Site Layout and Concrete Formwork

Introduction

Site layout and concrete formwork are two critical steps in any construction project. Layout determines the exact position of structures on the ground, while formwork defines the shape, dimensions, and surface quality of the poured concrete. For the Red Seal exam, you must master measuring instruments, levelling methods, slope calculations, tolerances, as well as formwork design and installation principles. This chapter covers all the theoretical and practical knowledge required, with an emphasis on Canadian standards and common pitfalls.

1. Site Layout

1.1 Definitions and Fundamental Principles

Layout consists of transferring the dimensions, axes, elevations, and exact locations of the structures to be built from the engineering drawings onto the ground. It is the first physical step of the construction site and conditions all subsequent operations.

The three fundamental principles of layout are:

10.Accuracy: layout tolerances are generally ±3 mm for main axes and ±5 mm for finished elevations.

11.Repeatability: every measurement must be verified by a second independent reading.

12.Referencing: all measurements must relate to fixed, permanent reference points.

1.2 Measuring and Alignment Instruments

Instrument	Primary Use	Typical Accuracy	Key Advantage
Optical level	Levelling and elevation transfer	±1 mm at 30 m	Simple and reliable
Theodolite	Horizontal and vertical angle measurement	±2 arc seconds	Precise angles
Total station	Distance, angle, coordinates	±2 mm + 2 ppm	Complete integration
Rotary laser level	360° horizontal alignment	±1.5 mm at 30 m	Speed of use
Calibrated steel tape	Linear measurements	±1 mm per 30 m	Independent of battery
GPS (RTK)	Geographic coordinates	±10 mm horizontal	Long distances

Golden rule: the steel tape must be pulled with constant tension (generally 50 N) and corrected for temperature. A 10 °C variation produces approximately 0.1 mm per metre of expansion on a steel tape.

1.3 Establishing Reference Points

Benchmarks are fixed points whose elevation is known with precision. They serve as the reference for all site levelling.

Installation procedure:

19.Choose a stable location, outside the work zone and protected from frost.

20.Use a permanent benchmark (concrete monument, benchmark nail, steel plate).

21.Document the exact elevation relative to the national geodetic benchmark (CGVD28 or CGVD2013).

22.Install a minimum of two independent benchmarks to allow cross-verification.

Stakes mark the axes and boundaries. They are installed outside the excavation zone, generally 1.5 to 2 m from the edge, to prevent displacement by machinery.

1.4 Levelling Methods

Direct Levelling (Optical Level)

The direct sighting levelling method consists of measuring the elevation difference between two points using a graduated rod.

Fundamental formula:

ΔH = Backsight reading − Foresight reading

Where:

ΔH = elevation difference between the two points

Backsight reading = reading on the rod at the known point

Foresight reading = reading on the rod at the point to be determined

Example: Point A has an elevation of 102.450 m. The backsight reading on A is 1.245 m. The foresight reading on B is 2.310 m.

Elevation of B = 102.450 + 1.245 − 2.310 = 101.385 m

Indirect Levelling (Trigonometric)

Used with a total station, indirect levelling calculates the elevation difference from the vertical angle and the slope distance:

ΔH = D × sin(α)

Where:

D = measured slope distance

α = vertical angle (zenith or elevation angle)

Rotary Laser Levelling

The rotary laser level projects a horizontal reference plane. The electronic rod detects the beam and displays the height difference. This method is fast for large surfaces and formwork.

1.5 Calculating Slopes and Elevations

Slope is expressed as a percentage (%) or as a ratio (1:50, 1:100, etc.).

Slope (%) = (Height difference ÷ Horizontal distance) × 100

Example: A sewer pipe 25 m long must have a 2% slope. The height difference between the two ends is:

ΔH = 0.02 × 25 = 0.50 m

Intermediate elevation calculation: For a uniform slope, the elevation at a distance x from the starting point is:

Elevation(x) = Elevation(start) − (slope × x)

Caution: drainage slopes are expressed as fall (negative in the direction of flow). A positive slope indicates a rise.

1.6 Layout Tolerances and Standards

Acceptable layout tolerances are defined by the drawings and specifications, but common values are:

Element	Vertical Tolerance	Horizontal Tolerance
Foundation axes	±5 mm	±3 mm
Foundation elevations	±10 mm	—
Foundation walls	±10 mm	±5 mm
Slabs on grade	±10 mm	±10 mm
Drainage slopes	±5 mm per 10 m	—

Common pitfall: layout tolerances are stricter than construction tolerances. A layout error propagates and amplifies in the upper floors.

1.7 Control and Verification

Cross-verification is mandatory. Every layout must be checked by a second person or an independent second method.

