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Repair and Rehabilitation

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Concrete Technology

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Repair and Rehabilitation

  1. 1. Repair and Rehabilitation Concrete Technology
  2. 2. Syllabus • Distress in structure: Causes and precautions, damage assessment of structural elements, repairing techniques and repairing materials.
  3. 3. Distress In Structure • Distress means Damage • Concrete may suffer distress or damage during its life period due to a number of reasons. Because of the varying conditions under which it is produced at various locations, the quality of concrete suffers occasionally either during production or during service conditions resulting in distress.
  4. 4. Distress In Structure Causes of distress of concrete: • Structural causes – Externally applied loads – Environmental loads – Accidents – Subsidence's, etc. • Error in design and detailing • Poor Construction practices • Construction Overloads • Drying Shrinkage • Thermal Stresses • Chemical Reactions • Weathering • Corrosion
  5. 5. Distress In Structure
  6. 6. In addition to the distress in hardened concrete, the plastic concrete may also suffer damage due to, • Plastic Shrinkage • Settlement Cracking • Early removal of formwork • Improper design of formwork.
  7. 7. Distress In Structure
  8. 8. Evaluation Procedure for Repair and Strengthening of Concrete Structures • Before finalizing any scheme for repairs and rehabilitation of a distressed concrete structure, the concerned engineer has to be fully aware of the causes of causes of distress, extent of damage to the structure and the present condition of the concrete in the structure for repairs to be effective and lasting. The extent of distress has to be categorized so that the repair schemes can be formulated according to the distress in a particular structural element. So that, pre-repair evaluation and assessment of a structure is pre-requisite for working out effective repair schemes.
  9. 9. Evaluation Procedure for Repair and Strengthening of Concrete Structures • Once the repairs have been carried out on a distressed structure, the post repair evaluation and assessment of the structure can be carried out for checking the efficacy of the repair. The post repair assessment is a tool with the engineer to evaluate whether the parent material and the repair material have obtained bond or whether the cracks or the voids have been filled up by the grouting materials. Thus, any scheme for effective repairs can be based on the pre-repair and post- repair evaluation of concrete structures
  10. 10. Tools for Evaluation of Concrete Structures • The various tools available for evaluation of concrete structures are as follows: • Visual inspection and observations • Questioning of concerned personnel • Scrutiny of field data and records • Design Checks • Non- destructive testing (NDT) • Extraction of cores and testing • Supplementary laboratory techniques • Load testing of a structural member • The general approach adopted for pre-repair evaluation of distressed concrete structures is given below:
  11. 11. Visual Inspection and Observations • The first step in the process of evaluation of a distressed concrete structure is visual inspection and observations. A through visual inspection and observations. A through visual inspection leads to proper approach to be adopted during investigation. It determines the number of field and laboratory tests required to be carried out. Visual inspection generally include the study of the following.
  12. 12. Visual Inspection and Observations • Ambient conditions • Crack width and patterns • Spelling of Concrete • Color, texture and rust stains • Sinking of columns • Failure of beam-Column junctions. • Mal-functioning of machinery, structural components etc. • Condition of fixtures • Deposits/ splashes on structural components.
  13. 13. Questioning of Personnel/ Scrutiny of field Data and Records • The questioning of personnel and the scrutiny of field data and records is carried out for the following: • Grade of concrete adopted • Cube test results • Type of material and sources • Constructional details • Environmental Conditions
  14. 14. Questioning of Personnel/ Scrutiny of field Data and Records • The Scrutiny of the grade of concrete and cube test results brings out adequacy of strength of concrete and the degree of quality control exercised during constructions. • The study of the type of materials used particularly cement, coarse aggregate, fine aggregate, additives etc. also focuses the direction of investigations. The scrutiny of other constructional details e.g. removal of formwork, shifting of formwork for slip form construction, the height of pouring of concrete, use of compaction devices etc. are useful information for further investigation in many cases.
  15. 15. • Rebound hammer test • Ultrasonic pulse velocity test • Pull- out test • Pull- off test • Carbonation test • Cover measurement • Break off test • Endoscopy • Radar techniques • Rapid chloride, alkali and sulphate kits, etc.
