1. CE2071 - REPAIRS AND REHABILITATION OF STRUCTURES OBJECTIVE To get the knowledge on quality of concrete, durability aspects, causes of deterioration, assessment of distressed structures, repairing of structures and demolition procedures.

2. UNIT – I MAINTENANCE AND REPAIR STRATEGIES Syllabus: Maintenance, repair and rehabilitation, Facets of Maintenance, importance of Maintenance various aspects of Inspection, Assessment procedure for evaluating a damaged structure, causes of deterioration

3. Definition: Maintenance of building is defined as the work done to keep the civil engineering structures in working condition so as to enable them to carry out the functions for which they are constructed. MAINTENANCE

4. Necessity of maintenance The maintenance of structures is done to meet the following objectives. · Prevent damages and decay due to natural agencies and wear & tear - to keep them in good appearance and working condition. · Repair defects occurred in structures and strengthen them, if necessary.

5. Maintenance work on dam structure

6. Classification of Maintenance Work: · Preventive Maintenance · Remedial Maintenance · Routine Maintenance · Special Maintenance

7. Preventive Maintenance: -before the defects occurred or damages developed

8. Remedial Maintenance: -after the defects or damage occurs in the structures. It involves the following basic steps. Finding the deterioration. Determining the causes. Evaluating the strength of the existing structures. Evaluating the need of the structures. Selecting and implementing the repair procedure

10. Routine Maintenance: maintenance attended to the structure periodically. It depends upon specifications and materials of structures,purpose, intensity and condition of use. It is done by the fund provided annually, which is normally 1.5% of the cost of construction. It includes the inspection, planning the programme and execute. It includes white washing, patching repair to plaster, replacement of fitting and fixtures, blinding of road surface.

11. Special Maintenance: work not covered in routing programme or the annual repair - is done under special condition It may be done for strengthening and updating of the structure - to meet the new condition of usage or to increase its serviceability. It include particular or complete renewal, such as renewal of floors, roofs etc

12. Special Maintenance

13. Causes which necessitate the maintenance: The causes which effects the service and durability of the structure are as follows: · Atmospheric Agencies 1. Rain 2. Wind 3. Temperature. · Normal wear and tear. · Failure of structure.

14. Atmospheric Agencies 1. Rain: -important source of water which affects the structure in the following ways. Physical: Dissolving and carrying away minerals as it is universal solvent.

15. Expansion and contraction: - materials subjected to repetitive expansion and contraction - while they become wet and dry and develop the stresses. Expansion of water: - variation of temperature causes the expansion and construction of absorbed water affects the micro structures of the materials.

16. Erosion: Abrasion of the structure materials is quite evident effect of the water. Spillway damage

17. Chemical: Water contains acids and alkali and other compound in dissolved form, acts over the material -which is known as chemical weathering. H 2 S corrosion on the concrete walls

18. 2.Wind:which transports the abrasive materials and assists the physical weathering. 3.Temperature: The temperature variation may also change in the structure and chemical composition of the materials.

19. Normal wear and tear: During the use of structure it is subjected to abrasion and there by it losses appearance and serviceability. Concrete damage was found to be a serious threat to the structural integrity of spillway.

20. Failure of structure: -behavior of structure not in agreement with expected condition of stability -or lacking freedom from necessary repair or non- compliance with desired use of and occupancy of the completed structure.

21. The causes of failure may be broadly grouped as: · Improper design. · Defective construction. · Improper use of structure · Lack of maintenance.

22. Inspection of a Building: Inspection of a building and any other civil engineering structure is routine duty of person in charge maintenance. -It means keen, analytic and dynamic observation regarding the change in condition of the structure deterioration reason and the causes, failure concluding the remedies for the same.

23. The inspections should be made on the following points. Condition of wall paint. Condition of paint on wood- work. Condition of flooring Roof leakage, leakage. if any Dampness in wall or floors, if any C ondition of service fittings. Drainage from terrace or pitched like

24. · Growth of vegetation, if any · Structural defects like 1. Crack 2. Settlement 3. Deflection (sagging)

25. CAUSES OF DETERIORATION 1. Local settlement of sub-grade. 2. Movement of formwork. 3. Vibrations 4. Internal settlement of concrete suspension. 5. Setting shrinkage.

