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Engineering materials

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  • 1. Engineering Material 1 PaintsPaints are used to protect metals, timber, or plastered surfaces from the corrosive effectsof weather, heat, moisture or gases etc and to improve their appearance.Composition of paintsFundamental components of an oil-based paint are:1. Body2. Vehicle3. Pigment4. Thinner5. Dryer6. Additives1. Body Forming the main body of a paint Make the paint film harder and more resistant to abrasion Reduce shrinkage cracks on dryingExamples: White lead, Zinc oxide, Iron oxide, Metallic powder such as Al, Cu, Br.Types of paints on the basis of body:Lead paint,Zinc paint, andAluminum paint.2. Vehicle Oily liquid in which the body and pigment are soluble Facilitates the paint to be conveniently spread over the surface Oils most commonly used as vehicle areExamples: Linseed oil, Soya bean oil, Fish oil, dehydrated castor oil.3. PigmentPigments are materials which gives the paint its color. In white paint the body is thepigment.Types of Pigments:Natural pigcentrements (natural iron oxides, chrome oxides). Synthetic pigments (phthalocynines ► coal tar derivatives)4. ThinnerLiquid thinner is added in the paint to Increase fluidity Making paint more smooth Help penetration into porous surfacesExamples ► turpentine (made by distilling gum from a number of pine trees) Cecos University Peshawar |
  • 2. Engineering Material 25. Dryers Added to quicken the drying of vehicle Organic salts of Iron, zinc, lead, manganese, Ca To accelerate the oxidation and hardening of vehicle.6. AdditivesAdditives,usually added in very small amount to improve some properties.Examples:v.v stabilizersBiocidesCatalystProperties:Some examples include additives to improve wet edge, to impart antifreeze properties,control foaming, control skinning, fight bacterial growth, or improve pigmentstability. There are various additives, which are added to improve some properties, suchas color opacity and matness, pigment dispersion, or stability.Preparation of paints Base/ body is thoroughly grounded in the vehicle. Mixed with the thinner to impart necessary workability. Pigments and dryers are separately mixed to a thin consistency. The two are then thoroughly mixed to form the desired paint.Removal of old paints1. Burning the paint: Which is economical and quick.2. Applying the paint removal2a. Hot solution of an equal part of soap, potash, quick lime applied keep it for 24 hours.2b. Two parts of quick lime one part of washing soda mixed with water. Cream spread onpaint surface, leave for an hour wash and then remove the paint.2c. Solution of caustic soda in water applied to the suface.Characteristics of good paintThere are a number of ways to judge paint quality:1. Wearability2. Covering ability3. Ease of cleaning4. Environmentally Friendly5. Aesthetic Cecos University Peshawar |
  • 3. Engineering Material 36. Practical and Cost Effective1. Wearability:Paint must be resistant to the wear and tear of the atmosphere and should maintain itscolor, smoothness and finish for a long time.2. Covering ability:Paints should cover the body uniformly and homogeneously on which it is applied andthe finish should be smooth and uniform.3. Ease of cleaning:When it is required to clean the paint, it shold be easy to remove i.e A good paint shouldnot react chemically with the materials but should only cover its surface.4. Environmentally Friendly:Paint should be water based and must not have any plasticizers or biocides as solvents.5. Aesthetic:It should provide a comfortable room climate and must not allow moulds and algae togrow on it.6. Practical and cost effective:The other qualities of a good paint are that they must be cheap, ready to use, long lastingand should color fast. In most cases Price is the decisive factor in selection of paints. BITUMEN:Bitumen is a generic name applied to the various mixtures of hydrocarbons.They may be gases, liquid ,semisolid and solid in nature and completely soluble incarbondisulphide.The most common material in the family of bitumen are tar, pitch and asphalts.Tar, Pitch and Asphalts forms a group of interrelated material widely used in the field ofcivil engineering in damp proofing building, water proofing roofs, water proofingbasement, painting, timber and steel and for setting out metalled roads.Composition of Bitumen:Materials in bituminous family are:1) Tar:Coal tar is a brown or dark black liquid of high viscosity, which smells of naphthaleneand aromatic hydrocarbons. Being flammable, coal tar is sometime used for heating or tofire boilers. It can be used in coal tar soap, and is used in medicated shampoo to kill andrepel head lice, and as a treatment for dandruff.Depending upon its source of origin, TAR is classified as: Cecos University Peshawar |