  16. 16. Questioning of Personnel/ Scrutiny of field Data and Records • Scrutiny of field data and records: • Design checks: • Non-Destructive testing (NDT): • Any visual inspection and scrutiny of the field data, the in-situ testing is carried as per the approach finalized. Various in-situ non- destructive tests available are:
  17. 17. • X-ray diffraction analysis • Differential thermal analysis • Chloride permeability test • Optical and Scanning microscopy • Chloride permeability test. • Compressive strength, density and modulus of elasticity determination on core samples, etc. • Load testing of a structural member: • Extraction of Cores and testing:
  18. 18. In addition to various in-situ tests carried out, it becomes necessary to support the findings with laboratory tests • The laboratory tests generally adopted are: • Cement Content of Hardened Concrete • Chemical Analysis • Chlorides • Sulphates • pH • Nitrates
  19. 19. Corrosion of Reinforcement in Concrete The damage to the concrete due to corrosion of reinforcement is considered to be one of the most serious problems. It is an universal problem and property worth of crores of rupees is lost every years. Due to corrosion problem in bridges, buildings and other RCC structures, India incurs heavy loss of about Rs. 1500 cores annually. This paper deals with various causes of corrosion and remedial measures thereon.
  20. 20. Corrosion of Reinforcement in Concrete
  21. 21. Corrosion of Reinforcement in Concrete
  22. 22. Corrosion Process and Mechanism: • Corrosion of reinforcement steel is a complex phenomenon involved chemical, electrochemical and physical process. When reinforcement steel rusts, the volume of iron oxide formed is 2-3 times greater than the steel corroded, which results in bursting stresses in the concrete surrounding the bar. This causes cracking, spalling and delimination of concrete. Another consequence of corrosion is reduction in cross- sectional area of the steel at anode, thus reducing its loads carrying capacity.
  23. 23. Causes of Corrosion and Remedial Measures • Various causes of corrosion and remedial measures are discussed below: • Presence of Cracks in Concrete: Certain amount of cracking always occurs in the tension zone of RCC depending upon the stresses in the reinforcing steel. Through these cracks, oxygen or sea water ingress into concrete and set up good environment for corrosion of reinforcement. Maximum permissible width of elastic cracks in RCC members would depend upon environmental and other factors. For normal environmental conditions, a maximum crack width of 0.3 mm for protected internal members and 0.2 mm for unprotected external members may be recommended.
  24. 24. Presence of Cracks in Concrete
  25. 25. Causes of Corrosion and Remedial Measures • Presence of moisture: Presence of moisture is a precondition for corrosion to take place because concrete can act as electrolyte in electrochemical cell only if it contains some moisture in pores. Corrosion can neither occurs in dry concrete or in submerged concrete. • The worst combination for corrosion to process is when the concrete is slightly drier than saturated i.e. about 80 - 90 % relative humidity with a low resistivity and the oxygen can still penetrate to the steel. Hence in high humidity areas like coastal India, low permeability concrete is recommended.
  26. 26. Causes of Corrosion and Remedial Measures Permeability of Concrete: • This is also an important factor affecting corrosion of reinforcement. Ingress of moisture, sea water, oxygen, CO2 etc. is easier in porous concrete than in dense and impermeable concrete. It is worth mentioning than with each increases of W/C ratio 0.1, permeability of concrete increases 1.5 times. Poor curring increases permeability 5 to 10 times in comparison to good cured concrete and poor compaction increases permeability 7 to 10 times in comparison to good compacted concrete. • For this consideration, quantity of cement in concrete should not be less than 350 kg/ m3 and W/C ratio should not exceed 0.55 for ordinary structures and 0.45 for marine structures. All other normal requirements of good quality concrete, namely, grading, and cleanliness of aggregates, through mixing, proper compaction and curing should be taken care of.
  27. 27. Permeability of Concrete
  28. 28. Causes of Corrosion and Remedial Measures • Carbonation: Hydrated cement paste forms a thin passivity layer of Gamma iron oxide (Fe2 O3) strongly adhering to the underlying steel and gives complete protection from reaction with the oxygen and water, that is from corrosion. • Hydration of cement liberates some calcium hydroxide which sets up a protective alkaline medium inhibiting electrochemical cell action and preventing corrosion of reinforcement.
