Welding and Thermal Cutting for Reinforcing

Welding and Thermal Cutting of Reinforcing Steel

Introduction to Reinforcing Steel Welding

Welding reinforcing steel bars is a permanent joining process that must be performed according to strict rules, as it directly affects the structural integrity of reinforced concrete. Unlike conventional structural welding, reinforcing steel welding presents particular challenges: reinforcing bars have a chemical composition that makes them susceptible to hot cracking and embrittlement, and the service stresses in reinforced concrete are cyclic and dynamic.

For the Red Seal exam, you must master the permitted welding processes, the precautions to take, qualification testing, and the rules of the Canadian Electrical Code, Part I (CE Code) as well as applicable CSA standards, primarily CSA W186 (Welding of Reinforcing Bars in Reinforced Concrete Construction) and CSA G30.18 (Carbon Steel Bars for Concrete Reinforcement).

Essential Definitions

Reinforcing bar (rebar) : deformed or plain steel bar used to reinforce concrete. Common designations are 10M, 15M, 20M, 25M, 30M, 35M, 45M, 55M (nominal diameters in millimetres).

Fusion welding : process where the parts to be joined are melted locally, with or without filler metal.

Resistance welding : process where heat is generated by electrical resistance to current flow through the parts.

Heat-affected zone (HAZ) : region of the base metal adjacent to the weld that has undergone microstructural changes due to heat.

Preheating : raising the temperature of the base metal before welding to reduce the cooling rate and prevent cracking.

Heat input : amount of thermal energy transferred to the workpiece per unit length of weld, expressed in kJ/mm.

Properties of Reinforcing Steels and Weldability

Steel Grades According to CSA G30.18

Standard CSA G30.18 defines the requirements for carbon steel reinforcing bars. Common grades are:

Grade	Minimum Tensile Strength (MPa)	Minimum Yield Strength (MPa)	Weldability
300R	500	300	Good (low carbon)
400R	540	400	Good
500R	620	500	Moderate (higher carbon)
500W	620	500	Excellent (low carbon equivalent)

The W suffix indicates a bar with low carbon equivalent (CE), specifically designed for welding. Carbon equivalent is calculated using the formula:

CE = C + (Mn/6) + (Cr + Mo + V)/5 + (Ni + Cu)/15

For welding without special precautions, the CE must be less than 0.45%. Above 0.55%, welding becomes problematic and requires significant preheating.

Effects of Chemical Composition

Carbon (C) : increases strength but decreases weldability. Above 0.30% C, there is a risk of martensite in the HAZ.

Manganese (Mn) : improves strength and hardenability, but increases the CE.

Sulphur (S) and phosphorus (P) : impurities that promote hot cracking. Limited to 0.05% each.

Vanadium (V) and niobium (Nb) : added for precipitation hardening, but can reduce HAZ ductility.

Identification of Weldable Bars

On the job site, weldable grade 500W bars are identified by a distinctive marking. Non-weldable bars (500R) must never be welded without written authorization from the engineer. Golden rule: always check the bar marking before welding.

Permitted Welding Processes for Reinforcing Steel

Shielded Metal Arc Welding (SMAW)

SMAW is the most commonly used process on construction sites. For reinforcing steel, E7018 (basic low-hydrogen electrode) or E7016 electrodes are used. These electrodes produce low diffusible hydrogen, which reduces the risk of cold cracking.

Typical SMAW parameters for reinforcing steel:

Bar Diameter	Electrode	Current (A)	Voltage (V)	Position
10M – 15M	E7018 Ø 3.2 mm	90 – 130	20 – 24	All
20M – 25M	E7018 Ø 4.0 mm	130 – 180	22 – 26	All
30M – 35M	E7018 Ø 5.0 mm	180 – 250	24 – 28	Flat only

Gas Metal Arc Welding (GMAW)

GMAW (MIG/MAG process) is used for shop welding, particularly for cross joints or lap joints. The shielding gas is typically an argon/CO₂ mixture (75% Ar / 25% CO₂). The filler wire must be ER70S-6 or E70C-6M (flux-cored wire).

Resistance Spot Welding

This process is used to attach transverse reinforcing bars to longitudinal bars, particularly in the manufacture of welded wire mesh. It requires no filler metal and produces a weld by local fusion of the two bars.

