Shear Strengthening

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Shear strengthening is required for structural members when the shear stress applied is higher than the corresponding design shear resistance. Shear capacity of FRP-strenghtened member must be calculated by also considering the role of the concrete and any transversal reinforcement present (ref. CNR DT 200/2004 par. 4.3).

Vsd ≤ Vrd

 

Shear strengthening is achieved by bonding one or more layers of strips of fabric to the external surface of the member to be strengthened. The fabric may be bonded on the two sides of the beam using “U” shaped pieces and by wrapping fabric around the section of the beam. The strips may be applied in either an irregular pattern with gaps between each strip or by winding them continuously alongside each other.

 

Figure 4.10 – Shear strengthening using composite fabrics

 

Shear strengthening of beams

This type of strengthening may be achieved by applying one or more layers of uniaxial fabric, such as WRAP C UNI-AX, WRAP C UNI-AX HM, WRAP C BI-AX, WRAP C QUADRI-AX, WRAP G UNI-AX, WRAP B UNI-AX or WRAP S FABRIC.

 

FRP FORMULA – DESIGN OF SHEAR STRENGTHENING FOR A “T” SECTION BEAM

The objective of the intervention is to supplement the stirrups in a beam.

 

Characteristics of the materials used in the calculation

Width of slab: W = 500 mm;

Width of web: b = 300 mm;

Depth of member: H = 450 mm;

Depth of slab: h = 100 mm

Stirrups: ø 6/25 mm

Steel: FeB38k (fyk ≥ 375 N/mm²)

Concrete class: C 20/25 (Rck 25 N/mm²)

 

Design shear force acting on the section:

Vsd = 75 kN

 

 

SELECTING THE APPROPRIATE STRENGTHENING

 

TYPE OF STRENGTHENING ➠ WRAP C UNI-AX 600 fabric

Shear capacity of FRP-strengthened member:

Vrd, post-strengthening = 140.88 kN ➠ Vsd < Vrd

 

APPLICATION TECHNIQUE FOR “WRAP” FABRICS USING THE “WET TECHNIQUE”

Procedure

1) Prepare the substrate (as per procedure on page 26).

2) Remove all sharp corners and create a rounded edge with a radius of at least 20 mm (in compliance with CNR DT 200/2004 par. 4.3.3.3).

3) Apply an even coat of WRAP PRIMER 1 by brush or with a roller. If the substrate is particularly absorbent, apply a second coat of primer once the previous one has been completely absorbed.

4) Skim and even out the surface with a 1 to 2 mm thick layer of WRAP 11 applied with a notched trowel while the WRAP PRIMER 1 is still wet If epoxy adhesive with a longer workability time is required, WRAP 12 may be used.

5) Smooth over the surface of the adhesive with a flat trowel to eliminate even the smallest irregularities from the surface. Fill and round off the corners with the same product to create a rounded edge with a radius of at least 20 mm (in compliance with CNR DT 200/2004 par. 4.3.3.3).

6) Impregnate the pieces of fabric before laying them on the surface. This step may be carried out either manually or with suitable tools and equipment. For manual impregnation, cut the WRAP fabric to the size required with scissors and soak it for a few minutes in a plastic container (preferably rectangular) filled to around 1/3 with WRAP 21. Remove the fabric from the container, leave it to drip for a few seconds and then press it lightly without twisting to completely remove all the excess resin. Wear protective rubber gloves when carrying out this operation. As an alternative to manual impregnation, simple equipment with a bowl and a series of rollers may be used; this makes it easier and safer for the operator to saturate the fabric and remove the excess resin. This system is particularly recommended when a large number of interventions on large surfaces need to be carried out and guarantees that the resin is distributed evenly in every part of the fabric.

7) Position the WRAP fabric immediately after impregnation, making sure that it is spread on evenly without creases. Wear protective rubber gloves for this operation.

8) Go over the surface of the fabric with a WRAP ROLLER to make sure the adhesive and resin completely penetrate through the fibres. This operation is also necessary to eliminate air bubbles trapped in the fabric.

9) Wash the WRAP ROLLER with thinners immediately after use.

(ref. “Design Guide” procedure G.1.4 and technical specifications G.1.4.1 to G.1.4.7)*.

 

APPLICATION TECHNIQUE FOR “WRAP” FABRICS USING THE “DRY TECHNIQUE”

Procedure

1) Prepare the substrate (as per procedure on page 26).

2) Remove all sharp corners and create a rounded edge with a radius of at least 20 mm (in compliance with CNR DT 200/2004 par. 4.3.3.3).

3) Apply an even coat of WRAP PRIMER 1 by brush or with a roller. If the substrate is particularly absorbent, apply a second coat of primer once the previous one has been completely absorbed.

4) Skim and even out the surface with a 1 to 2 mm thick layer of WRAP 11 applied with a notched trowel while the WRAP PRIMER 1 is still wet. If epoxy adhesive with a longer workability time is required, WRAP 12 may be used.

