![ADVANTAGES OF PPC IN HARDENED CONCRETE
Portland pozzolana cement (PPC) has following advantages when concrete is in its hardened state.
1. COMPRESSIVE STRENGTH & RATE OF STRENGTH GAIN
The strength & rate of strength gain of concrete made with PPC will be equivalent to ordinary concrete at
28 days. The silicate formation of PPC continues even after the rate of hydration of ordinary cement slows
down. This results in increased strength gain at later ages. This higher rate of strength gain will continue
with time and result in higher later age strength.
2. MODULUS OF ELASTICITY
The modulus of elasticity of PPC concrete is somewhat lower at early ages and little higher at later ages
than ordinary concrete.
3. BOND OF CONCRETE TO STEEL
The bond or adhesion of concrete to steel is dependent on the contact area of steel with concrete, the
depth of reinforcement & density of concrete. PPC being finer in nature usually increases paste volume &
reduce bleeding thus the contact will be increased, resulting into improved bond with steel.
4. HEAT OF HYDRATION
The hydration of PPC is a slower process than hydration of ordinary cement, resulting into slower heat
generation and lower internal stresses in concrete. Thus PPC becomes ideal cement for mass concreting
like dams, retaining walls, large foundation etc.
5. REDUCED SHRINKAGE
PPC in concrete helps to reduce drying shrinkage & plastic shrinkage. Drying shrinkage is reduced because
of lower internal concrete stresses & slower heat generation. Plastic shrinkage is also reduced
considerably because concrete bleeds less at a given slump or workability by using PPC.
6. PERMEABILITY
If concrete has interconnecting void spaces, then the concrete becomes permeable. In PPC concrete, the
lime [Ca (OH)2] liberated during initial hydration is consumed by reactive silica & forms an insoluble
cementitious compound instead of leaching on the concrete surface. This helps in reducing void spaces &
also blocks capillary channels & subsequently reduces permeability of concrete.](https://image.slidesharecdn.com/ctlecturenotes-220822011343-ed383c36/85/Concrete-Technology-Lecture-Nots-39-320.jpg)
Concrete Technology Lecture Nots
- 1. INTRODUCTION Concrete is the second most widely consumed building material in the world because of its beauty, strength and durability. And in an era of increased attention on the environmental impact of construction, engineered concrete performs well when compared to other building materials. Concrete and its ingredients do require energy to produce that in turn results in the generation of carbon dioxide. The billion tons of natural materials mined and processed each year, by their sheer volume, are bound to leave a substantial mark on the environment. Ingredient of Concrete •Cement •Fine Aggregate •Coarse Aggregate •Water •Admixture 1
- 2. Cement • A cement is a binder, a substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement mixed with fine aggregate produces mortar for masonry, or with sand and gravel, produces concrete. Concrete is the most widely used material in existence and is only behind water as the planet's most-consumed resource. • Cements used in construction are usually inorganic, often lime or calcium silicate based, which can be characterized as non- hydraulic or hydraulic respectively, depending on the ability of the cement to set in the presence of water • The word "cement" can be traced back to the Roman term opus caementicium, used 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. In modern times, organic polymers are sometimes used as cements in concrete. • If the cement industry were a country, it would be the third largest carbon dioxide emitter in the world with up to 2.8bn tonnes, surpassed only by China and the US. 2
- 3. Cement • What is cement made of? Cement is manufactured through a closely controlled chemical combination of calcium, silicon, aluminum, iron and other ingredients. Common materials used to manufacture cement include limestone, shells, and chalk or marl combined with shale, clay, slate, blast furnace slag, silica sand, and iron ore. • History of Cement Joseph Aspdin took out a patent in 1824 for "Portland Cement," a material he produced by firing finely- ground clay and limestone until the limestone was calcined. He called it Portland Cement because the concrete made from it looked like Portland stone, a widely-used building stone in England. 3
