Lecture notes on Advanced Concrete Technology

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Published Date:15-07-2017
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ADVANCED CONCRETE TECHNOLOGY ADVANCED CONCRETE TECHNOLOGY UNIT – 1 CEMENT • Cement, any material that hardens and becomes strongly adhesive after application. • Manufactured substance consisting of gypsum plaster, or Portland cement • Portland cement hardens and adheres after being mixed with water. HISTORY OF CEMENT: • The term “Portland cement” was first used in 1824 by Joseph Aspdin, a British cement-maker, because of the resemblance between concrete made from his cement and Portland stone, which was commonly used in building in Britain. • The first modern Portland cement, made from lime and clay materials heated until they formed clinkers was produced by Isaac Charles Johnson in Britain in 1845. • At that time cements were usually made in upright kilns where the raw materials were spread between layers of coke, which was then burnt. • The first rotary kilns were introduced about 1880. Portland cement is now almost universally used for structural concrete. Dept. of civil engineering, ACE,Banglore Page 1 ADVANCED CONCRETE TECHNOLOGY HOW IS IT MADE?  Limestone for calcium and Clay or shale for Silica/Alumina is used as raw materials.  The manufacturing process of Portland cement clinker consist essentially of grinding the raw materials, mixing them in appropriate proportion, burning the raw material in a kiln at a temperature of 1400-1500 oC until material partially fuses into balls known as clinker and grinding to cooled clinker together with a small amount of gypsum rock.  The mixture of raw material is burned in a rotary kiln. The Kiln: • The heart of the cement plant o Largest moving part of any machine o inclined, rotates o up to 50m long and 5m diam. Heated by fire jet • The rotary kiln is along steel cylinder lined with refractory brick (length /diameter 30).Modern kilns may reach 6m in diameter and over 180m in height with a production capacity exceeding 1000 tones a day. Dept. of civil engineering, ACE,Banglore Page 2 ADVANCED CONCRETE TECHNOLOGY • The kiln is inclined a few degrees from the horizontal ( about 4 cm\m ) and is rotated about its axis at a speed of about 60 to 150 revolution \ hour ). • Pulverized coal or gas is used as the source of heat. The heat is supplied from the lower end of the kiln. The max. Temperature near the lower end of the kiln is generally about 1400-1500 OC. • The upper end of the kiln the temperature is around 150 OC. • The mixture of the raw material is fed from the upper end of the kiln.This material move toward the lower end by effect of inclanation and rotation of the kiln. Thus the material is subjected to high temperature at lower end of the kiln. • The materials that are introduced into the rotary kiln are subjected to several distinct process as they move downward. • When the raw materials are fed into the kiln, drying of the material takes place, and any free water in the raw material is evaporated. • Clay losses its water about 150 to 350 OC. • Clay decompose at a range of 350 to 650 OC. • Magnesite in raw material loss about 600 OC. • The limestone losses its CO2 at about 900 OC. • At 1250 to 1280 OC some liquid formation begins and compound formation start to takes place. • Clinkering begins at about 1280 OC. The liquid that forms during the burning process causes the charge to agglomerate into nodules of various size, usually 1-25 mm in diameter known as Portland cement clinker. • All exhaust gases produced during the burning process of the materials leave the kiln through the stack. Cooling & grinding • Rapid cool - glassy • Grinding starts golf ball size. Ends about 2-80 microns, 300 m2/kg. Grinding Dept. of civil engineering, ACE,Banglore Page 3 ADVANCED CONCRETE TECHNOLOGY depends on application. Typical plant capacity is about: 1 MT/y • Portland cement is manufactured by inter grinding the Portland cement clinker with some (3 to 6 %) gypsum rock. CEMENT: PHYSICAL, CHEMICAL PROPERTIES AND HYDRATION Physical Properties Portland cements are commonly characterized by their physical properties for quality control purposes. Their physical properties can be used to classify and compare Portland cements. The challenge in physical property characterization is to develop physical tests that can satisfactorily characterize key parameters. The physical properties of cement • Fineness • Soundness • Setting Time • Strength Fineness • Fineness or particle size of Portland cement affects Hydration rate and thus the rate of strength gain. The smaller the particle size, the greater the surface areato- volume ratio, and thus, the more area available for water-cement interaction per unit volume. The effects of greater fineness on strength are generally seen during the first seven days. • When the cement particles are coarser, hydration starts on the surface of the particles. So the coarser