Concrete Formwork and Placement

Formwork and Concrete Placement

Introduction to Formwork and Concrete Placement

Formwork is a temporary or permanent mold designed to contain fresh concrete, give it its shape, dimensions, and surface texture, and support it until it has gained sufficient strength to support itself. Concrete placement, on the other hand, encompasses all operations from the delivery of fresh concrete to its consolidation and curing. For the Construction Craft Worker (CCW), mastering these operations is essential, as they represent a significant portion of tasks on construction sites across Canada.

Concrete is a composite material consisting of cement, water, fine aggregates (sand) and coarse aggregates (gravel or crushed stone), and admixtures. Its strength is expressed in megapascals (MPa) at 28 days, according to CSA A23.1 (Concrete: Constituent Materials and Execution of Work) and CSA A23.2 (Test Methods and Standard Practices for Concrete). Formwork must be designed and constructed to resist the lateral pressure of fresh concrete, which is a horizontal force acting on the mold walls.

Fundamental Principles of Formwork

Role and Requirements of Formwork

Formwork must satisfy four main requirements:

8.Strength: It must support the weight of fresh concrete, construction loads (workers, equipment, vibration), and lateral pressure without excessive deformation or failure.

9.Tightness: It must prevent the leakage of laitance (water + cement) through joints, which would create honeycombing and surface defects.

10.Shape and dimensions: It must maintain the precise dimensions and alignment specified on the drawings, with tolerances conforming to CSA A23.1 (typical tolerances of ±10 mm for section dimensions).

11.Easy stripping: It must be removable without damaging the hardened concrete or compromising its surface.

Types of Formwork

Type	Material	Typical Use	Advantages	Disadvantages
Timber formwork	Plywood, boards	Small elements, curved shapes	Versatile, economical	Limited lifespan, possible deformation
Metal formwork	Steel, aluminum	Walls, columns, repetitive elements	Reusable, smooth surface	Heavy, expensive to purchase
Insulated formwork (ICF)	Expanded polystyrene + lintels	Foundation walls, residential buildings	Built-in insulation, lightweight	Higher cost, requires finishing
Lost formwork	Cardboard, precast concrete, fiber	Columns, foundations, buried elements	No stripping required	Not reusable
Slip formwork	Steel, timber	Silos, chimneys, elevator cores	Continuous construction	Requires specialized crew

Lateral Pressure of Fresh Concrete

Lateral pressure (Ω) is the horizontal pressure exerted by fresh concrete on the formwork walls. It depends on several factors:

The height of concrete in the formwork (hydrostatic pressure)

The density of the concrete (typically 2400 kg/m³)

The temperature of the concrete (the lower the temperature, the slower the setting, therefore the higher the pressure)

The rate of placement (height of concrete placed per hour)

The consistency (slump) of the concrete

The vibration method (internal or external)

The theoretical maximum pressure is calculated using the hydrostatic formula:

Ω = ρ × g × h

Where:

Ω = pressure (Pa or N/m²)

ρ = density of concrete (kg/m³)

g = gravitational acceleration (9.81 m/s²)

h = height of fresh concrete (m)

Example: For a 3 m high wall, with concrete at 2400 kg/m³:

Ω = 2400 × 9.81 × 3 = 70,632 Pa ≈ 70.6 kPa

This pressure is maximum at the bottom of the formwork and decreases linearly toward the top. Shores and ties must be sized to resist this pressure, with an appropriate safety factor according to CSA S269.1 (Formwork for Concrete) and CSA S269.2 (Steel Formwork).

Formwork Design and Construction

Formwork Materials

Formwork plywood (phenolic) is the most common material for the contact surface with concrete. It is available in 1.22 m × 2.44 m (4 ft × 8 ft) panels with thicknesses of 12.7 mm (1/2 in), 15.9 mm (5/8 in), and 19.05 mm (3/4 in). The plywood must be classified according to CSA O121 (Douglas Fir Plywood) for formwork use.

Members (studs, stiffeners) are typically dimensional lumber (2×4, 2×6, 2×8) or engineered beams (I-joists, LVL). They transfer loads to the support system.

Ties (anchor rods) are threaded steel rods that pass through the formwork to hold the two opposite faces at the correct distance. They are spaced according to load calculations and are removed after stripping. Holes left by ties must be filled with mortar or an approved repair product.

Shores (vertical supports) and braces (inclined supports) hold the formwork in vertical and horizontal position. They must be securely anchored to resist lateral loads (wind, vibration, impact).