- 4. First Cement Factory of India India entered into the Cement Era in 1914, when the Indian Cement Company Ltd. started manufacturing Cement in Porbundar in Gujarat. However, even before that a small cement factory was established in Madras in 1904 by a company named South India Industrial Ltd. Upto 1st century B.C “Clay” is the most used binding material in construction with rocks and mud bricks. ... But most of the ancient temples are made from big boulders of rocks. Key Other Landmarks in History of Cement • In 1925, first association of the cement manufacturers was formed as “Cement Manufacturers Association“. • It was followed by “Concrete Association of India” in 1927. • In 1930 “Cement Marketing Company of India” was started • In 1936, all the cement companies except one i.e. Sone valley Portland Cement Company agreed and formed Associated Cement Companies Ltd. (ACC).This was the most important even in the history of cement industry in India. Many more companies were established in the following years. • Before partition India had 24 factories, out of which India retained 19 factories, which annual production of 2.1 million tons. 4
- 5. Chemical Composition of Cement • Portland cement gets its strength from chemical reactions between the cement and water. The process is known as hydration. This is a complex process that is best understood by first understanding the chemical composition of cement. Manufacture of cement Portland cement is manufactured by crushing, milling and proportioning the following materials: – Lime or calcium oxide, CaO: from limestone, chalk, shells, shale or calcareous rock – Silica, SiO2: from sand, old bottles, clay or argillaceous rock – Alumina, Al2O3: from bauxite, recycled aluminum, clay – Iron, Fe2O3: from from clay, iron ore, scrap iron and fly ash – Gypsum, CaSO4.2H20: found together with limestone • The materials, without the gypsum, are proportioned to produce a mixture with the desired chemical composition and then ground and blended by one of two processes - dry process or wet process. The materials are then fed through a kiln at 2,600º F to produce grayish-black pellets known as clinker. The alumina and iron act as fluxing agents which lower the melting point of silica from 3,000 to 2600º F. After this stage, the clinker is cooled, pulverized and gypsum added to regulate setting time. It is then ground extremely fine to produce cement. 5
- 6. Lime (CaO) 60 to 67% Silica (SiO2) 17 to 25% Alumina (Al2O3) 3 to 8% Iron oxide (Fe2O3) 0.5 to 6% Magnesia (MgO) 0.1 to 4% Sulphur trioxide (SO3) 1 to 3% Soda and/or Potash (Na2O+K2O) 0.5 to 1.3% Chemical Composition of Cement
- 7. Flow Chart of Manufacturing of Cement
- 8. Lime Stacker
- 11. Manufacturing of Cement • https://www.youtube.com/watch?v=SMnrszA hZuY
- 12. Functions and Limitations of Cement Ingredients Lime (CaO) • Lime or calcium oxide is the most important ingredient of cement. The cement contains 60 to 67% of lime in it. It is obtained from limestone, chalk, shale etc. Adequate quantity of lime in cement is helpful to form the silicates and aluminates of calcium. • If lime is added in excess quantity the cement becomes unsound as well as expansion and disintegration of cement will occur. • If lime content is lower than the minimum requirement strength of cement will reduce and also setting time of cement will decrease. Powdered Lime
- 13. Silica (SiO2) • Silica or silicon dioxide is the second largest quantity of cement ingredients which is about 17 to 25%. Silica can be obtained from sand, argillaceous rock etc. Sufficient quantity of silica helps for the formation of di-calcium and tri-calcium silicates which imparts strength to the cement. • Excess silica in cement will increase the strength of cement but at the same time setting time of cement also increased. Powdered Silica
- 14. Alumina (Al2O3) • Alumina in cement is present in the form of aluminum oxide. The range of alumina in cement should be 3 to 8%. It is obtained from bauxite, alumina contain clay etc. Alumina imparts quick setting property to the cement. • In general, high temperature is required to produce required quality of cement. But alumina when added with cement ingredients it behaves as a flux and reduces the clinkering temperature which finally weakens the cement. So, to maintain the high temperature alumina should not be used in excess quantity. Alumina
