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Anna Superkings
Anna Superkings
SRI VIDYA COLLEGE OF ENGINEERING & TECHNOLOGY LECTURE NOTES UNIT – I CONCRETE TECHNOLOGY A cement is a binder, a substance that sets and hardens and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed rock with burnt lime as binder. The volcanic ash and pulverized brick supplements that were added to the burnt lime, to obtain a hydraulic binder, were later referred to as cementum, cimentum, cäment, and cement. Cements used in construction can be characterized as being either hydraulic (pozzolan) or non-hydraulic, depending upon the ability of the cement to be used in the presence of water (see hydraulic and non-hydraulic lime plaster). Non-hydraulic cement will not set in wet conditions or underwater; rather, it sets as it dries and reacts with carbon dioxide in the air. It can be attacked by some aggressive chemicals after setting. Hydraulic cement is made by replacing some of the cement in a mix with activated aluminium silicates, or pozzolans, such as fly ash. The chemical reaction results in hydrates that are not very water-soluble and so are quite durable in water and safe from chemical attack. This allows setting in wet condition or underwater and further protects the hardened material from chemical attack (e.g., 1824 Portland cement). The chemical process for hydraulic cement found by ancient Romans used volcanic ash (activated aluminium silicates). Presently cheaper than volcanic ash, fly ash from power stations, recovered as a pollution control measure, or other waste or by products are used as pozzolanas with plain cement to produce hydraulic cement. Pozzolanas can constitute up to 40% of Portland cement. The most important uses of cement are as a component in the production of mortar in masonry, and of concrete, a combination of cement and an aggregate to form a strong building material. Non-hydraulic cement, such as slaked lime (calcium hydroxide mixed with water), hardens by carbonation in the presence of carbon dioxide which is naturally present in the air. First calcium oxide is produced by lime calcination at temperatures above 825 °C (1,517 °F) for about 10 hours at atmospheric pressure: CaCO3 → CaO + CO2 The calcium oxide is then spent (slaked) mixing it with water to make slaked lime: CaO + H2O → Ca(OH)2 Once the water in excess from the slaked lime is completely evaporated (this process is technically called setting), the carbonation starts: CE6506 / CONSTRUCTION TECHNIQUES,EQUIPMENT AND PRACTICES V SEM/III YEAR
SRI VIDYA COLLEGE OF ENGINEERING & TECHNOLOGY LECTURE NOTES Ca(OH)2 + CO2 → CaCO3 + H2O This reaction takes a significant amount of time because the partial pressure of carbon dioxide in the air is low. The carbonation reaction requires the dry cement to be exposed to air, for this reason the slaked lime is a non-hydraulic cement and cannot be used under water. This whole process is called the lime cycle. Conversely, the chemistry ruling the action of the hydraulic cement is hydration. Hydraulic cements (such as Portland cement) are made of a mixture of silicates and oxides, the four main components being: Belite (2CaO·SiO2); Alite (3CaO·SiO2); Tricalcium aluminate (3CaO·Al2O3) (historically, and still occasionally, called 'celite'); Brownmillerite (4CaO·Al2O3·Fe2O3). The silicates are responsible of the mechanical properties of the cement, the tricalcium aluminate and the brownmillerite are essential to allow the formation of the liquid phase during the kiln sintering (firing). The chemistry of the above listed reactions is not completely clear and is still the object of research. What are Different Grades of Cement? The grade 43 and 53 in cement mainly corresponds to the average compressive strength attained after 28 days ( 6724 hours) in mega pascals (Mpa) of at least three mortar cubes ( area of face 50 cm squared) composed of one part cement, 3 parts of standard s and ( conforming to IS 650:1966) by mass and P/4 ( P is the percentage of water required to produce a paste of standard consistency as per IS standard) + 3 percentage ( of combined mass of cement plus sand) of water , prepared, stored and tested in the manner described in methods of physical test for hydraulic cement. 721 hr not less than 23 MPa for 43 grade, 27 MPa for 53 grade 1682 hrs not less than 33MPa for 43 grade, 37MPa for 53 grade 6724 hrs not less than 43MPa for 43 grade, 53 MPa for 53grade Physical properties of Ordinary Portland Cement Cement should be tested for its following properties: 1. Fineness Fineness, or particle size of portland cement affects rate of hydration, which is responsible for the rate of strength gain. The smaller the particle size, the greater the surface area-to-volume ratio, which means more area available for water-cement reaction per unit volume. Approximately 95% of cement particles are smaller than 45 micron with the average particle size about 15 micron. Fineness is measured in terms of surface area per unit mass. Fineness can be tested by /Wagner turbidimeter/ test, /Blaine Air-permeability /test, 45-micrometer sieve CE6506 / CONSTRUCTION TECHNIQUES,EQUIPMENT AND PRACTICES V SEM/III YEAR
