CHAPTER – I INTRODUCTION TO CONCRETE AND CONCRETE MATERIALS “These days, there are two commonly used structural materials: concrete and steel. They sometimes complement one another, and sometimes compete with one another, so that many structures of a similar type and function can be built in either of these materials. And yet, universities, polytechnics and colleges teach much less about concrete than about steel. This in itself would not matter were it not for the fact that, in actual practice, the man on the job needs to know more about concrete than about steel.” - - Concrete Technology (A. M. Neville CBE, J. J. Brooks)
1.1 Use of Concrete in Structure and Types of Concrete Concrete, in the broadest sense, is any product or mass made by the use of a cementing medium. Generally, this medium is the product of reaction between hydraulic cement and water. Concrete has been the most common building material for many years. It is expected to remain so in the coming decades. Concreting is widely used in domestic, rural, commercial, recreational and educational constructions. Communities around the world rely on concrete as a safe, strong and simple building material. Concrete is used in many types of civil engineering structures such as such as buildings, bridges, dam, plates and shell structures, etc. Based on ingredients present in concrete, it can be classified into two types: a. Normal Concrete b. Special Concrete Normal Concrete has the ingredients namely cement, sand, aggregates (coarse) and water mixed in some proportion to achieve concrete of desired strength and property. Modern concrete, also called special concrete, invariably has additional components other than these ingredients namely admixtures. Admixtures are added to concrete to achieve special properties like ultra high strength or resistance to tensile forces.
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Based on strength, concrete can be classified as: S.N.
Types of Concrete
Grade Designation
Characteristic Strength (N/mm2)
1.
Ordinary Concrete
M10
10
M15
15
M20
20
M25
25
M30
30
M35
35
M40
40
M45
45
M50
50
M55
55
M60
60
M65
65
M70
70
M75
75
M80
80
2.
Standard Concrete
3.
High Strength Concrete
Designation of Concrete:
Concrete is designated through 28-day Standard Cube Compressive Strength.
Each cube has a side of 150 mm.
The cube is cast, cured and tested in a standard manner.
Concrete designated as M25 has a 28 day characteristic standard cube strength of 25 MPa.
Advantages of Concrete: 1.
Lower life cycle cost (production + maintenance): Although cement is costly, cement paste occupies only 25 to 40 % of total volume of concrete.
2. Mould-ability: Initially, the concrete is in plastic stage so that it can be cast into any shape. 3. Robustness: property associated with the massiveness of concrete 4. Can be designed for desired property: by changing proportions of ingredients Disadvantages of Concrete: 1.
Low tensile strength – Concrete is derived from particulate system – particulate systems are bonded by some type of bonding which is mostly physical type e.g. Van der Waal’s forces.
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This can be overcome by conventional reinforcements like steel (reinforced concrete) or by using small discrete randomly oriented fibres (fiber reinforced concrete). 2. Low ductility (brittle) – not much capacity to deform before failure unlike steel These disadvantages can be overcome by adding additional materials.
1.2 Concrete Materials – Role of Different Materials (Aggregates, Cement, Water and Admixtures) 1.2.1 Aggregates – Properties of Aggregates and their Gradation
Crushed or uncrushed materials derived from natural or artificial sources as in the form of boulder, gravel or sand for production of concrete are known as aggregates.
Aggregates occupy 70 to 80 % of the volume of concrete.
Earlier, aggregates were considered as chemically inert materials but now it has been recognized that some of the aggregates are chemically active and also that certain aggregates exhibit chemical bond at the interface of aggregate and paste.
Aggregates should be free from wild acids, alkalies and inorganic materials. Classification
Aggregates can be classified as: (i) Normal weight aggregates, (ii) Light weight aggregates and (iii) Heavy weight aggregates.
Normal Weight Aggregates
Natural E.g. derived from river bed, mountain quarry
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Artificial E.g. derived from pieces of burnt bricks, air-cooled slag, sintered fly ash
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Almost all natural aggregate materials originate from bed rocks. Since there are three types of rocks, normal weight aggregates can also be classified into three types based on the rock these are derived from: i.
Aggregates from Igneous Rocks: Bulk of the concrete aggregates is igneous in origin. These are the most chemically active concrete aggregates. They are also highly satisfactory because of hard, tough and dense nature. E.g. Basalt, Granite
ii.
