Unit i (1.2)
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The document provides information on various topics related to advanced construction technology: 1. It discusses different types of admixtures used in concrete, their functions, classifications and typical quantities used. 2. Lightweight concrete and the production of lightweight aggregates are described. 3. Shotcrete/gunite installation techniques and special concretes like ferrocement and fiber reinforced concrete are outlined. 4. Principles of pre-stressed concrete including materials, methods, advantages and causes for losses in pre-stress are summarized.
Unit i (1.2)
- 2. UNIT I ➢ 1.2 MODIFIED CONCRETE ➢ Admixtures – definition – function – classification - uses of different types - quantity to be used ➢ light weight concrete - light weight aggregate - production of light weight aggregate ➢ shotcrete or guniting – definition - typical arrangement for gunite system ➢ special concrete – Ferro cement- production process – curing - advantages and limitations fibre reinforced concrete - production process – uses ➢ Pre-stressed concrete - General principle of pre stressing - advantages of pre stressed Concrete - materials used - methods of pre- stressing - steel used - pretension method - post tension method - system of prestressing - freyssinet system - Magnel Blaton system - Leemc-call system - Causes for losses in pre-stress – remedial measures – Composite member
- 3. Admixture • Chemicals added along with concrete ingredients (fine aggregate, coarse aggregate, cement and water) to alter the properties of fresh and harden concrete.
- 4. Admixture functions & uses • Accelerate initial setting time • Retard initial setting time • Inc. strength • Inc. durability • Improve workability • Reduce heat of evolution • Improve pumpability and penetration • Reduce segregation • Inc. bond b/w old and new concrete • Inc. resistance to chemical attack • Produce non skid surface • Dec. wt. of concrete • Dec. capillarity flow • Produce colored concrete
- 5. classification • Accelerator • Retarders • Water reducing accelerators • Water reducing retarders • Air entraining agents • Water proof • Plasticizers • Super plasticizers
- 6. Sl.no. Admixture Functions Quantity used Chemicals used 1. Accelerators Rapid gain of strength 0.1% Calcium chloride, Sodium nitrite 2. Retarders Delay setting 0.1 to 0.28% of cement wt. Sugar, hydroxy carboxylic acid 3. Water reducing accelerator Inc. workability with gain of faster strength <0.25% Cacl2 & ligno sulphonate 4. Water reducing retarders Inc. workability & delay setting 0.1 – 0.25% Sugar, hydroxy carboxylic acid & ligno sulphonate 5. Water proofers Reduce permeability, prevent water entering - Potash soaps, butyl stated petroleum wax 6. Air entraining Entrainment of air into concrete 15- 130ml for 100kg ligno sulphonate & wood resin fats 7. Plasticizers Inc. workability 0.1-0.4% Calcium &sodium ligno sulphonate 8. Super plasticizers Greatly inc. workability 0.5-3% Sulphonated melamine formaldehyde resin
- 7. Light weight concrete • Ordinary concrete density 24000 kg/m3 • Light weight concrete density 300 – 1850 kg/m3
- 8. Applications of light wt. concrete • Precast floors & roof panels • Partition wall • Precast composite wall • Filler wall (precast rcc panel wall) • Load bearing masonry walls • Roof and floor slab
- 9. Advantage of light wt. concrete • Reduce dead load • Inc. progress of work • Decrease material cost • Tall structure in weak soil • Economy • Low thermal conductivity • Use of industrial waste
- 10. Light wt. aggregate • Used in light wt. concrete production • Weighs less than 1100 kg/m3 • Less wt. due to high internal porous micro structure • Two types • Natural • Artificial
- 12. • Natural light weight aggregate are not found in many places and they are also not in uniform size and quality • Pumice is most commonly used one • Artificial aggregates are derived from industrial waste such as fly ash, slag, etc.,
- 13. Production of light wt. aggregate • Cement & pulverized sand are first mixed in proportion • Then the mixture is made into slurry with addition of predestined quantity of water • Then the slurry is foamed with foam compounds • Then the foam product is poured into moulds • Then the mould is cured under elevated hydrothermal condition in autoclave to impart strength, reduce shrinkage & gives creamy color
- 14. Shotcrete or guniting • Shotcrete refers to spraying of concrete in wet mix process • guniting refers to spraying of concrete in dry mix process • Excellent binding in nature makes layers very strong • Require less formwork • Economical than conventional one • Applied in canal, tunnels of thin vertical & horizontal surface • Repair and restoration of old & fire damaged structures, water proofing walls
- 16. Typical arrangements • Gunned surface should be cleaned by applying either air blast or high pressure water jet • For removing loose rust in reinforcement sand blasting is done • Surface is likely to absorb water so it should be kept wet up to 6 hours before guniting • Mix ratio 1:3 to 1:4.5 with water cement ratio 0.3 • Max. size of sand is limited to 10mm
- 17. • Each layer of gunite is provided with a spot welded wire mesh fabric of 5mm to reduce initial shrinkage and prevent cracks in freshly placed material • Due to the good bond between reinforcement and gunite it act as a part of structure
- 19. Special concrete • Special Concrete means the concrete used or made for special cases for which special properties are more important than those commonly considered. • Sometimes, it may be of great importance to enhance one of the ordinary properties. • These special applications often become apparent as new development using new materials or as improvements using the basic materials. Some utilize special aggregates (lightweight aggregate, steel fiber, plastic fiber, glass fiber, and special heavy aggregate).
