Types of cracks in concrete

this presentation deals with the different types of cracks generated in concrete during its usage and after its application and also methods to retrofit these cracks

1. cracks Introduction Types of cracks Repairing techniques

2. introduction If this stress and the restrained strain are allowed to develop to such an extent that they exceed the strength or strain capacity of concrete, then cracks will take place. Generally, 1/16 to 1/4-inch-wide cracks is acceptable limits (if its not leaking water ). A 100-foot-long regular-weight concrete slab normally would shrink by about 3/4 inch . In other words, you should expect cracks totaling in widths up to 3/4 inch in every 100 feet of concrete.

3. TYPES OF CRACKS Structural Cracks Structural cracks are those which result from incorrect design, faulty construction or overloading and these may endanger the safety of a building. Non-Structural Cracks Non Structural cracks occur mostly due to internally induced stresses in building materials. These cracks normally do not endanger the safety but may look unsightly, create an impression of faulty work or give a feeling of instability.

4. Non structural cracks Plastic shrinkage cracks Cracking caused by plastic shrinkage in concrete occurs most commonly on the exposed surfaces of freshly placed floors and slabs with large surface areas when they are subjected to a very rapid loss of moisture caused by low humidity and wind or high temperature or both. Plastic shrinkage usually occurs prior to final finishing, before curing starts. When moisture evaporates from the surface of freshly placed concrete faster than it is placed by curing water, the surface concrete shrinks. In most cases, these cracks are wide at the surface. They range from a few millimeters to many meters in length and are spaced from a few centimeters to as much as 3 m apart .

5. Hairline   cracks  cracks may develop in concrete foundations as the conc r ete cures is called hairline cracks. If the cracks appear shortly after pouring the concrete foundation, concrete may have been mixed poorly or poured too quickly. In poured concrete foundations, hairline crack frequently appear in the center of the walls because the wall corners have a greater stability. 

6. Settlement   cracks They appear when the underlying ground has not been compacted or prepared properly or if the subsoil was not of the proper consistency. Settlement cracks are usually wider at the top of the crack than the bottom as the foundation "bends" over a single point, allowing differential settlement.

7. Thermal Cracking of Concrete Temperature difference within a concrete structure may be caused by portions of the structure losing heat of hydration at different rates or by the weather conditions cooling or heating one portion of the structure to a different degree or at a different rate than another portion of the structure. These temperature differences result in differential volume change, leading to cracks. This is normally associated with mass concrete including large and thicker sections of column, beams and slabs

8. Repairing techniques 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

9. 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. The stitching of cracks is not a method of crack repair or to gain the lost strength, this method is used to prevent the cracks from propagating and widening.

10. 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(routing) and sealing it with crack fillers. Care should be taken to ensure that the entire crack is routed and sealed.

11. RESIN INJECTION The smaller cracks less than 0.75 mm width can be effectively repair by using pressure injection of epoxy. Surface of the member near cracks is thoroughly cleaned. Loose materials are removed and plastic injection ports are placed along the length of crack. These ports are placed on both sides of the member and secured in placed with the help of epoxy seal. Low viscosity resin is injected into one port at a time starting from the port at lowest level and moving upwards. This method can be used for all types of structural members such are beams, columns, walls and slabs.

12. Repair of Large Cracks Concrete: 1)The surface of cracks is cleaned and all the loose materials are removed. These are then filled with quick setting cement mortar . 2) For cracks which are very large, filler materials such as stone chips can be used. 3) Additional reinforcement and shear reinforcements can be used for heavily damaged concrete members or wherever necessary based on requirements.

13. Cont…. 4) For damaged walls and roofs, additional reinforcement in the form of mesh is used on one side or both sides of the members. These mesh should sufficiently tied with existing members.