Carbon Fiber 'Band-Aids': Minimally Invasive Reinforcement for Bridges and Buildings

In daily life, we use plasters to treat minor cuts and grazes – simple and convenient. Now, however, when bridges develop fine cracks in their beams and old buildings' walls gradually loosen with age, a seemingly lightweight material is emerging as the 'main force' in structural reinforcement. Engineering technology has also embraced a similar 'band-aid' solution—carbon fibre fabric—which is quietly fortifying the bridges and buildings around us through minimally invasive methods.
Unidirectional fabric manufacturing process

Unidirectional carbon fibre fabric is a commonly used material in carbon fibre reinforcement projects. Its manufacturing process primarily involves the arrangement, fixation and moulding of carbon fibre filaments, with the core objective being to ensure the filaments are neatly aligned in a single direction (typically longitudinal) while maintaining high strength properties. The specific steps are broadly as follows:

Step One: Preparation of Carbon Fibre Filament
PAN-based carbon fibre precursor filaments (the market mainstream) are manufactured from polyacrylonitrile fibres through pre-oxidation and carbonisation processes. These filaments feature a diameter of 5–7 micrometres and exhibit exceptionally high tensile strength.
Step Two: Warping Process
Through the warping machine, thousands to tens of thousands of carbon fibre filaments are arranged longitudinally in parallel and uniformly into warp layers. Tension and alignment precision are strictly controlled to ensure unidirectional properties and prevent crosswise skewing.
Step Three: Weaving / Fixing Process
Using extremely fine transverse fibres (such as glass fibre or nylon), the longitudinal threads are secured by weaving or bonding. During weaving, the weft threads are interlaced with the warp threads at a low density (a few threads per centimetre), forming a "tight warp, loose weft" structure that provides reinforcement without compromising strength. Alternatively, an epoxy resin emulsion may be used to bond the material into a weftless fabric, though weaving remains the more common method.
Step Four: Surface Treatment
Remove impurities, increase surface roughness, or apply a coupling agent to enhance adhesion to the adhesive and improve compatibility with the resin.
Step Five: Setting and Winding
Following drying and setting, the product undergoes quality inspection and packaging to become the finished item. Throughout this process, the focus is on maximising the high strength of the longitudinal carbon fibre strands, with the transverse strands serving only an auxiliary fixing function.

The core of the entire process lies in ensuring the parallel alignment and high tensile strength of the longitudinal carbon fibre strands, with the transverse fibres serving only an auxiliary fixing function. Consequently, unidirectional carbon fibre fabric fully exploits the tensile properties of carbon fibre along its longitudinal axis, making it highly suitable for applications in structural reinforcement where tensile stresses are encountered.

Principles of Reinforcement
Carbon fibre fabric is bonded to the structural surface using a compatible resin adhesive, forming a composite material body (CFRP) that works in concert with the original structure. Possessing exceptionally high tensile strength (approximately ten times that of steel), the carbon fibre fabric is tightly integrated with the concrete through the resin adhesive. This transfers external loads to the carbon fibre fabric, leveraging its high-strength properties to enhance the structure's resistance to bending, shear forces, and seismic activity, while simultaneously inhibiting crack propagation.

Construction Procedures

Step One: Surface Preparation
Clean the concrete surface, grind away loose layers and oil stains to create a smooth base surface.
Step Two: Apply the epoxy primer
Apply a coat of primer to the concrete to ensure the fabric adheres securely.
Step Three: Apply the carbon fibre fabric
Dip the pre-cut carbon fibre fabric into epoxy resin and apply it to the structural areas requiring reinforcement (such as beam bottoms, columns, floor slabs, and bridge webs).
Step Four: Roller Compaction and De-aeration
Press repeatedly with a roller to ensure the adhesive permeates evenly into the fibres and expels any air bubbles, much like firmly pressing down a plaster.
Step Five: Curing and Maintenance
Once the adhesive has cured, the carbon fibre fabric and concrete become fused together, forming an exceptionally strong external layer.

Reinforcement characteristics

Enhancing load-bearing capacity: Applied to the underside of beams → improved bending resistance; applied to columns → enhanced compressive strength and seismic p erformance.

Crack control: Carbon fibre fabric restrains further cracking in concrete, much like a plaster protects a skin wound from worsening.

Extended service life: Corrosion-resistant, unlike steel which rusts, making it particularly suitable for environments such as bridges that are exposed long-term.

Lightweight and efficient: Rapid construction speed, requiring no heavy machinery and imposing no significant additional structural weight.

Why has carbon fibre fabric become the new choice for reinforcement?

Compared to traditional reinforcement methods such as steel plate reinforcement or section enlargement, carbon fibre fabric offers distinct advantages:

Its lightweight nature imposes virtually no additional load on the original structure. Weighing less than 300 grams per square metre, it alters the building's load distribution minimally after application, making it particularly suitable for reinforcing structures sensitive to self-weight, such as ancient buildings and bridges.

Its corrosion resistance ensures stable performance in harsh environments. Whether confronting salt spray corrosion on coastal bridges or damp conditions in basement beams and columns, carbon fibre fabric maintains consistent performance with a service life exceeding 50 years.

Efficient installation significantly reduces project timelines. Requiring no heavy machinery—only manual application—reinforcement of a thousand-square-metre building typically takes just 1–2 weeks, substantially shorter than conventional methods.