Placement of Reinforcing Steel (Rebar)
Introduction to Rebar Placement
Rebar placement is a fundamental skill for the Ironworker (Generalist) . Your role is to transform engineering drawings into safe, durable reinforced concrete structures. Concrete has excellent compressive strength but weak tensile strength; reinforcing steel compensates for this weakness. Incorrect placement can lead to cracking, collapse, or serious structural failures.
This chapter covers the principles, procedures, calculations, and best practices you must master for the Red Seal exam. The primary reference standards are CSA A23.1 (Concrete: Constituent Materials and Execution of Work) and CSA A23.2 (Test Methods and Standard Practices for Concrete). You must also be familiar with the requirements of the National Building Code of Canada (NBC) , particularly Section 4 (Structural Design).
Roles and Responsibilities of the Ironworker
The generalist ironworker is responsible for:
Reading and interpreting reinforcing drawings (shop and site drawings).
Receiving, storing, and handling reinforcing bars.
Cutting, bending, and assembling bars according to specifications.
Installing spacers (chairs, supports) to maintain the required concrete cover.
Assembling rebar cages for columns, beams, slabs, walls, and foundations.
Verifying placement compliance before concrete placement.
You must work in coordination with other trades (formworkers, concrete finishers) and comply with the occupational health and safety regulations of your jurisdiction, as well as applicable CSA standards.
Materials: Reinforcing Steel
Bar Types
Reinforcing bars are designated by their nominal diameter in millimetres (metric system) and their steel grade. Common grades are:
| Grade | Minimum Yield Strength (MPa) | Typical Use |
|---|
| 300R | 300 | Secondary structures, slabs on grade |
| 400R | 400 | General use (most common) |
| 500R | 500 | High-performance structures, seismic zones |
The grade is identified by a marking number on the bar. For example, a bar bearing the number "4" indicates grade 400R. Bars are also marked with the manufacturer's logo and a symbol indicating the type of steel.
Dimensions and Designations
Bars are designated by a number (10M, 15M, 20M, 25M, 30M, 35M, 45M, 55M) that corresponds approximately to the diameter in millimetres. The following table gives the essential properties:
| Designation | Nominal Diameter (mm) | Cross-Sectional Area (mm²) | Linear 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 |
| 45M | 43.7 | 1500 | 11.775 |
| 55M | 56.2 | 2500 | 19.625 |
Exam tip: The cross-sectional area is approximately equal to the linear mass divided by 7.85 (density of steel). For example, a 20M bar weighs 2.355 kg/m; its area is 2.355 / 7.85 × 1000 ≈ 300 mm². This relationship lets you quickly verify your calculations.
Coatings and Protection
Epoxy-coated bars: used in corrosive environments (bridges, parking garages). They are identified by their green or grey colour. Never bend an epoxy-coated bar after the coating has been applied without approved repair.
Galvanized bars: for marine or industrial environments. Hot-dip galvanizing provides sacrificial protection.
Stainless steel bars: for special applications where corrosion is critical (pools, chemical plants).
Reading Reinforcing Drawings
Symbols and Abbreviations
Reinforcing drawings use standardized symbols. You must master:
# followed by a number: designates the bar (e.g. #15M).
@ : spacing (e.g. 300 @ 300 means bars spaced 300 mm in both directions).
B : bottom bar.
T : top bar.
E : end bar.
W : wall bar.
S : slab bar.
Example of notation: 15M @ 300 E.F. means 15M bars spaced at 300 mm, each face.
Rebar Placement Drawings
A typical rebar placement drawing includes:
A plan view (top view) showing the arrangement of bars in a slab or footing.
Sections (cross-sections) showing the vertical position of the bars.
A bar bending schedule listing each bar with its number, shape, dimensions, quantity, and total length.
Reading procedure:
44.Identify the drawing scale.
45.Locate the construction grid lines (references A, B, C, etc.).
46.Identify areas of high rebar density (often at supports).
47.Check the indicated splices (overlaps).
48.Confirm the anchorages at the ends.
