Unit5(Pt1)_CC(A)

Unit 5(Pt1) CEMENT CONCRETE (Part A)(Also including Parts of Unit 6)PresentationsbyPV ESWAR9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 1CONCRETE Concrete a Composite Man-made Material, is the most widely used building material in the construction industry. It consists of a rationally chosen mixture of binding material such as Lime or Cement Well Graded Fine and Coarse Aggregates, Water Admixtures (to produce Concrete with Special properties). In a concrete mix, Cement and Water form a paste or Matrix which Fills the Voids of the FA Coats the surface ofFA and CA and binds them together. The matrix is usually 22-34% of the total volume. Wet or Green Concrete.Freshly mixed concrete before set Hardened or Set Concrete.After setting and hardening9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 3CONCRETE (Contd) Moulded Concrete Mix, after sufficient Curing becomes hard like Stone due to Chemical action between Waterand Binding material. With the advent of Cement, the use of lime concrete has been confined to making bases for Concrete Foundations and Roof Terracing. Major Factors responsible for wide usage of cement-concrete are Mouldability, Early Hardening High Early Compressive Strength Development of desired properties with Admixtures to be used in Adverse Situations Suitability for Guniting Pumpability Durability. 9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 4 Reason for its extensive use.properties can be controlled within a wide range by using Appropriate ingredients and by special Mechanical, Physical and Chemical Processing Techniques. Some Examples of use of CC Buildingsfrom single storey to multi storey Bridges, Piers, Dams, Weirs, Retaining Walls, Liquid Retaining Structures Reservoirs, Chimneys, Bins, Silos Runways, Pavements Shells, Arches Railway Sleepers9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 5Various ingredients of cement concrete and their Importance9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 6WATER9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 7Introduction PURPOSE. of using Water with Cement is to cause Hydration of the Cement. Lubricant between coarse and fine aggregates, producing a Workable and Economical concrete. W/C Ratio.Optimum water requirement for a particular Concrete and Mortar. EXCESSWATER (along with Cement) comes to the surface by Capillary action, forms Laitance, a thin layer over surface Laitance weakens bond between the successive lifts of concrete. excess water may leak through the form work, resulting in Honeycombed concrete; and on Evaporation makes the concrete Porous. LESSER WATER Workability decreases. Non-Uniform mixing results in Weaker Concrete Water is also used for Washing Aggregates and Curing.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 8Quality of Water Almost any Natural Potable Water that has no pronounced taste or odouris acceptable for the concrete mix. Many sources of water unsuitable for drinking may also be used. However, in case of a doubt, water samples should be tested for suitability. Excessive impurities may affect Setting Time Strength Durability may cause Efflorescence Surface Discolouration Corrosion of Steel. Testing Water wrt Setting Time The Initial Setting Time of the mixes with Impure Water and that with the Pure Water are obtained. 30 minutesacceptable. Testing water wrt Compressive Strength 7 day and 28 day compressive strengths of the Cube/Cylinder specimens prepared with impure water should not differ by 10 % from that of cubes/cylinders prepared with pure water. The tolerable concentrations of some of the impurities in water are given in Table 7.1.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 9Tolerance Conc of Impurtities in Mixing Water9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 10*Quality of WATER for CONCRETEWater in Concrete has 2 purposes: Hydrate the Cement Lubricate the mix so as to aid Compaction.Quality Guidelines: Clean and free from Oils, Acids, Alkalies, Vegetables or other Organic impurities In gen, Water fit for drinking is fit for Concreting. Excess of Acidity or Alkalinity. Harmful. Can be tested with Litmus paper. Soft waters. Produce weaker Concrete than Hard waters (having Ca(HCO3)2)Eswar/ BTech2yr/ BMCT/ Unit5 11*Quality of WATER for CONCRETEQuality Guidelines (Contd) Marsh waters. Harmful Water containing decayed Veg matters to be avoided.They interfere with setting of Cement Sea water NOT to be used in RCC or PCC wks may be used in Mass concreting, if no alternative will retard the Setting and Hardening causes Efflorescence will not affect theUltimate Str of Concrete,unless Salt is present in excess qtys. Salt in water corrodes Rft. Brackish water although not always potable, is NOT harmful for concreting.Eswar/ BTech2yr/ BMCT/ Unit5 12Practical Fd Test Make 2 identical Pats of size 75mm dia, and 12 mm th of neat Cement paste, one with the water under test, and the other with water of known suitability Place the Pats on a clean non-absorbant surface and leave for 48 hrs, and Setting and Hardening time obs for both the Pats. Should the Pat made with the water under test not be up to the std of the other, then water should be deemed unsuitable.