  • 4. Engineering Material 4a) Coal Tar:It is the liquid by-product of the distillation of coal to make coke. The gaseous by-productof this process is commonly known as town gas. It is used for coating of wooden polesand sleepers, iron poles.b) Wood Tar:It is obtained by the distillation of resinous wood. Wood tar contains creosote and as suchhas strong preservative properties.c) Mineral Tar:It is obtained by the distillation of bituminous shale.2) Pitch / Coal Tar Pitch:It is the residue of the direct distillation of crude tar produced by the high temperaturecarbonization of coal. It is used as a water proofing compound in masonry, steel andtimber structure. It is also used for water proofing concrete structures.3) Asphalts: Asphalts also known as bitumen. Asphalts are formed in natural state and are alsoproduced by refining of petroleum.There are main three groups of asphalts.1) Hot asphalts: Hot asphalts are softened by heat.2) Cut back asphalts: Cut back asphalts are dissolved in mineral solvents.3) Emulsion asphalts: Emulsion asphalts suspend in water base.PROPERTIES OF BITUMEN: 1. Adhesion: Bitumen has the ability to adhere to a solid surface in a fluid state depending on the nature of the surface. The presence of water on the surface will prevent adhesion. 2. Resistance to Water: Bitumen is water resistant. Under some conditions water may be absorbed by minute quantities of inorganic salts in the bitumen or filler in it. 3. Hardness: To measure the hardness of bitumen, the penetration test is conducted, which measures the depth of penetration in tenths of mm. of a weighted needle in bitumen after a given time, at a known temperature. Commonly a weight of 100 gm is applied for 5 sec at a temperature of 77 °F. The penetration is a measure of hardness. Typical results are 10 for hard coating asphalt, 15 to 40 for roofing asphalt and up to 100 or more for water proofing bitumen. 4. Viscosity and Flow: The viscous or flow properties of bitumen are of importance both at high temperature during processing and application and at low temperature to which bitumen is subjected during service. Cecos University Peshawar |
  • 5. Engineering Material 5 5. Ductility: Ductility test is conducted to determine the amount of bitumen will stretch at temperature below its softening point. 6. Softening point: The temperature at which a steel ball falls a known distance through the bitumen, when the test assembly is heated at known rate. Usually the test consist of (3/8) inches diameter steel ball, weight 3.5 gram,which is allowed to sink through out (5/8) inches diameter (1/4) inch the disc of thebitumen in a brass ring. When the whole assembly is heated at a rate of 9 F/min. Typical values would be 240F for coating the grade asphalts, 140-220F forroofing asphalts and 115F for bituminous water to fill material. RUBBER:It is essentially an elastic material and is either Natural Rubber or Indian Rubber Synthetic Rubber1. Natural RubberIt is present as an emulsion in the latex of rubber trees mostly grow in hot, moist climate► Malaysia, Venezuela, Mexico. Latex is milky fluid oozing (dropping) from verticalgrooves having an upward inclination cut around the trunk of the rubber tree. Pots aretied to the trunk of the trees for collecting latex. Crude rubber is obtained by coagulationof latex with alcohol, alum or lime.Crude rubber becomes hard and brittle in winter and soft in summer, therefore somecompounds are added to modify its properties.1. 1. COMPOUNDING RUBBER:Crude rubber is mixed with filler. There are number of fillers in common use eachimparting specific properties. Some of which are lamp black, oxides of iron, slaked lime,sulphur. Sulphur is the most common, with which rubber forms a mass which is hard,tough and resistant to water and ordinary changes of temperature. The process of mixingrubber with sulphur is called as Vulcanizing. 1. 2. RECLAIMING RUBBERRubber from old, worn out articles can be reclaimed either by mechanical or chemicalprocess. Mechanical ► by grinding the articles into fine particles, removing any trace ofiron with the help of electromagnets, washing. Chemical ►Ground articles are dissolvedeither in an acid or alkali to loosen and remove other materials.2. Synthetic RubberRecently rubber is produced on a large scale artificially from acetylene gas under tradenames of Neoprene Choroprene Cecos University Peshawar |
  • 6. Engineering Material 6 ButylArtificial rubbers possess certain properties that are hard to be achieved with naturalrubber such as resistance to acids particularly to grease, kerosene and petroleum.Uses of Rubber:About 70% of the total world production of rubber is consumed by automobile tyreindustry1. Rubber is used as a flooring material. Tiles of sheets of rubber for covering offloors are used in public and industrial buildings, buses and ships because of their fairweather resistance quality.2. Synthetic rubber is used for packing oil-working machinery, hose pipes forcarrying petrol.3. Rubber bearing pads are used under girders for bridges4. Rubber is also used for expansion joints.5. Rubber is made into articles as diverse as raincoats and sponges, bowling ballsand pillows, electrical insulation and erasers. People ride on rubber tires and walk onrubber heels.6. Rubber is also used in toys, balls, rafts, elastic bandages, adhesives, paints, hoses,and a multitude of other products.7. Ground or "crumb" rubber is used as a binder for asphalt pavements and forvarious asphalt sealants and crack fillers.8. The single most important use of rubber is for tires. Most tires contain severalkinds of rubber, both natural and synthetic. Radial automobile tires contain a greaterpercentage of natural rubber than other types of automobile tires because radial tires haveflexible sidewalls that tend to produce a buildup of heat, to which natural rubber has asuperior resistance.9. Either natural or synthetic rubber is suitable for most uses, and price determineswhich is used. SOME IMPORNTANT DEFINITION:DUCTILITYAbility of a material to deform easily under stress of temperature‚ pressure and Speed;especially‚ ability of a metal to stretch easily. Ductility is characterized by a weak elasticlimit and significant lengthening. (Example: The ductility of Bituminous products ismeasured by lengthening a test specimen of determined form that is stretched at astandardized speed and temperature to the precise instant of its breaking.) Syn. withMALLEABILITY. Cecos University Peshawar |