  29. 29. Carbonation • Carbon Dioxide (CO2) from the atmosphere diffuse inside the concrete, react with calcium hydroxide (Ca (OH)2) to form calcium carbonate which is water soluble. This reaction is known as carbonation. Carbonation lowers the alkalinity of concrete and reduce its effectiveness as protective medium. The pH value of pore water in concrete is generally between 10.5 to 12.0 but if due to carbonation it is lowered to 9.0 and below, the medium converts to acidic type and corrosion of reinforcement begins.
  30. 30. Carbonation
  31. 31. Causes of Corrosion and Remedial Measures • Chlorides: Chlorides can enter in the concrete during concreting or during service conditions. During concreting, the chloride can enter via aggregates, gauging water and admixtures like (CaCl2) In service conditions, the chloride ions entry is due to ingress of sea water, de-icing and other salts. The chloride ions (Cl-) attack the iron oxide film leading to corrosion. Chloride ions activate the surface of the steel to form an anode, the passivated surface being the cathode. The rate of corrosion depends upon chloride ion concentration.
  32. 32. Chlorides
  33. 33. Causes of Corrosion and Remedial Measures • Sulphate Attack: Solubility sulphates like sodium, potassium, magnesium and calcium are sometimes present in soil, ground water or clay bricks, react with tricalcium aluminate- 3CaO,Al2O3 (C3A) content of cement and hydraulic lime in the presence of moisture and from products which occupy much of cement and hydraulic lime in the presence of moisture and from products which occupy much bigger volume than the original constituent. This, expansive reaction results in weakening of concrete, masonry and plaster and formation of cracks as well as corrosion of reinforcement. • Severity of sulphate attack depends upon amount of soluble sulphate present in soil, water or clay bricks, permeability of concrete, amount of C3 A content in cement and duration for which concrete remains damp.
  34. 34. Sulphate Attack
  35. 35. Causes of Corrosion and Remedial Measures • Alkali Aggregate Reactions: OPC contains alkalies like sodium oxide (Na2 O) and potassium Oxide (K2O) to some extent these alkalies chemically reacts with reactive siliceous minerals in some aggregate and cause expansion, cracking and disintegration of concrete give rise to the corrosion of reinforcement. • Preventive Measure Consists of: • Avoid use of alkali-reactive aggregate in concrete. • Cement with alkali content more than 0.6 % should not be used. • Portland Pozzolana cement is recommended.
  36. 36. Alkali Aggregate Reactions
  37. 37. Causes of Corrosion and Remedial Measures • Inadequacy of Cover: If Concrete cover to reinforcement is inadequate, reinforcement is liable to get corroded soon due to various factors such as Carbonation, ingress of sea water, moisture penetration etc. It is therefore necessary that RCC works should have a minimum clear cover as recommended by IS 456: 2000. Reinforcement shall have concrete cover and thickness of such cover (exclusive of plaster or other decorative finish) shall be as follows:
  38. 38. Causes of Corrosion and Remedial Measures • At each end of reinforcing bars not less than 25 mm nor less than twice the dia. of such bar. • For longitudinal reinforcing bars in column, not less than dia. of such bar. In case of columns of dimensions 200 mm or under, whose reinforcing bars do not exceed 12 mm in dia. a cover of 25 mm may be used. • For longitudinal reinforcing bars in beams, not less than dia. of such bar. • for tensile, shear, compressive or other reinforcement in a slab, not less than 15 mm, not less than dia. of such bar.
  39. 39. Causes of Corrosion and Remedial Measures • Increased cover thickness may be provided when surface of concrete members are exposed to the action of harmful chemicals (as in case of concrete in contact with earth faces contaminated with such chemicals) acid vapours, saline atmosphere, sulphurous smoke etc. and such increase of cover may be between 15 mm to 40 mm beyond. • For RCC members totally immersed in sea water, cover shall be 40 mm more than the normal cover.
  40. 40. Inadequacy of Cover
  41. 41. Causes of Corrosion and Remedial Measures • In all such cases cover should not exceed 75 mm. Based on part research, concrete, cover more than 50 mm is, however, not recommended as it give rise to increase crack width which may further allow direct ingress of deleterious materials to the reinforcement. • Apart from the remedies discussed above other preventive measures suggested in various literature are: • Application of protective coating • Modification of concrete • Change in metallurgy of reinforcing steel • Cathodic protection system
  42. 42. Causes of Cracks in Concrete • The principle causes of cracking are discussed below. It will be benefit of informative to professional working either for design, construction or maintenance and repair. • Temperature and Plastic Shrinkage: • It is often seen relatively straight parallel with the span of floors. This is mainly with one way slabs for corridors of large length and is due to inadequate provision of distribution steel. IS 456-2000, suggests that minimum reinforcement in slabs in either direction shall not be less than 0.15 % for mild steel reinforcement and 0.12 % for high strength deformed bars, of the total cross sectional area, to avoid shrinkage cracks.