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 13*Quality of WATER for CONCRETE (Contd)AGGREGATES9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 14Cl of AGGREGATES9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 15 Aggregates are the materials basically used as Filler with binding material in the production of Mortar and Concrete. They are derived from Igneous, Sedimentary or Metamorphic rocks Or manufactured from Blast Furnace Slag, etc. Aggregates form the Body of the Concrete, reduce the Shrinkage They occupy 70-80 % of the Vol have considerable influence on the properties of the Concrete. important to obtain right type and quality of Aggregates at site. They should be Clean, Hard, Strong, Durable Graded INERTNESS. Earlier Aggregates were considered to be chemically inert but the Latest research has revealed that some of them are chemically active certain types exhibit Chemical Bond at the interface of Aggregates and Cement paste. To increase the bulk density of Concrete, Aggregates are used in two (02) markedly different sizes the Bigger ones known to be CA (grit) the Smaller ones FA (Sand). CA form the main Matrix of Concrete andFA from the filler Matrix between the CA9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 16Cl based on Geo OriginNATURAL AGGREGATES. Obtained by crushing of Igneous, Sedimentary or Metamorphic rocks. Gravels and Sand reduced to their present size by the Natural Agencies also fall in this category. Most widely used Aggregate are from Igneous origin. Aggregates obtained from Pits or dredged from River, Creek or Sea are most often NOT Clean enough or Well Graded to suit the Quality Requirement. They therefore require Sieving and Washing before their use.ARTIFICIAL AGGREGATES Broken Bricks, Blast Furnace Slag and Synthetic Aggregates Broken bricks known as brick bats are suitable for Mass Concreting, e.g., Foundation Bases. They are NOT for RCC wks. Blast Furnace Slag Aggregate obtained from Slow Cooling of the Slag,followed by Crushing. The dense and strong particles as obtained are used for making Precast Concrete Products. Sp. Gr.22.8 BulkDensity . 11201300 kg/m3. Fire Resisting Corrosion ofRft, due to Sulphur content of Slag. Synthetic Aggregates are produced by Thermally processed materials such as Expanded Clay and Shale used for making Light Wt Concrete.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 17Cl based on Size CA FA All-in- Aggregate.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 18CA - Cl based on Size Retained on 4.75 mm Sieve Obtained by Natural disintegration Artificial crushing of rocks. Max size of Aggregate.80 mm. Size of Aggregate is governed by Thickness of Section Spacing of Rft Clear Cover, Mixing, Handling and Placing methods. For Economy . Maximum Size of Aggregate should be As large as possible But, < 1/4thof min th of the Member. For Reinforced Sections Maximum size should be: at least 5 mm less than the Clear Spacing between the Rft clear Cover. Aggregate > 20 mm size are gen NOT USED for RCC structural members.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 19Cl based on Size (Contd)ALL IN AGGREGATES Naturally available aggregates of different fractions of FA and CA are known as All-in-Aggregate. The deficiency of any particular fraction can be corrected for use in the mix NOT RECOMMENDED for Quality Concrete.GRADED AGGREGATE Consists of selected proportions of different particle sizes usually chosen to produce a max density Concrete or Soil. e.g., a Graded Aggregate of Nominal Size 20 mm means an Aggregate most of which passes IS sieve 20 mm.However, grading will be based on above definition.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 20Cl based on Size (Contd)FINE AGGREGATE Aggregate passing through 4.75 mm Sieve are defined as FA. They may be Natural Sanddeposited by Rivers, Crushed Stone Sandobtained by Crushing Stones and Crushed Gravel Sand. The Smallest Size of FA (sand) is 0.06 mm. Depending upon the Particle Size, FA are described as Fine Sand MediumSand Coarse Sand On the basis of Particle Size Distribution, FA are Cl into Four (04) zones Progressively Finer from Grading Zone I to IV (IS: 383).9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 21Cl based on ShapeCLASSIFICATION Aggregates are Cl as Rounded Irregular, Angular Flaky.ROUNDED AGGREGATES These are generally obtained from River or Sea shore Produce Minimum Voids (about 32%) in the concrete. They have MINIMUM Ratio ofSurface Area to Volume Cement paste required is MINIMUM Poor Interlocking Bond makes it unsuitable for High Strength Concrete and Pavements.IRREGULAR AGGREGATES Voids about 36% Require MORE Cement Paste as compared to Rounded Aggregate. Because of irregularity in shape they develop GOOD BOND and are suitable for making Ordinary Concrete.