Concrete, by nature, shrinks as it cures — and without proper control, this shrinkage leads to random cracking. To manage this, we introduce contraction joints, deliberate weakened planes that guide cracking predictably. For steel fiber reinforced concrete (SFRC), this process demands additional precision. The presence of steel fibers changes how stresses distribute across the slab, requiring careful attention to cut depth, timing, and technique to achieve proper joint activation.

Understanding the mechanics

In conventional concrete, contraction joints are cut to a depth of roughly one-quarter the slab thickness, following standard ACI 302.1R-15 guidelines. However, in SFRC slabs, the embedded steel fibers create a tougher, more cohesive matrix. This reinforcement resists crack initiation longer — a structural benefit — but it also means the joint must be cut deeper to intentionally weaken the slab at the desired location.

Bekaert’s field-validated best practice recommends cutting SFRC joints to one-third the slab depth. This ensures the joint fully activates, preventing random cracking and avoiding stress concentration between panels. For an 8-inch (203 mm) slab, this translates to approximately a 2.7-inch (70 mm) cut.

The structural logic of depth — Why one-third?

Because fibers bridge the crack plane, holding the matrix together even as microcracks form. A shallow joint (¼ depth) may not sufficiently weaken the slab, causing the crack to initiate outside the intended joint line. By increasing cut depth, the joint remains the path of least resistance — ensuring controlled cracking exactly where designed.