Aggregates from Sedimentary Rocks: Quality varies depending upon the cementing material and the pressure under which the rocks were originally compacted. E.g. Limestone, Sandstone
iii.
Aggregates from Metamorphic Rocks: E.g. Quartzite, Gneiss. If the thickness of the foliation in metamorphic rocks is less, then individual aggregate may exhibit foliation which is not a desirable characteristic in aggregate.
Based on Size: Based on size, aggregates can be classified into coarse (retained on IS 4.75 sieve) and fine (passing through IS 4.75 sieve). The largest maximum size of aggregate practicable to handle under a given set of conditions should be used. This results in: i.
Reduction in the cement content
ii.
Reduction in water requirement
iii.
Reduction of drying shrinkage
The maximum size of aggregates that can be used in any given condition may be limited by the following conditions: i.
Thickness of section
ii.
Spacing of reinforcement
iii.
Clear cover
iv.
Mixing, handling and placing techniques
Maximum size of aggregate < 1/4th of maximum thickness of the member.
Based on Shape: Classification 1. Rounded
Description Fully water worn or completely shaped by attrition
Examples River
or
seashore
gravels; desert, seashore Shuvanjan Dahal (068-BCE-147)
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and wind blown sands 2.
Irregular
or Naturally irregular or partly shaped by attrition, Pit sands and gravels;
Partly Rounded
having rounded edges
land
or
dug
flints;
cuboidal rock 3. Flaky
Thickness is small relative to the width and/or length
Laminated rocks
Thickness (d) < 0.6 x mean size (average of lengths) 4. Elongated
Mean Size = Mean Thickness x 1.8
Based on Density: Light weight aggregates: Density ≤ 1120 kg/m3
1.
2. Normal weight aggregates: Density = 1520 to 1680 kg/m3 3. Heavy weight aggregates: Density ≥ 2080 kg/m3
Based on Mineralogical Composition: 1.
Siliceous – containing silica
2. Calcareous – containing clay Properties of Aggregates 1.
Physical Properties
2. Chemical Properties 3. Mechanical Properties
Mechanical Properties 1. Strength
The properties of concrete are based primarily on the quality of the cement paste.
The strength is dependent also on the bond between the cement paste and the segregate.
If either the strength of the paste or the bond between the paste and aggregate is low, a concrete of poor quality will be obtained irrespective of the strength of the rock or aggregate.
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AGGREGATE CRUSHING VALUE
The test specimen is cylindrical in shape of size 25 mm diameter and 25 mm height.
The aggregate is placed in the cylindrical mould and a load of 40 tonnes is applied gradually through a plunger.
The material crushed to finer than 2.36 mm is separated and expressed as a percentage of original weight taken in the mould. This % is referred as aggregate crushing value.
The crushing value of aggregate is restricted to 30% for concrete used for roads and pavements and 45% may be permitted for other structures.
The crushing value of aggregate is rather insensitive to the variation in strength of weaker aggregate. This is so because having been crushed before the application of the full load of 40 tons, the weaker materials become compacted, so that the amount of crushing during later stages of the test is reduced.
2. Hardness Abrasion strength or hardness is defined as the resistance to wear, which is expressed in percentage loss in weight on abrasion. It can be determined by Los Angeles Abrasion Value Test (LAAVT). LOS ANGELES ABRASION VALUE TEST (LAAVT)
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5 to 10 kg of aggregate is placed in a cylindrical drum mounted horizontally with a shelf inside.
11 spheres each of 11 to 14 kg are charged.
The drum is rotated for about 500 cycles.
The tumbling and dropping of the aggregate and of the balls result abrasion and attrition of the aggregate.
The proportion of broken material expressed as a percentage is measured.
3. Toughness Toughness can be defined as the resistance of aggregate to failure by impact. AGGREGATE IMPACT VALUE (ATV)
Weight of aggregate taken – 5 to 10 kg
A sample of standard aggregate kept in a mould is subjected to 15 blows of a metal hammer of weight 14 kgs falling from a height of 38 centimeters.
The quantity of finer material (passing through 2.36 mm sieve) resulting from pounding will indicate the toughness of the sample of aggregate.