- 21. Uses and Applications of Special Concrete • Special concrete is used in extreme weather. • Good cohesiveness or sticky in mixes with very high binder content • Some delay in setting times depending on the compatibility of cement, fly ash and chemical admixture • Slightly lower but sufficient early strength for most applications • Comparable flexural strength and elastic modulus • Better drying shrinkage and significantly lower creep • Good protection to steel reinforcement in high chloride environment • Excellent durability in aggressive sulphate environments • Lower heat characteristics • Low resistance to de-icing salt scaling • PC pipes with good resistance to chemical attack from both acidic and caustic effluents inside the pipe, and from chemical attack on the outside of the pipe
- 22. Ferro cement concrete • Consisting of cement mortar and wire mesh • Closely spaced wire mesh with rich cement mortar mix • Wire mesh is 0.5 to 1mm dia. wire at 5mm to 10 mm spacing • Cement mortar 1: 2 or 1 : 3 with W/C ratio of 0.4 to 0.45 • Elements wide 2 to 3 cm and thickness 2 mm to 3 mm • Steel content 300 to 500 kg per cubic meter
- 23. Production or casting methods • Hand plastering • Semi mechanized process • Centrifuging • guniting
- 24. Hand plastering • Reinforcement cage (bent to required shape) is made using small dia. steel rod • Then required number of wire mesh layer are tied to the reinforcement cage • Mortar is dashed from outside against the plain curved G.I sheets held on other side • Flexible G.I sheet is move all around the mortar is dashed • The closely spaced wire mesh adhere the mortar when dashed • Results in slightly increased in thickness
- 25. Semi mechanized process • Using hand plastering over form work • One layer wire mesh is wounded over inner cylindrical mould • Over this 4mm wire is fixed at spacing of 15 cm in both directions • Again one layer of chicken mesh is wounded over that • This forms complete reinforcement system • Cement plastering is done layer by layer • Mesh is tightly wound round the formwork so the thickness is reduced
- 26. centrifuging • Adopted for cylindrical units like pipe • The mild steel reinforcement cage is replaced by wire mesh layer cage • Because of compaction it can be used as pressure pipes
- 27. guniting • Process adopted for applying the mortar to the wire mesh system • Applied properly by experienced gunman to give good compact and uniform surfaces • Suitable for mass production of pre fabricated units
- 28. curing • Process by which loss of water from concrete is prevented • Methods • Water / moist curing • Membrane curing • Steam curing • Miscellaneous
- 29. Water curing • Exposed concrete surface shall be kept continuously in a damp or wet condition for atleast 7 days from the date of placing of concrete • Ponding is done for basement, slabs • Precast concrete members can be immersed in water tanks
- 30. Membrane curing • Coated with approved curing compounds • Applied to all exposed surface of concrete as soon as possible after the concrete sets. • Used in hot weather and water supply short conditions
- 31. Stream curing • Hydration accelerate with temperature
- 32. Miscellaneous curing • Calcium chloride • This absorbs moisture from surroundings and retain at concrete surface • Keep wet for concrete long time
- 33. Curing advantage and limitation • Protect concrete surface from sun and wind • Presence of water is essential to cause chemical action of concrete to set • Efficient curing is done strength is increased gradually with age • Shrinkage reduced • Durability and impermeability increased • Abrasion resistance increased • Keep concrete moist and warm for continue of hydration
- 34. Curing advantage and limitation • Improved bond and strength increased • More strong to withstand plastic shrinkage • Avoids cracking • Generation of heat is reduced
- 35. Period of curing • Depends upon type and nature of work • 7-14 days • Rapid cement considerably reduced curing period
- 36. Fiber reinforced concrete • Fiber Reinforced Concrete can be defined as a composite material consisting of mixtures of cement, mortar or concrete and discontinuous, discrete, uniformly dispersed suitable fibers. • Fibers include steel fibers, glass fibers, synthetic fibers and natural fibers. • Fiber is a small piece of reinforced material • Circular or flat • Prevent cracks formed due to shrinkage in concrete
- 37. Production of fiber reinforced concrete • Cement = 325 – 550 kg/cm3 • w/c ratio = 0.4-0.6 • % sand = 50 – 100% • Max. aggregate size = 10mm • Fiber content = 0.5 -2.5%
- 38. Glass fibers • Improve concrete strength at low cost. • Adds tensile reinforcement in all directions, unlike rebar. • Add a decorative look as they are visible in the finished concrete surface.
