Shrinkage Cracking in Concrete

Posted by in Buying/Maintaining a Home, Concrete, Shrinkage | January 30, 2018

Why Does Concrete Crack?

There are many causes for cracking in concrete: overload; earth movements (settlement, frost heave, consolidation, soil shrinkage, soil expansion, landslide, etc.); material shrinkage; impact; etc.  This article considers shrinkage.

What is shrinkage in concrete?

Shrinkage occurs when the concrete material becomes shorter due to moisture loss.  That is, when concrete cures, the moisture that does not chemically react with the cement powder evaporates from the slab.  The evaporation causes the concrete material to shrink similar to a drying sponge.  As the concrete shrinks the subgrade and the abutting sidewalk and street pavement restrain the concrete slab from shortening.  The combination and the shrinkage of the concrete material cause the concrete to be stretched: this is commonly called tension.  When the tension stresses from the stretching exceed the strength of the concrete, it cracks.  Concrete is relatively weak in tension and easily cracked.  This mechanical phenomenon begins immediately and progresses over a period of years.

The effects of shrinkage are normally reduced by placing expansion (isolation) joints and/or control joints in the slab.  Expansion joints, which are usually formed with the black felt material commonly seen in sidewalks and driveways, divide the slab into smaller pieces, and thus, reduce the length over which the concrete is stretched.  The reduced length reduces the magnitude of the tension in the concrete, thus reducing the probability of cracking.  Control joints are the tooled or sawcut joints (commonly spaced at four to ten feet in sidewalks and driveways), which reduce the thickness of the slab, and thus, provide a weakened section for cracking to occur.  See the featured photograph and Figure 1 for cracking due to shrinkage.  Control joints promote cracking at the bottom of the joint where they cannot be readily observed.  Thus, the cracking in the control joints of the subject driveway is normal and expected.

Figure 1 Shrinkage cracking at bottom of control joint in slab.

But how does one know if the cracking was caused by shrinkage or some other phenomena?

Cracking due to shrinkage is commonly evidenced by a lack of crushing, sinking, dropping, vertical offsets, or a pattern of cracking coinciding with the path of the vehicle.  It is also evidenced by a relatively uniform width in the cracking (unless there is an uneven restraint), being located in the control joints, or a relatively uniform spacing that divides the concrete slab into smaller, relatively equal sections.

Is that all there is to diagnosing shrinkage cracking?

No.  Sometimes the stresses and/or cracking induced by shrinkage are enhanced by other phenomena or a vertical movement, horizontal movement, or a pitch is introduced to make the evaluation more challenging.  The examination and consideration other nearby components and/or their movements and understanding of the effects of these can often shed light on the contributions of these other phenomena.

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