RCC lecture Shivang 1.pdf
- 1. Fresh Concrete Prepared By:- Prof. Shivang N. Dabhi Assistant Professor, CED, SSASIT.
- 2. Definition • Fresh concrete or plastic concrete is a freshly mixed material which can be moulded into any shape. • The relative quantities of cement, aggregates and water mixed together, control the properties of concrete in the wet state as well as in the hardened state.
- 3. Workability The diverse requirement of partial of partial properties of concrete like mixability, stability, transportability, mobility, compatibility and finishability are collectively referred to as Workabilty. IS 6461 (Part – VII) 1973 defines workability as that property of freshly mixed concrete or mortar which determine the ease and homogeneity with which it can be mixed, placed, compacted and finished. As per Road Research Laboratory, U.K. workability is defined as “the property of concrete which determine the amount of useful internal work necessary to produce full compaction.”
- 4. Stability: Mix should not segregate during transportation and placing when it is subjected to forces during handling operation. Transportability: Capacity of concrete mix to keep the homogeneous concrete mix free from segregation during a limited time period of transportation. Mobility: Ability of concrete a mix to be placed in the form around the reinforcement and should be able to cast in to the required shape without losing continuity or homogeneity under the available techniques of placing Mixability: Ability of the mix to produce a homogeneous green concrete Compactability: Ability of concrete mix to be compacted in to a dense, compact concrete with minimum voids under the existing facilities of compaction at site.
- 5. Workability and Consistency • Workability: ease with which concrete can be moved and placed in forms without segregation; • Consistency: degree of wetness of concrete. Means how much wet the concrete is. • Consistency cannot measure workability but it can give indication of workability of concrete. • It is often said that increasing water increases workability of the concrete. However, this is not always true. Increasing water or degree of wetness doesn’t always increases the workability. If water is increased then there are greater chances of segregation.
- 7. Factors Affecting Workability • Water Content • Mix Proportions • Size of Aggregates • Shape of Aggregates • Surface Texture of Aggregate • Grading of Aggregate • Use of Admixtures • Time • Temperature
- 8. Water Content: • Water is lubricant for fresh concrete; • The addition of water to concrete increases the fluidity of concrete; • increases of water may lead segregation which result in less strength of concrete; • it is necessary to maintain W/C ratio. • Proper w/c ratio is responsible for proper strength of concrete.
- 9. Mix Proportions: • Aggregate/cement ratio is an important factor influencing workability. The higher the a/c ratio, the leaner is the concrete. • In lean concrete, less quantity of paste is available for providing lubrication, per unit surface area of aggregate and hence the mobility of aggregate is restrained. • concrete with lower a/c ratio, more paste is available to make the mix cohesive and fatty to give better workability.
- 10. Size of Aggregate: • The bigger the size of the aggregate, the less is the surface area and hence less amount of water is required for wetting the surface and less paste is required for lubricating the surface to reduce internal friction. • For a given quantity of water and paste, bigger size of aggregates will give higher workability. The above, of course will be true within certain limits.
- 11. Shape of Aggregate: • Rounded and cubical shaped aggregate – better workability; • Angular and rough aggregate – lesser workability; • Flakiness and elongation of aggregate also reduces workability
- 12. Texture of Aggregate: • Rough texture area - less workability; • Smooth aggregate – high workability; • Smooth aggregate offer less inter particle frictional resistance and contributes to higher workability.
- 13. Grading of Aggregate: • Well graded aggregate – least amount of voids in a given volume; • Less voids – higher workability; • Gap graded – medium workability; • Poor graded – less workability due to higher voids in sample;
- 14. Use of admixture • Use of plasticizers and admixture – increase workability; • Air entraining agents also plays important role in workability of concrete.
- 15. Time • Fresh concrete loses the workability with time because of loses of moisture due to evaporation. • A part of mixing water is absorbed by the aggregate or lost by evaporation and part of it is consumed in hydration of cement; • On an average a 125 mm slump of concrete may lose about a 50 mm slump in the first hour.
- 16. Temperature • Workability is also reduced at higher temperature.
- 17. Measurement of workability • Slump test • Compacting factor test • Flow test • Vee bee consistometer test • Kelly ball test
- 18. Slump test
- 19. Slum pattern No. Slump Workability 1 25-75 mm Low workability 2 50-100 mm Medium workability 3 100-150 mm High workability
- 20. Ranges of slump Placing Conditions Degree of workability Slump (mm) Building concrete, shallow sections, pavements using pavers Very low Depends on quality (around 25) Mass concrete, lightly reinforced section in slabs, beams, walls, columns, floors, canal lining, strip footing Low 25-75 Heavily Reinforced section in slab, beams, walls, columns Medium 50-100 Slip formwork, pumped concrete Medium 75-100 Trench fill, in situ piling High 100-150 Tremie concrete Very high Flow determination is required
- 22. Dimension for CFT apparatus Details Dimensions (mm) Upper Hopper Top internal diameter 250 Bottom internal diameter 125 Internal height 275 Lower Hopper Top internal diameter 225 Bottom internal diameter 125 Internal height 225 Cylinder Internal diameter 150 Internal height 285 Distance between two hopper and cylinder 200
- 23. Segregation
- 24. • Defined as separating out the ingredients of concrete mix, so that mix is no longer in a homogeneous and stable condition. 1. Coarse aggregate are separating out from the mix; 2. The paste separating out from the paste; 3. Water is separating out from the mix. 4. A homogeneous, cohesive and stable mix can be obtained by taking into consideration various parameters like grading of aggregate, size, shape and surface texture of aggregate with optimum water content ratio.