  43. 43. Causes of Cracks in Concrete • Plastic shrinkage cracking occurs when subjected to a very rapid loss of moisture caused by combination of factors which include air and concrete temperature, relative humidity, and wind velocity at the surface of the concrete. These factors can combine to cause high rates of surfaces evaporation in either hot or cold weather. • When moisture evaporates from the surface of freshly placed concrete faster than it is replaced by bleeding water, the surface concrete shrinks, Due to the restraint provide by the concrete below the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete, resulting in shallow cracks of varying depth which may form are often fairly wide at surface. Plastic shrinkage cracks begins as shallow cracks but can become full depth cracks. • It is usual to see a crack parallel to main steel 4 to 7 m apart. This particularly creates problem when the slab is for terrace as leakage starts from these cracks only.
  44. 44. Cracks Repair By Routing and Sealing • The Crack sealers should ensure the structural integrity and service ability. In addition they provide protection from the ingress of harmful liquids and gases. • Routing and sealing of cracks can be used in condition requiring remedial repair and where structural repair is not necessary. The method consists of enlarging remedial repair and where structural repair is not necessary. The method consists of enlarging the crack along its length on the exposed surface, called chasing or routing, and sealing it with a suitable joint sealant.
  45. 45. Cracks Repair By Routing and Sealing • This is a common technique for crack treatment and is relatively simple in comparison to the procedures and the training required for epoxy injection. The procedure is most applicable to flat horizontal surfaces such as floors and pavements. However, this method can be accomplished on vertical surfaces as well as on curved surfaces. • This method is used to repair both fine pattern cracks and larger, isolated cracks. A common and effective use is for waterproofing by sealing cracks on the concrete surface where water stands, or where hydrostatic pressure is applied.
  46. 46. Cracks Repair By Routing and Sealing • The sealant may be of several materials, including epoxies, silicones, urethanes, polysulfides, asphaltic materials polymer mortars. Cement grouts should be avoided due to the likelihood of cracking. For floors, the sealant should be sufficiently rigid to support the anticipated traffic. • The procedure consists of preparing a groove at the surface ranging in depth, typically from 6 to 25 mm. A concrete saw, hand tools or pneumatic tools may be used. This groove is then cleaned by plastic or sir blasting and allowed to dry. A sealant is placed into the dry groove and allowed to cure.
  47. 47. Cracks Repair By Routing and Sealing
  48. 48. Cracks Repair By Routing and Sealing
  49. 49. Crack Repair by Stitching • The stitching procedure consists of drilling holes on both sides of the cracks, cleaning the holes and anchoring the legs of the stitching dogs that span the crack, which either a non-shrink grout or an epoxy- resin-based bonding system. The stitching dogs should be variable in length and orientation or both, and should be so located that the tension transmitted across the crack is not applied to a single plane but spread over area. • Stitching may be used when tensile strength must be reestablished across major cracks. Stitching a crack tends to stiffen the structure and the stiffening may increase the overall structural restraint, causing the concrete to crack elsewhere.
  50. 50. Crack Repair by Stitching
  51. 51. Providing Additional Reinforcement • The cracked reinforced concrete bridge gird can be successfully repaired by using epoxy injection and reinforcing bars. This techniques consists of sealings the crack, drilling holes of 20 mm diameter that intersect the crack plane at approximately 90 0, filling the hole and crack with injected epoxy and placing a reinforcing bar into drilled hole. Typically, 12 to 16 mm diameter bars extending at least 500 mm on each side of the crack are used. The epoxy bonds the bar to the sides of the hole. The epoxy used to rebond the crack should have a very low viscosity.
