CONCRETE AND FORMWORK SAFETY — HAZARDS EVERY WORKER SHOULD KNOW

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Concrete is so common on construction sites that workers often forget it is one of the most hazardous materials they handle. Wet concrete has a pH between 12 and 13 — making it almost as caustic as bleach — and can cause severe chemical burns that do not become apparent until hours after exposure. Formwork, the temporary structures that hold concrete in place while it cures, presents its own set of dangers: collapse, falls, struck-by hazards, and crush injuries. Together, concrete and formwork operations are responsible for some of the most serious injuries and fatalities on Ontario construction projects.

This guide covers the hazards that every worker involved in concrete and formwork operations should understand, along with the Ontario regulatory requirements that govern this work.

Chemical Burns from Wet Concrete

The most underestimated hazard in concrete work is the concrete itself. Portland cement, the binding agent in concrete, is highly alkaline. When mixed with water, it produces calcium hydroxide, which has a pH of 12 to 13. For comparison, household ammonia has a pH of about 11. At this level of alkalinity, wet concrete causes chemical burns through direct contact with skin.

What makes concrete burns particularly dangerous is the delay. Unlike thermal burns, which are immediately painful, chemical burns from concrete may not produce noticeable symptoms for several hours. A worker who kneels in wet concrete without waterproof protection might not feel anything during the pour — but by the end of the shift, the skin on their knees is red, blistered, and in the worst cases, ulcerated down to the underlying tissue.

• Skin contact — even brief contact with wet concrete can cause irritation. Prolonged contact — such as concrete seeping into boots, saturating clothing, or accumulating under knee pads — can cause full-thickness chemical burns that require skin grafts and weeks of recovery.
• Eye contact — concrete splashed into the eyes is a medical emergency. The alkaline material can cause permanent corneal damage and vision loss. Immediately flush the eyes with clean water for at least 15 minutes and seek emergency medical attention.
• Inhalation — dry cement dust and concrete cutting dust contain crystalline silica and calcium compounds that can cause respiratory irritation, chemical burns to the airways, and long-term lung disease. Wet methods and respiratory protection are essential during concrete cutting and finishing operations.
• Chromium sensitivity — some cement formulations contain hexavalent chromium, a known allergen and carcinogen. Workers who develop allergic contact dermatitis from cement exposure may experience chronic skin rashes that worsen with each subsequent exposure.

Formwork Collapse — Causes and Prevention

Formwork collapse is one of the most catastrophic events that can occur on a construction site. When forms fail under the weight of wet concrete, the results are immediate and devastating — workers are buried, crushed, or swept away by the flowing concrete. The forces involved are enormous: a cubic metre of wet concrete weighs approximately 2,400 kg (5,300 lbs).

The most common causes of formwork collapse are preventable:

• Inadequate design — formwork must be engineered to support the full weight of the wet concrete, plus the weight of workers, equipment, and any impact loads from the concrete placement process. Generic formwork designs that do not account for the specific dimensions, pour height, and rate of placement are a recipe for failure.
• Improper shoring — shores (the vertical supports that hold up horizontal formwork) must be plumb, properly braced, and bearing on a stable surface. Shores placed on soft ground, uneven surfaces, or unstable bases can shift or sink under load, causing progressive collapse.
• Premature stripping — removing formwork before the concrete has reached adequate strength is one of the leading causes of collapse. The concrete must be tested (typically by cylinder break tests) to confirm it has reached the specified strength before any forms or shores are removed.
• Overloading — pouring concrete too fast increases the lateral pressure on wall forms beyond their design capacity. The rate of placement must be controlled and must not exceed the rate specified in the formwork design.
• Missing or damaged components — a single missing tie, wedge, or brace can create a weak point that cascades into a full system failure. Every component specified in the formwork design must be in place before the pour begins.

