Concrete Cracks: Identification, Monitoring, and Repair Methods
Understanding Concrete Cracks: Types and Causes
A “crack” is a complete or incomplete separation of a structure into two or more parts due to breaking or fracturing.
- Active Crack: Any crack where the mechanism causing the cracking is still active.
- Dormant Crack: Any crack unlikely to become active in the future, or whose movement is insignificant.
- Structural Crack: Caused by dead loads, applied forces, or other external forces, resulting from incorrect design, faulty construction, or overloading.
- Non-Structural Crack: Not caused by external forces and has no effect on structural resistance or integrity; usually the result of external forces and has no effect on structural resistance or integrity.
Monitoring Cracks: Techniques and Tools
Cracks can be monitored by considering:
- Required accuracy of measurement
- Recommended method of measurement
- The “two screws” method
- The “three screws” method
“Tell-Tale” is a generic name for crack gauges. The use of glass tell-tales stuck across a crack to determine if future movement takes place is deprecated. They often fail owing to anchorage slip and are subject to vandalism. Even when they do crack, it often shows little more than that the glass has cracked.
Repair Methods for Dormant Cracks in Concrete
1. Sealing of Cracks
For standalone crack repairs where structural repair isn’t needed, isolated cracks in concrete should be sealed at the surface. A slot approximately 25mm wide and up to 10mm deep is saw-cut along the crack, keeping the crack centered in the slot. The concrete between the saw-cut edges is then chiseled out. After cleaning and soaking the slot with water for 10 hours and drying the surface, a latex bonding compound primer is applied. Once tacky, high-strength polymer-modified cementitious mortar is filled into the slot, compacted, and finished. A curing compound is applied once the surface is touch dry, followed by 7 days of wet curing using wet Hessian and a polythene sheet.
2. Routing and Sealing of Cracks
An alternative method for repairing cracks involves preparing a V-groove along the crack surface, ranging in depth from 6 to 25mm with a minimum opening of 6mm at the surface.
Tools such as a concrete saw, hand tools, or pneumatic tools can be used for this. The groove is then cleaned using air blasting, sand blasting, or water blasting and dried. A sealant, which can be epoxies, urethanes, silicones, polysulphide, asphaltic materials, or polymer mortars, is applied to the dry groove and allowed to cure. To prevent stress concentration at the bottom of the groove, a bond breaker like a polyethylene strip or tape, which won’t bond to the sealant, may be used.
3. Bond Breaking Method
In certain situations, over bonding or strip coating is used either alone or alongside sealing. This involves sand blasting or cleaning an area approximately 25 to 75mm on each side of the crack and applying a 1 to 2mm thick coating, like urethane, over the crack. A bond breaker can be used over the crack or a previously sealed crack. Cracks with minimal movement can be over banded, but for cracks with significant movement, over bonding should be combined with sealing to ensure a waterproof repair.
4. Epoxy Injection Method
Cracks as narrow as 0.3mm in concrete structures like buildings and bridges can be successfully bonded by injecting epoxy. However, unless the underlying cause of the cracking is addressed, the crack may reappear near the original site. If the cause can’t be removed and doesn’t weaken the structure, the crack can be sealed with flexible sealant or treated as a joint that accommodates movement before grouting with epoxy. This method isn’t suitable for actively leaking cracks that can’t be dried out, except for specific moisture-tolerant epoxies. Epoxy injection requires high skill levels for effective application, and ambient temperature can limit its use. (push caulking tube into the vent, fill until epoxy begins to flow from vent above, plug vent tube to keep epoxy from running out move to next vent above)
Repair Methods for Active Cracks in Concrete
1. Drilling and Plugging through Crack
The drilling and plugging method involves drilling holes perpendicular to the cracks and filling them with epoxy or epoxy-mortar. Reinforcement bars are then inserted into these holes to bridge the cracks. Care must be taken to avoid trapping air when placing the bars before filling with epoxy. This technique seals the crack, drills holes intersecting the crack at about 90 degrees, fills both the hole and crack with injected epoxy, and inserts reinforcing bars into the drilled holes. Typically, No. 4 or 5 (10M or 15M) bars are used, extending at least 18 inches (0.5m) on each side of the crack. The spacing and pattern of the reinforcing bars can be adjusted based on repair needs, design criteria, and the location of existing reinforcement.
2. Stitching of Concrete Cracks
Stitching is a method where holes are drilled on both sides of a crack and U-shaped metal units with short legs, known as staples or stitching dogs, are grouted in to span the crack. This technique is used to restore tensile strength across significant cracks.
3. External Prestressing
External prestressing, specifically the post-tensioning method, can arrest and correct flexural cracks in reinforced concrete. This method is particularly useful when a significant portion of a member needs strengthening or when existing cracks must be closed. It closes cracks by applying compression force to counteract tension and introduces residual compression. It utilizes prestressing strands or bars to apply this compressive force. Proper anchorage for the prestressing steel is crucial, and care must be taken to prevent the problem from simply moving to another part of the structure.
4. Drilling and Plugging
This method is suitable for repairing cracks that run in reasonably straight lines and are accessible at one end. This involves drilling along the length of the crack and grouting it to form a key using precast concrete or mortar plugs set in bitumen. The bitumen serves to break the bond between the plugs and the hole, preventing the plugs from cracking due to subsequent movement of the opening. For an enhanced seal, a second hole can be drilled and plugged with bitumen alone, using the first hole as a key and the second as a seal.
Understanding Concrete Jacketing
Reinforced concrete jacketing involves restoring or increasing the size of an existing structural member by encasing it with suitable materials. The process enhances the load-carrying capacity and stiffness of the structure. However, it results in an increase in the size of the concrete member after jacketing and requires constructing new formwork, which are the main disadvantages of this method.
Creating New Openings in Masonry Walls
General procedure when working from the upper floor is not possible:
- Determine if the wall is load-bearing or not.
- If load-bearing, set up temporary support to carry weight from above.
- Protect the floor from damage.
- Measure and mark the new opening’s position.
- Outline the lintel opening, ensuring it extends at least 150mm beyond the door frame.
- Erect scaffolding and temporary props as required.
- Place the lintel within the temporary supports before securing them.
- Use appropriate tools to cut the opening, considering safety and cleanliness.
- Fit Lintel: Position the lintel, ensuring it extends adequately beyond the opening.
- Fill gaps and voids around the lintel securely.
- Insert insulation if the wall is external.
- Once the lintel is set and mortar cured, cut the opening.
- Carefully remove bricks below the lintel.
- Place the frame securely within the opening.
- Complete any necessary finishing touches such as plastering and painting.
- Tidy and clean the work area thoroughly.
Removing Load-Bearing and Non-Load-Bearing Walls
General procedure when working from the upper floor is not possible:
Non-Load-Bearing Walls
Removing of masonry partition with a thickness of 100, 125 or 150mm:
- Using a temporary timber propping to support the partition.
- Using a temporary propping made of rolled steel joists (RSJ) to support the partition.
- The partition is supported only by finally fitted lintels made of rolled steel joists (RSI).
Load-Bearing Walls
Removing of load bearing wall on the higher floor:
- When removing a load-bearing wall, the new steel beam will have to rest on something.
- New lintels are made of rolled steel joists (RSJ) and located under the floor structure – they provide definitive support (propping).
- Temporary propping is made of timber structure.
Creating New Openings Greater Than 2.5m in Load-Bearing Walls
Scheme of Work Steps for Forming New Opening Wall Removal
- Preparation works