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 22Cl based on ShapeANGULAR AGGREGATES They have Sharp, Angular and Rough Particles Maximum VOIDS (40%). provide Very Good BOND than the earlier two Most Suitable for High Strength Concrete and Pavements Requirement of Cement Paste. Relatively More.FLAKY AGGREGATES These are sometimes wrongly called as elongated aggregate. However, both of these influence the concrete properties adversely. Least Lateral Dimension (thickness) < 0.6 x Mean Dimension. e.g. Mean Sieve Size or Mean Dimension for an Aggregate piece passing through 50 mm and retained on 40 mm sieve is (50 + 40)/2 = 45.0 mm. If the Least Lateral Dimension < 0.6 45 = 27.0 mm, the aggregate is classified as FLAKY. Flaky aggregate generally orient in one plane with Water and Air Voids underneath. They Adversely affect Durability. Restricted to 15% max.ELONGATED AGGREGATES Length = 1.8 xMean Dimension. Influence the Concrete properties adversely. 9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 23Note9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 24Cl based on Unit Wt9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 25*Quality of Aggregates Aggregates for Load bearing Concrete should be Hard, Strong Non-Porous free from Elongated and Laminated particles Should be Suitable for the purpose required. Porosity. 10% water after 24 hrs immersion in water are considered Porous. Porous materials corrode rft. Elongated and Laminated particles.Weak in Shear Mica.Stones having inclusion should be avoided Granite, Basalt, Quartzite, Trap and those with rough non-glossy surface.BEST9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 26Characterstics of Aggregate The Properties to be Considered while selecting aggregate for concrete are Strength, Particle Shape Specific Gravity Bulk Density Voids Porosity, Moisture Content Bulking.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 27STRENGTH (Characteristics of Aggregate)STRENGTH Strength should be at least equal to that of the Concrete. Rocks commonly used as Aggregates have a Compressive Strength >> usual range of Concrete Strength. A typical Stress-Strain Curve for Aggregate is shown in Fig. 6.1. The Tests conducted for strength evaluation are Crushing test Impact-test 10% Fines test. Of these the Cushing Test is the most reliable. 9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 28STRENGTH (Characteristics of Aggregate)STRENGTH(Contd) Limit for the Crushing Value. Generally the specifications prescribe 45 % for Aggregate used for Concrete other than Wearing Surface 30 % for Aggregate used for Concrete for Wearing Surfaces, such as Runways, Roads etc.. Toughness is measured by Impact Test. Impact Value < 30 % for Wearing Surface Impact value < 45 % for remaining Concretes. Hardness is tested by Abrasion test. The Abrasion value is restricted to < 30 % for Wearing surfaces and < 50 % for Concrete for other purposes. 9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 29Stress Strain Curve for Aggregate9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 30 Modulus of Elasticity of Concrete is approximately equal to the Weighted Average of the moduli of the Cement Paste and the Aggregate, as such. the modulus of the Coarse Aggregate has an important influence on the Stiffness of Concrete. A high value of Modulus reduces the Dimensional Changes due to Creep and Shrinkage of Cement Paste But,at the cost of higher Internal Stresses. In Concrete that is to be subjected to Wide Variations of Temperature and Humidity Internal Cracking is reduced by the use of a more Compressible Aggregate9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 31STIFFNESS (Characteristics of Aggregate ) Concrete is in a state of Internal Stress even if no external forces are present due to Difference between the Coefficients of Thermal Expansion of Paste and Aggregate The Shrinkage of Cement Paste during Hardening It is reported that the Stresses are likely to be Greatest at the Paste-Aggregate Interfaces where minute Cracks exist, even in concrete that has never been loaded. Under increasing External Load, these Cracks spread along the interfaces before extending into the Paste or Aggregate particles. The Str of the Bond between Aggregate and Cement paste thus has an important influence on the Str of Concrete. There is no standard test for bond but it is known that the rougher the surface texture of the particles, the better the bond. The role of particle shape is less well understood; the greater specific surface of Angular Particles should enable Greater Adhesive Force to be developed, But,the Angular Shape probably causes more severe Concentrations of Internal Stress.