Flash Butt Welding

Used in the shop to join bars end to end. This process produces very high-quality welds but requires specialized equipment and bars of the same diameter.

PROHIBITED Processes for Reinforcing Steel

Oxy-fuel welding : prohibited for load-bearing connections due to excessive heat input and risk of decarburization.

Submerged Arc Welding (SAW) : reserved for shop welding on thick sections, rarely used for reinforcing bars.

Unqualified spot welding : any welding process must be qualified according to CSA W186.

Joint Preparation and Welding Techniques

Joint Types for Reinforcing Steel

Joint Type	Description	Use
Lap joint	Bars overlap and are welded on the sides	Most common on site
Butt joint	Bars are aligned and welded with or without bevel	Requires full access
Cross joint	One bar is welded perpendicular to another	For stirrup assemblies
Sleeve joint	A tubular sleeve is welded over both bars	For large-diameter bars

Surface Preparation

Before welding, surfaces must be free of rust, mill scale, oil, grease, paint, or any contaminant. Preparation is done by:

46.Wire brushing (rotary stainless steel brush)

47.Grinding to remove deformations on surfaces to be welded (for butt joints)

48.Solvent cleaning for organic contaminants

Important : the deformations on reinforcing bars must be ground over a length of at least 25 mm on each side of the joint for butt welds.

Position Welding Technique

Flat position (1G) : easiest, allows better control of the weld pool.

Horizontal position (2G) : used for cross joints.

Vertical position (3G) : requires an uphill technique (push) with an electrode angle of 10° to 15° upward.

Overhead position (4G) : most difficult, requires lower current and faster travel speed.

Weld Length and Size

For lap joints, the total length of fillet welds must be at least 8 times the bar diameter for bars 20M and smaller, and 10 times the diameter for larger bars. The weld throat must be at least 0.5 times the bar diameter (minimum 6 mm).

Calculation example : For a 25M bar (25 mm diameter) welded in a lap joint, the total weld length must be 10 × 25 = 250 mm, distributed on both sides (125 mm on each side).

Preheating and Temperature Control

Why Preheat?

Preheating slows the cooling rate of the weld and the HAZ, allowing hydrogen to diffuse out of the metal and preventing the formation of brittle martensite. It also reduces residual stresses.

Recommended Preheating Temperatures

Carbon Equivalent (CE)	Bar Diameter	Minimum Preheating Temperature
< 0.45%	All	5 °C (ambient temperature)
0.45 – 0.55%	≤ 20M	40 °C
0.45 – 0.55%	> 20M	95 °C
> 0.55%	All	150 °C (welding not recommended)

Rule of thumb : if the ambient temperature is below 5 °C, always preheat to a minimum of 40 °C, regardless of steel grade.

Preheating Methods

Oxy-fuel torch : most common method on site; heat uniformly over an area of at least 75 mm on each side of the joint.

Electric resistance heaters : used in the shop for precise control.

Induction heating : modern and efficient, but expensive.

Temperature Control

The preheating temperature must be verified using:

Temperature-indicating crayons (thermo-crayons): melt at a precise temperature.

Infrared pyrometers : non-contact measurement.

Thermocouples : for continuous monitoring in the shop.

Exam trap : the preheating temperature must be maintained throughout the entire welding process, not just before starting. The interpass temperature (between passes) must not drop below the minimum preheating temperature.

Heat Input and Its Calculation

Heat input is a critical parameter that influences the HAZ microstructure. It is calculated as follows:

Heat input (kJ/mm) = (Voltage (V) × Current (A) × 60) / (Travel speed (mm/min) × 1000)

Example : Welding with 24 V, 150 A, travel speed of 200 mm/min:

Heat input = (24 × 150 × 60) / (200 × 1000) = 216,000 / 200,000 = 1.08 kJ/mm

Heat Input Limits

For grade 500W reinforcing bars, the maximum recommended heat input is 2.0 kJ/mm. Beyond this, the HAZ becomes too wide and mechanical strength decreases. For grade 400R bars, the limit is 1.5 kJ/mm.

Exam tip : if the heat input is too high, increase the travel speed or reduce the current. Never reduce the voltage below the electrode specifications.

Qualification of Welders and Procedures

CSA W186 Standard

Standard CSA W186 (Welding of Reinforcing Bars in Reinforced Concrete Construction) is the Canadian reference for qualifying welding procedures (WPS) and welders. It applies to all reinforcing bar welding work in reinforced concrete structures.