SRI VIDYA COLLEGE OF ENGINEERING & TECHNOLOGY LECTURE NOTES and electronic particle size analyzer. 2. Soundness Soundness refers to the ability of a hardened cement paste to retain its volume after setting. Lack of soundness is observed in the cement samples containing excessive amounts of hardburnt free lime or magnesia. /Autoclave expansion test/ is used to determine soundness of cement. 3. Consistency Consistency of a cement paste refers to its ability to flow. Normal consistency pastes are required to be prepared for testing cement specimens. A paste is said to have a normal consistency when the plunger of /Vicat apparatus/ penetrates it by 10±1 mm. the corresponding water-cement ratio is reported. 4. Setting Time Initial setting time is the time that elapsed from the instance of adding water untill the pastes ceases to behave as fluid or plastic. Whereas final setting time referred to the time required for the cement paste to reach certain state of hardness to sustain some load. Setting time is tested by /Vicat apparatus/ or /Gillmore needle/. 5. Compressive Strength Compressive strength of cement is tested by 50 mm mortar cubes made by using standard sand and cured in a prescribed way. the cubes are tested under a /compression testing machine/. The strength of cement varies with time, therefore in general it is reported as 3 day, 7 day or 28 day strength. 6. Heat of hydration The heat generated during the reaction of cement and water is known as heat of hydration. The factors affecting heat of hydration are C3A, C2S, water-cement ration, fineness of cement and curing temperature. /Conduction calorimeter /is used to test heat of hydration. 7. Loss on Ignition A cement sample of known weight is heated between 900 - 1000°C (1650 1830°F) until a constant weight is obtained. The weight loss of the sample due to heating is then determined. A high loss on ignition (more than 3%) indicates prehydration and carbonation, which may be due to CE6506 / CONSTRUCTION TECHNIQUES,EQUIPMENT AND PRACTICES V SEM/III YEAR
SRI VIDYA COLLEGE OF ENGINEERING & TECHNOLOGY LECTURE NOTES inappropriate storage or adulteration. 8. Specific gravity (relative density) Specific gravity is generally required in mix proportioning for concrete. The particle density (measured by excluding the air between particles) of OPC is found to be in the range of 3.1 to 3.25 Megagram per cubic meter. The relative density of OPC is assumed as 3.15. The density of cement is determined by Le Chatelier apparatus. 9. Bulk Density The bulk density can be determined by dividing the mass of cement particles and air between particles by the volume of cement sample. Bulk density of OPC ranges from 830 kg/cu.m to 1650 kg/cu.m. This test can be done with the help of two beakers having same amount of cement. The cement in one beaker is slightly vibrated which shows a decrease in the volume. Types of Cement in India There are some varieties in cement that always find good demand in the market. To know their characteristics and in which area they are most required, it will be better to take a look at some of the details given below. Portland Blast Furnace slag cement (PBFSC): The rate of hydration heat is found lower in this cement type in comparison to PPC. It is most useful in massive construction projects, for example - dams. Sulphate Resisting Portland Cement: This cement is beneficial in the areas where concrete has an exposure to seacoast or sea water or soil or ground water. Under any such instances, the concrete is vulnerable to sulphates attack in large amounts and can cause damage to the structure. Hence, by using this cement one can reduce the impact of damage to the structure. This cement has high demand in India. Rapid Hardening Portland Cement: The texture of this cement type is quite similar to that of OPC. But, it is bit more fine than OPC and possesses immense compressible strength, which makes casting work easy. Ordinary Portland Cement (OPC): Also referred to as grey cement or OPC, it is of much use in ordinary concrete construction. In the production of this type of cement in India, Iron (Fe2O3), Magnesium (MgO), Silica (SiO2), Alumina (AL2O3), and Sulphur trioxide (SO3) components are used. CE6506 / CONSTRUCTION TECHNIQUES,EQUIPMENT AND PRACTICES V SEM/III YEAR

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