- 15. Iron oxide (Fe2O3) • Iron oxide quantity in cement is ranges from 0.5 to 6%. It can be obtained from fly ash, iron ore, scrap iron etc. The main function of iron oxide is to impart color to the cement. • At high temperatures, Iron oxide forms tricalcium aluminoferrite by reacting with aluminum and calcium. The resultant product imparts the strength and hardness properties to the cement. Iron oxide
- 16. Magnesia (MgO) • Cement contains Magnesia or Magnesium oxide in the range of 0.1 to 3%. Magnesia in cement in small quantities imparts hardness and color to the cement. • If it is more than 3%, the cement becomes unsound and also strength of the cement reduces. Magnesium Oxide
- 17. Calcium sulfate (CaSO4) • Calcium sulfate is present in the form of gypsum in the cement. It is found together with limestone. It ranges between 1 to 3%. • The function of calcium sulfate in cement is to increase the initial setting time of cement. Gypsum Powder
- 18. Sulfur (SO3) • Sulfur or sulfur trioxide in the cement is about 1 to 3%. Its function is to make the cement sound. If it is in excess quantity the cement becomes unsound. Sulfer Trioxide
- 19. Alkalis • Alkalis like soda and potash are present in the cement which normally ranges from 0.1 to 1%. During manufacturing process of cement most of the alkalis are carried away by the flue gases at the time of heating. Hence cement contains very small quantities of alkalis in it. • If alkalis content is more than 1% then it will cause several problems like alkali aggregate reaction, efflorescence, staining etc. Efflorescence Due to Excess Alkali
- 20. R. H Bogue’s
- 21. Compound Composition Name of Compound Chemical Formula Abbreviated Formula Compound Composition Tri Calcium Silicate (Alite) 3 CaO SiO2 C3S 54.1 Di Calcium Silicate (Belite) 2 CaO SiO2 C2S 16.6 Tri Calcium Aluminate (Celite) 3 CaO Al2O3 C3A 10.8 Tetra Calcium Aluminium Ferrite (Felite) 4 CaO Al2O3 Fe2O3 C4AF 9.1
- 22. Properties of Bogue’s Compound C3S: • It is responsible for early strength • First 7 Days strength is due to C3S • It produce more Heat of hydration • A Cement with more C3S Content is better for cold weather concreting C2S: • The Hydration of C2S starts after 7 Days. Hence it gives strength after 7 Days • C2S hydrates and harden slowly and provide much of the ultimate strength • It produce less Heat of hydration • It is responsible for the later strength of hydration
- 23. Properties of Bogue’s Compound C3A: • The reaction of C3A with water is very fast and may leads to an immediate stiffening of paste and this process is termed as Flash set • To Prevent this flash set, 2 to 3% Gypsum is added at the time of grinding the cement clinkers. • These hydrates contribute little to strength development C4AF: • C4AF hydrates rapidly • It gives colour to Cement
- 25. 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.
- 26. Chemistry: Chemical reaction of OPC cement in concrete is follows: OPC is made up of four principal mineralogical phases symbolically represented by C3S, C2S, C3A and C4AF. The hydration reaction of these chemical compounds as mentioned in the respective section are as, For C3S: 2C3S + 6H -------> C3S2H3 + 3 Ca(OH)2 For C2S: 2C2S + 4H -------> C3S2H3 + Ca(OH)2 For C3A: C3A + 6H --------> C3AH6 Cement + Water --------> C-S-H Gel + Ca(OH)2 Liberate Heat 26
- 27. Flow sheet
- 31. Water Requirement for Hydration of Cement: • For fully hydration of Portland cement on an average 23% of water by weight is required, whereas C3S requires 24% of water by weight of cement and C2S requires 21%, This 23% of water chemically combines with cement and is called bound water. A certain amount of water is filled with in the pores of the gel and is known as gel water. The bound water and gel water are complimentary to each other. • If the quantity of water is inadequate to fill the gel pores, the gel formation will stop. If the gel formation stops, there is no question of the gel pores presence. It has been estimated that about 15% water by weight of cement is required to fill up the gel pores. Thus for the complete chemical reaction and to fill up the gel pore space, a total 23% + 15% = 38% of water by weight of cement is required.
