Concrete Cracking

I heard the statement one time that "all concrete cracks."  It is true that concrete is a composite that is very strong in compression, but weak in tension.  This makes it susceptible to cracking.  And, reinforced concrete is actually designed for cracking.  The reinforcement is engaged as the concrete section bends and yes, cracks.  But the statement above is not all its cracked up to be.

There are several different types of cracks, and most of them can be avoided with a proper mix design and best production practices.  The following is a list of types of cracks that are typical in concrete:

• Plastic Shrinkage
• Dry Shrinkage
• Alkali-Aggregate Reaction
• Alkali-Silica Reaction
• Corrosion
• Structural Overload

For this blog, I want to focus on Dry Shrinkage cracking.

Concrete is typically made with more water than is necessary for hydration.  The excess water evaporates or "bleeds" out.  As this occurs, there is noting to fill the volume left behind.  As the concrete drys it will naturally shrink in volume.  But, if the concrete is constrained by the formwork or reinforcement, then the concrete cannot shrink which causes internal stressing.  

Some times drying shrinkage occurs where the concrete is exposed to the air while still in the form.  The water evaporates more rapidly on exposed surfaces, and therefore the precast element contains small micro-cracks.  In reinforced concrete, due to the constraint of the steel, the cracks will appear somewhat equally spaced along an edge.  If the concrete is fiber reinforced, it may still crack, but instead of a 0.002" crack every 6", the cracks are not visible, and there may be dozens within a 6" span.

If you are a precaster who manufacturers septic tanks, you may notice drying shrinkage more in a certain season.  This is easily explained by the theory of drying shrinkage.  When the dew points are low, and the day time high is 30+ degrees above it, then the relative humidity is low.  New concrete contains moisture that is necessary for hydration, and as long as the water is present, the concrete hydration cycle will continue for weeks.  New concrete placed on the yard in storage will dryout where the surface is exposed to air.  A small septic tank is usually stored as a complete, sealed unit, ready for installation.  The relative humidity inside is high, but the outside surface is losing moisture.  This internal stress will lead to hairline cracking on the outside.

Sometime a crack is misdiagnosed as a dry shrinkage crack.  Here is a scenario from my past that will illustrate this.  A 1,250 gallon mid-seam septic tank was developing a "smiley" crack after 3-7 days in the yard.  We had two sets of forms to make this same size tank, and the crack only appeared on the castings from one of these forms.  We thoroughly inspected the castings after they were stripped.  No cracks.  Since we did not see a crack until the casting was about a week old, we were certain that it was caused by drying shrinkage.  But why did it only affect the castings off one of the two forms?

I realized that any cracks in the castings were most apparent after a rain.  As the concrete dried, the moisture retained in the crack longer, making it visible.  With this knowledge, I established a new QC procedure: using a wet rag, the QC inspector would wipe the casting walls prior to removal from the plant.  Sure enough, we found the "smiley" crack that had eluded us for several weeks.  It was there all along, and was not caused by drying shrinkage.  The actual cause was discovered to be a faulty air regulator on the air cylinder for the collapsing core of the form.

If you are seeing cracks your concrete products, perform a root cause analysis to identify the primary and contributing causes.  One or more of these will be the root cause of the cracking.  Don't just accept the statement that "all concrete cracks."  Many of the causes can be eliminated with minor, inexpensive changes.  The customer perception of the quality of your precast products will improve as a result.