- 39. Polypropylene and nylon fibers • Improve mix cohesion, improving pumpability over long distances • Improve freeze-thaw resistance • Improve resistance to explosive spalling in case of a severe fire • Improve impact– and abrasion–resistance • Increase resistance to plastic shrinkage during curing • Improve structural strength • Reduce steel reinforcement requirements • Improve ductility • Reduce crack widths and control the crack widths tightly, thus improving durability
- 40. Steel fibers • Improve structural strength • Reduce steel reinforcement requirements • Reduce crack widths and control the crack widths tightly, thus improving durability • Improve impact– and abrasion–resistance • Improve freeze-thaw resistance
- 41. Uses of fiber reinforced concrete • Road pavements • Bridge decks • Canal lining • Industrial flooring • Explosive resistant structure • Refractory lining etc.
- 42. Pre stressed concrete • It is substantially "prestressed" (compressed) during production • This compression is produced by the tensioning of high-strength "tendons" located within or adjacent to the concrete and is done to improve the performance of the concrete in service. • Tendons may consist of single wires, multi-wire strands or threaded bars that are most commonly made from high-tensile steels, carbon fiber
- 44. Uses of prestressed concrete • longer spans, • reduced structural thicknesses, and material savings compared with simple reinforced concrete. • high-rise buildings, • residential slabs, • foundation systems, • bridge and dam structures, • silos and tanks, • industrial pavements and nuclear containment structures.
- 45. Advantage of pre stressing • Eliminates cracking in all types of loading • Possibility of steel to rust and concrete detoriate is minimized • Resistant to impact, vibration and shock • Dead load are reduced so foundation are cost saving • Curved tendons are shear resistant • Steel quantity of 1/3 of ordinary concrete • Low deflection • Economical for long span structures
- 46. Materials used for prestressed concrete • Cement • Concrete • High strength concrete (M40) • Steel • High tensile steel (200mpa)
- 47. Terms in prestressed concrete Tendon • basically a steel cable or wire used in Prestressed Concrete structural elements like beam, column etc. • It is a medium through which tensile stresses are induced into the concrete. • It maybe one single steel wire or a group of wires twisted together to pass through required amount of tensile stresses.
- 48. Anchorage • used to anchor the tendons into the concrete while terminating or joining two tendons. • Main function of anchorage is to transfer the stressing force to the concrete once the stressing process is completed. • Anchorage is an inevitable part of prestressing system. • Efficiency of anchorages affects the service life of a prestressed structure.
- 49. Pre tensioning • Tendon is stressed before concrete is placed
- 51. Post tensioning • Tendon are stressed after concrete harden
- 61. causes • Due to elastic deformation of concrete • Creep in concrete • Shrinkage in concrete • Friction loss • Slip of anchorage • Creep in steel
- 62. Remedial measure • Providing prefect and slip free anchorage • Early setting concrete for good bond b/w tendon and concrete • Prevent shrinkage and creep of concrete by rich mix, effective mixing, pouring and consolidation
- 63. Composite member • Made of two or more materials • Cost of steel appreciable • Easy and fast execution of work