26. 6.Premature removal of forms. 7.Drying shrinkage 8. Temperature stresses This may be due to (i)Difference in temperature between the inside and outside of the building. (ii)Variation in internal temperature of the building structure. 9. Absorption of moisture by concrete

27. 10. Corrosion of reinforcement This could be caused by (i)Entry of moisture through cracks or pores (ii)Electrolytic action 11.Aggressive action of chemical 12. Weathering action 13. Action of shockwaves 14. Erosion

28. 15. Weathering action 16. Action of shockwaves 17. Erosion 18. Poor design details at (i) Re- entrant corners (ii) Changes in cross section (iii)Rigid joint precast elements (iv)Deflections

29. Poor design details leads to 1. Leakage through joints 2. Inadequate drainage 3. Inefficient drainage slope 4. Unanticipated shear stresses in piers, column and abutments etc.., 5. Incompatibility of materials of sections. 6. Neglect in design 7. Errors in design 8. Errors in earlier repairs 9. Overloading 10. External influences such as(a) Earthquake (b) Wind(c) Fire(e) Cyclone(f) Flash floors etc..,

30. REPAIRS AND REHABILITATION OF STRUCTURES UNIT – II SERVISIBILITY AND DURABILITY OF CONCRETE Syllabus: Quality assurance for concrete construction concrete properties- strength, permeability, thermal properties and cracking. - Effects due to climate, temperature, chemicals, corrosion –design and construction errors - Effects of cover thickness and cracking

31. Quality Strength: Strengthening measures are required in structures when they are required to accommodate increased loads. This leads to a redistribution of forces and the need for local reinforcement. In addition, structural strengthening may become necessary owing to wear deterioration arising from normal usage or environment factors.

32. Need for strengthening: Load increases due to higher live loads, increased wheel loads, installations of heavy machinery, or vibrations. Damage to structural parts due to aging of construction materials or fire damage, corrosion of the steel reinforcement, and impact of vehicles. Improvements in suability for use due to limitation of deflection, reduction of stress in steel reinforcement and reduction of crack widths. Modification of structural system due to the elimination of walls /columns and opening cut through slabs. Errors in planning or construction due to insufficient design dimensions and insufficient reinforcing steel.

33. Construction Errors Failure to follow specified procedures and good practice or outright carelessness may lead to a number of conditions that may be grouped together as construction errors. Typically, most of these errors do not lead directly to failure or deterioration of concrete Instead, they enhance the adverse impacts of other mechanisms.

34. Each error will be briefly described below along with preventive methods. In general, the best preventive measure is a thorough knowledge of these construction errors are plus an aggressive inspection program

35. CRACKING: Load – induced tensile stresses may result cracks tensile forces. Flexural and either sustained or repetitive loading.

36. A well – distributed reinforcing arrangement offers the best protection against undesirable cracking.

37. DESIGN ERRORS: resulting from in adequate structural design and those resulting from lack of attention to relatively minor design details. The two types of design errors. ·Inadequate structural design. · Poor design details.

38. Chemical Attack on Concrete Acid attack Alkali attack Carbonation Chloride attack Leaching Salt attack Sulphate attack

39. Acid attack

40. CORROSION The nature of reinforcement corrosion mechanism can be attributed to three predominant process, namely Chemical Electro chemical and Physical.

41. CORROSION INFLUENCING FACTOR The cover thickness. The quality of concrete in the cover region, especially in terms of permeability and diffusivity. Environmental conditions pH value in concrete Chloride level in concrete and presence of cracks etc.

42. CORROSION PROTECTION TECHNIQUES 1. Coating to reinforcement 2. Galvanized reinforcement 3. Improving metallurgical by addition of certain elements.