Wall Formwork Construction Steps

39.Layout: Mark the wall location on the foundation or slab using a transit or laser level. Mark the axes and dimensions with chalk or a marker.

40.Installation of the first face: Position the plywood panels against the positioning stops, aligning them with the layout marks. Attach the vertical members (studs) to the outside of the plywood, spaced 300 to 600 mm (12 to 24 in) apart according to calculations.

41.Installation of ties: Drill holes through the plywood and members, insert the ties with their cones or nuts, and tighten them by hand.

42.Installation of the second face: Place the second formwork face, aligning it with the ties and fastening it in the same manner as the first.

43.Leveling and alignment: Use spirit levels, plumb bobs, and lasers to check plumbness and alignment. Adjust with shores and braces.

44.Installation of shores: Place vertical and inclined shores to stabilize the formwork. Shores should be spaced 1.2 to 1.8 m (4 to 6 ft) apart and anchored to the ground or existing structure.

45.Final inspection: Inspect all joints, fasteners, and supports. Ensure the formwork is clean, free of debris, and that release agents have been applied.

Special Formwork

Column formwork: Square or rectangular columns use metal or plywood formwork with column clamps spaced vertically. Round columns use cardboard tubes (lost formwork) or segmented metal forms. Lateral pressure in a column is particularly high due to the height of fresh concrete.

Beam and slab formwork: Beams are formed with bottoms and sides supported by shores. Slabs use a table system (pre-assembled panels) or traditional formwork with joists, stringers, and plywood decking. Formwork tables are efficient for repetitive slabs, as they can be moved with a crane.

Slip formwork: Used for continuous vertical structures (silos, chimneys, pylons), slip formwork is raised hydraulically or mechanically as concrete is placed. The raising speed must be synchronized with the concrete setting time to avoid deformations.

Concrete Placement

Preparation Before Pouring

Before placing concrete, several checks are necessary:

53.Formwork inspection: Check cleanliness, tightness, strength, and alignment. Remove all debris, ice, snow, or standing water.

54.Release agent application: Apply a release agent (oil or chemical) to the inside surface of the formwork to facilitate stripping and improve surface finish. Do not apply to reinforcement.

55.Reinforcement check: Ensure reinforcement is correctly positioned, with appropriate spacers (chairs) and supports, according to the drawings and CSA A23.1.

56.Access preparation: Provide ramps, walkways, or hoppers to deliver concrete without allowing it to fall more than 1.5 m (5 ft) in free fall, to avoid segregation.

57.Equipment check: Test vibrators, pumps, buckets, and chutes to ensure they are working properly.

Placement Methods

Method	Equipment	Advantages	Disadvantages
Concrete bucket	Crane + bucket	Versatile, precise	Requires a crane, limited output
Concrete pump	Stationary or mobile pump	High output, difficult access	High cost, risk of blockage
Chute	Mixer truck	Simple, economical	Limited reach, segregation risk
Hopper and hose	Hopper + flexible hose	Free-fall control	Requires setup
Wheelbarrow / cart	Wheelbarrow, concrete cart	Small volumes, interior work	Labour-intensive

Placement Rules

61.Pouring rate: The rate of placement (height of concrete placed per hour) must be controlled to avoid excessive pressure on the formwork. The maximum rate is generally 1.2 m/h for walls, but it may vary depending on temperature and formwork design.

62.Layered placement: Concrete must be placed in horizontal layers 300 to 500 mm (12 to 20 in) thick, to facilitate vibration and avoid segregation.

63.Free fall: The free fall of concrete must not exceed 1.5 m (5 ft) to avoid aggregate segregation. For higher drops, use hoppers, tremie tubes, or chutes.

64.Continuous pouring: Concrete must be placed continuously to avoid construction joints (pour stops). If a stop is necessary, the joint must be prepared according to specifications (cleaning, wetting, application of a bonding agent).

65.Temperature: The concrete temperature must be between 5 °C and 35 °C at placement. In cold weather, use heated concrete or anti-freeze admixtures. In hot weather, use cooled concrete or retarding admixtures.

Concrete Vibration

Vibration is the process of consolidating fresh concrete to eliminate trapped air bubbles and ensure complete encapsulation of reinforcement. It is essential for achieving dense, strong concrete.

