Dr. Mor & Associates

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• Cracking
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Concrete cracking generates more complaints than all other defects combined

Concrete cracking is widespread and can be seen in all types of construction - from unreinforced slabs to heavily reinforced or post-tensioned structural elements. The first explanation you are likely to get from concrete professional is that "all concrete cracks". 
Is that true and is there a way to prevent it?

The short answer is that yes, all concrete cracks - but there are degrees of cracking and if done correctly these cracks need not be visible or detrimental.

Cracking Explained

The main reasons concrete cracks are:

• Drying shrinkage
• Structural loads
• Chemical attacks
• Corrosion of reinforcement

Drying shrinkage - Concrete will shrink and crack when it dries. In fresh concrete, the volume of water mixed with the cement and aggregates can be significant. When concrete dries, the excess water is removed - causing shrinkage and leaving behind voids of various sizes. In typical concrete this change amounts to about 1/16 of an inch in 10 feet (.4 cm in 3 meters).
To keep these cracks under control a contractor puts joints in concrete pavements and floors to allow the concrete to crack in a neat, straight line at the joint (see diagram). In structural elements that cannot have cracks (or joints) the engineer will specify reinforcement which will keep the cracks closed and invisible.  In post-tensioned elements the highly tensioned cables will squeeze the concrete and close the cracks before they have a chance to propagate.
 

So what can be done to minimize shrinkage cracking?

The first defense is the use of dry concrete mix.  Obviously, if we do not start with a lot of water  we are not going to lose as much water.  The problem is that we need to achieve a delicate balance between workability requirements (more water) and strength/cracking requirements (less water).  However, even the optimized mix will still have enough water to cause shrinkage.

The next defense is to allow the concrete enough time to gain strength before drying starts.  As the concrete gets stronger it is able to better resist crack growth and propagation. The drying delay is achieved through a process called curing.  What it does is either prevent the water from leaving the concrete by sealing the surface, or minimize evaporation by keeping the concrete wet for extended periods.  Sealing is done by spraying the concrete with "curing agents" which create an impermeable film on the surface.  Wet curing is done by a combination of watering (sprinklers or flooding) and minimizing losses through the use of wet blankets and plastic film.

A common recommendation is for a minimum of seven days wet curing.

Unfortunately, quite often the construction schedule does not allow such delays and any number of reasons can keep the contractor from providing proper curing.  Even when curing is attempted, environmental conditions such as strong winds and high temperatures may cause rapid loss of moisture and cracking before curing starts.

Structural loads - Concrete elements are designed to resist a given maximum load.  When that value is exceeded the concrete will start cracking.

Problems can start from multiple causes during the life of the structure.  The first culprit can be the design itself.  Errors in estimating the expected loads, calculation errors, and even the transfer of calculations into plans and specifications can result in a structure that is unable to carry the loads.

The next potential problem is errors by the construction crew.  These may include misplaced reinforcement, undersized elements, and other construction errors.

The concrete itself may fail to reach the design strength for various reasons.  Sometimes because the ready-mixed concrete supplier used the wrong mix design or made measurements errors.  Other times it can be defective, sub-standard materials (such as cement and aggregates) used in the mix or application by the contractor who chose to add more water than allowed.

Chemical attacks - are rare compared to the first two items because the codes and standards create a solid foundation of good practice.  However, it is possible to encounter reactions such as ASR where the cement and the aggregate react together, or when external attacks by acids or sulfates reduce the concrete's strength enough to cause failure.

Corrosion of reinforcement - can damage an essential part of the concrete element.  Its rusting provide two mechanisms for cracking and failure.  The first is through loss of reinforcing strength and reduction in the element's load capacity.  The other, which will normally be noticed first, is through the creation of expansive rust inside the concrete.  The resultant stress will lead to cracking and loss of concrete cover over the steel, allowing more rapid corrosion and failure.  This process is more commonly seen near the ocean where the presence of moisture, oxygen and salt tend to accelerate the corrosion.  Normally, corrosion can be prevented through the application of denser concrete with adequate cover to protect the steel from the elements.  In extreme cases it may be necessary to apply cathodic protection. 

A combination of causes - such as poor curing leading to lower strength and structural failure are more likely, as are various other, less common, processes that will not be discussed here..

Moree

• Misplaced reinforcement -