Concrete is created when cement, water, and rock aggregate are mixed together. The cement and water mixture turns into a paste that bonds with the aggregate as it sets. This happens through a curious process called hydration: the cement chemically cracks open the water molecules and uses one hydrogen atom and one oxygen atom to create the crystalline structure that forms as the material hardens. This process creates some initial heat, and also leftover hydrogen atoms are released as a gas into the atmosphere. 

   But a side-effect of the breaking apart of the H2O molecules and reconfiguration of the atoms is a reduction in the volume of the concrete. This shrinkage happens gradually over time, with most of it occurring within the first 28 days after the material is mixed and begins to set up. It creates tension within the concrete mass, which leads to cracking, releasing some of the tension. 

    The cracks are usually hairline, but can be larger if too much water is added to the mix. The cement can only use so much water in the hydration process, and the excess evaporates away, causing even more shrinkage.

   Because concrete shinkage is normal, some amount of it is always expected. 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?  

    Also, shallow score lines cut into a concrete floor slab shortly after placement are often used to manage—but not eliminate—the cracks. It makes them likely to occur at the bottom of the score slot, where they are less visible. This is often done for garage floors. Go to Why are there score line grooves in the concrete floor of the garage? for details.

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

• What causes honeycomb in concrete?

• 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.