  • 7. Engineering Material 7WORKABILITYThe ability of a mortar or a fresh concrete to fill correctly a mold or a formwork thanks toa well-studied batching of its constituents that give him a sufficient fluidity withoutharming its strength and its homogeneity. The workability is a factor of the first magnitude because it conditions amongother things: good filling, simplicity of placing, good covering of reinforcements. Theworkability depends on the batching in fine elements, quantity of water, temperature,batching in cement, and so on; it is given by means of measuring instruments such as theconcrete workability meter, slump cone, and so on.BLEEDTo go up on the surface, speaking of the water contained in a mortar, a concrete. To rejectinternal water.BLEEDINGThe appearance of a film of water or laitance on the surface of a slab or a concrete ormortar screed after troweling or vibration. The vibration, closing between them thevarious grains of the components of concrete, brings about the expulsion of a part ofwater that occupies the empties. Water, having lower density than the other components,goes up on the surface. Syn. with BLEED-THROUGH; SWEATING; WATER GAINSEGREGATION1. An imbalance in the chemical composition of the different components of an alloy.2. A preferential aggregation of the chemically alike components between them duringthe solidification phase of an alloy; this separation results in a chemically heterogeneousstructure.3. A selective dissociation, in distinct heaps, of different previously mixed bodies as aresult of vibration, brewing, etc.4. A phenomenon of dissociation of the concrete ingredients that can be due to variouscauses (excessive vibration, carriage, falls from critical height, etc.). Elements are dividedand rearranged by order of density. The heaviest aggregates go down to the bottom whileconversely, the mortar goes up to the surface. Segregation can occur: inside the concretemixer (malaxation was too long); at the time of the emptying of the vat or placing (tooimportant brutal free falls of the concrete (too distant from unloading point)); during thetransport (shakes, etc.); during the pouring (lateral projections and important falls, toolong vibration of concrete, etc.).HYDRATEA chemical body resulting from the combination of a body with water molecules.HYDRATIONPhenomenon of water absorption by a chemically receptive body. The process ofchemical reaction between water and cement is also called hydration. Cecos University Peshawar |
  • 8. Engineering Material 8DURABILITYResistance to weather conditionSHRINKAGEDecrease in volume of ConcreteCOMPACTIONCompaction is removing the air from concrete. Proper compaction results in concretewith an increased density which is stronger and more durable.PROPORTIONING AND MIXING CONCRETE:A concrete mix is designed to produce concrete that can be easily placed at the lowestcost. The concrete must be workable and cohesive when plastic, then set and harden togive strong and durable concrete. The mix design must consider the environment that theconcrete will be in; i-e exposure to sea water, trucks, cars, forklifts, foot traffic orextremes of hot and cold. 1. PROPORTIONING: Concrete is a mixture of Cement, Water, Coarse and FineAggregates and Admixtures. The proportions of each material in the mixture affects theproperties of the final hardened concrete. These proportions are best measured by weight.Measurement by volume is not as accurate, but is suitable for minor projects.Cement content: As the cement content increases, so does strength and durability.Therefore to increase the strength, increase the cement content of a mix.Water content: Adding MORE WATER to a mix gives a WEAKER hardenedconcrete. Always use as little water as possible, only enough to make the mix workable.Water to cement ratio: As the Water to Cement ratio INCREASES, the strength anddurability of hardened concrete DECREASES. To increase the strength and durability ofconcrete, decrease the Water-Cement ratio.Aggregates: Too much fine aggregate gives a sticky mix. Too much coarse aggregategives a harsh or boney mix.2. MIXING: Concrete must be mixed so the Cement, Water, Aggregates andAdmixtures blend into an even mix. Concrete is normally mixed by MACHINE. Machinemixing can be done on-site or be a Pre-Mixed concrete company. Pre-Mixed concrete isbatched (proportioned) at the plant to the job requirements. Cecos University Peshawar |