5) Smooth over the surface of the adhesive with a flat trowel to eliminate even the smallest irregularities from the surface. Fill and round off the corners with the same product to create a rounded edge with a radius of at least 20 mm (in compliance with CNR DT 200/2004 par. 4.3.3.3).

6) Apply an even coat of WRAP 31 with a brush or short-haired roller on the WRAP 11 or WRAP 12 while it is still wet.

7) Position “U” shaped pieces of WRAP fabric immediately after applying the resin, making sure that it is spread on evenly without creases. Wear protective rubber gloves for this operation.

8) Go over the surface of the fabric with a WRAP ROLLER to make sure the adhesive and resin completely penetrate through the fibres. This operation is also necessary to eliminate air bubbles trapped in the fabric.

9) Apply another coat of WRAP 31 on the WRAP fabric. Go over the surface of the impregnated fabric with a WRAP ROLLER to eliminate air bubbles trapped in the fabric.

10) If a different product is to be applied on the surface of the fabric, we recommend sprinkling a layer of sand over the WRAP 31 while it is still wet.

(ref. “Design Guide” procedure G.1.4 and technical specifications G.1.4.1 to G.1.4.7)*.

 

Shear/tensile strengthening of masonry structures

The strengthening of masonry structures (vertical members and vaults) may be achieved by applying the composite FRG System which consists of pre-primed, alkali-resistant A.R. glass fibre mesh (GRID G 220) or pre-primed basalt fibre mesh (GRID B 250), and an inorganic, cementbased matrix (PLANITOP HDM / PLANITOP HDM MAXI) or lime-based matrix (PLANITOP HDM RESTAURO).

The need for sustainable renovation installations to upgrade weak constructions and the intrinsic mechanical characteristics of masonry has led to the study and development of innovative structural strengthening materials and technology that are more compatible with the physical and mechanical characteristics of masonry and its intrinsic durability. Recent developments in the standardisation system also imply installations using innovative materials to upgrade weak areas that are located in buildings and structures after an assessment of their structural capacity. These include employing strengthening techniques based on the use of composites, which offer a number of significant advantages (high mechanical properties, low architectonic impact, high durability, ease of application and reversibility) for countries with such a rich patrimony of historical buildings and monuments as Italy. The application of this type of system overcomes the problem of the inherently low tensile strength of masonry and increases the overall ductility of structures.

This innovative, technologically advanced consolidating system is used in this sector through a series of inorganic matrix composites developed by consisting of pre-primed, alkali-resistant (A.R.) glass fibre mesh (GRID G 220) or pre-primed basalt fibre mesh (GRID B 250) positioned on the structure using two-component, high ductility, fibre reinforced cementitious mortar (PLANITOP HDM / PLANITOP HDM MAXI) or two-component, high ductility, ready-mixed hydraulic lime (NHL) and Eco-pozzolan-based mortar (PLANITOP HDM RESTAURO). Also, recent developments in composites, in which the matrix is made up of a base of eco-compatible pozzolan, allows these types of material to be used even on listed and historical buildings and monuments.

The success of these systems on masonry structures has been evaluated by means of a “laboratory test campaign” carried out at the Department of Structural Analysis and Design at the “Federico II” University of Naples.

 

Figures 5.2 and 5.3 – Strengthening on the outer face of stone vaults using PLANITOP HDM MAXI and GRID G 220

 

Laboratory testing: “Diagonal compression tests on tuff masonry panels strengthened with CMF: PLANITOP HDM + GRID G 220”

Testing was carried out on tuff masonry panels made with specially prepared construction mortar to reproduce the mechanical characteristics of the pozzolan-based mortar used on ancient monuments and buildings in the south of Italy’s area. Strengthened panels and panels without strengthening were then subjected to diagonal compression tests. The strengthening system was made of a pre-primed, alkali-resistant (A.R.) glass fibre mesh (GRID G 220) and a matrix based on a two-component, high ductility, fibre reinforced, pozzolanic-reaction cementitious mortar (PLANITOP HDM).

 

TEST RESULTS

The results of the tests demonstrated that the strengthening offered considerable benefits in terms of shear strength and ductility, and also had the capacity to distribute loads to limit the extremely fragile post-peak behaviour of the strengthened member, with significant advantages in the event of seismic activity. On the panels without strengthening, the tension induced by the loads generated cracks mainly along the surface of the tuff/mortar interface rather than in the direction of the load applied.

Failure was characterised by the beds of mortar slipping from the tuff stones. The presence of strengthening materials on both wall faces, on the other hand, modified the failure mode of the panels, moving the shear failure caused by the mortar slipping to shear failure characterised by widespread cracking in the direction of the compressive load, with small to medium size cracks intercepting both the layers of mortar and the tuff stones.