Shoring Requirements Under O. Reg. 213/91

Ontario Regulation 213/91 (Construction Projects) sets specific requirements for formwork and shoring on Ontario construction sites:

• Engineering drawings — formwork and falsework (temporary support structures) for cast-in-place concrete must be designed by a professional engineer when the height of the concrete pour exceeds a specified threshold or when the formwork is classified as complex. The engineering drawings must be available on site during construction and during the concrete pour.
• Competent supervision — formwork erection, concrete placement, and form stripping must be supervised by a competent person who understands the formwork design and can identify deviations or hazards.
• Inspection before pour — formwork must be inspected immediately before concrete placement to confirm that all components are in place, connections are secure, shores are plumb and braced, and the formwork conforms to the engineering drawings.
• Reshoring — when formwork is stripped from a freshly poured slab that has not yet reached full design strength, reshores must be installed to support the slab until the concrete reaches the required strength. The reshoring layout must follow the engineer's specifications.

Stripping Procedures

Form stripping — the process of removing formwork after the concrete has cured — is a high-risk activity that must be done in a controlled, systematic manner:

• Strength verification — do not strip forms until concrete cylinder test results confirm the concrete has reached the minimum strength specified by the engineer. Environmental conditions (cold weather, in particular) can significantly slow the curing process.
• Sequence — strip forms in the sequence specified in the engineering drawings. Removing components out of order can redistribute loads in ways the formwork was not designed to handle.
• Gradual release — lower shores gradually to transfer load to the cured concrete in a controlled manner. Do not knock out shores abruptly — the shock load can crack or damage the concrete.
• Exclusion zone — keep workers who are not involved in stripping operations clear of the area below. Falling formwork panels, hardware, and debris are a struck-by hazard during stripping.

PPE for Concrete Work

The personal protective equipment requirements for concrete work go beyond the standard hard hat and safety boots:

• Waterproof rubber boots — standard leather work boots will absorb moisture from wet concrete and hold it against the skin, causing chemical burns. Workers who will be standing in or near wet concrete must wear waterproof rubber boots that are tall enough to prevent concrete from spilling over the top.
• Alkali-resistant gloves — standard leather or cotton gloves are useless against wet concrete. Workers must wear gloves that are rated for alkali resistance — typically nitrile, neoprene, or PVC-coated gloves. The gloves must be long enough to prevent concrete from entering at the wrist.
• Eye protection — safety glasses or goggles must be worn during concrete placement, finishing, and any operation that may generate splashes. Full-face shields are recommended when using concrete saws, grinders, or when working above an active pour.
• Long sleeves and waterproof coveralls — all skin should be covered during concrete placement. Waterproof coveralls or aprons provide the best protection against splash and extended contact.
• Respiratory protection — required when cutting, grinding, or drilling cured concrete (silica dust exposure) and when working near dry cement handling operations. At minimum, a NIOSH-approved N95 filtering facepiece; P100 half-mask or full-face respirators for higher-exposure tasks.

Post-Tensioning Hazards

Post-tensioned concrete involves steel tendons (cables or bars) that are tensioned after the concrete has cured to put the concrete under compression, increasing its load-carrying capacity. This technique is widely used in parking structures, bridge decks, and high-rise slabs. The hazards are severe:

• Stored energy — a tensioned tendon stores enormous elastic energy. If a tendon fails during stressing, or if a worker accidentally cuts or damages a stressed tendon, the released energy can propel the tendon or its anchorage at lethal velocity. Workers have been killed by tendon blowouts.
• Exclusion during stressing — the area behind the stressing jack and along the tendon path must be cleared of all workers during the stressing operation. No one should stand in line with a tendon being stressed.
• Identification and marking — post-tensioned tendons must be clearly marked and identified so that future trades (electricians, plumbers, mechanical contractors) do not inadvertently drill into or cut a stressed tendon. Ground-penetrating radar (GPR) scans are used to locate tendons before any coring or cutting operations.

Concrete and formwork are the structural foundation of most construction projects. The hazards they present — chemical, structural, and mechanical — demand respect, training, and rigorous adherence to procedures. There are no shortcuts in concrete work that do not eventually result in someone getting hurt.