  • 9. Engineering Material 9Truck Mixing: The materials are normally added to the trucks at batching plants andmixed for required time and speed at the plant. The trucks drum continues to rotate toagitate the concrete as it is delivered to the site.Site Mixing: When site mixing begin by loading a MEASURED AMOUNT of coarseaggregate into the mixer drum. Add the sand before the cement, both in measuredamounts.NEVER USE A SHOVEL AS A MEASURE AS VOLUMES CANVARY WIDELY:Mix materials together until there is no visible sand in the mix. Add enough water to get aworkable mix. Be careful not to overload the mixer. Too much concrete in the mixermeans each batch takes longer to be properly mixed, which causes costly delays in thelong run or it will not mix at all. Always check how much the mixer holds so you knowhow much concrete can be produced at once. Avoid delays between batches to getmaximum output. Small quantities of concrete may be mixed by hand with a shovel.Mixing should be done on a clean board, or plate, or in a wheelbarrow. Mix the materialstogether until they are even. Then dish the material and add water. Use only enoughwater to get an even, workable mix.TRANSPORTING AND PLACING CONCRETE:When transporting and placing concrete, avoid :DELAY SEGREGATION andWASTAGE.3. TRANSPORTATIONThe method used to transport concrete depends on which one is the lowest cost andeasiest for the job size. Some ways to transport concrete include: a concrete truck, aconcrete pump, a crane and bucket, a chute, a conveyor or a hoist. On small jobs awheelbarrow is the easiest way to transport concrete. Always transport concrete as littleas possible to reduce problems of segregation and wastage.4. PLACINGWhen placing concrete be careful not to damage or move the formwork andreinforcement. Place concrete as near to its final position as possible. Start placing fromthe corners of the formwork or, in the case of a sloping site, from the lowest level.DELAYDelay can cause the concrete to dry-out and stiffen. Delay is more of a problem on a hot,and/or windy, day when the concrete will dry-out and stiffen more quickly.SEGREGATIONSegregation is when the coarse and fine aggregate, and cement paste, become separated.Segregation may happen when the concrete is mixed, transported, placed or compacted. Cecos University Peshawar |
  • 10. Engineering Material 10Segregation makes the concrete: WEAKER,LESS DURABLE and will leave A POORSURFACE FINISH.To avoid segregation:Check the concrete is not too wet or too dry. Make sure the concrete is properly mixed.It is important that the concrete is mixed at the correct speed in a transit mixer for at leasttwo minutes immediately prior to discharge. The concrete should be placed as soon aspossible. When transporting the mix, load carefully. If placing concrete straight from atruck, pour vertically and never let the concrete fall more than one-and-a-half meters.Always pour new concrete into the face of concrete already in place. When compactingwith a poker vibrator be sure to use it carefully.Never spread concrete sideways with a poker vibrator as this may cause segregation ofthe mix. Always be sure to vibrate concrete evenly.WASTAG EWastage can be costly, especially on small jobs. To minimize wastage; mix, load,transport and place carefully.5. COMPACTION: Compaction is the process which expels entrapped air fromfreshly placed concrete and packed the aggregate particle together so as to increase thedensity of concrete.ORRemoval of entrapped air from the voids of concrete by using some mechanical ways. It increases the significantly the ultimate strength of concrete and enhances thebond with reinforcement. It also increase the abrasion resistance and durability of concrete. It decreases the permeability and helps to minimize the shrinkage and creepcharacteristics.TWO STAGES PROCESS: 1. The aggregate particles are set in motion and slump to fill the form giving a level top surface. 2. In the second stage entrapped air is expelled.METHOD AND EQUIPMENT:VIBRATION: Vibration of the concrete is one of the important method ofcompaction. It is done with the help of vibrator.Types of Vibrators: 1. Internal/ Immersion/ Poker Vibrator 2. Surface Vibrator Cecos University Peshawar |
  • 11. Engineering Material 111. INTERNAL VIBRATIONInternal vibration is done with a mechanical vibrator or poker vibrator. The POKER isput into concrete and vibrates it from the inside.MethodMake sure there are enough workers so some can compact while others continue to place.Put the poker into the concrete QUICKLY. Take the poker out very SLOWLY otherwisea hole, or weak spot, may be left in the concrete. The SIZE of the poker determines howmuch concrete is vibrated at one time. The area vibrated at one time is called theRADIUS OF ACTION. This can be seen by over what radius air bubbles rise to thesurface. The radius of action will be greater with a LARGER poker and more-workableconcrete. Always compact in a definite pattern so the radius of action overlaps and coversthe whole area of the concrete. The poker should be long enough to reach and enter intothe layers of concrete under the one being compacted.HOW LONG TO COMPACTFor concrete of average workability (i-e slump of 80 mm)with a poker size between 25–75 mm, concrete should usually be vibrated for between5 and 15 seconds. It is worse toUNDER-VIBRATE than to OVER-VIBRATE concrete.2. SURFACE VIBRATOR: Surface Vibrators are applied are applied at the topsurface of the concrete and act downward from there. They are very useful for compactedslabs, industrial floors, road pavements and similar flat surfaces. They also aid inlavelling and finishing the surfaces. There are number of types of vibrators including 1) Vibrator roller screed 2) Vibrator beam screed 3) Pan type vibratorThey are used mainly on very specialized equipment such as road paving plant. The mostcommon type is the single or double vibrating beam screed.WHAT IS FINISHINGFinishing is screeding, floating or trowelling the concrete surface to density and furthercompact the surface of concrete, as well as giving it the look you want.STAGES: Finishing takes place in two stages: INITIAL and FINAL finishing.INITIAL FINISHINGConcrete is first screeded to the level of the formwork, then bull floated and left to set. Insome cases screeding leaves a good enough finish, especially if floor coverings are to beused over the concrete. Water then appears on the surface of the concrete. This water iscalled bleed water.No final finishing can begin until the bleed water has dried up. Mixing bleed water withthe surface paste will weaken it, possibly resulting in a dusty surface. Cecos University Peshawar |
