This chapter covers all concrete placement and finishing techniques as required for the Red Seal exam. You will find essential definitions, standard procedures, yield calculations, Canadian standards requirements (CSA A23.1, CSA A23.2, National Building Code) as well as common pitfalls. Mastering this content is crucial: questions on placement and finishing typically represent 15 to 20% of the exam.
2.1 Preparation Before Placement
2.1.1 Formwork and Reinforcement Inspection
Before any pour, you must verify:
Formwork stability: Shores, ties, and bracing must be in place and tightened. Formwork that deflects under the pressure of fresh concrete causes permanent deformations.
Joint tightness: Laitance leaks result in a loss of fine particles and create honeycombing (surface voids).
Reinforcement position: Bars must be supported by chairs (concrete, plastic, or metal supports) to maintain the minimum required concrete cover. According to CSA A23.1, the minimum cover is 75 mm for concrete cast against the ground, 50 mm for surfaces exposed to weather, and 30 mm for interior surfaces not exposed to weather.
Formwork cleanliness: Remove debris, ice, snow, and standing water. An air jet or water cleaning is required before the pour.
2.1.2 Concrete Verification at Delivery
At each ready-mix concrete delivery, you must:
14.Read the delivery ticket: Verify the concrete class (e.g., 30 MPa), specified slump, cement type, quantity (m³), and loading time.
15.Measure the slump: Perform the slump cone test in accordance with CSA A23.2-5C. The measured slump must be within ± 25 mm of the specified value.
16.Take samples: Test cylinders (150 mm × 300 mm) must be taken in accordance with CSA A23.2-3C and cured according to CSA A23.2-9C.
17.Check the temperature: Concrete temperature at delivery must be between 10 °C and 30 °C, unless otherwise specified. In cold weather, the minimum temperature at placement must be 10 °C for elements less than 300 mm thick, and 5 °C for more massive elements.
> Rule of thumb: Concrete must be placed within 90 minutes of truck loading, or before 300 drum revolutions, whichever comes first. Beyond that, adding water is prohibited without the supplier's authorization and only to restore slump, never to increase the water-cement ratio beyond specification.
2.1.3 Calculating Required Concrete Volume
Concrete volume is calculated in cubic meters (m³). For a rectangular slab:
V = L × W × T
Where:
V = volume (m³)
L = length (m)
W = width (m)
T = thickness (m)
Example: A slab measuring 12 m × 8 m × 0.15 m = 14.4 m³.
Waste factor: Add 5 to 10% for waste, formwork irregularities, and leveling. For foundations or complex pours, allow 10 to 15%.
Calculating the number of trucks: A standard mixer truck carries 8 to 10 m³. For 14.4 m³ + 10% = 15.84 m³, you will need 2 trucks (2 × 8 m³ = 16 m³).
2.2 Transport and Placement of Concrete
2.2.1 Transport Methods
Method
Distance/Reach
Advantages
Limitations
Wheelbarrow
< 50 m
Flexible, low cost
Labour-intensive
Concrete pump
Up to 300 m horizontal, 100 m vertical
Difficult access, high output (20-80 m³/h)
High cost, requires qualified operator
Truck chute
1-3 m
Fast, integrated into truck
Limited reach
Concrete bucket (crane)
Variable
Ideal for deep formwork
Requires a crane
Conveyor belt
10-30 m
Continuous flow
Can cause segregation if poorly adjusted
2.2.2 Placement Rules
Free-fall height: Maximum free fall is 1.5 m to prevent segregation. Beyond that, use a drop chute (tremie) or pump hose.
Pouring rate: Do not exceed 1.5 m/h for walls and columns, to avoid excessive lateral pressure on formwork and the formation of laitance on the surface.
Placement in layers: For thick elements, pour in horizontal layers 300 to 500 mm thick. Each layer must be vibrated before the next to avoid cold joints.
Pouring direction: Always pour toward the discharge point, never away from it, to prevent concrete from flowing over an already partially set surface.
