used for concrete in contact with the ground (soil/footings) Type SL – shrinkage limited cement does not change in volume owing to drying shrinkage, e.g. road pavements and bridge structures. Off-white and White Portland Cement – characterised by relatively high C3A contents (9-14%) and low C4AF contents (3% for off-white and 0.3-0.4% for white cements). Principally used for architectural purposes. Coloured cements – consists of cement and inorganic pigments interground or mixed together.
Strength development – Ultimate compressive strength may take several years to achieve, but for practical purposes 28-day strengths are used as indicators for the ‘final’ strength of most cements and are specified in cement standards all over the world. - Compressive strengths of cement is determined by crushing tests on prisms made from the standard mortar (1:3 cement: sand mix of 0.5 water-cement ratio) – these ratios are by WEIGHT not volume Strength development of blended cements is dependent on the nature and proportion of the component materials, i.e. the type and properties of the Portland cement, and the properties of the fly ash, slag and silica fume. Fly ash and slag blended cement gain strength more slowly than Portland cements at early ages, but they exhibit more strength gain over a longer period. Ultimate strength of blended cements can be higher than that of the Portland cement it incorporates.
Aggregates Aggregates are of 2 basic types: •COARSE: – crushed rock, gravel or screenings •FINE: – fine and coarse sands and crusher fines. – sand should be concreting sand and not brickies sand or plasterers sand – has to be durable and chemically inactive •Aggregates should be: STRONG and HARD & DURABLE – A stronger, harder aggregate will give a stronger final concrete. Never use a crumbly or flaky rock like sandstone CHEMICALLY INACTIVE so the aggregates do not react with the cement. CLEAN – Dirt or clay sticking to the aggregates will weaken the bond between paste and aggregates. GRADED – Aggregates should range in size so that they fit together well. This gives a stronger and denser concrete. – Rounded aggregates give a more workable mix. – Angular aggregates make concrete harder to place, work and compact, but can make concrete stronger The alkali–silica reaction (ASR) occurs over time in concrete between the highly alkaline cement paste and reactive non-crystalline (amorphous) silica (found in many common aggregates) This reaction causes the expansion of the altered aggregate by the formation of a swelling gel of calcium silicate hydrate (C-S-H). This gel increases in volume with water, and exerts an expansive pressure inside the material, causing spalling and loss of strength of the concrete, finally leading to its failure. ASR can cause serious cracking in concrete, resulting in critical structural problems that can even force the demolition of a particular structure
Used in insulation
Used in radiation shielding
Aggregate Grading = distribution of particle sizes in a particular batch of an aggregate. Influences the water demand of concrete and its subsequent tendency to bleed and segregate Influences the mix proportions for a desired workability and water-cement ratio The coarser the grading (lower proportion of fine aggregate) the lower the cement content required for a given workability and water-cement ratio Aggregates having a continuous relatively smooth grading curve will generally produce mixes with fewer large voids between particles Grading is determined by sieve analysis Bleeding = cement paste separates from mix, if over compacted = paste goes to top Segregation = all elements separate, larger aggregates sink
Water for Concrete Water is mixed with the cement powder to form a paste which holds the aggregates together like glue. Water must be clean, fresh and free from any dirt, unwanted chemicals or rubbish that may affect concrete. •Many concrete plants now use recycled water. •Always check bore water before use. •Don’t use sea water as it may rust the steel reinforcement in the concrete. What should not be present = solids in suspension (oil & grease), organic matter, dissolved chloride salts, dissolved carbonates and bicarbonates, acid water, - recycled water is generally satisfactory
Admixtures = are classified either by their characteristics or principal effect on the concrete (e.g. Set retarding) or by the type of material or chemical that is the principal constituent (e.g.. polysaccharides) Types = Air entraining (AEA) Overdose of admixtures: Set controlling: set retarding (Re), Set accelerating (Ac) Re = too long setting time Water-reducing HWR = severe segregation Water reducing set-controlling WR = excessive air entrainment Thickening (pumpability aids – further distance) Ac = loss in ultimate strength Shrinkage-reducing/ shrinkage-compensating Permeability reducing Special purpose Concrete setting: INITIAL SET: - workman definition: standing on concrete leaves 6mm deep boot print, time to start machine float process - engineers definition: paste starts to lose plasticity, first particle turns solid (must stop mixing, transporting, placing, compacting) FINAL SET: - workman’s definition: boots leave only the slightest scuff on the concrete, concrete cannot be worked further - engineer’s definition: paste completely lost its plasticity, all solid now,
3 STAGES OF HARDENING = PLASTIC, SETTING AND HARDENING
History • Ancient Egyptians used calcined (thermal decomposition) impure gypsum (CaSO 4 ). • The Greeks and Romans used calcined limestone † and later learned to add lime and water, sand and crushed stone or brick and broken tiles. This was the first concrete in history. († thermal decomposition of limestone CaCO3 CaO + CO2 at 848°C) • The Romans ground together lime and a volcanic ash to produce what became known as pozzolanic cement from the name of a village of Pozzuoli, near Vesuvius. • Patent for ‘Portland cement’ was taken out by Joseph Aspdin, a Leeds builder, in 1824. Cement was prepared by heating a mixture of finely-divided clay and hard limestone in a furnace until CO2 had been driven off. The name Portland cement, given originally due to the resemblance of the colour and quality of the set cement to Portland stone – a limestone quarried in Dorset – has remained throughout the world. (Hence ‘OPC’) = ordinary Portland cement