  • 12. Engineering Material 12Excess bleed water can be removed by dragging an ordinary garden hose across thesurface of the concrete.Never try to dry up the bleed water using stone dust or cement as this will weaken theconcrete surface in the long run. Once the bleed water dries up and concrete can supporta person’s weight, with only as light marking to the surface, the final finishing can begin.FLOATINGThere may be two stages in floating:The BULLFLOAT, which is part of the initial float.The POWER or HAND FLOAT which is part of the final float.Floating helps compact and level the surface and close minor cracks. Floating can bedone by hand or with a power float. Power floating leaves a better finish than handfloating.FINAL FINISHINGThis involves floating, trowelling, edging, jointing or patterning the concrete. Specialfinishes such as brooming, coloring or patterned finishes can be applied to the surface.TowelingToweling leaves a dense, hard, smooth and durable surface. The surface should betrowelled TWICE. A well trowelled surface will be very smooth and can be slipperywhen wet. Trowelling can be done by hand or power trowel..Edging and GroovingAll the edges of a slab should be finished with a special edging tool. This gives a neaterand stronger edge, less prone to chipping. Joints should be planned before placing and areusually formed into the concrete during finishing.Once any surface has been finished it MUST be cured.7. CURING CONCRETE: The process of maintaining a satisfactory moisture content and a favorabletemperature in concrete during the period immediately following placement so hydrationmay continue until the desired properties are developed to the sufficient degree to meetthe service requirement is called Curing.OR: Keeping the concrete moist or warm enough so that hydration may continue untilthe desired properties are developed to the sufficient degree to meet the servicerequirement is called Curing. Curing means to cover the concrete so it stays MOIST. By keeping concretemoist the bond between the paste and the aggregates gets stronger. Concrete doesn’tharden properly if it is left to dry out.WHEN TO CURE Cecos University Peshawar |
  • 13. Engineering Material 13Curing is done just after finishing the concrete surface, as soon as it will not be damaged.WHY CUREConcrete that is cured is: LESS LIKELY TO CRACK. More DURABLE. Curedconcrete has a surface that wears better, lasts longer and better protects the steelreinforcement.StrongerThe concrete can carry more weight without breaking.HOW TO CUREConcrete is cured by:1)APPLYING EXTRA WATER to the surface of the concrete, or2)STOPPING water loss from the concrete3)To cover the concrete with PLASTIC SHEETS to slow down water loss4)Curing the concrete by applying curing compound that is sprayed or brushed on.The most important thing in curing is to keep the concrete moist at all times. Hosing inthe morning and again at night and letting the concrete dry out in between is no good.Another way to cure concrete is to cover with PLASTIC SHEETS to slow down waterloss. This method is easy and cheap. The only problem is that the sheets may causeconcrete to become darker in places. To avoid this keep concrete EVENLY moist..HOW LONG TO CUREConcrete keeps getting HARDER AND STRONGER over TIME.Household concrete jobs MUST be cured for at least 3 DAYS.For better strength and durability, cure concrete for 7 DAYS.The LONGER concrete is cured, the closer it will be to its best possible strength anddurability.PROPERTIES OF CONCRETE:Properties of concrete are divide into two major groups Properties of Fresh Concrete Properties of Hardened ConcreteFRESH CONCRETE:Fresh concrete is that stage of concrete in which concrete can be moulded and it is inplastic state. This is also called "Green Concrete". Another term used to describe the stateof fresh concrete is consistence, which is the ease with which concrete will flow.Properties of Fresh ConcreteFollowing are the important properties of fresh concrete1. Setting2. Workability3. Bleeding and Segregation Cecos University Peshawar |
  • 14. Engineering Material 14a) Bleedingb) Segregation4. Hydration5. Air Entrainment1. Setting of ConcreteThe hardening of concrete before its hydration is known as setting of concrete. ORThe hardening of concrete before it gains strength. ORThe transition process of changing of concrete from plastic state to hardened state.Setting of concrete is based or related to the setting of cement paste. Thus cementproperties greatly affect the setting time.2. Workability of ConcreteWorkability is often referred to as the ease with which a concrete can be transported,placed and consolidated without excessive bleeding or segregation.Segregation in concreteSegregation can be defined as the separation of the constituent materials of concrete.Hardened Concrete PropertiesFollowing are the properties of hardened concrete:1. Strength of concrete2. Concrete Creep3. Shrinkage4. Modulus Of Elasticity5. Water tightness (impermeability)TYPES OF CONCRETE:Some common and main types of concrete are:1. Normal concrete2. High Strength Concrete3. High Performance Concrete4. Air Entrained Concrete5. Light Weight Concrete6. Self Compacting Concrete7. Shotcrete Concrete8. Pervious Concrete9. Roller Compacted Concrete10. Hot Weather Concrete11. Cold Weather Concrete1. NORMAL CONCRETE: The concrete in which common ingredients i.e. aggregate, water, cement are usedis known as normal concrete. It is also called normal weight concrete or normal strengthconcrete. Cecos University Peshawar |