AIV – ratio of the weight of the fines formed to the weight of the total sample taken
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Physical Properties 1. Specific Gravity Specific gravity is defined as the ratio of mass (or weight) in air of a unit volume of material to the mass of the same volume of water at the stated temperature. With the specific gravity of each constituent known, its weight can be converted into solid volume and hence a theoretical yield of concrete per unit volume can be calculated. Specific gravity of aggregate is also required in calculating the compacting factor in connection with the workability measurements.
2. Bulk Density The bulk density or unit weight of aggregate is measured by filling a container of known volume in a standard manner and weighing it. Bulk density shows how densely the aggregate is packed when filled in a standard manner. The bulk density depends on the particle size distribution and shape of the particles.
3. Absorption and Moisture Content The porosity, permeability and absorption of aggregate influence the bond between it and the cement paste, the resistance of concrete to freezing and thawing as well as chemical stability, resistance to abrasion and specific gravity. When we deal with aggregates in concrete, the 24 hour absorption during may not be of much significance; on the other hand, the percentage of water absorption during the time interval equal of final set of cement may be of more significance. The aggregate absorbs water in concrete and thus affects the workability and final volume of concrete.
Bulking of Sand Bulking occurs when there is a change in volume due to the addition of water to the sand particles. When dry sand comes in contact with moisture, thin film is formed around the particles, which causes them to get apart from each other. This results in increasing the volume of sand. This phenomenon is known as “bulking of sand”. When mixes are specified by volume, the sand is assumed to be dry. The volume of a given weight of sand, however, varies according to its moisture content.
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The water molecules grab onto the sand particle surface and also fill the air spaces to create a bulking effect. Fine sands tend to bulk more than coarse sands due to the increased ratio of surface area to volume available for water molecules to interact with the sand particles.
The extent of bulking can be estimated by a simple field test. A sample of moist fine aggregate is filled into a measuring cylinder in the normal manner. Let the level be h1. Water is poured into the measuring cylinder and sand is completely inundated. The cylinder is shaken. Since the volume of the saturated sand is the same as that of the dry sand, the inundated sand completely offsets the bulking process. Let the level of sand be h2. The difference of these two levels shows the bulking of the sample of sand under test.
Fineness Modulus of Aggregates It is a numerical index to measure the average particle size of aggregates.
Higher the F.M., higher will be coarser particles. Many a time, fine aggregates are designated as coarse sand, medium sand and fine sand. These classifications do not give any precise meaning. What the supplier terms as fine sand may be really medium or even coarse sand. To avoid this ambiguity, fineness modulus could be used as a yardstick to indicate the fineness of sand. The following limits may be taken as guidance: Shuvanjan Dahal (068-BCE-147)
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F.M. of Fine Sand: 2.2 – 2.6 F.M. of Medium Sand: 2.6 – 2.9 F.M. of Coarse Sand: 2.9 – 3.2 Sand having a fineness modulus more than 3.2 will be unsuitable for making satisfactory concrete. F.M. for coarse aggregates (10 kg) I.S. Sieve
F.M. for fine aggregates (1 kg)
Wt.
Cumulative
% of
retained
wt.
(kg)
I.S. Sieve
Wt.
Cumulative
% of
cumulative
retained
wt.
cumulative
retained
wt.
(kg)
retained
wt.
(kg)
retained
(kg)
retained
80 mm
0.0
0.0
0
80 mm
0.0
0.0
0
40 mm
0.0
0.0
0
40 mm
0.0
0.0
0
20 mm
3.5
3.5
35
20 mm
0.0
0.0
0
10 mm
3.0
6.5
65
10 mm
0.0
0.0
0
4.75 mm
2.8
9.3
93
4.75 mm
0.0
0.0
0
2.36 mm
0.70
10.0
100
2.36 mm
0.1
0.1
10
1.18 mm
0.0
10.0
100
1.18 mm
0.25
0.35
35
600 μ
0.0
10.0
100
600 μ
0.35
0.70
70
300 μ
0.0
10.0
100
300 μ
0.20
0.90
90
150 μ
0.0
10.0
100
150 μ
0.10
1.0
100
Σ=693 F.M. for coarse aggregates
= 693/100 = 6.93
F.M. for fine aggregates
= 305/100 = 3.05
Σ=305
1.2.2 Cement – Manufacturing of Cement, Compound Composition of Portland Cement, Structure and Reactivity of Compounds
Cement is a material having cohesive and adhesive properties which provides bonding action to other concrete ingredients.
It is the most important ingredient of concrete.
Water is required for chemical action (known as hydration).