Essential control points:

Verify rectangle diagonals (Pythagorean theorem: D² = L² + W²)

Verify 90° angles using the 3-4-5 method (right triangle)

Check elevations by double levelling (out and back)

Document all measurements in a layout logbook

3-4-5 method: On one side of the angle, measure 3 m; on the other side, measure 4 m; the diagonal must measure exactly 5 m. This method verifies perpendicularity.

2. Concrete Formwork

2.1 Role and Requirements of Formwork

Formwork is a temporary mould that gives concrete its shape, dimensions, and surface texture. It must resist the pressures of fresh concrete, maintain its geometry during placement, and allow stripping without damaging the concrete.

Fundamental requirements:

68.Strength: support the weight of fresh concrete, placement loads, and vibrations.

69.Tightness: prevent leakage of cement grout.

70.Rigidity: limit deformation under load (maximum deflection of L/360).

71.Flatness: produce a surface conforming to tolerances.

72.Strippability: allow removal without damage.

2.2 Fresh Concrete Pressures on Formwork

The lateral pressure of fresh concrete is the horizontal load exerted on formwork walls. It depends on several factors:

Height of concrete drop

Placement rate (m³/h)

Concrete temperature

Consistency (slump)

Concrete density (generally 2400 kg/m³)

Presence of vibrators

Simplified formula for normal-weight concrete (density 2400 kg/m³):

P = 23.5 × h

Where:

P = lateral pressure (kPa)

h = height of fresh concrete above the point in question (m)

Limitation: the maximum pressure generally does not exceed 100 kPa for conventional formwork, regardless of height, due to concrete setting.

Factors increasing pressure:

Rapid placement (more than 1.5 m/h)

Fluid concrete (slump > 100 mm)

Low temperature (slow setting)

Excessive vibration

Factors decreasing pressure:

Slow placement

Stiff concrete

High temperature

Set-accelerating admixtures

2.3 Types of Formwork

Type	Material	Typical Use	Advantages	Disadvantages
Timber formwork	19 mm plywood, 2×4, 2×6	Foundations, small walls	Versatile, economical	Limited service life
Metal formwork	Steel or aluminum panels	Repetitive walls, columns	Reusable, smooth surface	High initial cost
Insulating formwork (ICF)	Expanded polystyrene	Foundation walls, residential	Integrated insulation	High material cost
Lost formwork	Cardboard, polystyrene, precast concrete	Columns, beams	Stays in place	Not reusable
Slip formwork	Steel, hydraulic	Silos, towers, cores	Continuous placement	Complex, specialized

2.4 Timber Formwork Design

Step 1: Determine Loads

Vertical loads (on supports):

Concrete weight: 24 kN/m³

Formwork weight: 0.5 to 1.0 kN/m²

Placement load: 2.5 to 5.0 kN/m²

Vibration load: 1.0 kN/m²

Horizontal loads (on walls):

Lateral pressure of fresh concrete (calculated above)

Wind pressure (for exterior formwork)

Step 2: Support Spacing

The spacing of studs and shores is determined by the strength and allowable deflection of the plywood and framing members.

Maximum deflection formula for a simply supported beam:

δ = (5 × w × L⁴) ÷ (384 × E × I)

Where:

δ = deflection (m)

w = uniform load (N/m)

L = span (m)

E = modulus of elasticity (Pa)

I = moment of inertia (m⁴)

Rule of thumb: for 19 mm (3/4 in) plywood supported at 400 mm (16 in) on centre, deflection is generally acceptable for pressures up to 60 kPa.

Step 3: Anchoring and Bracing

Wall formwork must be anchored to resist the thrust of concrete. Tie rods pass through the formwork and are held by nuts and plates.

Typical tie rod spacing:

50 kPa pressure: 16 mm rods at 600 mm × 600 mm

75 kPa pressure: 16 mm rods at 450 mm × 450 mm

100 kPa pressure: 20 mm rods at 400 mm × 400 mm

Inclined braces brace the formwork vertically. A brace at 45° with a ground anchor is the most effective.

2.5 Formwork Installation

Preparation

129.Verify that the footing or base is clean and level.

130.Apply a release agent (form oil) to surfaces in contact with concrete.

131.Check the squareness and plumbness of the panels.

132.Install spreaders to maintain wall thickness.

Installation Procedure

134.Install panels on one side of the wall, aligned and levelled.

135.Install tie rods through the first panel.

136.Install spreaders at the tie rod locations.

137.Install panels on the opposite side.

138.Tighten tie rod nuts progressively.

139.Install inclined braces and bracing.

140.Check plumbness (spirit level or plumb bob) and alignment.

141.Check interior dimensions with a tape measure.

