Flexural Strengthening of Concrete: CFRP vs. Steel Plates and More

Concrete structures are not static.
Loads increase, codes evolve, durability issues appear, or sometimes the original design simply no longer meets today’s requirements.

When this happens, flexural strengthening becomes necessary. In 2025, engineers have several well-proven methods to draw on.

This post looks at the two most common techniques – CFRP (carbon fibre reinforced polymer) and bonded steel plates – and where other methods may be more suitable.

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Why Flexural Strengthening?

Flexural strengthening increases the bending capacity of beams, slabs, girders, and other members.

Common triggers include:

• Increased live loads (e.g. change of use, heavier traffic, retrofits)
• Damage from corrosion, fire, or impact
• Service life extension of critical assets
• Code upgrades, seismic provisions, or fatigue checks

The principle is simple: add a material with high tensile capacity to the tension face of the member. The right choice of material is what determines the outcome.

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CFRP (Carbon Fibre Reinforced Polymer)

CFRP has transformed strengthening practice. Laminates or fabrics are bonded with epoxy to provide extremely high tensile strength with only a few millimetres of thickness.

Advantages:

• Corrosion resistance – CFRP does not rust, even in marine or chloride-exposed environments
• Lightweight, thin profile – easy to transport, cut, and install, with minimal impact on headroom
• High strength-to-weight – tensile strength several times higher than steel
• Efficient for overhead works – simpler installation compared with steel
• Fatigue: CFRP is particularly effective for fatigue strengthening (e.g. bridges), where steel plates may not perform as well.

Limitations:

• Fire performance – capacity is quickly lost without fireproofing
• Anchorage – relies entirely on adhesive bond, no inherent mechanical anchoring
• Material cost – higher than steel, though labour savings often offset this
• Design familiarity – codes now exist (AS 5100.8, ACI 440, fib), but not all practitioners are confident with them

In most building and civil applications where fire risk is managed, CFRP is the preferred solution.

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Bonded Steel Plates

Bonded steel plates were the benchmark method for decades. Steel plates are epoxy-bonded and often mechanically anchored to the concrete soffit.

Advantages

• High tensile capacity – significant strengthening achievable
• Fire performance – retains strength at elevated temperature when mechanically anchored
• Mechanical anchoring – bolts or anchors provide redundancy beyond adhesive bond
• Designer familiarity – well-understood material behaviour and detailing

Limitations

• Corrosion – vulnerable unless well protected
• Weight and clearance – heavy to handle, reduces headroom
• Installation – labour-intensive, drilling and access required

Steel plates are now more of a niche choice, suited to projects where fire, anchorage, or very large strengthening demands rule out CFRP.

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Table below shows a practical comparison between CFRP and Steel Plates

Factor	CFRP Laminates	Steel Plates
Corrosion Resistance	Excellent	Poor without coatings
Weight	Very light, easy to handle	Heavy, difficult overhead
Profile / Headroom	Minimal thickness	Bulky, reduces clearance
Strength-to-Weight	Extremely high	High, but heavier
Fire Performance	Poor without fireproofing	Good with anchors
Anchorage	Adhesive bond only	Can be mechanically anchored
Durability	Proven long-term	Dependent on corrosion protection
Installation	Easier, faster, less intrusive	Labour-intensive
Best Use Cases	General flexural upgrades, tight spaces, marine/chloride	Fire-critical, extreme loads, anchorage needed

A Practical Note
Both CFRP and steel rely on proper substrate preparation and installation. Surface cleanliness, moisture control, and achieving the right concrete surface profile (CSP) are critical to bond performance. Without this, even the best material choice will underperform.

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Other Strengthening Techniques

Beyond CFRP and steel plates, engineers may use:

• External post-tensioning – effective for long spans such as bridges; anchorage zones can be complex inside buildings
• Section enlargement – adding reinforced concrete to the member; intrusive but delivers large capacity gains
• Supplementary steel sections – bolted or welded steel beams/angles assisting the concrete member

Each method has trade-offs, depending on access, durability, aesthetics, and load demand.

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Choosing the Right Method

The decision depends on project-specific factors:

• Is the structure in a corrosive or marine environment?
• How much extra flexural strength is required?
• Is fire performance critical?
• Is clearance or architectural intent a constraint?
• Can the works be carried out safely and economically?
• What is the balance between upfront cost and whole-of-life cost?

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The Takeaway

Steel plates are not obsolete. They still play a role in fire-critical or very high-demand applications.

But in most projects today, CFRP is the first choice: faster, less intrusive, corrosion-resistant, and increasingly well supported by design codes.

At Vulcan, we do not prescribe a trendy solution. We design what the project demands – whether that means CFRP, steel, post-tensioning, or a hybrid approach.