43. REPAIRS AND REHABILITATION OF STRUCTURES UNIT III MATERIALS AND TECHNIQUES FOR REPAIR

44. SPECIAL TYPES OF CONCRETE : -with out-of-the-ordinary properties or those produced by unusual techniques. Light: Transparent Concrete

45. STRUCTURAL LIGHTWEIGHT CONCRETE Structural lightweight concrete is similar to normal- weight concrete except that -it has a lower density. -it is made with lightweight aggregates or -it is made with a combination of lightweight and normal- weight aggregates. Lightweight concrete

46. The term "sand lightweight“ - made with coarse lightweight aggregate and natural sand. Structural lightweight concrete has -air-dry density in the range of 1350 to 1850 kg/m 3 -28-day compressive strength in excess of 17 MPa sand sandwich board light weight concrete

47. -for comparison, normal-weight concrete containing regular sand, gravel, or crushed stone has a dry density in the range of 2080 to 2480 kg/m 3 Structural lightweight concrete is used primarily to reduce the dead-load weight in concrete members, such as floors in high- rise buildings.

48. Structural Lightweight Aggregates -usually classified according to their production process because various processes produce aggregates with somewhat different properties, which includes: Rotary kiln expanded clays, shales, and slates Chipped Shale Expanded clay Expanded slate

49. Sintering grate expanded shale and slates Pelletized or extruded fly ash Expanded slag Fly ash Pellet Expanded slag

50. Structural lightweight aggregates can also be produced by processing other types of material, such as naturally occurring pumice and scoria. Pumicescoria

51. Structural lightweight aggregates have densities significantly lower than normal- weight aggregates, ranging from 560 to 1120 kg/m 3, compared to 1200 to 1760 kg/m 3 for normal-weight aggregates. These aggregates may absorb 5% to 20% water by weight of dry material. To control the uniformity of structural lightweight concrete mixtures, the aggregates are pre wetted (but not saturated) prior to batching.

52. HIGH-DENSITY CONCRETE High-density (heavyweight) concrete has a density of up to about 6400 kg/m 3. Heavyweight concrete used for radiation shielding As a shielding material, heavyweight concrete protects against the harmful effects of X-rays, gamma rays, and neutron radiation. Heavyweight concrete to protect & shield places with greater risk of radiation.

53. Selection of concrete for radiation shielding is based on space requirements and on the type and intensity of radiation. Where space requirements are not important, normal-weight concrete will generally produce the most economical shield; Where space is limited, heavyweight concrete will allow for reductions in shield thickness without sacrificing shielding effectiveness.

54. Properties of High-Density Concrete Both the freshly mixed and hardened states can meet job conditions and shielding requirements by proper selection of materials and mixture proportions. Except density, the physical properties are similar to normal-weight concrete. Strength is a function of water-cement ratio; thus, for any particular set of materials, strengths comparable to those of normal-weight concretes can be achieved. radiation shield has special requirements, trial mixtures should be made with job materials and under job conditions to determine suitable mixture proportions.

55. EXPANSIVE CEMENT: Concrete made with ordinary Portland cement shrinks due to loss of free water. Concrete also shrinks continuously for long time. This is known as drying shrinkage. Cement used for -grouting anchor bolts or -grouting machine foundations or -grouting the prestress concrete ducts, if shrinks, There has been a search for such type of cement which will not shrink while hardening and thereafter. As a matter of fact, a slight expansion with time will prove to be advantageous for grouting purpose. This type of cement which suffers no overall change in volume on drying is known as expansive cement.

56. Cement of this type has been developed by using an expanding agent and a stabilizer very carefully. Proper material and controlled proportioning are necessary in order to obtain the desired expansion. One type of expansive cement is known as shrinkage compensating cement. This cement when used in concrete, with restrained expansion, induces compressive stresses which approximately offset the tensile stress induced by shrinkage. Another similar type of cement is known as self-stressing cement. This cement when used in concrete induces significant compressive stresses after the drying shrinkage has occurred. Fibre content :0.5 to 2.5% by volume of mix

57. POLYMER CONCRETE: Continuous research by technologists to understand, improve and develop the properties of concrete has resulted in a new type of concrete known as “polymer concrete”. This concrete is porous in nature and this porosity due to air-voids, water voids or due to the inherent porosity of gel structure itself. The development of concrete-polymer composite material is directed at producing a new material by combining the ancient technology of cement concrete with the modern technology of polymer chemistry.

