Techniques for Repair and Strengthening of Structures

1. CHAPTER V Techniques For Repair And Repair Of Structures

2. This section includes: • Introduction • Guniting • Shortcreting • Repair for cracks • Shoring and underpinning • Repairs to overcome low member strength

3. Introduction: The success of repair activity depends on the identification of the root cause of the deterioration of the concrete structures. If this cause is properly identified, satisfactory repairs can be done for the improvement of strength and durability, thus extending the life of the structure, is not difficult to achieve.

4. General procedure in the repair of distressed concrete structure: Support the structural members properly as required. Remove all cracked, spalled and loose concrete. Clean the exposed concrete surfaces and steel reinforcement. Provide additional reinforcing bars, if the loss in reinforcement is more than 10% Apply shortcreting/polymer concrete for patch repair work and grouting for porous/honeycombed concrete. Apply protective coatings over the exposed/repaired surface.

5. Gunite & Shortcrete

6. Guniting Guniting is mechanically applied material consisting of cement, aggregates and water. The cement and sand are batched and mixed in the usual way and conveyed through a hose pipe with the help of compressed air. A separate pipe line brings water under pressure and the water and cement aggregate mix are passed through and intimately mixed in a special manifold and then projected at high velocity to the surface being repaired.

7. In good quality work, a density around 2100kg/m3 is achieved. For effective guniting, the nozzle should be kept at 60cm to 150cm from the work normal to the surface. Before guniting is applied, the old concrete surface is prepared properly, all the cracks treated and the new reinforcement fixed in position. Cracks wider than about 0.5 mm should be cut out and filled with hand-applied mortar or with gunite.

10. Shortcreting Shortcrete is defined as “ mortar or concrete pneumatically projected at high speed onto a surface”. Types of shorcrete: i) Dry mix ii) Wet mix Dry mix: Dry cement, sand and coarse aggregate are premixed with only sufficient water to reduce dusting. ii) Wet mix: The cement, sand and coarse aggregate are mixed with water and the resulting concrete is then pumped to the nozzle where compressed air propels the wet mixture onto the surface.

11. Applications : Shortcrete has been used to repair : canal and spillway linings and walls the faces of dams, tunnel linings highway bridges and tunnels deteriorating natural rock walls and earthen slopes to thicken and strengthen existing concrete surfaces

12. REPAIR FOR CRACKS

13. Repair for cracks: i) Stitching ii) Routing and sealing iii) Resin injection iv) Dry packing v) Polymer impregnation vi) Vacuum impregnation vii) Autogenous healing viii) Flexible sealing ix) Drilling and plugging x) Bandaging

15. STITCHING • In this technique, the crack is bridged with U-shaped metal units called stitching dogs before being repaired with a rigid resin material. • A non- shrink grout or an epoxy resin based adhesive should be used to anchor the legs of the dogs. • Stitching is suitable when tensile strength must be re established across major cracks. • Stitching dogs should be of variable length and orientation.

17. BENEFITS OF CRACKED STITCHING • Quick, simple, effective and permanent. • The grout combination provides an excellent bond within the substrate. • Masonry remains flexible enough to accommodate natural building movement. • Non-disruptive structural stabilization with no additional stress

18. ROUTING AND SEALING • This is the simplest and most common method of crack repair. • It can be executed with relatively unskilled labor and can be used to seal both fine pattern cracks and larger isolated cracks. • This involves enlarging the crack along its exposed face and sealing it with crack fillers. • Care should be taken to ensure that the entire crack is routed and sealed.

20. RESIN INJECTION • Epoxy resins are usually selected for crack injection because of their high mechanical strength and resistance to most chemical environments encountered by concrete. • Epoxies are rigid and not suitable for active cracks. • This method is used to restore structural soundness of members where cracks are dormant or can be prevented from further movements.

22. SHORING

23. INTRODUCTION Definition • It is the method of providing temporary support (shores) to an unsafe structure. Types of Shoring • Horizontal shoring or flying shoring • Vertical shoring or dead shoring • Inclined Shoring or flying shoring

24. Horizontal shoring • It consists of • Horizontal beam or strut • Wall plates • Cleats • Straining beams • Used to support two adjacent buildings.

25. Horizontal shoring

26. Single Flying Shoring

27. Double Flying Shoring

28. Vertical shoring • It consists of • Dead shores • Sole plates • Needles • Props • Used for rebuilding of walls.

29. Vertical shoring

30. Dead or Vertical Shoring

31. Inclined Shoring • It consists of • Rackers • Needles • Cleats • Braces • Sole plate • Used to strengthen a wall.

33. Inclined or Raking Shoring (unsymmetrical)

35. UNDERPINNING

36. DEFINITION •Underpinning is the process of strengthening and stabilizing the foundation of an existing building or other structure • Foundation underpinning is a means of transferring loads to deeper soils or bedrock.

37. PURPOSE OF UNDERPINNING • To obtain additional foundation capacity • To modify the existing foundation system • To create new foundations through which the existing load may be wholly or partially transferred into deeper soil • To arrest the excessive settlement • To improve the future performance of the existing foundations

38. WHEN UNDERPINING IS REQUIRED? Underpinning is required when: • Construction of a new project with deeper foundation adjacent to an existing building. • Change in the use of structure • The properties of the soil supporting the foundation may have changed or was mischaracterized during planning. • To support a structure which is sinking or tilting due to ground subsidence or instability of the super structure

39. METHODS USED FOR UNDERPINNING • Pit Underpinning • Push Piers System • Helical Pier System • Pile Underpinning • Other Methods • Chemical Grouting • Microfine Grouting • Micropiles

40. PIT UNDERPINNING

42. HELICAL PIER SYSTEM Repaired Foundation Damaged Foundation

43. DEFLECTION

44. DEFLECTION Equations for calculating Dinst for common cases

46. DAMAGE DUE TO FIRE

49. Fire Damage Repairs • Timber structures may be repaired with new timbers or composites of steel and timber members • Steel structures are normally repaired with steel. • Both concrete and masonry structural elements are frequently repaired with fiber reinforced polymers (FRP). • Concrete structures are occasionally repaired with shortcrete.

50. Repair of leakage structure