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 32BOND STR (Characteristics of Aggregate)SHAPE and TEXTURE (Characteristics of Aggregate ) The Shape influences the properties of Fresh Concrete more than when it has Hardened. Rounded Aggregate and Irregular shaped Aggregate Highly Workable, but yield Low Str Concrete. Flaky aggregate require more Cement Paste, produce Max Voids Not Desirable. Angular shape is the best. Crushed and Uncrushed Aggregates generally give essentially the same Str for the same Cement Content. The Shape and Surface Texture of FA govern its Void Ratio and significantly affect the Water requirement.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 33Sp Gr (Characteristics of Aggregate ) 2.6-2.7. Sp Gr and Porosity of Aggregates greatly influence the Str and Absorption of Concrete. Sp Gr of Aggregates indicates its Quality. A low Sp Gr may indicate High Porosity and Therefore, Poor Durability and Low Str. Concrete Density will greatly depend on Sp Grof Aggregate9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 34Bulk Density (Characteristics of Aggregate) The Bulk Density of Aggregate depends upon their Packing (Graded Aggregates are more Packed) the Particles Shape and Size the Grading the Moisture Content. For CA, a higher Bulk Density is an indication of fewer Voids to be filled by Sand and Cement.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 35VOIDS (Characteristics of Aggregate) The Void ratio is calculated asVoid ratio = 1 (Bulk density / Apparent Sp Gr) More the Voids, Less the Str9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 36Porosity (Characteristics of Aggregate) Entrapped Air Bubbles in the rocks during their formation lead to minute Holes or Cavities known as Pores. Porosity of Rocks is gen < 20% Due to Porosity, Concrete becomes Permeableand ultimately affects the Bond between Aggregate and Cement Paste, resistance to Freezing and Thawing of concrete and resistance to Abrasion of Aggregate. Porous Aggregate Absorb more Moisture, resulting in Loss of Workability of Concrete at a much faster rate.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 37Moisture Content (Characteristics of Aggregate) Moisture Content.The Surface Moisture expressed as a % of the Wt of the Saturated Surface Dry Aggregate A High Moisture Content Increases the effective W/C Ratio to an appreciable extent and may render the Concrete Weak.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 38Bulking (Characteristics of Aggregate) The increase in the Volume of a given mass of FA caused by the presence of Water is known as Bulking. REASON. Water forms a filmover the FA particles, exerts force of Surface Tension and pushes them apart increasing the Volume. EXTENT and VARIATION of BULKING. Extent of bulking depends upon the % Moisture present in the Sand and its Fineness. With Ordinary Sand bulking varies from 15-30 %. It increases with Moisture Content up to a certain point (4-6%), reaches Maximum, the Film of water on the Sand surface breaks, and then it starts Decreasing. Figure 6.2 shows the bulking of sand with Moisture Content. In preparing concrete mixes if Sand is measured by Volume and no allowance is made for bulking, the Moist Sand will occupy considerably Larger Volume than that prepared by the Dry Sand and consequently the Mix will be Richer. This will cause, Less Qty of Concrete per Bag of Cement. 9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 39Effect of Moisture Content on Bulking of Sand9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 40Bulking (Characteristics of Aggregate) For example, ifthe Bulking of Sand is 10% and if Mix Ratio is 1:2:4, the Actual Volume of Sand to be used should be 1.1 2 = 2.2%, instead of 2% Volume of Cement. If this correction is not applied, the Actual Dry Sand in the concrete will be (1 /1.1) x 2 = 1.82, instead of 2 per unit volume of cement. The Mix Proportion then would be 1:1.82:4 rather than 1: 2: 4. Which indicates Lesser Production of Concrete. Also, there will be chances of Segregation Honey Combing and Reduced Yield of Concrete. 9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 41Bulking (Contd) (Characteristics of Aggregate) Bulking of Sand can be determined, in field, by filling a Container of known Volume (A) with Damp Sand in the manner in which the mixer hopper will be filled. The ht of Sand in the container is measured. The Sand is then taken out of Container carefully, ensuring no Sand is lost during this transaction. The Sand is then either Dried and Filled Back into the gauge box, (or)the Container is filled with water and the damp sand is poured in to displace the water. Whichever method is adopted, the new depth of aggregate in the container gives the Unbulked Volume (B). Then % bulking expressed as a % of the Dry Vol% bulking = ((A-B)/B) x 1009/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 42Fineness Modulus (Characteristics of Aggregate) It is a Numerical Index of Fineness, giving some idea about the Mean Size of the Particles in the Aggregates. The fineness modulus (F.M.) varies 2.0to 3.5 for