CFRP LAMINATES FOR SHEAR STRENGTHENING OF CONCRETE BEAMS

The use of Near Surface mounted Reinforcement for concrete structures are not a new invention. A type of NSMR has been used since the 1940s, where steel reinforcement is placed in slots in the concrete cover or in addition concrete cover that is cast onto the structure. Here steel bars are placed in slots in the concrete structure and then the slots are grouted. It has also been quite common to use steel bars, fastened to the outside of the structure covered with shotcrete. However in these applications it is often difficult to get a good bond to the original structure and in some cases, it is not always easy to cast the concrete around the whole steel reinforcing bars. From 1960s the development of strong adhesives such as epoxies, for the construction industry moved the method further ahead by bonding the steel bars in sawed slots in the concrete cover. However, due to the corrosion sensitivity of steel bars an additional concrete cover is still needed. For these applications, epoxy coated steel bars are not always corrosion resistant for various reasons that will not be discussed here. The use of steel NSMR cannot be said to have shown great success. Nevertheless, by using CFRP NSMR some of these drawbacks that steel NSMR posses can be overcome.

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Fig. 5.1. Near surface mounted FRP, rectangular shapes and rods

Firstly, CFRP NSMR does not corrode, so thick concrete covers are not needed. Secondly, the CFRP laminate can be tailor made for near surface applications and moreover the lightweight of the CFRP laminates makes them easy to mount. Finally, depending on the form of the form of the laminate air voids behind the laminates can be avoided. Both epoxies and systems using high quality cement mortar can be used. However, before proceeding, a short description of how to undertake a strengthening work with NSMR will be given. In practical execution the following steps must in general be performed during strengthening:

Sawing slots in the concrete cover, with the depth depending upon product used and the depth of concrete cover.
Careful cleaning of the slots after sawing using high-pressurized water, approximately 100-150 bars is recommended. No saw mud is allowed in the slot.
If an epoxy system is used, the slot must be dry before bonding. If a cement system is used it is generally recommended that the existing surfaces are wet at the time of concrete mortar casting.
Adhesive is applied in the slot, or with a cement system, cement mortar is applied in the slot.

Table 5.1 Characteristics and aspects of externally bonded FRP reinforcement.

Properties	NSMR
Shape	Rectangular strips or laminates
Dimensions: Thickness Width	Simple bonding of factory made profiles with adhesive or cement mortar in presawed slots in the concrete cover
Application aspects	For flat surfaces Depends on the distance to steel reinforcement A slot needs to be sawn up in the concrete cover The slot needs careful cleaning before bonding Bonded with a thixotropic adhesive. Possible to use cement mortar for bonding Protected against impact and vandalism Suitable for strengthening in bending Minor protection against fire

ASSUMPTIONS IN NSMR STRENGTHENING

In the design for strengthening with NSMR the following assumptions are made:

Bernoulli’s hypothesis applies, i.e.; linear strain across the section varies rectilinearly. This implies that the linear strain in the concrete, steel reinforcement and laminate that occurring at the same level is of the same size. Composite action applies between all the materials involved.
Concrete stresses are obtained from the materials characteristic curve. Concrete compression strain is limited to an approved failure strain of €=3.5%.
For a cracked cross section, the concrete’s residual ensile strength is ignored.
The stress in tensile and compression steel reinforcement are taken from the reinforcement’s characteristics curve corresponding to the total strain. The total strain in laminate may not exceed the failure strain.
The laminate is assumed to be linearly elastic until breakage i.e.; Hooke’s law applies.

In addition to this it is important to notice that if there exists a strain field on the structure, due to for example the dead load, this must be considered in design.

STRENGTHENING TECHNIQUES

The NSMR strengthening is based on fixing,by epoxy adhesive,Carbon Fibre Reinforced Polymers into precut slits opened in the concrete cover of lateral surfaces of beams.The EBR and NSM strengthening techniques are represented infig6 .

Strengthening techniques (a) external bonded,(b)near surface mounted

Fig.Strengthening techniques (a) external bonded,(b)near surface mounted

Detailing of the near surface mounted reinforcement is an important issue; we need to select the most suitable FRP cross-section and adhesive. In design there should be considered the minimum distance between adjacent reinforcement to avoid horizontal propagation of the splitting cracks, and the minimum distance from the edge of the member to avoid edge splitting effect.

