David Coles gives an overview of the causes of cracking to reinforced concrete and why it is important to have fresh cracks assessed as soon as they become evident.
David is a Chartered Structural Engineer with over 30 years’ experience. He is a Chartered Member of Engineering New Zealand and a Senior Structural Engineer at EDC. Do not hesitate to contact him if you need assistance with concrete repair issues on 021637592 or david.coles@edc.co.nz
Cracking to reinforced concrete structures can result from various factors and it is important to diagnose what has caused the cracking to determine underlying structural issues. Reinforced concrete is designed to crack to a certain extent, but fresh cracking, or large cracks can indicate a problem.
Drying Shrinkage
Particularly for floor slabs, the most common cause of cracking is drying shrinkage. Wet concrete will set within a few hours of being poured but then goes through a month-long period of curing while it gains full strength. As the concrete cures it can shrink due to the evaporation of water. Good contractors will control this issue by applying a curing compound film to the slab surface after casting, or by keeping the surface wet during the curing process. At EDC, we also carefully consider the locations of saw cuts to help control where any cracking may occur.
The cracks are usually non-structural (they generally will not affect the structural capacity of the element), however when exposed to the elements, should be sealed to prevent chloride attack. Repairs are typically straight forward to protect the long-term durability of the concrete and in turn protect the steel reinforcement.
Drying shrinkage cracks can be a significant issue aesthetically if the slab is exposed.
Structural Performance
Of more concern is cracks that form suddenly from structural actions.
- Overloading – Exceeding the designed load capacity;
- Settlement of Foundation – Uneven soil settlement can create differential movement of the structure;
- Seismic Activity – Seismic Ground movement can cause excessive movement in the structure;
- Vibrations – Vibrations from traffic or machinery or people walking can set up simple harmonic motions which have not been designed for and can cause excessive deformation;
- Thermal expansion and contraction – Thermal movement can cause cracking if control joists There are many causes of cracking to concrete, and an assessment must be undertaken to correctly diagnose the cause.
Any time fresh cracks form it is important to get a structural review as the damage could indicate a weakening of the structure.
Once the cause is known, the appropriate repairs can be specified and should this be done early, this can result in more cost-effective repairs. Common repair methods include the use of epoxy resin injection and strengthening with additional reinforcing or carbon fibre.
What if cracks are left untreated?
Carbonation
Reinforced concrete structures in New Zealand are generally designed for a 50-year design life. The alkalinity of the concrete protects the steel reinforcement from corrosion by providing a passive oxide layer on the steel surface. However, this passive oxide layer is lost over time due to exposure to the environment.
When concrete is first cast it is highly alkaline with a pH value of 12 to 13.5 this high pH is due to the presence of calcium hydroxide, which forms when cement reacts with water. Carbonation is a chemical process that occurs when carbon dioxide from the atmosphere reacts with the calcium hydroxide in concrete, forming calcium carbonate. This reaction gradually reduces the pH of the concrete. The carbonation starts from the external (i.e. exposed) faces of the concrete and has a typical rate of 4-8mm per square root of time (for New Zealand). Once all of the cover concrete has been carbonated (the pH is less than 9) then the reinforcing bar is no longer protected.
Once the passive oxide layer has gone, corrosion can occur if the reinforcement is exposed to oxygen and moisture. Steel reinforcement corrodes in the presence of water and oxygen, with iron oxide forming on the surface giving the brown “rusty” colour to the bars. The iron oxide produced is seven times greater in volume than the original steel and this will expand into the concrete, potentially causing the cracks to widen, and then for concrete to spall off.
The cracks can start off as hairline cracks (<0.3mm) and this gives the warning sign that the steel is beginning to corrode. This can progress to spalling of the cover concrete as the corrosion progresses. Once spalling has occurred it’s likely that significant corrosion has occurred to the reinforcing bars and their loss of section size can reduce the load resisting capacity of the structure.
The whole of the reinforced concrete structure should be assessed for carbonation as re-alkalisation of the concrete may be required to prevent corrosion occurring to other areas of the building. Remedial repairs requiring the corroded reinforcement to be replaced can occur, however this is significant and can be prevented should the problem be detected at an early stage.
Chlorides
Chlorides, most commonly from sea water/sea spray, can also cause corrosion even with the concrete having high pH levels by breaking down the passive film locally to the reinforcing bar.
Chlorides can be “ingressed” from the environment or “cast in” during the construction process.
Once the passive layer has been breached the presence of moisture and oxygen can cause the reinforcement to corrode and expand and cause cracking and then spalling of the concrete.