FA 5.5to 8.0 for CA 3.5 to 6.5 for All-in Aggregate. PROCEDURE Aggregate, whose F.M. is required, is placed on a Standard Set of Sieves (80, 63, 40, 20, 12.5, 10, 4.75, 2.36, 1.18 mm and 600, 300, 150 m) and the Set Vibrated. The material Retained on each Sieve after Sieving represent the fraction of Aggregate Coarser than the Sievein question, but, Finer than the Sieve Above. FM = % Retained on the Sieves /1009/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 43Fineness Modulus (Characteristics of Aggregate) A FM of 3.0 means the 3rdsieve i.e., 600 m is the Average Size. The test procedure is given IS: 2386 (Part I). The AIM of finding F.M. is to Grade the given Aggregate for the required Strand Workability of Concrete Mix with Minimum Cement. Higher F.M. Aggregate result in Harsh Concrete Mixes Lower F.M. result in Uneconomical Concrete Mixes.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 44*Quality of Aggregates Sand stones. are Porous Soft varieties of Sand-stones make Poor Concretes and also produce Shrinkage Cracks Lime stone. Quite Good provided it is Hard, Crystalline and entirely free from Dust. Should not be used in wks subject to Excessive Heat. Lime stones and Sand-stones and other Porous stones. are not suitable for Water Retaining Structures Aggregate must be Clean free from Clay, Loam, Vegetable and other Organic mtrl. Clay and Dirt coating on Aggregates prevents Adhesion of Cement to Aggregate, slows down the Setting and Hardening of the Cement(Concrete) reduce the Strength of the Concrete. 9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 45*Aggregates - SizesSIZES CA.Mtrl retained on a 4.75mm IS Sieve FA or Sand.Mtrl below 4.75mm IS Sieve. Mtrl passing a 75 micron IS Sieve (No 200 BS sieve) is gen considered as Clay, Fine Silt or Fine Dust in an Aggregate.Ordering of CA Ordered in Separate Sizes and Recombined in proper proportion, while batching. A 40 mm nominal max size aggregate will be ordered in 3 different Sizes 40mm to 20mm 20mm to 10mm 10mm to 4.75mm. Separate Stockpiles should be maint for different sizes Angular and roughly Cubicle particles are ideal.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 46*Shape and Surface Texture Shape is Cl as: Rounded (or Spherical).Fully water worn; River or Seashore Gravels Irregular.Partly shaped having rounded edges; Pit Sands and Gravels Flaky (or Elongated ). Usually Angular Thickness is small relative to the Width and/or Length. Surface Texture is Cl under 6 headings: Glassy Smooth Rough or Pitted Crystalline Honey Combed Porous Rounded and Irregular Gravels are smooth Crushed Stones are Rough9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 47Both Shape and Surface Texture affect Workability Density Str of ConcreteEffect of Shape. It is more imp factor. Workability.increases as the Aggre particles become Smoother and Rounder. Roughly Spherical (or, Rounded) aggregate produce the most Workable Concrete for the same Mix Proportions and the W/C ratio Concrete made with Sharp Angular Aggregate (Crushed Rock or Crushed Gravel) is considerably less Workable and also needs more Sand and more Cement; Interlocking. Angular particles interlock better. Voids. Angular pieces have more Voids than rounded ones. Crushing Str.For the same deg of Workability,Angular Aggregate may produce a Concrete having a Crushing str some 50% lower than a water worn and relatively rounded aggregate.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 48*Shape and Surface Texture (Contd) Excess of thin, flat, elongated or flaky particles should be avoided as they produce Harsh Unworkable mix and are not suitable for Strength Bearing Concrete Wks. Rounded Aggregate should be preferred where available, because, they are Economical than Angular pieces to produce the same Workability.... require about 1 Cum less Cement,per 100 Cum of Concrete about 2 Cum less Sand,per 100 Cum of Concrete about 1.5 ltrs less Water,per Bag of Cement Effect of TEXTUREAggre with Rough surface produce Weaker Concrete.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 49*Shape and Surface Texture (Contd) SAND9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 50FA or Sand Sand is Round/ Angular in Grains often found Mixed in various Gradation of Fineness. Sand used for Mortars should consist of Sharp (i.e. Angular) Grains of various sizes. Angular Grained Sands best results of Tensile Str. Round Grained Sandshighest Compression Str. STR of Mortars is effected by the variation in the size of Sand particles. form and variety of Sand particles causeUp to 50% Variation in Str Sand particles should be Hard. Color. Varies from Deep Brown to White.Deep Brown color is due to traces of Iron.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 51FA or Sand Impurities in Sand. Clay, Silt, Salts, Mica and Organic matter cause Weakness. All Sands gen contain some % of Silt and Clay. Mica is easily discernable from its shining surface. A certain % of impurities are inevitable in Sand. Permissive limits. Silt6% Mica 2 to 3% Free from Shell. Coal residues have a Corrosive effect on Rft.