  • 15. Engineering Material 15 It has a setting time of 30 - 90 minutes depending upon moisture in atmosphere,fineness of cement etc. The development of the strength starts after 7 days the common strength values is10 MPa (1450 psi) to 40 MPa (5800 psi). At about 28 days 75 - 80% of the total strengthis attained. Almost at 90 days 95% of the strength is achieved.Properties of Normal Concrete Its slump varies from 1 - 4 inches. Density ranges from 140 pcf to 175 pcf. It is strong in compression and weak in tension. Air content 1 - 2 %. Normal concrete is not durable against severe conditions e.g. freezing andthawing.2. HIGH STRENGTH CONCRETE: Compressive strength of high strength concrete mix is usually greater than 6,000pounds per square inch. High strength concrete is made by lowering the water cement (W/C) ratio to 0.35or lower. Often silica fume is added to prevent the formation of free calcium hydroxidecrystals in the cement, which might reduce the strength at the cement aggregate bond. Low w/c ratios and the use of silica fume make concrete mixes significantly lessworkable, which is particularly likely to be a problem in high-strength concreteapplications where dense rebar cages are likely to be used. To compensate for thereduced workability in the high strength concrete mix, super plasticizers are commonlyadded to high-strength mixtures. Aggregate must be selected carefully for high strength mixes, as weakeraggregates may not be strong enough to resist the loads imposed on the concrete andcause failure to start in the aggregate.3. HIGH PERFORMANCE CONCRETE:This mix has the following main properties: High strength. High workability. High durability. Ease of placement. Compaction without segregation. Early age strength. Long-term mechanical properties. Permeability. Density. Heat of hydration. Toughness. Cecos University Peshawar |
  • 16. Engineering Material 16 Volume stability. Long life in severe environmentsPreparationHigh strength concrete mix can be prepared with careful selection of ingredients andoptimization of mix design. High workability is attained by super plasticizers, they lower the water cementratio to 0.25 which is the amount required only for hydration process. High durability is attributed to fly ash and silica fume which modify the emineralogy of the cement; it enhances the compatibility of ingredients in concrete massand reduces the CH amount. Fly ash also causes ball bearing effect increasingworkability. The admixtures are 20-25% fly ash of partial replacement of cement and rest 70%is Ordinary Portland Cement. As it is not usually durable against freezing and thawing so air entrained agentscan also be utilized.Properties of high performance concrete mix Strength of high performance concrete ranges from 10000 psi - 15000 psi Water cement ratio can be reduced to 0.25.4. AIR ENTRAINED CONCRETE: One of the greatest achievements in field of concrete technology is developmentof air entrained concrete. It is used where the concrete is vulnerable to freezing andthawing action. It is used where the concrete is vulnerable to freezing and thawing action. It isprepared by adding the air entraining admixture.The air entrainment in concrete does the following functions:.1. It lowers the surface tension of water and thus bubbles are created.2. Secondly the air entraining agents prevents coalescing i.e. the combining ofbubbles. The diameter of these bubbles ranges form 10 micrometer to 1000 micrometerand in entrapped air the diameter of bubble is greater than 1mm.Air entraining agents OR air entrained admixtures are used for the purpose of makingentrained air in concrete.5. LIGHT WEIGHT CONCRETE The concrete which has substantially lower mass per unit volume then theconcrete made of ordinary ingredients is called lightweight concrete. The aggregates usedare lighter in weight. Density of light weight concrete is 240 kg/m³ (15pcf) -1850 kg/m³ (115 pcf). Strength of light weight concrete blocks varies from 7 MPa (1000 psi) - 40 MPa(5800 psi). Some times Air Entrained Admixtures are also added to it giving resistance tofreezing and thawing along with strength. Cecos University Peshawar |
  • 17. Engineering Material 17Uses of Light weight concrete: Used where extra load is not applied e.g. parapet wall, road lining etc. or toreduce dead load.6. SELF COMPACTING CONCRETEThe concrete where no vibration is required. The concrete is compacted due to its ownweight. It is also called self consolidated concrete or flowing concrete. It can be alsocategorized as high performance concrete as the ingredients are the same, but in this typeof concrete workability is increased. This self-consolidating concrete is characterized by: Extreme fluidity as measured by flow, typically between 650-750 mm on a flowtable, rather than slump (height). No need for vibrators to compact the concrete. Placement being easier. No bleed water, or aggregate segregation.Uses and Applications of Self Compacting Concrete:1. It is used in location unreachable for vibrations. e.g. underground structure, deepwells or at bottom of deep sea.2. SCC can save up to 50% in labor costs