58. TYPES OF POLYMER CONCRETE: Four types of polymer concrete materials are being developed, Polymer Impregnated Concrete(PIC) Polymer Cement Concrete(PCC) Polymer Concrete(PC) Partially Impregnated and Surface coated polymer concrete

59. POLYMER IMPREGNATED CONCRETE (PIC): The monomers used in this type are, Methylmethacrylate(MMA) Styrene Acrylonitrile t-butyl styrene Other thermoplastic monomers POLYMER CEMENT CONCRETE (PCC) The monomers that are used in PCC are, Polyster-styrene Epoxy-styrene Furans Vinylidene chloride

60. APPLICATIONS OF POLYMER IMPREGNATED CONCRETE: The following are the application of the PIC, Prefabricated structural elements Prestressed concrete Marine works Desalination plants Nuclear power plants Sewage works-pipe and disposal works Ferrocement products For water proofing of structure Industrial applications

61. SULPHUR-INFILTERATED CONCRETE: New types of composites have been produced by the recently developed techniques of impregnating porous materials like concrete with sulphur. Sulphur impregnation has shown great improvement in strength. In the past, some attempts have been made to use sulphur as a binding material instead of cement. Sulphur is heated to bring it into molten condition to which cores and fine aggregates are pored and mixed together. On cooling, this mixture gave fairly good strength, exhibited acid resistance and also other chemical resistance, but it proved to be costlier than ordinary cement concrete. Recently, use of sulphur was made to impregnate lean porous concrete to improve its strength and other useful properties considerably. In this method, the quantity of sulphur used is also comparatively less and thereby the processes is made economical. It is reported that compressive strength of about 100MP a could be achieved in about two days time.

62. Two procedures are adapted “A” after 24hours of moist curing, the specimen is dried in heating cabinet for 24hours at 121 0 C. Then the dried specimens are placed in a container of molten sulphur at 121 0 C for 3 hours. Specimens are removed from the container, wiped clean of sulphur and cooled to room temperature for 1hour and weighed to determine the weight of sulphur in filtrated concrete. In procedure “B”, the dried concrete specimen is placed in an airtight container and subjected to vacuum pressure of 2mm mercury for 2hours. After removing the vacuum, the specimens are soaked in the molten sulphur at atmospheric pressure for another half an hour. The specimen is taken out, wiped clean and cooled to room temperature in about 1hour.The specimen is weighed and the weight of sulphur-impregnated concrete is determined.

63. FERROCEMENT CONCRETE: Ferro-cement technique though of recent origin, have been extensively used in many countries, notably in U.K., New Zealand and China. There is a growing awareness of the advantages of this technique of construction all over the world. It is well known that conventional reinforced concrete members are too heavy, brittle cannot be satisfactorily repaired if damaged, develop cracks and reinforcements are liable to be corroded. Ferrocement is a relatively new material consisting of wire meshes and cement mortar. It consists of closely spaced wire meshes which are impregnated with rich cement mortar mix. The wire mesh is usually of 0.5 to 1.0mm dia wire at 10mm spacing and cement mortar is of sand ratio of 1:2 or 1:3 with water /cement ratio of 0.4 to 0.45.

64. The ferrocement elements are usually of the order of 2 to 3cm in thickness with 2 to 3mm external cover to the reinforcement. The steel content varies between 300kg to 500kg per cubic meter of mortar. The main advantages are simplicity of its construction, lesser dead weight of the elements due to their small thickness, its high tensile strength, less crack widths compared to conventional concrete, easy reparability, non corrosive nature and easier mould ability to any required shape. There also saving in basic materials namely cement and steel. This material is more suitable to special structures like shells which have strength through forms and structures like roofs, silos, water tanks and pipelines. The development of ferrocement depends on suitable casting techniques for the required shape. Development of proper prefabrication techniques for ferrocement is still not a widely explored area and gap needs to be filled.