- 32. • Thus it will be seen if only 38% by weight of cement water is used, the resultant paste will go full hydration and no extra water will be available for the formation of undesirable capillary cavities. On the other hand if more than 38% of water is used, then the excess water will form undesirable capillary cavities. Thus greater the amount of water above 38%, larger the undesirable capillary cavities. • On hydration the volume of cement particles increases to 2.06 to 2.1 times of its un-hydrated volume. Upto water cement ratio of about 0.6, the gel fully fills up the space occupied by water and the structure is homogeneous. If the cement ratio is more than 0.7, the increase in volume of hydrated product will not be sufficient to fill up the voids produced by water in the paste, resulting porous mass
- 33. Types of Cement • Ordinary Portland Cement (OPC) • Portland Pozzolana Cement (PPC) • Rapid Hardening Cement • Quick setting cement • Low Heat Cement • Sulfates resisting cement • Blast Furnace Slag Cement • High Alumina Cement • White Cement • Colored cement • Air Entraining Cement • Expansive cement • Hydrographic cement
- 34. Ordinary Portland cement (OPC) is one of the most widely used type of Cement. Types, properties, constituents, manufacture, uses and advantages of Ordinary Portland Cement is discussed. In 1824 Joseph Aspdin gave the name as Portland cement as it has similarity in colour and quality found in Portland stone, which is a white grey limestone in island of Portland, Dorset. Constituents of Ordinary Portland Cement • The principal raw materials used in the manufacture of Ordinary Portland Cement are: • Argillaceous or silicates of alumina in the form of clays and shales. • Calcareous or calcium carbonate, in the form of limestone, chalk and marl which is a mixture of clay and calcium carbonate. • The ingredients are mixed in the proportion of about two parts of calcareous materials to one part of argillaceous materials and then crushed and ground in ball mills in a dry state or mixed in wet state. • The dry powder or the wet slurry is then burnt in a rotary kiln at a temperature between 1400 degree C to 1500 degree C. the clinker obtained from the kiln is first cooled and then passed on to ball mills where gypsum is added and it is ground to the requisite fineness according to the class of product.
- 35. • Ordinary Portland Cement(OPC) is the most widely used cement in the world for producing concrete, mortar, stucco, and non-specialty grouts. Ordinary Portland Cement has 3 grades based on its strength namely 33, 43 and 53 grade that indicates the compressive strength obtained after 28 days of setting. Here 33, 43 and 53 represents the 28 days characteristic strength of cement Grade
- 36. Portland Pozzolana Cement (PPC) • Portland pozzolana cement is prepared by grinding pozzolanic clinker with Portland cement. It is also produced by adding pozzolana with the addition of gypsum or calcium sulfate or by intimately and uniformly blending Portland cement and fine pozzolana. • This cement has a high resistance to various chemical attacks on concrete compared with ordinary portland cement, and thus, it is widely used. It is used in marine structures, sewage works, sewage works, and for laying concrete underwater, such as bridges, piers, dams, and mass concrete works, etc.
- 37. Pozzolonic Reaction Cement + Water --------> C-S-H Gel + Ca(OH)2 Liberate Heat Action of Pozzolonic Material Ca(OH)2 + SiO2 (Fly ash) --------> C-S-H Gel (Additional) Pozzolonic Material Rich in Silica Content
- 38. ADVANTAGES OF PPC IN FRESH CONCRETE Portland pozzolana cement (PPC) has following advantages when concrete is in its fresh state. 1. WORKABILITY Portland pozzolana cement has spherical cement particles and they have higher fineness value. Due to the spherical shape concrete move more freely and more fineness of particles allows better filling of the pores. This type of cement also gives better cohesiveness to concrete. PPC cement also reduces the rate of slump loss of concrete as compared to concrete made with ordinary cement, particularly in hot weather condition. 2. BLEEDING Bleeding is a type of segregation in which some of the water in the concrete mix tens to rise to the surface of fresh concrete. As a result of bleeding, the top surface becomes too wet and concrete will become porous, weak and non durable. PPC cement reduces bleeding by providing greater fines volume and lower water content for a given workability. This also helps to block bleed water channels. 3. PUMPABILITY PPC cement helps to produce more cohesive concrete and is less prone to segregation & bleeding. The spherical shape of particles serves to increase workability and pumpability by decreasing friction between aggregate particles and between concrete & pump line. 4. SETTING TIME & FINISHABILITY PPC cement slightly prolongs the setting time of concrete which helps the mason for good finishing of concrete or cement mortar. The cohesiveness of concrete mix helps for better finishing of concrete.
