Basic construction element CONCRETE

• 1.Concrete Basic and most important material ofconstruction technique Most widely used and major building material.

2. Why concrete is so widely used ? It can be easily moulded/casted in various sizes and shapes 2. It can easily be modified and changed as per the requirements of the structure. 3. It is simple to prepare and place; also readily available. 1. 3. TYPES OF CONCRETE Plain cement concrete (PCC):- It is simple mixof Cement : Fine aggregate : Coarse aggregate used for preparing level surfaces, stages, filling of various structures. Reinforced cement concrete (RCC):- It is similar to PCC only steel (also wood, resin, plastic, etc. in special cases) is reinforced in designed manner increasing the tensile strength of concrete. 4. Concrete preparations Various proportion of Cement:Sand:Aggregate are used to prepare concrete as per its importance and uses. Following are the various methods used for determining the volumetric proportion of concrete. Arbitrary Method Fineness modulus method Minimum voids method Maximum density method Water-cement ratio method. 5. Concrete mixes and purpose Type of concreteProportion of mixNature of work.M51 : 5 : 10Mass concrete for heavy walls, foundation, footings.M7.5 M101:4:8 1:3:6Mass concrete and foundations of less importance.M151:2:4For general RCC works (slab, beams, columns, etc.)M201 : 11/2 : 3Water retaining structures, piles, and general RCC structure.M251:1:2Heavily loaded RCC structure long span slab, beams etc. 6. Batching of Concrete There are twomethods of batching of concrete. 1. Volume batching 2. Weight batching Depending on the site conditions and importance of structure the suitable batching method is decided. 7. Weight Batching 8. Volume Batchers (Farma) 9. Procedure for preparing Concrete. After procuring the materials and deciding theproportion (mix-design) following are two methods of mixing: Hand mixing Machine mixing 10. Hand mixing In hand mixing similar to mortar cement : fineaggregates : Coarse aggregates. Are taken in desired quantity on water tight clean platform and dry mixed thoroughly 3 times. Then water is added as per the water cement ratio and thoroughly mixed by spade. It is suitable for only small quantities and where quality can be monitored on site. It has to be consumed within 30 mins of its preparation. 11. Advantages and Disadvantages of hand mixing of concrete. Advantages 1. Process is done on2. 3. 4. 5.site hence no time constraint can be prepared as per the site requirements. No power is required No heavy machinery is required Process is less noisy. Quality can be monitored on site always recommended to use 10% more cement then specified. Disadvantages 1. Cannot be adopted for2.3.4.large quantity of concrete. Since the mixing is done by hand proper supervision on mixing is also required The work area gets untidy and cleaning work is increased. Process consumes more time. 12. MACHINE MIXING 13. Machine mixing All the materials are added into the mechanicalmixer and rotated for certain time(1- 4 mins). The mixture is then taken out of the drum and transported. It is to be noted that if water is added 1st in the mixer it ensures evenly distribution of water in the mix. The speed and time of mixing is crucial(important) and has to be supervised because it affects the quality and strength of concrete. The concrete discharged from mixer should also be consumed within 30 mins. 14. Advantages and Disadvantages of machine mixing of concrete. AdvantagesDisadvantagesThe quality of concrete is good and fast. 2. Since mixing is done in machine; area is less untidy and cleaning is easy. 3. Since machine is movable(potable) concrete can be produced at various location on site reducing the Requires1.maintenance, cleaning is difficult, blades need to be checked for any damage. Requires power and is noisy. Skilled operator is required for operation. 15. Transportation of concrete. The concrete ready to use should be placed andfinished within 30 min of its preparation; hence transportation of concrete is also an important activity. The type of equipment for transportation of concrete depends on the nature of work, height above/below ground level and distance between point of preparation and placing of concrete. For ordinary work human ladder is formed and concrete is conveyed(passed on) in pans hand to hand. For important works various mechanical devices such as dumpers, trucks, pumps, hoist, etc. 16. Precautions to be taken while transportation of concrete. Concrete should be transferred such that there isno segregation of aggregates. Under no circumstances water should be added to concrete during its passage from mixer to formwork. Spillage and wastage should be minimum. It should be easily accessible at both the points. 17. Placing of Concrete After transporting the concrete another activity isplacing of concrete. Concrete needs to be placed with great care to avoid segregation and to achieve proper compaction. It is advisable to deposit small small quantity of concrete at various location rather than deposit huge chunk at one location. 18. Precautions to be taken while placing of Concrete. 1.2. 3. 4. 5.Formwork should be cleaned and well watered if applicable shuttering oil should be applied before laying reinforcement to avoid its contact with steel. Concrete to be deposited as near as possible to its final position. Concrete to be deposited from a maximum height of 1.5m. The mix should be hard enough or else it will start bleeding form the formwork. Concrete should be laid in layers ranging from 150 mm to 500 mm thick. 19. 1. Compaction of concrete to be done properly with various compacting equipments to avoid honey combing 2. No concrete should be placed after 30 mins of its preparation. 3. During placing care should be taken of reinforcement and cover . 