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 52 Sand may be cl on the basis of Source Mineralogical Composition Size of the Particles Particle Size Distribution. Cl based on Source Natural Sandresulting from natural disintegration of Rocks or deposited by Streams Crushed Stone Sandproduced by crushing Hard Stones Crushed Gravel Sandproduced by crushing Natural Gravel. 9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 53Classisfication Cl based on Mineralogical Composition Quartz Sand Felspar Sand Carbonaceous Sand. Cl based on Size Coarse SandF.M. 2.90-3.20; Medium SandF.M. 2.60-2.90; Fine SandF.M. 2.20-2.60. Cl based on Particle Size Distribution divided in 4 Grades from Grading Zone I to Grading Zone IV given in Table 6.2.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 54ClassisficationGradings of FA9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 55FINER COARSERFINENESS INCREASING*FA or SAND FA or Sand is the mtrl most of which passes through 4.75mm IS sieve. It is used in the Concrete to fill the voids in CA to produce a dense Concrete and to reduce the qty of Cement.Sources Sea Sand. Particles too fine and too uniform for good class wk. Sea Sand should not be used in its natural state.Salts will attack rft ifSalt content is too high, (-) it will retard Setting and Hardening of Cement may cause Efflorescence but it may not have any deleterious effect on Ultimat Str of the Concrete. Washing will remove most of the Salt content. Sea Sand must be tested for Organic Impurities.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 56*FA or SAND Pit Sand. From Abandoned Beds ofRiver. Usually considered as the BEST. Has Sharp Angular Grains (vs) River Sand which is Finewith Rounded Grains. Fresh water, River or Lake Sand. Usually Quite Good, but, may be Contaminated with mud. Often found mixed with Clay, Silt and Mica Crushed Stone. Screenings from Crushed stone often contain a high %of Dust and Clay and may tend to be Flaky. Flaky or Angular Particles mayproduce a Harsh Concrete.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 57Proportioning of Concrete9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 58 The Principle Object of proportioning Concrete is to achieve requisite Strength with desired Workability To achieve this Object, due attention should be paid to the selection of Cement and Aggregates according to the specifications. Concrete Mix Design is dealt in detail in chapter 11. Some Imp Considerations are:1. The MIX must be Workable so that it can be placed and finished without extra effort.2. Low Cement Consumption, consistent with the attainment of desired properties.3. High Cement Content should improve Strength, Impermeability, Density and Workability.4. For the same Cement Content, Ingredients and Workability :increase in Density of Mix Strength andImpermeability9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 59Grades and Mix Proportioning of CC9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 60PRODUCTION9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 61Quality Concrete A Good Quality Concrete is essentially a Homogeneous Mixture of Cement, CA, FA, Water which consolidates into a Hard Mass, due to Hydration. Specific Functions of these four constituents CA acts as a Filler. FA fills up the voids between the Paste and the CA. Cement in conjunction with Water acts as a Binder. The Mobility of the Mixture is aided by the Cement Paste, Fines and nowadays, increasingly by the use of Admixtures. 9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 62Quality Concrete QUALITY CONCRETE Most of the properties of the hardened concrete depend on the Care exercised at Every Stage of Mfg. A Rational Proportioning of the ingredients of concrete is the essence of the Mix Design. However, it may not guarantee of having achieved the objective of the quality concrete work. The AIM of QC is to ensure the production of concrete of Uniform Strength from Batch to Batch. This requires some Rules to be followed in various stages of Concrete Production9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 63Stages of Concrete Production1. Batching, or measurement of materials2. Mixing3. Transporting4. Placing5. Compacting6. Curing7. Finishing9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 64BATCHING Ensuring Proper and Accurate Quantity of all ingredients to achieve Good Quality Concrete Accuracy of Batching. Aggregates, Cement and Water. 3% Admixtures. 5% 2 Methods of batching Volume batching Weigh batching. Factors affecting the Choice of Batching Method Size of job Production Rate Required Standards of Batching performance. For most important works Weigh Batching is recommended. 