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Manufacture of Portland Cement The raw materials required for manufacture of Portland cement are calcareous materials, such as limestone and chalk, and argillaceous materials such as shale or clay. Cement factories are established where these raw materials are available in plenty. The process of manufacture of cement consists of: Grinding the raw materials. Mixing them intimately in certain proportions depending upon their purity and composition. Burning them in a kiln at a temperature of about 1300 to 1500°C (at this temperature, the material sinters and partially fuses to form nodular shaped clinker). The clinker is cooled and ground to fine powder with addition of about 3 to 5 % Gypsum. The product formed by using this procedure is Portland Cement.
Compound Composition of Portland cement ASTM C 150 defines Portland cement as hydraulic cement produced by pulverizing clinkers consisting essentially of hydraulic calcium silicates. Clinkers are 5 to 25 mm diameter nodules of a
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sintered material which is produced when a raw mixture of predetermined composition is heated to a high temperature. The raw materials used for the manufacture of cement consist mainly of lime, silica, alumina and iron oxide. These oxides interact with one another in the kiln at high temperature to form more complex compounds. Ingredients of OPC
Percentage
Lime (CaO)
60 – 67
Silica (SiO2)
17 – 25
Alumina (Al2O3)
3–8
Iron Oxide (Fe2O3)
0.5 – 6
Magnesia (MgO)
0.1 – 4
Soda and Potash (K2O and Na2O)
0.2 – 3
Sulphur Trioxide (SO3)
1–3
Free Lime
0–1
Indian Standard specification for 33 grade cement specifies the following chemical requirements: (a) Ratio of percentage of lime to percentage of silica, alumina and iron oxide, known as Lime Saturation Factor, when calculated by the formula
should
not be greater than 1.02 and not less than 0.66. (b) Ratio of percentage of alumina to that of iron oxide – not less than 0.66 (c) Weight of insoluble residue
- not more than 4 per cent
(d) Weight of magnesia
- not more than 6 per cent
(e) Total sulphur content, calculated as sulphuric anhydride (SO3) – not more than 2.5% when C3A is 5% or less. Not more than 3% when C3A is more than 5%. (f) Total loss on ignition
- not more than 5 per cent
Bogue’s Compounds The oxides present in the raw materials when subjected to high clinkering temperature combine with each other to form complex compounds. The calculation of the potential composition of Portland cement is based on the work of R. H. Bogue and others, and is often referred to as ‘Bogue’s Compounds’. Shuvanjan Dahal (068-BCE-147)
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S.N.
Name of Compound
Chemical Formula
Abbreviation
Commercial Name
1.
Tricalcium Silicate
3CaO.SiO2
C3S
Alite
2.
Dicalcium Silicate
2CaO.SiO2
C2S
Bellite
3.
Tricalcium Aluminate
3CaO.Al2O3
C3A
Cellite
4.
Tetracalcium alumino-ferrite
4CaO.Al2O3.Fe2O3
C4AF
Fellite
The equations suggested by Bogue for calculating the percentages of major compounds are given below. (
)
(
)
(
(
(
( )
(
)
(
)
(
)
(
)
) )
)
The oxide shown within the brackets represent the percentage of the same in the raw materials.
The silicates, C3S and C2S, are the most important compounds, which are responsible for the strength of hydrated cement paste.
The presence of C3A in cement is undesirable; it contributes little or nothing to the strength of cement except at early ages, and when hardened cement paste is attacked by sulphates, the formation of calcium sulphoaluminate (ettringite) may cause disruption. However, Tricalcium aluminate is beneficial in the manufacture of cement in that it facilitates the combination of lime and silica.
C4AF reacts with gypsum to form calcium sulphoferrite and its presence may accelerate the hydration of the silicates.
