Demystifying Structural Strengthening with CFRP

Guidance regarding the design life of CFRP structural strengthening is available in  concrete Society Technical Report 55: Design guidance for strengthening concrete structures using fibre composite materials (TR-55). Epoxy materials and carbon fibres are inert materials and it is noted that design lives are significantly dependent on the condition of the concrete to which the CFRP is bonded. It is crucial that any issues affecting the strengthened area, as well as the structure more broadly are addressed and prevented from reoccurring. Issues that could affect the local concrete strength or bond strength of the adhesive include water penetration, corrosion and surface cracking. 

This comes down to several factors including:

• The wrap or plate being correctly positioned so that the fibres it contains are put into tension by the new proposed loads. This is a design consideration, but can also be influenced by site workmanship.
• The CFRP being fully bonded to the substrate material. For a reinforced concrete structure, this would mean undertaking adequate surface repair and preparation which removes unsound concrete, oil, laitance, dust and other contaminants and leaves behind clean structural concrete to bond to. For column confinement, the corners of the column must also be rounded to a consistent radius of >20 mm to increase efficiency and reduce the shear impact on fibres.
• Adequate anchorage is fully considered in the design and that any additional mechanical anchorages are correctly specified and installed on site.
• Following preparation, it is critical that we ensure the concrete is capable of transferring load from the structure to the fibres. This is done by testing the concrete strength in tension by completing representative pull-off testing to BS EN 1542.
• Surface flatness is also an important factor that must be addressed to ensure effective load transfer. FIB Bulletin 14 recommends no more than 10 mm over a 2m length, or 4mm over 300mm for laminate plate applications.
• It is essential that the correct adhesive and the correct amount of adhesive or resin is used to apply the CFRP. This ensures strong and complete bonding, no voids and that every fibre contributes to the strengthening of the element.
• The work is carried out in the right environmental conditions: temperature, relative humidity and substrate moisture conditions etc., to ensure that the adhesive or resin performs as specified once cured.
• Overlaps when wrapping columns, or four-sided shear strengthening, must be greater than 200 mm (TR-55) and distributed around the column when applying adjacent wraps.

Please refer to Sika Method Statements for complete installation guidance – these can be found on our website, or email for more details.

It is very often the case that the CFRP is not required to achieve the required Fire Resistance of a structure due to the reduction infactors accompanying an Accidental Load scenario.

Where required, fire protection can be provided by specialist fire protection mortars and fire boarding techniques. Both methods have the potential to improve both the Fire resistance and Reaction to Fire of the structure and the FRP, although they both have limitations.

Sika offers Sikacrete-213 F fire protection mortar which when applied to CFRP installed on a reinforced concrete element will dramatically slow the penetration of heat into the structure. Multi-layer fire-boarding techniques may also offer a solution.

The increase in Fire Resistance of CFRP strengthening is limited as it is usually surface-mounted and epoxy adhesives/resins begin to soften at ~60°C. In contrast, fire protection mortars and boards can be very effective at increasing the Fire Resistance of the reinforced concrete section, as the temperatures at which the strength of concrete and steel begin to degrade are significantly higher. Independent testing has shown that a 40mm application of Sikacrete-213 F can achieve a Fire Resistance of 240 minutes under RWS curve conditions.[1]

Both fire protection mortars and boarding can also help with Reaction to Fire of the epoxy materials by effectively isolating and preventing them from contributing to the fire. 

We have a wide array of EPDs and LEEDv4 Attestations available for our materials, including all the resins, adhesives and epoxy-based repair materials used for CFRP structural strengthening. Please use the search function here to download these documents, or email us.