Manufacture • Dry Method – most common 1) Limestone (CaCO3) is crushed, usually in two progressively smaller crushers 2) Add iron ore or fly ash and ground, mixed and feed to a cement kiln. Heat to about 1500°C. 3) Elements unite to form a new substance called clinker, grey balls about the size of marbles. - manufacturers now import clinker to move CO2 emissions elsewhere 4) The cooled clinker is inter-ground with gypsum in a ball mill consisting of several compartments with progressively smaller steel ball – cooled faster = better quality. Gypsum = delays setting time 1) limestone ball mill + water = slurry • Wet Method – raw materials are ground with water before being fed into the kiln (pumped storage 1) Limestone (CaCO3) is crushed, usually in two progressively smaller crushers, then fed tanks) - lime content into a ball mill with the clay dispersed in water. There the comminution of the limestone adjusted (to the fineness of flour) is completed, and the resultant cement slurry is pumped into storage tanks. The slurry is a liquid of creamy consistence, with a water content of 2) slurry rotary kiln between 35 to 50%. The lime content of the slurry is adjusted by blending with slurries - water driven off + CO2 from different storage tanks. liberated = lime + 2) The slurry is passed into a rotary kiln where pulverised coal is blown in by an air blast silica + alumina = to temperatures from 1300 to 1500°C. CLINKER (hard ceramic3) At first water is driven off and CO2 is liberated: further on the material becomes liquid like balls) and lime, silica and alumina recombine to become ‘clinker’ – hard balls of ceramic-like 3) clinker cooled, material. The cooled clinker is inter-ground with gypsum in a ball mill consisting of interground with several compartments with progressively smaller steel balls gypsum in ball mill
Chemical Composition • Raw materials consists mainly of lime, silica, alumina and iron oxides • After processing, the four major constituents of cement are:
= Light colour, early strength = light colour, late strength = light colour, quickly sets = dark colour, little value (SiO2 = silica, Al2O3 = alumina, Fe2O3 = iron oxide)
Hydration of Cement • In the presence of water, the silicates and aluminates form products of hydration, which in time produce a firm and hard mass – the hardened cement paste •The C2S provides most of the ultimate long term strength and the C3S gives the high early strength •Since hydration starts at the surface of the cement particles, it is the total surface area of cement that represents the material available for hydration •The rate of hydration depends on the fineness of the cement particles, and for a rapid development of strength high fineness is necessary (320-380 m2/kg for GP to 450-650 m2/kg for HE) – HE = high early strength - finer particles = higher quality = more surface area to react where the water goes = in the hydration process, the water is ‘used’ in the chemical reaction – if there’s too much water it won’t set = doesn’t need evaporation
Types of Cement The cement powder, when mixed with water, forms a paste. This paste acts like glue and holds or bonds the aggregates together. There are 6 major types of cement sold in Australia: Type GP (General Purpose Portland cement) Type GB (General Purpose Blended Cement) Type HE (High Early Strength cement) Type LH (Low Heat cement) Type SR (Sulphate Resisting cement) Type SL (Shrinkage Limited cement) The most common types of cement are Type GPand Type GB. Blended cements contain Portland cement and more than 5% of either fly ash, ground slag, silica fume, or a combination of these (supplementary cementitious material)
Type GP = General Purpose Portland Cement Type GP is intended for use in most forms of concrete construction when there is no exposure to sulphates in the soil or ground water, and where special properties of other types – such as high early strength, low heat of hydration – are not required. May contain up to 5% of approved mineral additions – CaCO3, fly ash and slag. More economical and more uniform product. LESS THAN 5% MINERAL ADDITIONS - less environmental but faster strength gain Type GB – general purpose Blended cement Contains greater than 5% of slag, fly ash or both and/or up to 10% silica fume. Indeed, there is very little concrete used in general building construction which does not contain a proportion of fly ash and/or slag. - MORE THAN 5% MINERAL ADDITIONS. – more environmental but slower strength gain & CHEAPER Type HE – high early strength cement Is where the 3-day strength is of the same order as the 7-day strength of the GP cement. The increased rate of gain of strength is achieved by a higher C3S content and by finer grinding of the cement clinker. The use of HE cement is indicated where a rapid development is desired, e.g. early removal of formwork. The rapid strength development is usually accompanied by a higher rate of heat evolution, and should not be used in thick concrete sections or in mass construction Type LH – low heat cement was first produced for use in large gravity dams in the US and is usually used in large concrete pours to limit the rate of heat generation – Heat of Hydration= energy/heat released when cement and water react Type SR – sulfate resisting cement has a low C3A content (less than 5%). Type SR cement is intended primarily for use where resistance to ground waters containing sulfates in solution is required. AS3972 replaced the limit on C3A for sulfate resisting Portland cement by a performance test and a performance limit after tests have shown that limiting the C3A content was not appropriate. - used for concrete in contact with the ground (soil/footings) Type SL – shrinkage limited cement does not change in volume owing to drying shrinkage, e.g. road pavements and bridge structures. Off-white and White Portland Cement – characterised by relatively high C3A contents (9-14%) and low C4AF contents (3% for off-white and 0.3-0.4% for white cements). Principally used for architectural purposes. Coloured cements – consists of cement and inorganic pigments interground or mixed together.