Microstructure The chemical composition of compounds present in the industrial Portland cements is not exactly what is expressed by the commonly used formulas, C3S, C2S, C3A and C4AF. This is because at the high temperatures during clinker formation, the elements present in the system, including the impurities such as magnesium, sodium, potassium and sulphur possess the capability of entering into solid solutions with each of the major compounds in clinker. Small amount of impurities in solid solution may not significantly alter the crystallographic nature and the reactivity of a Shuvanjan Dahal (068-BCE-147)
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compound with water, but larger amounts can do so. The crystal structure of cement compounds is highly complex; however some of their essential features that account for differences in the reactivity are described here. CALCIUM SILICATES Tricalcium Silicate (C3S) and beta-dicalcium silicate (βC2S) are the two hydraulic silicates commonly found in industrial Portland cement clinkers. Both invariably contain small amounts of magnesium, aluminium, iron, potassium, sodium and sulphur ions; the impure forms of C 3S and βC2S are known as alite and belite respectively. Normally, C3S is found in largest quantity in any cement. It occurs in small dimensional colourless grains. On cooling below 1250° C, it decomposes slowly, but if cooling is not very slow, it remains unchanged and relatively is stable at ordinary temperatures. C2S is found to have three or even four forms depending on temperature, such as αC2S, which exists at high temperatures; αC2S changes to βC2S at about 1450° C, βC2S further undergoes change to γC2S at about 670° C. However, at the cooling rate of commercial cements, βC2S is preserved in the clinker. It forms rounded grains. CALCIUM ALUMINATE AND FERROALUMINATE Several hydraulic calcium aluminates can occur in the CaO-Al2O3 system; however, C3A is the principal aluminate compound in Portland cement clinker. Calcium ferrites are not found in normal Portland cement clinker; instead, calcium ferroaluminates which belong to the C2A-C2F series are formed and the most common compound corresponds approximately to the equimolecular composition C4AF. Similar to the calcium silicates, both C3A and C4AF in industrial clinkers contain in their crystal structures significant amounts of such impurities as magnesium, sodium, potassium and silica. The crystal structure of pure C3A is cubic; however, C4AF and C3A containing large amounts of alkalies are orthorhombic. The crystal structures are very complex but are characterized by large structural holes which account for high reactivity. MAGNESIUM OXIDE AND CALCIUM OXIDE The source of magnesium oxide in cement is usually dolomite, which is present as an impurity in most limestone. A part of the total MgO in Portland cement clinker (i.e. up to 2 percent) may enter into solid solution with the various compounds; however, the rest occurs as crystalline MgO, also called periclase. Hydration of periclase to magnesium hydroxide is a slow and expansive reaction
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which under certain conditions can cause unsoundness (i.e. cracking and pop-outs n hardened concrete). Uncombined or free calcium oxide is rarely present in significant amounts in modern Portland cements. Improper proportioning of raw materials, inadequate grinding and homogenization and insufficient temperature or hold time in the kiln burning zone are among the principal factors that account for the presence of free or crystalline calcium oxide in Portland cement clinker. Like MgO, a crystalline CaO exposed to higher temperature in the cement kiln hydrates slowly and the hydration reaction is capable of causing unsoundness in hardened concrete. Both MgO and CaO form cubic structure, with each magnesium or calcium ion surrounded by six oxygen in a regular octahedron. The size of the Mg2+ ion is such that in the MgO structure the oxygen ions are in close contact with the Mg2+ ion and well packed in the interstices. However, in the case of the CaO structure, due to the much larger size of the Ca2+ ion, the oxygen ions are forced apart so that the calcium ions are not well packed. Consequently, the crystalline MgO formed from a high temperature (>1400° C) melt in a Portland cement kiln is much less reactive with water than the crystalline CaO, which has been exposed to the same temperature condition. This is the reason why under ordinary curing temperatures, the presence of significant amount of crystalline free CaO in Portland cement may cause unsoundness in concrete, whereas a similar amount of crystalline free MgO generally proves harmless. ALKALI AND SULPHATE COMPOUNDS The alkalies, sodium and potassium in Portland cement clinker are derived mainly from the clay components present in the raw mix and coal. Their total amount, expressed as Na2O equivalent (Na2O + 0.64 K2O), may range from 0.3 to 1.5 percent. The sulphates in a cement kiln generally originate from fuel. Depending on the amount of sulphate available, soluble double sulphates of alkalies such as langbeinite and aphthitalite are known to be present in Portland cement clinker. Their presence is known to have a significant influence on the early hydration reactions of the cement. When sufficient sulphate is not present in the kiln system, the alkalies are preferentially taken up by C3A and C2S, which may then be modified to compositions of the type NC8A3 and KC23S12, respectively. Sometimes large amounts of sulphate in the form of gypsum are purposefully added to the raw mix either for lowering the burning temperature or for modification of the C3A phase to C4A3S, which is an important constituent of certain types of expansive as well as rapid-hardening cements. Shuvanjan Dahal (068-BCE-147)