2.2.3 Concrete Vibration
Vibration is essential to remove entrapped air and coat the reinforcement. Two types:
Internal vibrator (poker): Diameter 25 to 75 mm. Frequency 10,000 to 12,000 vibrations/min. Radius of action: 6 to 10 times the poker diameter.
External vibrator (form vibrator): Used for thin elements or heavily reinforced sections.
Correct procedure:
45.Insert the poker vertically, quickly, to the bottom of the layer.
46.Hold in place 5 to 15 seconds, until laitance rises to the surface and air bubbles stop.
47.Withdraw slowly (about 75 mm/s) to allow concrete to close the hole.
48.Space insertions 450 to 600 mm apart, slightly overlapping previously vibrated zones.
> Common pitfall: Over-vibration causes segregation (coarse aggregate settles to the bottom, laitance rises). Under-vibration leaves honeycombing and air bubbles on the surface. Properly vibrated concrete presents a smooth, shiny surface without excessive bugholes.
2.3 Finishing Horizontal Surfaces (Slabs)
2.3.1 Finishing Operations — Sequence and Timing
Slab finishing follows a precise sequence dictated by the setting time of the concrete. Ambient temperature, humidity, and cement type influence this schedule.
Operation
Timing (at 20 °C)
Tool
Purpose
Screeding
Immediately after pouring
Vibrating screed, straightedge
Establish level and slope
Bull floating
1-2 h after
Bull float
Embed aggregates, flatten surface
Troweling — 1st pass
2-4 h
Hand trowel or power trowel
Compact surface, close pores
Troweling — 2nd pass
3-5 h
Steel trowel
Dense, smooth surface
Broom finishing
4-6 h
Broom
Non-slip texture
Finger test: Press your finger on the surface. If the imprint remains visible but concrete does not stick to your finger, the surface is ready for bull floating. If your finger sinks in, wait. If no imprint forms, it is too late for bull floating.
Steel trowel finish: Dense, smooth surface. Suitable for industrial floors and interiors. Two or three steel trowel passes are required.
Broom finish: Parallel grooves obtained by pulling a broom across the still-plastic surface. Excellent traction for slopes and exterior pedestrian surfaces.
Exposed aggregate finish: A surface retarder is applied, then the laitance is washed away with pressurized water to reveal the aggregates. Used for driveways and decorative sidewalks.
2.3.3 Control and Expansion Joints
Joints are essential for controlling cracking due to shrinkage and thermal variations.
Control joints (contraction joints): Grooves created in fresh concrete (with a jointing tool) or sawed after hardening. They create a weakened plane where cracking will occur cleanly.
Spacing: 24 to 36 times the slab thickness. For a 100 mm slab, space joints 2.4 to 3.6 m apart.
Depth: 1/4 to 1/3 of the slab thickness.
Saw-cutting: Sawing must be done 6 to 24 hours after pouring, before shrinkage causes random cracking.
Expansion joints: Separate the slab from structural elements (walls, columns, foundations). They allow thermal movement. Material: compressible fibre board, 10 to 25 mm thick.
Construction joints: Created intentionally to interrupt a pour. They must be located at points of least stress and may be reinforced (shear keys) to transfer loads.
2.3.4 Slopes and Leveling
Exterior slabs must have a minimum slope of 2% (2 cm per meter) to ensure drainage. Interior garage slabs: 1.5 to 2%. Slopes are checked with a laser level or a 2 m spirit level.
Slope calculation: For a 6 m long slab with a 2% slope, the height difference between the two ends is:
ΔH = 6 m × 0.02 = 0.12 m (120 mm)
2.4 Finishing Vertical Surfaces (Walls, Columns)
2.4.1 Form Removal and Repairs
Form removal time: According to CSA A23.1, wall forms can be removed when the concrete has reached sufficient strength to support its own weight, typically 12 to 24 hours at 20 °C. For columns, 24 to 48 hours.
Surface repairs: Honeycombing and bugholes must be repaired with a compatible repair mortar (e.g., polymer-modified cement mortar). Clean the area, apply a bonding agent, then fill and finish.
2.4.2 Wall Surfacing
Form finish: The surface is left as-is after form removal. Minor defects are tolerated.