- 25. Causes of segregation • Bad proportioned mix; • In sufficient mixing of concrete with excess amount of water; • Dropping of concrete from height; • Discharging concrete against an obstacles like reinforcing bars, form works etc.; • When concrete is discharge from badly designed mixer, or from a mixer with wornout blades; • Conveyance of concrete by wheel borrow, conveyor belts, long distance haul by dumper etc..; • If too wet a mix is excessively vibrated, it is likely that the concrete gets segregated.
- 26. Control measure for segregation • Using correctly proportioned mix; • Use of certain workability agents, pozzolanic materials makes the mix cohesive and greatly help in reducing segregation; • Reducing the height of drop of concrete; • Concrete should not be caused to flow horizontally or discharged against a obstruction; • Reducing the continued vibration over a longer time than is optimum. • At any stage, if segregation is observed, remixing for a short time would make the concrete again homogeneous;
- 27. Bleeding • Is the form of segregation in which some of the water from the concrete comes out to the surface of the concrete. • It is sometime water gain; • Main causes are; – Highly wet mix; – Badly proportional mix; – Insufficiently mixed concrete; • Bleeding is due to rise of water in the mix to the surface because inability of the solid particles in the mix to hold all the mixing water during the setting of the particles under the effect of compaction.
- 28. • In case of lean concrete, water while rising up, create capillary channels. • If w/c ratio is more than 0.7, these capillary channels remain continuous and unsegment by the development of gel, increasing the permeability of concrete; • Bleeding water while coming from bottom to top, brings certain quantity of cement to the surface. The formation of cement paste at the surface is known as ‘Laitnce’ or ‘scum’; • Excess mortar at the top causes plastic shrinkage cracks; • Laitance formed on the roads produces dust in summer and mud in rainy season;
- 29. Remedies of bleeding • Using rich mixes; • Using finer cement; • Proper proportioning the mix; • Uniform and sufficient mixing of concrete; • Use of finely pozzolanic materials; • Use of air entraining agents is also effective in reducing bleeding
- 30. Workability and strength • The relation is inversely proportional.
- 31. Production of Concrete • Good quality of ingredients is not enough; • Proper mixture is essential; • The good quality concrete is a homogeneous mixture of cement, C.A., F.A., water and other admixture.
- 32. Stages of good quality production of concrete • Batching; • Mixing; • Transporting; • Placing; • Compacting; • Finishing; • Curing.
- 33. Batching • 1. Volume Batching • 2. Weigh batching
- 34. Volume batching
- 35. • Volume batching is not a good method for proportioning the material; • Material is taken by volume; • Gauge box or farmas is used for volume of material. • The volume of gauge box is made equal to one bag of cement means 35 liters. • Gauge box are made up of timber or steel plates; • Gauge should not be shallow it should deep enough; • When sand is moist, correction to the effect of bulking should be made; • Volume batching is preferable for small projects of concrete work
- 37. (ii) Weigh Batching: • Weigh batching is the correct method of measuring the materials. • Weigh batching system should be adopted. • Batching, facilitates accuracy, flexibility and simplicity. • Large weigh batching plants have automatic weighing equipment. • The use of this automatic equipment for batching is one of sophistication and requires qualified and experienced engineers. • Complication will come to adjust water content to cater for the moisture content in the aggregate.
- 38. Weigh batching equipment Manual Semi automatic Fully automatic
- 39. Bulk density of materials • Cement: 1.428 kg/lit; • F.A. : 1.5 kg/lit; • C.A. : 1.6 kg/lit
- 40. Mixing (i ) Hand mixing (ii )Machine mixing a. Tilting: 85 T, 100 T, 140 T, 200 T b. Non-Tilting: 200 NT, 280 NT, 375 NT, 500 NT, 1000 NT c. Reversing: 200 R, 280 R, 375 R, 500 R and 1000 R
- 41. Operations in Hand mixing • Prepare mixing platform; • Measured the quantity of sand; • Spread the cement on sand and mix it till colour of mixture is uniform; • Spread aggregate; spread the cement sand mixture and mix it at least three times by shovelling and turning over by twist from center to side, then back to center; • Make hollow in middle; add three quarters of total quantity of water required mix it.
