Formwork and Reinforcement

Formwork and Reinforcement

Introduction to Formwork and Reinforcement

Formwork and reinforcement form the invisible but essential framework of any concrete structure. As a concrete finisher, your role is not limited to surface finishing; you must understand how concrete is supported, reinforced, and prepared before it is even poured. Formwork is a temporary mold that gives shape to fresh concrete, while reinforcement is the set of steel bars embedded in concrete to resist tensile forces. This knowledge is crucial for the Red Seal exam, as it directly influences the quality, safety, and durability of the final structure.

Concrete has excellent compressive strength (typically 25 to 35 MPa for common concretes) but very low tensile strength (about 10% of its compressive strength). This is why steel reinforcement, with its high tensile strength (400 or 500 MPa), is essential. Formwork, for its part, must resist the lateral pressure of fresh concrete, which can reach considerable values, especially during rapid pours.

Types of Formwork

Wood Formwork

Wood is the most traditional formwork material and is still widely used. There are several types:

Wood has the advantage of being lightweight, easy to cut, and relatively economical. Its disadvantages are its limited durability (5 to 10 reuses for film-faced plywood) and its deformation under moisture.

Metal Formwork

Metal formwork (steel or aluminum) is used for repetitive structures: columns, beams, walls, precast stairs. It offers:

Aluminum is lighter than steel (about 1/3 of the weight) but more expensive and more sensitive to impact. Steel is more robust but requires cranes for handling.

Permanent Formwork

Permanent formwork remains in place after pouring and becomes an integral part of the structure. It is found in:

This type of formwork eliminates the stripping step but requires particular attention to flatness and watertightness, since defects cannot be corrected afterward.

Slip Formwork and Climbing Formwork

Slip formwork is used for tall vertical structures (silos, towers, elevator cores). It is raised continuously using hydraulic jacks, at a rate of 150 to 300 mm/h. The concrete must have controlled setting to support its own weight as soon as it leaves the formwork.

Climbing formwork is used for structures with variable geometry (bridge piers, tall walls). It is moved in stages, generally using a crane, and can be adjusted in height and width.

Fresh Concrete Pressure on Formwork

The lateral pressure exerted by fresh concrete on formwork is an essential design parameter. It depends on several factors:

The theoretical maximum pressure is calculated using the hydrostatic formula:

P = ρ × g × h

Where:

In practice, for concrete of normal consistency (80 to 100 mm slump) poured at a rate of 1.5 m/h at 20 °C, the maximum pressure is reached at a height of about 2 to 3 meters. Beyond that, concrete setting reduces the hydrostatic pressure.

Table 1: Approximate lateral pressure of fresh concrete (kPa)

Values for concrete with a density of 2400 kg/m³, 100 mm slump. These values are indicative; always consult the project specifications.

Practical Rule for the Exam

For the exam, remember that the maximum pressure on a wall formwork generally does not exceed 100 kPa under normal pouring conditions. If the pour rate exceeds 3 m/h or if the temperature is below 10 °C, the pressure can reach higher values, requiring formwork reinforcement.

Watertightness and Alignment of Formwork

Watertightness

Formwork must be watertight to prevent the loss of laitance (water + cement) which would weaken the concrete surface and create honeycombing. Measures to take:

Alignment and Plumbness

Formwork alignment must be checked before each pour. Common tolerances are:

For alignment, the following are used:

Tie Rods and Spreaders

Tie rods are steel rods that pass through the formwork to hold the two faces at the correct distance. They are fitted with nuts and bearing plates. After stripping, the ends of the tie rods are cut off or the rods are removed, leaving holes that must be patched.

Spreaders (or spacers) maintain the spacing between the two formwork faces during the pour. They can be plastic, precast concrete, or metal. Plastic spreaders leave conical holes that are patched after stripping.

Reinforcement: Principles and Arrangements

Role of Reinforcement

Steel reinforcement in reinforced concrete serves to:

Bar Types and Steel Grades

In Canada, reinforcing bars are designated by their metric number (10M, 15M, 20M, 25M, 30M, 35M, 45M, 55M) which corresponds approximately to the diameter in millimeters.

Table 2: Characteristics of standard reinforcing bars

Steel grades are designated according to CSA G30.18:

Bars are identified by raised marks: the producer's symbol, the grade (a number), and the bar number.

Minimum Concrete Cover

Concrete cover is the distance between the outer surface of the reinforcement and the concrete surface. It protects the steel from corrosion and ensures force transfer. The minimum values according to the Canadian Concrete Code (CSA A23.1/A23.2) are:

Table 3: Minimum cover according to exposure

Bar Spacing

The minimum spacing between parallel bars must be:

The maximum spacing is generally 3 × the slab thickness or 450 mm, whichever is smaller.

Reinforcement Installation

Supports and Chairs

Reinforcement must be held in the correct position using:

Supports should be spaced approximately 1 m apart in a staggered pattern to prevent reinforcement from sagging under its own weight.

Laps and Anchorages

Reinforcing bars are delivered in standard lengths of 6 m or 12 m. When longer lengths are required, laps are made:

Anchorage is the length of bar required to transfer forces to the concrete. It depends on the diameter, steel grade, concrete strength, and bar position (horizontal or vertical).

Hooks and Bends

Bars are bent cold on site or in the shop. Common bends are:

The minimum bend diameter is 6 × the bar diameter for bars 10M to 20M, and 8 × the diameter for bars 25M and larger.

Reinforcement Inspection and Control

Before concrete placement, reinforcement inspection must verify:

Installation Tolerances

Allowable tolerances for reinforcement installation are:

Formwork and Reinforcement: Interaction with the Finisher

Preparation Before Pouring

Before pouring, the finisher must verify:

During the Pour

The finisher must:

After Stripping

After stripping, the finisher must:

Safety: Essential Rules

Site Regulations

The Canada Labour Code (Part II) and provincial standards require:

Formwork Stability

Formwork must be designed to resist:

Stripping must only be done when the concrete has reached sufficient strength. As a general rule:

Practical Calculations for the Exam

Lateral Pressure Calculation

Example: A wall 3 m high is poured at a rate of 2 m/h. The concrete temperature is 20 °C. What is the maximum pressure on the formwork?

Solution:

Reinforcement Quantity Calculation

Example: A slab 6 m × 4 m × 0.15 m is reinforced with 15M bars spaced at 300 mm in both directions. How many bars need to be ordered?

Solution:

Lap Length Calculation

Example: What is the minimum lap length for 20M bars (19.5 mm) in grade 400 steel, 30 MPa concrete?

Solution:

Applicable Standards and Codes

Canadian Concrete Code

The Canadian Standard for Design of Concrete Structures (CSA A23.3) is the primary reference for design. Requirements for reinforcement are detailed in Chapters 7 (reinforcement) and 8 (detailing).

Standard CSA A23.1/A23.2 deals with concrete: constituents, execution of work, and testing. It specifies in particular:

Other Relevant Standards

Common Pitfalls to Avoid

Summary

Formwork and reinforcement are essential preparatory steps that determine the final quality of any concrete structure. For the Red Seal exam, remember:

Self-Assessment Questions

Answers: 1-b, 2-a, 3-b, 4-c, 5-c (5 mm/m × 3 m = 15 mm).