due to 80% faster pouring and reducedwear and tear on formwork7. SHOTCRETE CONCRETE: Shotcrete concrete uses compressed air to shoot concrete onto (or into) a frame orstructure. Shotcrete is mortar or (usually) concrete conveyed through a hose andpneumatically projected at through a shortcrete nozzle with high velocity onto a surface.Shotcrete undergoes placement and compaction at the same time due to the force withwhich it is projected from the nozzle. It can be impacted onto any type or shape of surface, including vertical oroverhead areas. Shotcrete is frequently used against vertical soil or rock surfaces, as it eliminatesthe need for formwork. It is sometimes used for rock support, especially in tunneling. Shotcrete is also used for applications where seepage is an issue to limit theamount of water entering a construction site due to a high water table or othersubterranean sources. This type of concrete is often used as a quick fix for weathering for loose soiltypes in construction zones.8. PERVIOUS CONCRETE: Pervious concrete contains a network of holes or voids, to allow air or water tomove through the concrete. This allows water to drain naturally through it, and can bothremove the normal surface water drainage infrastructure, and allow replenishment ofgroundwater when conventional concrete does not. Cecos University Peshawar |
  • 18. Engineering Material 18 It is formed by leaving out some or the entire fine aggregate (fines), the remaininglarge aggregate then is bound by a relatively small amount of Portland cement. When set, typically between 15% and 25% of the concrete volumes are voids,allowing water to drain. The majority of pervious concrete pavements function well with little or nomaintenance. Maintenance of pervious concrete pavement consists primarily ofprevention of clogging of the void structure. In preparing the site prior to construction, drainage of surrounding landscapingshould be designed to prevent flow of materials onto pavement surfaces. Soil, rock,leaves, and other debris may infiltrate the voids and hinder the flow of water, decreasingthe utility of the pervious concrete pavement.9. ROLLER COMPACTED CONCRETE: Roller compacted concrete, sometimes called rollcrete, is a low-cement-contentstiff concrete placed using techniques borrowed from earthmoving and paving work. The concrete is placed on the surface to be covered, and is compacted in placeusing large heavy rollers typically used in earthwork. The concrete mix achieves a high density and cures over time into a strongmonolithic block. Roller compacted concrete is typically used for concrete pavement. Rollercompacted concrete dams can also be built, as the low cement content causes less heat tobe generated while curing than typical for conventionally placed massive concrete pours.10. HOT WEATHER CONCRETE:ACI 305 “Hot Weather Concreting” defines hot weathers as any combination of thefollowing conditions that tends to impair the quality of the freshly mixed or hardenedconcrete:High ambient temperatureHigh concrete temperatureLow relative humidityWind speedSolar radiationThe success of many hot-weather concreting operations depends on the steps taken toslow the cement hydration reactions within the concrete and to minimize the rate ofevaporation of moisture from the freshly mixed concrete.Precautions in hot weather concreting:To control the high concrete temperature following steps may be taken:1. Schedule concreting. The concrete can be scheduled to time when the temperatureis not high e.g. in summer it can be schedule to night or early morning2. Material and mix proportions: use material and mix design proportion havinggood weather resistance e.g. select sand having low specific heat.3. Covered environment: The concrete can be done in a covered environment. Cecos University Peshawar |
  • 19. Engineering Material 194. Chilling: The chilling of aggregate can be done by watering or keeping themcovered.5. Efficient work force: employ efficient workforce and machinery to improve thehandling.6. Use of low heat cement: Use low heat cement.11. COLD WEATHER CONCRETE:ACI 306 “Cold Weather Concreting” defines cold weather concreting as a period whenfor more than three (3) consecutive days, the following conditions exist: The average daily air temperature is less than 5°C (40°F) and, The air temperature is not greater than 10°C (50°F) for more than one-half of any24 hour period.Concrete placed during cold weather will develop sufficient strength and durability tosatisfy intended service requirements only if it is properly produced, placed andprotected.Objectives of Cold Weather ConcretingThe objectives of cold weather concreting are to: Prevent damage to concrete due to freezing at early ages Assure that concrete develops the required strength for the safe removal of forms Maintain curing conditions that foster normal strength development without usingexcessive heat Limit rapid temperature changes in the concrete to prevent thermal cracking.Precautionary Measures for concrete in cold weather1. Using hot water2. Providing enclosures i.e. covered area3. Using air entraining agents4. Scheduling concreting5. Using admixtures Accelerators6. Type III or high early strength cement7. Using amount of cement AdmixturesA material other than water, aggregates, or cement that is used as an ingredient ofconcrete or mortar to control setting and early hardening, workability, or to provideadditional cementing properties.Types of Admixtures Chemical Admixtures1. Plasticizers2. Super Plasticizers Cecos University Peshawar |