Types of vibrators:

Internal vibrator (needle): The most common. A vibrating needle is inserted vertically into the concrete, spaced 300 to 500 mm (12 to 20 in) apart, and withdrawn slowly (about 75 mm/s or 3 in/s). The vibration duration is 5 to 15 seconds per insertion, until the surface becomes smooth and air bubbles stop rising.

External vibrator (form vibrator): Attached to the outside of the formwork for thin elements or heavily reinforced sections. Used with metal formwork, it transmits vibrations through the form.

Vibrating table: Used in precast production for flat elements (slabs, panels).

Vibration rules:

Do not vibrate concrete for more than 15 seconds in one location, to avoid segregation.

Do not use the vibrator to move concrete horizontally.

Do not vibrate reinforcement directly, as this could create voids around the bars.

Vibrate each concrete layer before placing the next layer, inserting the needle 50 to 100 mm (2 to 4 in) into the previous layer.

Construction Joints

A construction joint is a contact surface between two successive concrete pours. It must be designed and prepared to ensure adequate bonding between the two concretes.

Types of joints:

Construction joint: Planned in advance, generally at a location where stress is low (mid-span of a beam, third of a slab span).

Expansion joint: Allows thermal movement and concrete shrinkage, generally filled with a compressible material.

Contraction joint: Creates a weakened plane to control shrinkage cracking, generally made by saw-cutting or inserting a strip.

Construction joint preparation:

84.Clean the hardened concrete surface (high-pressure water jet, brushing, sandblasting).

85.Moisten the surface without saturating it (saturated surface dry, or SSD condition).

86.Apply a bonding agent (cement mortar, epoxy resin) if specified.

87.Place the new concrete immediately after preparation, before the surface dries.

Calculations and Tolerances

Concrete Quantity Calculation

The quantity of concrete required is calculated by volume (m³). For a simple element (wall, slab, beam), the volume is:

V = L × W × H

Where:

V = volume (m³)

L = length (m)

W = width or thickness (m)

H = height (m)

Example: A slab measuring 10 m × 5 m × 0.15 m:

V = 10 × 5 × 0.15 = 7.5 m³

You should order approximately 5 to 10% additional concrete to compensate for losses, irregularities, and overflows.

Tie Quantity Calculation

The number of ties required for a wall formwork is calculated based on lateral pressure and the capacity of each tie.

Example: A wall 3 m high and 10 m long, with a lateral pressure of 70.6 kPa. Each tie has a capacity of 20 kN (20,000 N).

The total force on the formwork is:

F = Ω × A = 70,600 Pa × (3 m × 10 m) = 2,118,000 N

The number of ties required is:

N = F / capacity = 2,118,000 / 20,000 = 106 ties

In practice, ties are spaced on a regular grid (for example 600 mm × 600 mm), which gives:

N = (10 / 0.6) × (3 / 0.6) = 17 × 5 = 85 ties (rounded up to 90 with edges).

Dimensional Tolerances

CSA A23.1 defines the following tolerances for concrete elements:

Element	Tolerance
Section dimensions (width, height)	±10 mm
Reinforcement position	±10 mm
Vertical alignment of walls	±10 mm per 3 m of height
Slab level	±10 mm
Position of openings and inserts	±10 mm
Finished surface (flatness)	3 mm under a 3 m straightedge

Stripping and Curing

Stripping Time

Formwork must only be stripped when the concrete has reached sufficient strength to support its own weight and construction loads. The minimum required strength is generally 70% of the specified 28-day strength, but it may vary according to specifications.

Stripping time depends on:

Ambient temperature (the higher the temperature, the faster the setting)

Cement type (rapid-setting vs. normal cement)

Admixtures (accelerators or retarders)

Specified concrete strength

Ambient Temperature	Walls (stripping)	Slabs (stripping)
20 °C	24 to 48 hours	7 to 14 days
10 °C	48 to 72 hours	10 to 21 days
5 °C	72 to 96 hours	14 to 28 days

Important: Premature stripping can cause cracking, deformation, or structural collapse. Late stripping can make formwork removal difficult and damage the concrete surface.

Concrete Curing

Curing is the process of maintaining concrete at adequate temperature and humidity during the setting and hardening period. It is essential for:

Developing concrete strength

Reducing shrinkage and cracking

Improving durability and impermeability

Protecting the surface against freezing and desiccation

Curing methods:

Method	Description	Minimum Duration
Water spraying	Continuous moistening of the surface	7 days
Wet burlap	Covering the surface with maintained-wet fabrics	7 days
Curing membrane	Application of a film-forming product (curing compound)	7 days
Steam	Steam curing for precast elements	12 to 24 hours
Thermal curing	Heating concrete by resistance or steam	Variable

The minimum curing duration is 7 days at a temperature above 10 °C, according to CSA A23.1. In cold weather, curing must be extended or the concrete must be protected with insulation.