Concrete Cover
Cover is the distance between the outer surface of the bar and the surface of the concrete. It protects the steel against corrosion and ensures stress transfer. Minimum values are specified in CSA A23.1 and the NBC.
| Element | Minimum Cover (mm) |
|---|
| Concrete cast against the ground | 75 |
| Concrete exposed to weather (walls, beams) | 50 |
| Interior slabs and walls | 20 |
| Columns | 40 |
| Pile foundations | 75 |
| Precast concrete | 25 (or per specifications) |
Golden rule: Cover must never be less than the diameter of the bar, nor less than 20 mm for main bars.
Spacers
To maintain cover, you use spacers (supports):
High chairs: for top bars in slabs.
Low chairs: for bottom bars.
Plastic supports: for walls and slabs.
Precast concrete supports: for foundations.
Requirements: Spacers must be spaced no more than 1.2 m apart in each direction for slabs, and no more than 0.6 m for vertical wall bars. They must be strong enough to support the weight of the bars and workers without deforming.
Bending and Shaping Bars
Minimum Bend Radii
Bending bars must respect minimum radii to avoid cracking the steel. CSA A23.1 specifies:
| Bar Diameter | Minimum Bend Radius (inside) |
|---|
| 10M to 20M | 3 × diameter |
| 25M to 30M | 4 × diameter |
| 35M to 55M | 5 × diameter |
Example: For a 20M bar (diameter 19.5 mm), the minimum inside radius is 3 × 19.5 = 58.5 mm.
Common Bend Types
Standard 90° hook: used for anchorages. Length of the bent portion = 12 × diameter.
Standard 180° hook: used for tension bars. Length of the bent portion = 4 × diameter (plus the straight length).
Stirrups: for beams, bent at 90° or 135°.
U-bends: for anchorages in footings.
Development length formula: The required straight length for an anchorage is calculated using the formula:
Ld = (0.19 × fy × db) / (√f'c)
Where:
Ld = development length (mm)
fy = yield strength of the steel (MPa)
db = bar diameter (mm)
f'c = compressive strength of the concrete (MPa)
Exam tip: This formula is often provided in the exam tables. You need to know how to use it, not memorize it.
Rebar Placement in Common Elements
Slabs on Grade
Slabs on grade (basement slabs, industrial floors) are typically reinforced with welded wire mesh or regularly spaced bars.
Maximum spacing: 3 × the slab thickness, but never more than 450 mm.
Bar position: in the bottom third of the slab to resist positive moments.
Cover: 75 mm if cast on ground, 50 mm if on a membrane.
Procedure:
85.Check the flatness of the ground (tolerance ± 10 mm over 3 m).
86.Install low chairs (50 mm).
87.Place bars in the longest direction first.
88.Space the bars using a template or tape measure.
89.Tie the bars at every intersection (tie wire No. 16 or 18).
Foundation Walls
Foundation walls are reinforced with vertical and horizontal bars.
Vertical bars: spaced 300 mm to 600 mm depending on wall height.
Horizontal bars: spaced 300 mm to 450 mm.
Vertical splice: 40 × diameter (for bars in tension).
Horizontal splice: 30 × diameter.
Trap to avoid: Vertical bars must be placed inside the horizontal bars (earth side) for retaining walls. Reversing this arrangement weakens the structure.
Columns
Columns are reinforced with longitudinal bars (vertical) and transverse reinforcement (ties or spirals).
Minimum number of longitudinal bars: 4 for rectangular columns, 6 for circular columns.
Maximum tie spacing: 16 × diameter of longitudinal bars, or 48 × diameter of ties, or the smallest column dimension.
Splice of longitudinal bars: 40 × diameter (tension), 30 × diameter (compression).
Column cage assembly procedure:
103.Cut longitudinal bars to the required length (including splice).
104.Install ties at regular intervals (often 100 mm at the ends, 200 mm in the middle).
105.Tie the ties to the longitudinal bars with tie wire.
106.Check the plumbness of the cage (± 10 mm over 3 m).
107.Install spacers to maintain the 40 mm cover.
Beams
Beams contain main bars (bottom and top) and stirrups.