particles may not be completely hydrated. This causes low strength and low durability. • For a rapid development of strength a high fineness is necessary. Dept. of civil engineering, ACE,Banglore Page 4 ADVANCED CONCRETE TECHNOLOGY Soundness When referring to Portland cement, "soundness" refers to the ability of a hardened cement paste to retain its volume after setting without delayed expansion. This expansion is caused by excessive amounts of free lime (CaO) or magnesia (MgO). Most Portland cement specifications limit magnesia content and expansion. Setting Time • Cement paste setting time is affected by a number of items including: cement fineness, water-cement ratio, chemical content (especially gypsum content) and admixtures. Setting tests are used to characterize how a particular cement paste sets. For construction purposes, the initial set must not be too soon and the final set must not be too late. Normally, two setting times are defined: • Initial set. Occurs when the paste begins to stiffen considerably. • Final set. Occurs when the cement has hardened to the point at which it can sustain some load. Dept. of civil engineering, ACE,Banglore Page 5 ADVANCED CONCRETE TECHNOLOGY • Setting is mainly caused by C3A and C3S and results in temperature rise in the cement paste. • False set :No heat is evolved in a false set and the concrete can be remixed without adding water • Occures due to the conversion of unhydreous/semihydrous gypsum to hydrous gypsum(CaSO4.2H2O) • Flash Set: is due to absence of Gypsum. Specifically used for under water repair. Strength • Cement paste strength is typically defined in three ways: compressive, tensile and flexural. These strengths can be affected by a number of items including: water-cement ratio, cement-fine aggregate ratio, type and grading of fine aggregate, curing conditions, size and shape of specimen, loading conditions and age. Duration of Testing Typically, Durations of testing are: • 1 day (for high early strength cement) • 3 days, 7 days, 28 days and 90 days (for monitoring strength progress) • 28 days strength is recognised as a basis for control in most codes. • When considering cement paste strength tests, there are two items to consider: • Cement mortar strength is not directly related to concrete strength. Cement paste strength is typically used as a quality control measure. • Strength tests are done on cement mortars (cement + water + sand) and not on cement pastes. Dept. of civil engineering, ACE,Banglore Page 6 ADVANCED CONCRETE TECHNOLOGY TESTS FOR CEMENT The physical principal tests on cement are; • Consistency, Setting time, Soundness, Compressive strength, Fineness CEMENT Chemical Composition Oxide Composition of Portlant Cement • Portland cement is composed of four major oxides: lime ( CaO ), silica ( SiO2 ), alumina ( Al2O3 ), and iron ( Fe2O3 ). • Also Portland cement contains small amount of magnesia ( MgO ), alkalies (Na2O and K2O ), and sulfiric anhydrite ( SO3 ). Approximate Composition Limits of Oxides in Portland Cement Oxide Common Name Content, % CaO Lime 60-67 SiO2 Silica 17-25 Al2O3 Alumina 3-8 Fe2O3 Iron 0,5-6 MgO Magnesia 0,1-4 Na2O and Alkalies 0,2-1,3 K2O Sulfuric 1-3 SO3 anhydride Dept. of civil engineering, ACE,Banglore Page 7 ADVANCED CONCRETE TECHNOLOGY Oxide composition Mass Percentage Oxide Cement 1 Cement 2 Cement 3 CaO 66 63 66 SiO2 20 22 20 Al2O3 7 7.7 5.5 Fe2O3 3 3.3 4.5 Others 4 4 4 Major Compounds of Portland Cement (Bogue’s Compound Composition) Name Chemical Abbreviation formula Tricalcium silicate 3CaO.SiO2 C3S • 2CaO.SiO2 C2S Dicalcium silicate • 3CaO.Al2O3 C3A Tricalcium aluminate • 4CaO.Al2O3. C4AF Tetracalcium alumino • Fe2O3 ferrite Dept. of civil engineering, ACE,Banglore Page 8 ADVANCED CONCRETE TECHNOLOGY Bogue’s Compound Composition • C3S=4.07(CaO)-7.6(SiO2)- 6.72(Al2O3)-1.43(Fe2O3 ) – 2.85( SO3 ) • C2S= 2.87 (SiO2) - 0.75( 3Cao. SiO2) • C3A= 2.65(Al2O3) – 1.69 (Fe2O3 ) • C4AF = 3.04 (Fe2O3 ) Significance of Compound Composition Mass Percentage Compound Cement 1 Cement Cement 2 3 C3S 65 33 73 C2S 8 38 2 C3A 14 15 7 C4AF 4 10 14 Hydration When Portland cement is mixed with water its chemical compound constituents undergo a series of chemical reactions that cause it to harden. This chemical reaction with water is called "hydration". Each one of these reactions occurs at a different time and rate. Together, the results of these reactions determine how Portland cement hardens and gains strength. Dept. of civil engineering, ACE,Banglore Page 9 ADVANCED CONCRETE TECHNOLOGY OPC hydration •Hydration starts as soon as the cement and water are mixed. • The rate of hydration and the heat liberated by the reaction of each compound is different. • Each compound produces different products when it hydrates. • Tricalcium silicate (C3S). Hydrates and hardens rapidly and is largely responsible for initial set and early strength. Portland cements with higher percentages of C3S will exhibit higher early strength. • Tricalcium