- 39. ADVANTAGES OF PPC IN HARDENED CONCRETE Portland pozzolana cement (PPC) has following advantages when concrete is in its hardened state. 1. COMPRESSIVE STRENGTH & RATE OF STRENGTH GAIN The strength & rate of strength gain of concrete made with PPC will be equivalent to ordinary concrete at 28 days. The silicate formation of PPC continues even after the rate of hydration of ordinary cement slows down. This results in increased strength gain at later ages. This higher rate of strength gain will continue with time and result in higher later age strength. 2. MODULUS OF ELASTICITY The modulus of elasticity of PPC concrete is somewhat lower at early ages and little higher at later ages than ordinary concrete. 3. BOND OF CONCRETE TO STEEL The bond or adhesion of concrete to steel is dependent on the contact area of steel with concrete, the depth of reinforcement & density of concrete. PPC being finer in nature usually increases paste volume & reduce bleeding thus the contact will be increased, resulting into improved bond with steel. 4. HEAT OF HYDRATION The hydration of PPC is a slower process than hydration of ordinary cement, resulting into slower heat generation and lower internal stresses in concrete. Thus PPC becomes ideal cement for mass concreting like dams, retaining walls, large foundation etc. 5. REDUCED SHRINKAGE PPC in concrete helps to reduce drying shrinkage & plastic shrinkage. Drying shrinkage is reduced because of lower internal concrete stresses & slower heat generation. Plastic shrinkage is also reduced considerably because concrete bleeds less at a given slump or workability by using PPC. 6. PERMEABILITY If concrete has interconnecting void spaces, then the concrete becomes permeable. In PPC concrete, the lime [Ca (OH)2] liberated during initial hydration is consumed by reactive silica & forms an insoluble cementitious compound instead of leaching on the concrete surface. This helps in reducing void spaces & also blocks capillary channels & subsequently reduces permeability of concrete.
- 40. Rapid Hardening Cement • Rapid hardening cement attains high strength in the early days; it is used in concrete where formworks are removed at an early stage and are similar to ordinary portland cement (OPC). This cement has increased lime content and contains higher C3S content and finer grinding, which gives higher strength development than OPC at an early stage. • The strength of rapid hardening cement at the three days is similar to 7 days strength of OPC with the same water-cement ratio. Thus, the advantage of this cement is that formwork can be removed earlier, which increases the rate of construction and decreases the cost of construction by saving formwork cost. • Rapid hardening cement is used in prefabricated concrete construction, road works, etc.
- 41. Quick setting cement • The difference between the quick setting cement and rapid hardening cement is that quick-setting cement sets earlier. At the same time, the rate of gain of strength is similar to Ordinary Portland Cement, while quick hardening cement gains strength quickly. Formworks in both cases can be removed earlier. • Quick setting cement is used where works is to be completed in very short period and for concreting in static or running water.
- 42. Low Heat Cement • Low heat cement is produced by maintaining the percentage of tricalcium aluminate below 6% by increasing the proportion of C2S. A small quantity of tricalcium aluminate makes the concrete to produce low heat of hydration. Low heat cement suitable for mass concrete construction like gravity dams, as the low heat of hydration, prevents the cracking of concrete due to heat. • This cement has increased power against sulphates and is less reactive and initial setting time is greater than OPC.
- 43. Testing of Cement • Field Testing • Laboratory Testing Field Testing Quality tests on cements at construction site (also called field tests on cement) are carried to know the quality of cement supplied at site. It gives some idea about cement quality based on colour, touch and feel and other tests.
- 63. 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. Portland cement hydration reactions are so highly exothermal that they heat the cement paste. Heat develops rapidly during setting and initial hardening and gradually declines and finally stabilises as hydration slows. Hence, 50% of the heat is generated in the first 3 days and 80% in the first 7 Test Video: https://youtu.be/k02fOFB2-iQ
- 65. X-ray fluorescence Test (XRF) (E1621-13) As a versatile characterization technique, XRF has become the most preferred method for material analysis. XRF entails exposing a sample of material to X-ray light, which excites the elements present in the sample. The elements emit light as they return back to their ground state. The light emitted as the elements relax is distinctive to the particular elements present in the sample and measuring the fluorescence makes it possible to calculate the exact chemical composition of the sample. While XRF analysis is non-destructive, quick, and simple to perform, correct sample preparation is required to realize accurate results. Concrete chemical composition can vary extensively and this can considerably influence performance. Elemental analysis of concrete mix using XRF can provide peace of mind concerning the quality and suitability of green material used for a given application.
