previous
  • Video: Install a Fence
    Video: Install a Fence
  • Design Inspiration
    Design Inspiration
  • All about Roofing
    All about Roofing
  • Energy-Smart Details
    Energy-Smart Details
  • Master Carpenter Videos
    Master Carpenter Videos
  • 7 Trim Carpentry Secrets
    7 Trim Carpentry Secrets
  • 9 Concrete Countertops Ideas
    9 Concrete Countertops Ideas
  • 7 Small Bathroom Layouts
    7 Small Bathroom Layouts
  • 7 Smart Kitchen Solutions
    7 Smart Kitchen Solutions
  • Magazine Departments
    Magazine Departments
  • Video Series: Tile a Bathroom
    Video Series: Tile a Bathroom
  • Basement Remodeling Tips
    Basement Remodeling Tips
  • Tips & Techniques for Painting
    Tips & Techniques for Painting
  • Clever daily tip in your inbox
    Clever daily tip in your inbox
  • Radiant Heat Comparison
    Radiant Heat Comparison
  • Remodeling Articles
    Remodeling Articles
  • Read FHB on Your iPad
    Read FHB on Your iPad
next

Control vs. isolation joints in a concrete driveway

Q: What’s the best way to pour a concrete driveway: with expansion joints cut into the concrete after it has cured, or with isolation joints integrated into the forms?





A: Paul Fisette, director of the building-materials and wood-technology program at the University of Massachusetts, replies:  Isolation joints typically are used where different building elements meet, such as where a horizontal slab meets a vertical wall. Control joints, or expansion joints as you call them, are better for driveways.

Control joints are a deliberate plane of weakness placed in the slab, either cut with a concrete saw or formed with a jointer while the concrete is still wet.  Spaced every 10 ft. to 12 ft., control joints don’t go all the way through the concrete; they just extend about one-quarter of the slab’s thickness. As the slab shrinks, it cracks along these predetermined lines (see drawings).

Control joints should transfer loads perpendicular to the surface of the slab. Typically, the cracks that develop as the slab shrinks follow the cement paste in the mix, and the aggregate particles from each side of the crack are still somewhat interlocked or knuckled together. This knuckling provides some load transfer between slab panels so that concentrated loads are not at an edge of a panel. Because they break continuity between panels, isolation joints don’t have the load-sharing capacity of control joints.



From Fine Homebuilding 172, pp. 102 July 1, 2005