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:
Lumber (25 mm or 38 mm thick boards): used for small structures, foundations, and simple shapes.
Formwork plywood (phenolic or film-faced): panels 12 to 19 mm thick, offering a smooth surface and good durability. Film-faced plywood is covered with a phenolic film that facilitates stripping and provides a superior finish.
Formwork beams (laminated wood or I-beams): used as horizontal supports for large slab formwork.
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:
High durability (200 to 1000 reuses).
A very smooth concrete surface.
High rigidity, allowing higher pouring rates.
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:
Foundations with insulating panels (insulating formwork).
Slabs on grade with polystyrene.
Columns with cardboard or thin metal tubes.
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:
Pour height (maximum hydrostatic pressure = density × height).
Pour rate (in meters per hour).
Concrete temperature (a higher temperature accelerates setting and reduces pressure).
Consistency (slump) of the concrete.
The presence of vibrators (vibration increases pressure).
The theoretical maximum pressure is calculated using the hydrostatic formula:
P = ρ × g × h
Where:
P = pressure (Pa)
ρ = density of concrete (≈ 2400 kg/m³)
g = gravitational acceleration (9.81 m/s²)
h = height of fresh concrete above the point in question (m)
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)
| Pour Rate (m/h) | Temperature 10 °C | Temperature 20 °C | Temperature 30 °C |
|---|
| 0.5 | 45 | 35 | 28 |
| 1.0 | 60 | 48 | 38 |
| 1.5 | 72 | 58 | 46 |
| 2.0 | 82 | 66 | 52 |
| 3.0 | 98 | 80 | 64 |
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:
Use panels in good condition, without holes or deformations.
Apply a release agent (form oil) to surfaces in contact with concrete. This product facilitates stripping and protects the wood.
Seal joints between panels with caulking, tape, or rubber profiles.
Check the watertightness of corners and connections.
Alignment and Plumbness
Formwork alignment must be checked before each pour. Common tolerances are:
Plumbness: ± 5 mm per meter of height, maximum 20 mm over the total height.
Horizontal alignment: ± 10 mm over 3 meters.
Dimensions: ± 10 mm for sections, ± 20 mm for lengths.
For alignment, the following are used:
Spirit levels and laser levels.
Plumb bobs for verticality.
Theodolites or total stations for large structures.
Shores and tie rods to hold formwork in position.
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:
Resist tensile forces that concrete cannot support.
Distribute cracks and limit their width.
Resist shear forces (stirrups, ties).
Resist compressive forces (longitudinal reinforcement in columns).
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
| Number | Diameter (mm) | Area (mm²) | Mass (kg/m) |
|---|
| 10M | 11.3 | 100 | 0.785 |
| 15M | 16.0 | 200 | 1.570 |
| 20M | 19.5 | 300 | 2.355 |
| 25M | 25.2 | 500 | 3.925 |
| 30M | 29.9 | 700 | 5.495 |
| 35M | 35.7 | 1000 | 7.850 |
Steel grades are designated according to CSA G30.18:
Grade 400 (yield strength of 400 MPa): the most common for buildings.
Grade 500 (yield strength of 500 MPa): used for heavy civil engineering structures and heavy loads.
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
| Exposure Condition | Minimum Cover (mm) |
|---|
| Concrete cast against the ground | 75 |
| Concrete exposed to weather (walls, beams) | 50 |
| Concrete not exposed to weather (interior) | 30 |
| Slabs and walls (bars 20M and smaller) | 20 |
| Columns | 40 |
| Pile foundations | 75 |
Bar Spacing
The minimum spacing between parallel bars must be:
At least equal to the nominal diameter of the bar.
At least 25 mm (or 1.33 times the maximum aggregate size).
At least 1.33 × the maximum aggregate size to allow proper concrete encasement.
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:
Concrete chairs (precast): for foundations and slabs on grade.
Plastic chairs: for walls and suspended slabs.
Metal supports (tripods, cradles): for thick slabs.
Spacers: to maintain spacing between reinforcement layers.