Bottom bars: resist positive moments (tension at the bottom).
Top bars: resist negative moments (tension at the top, at supports).
Stirrups: resist shear forces and hold the main bars in place.
Stirrup spacing: Varies by zone. At supports, spacing is reduced (often 100 mm); at mid-span, it is larger (200 mm to 300 mm).
Trap to avoid: Top bars must be held in position with high chairs or truss supports. Without them, they sag during concrete placement, reducing the effective depth of the beam.
Splices and Anchorages
Types of Splices
Lap splice: bars are placed side by side over a minimum length.
Mechanical splice: a threaded or compression coupler connects the bars.
Welded splice: permitted only with weldable steels and per specifications.
Minimum Splice Lengths
| Condition | Splice Length |
|---|
| Bars in tension (Class A) | 1.0 × Ld |
| Bars in tension (Class B) | 1.3 × Ld |
| Bars in compression | 0.071 × fy × db (minimum 300 mm) |
The splice class depends on the percentage of bars spliced at the same section:
Class A: 50% or less of the bars are spliced at the same section.
Class B: more than 50% of the bars are spliced.
Exam tip: For 20M bars and larger, the tension splice length is often 40 to 50 × diameter. For smaller bars (10M, 15M), it is 30 to 40 × diameter.
Quantity Calculations
Calculating Total Bar Length
For each bar, the total length is the sum of:
The straight length (span + anchorages).
The bends (developed according to the radius).
The splices (if applicable).
Formula for a 90° hook:
Hook length = 12 × db + 3 × db (for the radius)
Example: 15M bar (db = 16 mm) with a 90° hook:
Hook length = 12 × 16 + 3 × 16 = 192 + 48 = 240 mm.
Calculating the Number of Bars
For a 6 m × 4 m slab with 15M bars @ 300 mm in both directions:
Number of bars in the 6 m direction: (6000 / 300) + 1 = 21 bars.
Number of bars in the 4 m direction: (4000 / 300) + 1 = 14.33 → round up to 15 bars.
Length of each bar (6 m direction): 6000 + 2 × 240 (hooks) = 6480 mm.
Total length (6 m direction): 21 × 6480 = 136,080 mm = 136.08 m.
Total mass: 136.08 m × 1.570 kg/m = 213.6 kg.
Exam tip: Always round the number of bars up to the next whole number. Never round down.
Placement Tolerances
CSA A23.1 specifies placement tolerances:
| Parameter | Tolerance |
|---|
| Bar position (in plan) | ± 15 mm |
| Concrete cover | + 10 mm / - 0 mm (never less than the minimum) |
| Bar spacing | ± 10 mm |
| Height of top bars | ± 10 mm |
| Stirrup position | ± 25 mm |
Important rule: Concrete cover can never be less than the specified minimum value. A negative tolerance of 10 mm is permitted only if the cover remains above the absolute minimum.
Site Safety
Specific Rebar Hazards
Cuts and punctures: bar ends must be protected with protective caps (uncast vertical bars).
Falls: working on elevated rebar cages requires harnesses and lifelines.
Lifting: bundles of bars must be lifted with approved slings. Never exceed the rated load.
Electrocution: maintain a minimum distance of 3 m from overhead power lines.
Personal Protective Equipment (PPE)
Safety helmet with chin strap.
Safety glasses (against sparks when cutting).
Leather gloves (for handling).
Steel-toed boots.
Long-sleeved clothing (protection against cuts).
Quality Control and Inspection
Before Concrete Placement
The ironworker must perform a self-inspection before the inspector arrives:
163.Verify that all bars match the drawings (diameter, length, shape).
164.Confirm spacings using a tape measure.
165.Check splices (length and position).
166.Ensure spacers are in place and in good condition.
167.Verify that bars are clean (no oil, loose rust, or ice).
168.Confirm that ties are tight (tie wire twisted, not just wrapped).
169.Check the stability of cages (no movement possible during placement).
During Concrete Placement
Monitor bar displacement under the pressure of the concrete.
Do not walk directly on the bars (use walk boards).
Adjust displaced bars immediately (if possible).
