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Cracking of Concrete Toppings

Discussion

     Very thin (2” and less), high strength, concrete toppings (SCC type included) are usually installed with in situe conditions that can combine to create a risk of significant cracking of the newly placed concrete.  These toppings have a high surface area to volume ratio and are therefore particularly susceptible to both plastic and shrinkage cracking.  They are also subject to the normal movement of a wood frame subsurface.  Since they are normally not reinforced, they have little ductility to resist bending stress.

 

     Plastic cracking is the result of a loss of water during the critical setting period.  Water, essential to the chemical reaction, evaporates more quickly than it can be replaced by bleed water from the mix.  This causes tension in the material from the surface to a depth where the bleed water is evident.  The depths of these cracks will depend on the rate of evaporation but rarely exceed more than ½ of the depth of the material.  These cracks are seen as “squiggly” parallel lines in the surface that do not generally lead to the perimeter.  Plastic cracking generally occurs very soon after placing and is usually complete by time the material has set.  Any methods used that will reduce the evaporation rate of the surface water will reduce this type of cracking.  (see below)

 

     Drying or shrinkage cracking is the result of the loss of the volume of water / liquid contained in the mix.  This is a frequent occurrence in most concrete that is designed to flowable.  This also creates tension in the mix and like most structures, the stress is concentrated in areas where there are sharp changes in the shape of the perimeter (ie. corners, columns, etc) or stiffer sections of the material itself (ie. areas drying at a faster rate because of temperature differential, sun exposure or wind).  These cracks can be made worse by the presence of heating tubes from hydronic heating or waterlines which are very often sheathed.  These cracks tend to be longer and deeper and can occur or worsen days, even weeks, after placement.  Any methods that minimize rapid evaporation or that reduce the internal stress of the material will reduce this type of cracking.  (see below)

 

     Cracking due to settlement or dynamic movement of the subsurface is a common cause of cracking in wood frame structures.  Settlement of a wood frame structure can occur from normal or extraordinary loading, shrinkage of the materials used to construct the assemblies, normal design or extraordinary deflection of the floor assembly, or compressible material (ie sound mats, insulation, etc.) placed between the floor assembly and the placed concrete.  Settlement very often creates a combination of voids and heaved sections of the wood subsurface.  These cracks can be significant and are usually the depth of the material.  There is often a change in elevation observed on either side of the crack and these cracks usually follow a pattern that mimics the stiffer sections of the structure below. (ie beams or different materials like steel).  Any method that reduces the settlement below the placed material will reduce this type of crack. (see below)

Condition
Action to Reduce Effect

Plastic Cracking

  • Pre-wet floors to a SSD* condition.

  • Ensure doors and windows are installed to block wind.

  • Darken windows to prevent sunlight.

  • Avoid placing during high ambient temperatures.

  • Avoid placing during periods of low humidity.

  • Use fiber reinforcing.

  • Use anti shrinkage admixtures.

  • Use post placement cure & seal products or mist surface.**

Shrinkage Cracking

  • All actions described for plastic cracking plus:

  • Reinforce concrete at corners and columns.

  • Separate perimeter, corners and columns with a flexible material like insulation or sill gasket.

  • Install crack control (ie. cant strips to ½ depth of the concrete at critical or flooring transition locations.)

Settlement Cracking

  • All actions described for shrinkage cracking plus;

  • Design floor with minimum allowable deflection.

  • Allow for a slower drying period for wood structure.

  • Minimize loading on floor and structure post placement.

  • Use sound mats / insulation with minimal compression.           

  • Use sound mats / insulation that will not shrink over time or swell by absorbing water.

  • Ensure sound mats or any element below or encased by the concrete to be fully and completely secured.

Summary

     The cracking you may see in your floor is usually a combination of the types of cracks described above.  As you can see, the elements that lead to cracking are all present in abundance for a thin topping in a wood frame structure.  The trick to minimizing cracking is to do as many items from the “action” list as possible.  Obviously, it is rarely possible to do them all however, most items are relatively inexpensive and should be done wherever possible.

 

     We also always recommend that sufficient budget is allocated to the preparation of concrete toppings for final flooring.  The floors should be reviewed well in advance, and coordinated with your flooring Contractor so that the required floor will not delay the installation of the flooring.  The flooring Contractor may also have some suggestions like anti-fracture membranes or crack stabilization to offer.  These should be considered especially in areas of tile or natural stone or where the continued movement of the subsurface is observed.

 

If you have any questions, or require further information, please do not hesitate to contact us.

 

​

Steven Schwartz

President

*   SSD – saturated surface dry condition.   In this case, it means a saturated floor with no surface water (puddles)

** mist surface – caution:  only mist if the surface has not gone “dry”.  Misting a dry surface can create elastic tension as the concrete alternates between wet and dry.  This may actually create additional cracking.

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