We are yet to conduct any direct comparison studies which examine the impact of, for example, CFRP vs. steel plate bonding and aren’t

aware of any published studies. As discussed in the presentation, although the material energy intensity (MJ/kg) is significantly higher for CFRP laminate plate than for steel plate, its lower weight means that it has a lower contribution overall. It is also possible that the energy intensity of the CFRP plate installation is significantly lower due to:

• The reduced number of operatives and trades on site (less vehicle journeys and movements).
• No need for lifting equipment, temporary propping, bracketry etc.
• Reduced logistic demands – simple pallet or box delivery, vs. dedicated vehicle delivery.
• No need for grit-blasting/specialist preparation of the steel plate.

It is likely that reinforced concrete jacketing, a particularly labour-intensive form of column strengthening where new concrete or grout is placed in addition to new steel reinforcement, will be significantly more energy intensive than the equivalent CFRP wrap method for strengthening. 

If required, fire protection is always applied onto CFRP strengthening systems after they have been installed and the adhesive/resin has cured. If a fire protection mortar such as Sikacrete-213 F is to be used, then the installed plate or wrap must be broadcast with a suitable kiln-dried quartz sand to achieve the required mechanical bond with the mortar. For wrap applications, the sand would be broadcast into the wet resin of the outermost layer of the CFRP (i.e., no additional resin application is required). For plate applications, an additional layer of adhesive should be applied to the installed plate before the quartz sand is broadcast onto it.

Please note, fire protection is rarely required for the CFRP strengthening, but can be required for the reinforced concrete section (see more above). 

NSM is an acronym for Near Surface Mounted. NSM bars and rods come in a variety of circular and rectangular profiles and like the surface applied plates are manufactured by pultrusion. The key difference is that these plates are installed in channels in the cover concrete so that the strengthening system is fully ‘hidden’ in the structure. This is a major advantage when the likelihood of accidental damage is high, for example on exposed slabs and bridge decks. As they are recessed, the installation area can be covered by waterproofing or road surfacing with a much reduced risk of damage to the strengthening system. Due to the three-sided encapsulation NSM bars have excellent anchorage characteristics and they are widely used for to provide flexural strengthening against negative or hogging moments over supports. NSM profiles can also be used to strengthen timber structures, where they can be completely hidden – an important feature for historic structures.

It is difficult to state comparative costs of CFRP vs. traditional strengthening methods with any confidence as how they compare will be affected by many factors that are dependent on the site and project specifics. For example, available access, condition of the existing structure and the required increase in capacity.

As was noted in the Webinar, the main cost savings attributed to the use of CFRP are:

• Significant reduction in programme
• Less trades and labour
• Less temporary works
• Less material use

Anecdotally, the overall cost of strengthening a single column by CFRP wrapping may be similar to the cost of installing a reinforced concrete jacket. However, the programme for the latter may be 4-7 days, whereas for the wrapping method it may be 1-2 days. In addition, the CFRP will reach its full strength at seven days, whereas the concrete or grout will take 28 days. 

Sika provide detailed Method Statements for the application of our CFRP structural strengthening systems, along with details of the required substrate testing/checks and recommended quality assurance. These requirements generally align with the recommendations and guidance in TR-57. We do not provide specific advice on maintenance, inspection and testing post-installation, but this is covered by TR-57.

Drilling through plate or wrap creates stress concentrations which cause a disproportionate amount of damage to the composite section. TR-57 suggests that drilling a single hole through a laminate reduces its strength by an amount equivalent to a reduction in section of two to three hole diameters. The positioning of any damage to CFRP is also important; for example, a hole drilled through a laminate plate close to its end could have a much smaller impact than a hole at mid-span where the plate is carrying a higher tensile load.

Any required fixtures to the substrate post-CFRP strengthening should be considered in the design phase. It may be possible to allow space for these fixings to minimise damage to installed materials.

If you have any further questions that aren’t covered by the below, or have any design/specification queries regarding upcoming projects, please email in.

[1] Results of fire resistance experiments on sika-frp-strengthened reinforced concrete columns, N. Benichou, D. Creed, M.
Adelzadeh, M.F. Green, P. Leroux, and J.C. Latour, National Research Council of Canada