- 66. Chemical composition of various conventional and green mixes Sample ID SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2O Cl TiO2 P2O5 CM25 45.5 10.5 8.65 23.5 2.8 0.18 0.13 0.85 0.01 1.65 0.13 SCC 25 - CF4 47.2 10.19 8.09 20.9 3.68 0.19 0.14 0.87 0.01 1.74 0.14 CM30 45 10.51 8.53 21.5 3.62 0.16 0.14 0.81 0.01 1.89 0.15 SCC M30- CF4 47.7 10.35 8.33 20.8 3.54 0.12 0.14 0.86 0.01 1.81 0.15 CM40 46.3 10.48 8.55 21.2 3.57 0.12 0.14 0.84 0.01 1.89 0.15 SCC M40- CF4 50.6 10.43 8.18 20 3.06 0.11 0.14 0.89 0.01 1.76 0.14 CM50 46.8 10.1 7.98 21.8 2.99 0.16 0.13 0.87 0.01 1.69 0.14 SCC M50- CF4 49.1 10.27 8.05 20.8 3.21 0.15 0.14 0.91 0.01 1.69 0.14
- 67. Aggregate • What is an Aggregate? Aggregates are the important constituents of the concrete which give body to the concrete and also reduce shrinkage. Aggregates occupy 70 to 80 % of total volume of concrete. Classification of Aggregate: • Classification of aggregates based on: Grain Size • Classification of aggregates based on: Density • Classification of aggregates based on: Geographical Origin • Classification of aggregates based on: Shape
- 68. Classification of aggregates based on: Grain Size If you separate aggregates by size, there are two overriding categories: • Fine • Coarse The size of fine aggregates is defined as 4.75mm or smaller. That is, aggregates which can be passed through a number 4 sieve, with a mesh size of 4.75mm. Fine aggregates include things such as sand, silt and clay. Crushed stone and crushed gravel might also fall under this category. Typically, fine aggregates are used to improve workability of a concrete mix. Coarse aggregates measure above the 4.75mm limit. These are more likely to be natural stone or gravel that has not been crushed or processed. These aggregates will reduce the amount of water needed for a concrete mix, which may also reduce workability but improve its innate strength.
- 69. Classification of aggregates based on: Density There are three weight-based variations of aggregates: • Lightweight • Standard • High density Different density aggregates will have much different applications. Lightweight and ultra lightweight aggregates are more porous than their heavier counterparts, so they can be put to great use in green roof construction, for example. They are also used in mixes for concrete blocks and pavements, as well as insulation and fireproofing. High density aggregates are used to form heavyweight concrete. They are used for when high strength, durable concrete structures are required – building foundations or pipework ballasting.
- 70. Classification of aggregates based on: Geographical Origin Another way to classify aggregates is by their origin. You can do this with two groups: • Natural – Aggregates taken from natural sources, such as riverbeds, quarries and mines. Sand, gravel, stone and rock are the most common, and these can be fine or coarse. • Processed – Also called ‘artificial aggregates’, or ‘by-product’ aggregates, they are commonly taken from industrial or engineering waste, then treated to form construction aggregates for high quality concrete. Common processed aggregates include industrial slag, as well as burnt clay. Processed aggregates are used for both lightweight and high- density concrete mixes.
- 71. Classification of aggregates based on: Shape Shape is one of the most effective ways of differentiating aggregates. The shape of your chosen aggregates will have a significant effect on the workability of your concrete. Aggregates purchased in batches from a reputable supplier can be consistent in shape, if required, but you can also mix aggregate shapes if you need to. • The different shapes of aggregates are: • Rounded – Natural aggregates smoothed by weathering, erosion and attrition. Rocks, stone, sand and gravel found in riverbeds are your most common rounded aggregates. Rounded aggregates are the main factor behind workability. • Irregular – These are also shaped by attrition, but are not fully rounded. These consist of small stones and gravel, and offer reduced workability to rounded aggregates.
- 72. • Angular – Used for higher strength concrete, angular aggregates come in the form of crushed rock and stone. Workability is low, but this can be offset by filling voids with rounded or smaller aggregates. • Flaky – Defined as aggregates that are thin in comparison to length and width. Increases surface area in a concrete mix. • Elongated – Also adds more surface area to a mix – meaning more cement paste is needed. Elongated aggregates are longer than they are thick or wide. • Flaky and elongated – A mix of the previous two – and the least efficient form of aggregate with regards to workability.