Qualification Requirements

86.Welding Procedure Specification (WPS) : must be qualified by tensile and bend testing according to CSA W186.

87.Welder : must be qualified for each process, position, and joint type.

88.Required tests :

Tensile test on welded specimen (minimum strength = 100% of the specified bar strength)

180° bend test (no cracks in the weld or HAZ)

Visual examination (dimensions, porosity, lack of fusion)

Validity of Qualifications

A welder's qualification is valid for a period of 24 months if the welder has welded at least once every 6 months using the qualified process.

Any change in process, position, electrode type, or bar diameter requires a new qualification.

The welder must be requalified if a weld is rejected during inspection.

Quality Control and Weld Inspection

Visual Testing (VT)

Visual testing is mandatory on 100% of welds. Acceptance criteria according to CSA W186:

Defect	Acceptance Criterion
Cracks	No cracks permitted
Porosity	Maximum 5% of the weld surface
Lack of fusion	Not permitted
Undercut	Maximum 1 mm depth
Excessive reinforcement	Maximum 3 mm above the surface
Spatter	Must be removed

Non-Destructive Testing (NDT)

Magnetic Particle Testing (MT) : used to detect surface and subsurface cracks in reinforcing bar welds.

Ultrasonic Testing (UT) : used for large-diameter butt joints (≥ 25M).

Penetrant Testing (PT) : for detecting surface cracks on non-ferromagnetic materials (rare for reinforcing bars).

Destructive Testing

Tensile test : on specimens taken from qualification welds.

Bend test : to verify weld ductility.

Hardness test : to detect excessive hardening in the HAZ (maximum hardness of 350 HV).

Thermal Cutting of Reinforcing Bars

Cutting Processes

Thermal cutting is used to cut reinforcing bars to the required length. Permitted processes are:

111.Oxy-fuel cutting : most common, uses the combustion of iron in oxygen. Suitable for bars with diameter ≥ 15M.

112.Plasma cutting : faster and more precise, but requires electrical equipment.

113.Abrasive cutting : mechanical method, not thermal, preferred for small diameters.

Precautions for Oxy-Fuel Cutting

Preheating : the preheating temperature must be the same as for welding.

Cutting distance : the flame must be maintained at 3 – 5 mm from the surface.

Cutting speed : must be uniform; a speed that is too slow produces excessive slag, too fast produces an irregular cut.

After cutting : remove slag and burrs by grinding.

Effects of Thermal Cutting on Steel

Thermal cutting produces a HAZ similar to welding. Cut edges must be inspected for cracks. If the bar is to be welded after cutting, the cut area must be ground to remove the oxide layer.

Safety in Welding and Cutting

Personal Protective Equipment (PPE)

Welding helmet with filter lens of appropriate shade (shade 10 to 13 for SMAW).

Welding gloves in leather, heat-resistant.

Protective clothing : leather jacket or flame-resistant cotton, long sleeves.

Respiratory protection : if ventilation is insufficient, use a mask with cartridge for metal fumes.

Hearing protection : during grinding or gouging.

Fire Safety

Hot work permit : mandatory on construction sites.

Work area : clear all combustible materials within a 10 m radius.

Fire extinguisher : an ABC extinguisher must be available within 10 m of the work station.

Fire watch : monitor the area for 30 minutes after completing welding or cutting work.

Ventilation and Fumes

Welding fumes from reinforcing steel contain iron oxides, manganese oxides, and sometimes zinc compounds (if bars are galvanized). Local exhaust ventilation is required in confined spaces. Standard CSA W117.2 (Safety in Welding, Cutting, and Allied Processes) defines ventilation requirements.

Electrical Safety

Canadian Electrical Code, Part I (CE Code) : applies to temporary electrical installations on construction sites.

Rule 8-200 : welding circuits must be protected by ground fault circuit interrupters (GFCIs) rated at 30 mA maximum.

Grounding : the workpiece must be grounded through the return cable.

Cables : regularly inspect welding cables for cuts and abrasions.

Tolerances and Acceptance Criteria

Dimensional Tolerances for Welds

Parameter	Tolerance
Weld throat	± 1 mm
Weld length	± 10 mm
Joint position	± 5 mm from intended location
Angle between bars	± 5°

Weld Rejection Criteria

A weld is rejected if:

145.Visible cracks are present, either by the naked eye or by magnetic particle testing.