aluminate (C3A). Hydrates and hardens the quickest. Liberates a large amount of heat almost immediately and contributes somewhat to early strength. Gypsum is added to Portland cement to retard C3A hydration. Without gypsum, C3A hydration would cause Portland cement to set almost immediately after adding water. • Dicalcium silicate (C2S). Hydrates and hardens slowly and is largely responsible for strength increases beyond one week. • Tetracalcium aluminoferrite (C4AF). Hydrates rapidly but contributes very little to strength. Its use allows lower kiln temperatures in Portland cement manufacturing. Most Portland cement color effects are due to C4AF Dept. of civil engineering, ACE,Banglore Page 10 ADVANCED CONCRETE TECHNOLOGY Characteristics of Hydration of the Cement Compounds Heat Compounds Reaction Amount of Strength Liberation Rate Liberated C3S Moderate Moderate High High Low C2S Slow Low initially, Low high later C3A Fast Very high Low Very high C4AF Moderate Moderate Low Moderate Reactions of Hydration • 2C3S + 6H = C3S2H3 + 3Ca(OH)2 (100 + 24 = 75 + 49) • 2 C2S + 4H = C3S2H3 + Ca(OH)2 (100 + 21 = 99 + 22) • C3A + 6H = C3AH6 C3A + CaSO4. 2H2O = 3Cao. Al2O3. 3CaSO4. 31H2O Calcium Sulfoaluminate Dept. of civil engineering, ACE,Banglore Page 11 ADVANCED CONCRETE TECHNOLOGY Heat of Hydration The heat of hydration is the heat generated when water and Portland cement react. Heat of hydration is most influenced by the proportion of C3S and C3A in the cement, but is also influenced by water-cement ratio, fineness and curing temperature. As each one of these factors is increased, heat of hydration increases. •For usual range of Portland cements, about one-half of the total heat is liberated between 1 and 3 days, about three-quarters in 7 days, and nearly 90 percent in 6 months. • The heat of hydration depends on the chemical composition of cement. Dept. of civil engineering, ACE,Banglore Page 12 ADVANCED CONCRETE TECHNOLOGY STRENGTH, ELASTICITY & SHRINKAGE The "strength" of hardened concrete is its ability to resist strain or rupture induced by external forces. The resistance of concrete to compressive, tensile and bending stresses is known as compressive strength, tensile strength, and bending (or flexural) strength, respectively. The resistance of concrete to repeated stresses is called its fatigue strength. Strength is expressed in terms of kgf/cm2 or MPa. Dept. of civil engineering, ACE,Banglore Page 13 ADVANCED CONCRETE TECHNOLOGY The compressive strength of concrete is usually determined at an age of 28 days of the specimen. The 28-day compressive strength is the strength value used in concrete designs. Sometimes, the compressive strength at 7 days is also determined. The 7-day compressive strength is approximately 65-70% of its 28- day strength. At least three specimens should be tested; the average of their compressive strengths is found for determining the compressive strength of a concrete sample on a particular testing day. The compressive strength values obtained for cylinder specimens and cube specimens prepared from the same concrete sample are not the same. Compressive Strength Test (Drilling Core Method) This test is conducted on cylindrical concrete core specimens removed from the hardened concrete by a drilling operation. A core drilling machine is used for cutting and removing the concrete samples. This machine is equipped with diamond cutters located on the end of a cylindrical (tube-like) cutting device. As the machine is operated, the cylinder shaped cutter rotates at a high speed. Dept. of civil engineering, ACE,Banglore Page 14 ADVANCED CONCRETE TECHNOLOGY The diameter of the concrete core specimen removed from the hardened concrete depends on the inner diameter of the cylindrical cutting device. Usually concrete cores having diameters of 10 cm or 15 cm are obtained. The removed core specimens may have different lengths depending on the thickness of the hardened concrete that they are cut from. If the core specimen is too long, it is shortened so that it will have a length/diameter ratio of 2.0. Core specimens which have a length/diameter ratio of less than 2.0 can also be used for compressive strength testing purposes, but a specimen having a length/diameter ratio of less than 1.0 should not be used. If the ratio of the length to the diameter of the specimen is less than 2.0, allowance is made; the compressive strength found by the test should be multiplied with the correction factors shown in Table. Table. Strength Correction Factors for Core Specimens Having Length/Diameter2.0 L/d Correction Factor 1.75 0.98 1.50 0.96 1.25 0.93 1.00 0.87 Dept. of civil engineering, ACE,Banglore Page 15 ADVANCED CONCRETE TECHNOLOGY Determination of the compressive strength of concrete by testing core specimens is useful in finding the strength of concrete that is present in a structure. As is known, the strength of the concrete in the structure may be different from the strength found by