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Chemical Reactions when mixed with Water On adding water to the cement, the silicates and aluminates present in the cement start a chemical reaction and form a mass known as gel. During this process, a large quantity of heat is generated. The quantity of heat generated depends upon the amount of different constituents in the cement. The main hydrates of cement can be classified as calcium silicate hydrates and Tricalcium aluminate hydrate. The two calcium silicate hydrates are the main cementitious compounds in cement. HYDRATION OF SILICATES When water is added in a limited quantity as in the case of cement paste or cement concrete, C3S and βC2S undergo hydrolysis first, producing calcium silicate of lower basicity and ultimately C3S2H3 and releases lime as Ca (OH) 2. The reaction for fully hydrated C3S and C2S pastes may be expressed as:
From the above equations, it is clear that on weight basis, both silicates require approximately the same amount of water for their hydration, but C3S produces more than two times the calcium hydroxide as is formed by hydration of C2S. From the above reaction, it is also seen that if the surface area and consequently, the adhesive property of hydrated cement paste are mainly due to the formation of calcium silicate hydrate; it is expected that the ultimate strength of a high – C3S Portland cement would be lower than a high – C2S cement. Second, if the durability of a hardened cement paste to acidic and sulphate water is reduced due to the presence of calcium hydroxide, it may be expected that the cement containing a higher proportion of C2S will be more durable in acidic and sulphate environment than the cement containing a higher percentage of C3S.
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HYDRATION OF ALUMINATES
a) For C3A (
)
(
)
b) For C4AF ( (
)
)
If sufficient water and gypsum are added, then the reaction of C3A is as follows:
The product is known as ettringite. The typical characteristic of this reaction is that due to 32 molecules of water larger volume change takes place for the product on the right side in comparison to the volume of the compounds on the left side of the reaction. In plastic (green) stage of concrete, the volume change is not so dangerous.
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Volume change in large magnitude is dangerous when it happens in hardened stage. Such phenomenon happens when C4AF hydrates contacts with sulphate environment. In such case the reaction is as follows:
In this reaction, the product is 4.6 times increased in volume. Such reaction in the hardened cement paste induces stresses and destroys the internal structure (sulphate corrosion or deterioration).
1.2.3 Introduction to Special Types of Cement 1. Rapid Hardening Cement 7 days OPC Strength = 3 days RPC Strength
Higher C3S content and a higher fineness than OPC.
Used when formwork is to be removed early for re-use or where sufficient strength for further construction is required quickly.
RPC should not be used in large structural sections because of its higher rate of heat development.
Provides a satisfactory safeguard against early frost damage.
Used in road repair works.
2. Quick Setting Cement (QSC) Initial Setting Time (IST) – 5 minutes Final Setting Time (FST) – 30 minutes This name, as the name suggests, sets very early. The early setting property is brought out by reducing the gypsum content at the time of clinker grinding. This cement is required to be mixed, placed and compacted very early. It is used mostly in under water construction where pumping is involved. Use of QSC in such conditions reduces the pumping time and makes it economical. 3. Portland Pozzolana Cement (PPC) Pozzolana is a siliceous or siliceous and aluminous material which in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with lime at ordinary temperatures to form compounds possessing cementitious properties.
This cement gains strength slowly but the long-term strength is high.
Requires curing over a comparatively long period.
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Used when high strengths at early stages are not required.
4. Coloured Cement These cements fall into two groups; most are derived from pigment addition to white cement, but some are produced from clinkers having the corresponding colours. The addition of pigment should not interfere with the strength of the cement. 5. White Cement White cement is made from china clay, which contains little iron oxide and manganese oxide, together with chalk or limestone free from specified impurities.
Special precautions are required during the grinding of the clinker so as to avoid contamination.
The cost of this type of cement is high.
6. Sulphate Resisting Cement Low C3A content so as to avoid sulphate attack from outside the concrete.
Heat developed by this type of cement is not much higher but the cost is high due to the special composition of the raw materials.
The use of sulphate resisting cement is recommended under the following conditions: i.
Concrete to be used in marine condition;
ii.
Concrete to be used in foundation and basement, where soils is infested with sulphates;
iii.
Concrete used for fabrication of pipes which are likely to be buried in marshy region or sulphate bearing soils;
iv.