146.Excessive porosity (more than 5% of the surface).

147.Lack of fusion between the weld and the base metal.

148.Undercut greater than 1 mm.

149.Insufficient reinforcement (throat below the minimum value).

150.Excessive bar deformation (deflection greater than 2% of the length).

Practical Applications and Calculations

Calculating Lap Weld Length

Formula : L = k × d

L = total weld length (mm)

k = 8 for bars ≤ 20M, k = 10 for bars > 20M

d = nominal bar diameter (mm)

Example : 30M bar (d = 30 mm): L = 10 × 30 = 300 mm of total weld. If welding on two sides, 150 mm per side.

Calculating the Number of Welds for an Assembly

For a cross joint (stirrup on longitudinal bar), the weld must have a minimum length of 4 times the diameter of the smaller bar, with a minimum of 50 mm.

Calculating Preheat Based on Ambient Temperature

Simplified rule : T_preheat = 40 °C if T_ambient < 5 °C, otherwise T_preheat = 0 °C (no preheating required) for low-CE steels.

Pitfalls to Avoid

163.Welding non-weldable bars (grade R) : always check the marking. 500R bars must never be welded without written authorization from the engineer.

164.Neglecting preheating in cold weather : ambient temperature is measured in the shade, not in the sun. If the bar is at 4 °C, preheat to 40 °C.

165.Using damp electrodes : E7018 electrodes must be stored in an oven at 120 °C and used within 4 hours after removal from the oven. Damp electrodes produce hydrogen and cause cracking.

166.Confusing heat input and current : heat input depends on voltage, current, AND travel speed. A slow speed with low current can produce high heat input.

167.Forgetting visual inspection after welding : visual inspection must be done after complete cooling, not immediately after welding.

168.Welding without qualification : any reinforcing bar weld must be performed according to a qualified WPS and by a qualified welder. Unqualified welds must be removed.

169.Ignoring Rule 8-200 of the Canadian Electrical Code : welding stations must be protected by 30 mA GFCIs.

170.Cutting bars with a torch without preheating : thermal cutting requires the same precautions as welding.

171.Welding galvanized bars without precautions : zinc fumes are toxic and the weld quality is poor. Grind the zinc coating over 25 mm on each side of the joint.

172.Not documenting welding parameters : actual parameters (current, voltage, speed) must be recorded and compared to the WPS. A deviation of more than 10% requires a new qualification.

Summary

Reinforcing bar welding is governed by CSA W186 and CSA G30.18 in Canada.

Only steels with low carbon equivalent (grade W) are weldable without special precautions.

Permitted processes are SMAW (E7018 electrodes), GMAW, resistance welding, and flash butt welding.

Preheating is mandatory if the ambient temperature is below 5 °C or if the CE is high.

The maximum heat input is 2.0 kJ/mm for 500W bars.

Lap welds must have a total length of 8 to 10 times the bar diameter.

Visual inspection is mandatory on 100% of welds; magnetic particle testing is required for critical joints.

Welders must be qualified according to CSA W186 and requalified every 24 months.

Electrical safety is governed by the Canadian Electrical Code, Part I (CE Code), Rule 8-200 (30 mA GFCI).

Thermal cutting requires the same precautions as welding (preheating, PPE, ventilation).

Basic electrodes must be stored dry and used within 4 hours after opening the oven.

Final Exam Tips

186.Memorize key values : CE = 0.45% (limit), heat input = 2.0 kJ/mm, weld length = 8d/10d, preheat = 40 °C below 5 °C ambient.

187.Understand the formulas : carbon equivalent and heat input are frequent exam calculations.

188.Know the standards : CSA W186, CSA G30.18, CSA W117.2, Canadian Electrical Code, Part I (CE Code).

189.Identify the traps : questions about damp electrodes, non-weldable bars, and preheating temperatures are recurring.

190.Practice the calculations : weld length, heat input, number of welds required for a given assembly.

191.Review acceptance criteria : porosity 5%, undercut 1 mm, reinforcement 3 mm, hardness 350 HV.

Mastering this chapter will allow you not only to pass the Red Seal exam, but also to perform safe and compliant reinforcing bar welds on Canadian job sites. Good luck with your preparation!