The exterior window sills in concrete block homes are often precast in a factory and set in place by masons when the blocks are laid. Steel reinforcing bars are cast into the center of the sill to strengthen it. Concrete has high compression strength but low resistance to bending (tensile strength), so the steel reinforcement is necessary.
Water is sometimes able to penetrate the concrete because of damage to the surface or just deterioration over time. Once the moisture reaches the steel inside, rust begins. Because corrosion is an expansive process, the rusty steel bar eventually begins to open a crack, and later starts pushing off small chunks of concrete. The process is called “spalling,” and you can learn more about it at our blog post There's cracks running along the home's concrete tie beam. What's wrong?
Here’s another, more advanced, example of concrete spalling at a window sill.
Also, see our other posts How do I recognize serious structural problems in a house? and What causes a horizontal crack in a block or brick wall? and What are the places to look for structural cracks in a house?
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To learn more about exterior walls and structures, see these other blog posts:
• What is the average lifespan of a house foundation?
• What causes vertical cracks in fiber cement siding planks?
• What causes raised white lines of residue on a block wall that are crusty and crumbling?
• What is the difference between soil subsidence, heave, creep, and settlement?
• How much ventilation is required for the under-floor crawl space of a home?
• What causes stair-step cracks in a block or brick wall?
• What causes a horizontal crack in a block or brick wall?
• How can I tell if a diagonal crack in drywall at the corner of a window or door indicates a structural problem?
• What causes the surface of old bricks to erode away into sandy powder?
• What are the pros and cons of concrete block versus wood frame construction?
• Should I buy a house with a crawl space?
• Why is my stucco cracking?
• What would cause long horizontal lines of brick mortar to fall out?
• What is engineered wood siding?
• Should I buy a house that has had foundation repair?
• What is a "continuous load path”?
• Should I buy a house with asbestos siding?
• How can I tell if cracks in the garage floor are a problem or not?
• What do you look for when inspecting vinyl siding?
• Why is housewrap installed on exterior walls under the siding?
• 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?
• Should I buy a house with structural problems?
• What are those powdery white areas on my brick walls?
• What causes cracks in the walls and floors of a house?
• How can I tell if the exterior walls of a house are concrete block (CBS) or wood or brick?
• What are the common problems of different types of house foundations?
• What are the warning signs of a dangerous deck?
• How can I tell whether my house foundation problems are caused by a sinkhole or expansive clay soil?
Visit our EXTERIOR WALLS AND STRUCTURE page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.