The addition of water to a dry concrete mix activates a process called hydration, the chemical reaction in which water is absorbed to create a rock-like crystalline structure. It continues slowly for months after the inital set, and the concrete shrinks in size slightly over this time. Enough water must be used to cause the hydration, but too much water weakens the final product and leads to more shrinkage.

   Because the shinkage is normal, some amount of it is always expected and hairline shrinkage cracks can appear. Reinforcing steel bars or mesh in the concrete makes it stronger against tensile (bending) stress, but does not significantly reduce the hairline shrinkage cracks. Two ways that they can be controlled are addition of fibers to the mix or post-tensioning the slab, both of which you can read about at What is fiber reinforced concrete? and Why is there a "WARNING! POST-TENSION SLAB" sticker in my house?  

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Here’s links to a collection some of our other blog posts about CONCRETE AND CONCRETE BLOCK:

• What causes raised white lines of residue on a block wall that are crusty and crumbling? 

• Do stucco walls mean a house is concrete block?

• What causes stair-step cracks in a block or brick wall?

• Why is the concrete window sill cracking?

• What causes a horizontal crack in a block or brick wall? 

• What are the pros and cons of concrete block versus wood frame construction?

• What does CBS mean in house construction?

• What is "Ocala" block?

• There's cracks running along the home's concrete tie beam. What's wrong? 

• What would cause long horizontal lines of brick mortar to fall out?

• How do I recognize structural problems in a retaining wall? 

• Should I be suspicious about a concrete block house covered with siding?  

• How can I tell if cracks in the garage floor are a problem or not? 

• How do I recognize serious structural problems in a house?

• Why did so many concrete block homes collapse in Mexico Beach during Hurricane Michael? 

• How can I tell if the concrete block walls of my house have vertical steel and concrete reinforcement?

• How can I tell if the exterior walls of a house are concrete block (CBS) or wood or brick?

• What is concrete spalling? 

• What is a "slab on grade" house? 

• What causes a vertical crack in an exterior concrete block or brick wall? 

• How can I identify a home as ICF (Insulated Concrete Form) construction?

• How can I tell if cracks in the wall or floor are getting worse or staying the same?

• Why do concrete blocks have holes in them? 

• What are those powdery white areas on my concrete block wall? 

• Is efflorescence a serious problem? 

• What does concrete spalling look like? 

    Visit our CONCRETE AND CONCRETE BLOCK page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.