  • 20. Engineering Material 203. Accelerators4. Set Retarders Mineral Admixtures1. Cementitious2. Pozzolanic3. Blast Furnace Slag4. Fly ash5. Silica Fume6. Rice HuskChemical admixtures:1. Water-reducing admixture / Plasticizers:These admixtures are used for following purposes:1. To achieve a higher strength by decreasing the water cement ratio.2. To achieve the same workability by decreasing the cement content.3. To increase the workability so as to ease placing in accessible locations.4. Water reduction more than 5% but less than 12%5. The commonly used admixtures are Ligno-sulphonates and hydrocarbolic acid salts.2. Super Plasticizers:These are more recent and more effective type of water reducing admixtures also knownas high range water reducer. The main benefits of super plasticizers can be summarizedas follows:Increased fluidity: Flowing Self-leveling Self-compacting concreteReduced W/C ratio: Very high early strength Very high later age strengths Reduced shrinkage Improved durabilityThe commonly used Super Plasticizers are as follows: Sulphonated melamine formaldehyde condensates (SMF) Sulphonated naphthalene formaldehyde condensates (SNF) Polycarboxylate ether superplasticizers (PCE)3. Accelerators:An admixture which, when added to concrete, mortar, or grout, increases the rate ofhydration of hydraulic cement, shortens the time of set in concrete, or increases the rateof hardening or strength development. Cecos University Peshawar |
  • 21. Engineering Material 21Accelerating admixtures can be divided into groups based on their performance andapplication:1. Set Accelerating Admixtures,Reduce the time for the mix to change from the plastic to the hardened state.Set accelerators have relatively limited use, mainly to produce an early set.2. Hardening Accelerators,Hardening accelerators find use where early stripping of shuttering or very early access topavements is required. They are often used in combination with a high range waterreducer, especially in cold conditions.Examples:Calcium chloride is the most effective accelerator and gives both set and hardeningcharacteristicsChloride-free accelerators are typically based on salts of nitrate, nitrite, formate andthiocyanate.4. Set Retarders:The function of retarder is to delay or extend the setting time of cement paste in concrete.These are helpful for concrete that has to be transported to long distance, and helpful inplacing the concrete at high temperatures.When water is first added to cement there is a rapid initial hydration reaction, after whichthere is little formation of further hydrates for typically 2–3 hours. The exact timedepends mainly on the cement type and the temperature. This is called the dormantperiod when the concrete is plastic and can be placed.Retarding admixtures delay the end of the dormant period and the start of setting andhardening.The commonly known retards are Calcium Ligno-sulphonates and Carbohydratesderivatives used in fraction of percent by weight of cement.5. Air Entrained Admixtures:An addition for hydraulic cement or an admixture for concrete or mortar which causesair, usually in small quantity, to be incorporated in the form of minute bubbles in theconcrete or mortar during mixing, usually to increase its workability and frostresistance.Air-entraining admixtures are surfactants that change the surface tension of the water.Examples: fatty acid salts or vinsol resinMineral Admixtures:Types of Mineral Admixtures1. CementitiousThese have cementing properties themselves. For example: Ground granulated blast furnace slag (GGBFS). Cecos University Peshawar |
  • 22. Engineering Material 222. PozzolanicA pozzolan is a material which, when combined with calcium hydroxide (lime), exhibitscementitious properties. Examples are Fly ash Silica Fume Rice Husk Ash Metakaolin3.Ground Granulated Blast Furnace Slag (GGBFS)Ground granulated blast-furnace slag is the granular material formed when molten ironblast furnace slag (a by-product of iron and steel making) is rapidly chilled (quenched) byimmersion in water. It is a granular product, highly cementitious in nature and, groundto cement fineness, hydrates like Portland cement.Blast furnace slag is blended with Portland cement clinker to form PORTLANDBLASTFURNACE SLAG CEMENT).GGBFS has been widely used in Europe, and increasingly in the United States and inAsia (particularly in Japan and Singapore) for its superiority in concrete durability.4. Fly Ash:The finely divided residue resulting from the combustion of ground or powdered coal.it has POZZOLANIC properties e-g an addition to Portland cement concrete mixture toincrease the long term strength and other material properties of Portland cement concreteand in some cases reduce the material cost of concrete.5.Silica Fume:The terms condensed silica fume, microsilica, silica fume and volatilized silica are oftenused to describe the by-products extracted from the exhaust gases of silicon, ferrosiliconand other metal alloy furnaces, that are of high quality, for use in the cement and concreteindustry.Silica Fume is used in concrete to improve its properties. It has been found that SilicaFume improves compressive strength, bond strength, and abrasion resistance; reducespermeability of concrete and protecting reinforcing steel from corrosion.6. Rice Husk Ash:This is a bio waste from the husk left from the grains of rice. It is used as a pozzolanicmaterial in cement to increase durability and strength. Cecos University Peshawar |