65. FIBRE REINFORCED CONRETE: Plain concrete possess a very low tensile strength, limited ductility and little resistance to cracking. Internal micro cracks are inherently present in the concrete and its poor tensile strength due to the propagation of such micro cracks, eventually leading to brittle fracture of the concrete. In plain concrete and similar brittle materials, structural cracks(micro-cracks) develop even before loading, particularly due to drying shrinkage or other causes of volume change. When loaded, the micro cracks propagate and open up, and owing to the effect of stress concentration, additional cracks form in places of minor defects. The development of such microcracks is the main cause of inelastic deformations in concrete.

66. It has been recognized that the addition of small, closely spaced and uniformly dispersed fibres to concrete would act as crack arrester and would substantially improve its static and dynamic properties. This type of concrete is known as Fibre Reinforced Concrete. Fibre Reinforced Concrete can be defined as a composite material consisting of mixture of cement, mortar or concrete and discontinuous, discrete, uniformly dispersed suitable fibres. Continuous meshes, woven fabrics and long wires or rods are not considered to be discrete fibres

67. FIBRES USED: Many types of fibres used in cement and concrete, not all of them can be effectively and economically used. Each type of fibre has its characteristics properties and limitations. Some of the fibres that could be used are steel fibres, polypropylene, nylons, asbestos, coir, glass and carbon. The fibre is often described by a convenient parameter called “Aspect ratio”. The aspect ratio of the fibre is the ratio of its length to its diameter. It ranges from 30 to 150.

68. Steel fibres are most commonly used and the diameter may vary from 0.25 to 0.75mm. Glass fibre is a recent introduction in making fibre concrete. It has tensile strength of about 1020 to 4080N/mm² Polypropylene and nylon fibres are found to be suitable to increase the impact strength. They posses high tensile strength, but their low modulus of elasticity and higher elongation do not contribute to the flexural strength. Asbestos is a mineral fibre and has proved to be most successful of all fibres as it can be mixed with Portland Cement. Tensile strength would be from 560 to 980N/mm². Carbon fibres perhaps posses very high tensile strength 2110 to 2815N/mm².

69. UNIT IV TECHNIQUES FOR REPAIR AND DEMOLITION

70. GUNITE or SHOTCRETE: Gunite can be defined as mortar conveyed through a hose and pneumatically projected at a high velocity on to a surface. Recently the method has been further developed by the introduction of small sized coarse aggregate into the mix deposited to obtain considerably greater thickness in one operation and also to make the process economical by reducing the cement content. Normally fresh material with zero slumps can support itself without sagging or peeling off. The newly developed "Redi-set cement" can also be used for shotcreting process.

71. EPOXY COATED REINFORCEMENT: The object of coating to steel bar is to provide a durable barrier to aggressive materials, such as chlorides. The coatings should be robust to withstand fabrication of reinforcement cage, and pouring of concrete and compaction by vibrating needle. Simple cement slurry coating is a cheap method for temporary protection against rusting reinforcement in storage. Central Electro Chemical Research Institute, (CECRI) Karaikudi have suggested a method for preventing of corrosion in steel reinforcement in concrete. The steps involved in this process are

72. 1. Derusting: The reinforcements are cleaned with a derusting solution. This is followed without delay by cleaning the rods with wet waste cloth and cleaning powder. The rods are then rinsed in running water and air dried. 2.Phosphating: Phosphate jelly is applied to the bars with fine brush. The jelly is left for 45-60 minutes and then removed by wet cloth. An inhibitor solution is then brushed over the phosphate surface.

