Continuous Footing Reinforcement Mechanics (ACI 318 & IRC R403)
Concrete exhibits exceptional compressive strength (typically 3,000 to 4,000 PSI in residential footings), but possesses negligible tensile strength (approximately 10 percent of compressive rating). When expansive clay soils swell, subgrade settlements occur, or heavy point loads concentrate above, the bottom zone of a continuous strip footing experiences severe flexural tension.
To prevent transverse flexural cracks from propagating upward into foundation stem walls and framing, the International Residential Code (IRC Section R403.1.3) and the American Concrete Institute (ACI 318-19) mandate continuous longitudinal reinforcement. In residential light-frame construction, footings typically require two continuous rows of #4 (1/2-inch) or #5 (5/8-inch) deformed steel rebar conforming to ASTM A615 or ASTM A706 Grade 60 (minimum yield strength of 60,000 PSI).
Steel reinforcement placed in concrete cast directly against and permanently exposed to earth must have a minimum clear cover of 3.0 inches (76 mm). Placing rebar on the dirt, or elevating it on scrap 2x4 lumber or hollow CMU fragments, allows groundwater and soil chlorides to reach the steel, initiating rust expansion (which swells to 600 percent of steel volume) and destroying the footing.