Formwork installation tolerances:

Plumbness: ±5 mm per 3 m of height

Horizontal alignment: ±10 mm over 10 m

Interior dimensions: +10 mm / −5 mm

Top surface elevation: ±5 mm

2.6 Formwork for Specific Elements

Footings and Foundations

Footing formwork is generally simple: 2×8 or 2×10 boards held by stakes. The top face of the formwork serves as a guide for levelling the concrete.

Pitfall: stakes must be driven below the frost line or anchored deeply enough to resist the thrust of the concrete.

Foundation Walls

Wall formwork must resist the full lateral pressure of concrete. Panels are generally 1.2 m × 2.4 m (4 ft × 8 ft) of 19 mm plywood with vertical 2×4 framing members at 300 mm on centre.

Critical points:

Tie rods must be tightened uniformly.

Spreaders must be removed progressively as concrete is placed.

Openings (doors, windows) require additional formwork and reinforcement.

Columns

Column formwork is subjected to high lateral pressure due to the height of concrete drop. Column clamps are spaced from 300 mm at the base to 600 mm at the top.

Rule: pressure is maximum at the base of the column. For a 3 m column, the pressure at the base is approximately 70 kPa.

Slabs and Beams

Slab formwork includes:

Form panels (plywood)

Joists supporting the panels

Stringers

Vertical shores

Typical spacing:

Joists: 300 to 400 mm on centre

Stringers: 1.2 to 1.8 m on centre

Shores: 1.2 to 2.4 m on centre

Allowable deflection: L/360 for exposed surfaces, L/240 for concealed surfaces.

2.7 Stripping and Maintenance

Stripping must only be done when the concrete has reached sufficient strength to support its own weight and construction loads.

Minimum recommended strength:

Formwork sides (walls, columns): 2 MPa (generally 12 to 24 hours)

Underside of slabs and beams: 70% of specified strength (generally 7 to 14 days)

Reshores: according to engineering drawings

Factors influencing stripping time:

Ambient temperature (concrete hardens more slowly in cold weather)

Type of cement (high early strength cement hardens faster)

Admixtures (accelerators or retarders)

Element thickness

Rule of thumb: for every 10 °C increase in temperature, the setting rate approximately doubles.

Formwork maintenance:

Clean panels after each use

Remove nails and debris

Repair damaged surfaces

Reapply release agent

Store flat, protected from weather

2.8 Applicable Standards and Codes

Formwork in Canada is governed by several standards:

CSA A23.1/A23.2: Concrete — Materials and methods of concrete construction / Test methods and standard practices for concrete

CSA S269.1: Formwork for concrete (formerly CAN/CSA-S269.1)

National Building Code of Canada (NBC): structural and safety requirements

Labour standards regulations: worker safety on scaffolding and formwork

Key requirements of CSA S269.1:

Formwork must be designed to resist all anticipated loads

Maximum deflection must not exceed L/360

Dimensional tolerances are specified

Materials must conform to applicable standards

2.9 Safety on Formwork Sites

Safety is an absolute priority. Formwork accidents are often fatal.

Essential safety rules:

203.Wear personal protective equipment (hard hat, gloves, safety boots, safety glasses).

204.Install guardrails around openings and slab edges.

205.Use ladders or stairs to access elevated areas.

206.Never stand under a suspended load.

207.Verify shore stability before placement.

208.Never remove shores without authorization.

209.Report any deformation or abnormal noise of the formwork during placement immediately.

Warning signs during placement:

Swelling or bulging of panels

Grout leakage between panels

Cracking or creaking of framing members

Movement of shores or tie rods

3. Practical Calculations for the Exam

3.1 Concrete Volume Calculation

Volume (m³) = Length (m) × Width (m) × Height (m)

Example: A footing measuring 12 m × 1.5 m × 0.3 m.

Volume = 12 × 1.5 × 0.3 = 5.4 m³

Waste: add 5 to 10% for losses and waste.

3.2 Lateral Pressure Calculation

Example: A wall 3 m high is placed at a rate of 1.2 m/h. The concrete temperature is 20 °C.

Maximum pressure = 23.5 × h = 23.5 × 3 = 70.5 kPa

Verification: the maximum pressure for normal-setting concrete is limited to approximately 100 kPa. Here, 70.5 kPa is acceptable.

3.3 Tie Rod Spacing Calculation

Example: A wall formwork is subjected to a pressure of 60 kPa. The tie rods have a capacity of 50 kN each.

Area supported per tie = 50,000 N ÷ 60,000 Pa = 0.833 m²

If the vertical spacing is 600 mm, the horizontal spacing is:

0.833 ÷ 0.6 = 1.39 m → use 1.2 m maximum

3.4 Deflection Calculation for a Formwork Panel

Example: A 19 mm plywood panel (E = 10,000 MPa, I = 5.7 × 10⁻⁷ m⁴ per metre of width) supports a pressure of 50 kPa over a span of 400 mm.