- 73. Classification Based on Origin • All rocks originate as igneous rocks, which are formed through cooling and solidification of molten materials that are underlying the earth’s crust. If the solidification occurs slowly, the rocks will be labelled as intrusive rocks (coarse to medium grained) and in case the molten material forces its way to the surface and crystallize more rapidly the extrusive rocks will be formed (fine grained). • Sedimentary rocks are formed by chemical and mechanical breakdown of pre-existing rocks in a process of deposition and cementation of the materials. The layered structure of particles settlement can result in undesired shapes and flaky grains in the case of aggregates crushed from such rocks. • Lastly, metamorphic rocks are made of pre-existing igneous or sedimentary rocks when the original rock is subjected to high pressure and temperature usually at great depth. Hydrothermal metamorphism can result in the minerals in these rocks being re-formed and recrystallized which makes them less durable and less stable in some cases, for example the formation of alkali-reactive silicates. Metamorphic rocks such as Quartzite make important contributions to the production of concrete aggregates.
- 74. Factors Affecting Workability of Concrete Factors which affect workability of concrete are: • Cement content of concrete • Water content of concrete • Mix proportions of concrete • Size of aggregates • Shape of aggregates • Grading of aggregates • Surface texture of aggregates • Use of admixtures in concrete • Use of supplementary cementitious materials
- 75. Cement Content of Concrete • Cement content affects the workability of concrete in good measure. More the quantity of cement, the more will be the paste available to coat the surface of aggregates and fill the voids between them. This will help to reduce the friction between aggregates and smooth movement of aggregates during mixing, transporting, placing and compacting of concrete. • Also, for a given water-cement ratio, the increase in the cement content will also increase the water content per unit volume of concrete increasing the workability of concrete. Thus increase in cement content of concrete also increases the workability of concrete.
- 76. Water/Cement Ratio or Water Content of Concrete • Water/cement ratio is one of the most important factor which influence the concrete workability. Generally, a water cement ratio of 0.45 to 0.6 is used for good workable concrete without the use of any admixture. Higher the water/cement ratio, higher will be the water content per volume of concrete and concrete will be more workable. • Higher water/cement ratio is generally used for manual concrete mixing to make the mixing process easier. For machine mixing, the water/cement ratio can be reduced. These generalised method of using water content per volume of concrete is used only for nominal mixes. • For designed mix concrete, the strength and durability of concrete is of utmost importance and hence water cement ratio is mentioned with the design. Generally designed concrete uses low water/cement ratio so that desired strength and durability of concrete can be achieved.
- 77. Mix Proportions of Concrete • Mix proportion of concrete tells us the ratio of fine aggregates and coarse aggregates w.r.t. cement quantity. This can also be called as the aggregate cement ratio of concrete. The more cement is used, concrete becomes richer and aggregates will have proper lubrications for easy mobility or flow of aggregates. • The low quantity of cement w.r.t. aggregates will make the less paste available for aggregates and mobility of aggregates is restrained.
- 78. Size of Aggregates • Surface area of aggregates depends on the size of aggregates. For a unit volume of aggregates with large size, the surface area is less compared to same volume of aggregates with small sizes. • When the surface area increases, the requirement of cement quantity also increase to cover up the entire surface of aggregates with paste. This will make more use of water to lubricate each aggregates. • Hence, lower sizes of aggregates with same water content are less workable than the large size aggregates.
- 79. Shape of Aggregates • The shape of aggregates affects the workability of concrete. It is easy to understand that rounded aggregates will be easy to mix than elongated, angular and flaky aggregates due to less frictional resistance. • Other than that, the round aggregates also have less surface area compared to elongated or irregular shaped aggregates. This will make less requirement of water for same workability of concrete. This is why river sands are commonly preferred for concrete as they are rounded in shape.
- 80. Grading of Aggregates • Grading of aggregates have the maximum effect on the workability of concrete. A well graded aggregates have all sizes in required percentages. This helps in reducing the voids in a given volume of aggregates. • The less volume of voids makes the cement paste available for aggregate surfaces to provide better lubrication to the aggregates. • With less volume of voids, the aggregate particles slide past each other and less compacting effort is required for proper consolidation of aggregates. Thus low water cement ratio is sufficient for properly graded aggregates.