Application of near surface mounted FRP reinforcement consists

of the following working steps:

In the first step a groove is cut using a saw with one or two diamond blades or a grinder with dimensions in function of the reinforcement size and type. Further preparation of the groove consists of cleaning the surface from dust and lose parts using vacuum or compressed air, then the groove is filled halfway with adhesive, afterwards the FRP rod/strip is inserted and lightly pressed to let the adhesive flow around the FRP. Finally, the groove is filled with more paste and the surface is levelled .The minimum dimension of the grooves should be taken atleast 1.5 times the diameter of the FRP bar. When a rectangularbar (strip) with large aspect ratio is used, the minimum dimensionsmust be 3 times the bar width and 1.5 times the bar height. In other instances, the minimum groove dimension could be the result of installation requirements rather thanengineering. For example the groove width may be limited bythe minimum blade size and the depth by the concrete cover.We should always avoid cutting of the existing steel reinforcement.Optimal dimensions of the groove may depend on characteristicsof the adhesive, surface treatment of FRP, and concrete tensile strength, surface aggregates

Spacing of the NSM reinforcement

Fig. Spacing of the NSM reinforcement

Spacing of FRP shear reinforcement should not exceed lnet /2 or 600 mm

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Fig. Shear Strengthening

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Fig.Procedure for strengthening with NSMR

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Fig. Strengthening of a concrete joint with NSMR

ADVANTAGES AND DISADVANTAGES OF NSMR

The advantages of Near Surface Mounted Reinforcements are:-

a low weight of the fibre makes it easy to handle without lifting equipment at the site
negligible changes of crosssection, self weight and free height of a structure.
quick to apply

There also exist disadvantages such as:

Without protection the reinforcement is fire and impact sensitive.
Design consultants, contractors and clients have limited experience.

Depending on the structure going to be strengthened, different aspects might arise. For all strengthening methods it is of utmost importance to understand how the strengthening will affect the final structure.

FAILURE MODES

Several failure modes are known in general for elements strengthened with FRP. Their understanding is important, because they have significant effect on the ultimate load.

5.5.1 Failure Modes of Externally Bonded FRP Reinforcements

Bond is necessary to transfer forces from the concrete in to the FRP, bond failure implies complete loss of composite action. Four different bonding failures are discussed below:

debonding in the concrete cover near the surface along a weakened layer,
debonding at the interface between concrete and adhesive,
debonding in the adhesive, and
debonding between adhesive and FRP

Peeling-off failure is associated with the propagation of the localized debonding. Peeling-off failures can be distinguished according to the initiation of debonding. Debonding can result in peeling-off at: flexural cracks, shear cracks, unevenness of the concrete surface and in the anchorage zones

Interface bond failure modes for EBR FRP strips

Fig. Interface bond failure modes for EBR FRP strips

Failure Modes of Near Surface Mounted FRP Reinforcement

1.Interfacial Failure Modes

Interfacial failure modes can develop in two modes as a pure interfacial failure or as a cohesive shear failure in the adhesive. Pure interfacial failure can be identified by the absence of adhesive remained at the FRP surface after failure. Cohesive shear failure can be identified by the presence of adhesive on both FRPand concrete after failure.

(1) Failure at reinforcement adhesive interface

The pure interfacial mode can be critical for bars with smooth or lightly sand-blasted surfaces, when the bond relies on adhesion instead of mechanical interlock between bar and adhesive.

(2) Failure at the epoxy concrete interface

Interfacial failure was found critical only in case of precast grooves due to their even surface. When this type of failure develops the bond stress is lower than usual, but failure is more ductile due to the residual friction at adhesive and concrete interface.

2.Cover Splitting

The mechanism of cover splitting in case of round bars is similar to the splitting bond failure of steel deformed bars, but due to the softer deformations of the FRP bars the splitting tendency is not as intense. Splitting is caused by the radial component of the bond stress. Multiple types of cover splitting were observed, incase of epoxy adhesive concrete cracking and concrete cracking accompanied by longitudinal splitting of the adhesive, in case of cementitious mortar adhesive splitting of the adhesive was dominant influenced by the low tensile strength of the filler material. However, in case of NSM strips the perpendicular component of interactional stress acts towards the thick lateral concrete (exception are reinforcements close to the edge) so splitting failure is less likely to appear

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Fig. Failure at epoxy concrete interface

Cover splitting failure of NSM round bars a) concrete cracking b)concrete cracking accompanied by longitudinal splitting of the adhesive c) splitting of the adhesive

Fig. Cover splitting failure of NSM round bars a) concrete cracking b)concrete cracking accompanied by longitudinal splitting of the adhesive c) splitting of the adhesive

3.Edge Splitting

Edge splitting failure can be critical in elements where the reinforcement is close to the edge of the concrete member. It is induced also by the development of interactional stress. Edge splitting failure can be avoided by keeping a minimum distance from the edge; this should be considered in design Thermal expansion differences between epoxy and concrete can influence edge splitting.

4.FRP Tensile Rupture

Tensile rupture (it has been rarely observed by non prestressed strengthening) should be avoided according to its explosive nature. Structures strengthened with prestressed FRP more frequently fail by fibre tensile rupture because by prestressing the FRP we use a portion of its strain capacity .