Linear load w = 50,000 Pa × 0.4 m = 20,000 N/m

δ = (5 × 20,000 × 0.4⁴) ÷ (384 × 10 × 10⁹ × 5.7 × 10⁻⁷)

δ = (5 × 20,000 × 0.0256) ÷ (384 × 10⁹ × 5.7 × 10⁻⁷)

δ = 2560 ÷ 218,880 = 0.0117 m = 11.7 mm

The deflection of 11.7 mm is excessive (L/360 = 400/360 = 1.1 mm). The spacing must be reduced to 200 mm or a thicker plywood must be used.

4. Pitfalls to Avoid

240.Confusing lateral pressure and vertical pressure: the lateral pressure of fresh concrete is horizontal and acts on the formwork walls, while vertical pressure acts on the base.

241.Forgetting temperature correction: steel tapes measure correctly at 20 °C. In cold weather, the tape contracts and measurements are too long.

242.Neglecting diagonal verification: a rectangle can have correct sides but non-right angles. Always check the diagonals.

243.Using a positive slope for drainage: drainage slopes are negative in the direction of flow. A sign error causes a reverse slope.

244.Stripping too early: concrete must reach a minimum strength before stripping. Premature stripping causes cracking and sagging.

245.Forgetting spreaders: without spreaders, concrete pressure draws the panels together and reduces wall thickness.

246.Over-tightening tie rods: excessive tightening deforms the formwork and creates concave surfaces.

247.Ignoring allowable deflection: maximum deflection is L/360 for exposed surfaces. Excessive deflection produces wavy surfaces.

248.Confusing benchmarks: elevation benchmarks and layout stakes are different. Never use an elevation benchmark as an anchor point.

249.Not documenting measurements: layout without documentation is useless. All measurements must be recorded.

5. Exam Tips

252.Memorize the basic formulas: lateral pressure (P = 23.5 × h), slope (%), deflection (L/360), volume (L × W × H).

253.Know the tolerances: layout tolerances (±3 mm axes, ±5 mm elevations) and formwork tolerances (±5 mm plumbness, +10/−5 mm dimensions).

254.Understand the units: the metric system is used in Canada. Convert units correctly (1 kPa = 1000 Pa, 1 kN = 1000 N).

255.Identify pressure factors: placement rate, temperature, consistency, vibration. Low temperature increases pressure (slow setting).

256.Review the standards: CSA A23.1/A23.2 for concrete, CSA S269.1 for formwork, NBC for general requirements.

257.Practice the calculations: calculation questions represent approximately 20% of the exam. Practice with numerical examples.

258.Read questions carefully: multiple-choice questions often contain distractors that seem correct. Identify keywords like "ALWAYS", "NEVER", "EXCEPT".

6. Summary

Layout transfers drawing dimensions to the ground with tolerances of ±3 mm (axes) and ±5 mm (elevations).

Main instruments are the optical level, total station, rotary laser, and calibrated steel tape.

Direct levelling uses the formula ΔH = Backsight reading − Foresight reading.

Slopes are expressed as a percentage or ratio, and drainage slopes are negative.

Formwork must resist the lateral pressure of fresh concrete, be tight, rigid, and strippable.

Lateral pressure is calculated by P = 23.5 × h and depends on placement rate, temperature, and consistency.

Tie rods and braces ensure formwork stability.

Stripping is done only after the concrete has reached the required strength (2 MPa for sides, 70% for slab undersides).

Applicable standards are CSA A23.1/A23.2, CSA S269.1, and the NBC.

Safety is paramount: PPE, guardrails, shore verification, monitoring during placement.

7. Review Questions

273.What is the maximum lateral pressure exerted by concrete 3.5 m high on formwork?

274.A steel tape measures 30.000 m at 20 °C. What is the actual length at 0 °C? (coefficient of expansion = 0.0000112 m/m·°C)

275.What is the maximum allowable deflection for a formwork panel with a 2.4 m span?

276.What are the four main factors that influence the lateral pressure of fresh concrete?

277.What is the minimum concrete strength before stripping formwork sides?

278.A wall 8 m long and 2.5 m high is placed with concrete having a lateral pressure of 55 kPa. If the tie rods have a capacity of 40 kN and are spaced vertically at 500 mm, what is the maximum horizontal spacing?

279.What is the difference between an elevation benchmark and a layout stake?

280.Why does low temperature increase the lateral pressure of concrete on formwork?

This chapter covers all the essential knowledge for the Red Seal exam in carpentry, layout and formwork section. Review the formulas, tolerances, and standards, and practice the calculations until they become automatic.

Site Layout and Concrete Formwork