After Concrete Placement
Report any visible defects (exposed bars, cage sagging).
Participate in the final inspection if required.
Traps to Avoid
178.Confusing steel grades: A 400R bar cannot replace a 500R bar without engineer approval. Always check the markings.
179.Neglecting minimum cover: A 15 mm cover instead of 20 mm may seem insignificant, but it reduces corrosion protection and structural capacity. The tolerance is +10 mm / -0 mm.
180.Forgetting support bars: Support bars (top bars in slabs) are often omitted. They are essential for maintaining the position of the main bars.
181.Bending an epoxy-coated bar without repair: Bending cracks the epoxy coating. Any bent epoxy-coated bar must be repaired with an approved product or replaced.
182.Using ties that are too loose: A tie wire that is simply wrapped can come undone during concrete placement. It must be twisted firmly (minimum two turns).
183.Placing top bars without supports: Top bars in slabs and beams must be supported by chairs. Without them, they sag and the effective depth is reduced.
184.Ignoring high-density zones: At beam supports and column junctions, stirrup spacing is reduced. Always check the drawings for these zones.
185.Rounding quantities down: The number of bars must always be rounded up to the next whole number. A calculation of 14.33 bars means 15 bars.
186.Mixing bars from different lots: Bars from different manufacturers may have slightly different properties. Separate them and identify them clearly.
187.Not checking the plumbness of cages: A column cage leaning 20 mm can result in insufficient cover on one side. Use a spirit level or plumb bob.
Summary
Reinforcing bars are designated by their diameter (10M to 55M) and grade (300R, 400R, 500R). Grade 400R is the most common.
Concrete cover protects the steel against corrosion. Minimum values range from 20 mm (interior) to 75 mm (ground).
Spacers maintain the cover. They must be spaced no more than 1.2 m apart for slabs.
Minimum bend radii range from 3 × diameter (bars ≤ 20M) to 5 × diameter (bars ≥ 35M).
Development and splice lengths depend on the diameter, steel grade, and concrete strength. Use the formula Ld = (0.19 × fy × db) / (√f'c).
Placement tolerances are ± 15 mm for position, ± 10 mm for spacing. Cover can never be less than the minimum.
Safety is paramount: protect bar ends, wear full PPE, and maintain clearance from power lines.
Self-inspection before concrete placement is mandatory. Check bars, spacings, splices, and ties.
Key formulas to remember:
Cross-sectional area (mm²) ≈ Linear mass (kg/m) / 7.85 × 1000
Development length: Ld = (0.19 × fy × db) / (√f'c)
Compression splice: 0.071 × fy × db (minimum 300 mm)
Number of bars = (Length / Spacing) + 1, rounded up to the next whole number
Self-Assessment Questions
203.What is the development length of a 20M bar (fy = 400 MPa) in concrete with f'c = 30 MPa?
Ld = (0.19 × 400 × 19.5) / √30 = (1482) / 5.48 ≈ 270 mm.
205.How many 15M bars are needed for a 3.6 m × 2.4 m slab with 300 mm spacing in both directions?
3.6 m direction: (3600 / 300) + 1 = 13 bars.
2.4 m direction: (2400 / 300) + 1 = 9 bars.
Total: 13 + 9 = 22 bars.
209.What is the minimum splice length for a 25M bar in tension (Class B) in concrete with f'c = 25 MPa?
Ld = (0.19 × 400 × 25.2) / √25 = (1915.2) / 5 = 383 mm.
Class B: 1.3 × 383 = 498 mm.
212.True or false: Concrete cover can be reduced by 10 mm if the bar is stainless steel.
False. The minimum cover is specified in the drawings and the standard; it does not depend on the type of steel.
214.What is the minimum bend radius for a 30M bar?
30M has a diameter of 29.9 mm. For bars from 25M to 30M, the minimum radius is 4 × diameter = 4 × 29.9 = 119.6 mm.
This chapter prepares you for the theoretical and practical questions on the Red Seal exam regarding rebar placement. Review the sections on splices and tolerances, as they are frequently tested. Good luck with your preparation!
