Concrete and Masonry Construction Safety
Identifying and mitigating hazards related to formwork collapse, reinforcing steel impalement, precast concrete erection, and masonry wall stability.
Overview
Concrete and masonry construction involves massive temporary loads (wet concrete), dangerous protruding materials (rebar), and the handling of massive structural elements (precast panels). The hazards span from chemical burns from wet cement to catastrophic structural failures of formwork during pours.
Formwork and Shoring Hazards
Formwork must be designed, fabricated, erected, supported, braced, and maintained so that it will be capable of supporting without failure all vertical and lateral loads that may reasonably be anticipated to be applied to the formwork.
Wet concrete exerts extreme hydrostatic pressure on forms, similar to water. This pressure increases with the depth of the pour and the rate of placement.
Lateral Pressure of Concrete (ACI Formula)
Calculates the lateral pressure exerted by wet concrete on formwork.
$$
P = C_w \cdot C_c \cdot \left[ 150 + \frac{9000 \cdot R}{T} \right]
$$Note
Notice that as the rate of placement () increases, the lateral pressure () increases significantly. Pouring too fast is the leading cause of formwork blowout.
Controls for Formwork and Shoring
Checklist
- Engineered Drawings: All formwork and shoring must be erected in accordance with drawings prepared by a qualified engineer.
- Inspection: Shoring equipment must be inspected prior to erection to determine that it is as specified in the shoring layout. Damaged equipment must not be used.
- Pour Rates: Strictly adhere to the maximum allowable concrete placement rate specified by the formwork engineer.
- Removal: Forms and shores must not be removed until the concrete has gained sufficient strength to support its weight and superimposed loads, verified by testing (e.g., field-cured cylinders).
Reinforcing Steel (Rebar) Impalement
Protruding rebar poses a severe impalement hazard. A worker falling even from a few feet, or tripping on the same level, can be fatally impaled by unprotected rebar.
Procedure
- Elimination: Bend rebar over or design the pour sequence to eliminate vertical protruding dowels where workers must travel.
- Engineering Controls: Cover all protruding reinforcing steel onto which employees could fall with engineered, impact-resistant rebar caps (often square, steel-reinforced caps, commonly called "carnie caps") or wooden troughs. Standard plastic mushroom caps only protect against scratches; they provide zero impalement protection during a fall.
- Fall Protection: If workers are working at height above protruding rebar that cannot be capped, 100% fall protection (guardrails, nets, or PFAS) must be utilized to prevent the fall from occurring.
Masonry Construction Stability
Masonry walls under construction are highly susceptible to wind loads because the mortar has not yet cured, and the wall lacks the structural support of the final roof or intersecting walls.
Limited Access Zone (LAZ)
A designated area established alongside a masonry wall under construction, restricted to entry only by employees actively engaged in constructing the wall. The LAZ must be equal to the height of the wall being constructed plus four feet, and must run the entire length of the wall on the side without scaffolding.
Key Takeaways
- Formwork blowouts are catastrophic events primarily caused by exceeding the engineered concrete placement rate, which spikes lateral hydrostatic pressure.
- Protruding rebar must be guarded against impalement using steel-reinforced caps or troughs; standard plastic mushroom caps are insufficient.
- Unbraced masonry walls are highly vulnerable to wind loads and require the establishment of a Limited Access Zone (LAZ) to protect adjacent workers from collapse.
- Continuous inspection of shoring systems during pouring is critical to prevent catastrophic blowouts.