- 81. Surface Texture of Aggregates • Surface texture such as rough surface and smooth surface of aggregates affects the workability of concrete in the same way as the shape of aggregates. • With rough texture of aggregates, the surface area is more than the aggregates of same volume with smooth texture. Thus concrete with smooth surfaces are more workable than with rough textured aggregates.
- 82. Use of Admixtures in Concrete • There are many types of admixtures used in concrete for enhancing its properties. There are some workability enhancer admixtures such as plasticizers and superplasticizers which increases the workability of concrete even with low water/cement ratio. • They are also called as water reducing concrete admixtures. They reduce the quantity of water required for same value of slump. • Air entraining concrete admixtures are used in concrete to increase its workability. This admixture reduces the friction between aggregates by the use of small air bubbles which acts as the ball bearings between the aggregate particles.
- 83. Use of Supplementary Cementitious Materials • Supplementary cementitious materials are those which are used with cement to modify the properties of fresh concrete. Fly ash, fibers, silica fume, slag cements are used as supplementary cementitious materials. • The use of fly ash in improves the workability of concrete by reducing the water content required for same degree of workability or slump value. • The use of steel or synthetic fibers in concrete reduces the workability of concrete as it makes the movement of aggregates harder by reducing the lubricating effect of cement paste. • The workability of concrete is reduced and increased based on the quantity of silica fume. The use of silica fume in concrete can improves workability when used at low replacement rates, but can reduce workability when added at higher replacement rates. Silica fume are used as pumping aid for concrete when used as 2 to 3% by mass of cement. • The use of slag cement also improves workability but its effect depends on the characteristics of the concrete mixture in which it is used.
- 84. •Is the reaction between the active mineral constituents of some aggregate and the sodium and potassium alkali hydroxide in the Concrete. •Harmful only when it produce significant expansion Alkali- Aggregate Reaction
- 89. BULKING OF SAND: • Bulking in sand Occurs When dry sand interacts with the atmospheric moisture. Presence of moisture content forms a thin layer around sand particles. This layer generates the force which makes particles to move aside to each other. This results in the increase of the volume of sand
- 91. Test Procedure for Bulking of Sand • First fill the container about two-thirds full with the sand you are testing. Drop it loosely, do not pack it. • Level off the top of the sand and pushing the steel rule down through the bottom, measure its height. Suppose it is 15 cm. • So now you know the height of the damp, bulked sand. Next step is to find the height of the same sand when saturated with water. You can then compare the two. • Empty the sand in the other container, taking care to see that none of it is lost in the process, and half fill the first container with water. Now put the sand back into the water, bit by bit, so that it is entirely saturated. • First put back about half the sand and rod it thoroughly to remove any air. Then add the rest and rod again in the same way and level off top. Now push your rule through the sand as before and measure the new height then it is find that it has sunk noticeably. Say it now measures 12.5 cm. Calculation: • Result shows that when sand saturated, it is bulked to 15 cm. Therefore bulking is 2.5 cm on 12.5 cm of dry sand and that is to say 20 percent.
- 92. • In gauging sand for the mix, therefore you should add 20 percent more sand than that quoted in the specification or as suggested in concrete mix design. • Suppose the mix specified is 1:2:4, then the actual quantity of sand to be used will be 1.20 X 2 = 2.4 litres giving a field mix of 1:2.4:4. • If you neglect to make this correction for bulking the actual dry sand measured will be X 2 = 1.67 litres. The mix will then be 1:1.67:4 in terms of dry sand. • This reduction in the ratio of sand causes a reduction in the quantity of concrete produced with each bag of cement, and in most cases there will be insufficient fine material to give a workable mix which can make concrete with voids their by reducing its strength and durability both. It may be noted that coarse aggregate is little affected in volume by moisture. • Practically it has been seen that bulking of sand increase up to 30% by volume and up to 5% by weight. If more water is added the film around the sand particles is broken and the volume of sand comes to its original dry volume, and the water comes at its surface. Thus it is important that the volume of dry as well as wet sand is the same. The volume of moist sand is only more.