Float finish: After form removal, the surface is dampened and floated with a cement mortar to achieve a uniform texture.
Rubbed finish: The surface is rubbed with an abrasive stone or diamond disc to remove laitance and imperfections.
2.5 Concrete Curing
Curing is the most critical operation for concrete durability. It maintains the moisture and temperature conditions favorable to cement hydration.
2.5.1 Curing Duration
According to CSA A23.1, the minimum curing duration is:
3 days for high-early-strength concrete (Type HE cement)
7 days for ordinary concrete (Type GU cement)
14 days for concrete containing supplementary cementing materials (fly ash, slag)
2.5.2 Curing Methods
Method
Description
Advantages
Limitations
Wet curing
Continuous watering, wet burlap, straw
Excellent effectiveness
Labour cost, risk of thermal shock
Curing membrane
Liquid product (resin, wax) sprayed on the surface
Simple, economical
Must be removed before certain finishes
Curing sheets
Polyethylene sheets placed on the surface
Retains moisture
Risk of staining if placed too early
Steam curing
Enclosed chamber with steam (precast)
Accelerates setting
Specialized equipment
2.5.3 Temperature and Cold Weather Curing
Cold weather concreting (ambient temperature < 5 °C): Concrete must be protected from freezing for at least 48 hours or until it reaches a strength of 3.5 MPa. Use insulation, heated enclosures, or insulated forms.
Hot weather concreting (ambient temperature > 30 °C): Concrete temperature must not exceed 30 °C at placement. Use cold water, ice, or pour early in the morning. Wet curing must begin immediately after finishing to prevent rapid evaporation.
> Golden rule: Curing must begin as soon as finishing is complete, and certainly before the surface loses its shiny appearance (evaporated bleed water).
2.6 Quality Control and Testing
2.6.1 Fresh Concrete Tests
Slump — CSA A23.2-5C: Measures consistency. A 75 mm slump is typical for slabs, 100-125 mm for heavily reinforced elements.
Air content — CSA A23.2-4C: Measured by the pressure method. Concrete exposed to freeze-thaw cycles must contain 5 to 8% entrained air.
Temperature: Measured with a probe thermometer. Must be between 10 °C and 30 °C.
Density: Checked on a sample. Typical value: 2200 to 2400 kg/m³.
2.6.2 Hardened Concrete Tests
Compressive strength — CSA A23.2-9C: Cylinders 150 × 300 mm, cured and tested at 7 and 28 days. The specified strength (e.g., 30 MPa) must be achieved at 28 days.
Flexural strength — CSA A23.2-8C: Beams 150 × 150 × 500 mm, tested in 3-point bending. Used for slabs on grade.
According to the National Building Code of Canada (NBC) and CSA A23.1:
Element
Tolerance
Slab levelness
± 10 mm over 3 m
Wall plumbness
± 6 mm over 3 m
Slab thickness
+ 10 mm / - 5 mm
Reinforcement position
± 10 mm
2.7 Practical Site Calculations
2.7.1 Calculating the Number of Cement Bags
For concrete mix designed at 350 kg/m³ of cement, with 30 kg bags:
Number of bags = (350 kg/m³ × volume in m³) / 30 kg
For 1 m³: 350 / 30 = 11.7 bags, or 12 bags per m³.
2.7.2 Calculating Pump Output
A concrete pump with a rate of 40 m³/h can pump a 20 m³ slab in 30 minutes, plus 15 minutes for setup and cleanup. Plan truck logistics accordingly.
2.7.3 Calculating Fresh Concrete Pressure on Formwork
The maximum lateral pressure of fresh concrete on formwork is approximately:
P = ρ × g × h
Where:
P = pressure (Pa)
ρ = concrete density (≈ 2400 kg/m³)
g = 9.81 m/s²
h = height of concrete above the point in question (m)
Example: For a 3 m high wall, P = 2400 × 9.81 × 3 = 70,632 Pa ≈ 70.6 kPa. The formwork must resist this pressure, multiplied by a safety factor.