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:
Simple lap splice: the two bars overlap over a minimum length of 40 × the diameter (for grade 400 steel and 30 MPa concrete). Example: for a 15M bar (16 mm), lap = 40 × 16 = 640 mm.
Welded splice: bars are welded end to end (rare on site).
Mechanical couplers: bars are screwed into a steel coupler (for large diameters).
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:
90° hook (for end anchorages).
135° hook (for stirrups).
Closed stirrups (for ties).
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:
120.Positioning: bars are in the correct position according to the drawings.
121.Spacing: conforms to the drawings and code requirements.
122.Laps: sufficient lengths and staggered in sections.
123.Hooks: angles and lengths conform.
124.Cover: chairs and supports are in place.
125.Cleanliness: no loose rust, mud, oil, or ice on the bars.
126.Ties: bars are securely tied with tie wire (1.6 mm annealed steel wire).
Installation Tolerances
Allowable tolerances for reinforcement installation are:
Vertical position: ± 10 mm (or ± 15 mm if the slab thickness is greater than 300 mm).
Horizontal position: ± 50 mm.
Cover: -10 mm (never less than the minimum), +15 mm.
Formwork and Reinforcement: Interaction with the Finisher
Preparation Before Pouring
Before pouring, the finisher must verify:
That the formwork is clean (no debris, sawdust, or nails).
That the release agent has been applied.
That openings (blockouts, penetrations) are in place.
That level marks are visible on the formwork.
That the reinforcement is stable and will not move during the pour.
During the Pour
The finisher must:
Monitor the behavior of the formwork (deformations, leaks, unusual noises).
Not walk directly on the reinforcement; use walkways or distribution planks.
Guide the vibrator to avoid touching the reinforcement (direct vibration of the steel can create voids around the bars).
Immediately report any formwork movement or reinforcement settlement.
After Stripping
After stripping, the finisher must:
Inspect the surfaces for honeycombing, cracks, or defects.
Patch tie rod holes with repair mortar.
Verify that chairs in plastic or concrete are not visible on the surface (if so, cover them).
Safety: Essential Rules
Site Regulations
The Canada Labour Code (Part II) and provincial standards require:
Wearing a hard hat and steel-toed boots.
Wearing gloves when handling reinforcing bars (risk of cuts).
Inspection of shores and scaffolding by a competent person.
Prohibition of walking on reinforcement without protection (risk of falls and impalement).
Protection of bar ends with caps or guards.
Formwork Stability
Formwork must be designed to resist:
The self-weight of the formwork and reinforcement.
The weight of fresh concrete.
The lateral pressure of the concrete.
Construction loads (workers, equipment, vibrations).
Wind loads (for tall formwork).
Stripping must only be done when the concrete has reached sufficient strength. As a general rule:
Side formwork (walls, columns): 12 to 24 hours after pouring (if the temperature is above 10 °C).
Slab formwork: 7 to 14 days, depending on span and loads.
Shores: kept in place until the concrete reaches 75% of its specified strength.
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:
177.Calculate the maximum hydrostatic pressure: P = ρ × g × h = 2400 × 9.81 × 3 = 70,632 Pa ≈ 70.6 kPa.
178.According to Table 1, for 2 m/h at 20 °C, the pressure is approximately 66 kPa.
179.The actual pressure is the smaller of the two values: 66 kPa.
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:
183.Number of bars in the 6 m direction: 4000 / 300 + 1 = 14.33 → 15 bars of 6 m.
184.Number of bars in the 4 m direction: 6000 / 300 + 1 = 21 bars of 4 m.
185.Total length: (15 × 6) + (21 × 4) = 90 + 84 = 174 m.
186.Mass: 174 m × 1.570 kg/m = 273.2 kg.
187.Allow 10% for waste: 273.2 × 1.10 = 300.5 kg.
Lap Length Calculation
Example: What is the minimum lap length for 20M bars (19.5 mm) in grade 400 steel, 30 MPa concrete?
Solution:
Lap length = 40 × diameter = 40 × 19.5 = 780 mm.