Strength development – Ultimate compressive strength may take several years to achieve, but for practical purposes 28-day strengths are used as indicators for the ‘final’ strength of most cements and are specified in cement standards all over the world. - Compressive strengths of cement is determined by crushing tests on prisms made from the standard mortar (1:3 cement: sand mix of 0.5 water-cement ratio) – these ratios are by WEIGHT not volume Strength development of blended cements is dependent on the nature and proportion of the component materials, i.e. the type and properties of the Portland cement, and the properties of the fly ash, slag and silica fume. Fly ash and slag blended cement gain strength more slowly than Portland cements at early ages, but they exhibit more strength gain over a longer period. Ultimate strength of blended cements can be higher than that of the Portland cement it incorporates.
Aggregates Aggregates are of 2 basic types: •COARSE: – crushed rock, gravel or screenings •FINE: – fine and coarse sands and crusher fines. – sand should be concreting sand and not brickies sand or plasterers sand – has to be durable and chemically inactive •Aggregates should be: STRONG and HARD & DURABLE – A stronger, harder aggregate will give a stronger final concrete. Never use a crumbly or flaky rock like sandstone CHEMICALLY INACTIVE so the aggregates do not react with the cement. CLEAN – Dirt or clay sticking to the aggregates will weaken the bond between paste and aggregates. GRADED – Aggregates should range in size so that they fit together well. This gives a stronger and denser concrete. – Rounded aggregates give a more workable mix. – Angular aggregates make concrete harder to place, work and compact, but can make concrete stronger The alkali–silica reaction (ASR) occurs over time in concrete between the highly alkaline cement paste and reactive non-crystalline (amorphous) silica (found in many common aggregates) This reaction causes the expansion of the altered aggregate by the formation of a swelling gel of calcium silicate hydrate (C-S-H). This gel increases in volume with water, and exerts an expansive pressure inside the material, causing spalling and loss of strength of the concrete, finally leading to its failure. ASR can cause serious cracking in concrete, resulting in critical structural problems that can even force the demolition of a particular structure
Used in insulation
Used in radiation shielding
Aggregate Grading = distribution of particle sizes in a particular batch of an aggregate. Influences the water demand of concrete and its subsequent tendency to bleed and segregate Influences the mix proportions for a desired workability and water-cement ratio The coarser the grading (lower proportion of fine aggregate) the lower the cement content required for a given workability and water-cement ratio Aggregates having a continuous relatively smooth grading curve will generally produce mixes with fewer large voids between particles Grading is determined by sieve analysis Bleeding = cement paste separates from mix, if over compacted = paste goes to top Segregation = all elements separate, larger aggregates sink
Water for Concrete Water is mixed with the cement powder to form a paste which holds the aggregates together like glue. Water must be clean, fresh and free from any dirt, unwanted chemicals or rubbish that may affect concrete. •Many concrete plants now use recycled water. •Always check bore water before use. •Don’t use sea water as it may rust the steel reinforcement in the concrete. What should not be present = solids in suspension (oil & grease), organic matter, dissolved chloride salts, dissolved carbonates and bicarbonates, acid water, - recycled water is generally satisfactory
Admixtures = are classified either by their characteristics or principal effect on the concrete (e.g. Set retarding) or by the type of material or chemical that is the principal constituent (e.g.. polysaccharides) Types = Air entraining (AEA) Overdose of admixtures: Set controlling: set retarding (Re), Set accelerating (Ac) Re = too long setting time Water-reducing HWR = severe segregation Water reducing set-controlling WR = excessive air entrainment Thickening (pumpability aids – further distance) Ac = loss in ultimate strength Shrinkage-reducing/ shrinkage-compensating Permeability reducing Special purpose Concrete setting: INITIAL SET: - workman definition: standing on concrete leaves 6mm deep boot print, time to start machine float process - engineers definition: paste starts to lose plasticity, first particle turns solid (must stop mixing, transporting, placing, compacting) FINAL SET: - workman’s definition: boots leave only the slightest scuff on the concrete, concrete cannot be worked further - engineer’s definition: paste completely lost its plasticity, all solid now,
3 STAGES OF HARDENING = PLASTIC, SETTING AND HARDENING
