Understanding Different Types of Cracks in Concrete Structures
What is a ‘Crack’?
Basic Types
Describe complete or incomplete separation of structure into two or more parts produced by breaking.
- Active: Any crack for which the mechanism causing the cracking is still at work
- Dormant: Any crack not likely to become active in the future or whose movement is of such magnitude that a repair won’t be affected
- Structural crack: Caused by dead loads, applied forces, or other external forces
- Non-structural crack: Not the result of external forces and has no effect on structural resistance or integrity
How can Cracks be Monitored?
Methods:
- Required accuracy of measurement
- Recommended method of measurement
- The ‘2 screws’ method
- The ‘the 3 screws’ method
What does ‘Mean Tell-Tale’ Mean?
Are crack gauges. The use of glass tell-tales stuck across a crack to differentiate tensile, compressive, shear crack.
Principal Causes of Cracks in Concrete Structures
Permeability of concrete: As deterioration process in concrete begins with penetration of various aggressive agents, it dictates the ability of concrete to withstand weathering action, chemical attack or any process of deterioration. Therefore, low permeability is a key factor to concrete durability. Thermal movement, Creep movement
Repair Methods for Dormant Cracks in Concrete Structures
Sealing of Cracks: Is used when structural repair is unnecessary. Isolated cracks are sealed on concrete surfaces. A 25mm-wide slot, 10mm deep, is cut along the crack, cleaned, and soaked for 10 hours. A latex bonding primer is applied, followed by filling the slot with polymer-modified mortar. Curing compound is applied, and wet curing for 7 days follows.
Routing and Sealing of Cracks: Alternatively, a V-groove ranging from 6 to 25mm deep is made along the crack, with a minimum opening of 6mm at the surface, using concrete saws, hand tools, or pneumatic tools. The groove is then cleaned by air blasting, sand blasting, or water blasting and dried. A variety of sealant materials, such as epoxies, urethanes, silicones, polysulfides, asphaltic materials, or polymer mortars, can be used. To prevent stress concentration, a bond breaker like polyethylene strip or tape is placed at the bottom of the groove, ensuring the sealant can change shape without stressing the bottom.
Bond Breaking Method: Over bonding (strip coating) may be used alone or with sealing. An area around the crack is cleaned and coated with a 1 to 2mm thick layer. A bond breaker may be applied.
Epoxy Injection Method:
Cracks as narrow as 0.3mm can be fixed with epoxy injection, but the cause must be addressed to prevent recurrence. If the cause can’t be removed and the crack doesn’t weaken the structure, it can be sealed with flexible sealant or a joint established, followed by epoxy grouting. Epoxy injection requires skill and is limited by temperature and leakage. Common resins include epoxy, polyurethane, and acrylic, each with specific properties for different applications. Microcement grouts are less flexible and mainly used for structural joints. Injection methods vary based on factors like pressure and concrete health. Direct injection with surface packers is for low-pressure situations, while mechanical packers are common for healthy concrete, allowing for low or high-pressure injections.
Repair Methods for Active Cracks in Concrete Structures
Drilling and Plugging through Crack: One method involves drilling holes perpendicular to cracks and filling them with suitable epoxy or epoxy-mortar. Reinforcement bars are then placed in these holes to stitch across the cracks. Care must be taken to avoid air entrapment when placing the bars in the holes prior to filling with epoxy.
Stitching of Concrete Cracks: Stitching involves drilling holes on both sides of the crack and inserting U-shaped metal units (staples or stitching dogs) to span the crack. This method restores tensile strength across major cracks but may stiffen the structure, potentially causing cracks elsewhere. Proper investigation and reinforcement of adjacent sections may be needed.
External Prestressing: The ‘Post-tensioning’ method is used to arrest and correct flexural cracks in reinforced concrete. It is preferred when a significant portion of a member needs strengthening or when existing cracks need closing. Post-tensioning applies compression force to close cracks and adds residual compression. This technique utilizes pre-stressing strands or bars for compression. Proper anchorage for the prestressing steel is essential to prevent issues from relocating to another part of the structure.
Drilling and Plugging: For cracks running in straight lines and accessible at one end, drilling along the crack’s length and grouting it can repair them. Create a key by setting precast concrete or mortar plugs in bitumen. Bitumen breaks the bond between plugs and holes, preventing cracks from subsequent movement. For a better seal, drill a second hole and plug it with bitumen alone, using the first hole as a key and the second as a seal.