Safety and Regulations

Site Safety

Working with formwork and concrete presents several hazards:

Falls: Elevated formwork requires guardrails, safety nets, and harnesses.

Crushing: Heavy formwork and shores can collapse. Always check stability before working under or near them.

Burns: Fresh concrete is alkaline (high pH) and can cause chemical burns. Wear gloves, safety glasses, and protective clothing.

Cuts: Ties, nails, and metal edges can cause injuries. Wear work gloves.

Noise: Vibrators and concrete pumps generate high noise levels. Wear hearing protection.

Dust: Cement and dry concrete generate crystalline silica dust, which can cause silicosis. Wear an approved respirator (N95 or higher).

Applicable Canadian Standards

Standard	Title	Application
CSA A23.1	Concrete: Constituent Materials and Execution of Work	Requirements for concrete, formwork, placement, and curing
CSA A23.2	Test Methods and Standard Practices for Concrete	Strength, slump, and air content testing
CSA S269.1	Formwork for Concrete	Design and construction of timber formwork
CSA S269.2	Steel Formwork	Design and construction of steel formwork
CSA O121	Douglas Fir Plywood	Plywood classification for formwork
CSA A3000	Cement	Classification and requirements for cements
Canada Occupational Health and Safety Regulations	Part II	General safety requirements on federal worksites

Pitfalls to Avoid

143.Confusing lateral pressure and vertical pressure: Lateral pressure is horizontal and acts on the formwork walls. Vertical pressure is the weight of concrete acting on the formwork bottom. Both must be calculated separately.

144.Forgetting the safety factor: Formwork calculations must include a safety factor of at least 1.5 for dead loads and 2.0 for construction loads, according to CSA S269.1.

145.Neglecting concrete temperature: Temperature directly affects setting rate and therefore lateral pressure. Cold concrete (5 °C) exerts higher pressure than hot concrete (30 °C) for the same placement rate.

146.Using non-formwork-rated plywood: Plywood must be specifically rated for formwork (CSA O121) and have a phenolic surface to resist moisture and facilitate stripping.

147.Vibrating too long: Over-vibration causes aggregate segregation and laitance rising to the surface, which weakens the concrete.

148.Stripping too early: Concrete must reach at least 70% of its specified strength before stripping. Premature stripping can cause collapse or cracking.

149.Ignoring construction joints: A poorly prepared construction joint creates a structural weakness. The surface must be clean, moist, and rough before the new pour.

150.Confusing slump and strength: Slump measures concrete consistency, not strength. A fluid concrete (150 mm slump) may have lower strength than a stiff concrete (50 mm slump).

151.Forgetting tolerances: Dimensional tolerances in CSA A23.1 are strict. A wall that is out of plumb by more than 10 mm per 3 m must be corrected or demolished.

152.Not protecting concrete in cold weather: Concrete must be maintained at a minimum temperature of 10 °C during curing. Premature freezing destroys the concrete structure.

Summary

Formwork and concrete placement are essential skills for the Construction Craft Worker. The key points to remember for the Red Seal exam are:

155.Formwork must be strong, tight, precise, and easy to strip. Common materials are phenolic plywood, steel, and aluminum.

156.Lateral pressure of fresh concrete is calculated using the hydrostatic formula Ω = ρ × g × h, and depends on the height, density, and temperature of the concrete.

157.Placement must be done in layers of 300 to 500 mm, with a maximum free fall of 1.5 m, and adequate vibration to eliminate air bubbles.

158.Construction joints must be carefully prepared to ensure structural continuity.

159.Stripping must only be done after the concrete has reached 70% of its specified strength, and curing must last at least 7 days.

160.CSA A23.1, A23.2, S269.1, and S269.2 are the primary references for concrete and formwork in Canada.

161.Safety is paramount: fresh concrete is corrosive, formwork can collapse, and silica dust is hazardous to health.

162.Quantity calculations for concrete and tie counts are frequent exam questions. Master the basic formulas (volume, pressure, force).

By mastering these concepts and practicing the calculations, you will be well prepared to succeed on the formwork and concrete placement section of the Red Seal exam. Good luck!