20. Placing of concrete with boom placer. 21. Placing of concrete on slopes. 22. Placing of concrete on slopes. 23. Placing of concrete on slopes. 24. Placing of concrete on slopes. While placing on slopes some extra care hasto be taken following are the various points. 1. Workability of concrete should be less i.e. concrete should be stiff. 2. Concrete should be laid in such a manner that it does not flows downward under the action of gravity 3. Certain barriers should be made to resist its flow 4. Finishing should be done as soon as it gets settled (max after 1 hour). 5. Placing of concrete should always start from bottom. 25. Compaction of concrete. Compaction of concrete has to be done toeliminate air bubbles (voids) in concrete and obtain maximum density Also to achieve proper contact with reinforcement And to get proper shape of the structure by ensuring that concrete has reached every corners of the formwork shuttering. There are two basic methods of compacting 1. Hand Compaction. 2. Machine Compaction. 26. Hand Compaction of concrete. It is done for unimportant works byramming, tamping, spading, and slicing with suitable tools. Concrete should be little low in slump and highly workable Hand compaction is preferred because the use of vibrator may lead to the segregation of the aggregates. 27. Mechanical Compaction. Mechanical vibrators are machines used toachieve well compacted concrete It is easy to compact hard concrete with these methods Quality of concrete can be improved The time taken for compaction is less It is easy with vibrators to deposit well compacted concrete in complex designs of structure. Following are the four types of vibrators 1. Immersion vibrators 2. Surface vibrators 3. Shutter vibrators 4. Vibrating tables. 28. Immersion Vibrators 29. Surface Vibrators 30. Shutter Vibrators 31. Vibrating Tables These are the steel platforms mounted on flexiblesprings and they are operated by electromagnetic action or electric motors. The frequency of vibrations varies from 3000 to 7200 vibrations per minute. Frequency 1 / Time It means frequency is inversely proportional to the time of vibration i.e. if frequency is more the compacted concrete is achieved in less time. 32. Curing of Concrete. After attaining the final setting time fresh concreteis kept moist for certain period (min 7 days and max 23 days) so as to promote the hardening of cement (concrete). Cement contains C3S, C3A, C2S, C4AF these elements of cement react with water and hardens the concrete the content of these elements in different ratio determines the various property of cement Generally the 7 days curing is of utmost importance because initial 60-80 % strength of concrete is achieved in this period the other 4020 % strength of design concrete is achieved within 23 days. 33. Purpose of Curing It protects the finished surface fromdirect sun and wind. It helps in chemical reaction of concrete. It reduces the shrinkage, increases durability, impermeability 34. Methods of curing There are various methods but the main concernof every method is to reduce heat of hydration, the temperature should be kept minimum and the water loss should be prevented. 1. Ponding with water 2. Covering concrete with wet jute bags. 3. Covering concrete with wet sand, saw-dust, etc. 4. Covering concrete with polythene sheets. 5. Intermittent spraying / continuous sprinkling of water. 6. Applying curing compounds. 35. Application of curing compound. Various chemicals used are acrylates,chlorinated rubbers, bitumens, sodium silicates, linseed oils, wax, etc. Precautions for application of curing compound are 1. Concrete should be thoroughly wet, curing compound containers should be shaken well before use, it should be sprayed as soon as possible after casting of concrete, it should not be applied when water is on the surface. 36. Advantages of curing compound It relives from the continuous monitoring of structure. It is applied in a single operation and also aids in the quality control of concrete. They are used where there is shortage of water. They are useful in the areas which are nonaccessible for eg slip form tall chimneys etc. They are economical for industries manufacturing pre-cast products. 37. Workability of concrete The property of wet concrete to easily form andtake the shape of formwork/shuttering; its ability to get casted in the desired shape is known as workability. Generally to make a workable concrete water is added in access but it compromises with the strength of concrete. Water if added in required optimum quantity imparts maximum designed strength to concrete this is due to hydration of cement. If water is less reaction is incomplete cement gets wasted; if water is more reaction gets disturbed and again strength is disturbed. 38. Workability of concrete is achieved by addingIf the mix is too wet/flowable then by slightly changing the proportion of fine and coarse aggregate mix could be made dry. 2. If the mix is too stiff then by adding cement water paste of 1:1 the mix is corrected Various admixtures are also added to achieve workability in modern mix design. 1. 39. Hydration of cement The products of the reaction between cement and water are termed "hydration products." In concrete (or mortar or other cementitious materials) there are typically four main types: Calcium silicate hydrate: Calcium hydroxide - Ca(OH)2: AFm and AFt phases: Ettringite: 40. Hydration of cement 41. Hydration of cement 42. Finishing of concrete surface If proper compaction is done on concrete andproper curing then there is very less chances of improper finish. To increase the finish of surface concrete is prepared by adding more fines to it for e.g. slag, fly ash, porcelain etc.. It also increases the workability of concrete. Proper tamping on shuttering while laying of concrete and compaction also helps in achieving well finished surface.