STRENGTHENING WITH CFRP SYSTEMS – DETAILING CONSIDERATIONS
CFRP laminates represent a cost-effective solution for the flexural strengthening of concrete elements.
Laminates present as a wide but thin carbon fibre element.
Laminate CFRP systems come in a range of sizes, and we keep stock of the two most popular from the Sika Carbodur range, S812 and S1214.
In most situations where reasonable levels of strengthening are required, the S1214 systems will be the preferred option for overall economy.
CFRP laminates come in rolls of 50m,100m & 250m, which we typically guillotine cut on-site to length.
Laminates can be applied in horizontal applications, typically for carbon fibre strengthening beams and slabs, and in vertical applications for columns, walls and cores. As we dispense directly from the roll and cut to length on site, particularly long lengths spanning between multiple floors in vertical applications are possible, limited only by the application technique, minimising lap locations.
Laminates can be applied directly to a prepared surface, as is common in many soffit installations, or they can be recessed into the top surface and backfilled to allow a flush and durable finish on completion.
Rebate depths will be approximately 10mm for single-direction applications and up to 16mm for overlapping laminates in two directions. Slabtec always uses GPR scanners to confirm reinforcement/tendon covers prior to rebating.
CFRP woven fibre fabrics can be in unidirectional or bi-directional form.
The carbon fibre strengthening-based systems available in Australia are typically in grades from 230g/m2 to 900g/m2 in grams of carbon fibre per square meter.
Wrap weights over approximately 300g/m2 to 400g/m2 are normally laid wet, i.e., before being placed against the substrate, the fabric must be properly passed through a saturator machine.
Slabtec has purpose-built saturators, which the fabric is passed through to ensure complete saturation of the fabric occurs.
With the lighter fabrics, the adhesive can be placed against the surface and the fabric laid dry while working the adhesive through the fabric while in position.
Heavier fabrics cannot be laid in this way, as the penetration of the resin into the fabric cannot be guaranteed.
Different adhesives are available for wet and dry lay applications.
The primary advantage of fabric is the flexibility of the material that allows it to be placed around external corners as long as minimum radii are observed.
The classic example of strengthening for which fabric is a circumferential wrapping of columns, either circular or rectangular. Notwithstanding this, they can also be used effectively in vertical applications for column, wall or core elements.
CFRP rods have some advantages in certain applications, similar to laminates but in a circular cross-section.
The CFRP rods are rebated in the top of the slab or vertically into the wall.
Rebating of rods is relatively quick and easy as the rebate takes the form of a relatively narrow chase in the floor or wall also the narrow chase is quite robust when filled.
However, rod systems do have some disadvantages. The unit cost of the material is often more expensive than in laminate form, and, as the rods cannot be rolled in many of the larger and practical sizes they tend to be typically purchased in 6m or 12m lengths. These can be difficult to handle and sometimes leave significant waste offcuts, which is undesirable in a high-value material.
Obviously, being a round cross-section, they also cannot be surface mounted, and one of the other drawbacks is the depth of the chases the rods are mounted in, which is typically 1.5x the rod diameter.
So, for one of the most common rod sizes in Australia, 12mm, the chase’s depth (and width) will be 18mm. Given this is close to the typical minimum internal cover of 20mm in internal conditions as specified by AS13007522832, a great deal of care must be taken to determine the as-built covers to reinforcement and tendons prior to installation.
This also precludes the use of rod in two directions in the vast majority of cases where the chases would be nominally 36mm.
CFRP – Near Surface Mounted (NSM)
NSM systems (Near Surface Mount) generally employ smaller laminates up to 20mm x 2.5mm, the most common in Australia, and are installed by slotting into concrete surfaces.
The slots are quick and easy to facilitate, but given the depth of the slots required, two-directional applications are typically not possible.
Another complexity is due to the element’s stiffness in the vertical direction; any curvatures or displacements in the slab, particularly in applications requiring long laminate lengths, will require a deeper overall slot to maintain minimum depths.
This can also be a factor in rod applications, although to a lesser extent.
CFRP – L plates (Proprietary Sika CarboShear Shear Links)
Pre-formed L plates that have been purposefully designed for shear-strengthening applications.
Available in 3 leg lengths, combinations are available up to 1,500mm long with a 25mm radius at the bend zone.
The compact 40mm x 2.0mm cross-section allows for greater flexibility in placement options between cast-in components and penetrations through beams.
CFRP – Rope (Proprietary SikaWrap FX-50 C or custom fabric-based rolled systems)
Ropes are a unidirectional carbon fibre rope or string commonly used to provide anchorage for other FRP systems.
The small diameter rope is presaturated and installed into 14mm -16mm diameter holes containing the approved adhesive.
The free end of the laminate is then typically displaced in a fan arrangement as required by the anchoring requirements of the primary CFRP carbon fibre strengthening system.
Adhesion Testing – the importance of bond!
Any CFRP strengthening system’s effectiveness will always depend on achieving the required minimum adhesion levels to allow the FRP carbon fibre strengthening systems to provide the necessary design resistance to the member.
Slabtec always recommends that adhesion testing be undertaken on-site to confirm the design intent can be achieved.
Slabtec’s pull-off adhesion testing equipment allows us to perform adhesion testing of prepared substrates to accepted international standards.