7 Common Failure Types, Root Causes & Corrective Actions — Plus a Standardized Manufacturing Workflow

Rebar couplers are the hidden skeleton of reinforced concrete structures. Their performance directly affects structural integrity, seismic resistance, and long-term durability.

However, laboratory testing shows that nonconforming couplers occur frequently, with rebar pull-out from the sleeve accounting for the highest failure rate.

Today, we break down 7 major failure types, explain the root causes, provide practical corrective actions, and share a standardized production process to help you avoid costly construction and inspection pitfalls.

7 Major Nonconformity Types: Causes & Corrective Measures

1. Rebar Base Material Defects

Unqualified raw material makes everything else meaningless

Common causes:

• Rebar strength below design requirements

• Cracks, inclusions, or internal defects

• Non-uniform material at the first 500 mm of rebar ends (not removed)

Corrective actions:
✅ Perform incoming inspection of chemical composition and mechanical properties
✅ Mandatory removal of the first 500 mm at rebar ends
✅ Use compliant grades such as HRB400E / HRB500E only
✅ Never mix rebars of different grades or specifications

2. Poorly Machined Rebar Threads

A small machining error leads to a major connection failure

Common causes:

• Worn threading rollers → incomplete threads

• Thread ends not cut flat (excessive “horse-hoof” taper)

• Insufficient thread length

• Over-stripping or under-stripping of the rebar

Corrective actions:
✅ Replace threading rollers regularly and calibrate machines before operation
✅ Cut thread ends flat; control taper angle within ≤ 3°
✅ Machine thread length to ½ sleeve length + 1 full thread
✅ Measure each thread with calipers — nonconforming threads must be reworked

3. Upset Forging Transition Defects

Improper upsetting leads to cracking

Common causes:

• Insufficient upsetting temperature or pressure

• Cracks or folding in the transition zone

• Upset diameter below specification

Corrective actions:
✅ Perform visual inspection + magnetic particle testing (MT)
✅ Scrap any rebar with visible cracks
✅ Ensure upset diameter is 4–6 mm larger than original rebar diameter

4. Defective Coupler Material or Dimensions

Unqualified couplers must never be used

Common causes:

• Low-grade steel instead of qualified carbon steel (e.g., 45# steel)

• Dimensional deviation (inner diameter, length, wall thickness)

• Incorrect thread parameters

• Internal shrinkage cavities or cracks

5. Longitudinal Cracking of Couplers

Stress concentration is the main culprit

Common causes:

• Insufficient wall thickness

• Brittle material properties

• Eccentric alignment between rebar and coupler

Corrective actions:
✅ Verify coupler wall thickness against design requirements
✅ Prohibit material downgrading or weight reduction
✅ Ensure precise axial alignment during installation

6. Rebar Pull-Out from the Coupler (Most Common Failure)

This is the most frequent laboratory failure, typically caused by four critical details:

🔸 Cause 1: Rebar size deviation or excessive stripping
→ Loose engagement between threads
Fix: Control stripping length to ½ sleeve length and verify fit with gauges

🔸 Cause 2: Worn threading rollers
→ Incomplete, uneven threads
Fix: Replace rollers regularly; rework any defective threads

🔸 Cause 3: Uncut thread ends
→ Insufficient effective threads
Fix: Cut ends flat before threading to ensure full engagement

🔸 Cause 4: Insufficient thread length
→ Thread length < ½ sleeve length
Fix: Machine threads to ½ sleeve length + 1 thread, measure each piece

7. Other Common Oversights

Minor details can still cause major failures

Common causes:

• Inadequate tightening during installation

• Mismatched coupler and rebar sizes (e.g., M25 coupler with Ø22 rebar)

• Corrosion of threads

Corrective actions:
✅ Verify tightening torque using a torque wrench
✅ Match coupler size strictly to rebar diameter
✅ Protect threads from moisture and corrosion after machining