73. 3.Cement coating: Slurry is made by mixing the inhibitor solution with Portland cement and applied on the bar. A sealing solution is brushed after the rods are air cured. The sealing solution has an insite curing effect. The second coat of slurry is then applied and the bars are air dried. 4.Sealing: Two coats of sealing solution are applied to the bars in order to seal the micro-pores of the cement coat and to make it impermeable to corrosive salts. The above is a patent method evolved by CECRI and license is given to certain agencies.

74. CATHODIC PROTECTION: Principles of Cathodic Protection: Metal that has been extracted from its primary ore (metal oxides or other free radicals) has a natural tendency to revert to that state under the action of oxygen and water. This action is called corrosion and the most common example is the rusting of steel. Corrosion is an electro- chemical process that involves the passage of electrical currents on micro or macro scale. The change from the metallic to the combined form occurs by an“anodic” reaction.

75. Advantages and Uses of Cathodic Protection The main advantage of cathodic protection over other forms of anti-corrosion treatment is that it is applied simply by maintaining a dc circuit and its effectiveness may be monitored continuously. Cathodic protection is commonly applied to a coated structure to provide corrosion control to areas where the coating may be damaged. It may be applied to existing structures to prolong their life. Cathodic protection can, in principle, be applied to any metallic structure in contact with a bulk electrolyte (including concrete). In practice, its main use is to protect steel structures buried in soil or immersed in water. It cannot be used to prevent atmospheric corrosion on metals. However, it can be used to protect atmospherically exposed and buried reinforced concrete from corrosion, as the concrete itself contains sufficient moisture to act as the electrolyte.

76. Structures that are commonly protected by cathodic protection are the exterior surfaces of: Pipelines Ships' hulls Storage tank bases Jetties and harbor structures Steel sheet, tubular and foundation pilings Offshore platforms, floating and sub sea structures

77. DEMOLITION TECHNIQUES DEMOLITION MANAGEMENT: A construction removal and demolition company of tem of engineers, project managers and demolition technicians providing effective and efficient work. SELECTIVE DEMOLITION: Hydro demolition Marine demolition Underwater demolition Bridge demolition Building demolition Nuclear demolition Runway demolition

78. METHODS OF DEMOLITION: 1.Pneumatic and hydraulic breakers 2.Dismantling 3.Pressure bursting 4.Explosives 5.Ball and crane for demolishing masonry and concrete structures.

79. UNIT – V REPAIR, REHABILITATION AND RETROFFITNG OF STRUCTURES

80. REPAIRS TO OVERCOME LOW MEMBER STRENGTH: These guidelines have the following objectives; To indicate appropriate methods of repair and restoration taking into account the building type and the type of damage. To recommend methods of seismic strengthening to upgrade the strength of the building in line with the requirements of the seismic-zoning map of India (IS : 1893-1984) and Earthquake Resistance Codes (13 : 4326- 1993) and (18:13828-1993).

81. CONCEPTS OF REPAIR, RESTORATION AND RETROFITTING: REPAIR: It consists of actions taken for patching up superficial defects, re-plastering walls, repairing doors and windows and services such as the following : Patching up of defects as cracks and fall of plaster and re-plastering if needed. Repairing doors, windows and replacement of glass panes.

82. Checking and repairing electrical connections, gas connections, plumbing, heating, ventilation Rebuilding non-structural walls, chimneys, boundary walls. Relaying cracked flooring at ground level and roofing sheets or tiles. Redecoration work (White or colour washing etc.) It would be seen that the repairing work carried out as above does not add any strength to the structure.

83. RESTORATION : This includes actions taken for restoring the lost strength of structural elements of the building. This is done by making the columns, piers, beams and walls at least as strong as originally provided as follows: Removal of portions of cracked masonry walls and piers, and rebuilding them in richer mortar. Use of non-shrinking mortar will be preferable. Addition of reinforcing mesh on both faces of the cracked wall, holding it to the wall through spikes or bolts and then covering it suitably with micro-concrete or 1:3 cement -coarse sand plaster.

84. Injecting neat cement slurry or epoxy like material, which is strong in tension, into the cracks in walls, columns, beams etc. If the structural restoration is properly executed, the structure will be as strong as before the-earthquake. It is also possible to strengthen a structure to take increased vertical loading, if required.

