Internal Vibrator Spacing (ACI 309)
Spaces the internal-vibrator insertions that consolidate fresh concrete (ACI 309).
Example
You enter
- Vibrator radius of action R (in) 12
- Lift / run length to consolidate (ft) 20
You get
- Max insertion spacing 18.0 in
- Max distance from the form 9.0 in
- Insertion points 14
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
Scopemax spacing = 1.5 x radius of action; edge distance <= 0.75 R; insertions = ceil(lift length x 12 / spacing). A 12 in radius head spaces at 18 in, stays within 9 in of the form, and a 20 ft lift takes 14 stabs; a smaller head with an 8 in radius of action tightens to 12 in and 20 stabs. The head's radius of action, not the wall, sets the count. ACI 309 governs.
Formula or tablemax_spacing_in = 1.5 x radius_of_action_in; edge_max_in = 0.75 x radius_of_action_in; insertions = ceil(lift_length_ft x 12 / max_spacing_in).
Source and editionACI 309 internal-vibration spacing rule by name (spacing = 1.5 R; edge <= 0.75 R); first-principles count arithmetic.
Free accessThe 1.5 R / 0.75 R spacing rule is public ACI 309 guidance; the radius of action comes from the vibrator manufacturer.
What governsEstimate. AHJ and licensed professional govern.
API field namesradius_of_action_in, lift_length_ft, max_spacing_in, edge_max_in, insertions
- Radius of action from the vibrator manufacturer for the head diameter and the mix (~12 in default)vibrator manufacturer
- Spacing rule max spacing 1.5 x radius of action, head within 0.75 R of the form, overlapping circlesACI 309