Concrete to be used in the construction of sewage treatment works. 7. Portland Blast Furnace Slag Cement Made by intergrinding cement clinker with granulated blast furnace slag.
Amount of slag = 25 to 65 % of the mass of the mixture.
Early strengths are generally lower compared to OPC.
Typical uses are in mass concrete because of a lower heat of hydration and in sea water construction because of a better sulphate resistance.
8. Low Heat Cement In this cement, a low heat evolution is achieved by reducing the contents of C3A and C3S which are the compounds evolving the maximum heat of hydration and increasing C2S.
A reduction in temperature will retard the chemical action of hardening and so further restricts the rate of evolution of heat.
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Slow rate of gain of strength but ultimate strength is similar to that achieved by OPC.
9. Alumina Cement Obtained by grinding high alumina clinker.
High early strength, high heat of hydration and very high durability against chemical attack.
Used in hot weather is very limited due to high heat of hydration and increased porosity.
1.2.4 Use of Water in Concrete
Cheapest ingredient of concrete.
Water is required for: Mixing of ingredients of concrete. Washing of ingredients of concrete. Curing of concrete (7 to 10 days).
Requirements of Mixing Water 1.
The silt content should be less than 2000 ppm. It affects setting and hardening of concrete.
2. Drinking water may be unsuitable as mixing water when the water has a high concentration of sodium or potassium. 3. As a rule, any water with a pH of 6.0 to 8.0 which does not taste saline or brackish is suitable for use. 4. Water should be free from wild acids, alkalies and organic matter. 5. Alkali carbonates and bicarbonates in water should not exceed 1000 ppm. 6. As a rule, water suitable for drinking is suitable for concrete.
Test of Water The suitability of water for mixing can be determined by comparing the setting time of cement and the strength of mortar cubes using the water in question with the corresponding results obtained using known ‘good’ water or distilled water. BS 3148:1980 suggests a tolerance of 10 per cent to allow for change in variations in strength.
1.2.5 Admixtures It is the fifth ingredient of concrete other than cement, sand, aggregate and water and found invariably in modern day concrete. Admixtures are used to enhance the properties of concrete.
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Admixtures
Mineral Admixtures
Fly Ash
Rice Husk Ash
Chemical Admixtures
Accelerating Admixture
Retarding Admixture
Plasticizer
Super Plasticizer
Local Admixtures used in Nepal: sugar (causes slow setting), edible soda (accelerating admixture)
Admixtures modify properties either in wet state or after mix have been hardened.
Added less than 5% of cement (generally <2%) i.e. in very small amount.
Chemical Admixtures/Chemical Plasticizers Mechanism of Action – Chemical:
Chemical interaction in hydration (accelerators and retarders)
Adsorption on cement surface causing better particle dispersion (plasticizers and super plasticizers)
Affecting surface tension of water (increased air entrainment)
Affecting rheology of water (viscosity modifier)
1. Plasticizers (Water-reducing Admixtures) These admixtures are used for following purposes: i.
To achieve a higher strength by decreasing the water cement ratio at the same workability as an admixture free mix.
ii.
To achieve the same workability by decreasing the cement content so as to reduce the heat of hydration in mass concrete.
iii.
To increase the workability so as to ease placing in accessible locations.
iv.
Water reduction more than 5 % but less than 12 %.
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The commonly used admixtures are Ligno-sulphonates and hydro carbolic acid salts. Plasticizers are usually based on lignosulphonate, which is a natural polymer, derived from wood processing in the paper industry. MECHANISM OF ACTION: 1.
Dispersion: Water reducers are surface acting agents adsorbed on the cement and fine particles giving them –ve charges leading to repulsion.
2. Lubrication: The –ve charges also cause development of sheath of oriented water molecules around molecules. 3. Retardation: A thin layer formed over the cement particles protect them from hydration and increases the setting time. 2. Super Plasticizers These are more recent and more effective type of water reducing admixtures also known as high range water reducer. The main benefits of super plasticizers can be summarized as follows: Increased Fluidity:
Flowing
Self-leveling
Self-compacting concrete
Penetration and compaction around dense reinforcement
Reduced W/C ratio:
Very high early strength, > 200% at 24 hours or earlier
Very high later age strengths, > 100 MPa
Reduced shrinkage, especially if combined with reduced cement content.
Improved durability by removing water to reduce permeability and diffusion.