9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 65VOLUME BATCHINGVOLUME BATCHING for Small Jobs only. Amount of each Solid Ingredient is measured by Loose Volume; using standard box known as Gauge Box (Fig. 10.1). Gauge Box Vol = 0.035m3 Cement is always measured by Weight, irrespective of the method of batching. Volume of a Bag of Cement (50 kg) = 0.035 m3. The amount of Solid Granular material in a CuMwill be an indefinite quantity.e.g. Density of Sand in a Moist Loose condition = 3 2 2.52 2 1.5 23 th of Concrete Cover to the embedded Steel, Steel becomes Vulnerable to the attack of Moisture. This expedites Rusting of Steel as the protective Concrete Cover remains NO longer Alkaline in nature. Voids also REDUCE the Contact between embedded Steel and Concrete. This results in Loss of Bond Strength of Reinforced Concrete member and thus the member loses Str. Voids such as Honeycombs and Blowholes on the exposed surface produce Visual Blemish. Concrete surface is not good to look with all such blemishes. Concrete with smooth and perfect surface finish not only Looks good but is also Stronger and more Durable.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 113COMPACTION (Contd) Compaction is achieved by imparting external Work over the Concrete to overcome the Internal Friction between the Particles forming the Concrete, between Concrete and Rft and between Concrete and Forms and by reducing the Air Voids to a Minimum. The Compaction of Concrete can be achieved by the following Methodsgiven in succeeding Slides.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 114HAND COMPACTION Used for Small and Unimportant Jobs. Extremely useful for Thin Elements such as Slabs, and for members with Congested Rfts. for Mixes with any Workability except for very fluid or very plastic Mix. Hand Compaction is achieved by Rodding, Ramming, orTamping. Rodding is done with the help of 16 mm diameter, 2 m longSteel Rod to pack the Concrete between the Rft, Sharp Corners and Edges, continuously during Concreting. Ramming is permitted only for Unreinforced Concrete Constructions. Tamping. Roof and Floor Slabs are usually tamped for achieving Compaction.Tampers are 100 100 mmin section and about 1 m long. Tamping Bars not only Compact the Concrete but also Level the Top surface. Limitation. large W-C ratio is required for full compaction.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 115COMPACTION BY VIBRATION This is the Most Common and Widely Used method of compacting concrete for any Structural Element. Vibrations imparted to the Fresh Concrete reduce the internal friction between the particles of concrete by setting the particles in motion and thus produce a dense and compact mass. On vibration, Concrete mix gets Fluidized and Internal Friction between the Aggregate particles REDUCES resulting in Entrapped Air to rise to the Surface. On losing entrapped air the Concrete gets Denser. Vibrations do not affect the Str of Concrete; but Concrete of Higher Str and better Quality can be made with Lesser Water and given Cement Content. Therefore, with vibrations Stiff Concrete with low W-C ratio can also be Well Compacted. 9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 116COMPACTION BY VIBRATION For Full Compaction, Vibration can be considered to be Sufficient when the Air Bubbles Cease to appear and Sufficient Mortar appears to close the Surface Interstices and facilitate easy Finishing Op. Vibration Helps Entrapped Air to escape first from between the CA particles and later from the Mortar. When Vibration continues some more Entrapped Air from the mortar isDriven out. However, during this Second Phase, Concrete does not show any movement but it is in this phase that Maximum EntrappedAir is driven out and that is the time when Most of the Consolidation takes place. Plastic mixes need less time of Vibration than Harsh or Dry Mixes. Types of Vibrators in use are Needle, Formwork, Table or Platform, and Surface Vibrators.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 117NEEDLE VIBRATOR can be used for any type of concrete work. This consists of a Steel Tube, called Poker, having an Eccentric Vibrating Element inside The Needle Diameter varies from 20 to 75 mm and its Length from250 to 900 mm. In places where the Rft is congested , Blades are used instead of Needle. Flexible Shaft Power Unit. An average Frequency of Vibration is 3,500 to 5,000 CPM A Needle Vibrator is shown in Plate 2. USE Immersed into Concrete at a Spacing of < 600 mm (or) 8 to 10 times the Diameter of the poker. The Vibrator is immersed at a place for about 30 Secs to 2 Mins. Location of the Poker Insertion should be STAGGERED to ensure that every bit of concrete is compacted (Fig. 10.9). The Vibrator should be allowed to Penetrate the Concrete Vertically(inclination 10) under its Own Wt. Poker Position should be changed gradually from the Side of the Heap and moving away from the heap ALL ROUND (Fig. 10.10(a). Poker should NOT be introduced in the CENTER of the Heap (Fig. 10.10(b)9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 1189/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 119Method of Poker insertion near a heap9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 1209/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 121FORMWK VIBRATOR These are also known as External or Shutter vibrators. These are generally used under the following Circumstances: 1. Compaction of Concrete is required to be done in a Very Thin or Very Densely Congested Reinforced Section. 