Round up to 800 mm (multiple of 50 mm).
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:
Exposure classes (C-XL, C-1, C-2, C-3, C-4) which determine durability requirements.
Tolerances for reinforcement installation.
Test methods for fresh and hardened concrete.
Other Relevant Standards
CSA G30.18: Steel reinforcing bars.
CSA S16: Design of steel structures (for metal formwork).
CSA S269.1: Wood formwork (design and use).
CSA S269.3: Steel formwork.
Common Pitfalls to Avoid
208.Confusing hydrostatic pressure and actual pressure: The actual pressure is almost always lower than the hydrostatic pressure due to concrete setting. Do not calculate pressure using the total wall height if the pour rate is low.
209.Forgetting the effect of temperature: A lower temperature slows setting and increases pressure on the formwork. At 5 °C, the pressure can be 50% higher than at 20 °C for the same pour rate.
210.Neglecting concrete cover: Insufficient cover is a major cause of reinforcement corrosion. Always check the chairs before pouring.
211.Walking on reinforcement: Not only is this dangerous, but it displaces the bars and crushes the chairs. Use walkways.
212.Vibrating directly on reinforcement: Vibration of the steel can create voids around the bars and reduce concrete-to-steel bond. The vibrator should be inserted vertically and withdrawn slowly.
213.Stripping too early: Concrete must have reached a minimum strength (often 70% of the specified strength) before stripping load-bearing elements. Premature stripping can cause cracks and sagging.
214.Ignoring tie rod holes: They must be patched with compatible mortar, otherwise they become entry points for water and corrosion.
215.Confusing bar numbers: The metric number (10M, 15M) does not correspond exactly to the diameter in millimeters. A 15M bar has a diameter of 16 mm, not 15 mm.
216.Forgetting waste: Always allow 5 to 10% for cut-offs, laps, and cutting errors.
217.Not checking the drawings: Reinforcement drawings are complex; a reading error can lead to incorrect bar placement and compromise the structure.
Summary
Formwork and reinforcement are essential preparatory steps that determine the final quality of any concrete structure. For the Red Seal exam, remember:
Types of formwork: wood (traditional), metal (repetitive), permanent (left in place), slip (continuous vertical), and climbing (in stages).
Lateral pressure: calculated using the hydrostatic formula P = ρ × g × h, but limited by concrete setting. Typical values range from 30 to 100 kPa.
Watertightness and alignment: plumbness tolerances of ± 5 mm/m, joint sealing, release agents.
Reinforcing bars: metric numbers (10M to 55M), grades 400 and 500, minimum cover of 20 to 75 mm depending on exposure.
Lap splices: 40 × diameter for grade 400 steel, with installation tolerances of ± 10 mm in vertical position.
Chairs and supports: spaced approximately 1 m apart, in plastic, concrete, or metal.
Safety: hard hat, gloves, protection of bar ends, formwork stability.
Stripping: 12 to 24 h for side formwork, 7 to 14 days for slabs, depending on concrete strength.
Self-Assessment Questions
231.What is the maximum lateral pressure on a wall formwork 4 m high poured at 1 m/h at 15 °C?
a) 40 kPa
b) 60 kPa
c) 80 kPa
d) 94 kPa
236.What is the minimum concrete cover for a reinforcing bar in an interior slab not exposed to weather?
a) 20 mm
b) 30 mm
c) 50 mm
d) 75 mm
241.What is the minimum lap length for 15M bars (16 mm) in grade 400?
a) 400 mm
b) 640 mm
c) 800 mm
d) 1000 mm
246.What is the minimum bend diameter for a 25M bar?
a) 100 mm
b) 150 mm
c) 200 mm
d) 250 mm
251.What is the plumbness tolerance for a wall formwork 3 m high?
a) ± 5 mm
b) ± 10 mm
c) ± 15 mm
d) ± 20 mm
Answers: 1-b, 2-a, 3-b, 4-c, 5-c (5 mm/m × 3 m = 15 mm).