  52. 52. Drilling and Plugging • This method consists of drilling down the length of the crack and grouting it to form a key. A hole, typically 50 to 75 mm in diameter should be drilled, centered on and the following the crack. The drilled hole is then cleaned, made tight and filled with grout. The grout key prevents transverse movements of the sections of concrete adjacent to the crack. The key will also reduce heavy leakage through the crack and loss of soil from behind a leaking wall. • When structural strength is not the criteria but water- tightness is essential, the drilled hole, should be filled with a resilient material of low modulus in lieu of grout. If the keying effect is essential, the resilient material can be placed in a second hole, the first being grouted.
  53. 53. Drilling and Plugging
  54. 54. Cracking Repair by Prestressing Steel • When a major portion of a member is to be strengthened, or a crack is to be closed, post- tensioning is often the desirable solution. The technique uses prestressing strands or bars to apply a compressive force. Adequate anchorage must be provided for the prestressing steel. The method of correction crack in slab and beam.
  55. 55. Cracking Repair by Pre-stressing Steel
  56. 56. Cracking Repair By Grouting • Based on grouting material used, there are three methods: • Portland Cement Grouting • Chemical Grouting • Epoxy Grouting
  57. 57. Portland Cement Grouting • Wide Cracks, particularly in gravity dams and thick walls may be repaired by filling with portland cement grout. This method is effective in preventing water leakage, but will not structurally bond cracking sections. The procedure consists of cleaning the concrete along the crack by air jetting or water jetting; installing grout at suitable intervals, sealing the crack between the seats with sealant; flushing the crack to clean it and test the seal; and then grouting the whole area. Grout mixtures may contain cement and water or cement plus sand and water, depending upon the width of the crack. Water reducers or admixtures may be used to improve the properties of the grout. For large volumes, a pump is used and for small volumes, a manual injection gun may be used. After the crack is filled, the pressure should be maintained to ensure proper penetration of grout.
  58. 58. Portland Cement Grouting
  59. 59. Chemical Grouting • Chemicals used for grouting are sodium silicates, urethanes and acrylamides. Two or more chemicals are combined to form gel, a solid precipitate or a foam as opposed to cement grouts that consists of suspension of solids particles in a fluid. The advantages of chemical grouts include applicability in moist environments and their ability to be applied in very fine cracks.
  60. 60. Chemical Grouting
  61. 61. Epoxy Grouting
  62. 62. Column Jacketing • Column Jacketing is done to improve the load carrying capacity of the column. The procedure followed is: • Open the footing of the column by excavating soil around it. • Remove the plaster from the surface of the column. • Make the surface of column concrete rough by sand blasting. • Remove the corroded bars by cutting them. Add new bars from footing to the slab as per the instruction of engineers. • Apply bonding agent on the old concrete for proper bonding between old and new concrete. • Erect necessary shuttering around the column. • Pour minimum M-25 grade of concrete, vibrate and cure it.
  63. 63. Column Jacketing
  64. 64. Beam Jacketing • Before taking up the strengthening of a beam, the load acting on it should be reduced by removing the flooring tiles and bed mortar from the slab. Props are erected to support the slab. After clipping off the existing plaster on the beam, additional longitudinal bars at the bottom of the beam to- geather with new stirrups are provided. Stirrups are inserted by making holes from the slab. The longitudinal bars are passed through the supporting columns through holes of appropriate diameter drilled in the columns. The spaces between bars and surrounding holes are filled with epoxy grout to ensure a good bond.
  65. 65. Beam Jacketing • The surface of old concrete is cleaned by air jetting. Expanded wire mesh is fixed on the two sides and bottom of the beam. To ensure a good bond between old concrete and new polymer modified concrete, an apoxy bond coat is applied to the old concrete surface. The polymer modified mortar is applied, while the bond coat is still fresh. Sometimes 2 to 3 coats of polymer modified mortar are applied to achieve desired thickness. The mortar is cured for appropriate period in water. Epoxy resin grout is injected in the cracks along top of beams.
  66. 66. Beam Jacketing
  67. 67. Questions • State causes and precautions for distress in structure. • State new repair system or products. • Write a SN on properties of repair material and material for repair. • Describe the causes of cracks in concrete. • What is meant by jacketing? Discuss different types of jacketing? • Explain in short the procedure for the damage assessment of structural element.
  68. 68. References • Concrete Technology by: R.P. Rethaliya Atul Prakashan • Concrete Technology by . M.S. Shetty • Internet websites
  69. 69. Thanks Prepared and Presented By: Prof. Gaurav.H.Tandon Sal Institute of Technology and Engineering Research

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