2. In addition to Internal Vibration, Compaction is required to be done specially in the Cover Area where at times Needle or Poker Vibrator is Unable to do satisfactory Compaction. 3. Compaction of Very Stiff Concrete is required to be done because such Concrete cannot be Compacted by Internal Vibrators. Formwork Vibrators are used for Concreting Columns Thin Walls and Precast Units. These are Rigidly Clamped to the Formwork, causing it to Vibrate and consequently Transfer the Vibrations to Concrete. The performance of these vibrators when directly attached to Formwork is NOT generally satisfactory. The Vibrator is mounted on a Steel Plate, which is attached to a Channel or the I-beam, which runs along the formwork touching the Form Stiffeners. 9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 122FORMWK VIBRATOR (Contd) The Shuttering and the formwork have to be rigid, strong and watertight. The Vibrators operate at a Frequency of 3,000 to 9,000 CPM The Formwork requires to be properly designed to transfer the Vibrations to the Concrete without itself getting displaced or opening up. These consume more Power and are less Efficient than the needle vibrators The Compaction Time of Form Vibrators is generally between 1 to 2 Mins. Form Vibrators should NOT be used on Top of the Vertical Formwork. Generally they should be fixed 1000 mm below the top finished Level of Concrete. The Top Layer of Concrete must be vibrated by a Needle Vibrator. Use of Form Vibrator at top locations will generally cause Separation of Concrete from the formwork. Due to Formwork being inadequately stiffened at top it results in, in and out movement causing the Separation.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 123FORMWK VIBRATOR (Contd) Form vibrators if used in addition to Internal vibrators help removing Entrapped Air along the Concrete Surface giving a much Superior Finish. Generally, Tapping with a Wooden Mallet on the external face of the formwork also helps remove Entrapped Air which generally blemishes the concrete surface inspite of adequate internal vibration.VIBRATING TABLE. A Special Case of a Formwork Vibrator used in Labs and Factories is Vibrating Table. The Vibrating Table consists of a rigidly built Steel Platform mounted on Flexible Springs and driven by an Electric Motor. The average Frequency of vibration is 4000 CPM. The Moulds are clamped on the Platform. Vibrations are stopped as soon as the Concrete in the moulds develops a Level Smooth Surface. Medium Workable Mixes generally require Higher Frequency of Vibration than Stiff Mixes. It is observed that increasing the Frequency and decreasing the Amplitude as vibration progresses improves Consolidation by using this type of Vibrator. The Vibrating Tables are Very Efficient in compacting Stiff and Harsh Mixes required for Precast Elements.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 124SURFACE VIBRATOR These are also known as Screed Board Vibrators. Surface vibrators are used for Floor and Roof Slabs and Pavement Surfaces. These are effective only up to a th of 150 mm of Concrete but can be used up to 250 mm. Surface vibrators cause movement of fine particles to the top and hence aid the Finishing Op. The operating Frequency is 4,000 CPM.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 125COMPACTION BY SPINNING This method is also known as Centrifugationand is used for producing circular elements such as Pipes. The Plastic Concrete is SPUN into the horizontal mould at a Very High Speed. Water forced out of the mix during spinning flows out of the Mould. The initial W-C ratio required for effective Compaction is 0.35 to 0.4 which after spinning reduces to 0.3.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 126COMPACTION BY JOLTING In this method of Compaction, the mould containing Dry Concrete is subjected to Jolt at a frequency of 100 to 150 JPM. Jolting is a Vibrating action of low frequency and high amplitude. The mould filled with concrete is raised by about 12 mm and then allowed to fall under gravity. This method is used for Precast Units such as Hollow Blocks Cavity Blocks etc.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 127COMPACTION BY ROLLING Method of Compaction the Soft and Plastic Concrete is fed continuously between Rubber Rollers under Pressure up to 50 atm This forces out the excess water in concrete. Rolling is used to manufacture Precast Concrete Products such as Tiles.9/29/2014 Eswar/ BTech2yr/ BMCT/ Unit5 128