the standard test method. The operations applied to the concrete - in the structure such as placing, consolidation, and curing may lead to these differences in the strength. This method provides the possibility of finding the actual quality of the concrete in the structure. Factors Influencing Cube Compressive Strength • Platen effect • Rate of loading • Size of the specimen • Moisture content • Age of the specimen Modulus Elasticity Defining modulus of elasticity of concrete is difficult;Because concrete is not a linearly elastic material Since the slope of σ-ε curve of concrete is not constant. We must first describe modulus of elasticity (Ec). In general; Modulus of elasticity defined for concrete is the instantaneous Ec. This is not influenced by the time effect (mean Ec is function of many variables) Instantaneous Ec can be defined in 3 ways. – Initial Modulus of Elasticity, E – Secant modulus – Tangent modulus Dept. of civil engineering, ACE,Banglore Page 16 ADVANCED CONCRETE TECHNOLOGY Modulus Elasticity a) Initial modulus: tangent to curve at origin. b) Secant modulus: slope at secant at a given stress usually 0.5fc. c) Tangent modulus: tangent at a given stress; usually 40 to 50% of compressive strength. d) Depending on the problem, these three can be used in design and research. However, secant modulus is mostly used and codes referred. • Categorize the elastic behaviour of concrete in terms of the varius type of elastic behaviour of engineering materials.The definition of pure elasticity is that strains appear and disappear immediately on aplication and removal of stress. • The stress- strain curve of the figure illustrate two categories of pure elasticity. A) is linear and elastic B) is brittle materials, such as glass and most rocks are described as linear and non elastic Dept. of civil engineering, ACE,Banglore Page 17 ADVANCED CONCRETE TECHNOLOGY C) because seperate linear curves exist for the loading and unloading brunches of stress-strain diagram and permanent deformation exists after removal of load. D) described as non linear and non elastic behaviour. E-aggregate-concrete The properties of aggregate also influence the modulus of elasticity although they don‟t affect the compressive Strength. The relation between the modulus of elasticity of concrete and Strength depends also on age. Poisson’s ratio • The design and analysis of some type of structure requıre the knowledge of poissons ratio.viz. The ratio of the lateral strain.The sign of the strains is ignored.We are usually interested in applied compression and therefore have axial conctraction and lateral extension. • Generally poisson ratio for normal weight and light weight lies in range of 0.15 to 0.20 when determinerd from strain meausurements taken in the static modulus of elasticity tests • An alternative method of determinig poissons ratio is by dynamic means. CREEP • Creep is the time-dependent flow of concrete caused by its being subjected to stress. • This deformation, which occurs rapidly at first and then decreases with time, can be several times larger than the strains due to elastic shortening. • Using more scientific approach;When load is applied to concrete at time to, a deformation occurs immediately which can be expressed as the elastic strain, ε Dept. of civil engineering, ACE,Banglore Page 18 ADVANCED CONCRETE TECHNOLOGY (to). If this applied load is left on concrete producing a constant stress, the instantaneous elastic strain ε (to) begins to increase. • The rate of increase is fast during the first 3 months, after which it begins to slow down. • Whichever slowing rate, creep continues for years • Creep of Concrete resulting from the action of a sustained stress is a gradual increase in strain with time; it can be of the same order of magnitude as drying shrinkage. • Creep does not include any immediate elastic strains caused by loading or any shrinkage or swelling caused by moisture changes. • When a concrete structural element is dried under load the creep that occurs is one to two times as large as it would be under constant moisture conditions. Adding normal drying shrinkage to this and considering the fact that creep can be several times as large as the elastic strain on loading, it may be seen that these factors can cause considerable deflection and that they are of great importance in structural mechanics. Dept. of civil engineering, ACE,Banglore Page 19 ADVANCED CONCRETE TECHNOLOGY If a sustained load is removed, the strain decreases immediately by an amount equal to the elastic strain at the given age; this is generally lower than the elastic strain on loading since the elastic modulus has increased in the intervening period. This instantaneous recovery is followed by a gradual decrease in strain, called creep recovery. This recovery is not complete because creep is not simply a reversible phenomenon. Dept. of civil engineering, ACE,Banglore Page 20

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