- 42. Machine Mixing • According to the operating condition – Batch mixer – Continuous mixer • According to principle of mixing – Gravity type; – Type with forced mixing; • According to condition of use – Stationary concrete mixer – Portable concrete mixer
- 45. Transporting Concrete (a) Mortar Pan (b) Wheel Barrow, Hand Cart (c) Crane, Bucket and Rope way (d ) Truck Mixer and Dumpers (e) Belt Conveyors (f ) Chute (g) Skip and Hoist (h) Tansit Mixer (i ) Pump and Pipe Line ( j ) Helicoptor.
- 48. Placing Concrete (a) Placing concrete within earth mould. (example: Foundation concrete for a wall or column). (b) Placing concrete within large earth mould or timber plank formwork. (example: Road slab and Airfield slab). (c ) Placing concrete in layers within timber or steel shutters. (example: Mass concrete in dam construction or construction of concrete abutment or pier). (d ) Placing concrete within usual from work. (example: Columns, beams and floors). (e ) Placing concrete under water.
- 49. Compaction of Concrete (a) Hand Compaction (i ) Rodding (ii ) Ramming (iii ) Tamping (b) Compaction by Vibration (i ) Internal vibrator (Needle vibrator) (ii ) Formwork vibrator (External vibrator) (iii ) Table vibrator (iv ) Platform vibrator
- 50. (v ) Surface vibrator (Screed vibrator) (vi ) Vibratory Roller. (c ) Compaction by Pressure and Jolting (d) Compaction by Spinning. Continue…
- 51. • Compaction by Pressure and Jolting: This is one of the effective methods of compacting very dry concrete. • This method is often used for compacting hollow blocks, cavity blocks and solid concrete blocks. • The stiff concrete is vibrated, pressed and also given jolts. • With the combined action of the jolts vibrations and pressure, the stiff concrete gets compacted to a dense form to give good strength and volume stability. • By employing great pressure, a concrete of very low water cement ratio can be compacted to yield very high strength.
- 52. • Compaction by Spinning: • Spinning is one of the recent methods of compaction of concrete. • This method of compaction is adopted for the fabrication of concrete pipes. • The plastic concrete when spun at a very high speed, gets well compacted by centrifugal force. • Patented products such a “Hume Pipes”, “spun pipes” are compacted by spinning process.
- 53. General Points on Using Vibrators (a) Electric motors either driving the vibrator through flexible shaft or situated in the head of the vibrator. (b) Internal combustion engine driving the vibrator needle through flexible shaft, and (c) Compressed-air motor situated near the head of the vibrator.
- 54. Further Instructions on use of Vibrators • Height of Concrete Layer • Depth of Immersion of Vibrator • Spacing and Number of Insertion Positions • Speed of Insertion and Withdrawal of the Vibrating Head • Duration of Vibration • Vibrating Concrete at Junctions with Hardened Concrete
- 55. • Vibrating the Reinforced Concrete • Vibrating near the Formwork • Vibrating High Walls and Columns • Over-Vibration • Output of Immersion Vibrator • Re-vibration • Vibration of Lightweight Concrete Continue…
- 56. 56 Compacting Concrete WHAT IS COMPACTION: Compaction is done by shaking, or vibrating, the concrete which liquefies it, allowing the trapped air to rise out. The concrete settles, filling all the space in the forms WHEN TO COMPACT: Compaction must be done as concrete is placed, while it is still plastic. Never let concrete dry-out and stiffen because it will be too hard to compact WHY COMPACT: Properly compacted concrete is more dense, strong and durable. Off- form finishes will also be better.
- 57. 57 Compacting Concrete EXTERNAL VIBRATION Screeding: The Mechanical Screed: Concrete is screeded TWICE. The first screed levels the concrete roughly and compacts it. The second screed levels and compacts the concrete more. The screed is pulled along the top of the forms by two workers.
- 58. 58 Compacting Concrete INTERNAL VIBRATION is done with a mechanical vibrator or poker vibrator. The POKER is put into concrete and vibrates it from the inside. Method of internal vibrations: Make 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 otherwise a hole, or weak spot, may be left in the concrete
- 59. 59 Compacting Concrete INTERNAL VIBRATION • The SIZE of the poker determines how much concrete is vibrated at one time. • The area vibrated at one time is called the RADIUS OF ACTION. • This can be seen by over what radius air bubbles rise to the surface. • The radius of action will be greater with a LARGER poker and more- workable concrete. • Always compact in a definite pattern so the radius of action overlaps and covers the whole area of the concrete.
- 60. 60 Compacting Concrete PRECAUTIONS: 1.Taking the poker out TOO QUICKLY will leave a hole in the concrete. 2.To close the hole, vibrate near the hole and take the poker out VERY SLOWLY. 3.NEVER touch the form face with the poker as it can damage the formwork and the concrete. 4.NEVER touch the reinforcement with the poker. 5.NEVER spread or move concrete sideways with the poker, always use a shovel.
- 61. 61 Compacting Concrete HOW LONG TO COMPACT: For concrete of average workability ( slump of 60 mm) with a poker size between 25–75 mm, concrete should usually be vibrated for between 5 and 15 seconds. It is worse to UNDER-VIBRATE than to OVER-VIBRATE concrete.