Water reduction: 12 to 30 % Dosage: Low (0.6 to 2 % on cement) Examples of commonly used super plasticizers are Sulphonated Melamine Formaldehyde condensates (SMF), Sulphonated Naphthalene Formaldehyde condensates (SNF) and Prolycarboxylate Ether super plasticizer (PCE).
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3. Accelerators It is an admixture which when added to concrete, mortar or grout, increases the rate of hydration of hydraulic cement, shortens the time of set in concrete or increases the rate of hardening or strength development. They can be divided into two groups based on their performance and application. a. Set Accelerating Admixtures: They reduce the time for the mix to change from the plastic state to the hardened state. These admixtures have relatively limited use, mainly to produce an early set. b. Hardening Accelerators: They increase the strength at 24 hours by at least 120% at 20° C and at 5° C by at least 130% at 48 hours. These admixtures find use where early stripping of shuttering or very early access to pavements is required. They are often used in combination with a high range water reducer, especially in cold conditions. E.g. calcium chloride, chloride-free accelerators based on salts of nitrate, nitrite, formate and thiocyanate. 4. Set Retarders The function of retarder is to delay or extend the setting time of cement paste in concrete. These are helpful for concrete that has to be transported to long distance, and helpful in placing the concrete at high temperatures. When water is first added to cement there is a rapid initial hydration reaction, after which there is little formation of further hydrates for typically 2–3 hours. The exact time depends mainly on the cement type and the temperature. This is called the dormant period when the concrete is plastic and can be placed. At the end of the dormant period, the hydration rate increases and a lot of calcium silicate hydrate and calcium hydroxide is formed relatively quickly. This corresponds to the setting time of the concrete. Retarding admixtures delay the end of the dormant period and the start of setting and hardening. This is useful when used with plasticizers to give workability retention. Used on their own, retarders allow later vibration of the concrete to prevent the formation of cold joints between layers of concrete placed with a significant delay between them. E.g. Calcium Ligno-sulphonates, Carbohydrate derivatives
Mineral Admixtures These are generally of two types:
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1. Cementitious These have cementing properties themselves. E.g. Ground Granulated Blast Furnace Slag (GGBFS) 2. Pozzolanic A pozzolan is a material which, when combined with calcium hydroxide (lime), exhibits cementitious properties. Pozzolans are commonly used as an addition (the technical term is "cement extender") to Portland cement concrete mixtures to increase the long-term strength and other material properties of Portland cement concrete and in some cases reduce the material cost of concrete. Examples are
Fly ash
Silica Fume
Rice Husk Ash
Metakaolin
Fly Ash Coal from mines is generally contaminated with clay. When this coal is grinded to fine size and then burnt in thermal power plant, it turns into carbon dioxide. The clay contaminants form ash mainly of silica and alumina. The larger ash settles on the bottom whereas the finer ones called fly ash fly above. Thus, fly ash is the finely divided residue resulting form the combustion of ground or powdered coal. Fly ash is generally captured from the chimneys of coal-fired power plants. One of the most important fields of application for fly ash is PCC pavement, where a large quantity of concrete is used and economy is an important factor in concrete pavement construction. Fly ash increases cementitious property. These are spherical particles; thus, ball bearing action helps to reduce water demand. Silica Fume By-product of semiconductor industry. The terms condensed silica fume, microsilica, silica fume and volatilized silica are often used to describe the by-products extracted from the exhaust gases of silicon, ferrosilicon and other metal alloy furnaces. However, the terms microsilica and silica fume are used to describe those condensed silica fumes that are of high quality, for use in the cement and concrete industry. Because of its extreme fineness and high silica content, Silica Fume is a highly effective pozzolanic material. Silica Fume is used in concrete to improve its properties. It has been found that Silica Fume improves compressive strength, bond strength, and abrasion resistance; reduces permeability of Shuvanjan Dahal (068-BCE-147)
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concrete to chloride ions; and therefore helps in protecting reinforcing steel from corrosion, especially in chloride-rich environments such as coastal regions. Rice Husk Ash This is a bio waste from the husk left from the grains of rice. It is used as a pozzolanic material in cement to increase durability and strength. The silica is absorbed from the ground and gathered in the husk where it makes a structure and is filled with cellulose. When cellulose is burned, only silica is left which is grinded to fine powder which is used as pozzolana.
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