2.8 Safety During Placement and Finishing
Personal protective equipment (PPE): Hard hat, safety glasses, waterproof gloves, steel-toed boots. Fresh concrete is caustic (pH ≈ 12-13) and can cause severe chemical burns.
Working at heights: Use compliant scaffolding, guardrails, and harnesses if required.
Concrete pumps: Check hoses, couplings, and outriggers before use. Never point the hose at a person.
Vibrators: Use ground fault circuit interrupters (GFCIs) for all power tools. Never run a vibrator outside of concrete.
Curing compounds: Some products emit flammable vapors. Ensure adequate ventilation and prohibit ignition sources.
2.9 Applicable Canadian Standards
Standard
Title
Application
**CSA A23.1**
Concrete: Constituent materials and execution of work
Requirements for materials, placement, curing, and control
**CSA A23.2**
Concrete: Test methods and standard practices
Fresh and hardened concrete testing
**CSA A23.3**
Design of concrete structures
Structural design (reference)
**National Building Code (NBC)**
Parts 4 and 9
Structural requirements and buildings
**CSA A3000**
Cementitious materials
Cement classification (GU, HE, MS, HS, etc.)
2.10 Pitfalls to Avoid
139.Confusing floating and troweling: Bull floating is done with a wood or magnesium float, before troweling. Troweling is done with a steel trowel, after floating. Doing the reverse traps air and creates a weak surface.
140.Adding water to concrete without authorization: Adding water increases the water-cement ratio and reduces strength. A 30 MPa concrete with 20 L of added water per m³ can drop to 25 MPa.
141.Neglecting curing: Uncurated concrete loses up to 50% of its potential strength. Curing is not optional.
142.Vibrating too long: Over-vibration causes segregation. 5 to 15 seconds is sufficient.
143.Forgetting control joints: A slab without control joints will crack randomly. Maximum spacing: 36 × thickness.
144.Pouring in cold weather without protection: Freezing of fresh concrete destroys the internal structure. Protection is mandatory until 3.5 MPa.
145.Confusing cement types: Type GU (general use) cement does not set the same as Type HE (high early strength). Check the delivery ticket.
146.Ignoring concrete temperature: Concrete at 35 °C sets too quickly and develops plastic shrinkage cracks. Concrete at 5 °C sets too slowly and risks freezing.
147.Walking on freshly troweled surfaces: Leaves marks and deformations. Use walk boards or special soled shoes.
148.Not checking the slump: Concrete that is too dry (low slump) is difficult to place; concrete that is too wet (high slump) has reduced strength.
Placement must avoid segregation: free fall ≤ 1.5 m, layers 300-500 mm, proper vibration (5-15 s, spacing 450-600 mm).
Finishing follows a strict sequence: screeding → bull floating → troweling (1st and 2nd pass) → broom finishing. Timing depends on temperature and cement type.
Joints (control, expansion, construction) are essential for managing shrinkage and thermal movements.
Curing is mandatory: 3 to 14 days depending on cement type, by wet method, membrane, or sheets.
Testing (slump, air content, compressive strength) is governed by CSA A23.2.
Dimensional tolerances are defined by the NBC and CSA A23.1.
Safety: fresh concrete is caustic, pumps and vibrators present specific risks.
2.12 Exam Tips
Memorize key values: free fall 1.5 m, joint spacing 24-36 × thickness, curing 7 days (GU), temperature 10-30 °C, cover 75/50/30 mm.
Understand sequences: Questions on the order of finishing operations are common. Remember: screeding → bull floating → troweling → broom finishing.
Master volume calculations: Volume = L × W × T, with a 5-10% waste factor. Calculation questions are almost guaranteed.
Know the standards by name: CSA A23.1 (execution), CSA A23.2 (testing), NBC. Questions reference these standards.
Visualize situations: Scenario-based questions (e.g., "What to do if concrete arrives too dry?") require practical, code-compliant answers.
This chapter covers the essential competencies of the "Concrete Placement and Finishing" competency block for the Red Seal exam. Review the numerical values, sequences, and standards. Good luck with your preparation!