85. SEISMIC STRENGTHENING (RETROFITTING) It will involve actions for upgrading the seismic resistance of an existing building so that becomes safer under the occurrence of probable future earthquakes. The seismic behavior of existing buildings is affected by their original structural inadequacies, material degradation due to aging and alterations carried out during use over time. The complete replacement of such buildings in a given area is just not possible due to a number of social, cultural and financial problems. Therefore, seismic strengthening of existing undamaged or damaged buildings is a definite requirement.

86. Seismic strengthening structural restoration and cosmetic repairs may sometimes cost upto 25 to 30 per cent of the cost of rebuilding although usually it may not exceed 12 to 15 per cent. Hence justification of strengthening work must be fully considered from cost point of view. The main items of seismic strengthening could be some or all of that following actions: Modification of roofs, Substitution or strengthening of floors,

87. Modification in the building plan, Strengthening of walls including provision of horizontal and vertical bands or belts, introduction of header stones in thick stone walls, and injection grouting etc., Adding to the sections of beams and columns by casing or jacketing etc., Adding shear walls or diagonal bracings, Strengthening of foundations if found necessary (but very difficult and expensive).

88. STRENGTHENING OF FOUNDATIONS Seismic strengthening of foundations before or after the earthquake is the most involved task since it may require careful underpinning operations. Some alternatives are given below for preliminary consideration of the strengthening scheme. Introducing new load bearing members including foundations to relieve the already loaded members. Jacking operations may be needed in this process. Improving the drainage of the area to prevent saturation of foundation soil to obviate any problems of liquefaction which may occur because of poor drainage. Providing apron around the building to prevent soaking of foundation directly and draining off the water.

89. Adding strong elements in the form of reinforced concrete strips attached to the existing foundation part of the building. These will also bind the various wall footings and may be provided on both sides of the wall. To avoid digging the floor inside the building, the extra width could be provided only on the outside of external walls. The extra width may be provided above the existing footing or at the level of the existing footing. In any case the reinforced concrete strips and the walls have to be linked by a number of keys, inserted into the existing footing.

90. CRACKING Cracking, like corrosion of reinforcing steel, is not commonly a cause of damage to concrete. Instead, cracking is a symptom of damage created by some other cause. All Portland cement concrete undergoes some degree of shrinkage during hydration. This shrinkage produces multidirectional drying shrinkage and curing shrinkage cracking having a somewhat circular pattern. Such cracks seldom extend very deeply into the concrete and can generally be ignored.

91. Plastic shrinkage cracking occurs when the fresh concrete is exposed to high rates of evaporative water loss which causes shrinkage while the concrete is still plastic. Plastic shrinkage cracks are usually somewhat deeper than drying or curing shrinkage cracks and may exhibit a parallel orientation that is visually unattractive.

92. Thermal cracking is caused by the normal expansion and contraction of concrete during changes of ambient temperature. Concrete has a linear coefficient of thermal expansion of about 5.5 milli inch per inch per degree Fahrenheit (°F). This can cause concrete to undergo length changes of about 0.5 inch per 100 linear feet for an 80 °F temperature change.

93. Thermal cracking can also be caused by using Portland cements developing high heats of hydration during curing. Such concrete develops exothermic heat and hardens while at elevated temperatures. Cracking is also caused by alkali-aggregate reaction, sulfate exposure, and exposure to cyclic freeze-thaw conditions, as has been discussed in previous sections, and by structural overloads as discussed in the following section. Successful repair of cracking is often very difficult to attain.

94. The selection of methods for repairing cracked concrete depends on the cause of the cracking. First, it is necessary to determine if the cracks are "live" or "dead." If the cracks are cyclically opening and closing, or progressively widening, structural repair becomes very complicated and is often futile. Such cracking will simply reestablish in the repair material or adjacent concrete. If reflective cracking is intolerable, the repairs must be designed as separate structural members not bonded to the old existing concrete.