 

Figure 5.4 – A panel with a strengthening system applied: GRID G220 + PLANITOP HDM

 

 Figures 5.5 and 5.6 – Diagonal compressive strength test

The strengthening on both faces of the wall, therefore, managed to change the distribution of the tension over the entire surface of the wall. The tensile properties of the strengthening system were clearly visible through the presence of widespread micro cracking along the outer layer of mortar distributed on both faces of the wall, with a sub-vertical trend along the compressed diagonals of the wall. Graph 1 shows the stress-deformation behaviour of each sample

 

 Figure 5.8 – The behaviour of a masonry panel without strengthening

 

and, apart from a marked increase in maximum tension, the capacity of the strengthened panels to deform considerably more than the panels without strengthening is evident. The positive effect of the strengthening on the masonry is even more evident if we compare the areas below their relative tension-deformation curves. This comparison highlights the increased capacity of the strengthened samples to dissipate energy, a property that is particularly important in the event of seismic activity. The formation of a series of small cracks, as shown in Graph 1, allows the masonry panel to dissipate more energy and to deform much more without losing its resistance. Also, as far as stiffness is concerned, a single layer of strengthening means that it remains practically the same.

 

Graph 1 – The mechanical behaviour of a panel strengthened with the “reinforced” PLANITOP HDM + GRID G 220 system

 

The results obtained allow us to make the following observations:

  • from an examination of the values of shear strength obtained by the various panels, there is a net improvement in the load-bearing capacity of masonry with a strengthening system;
  • in the post-peak phase, the strengthening system allowed a high level of resistance to shear deformation to be maintained for a sufficiently long period in all cases;
  • the presence of a strengthening system delayed cracking being triggered off and increased tension at the elastic limit of the panels;
  • a strengthening system allows ductility to be increased considerably (around 140%) compared with the behaviour of panels without strengthening; also, ductility was around 2.25 times higher than the values obtained with samples without strengthening.

 

Laboratory testing: “Diagonal compression tests on tuff masonry panels strengthened with CMF: PLANITOP HDM RESTAURO + GRID G 220 and PLANITOP HDM RESTAURO + GRID B250”

In this case too diagonal compression tests were carried out on tuff masonry panels built in the same way as the previous examples. Diagonal compression tests were carried out on panels without strengthening and on panels with a strengthening system comprising mortar reinforced with pre-primed, alkali-resistant (A.R.) glass fibre mesh (GRID G 220) and a matrix of two-component, ready-mixed, high ductility hydraulic lime (NHL) and Ecopozzolan based mortar (PLANITOP HDM RESTAURO). Panels strengthened with pre-primed basalt fibre mesh (GRID B 250) were also tested.

 

TEST RESULTS

The following graphs show the relative curves of various strengthening systems, from which we can deduce how the “LIME+GLASS” system (PLANITOP HDM RESTAURO + GRID G 220) considerably increases shear strength (Graph 2), comparable to the “CEM+GLASS” system (PLANITOP HDM + GRID G 220), while the “LIME+BASALT” system (PLANITOP HDM RESTAURO + GRID B 250), apart from increasing the shear strength of the panel to a lower degree than the “LIME+GLASS” system (PLANITOP HDM RESTAURO + GRID G 220), notably increases its ductility (indicated by an extension in the branch of the curve).

 

 Figures 5.9 and 5.10 – Diagonal compressive strength test

 

Graph 2 – The mechanical behaviour of panels strengthened with a system made from basalt and glass fibre mesh

Graph 3 – The behaviour of tuff panels strengthened with GRID B 250 and PLANITOP HDM RESTAURO

 

APPLICATION TECHNIQUE FOR FRG SYSTEMS ON MASONRY MEMBERS (SHEAR STRENGTHENING FOR WALLS) (ref. RELUIS GUIDELINES par. 3.2.4.3)

Procedure

The application of a strengthening system is made on the presumption that the substrate has undergone adequate preparation and has no loose material or mechanically weak layers.

Shear strengthening of bay walls using the PLANITOP HDM / PLANITOP HDM MAXI + GRID G 220 (or GRID B 250) system

1) Level off the substrates to form a sufficiently flat layer with two-component, high ductility, fibre reinforced, pozzolanic-reaction cementitious mortar (PLANITOP HDM / PLANITOP HDM MAXI).

2) Apply pre-primed, alkali-resistant (A.R.) glass fibre strengthening mesh (GRID G 220) or pre-primed basalt fibre mesh (GRID B 250).

3) Apply a second layer of PLANITOP HDM /PLANITOP HDM MAXI mortar so that it completely covers the strengthening mesh evenly.

(ref. “Design Guide” procedure G.2.6 and technical specifications G.2.6.1 and G.2.6.2)*.

 

Shear strengthening of bay walls using the PLANITOP HDM RESTAURO + GRID G 220 (or GRID B 250) system.

1) Level off the substrates to form a sufficiently flat layer with two-component, high ductility, fibre reinforced natural hydraulic lime (NHL) and Eco-pozzolan based mortar (PLANITOP HDM RESTAURO).

2) Apply pre-primed, alkali-resistant A.R. glass fibre strengthening mesh (GRID G 220) or pre-primed basalt fibre mesh (GRID B 250).

3) Apply a second layer of PLANITOP HDM RESTAURO mortar so that it completely covers the strengthening mesh evenly.

(ref. “Design Guide” procedure G.2.6 and technical specifications G.2.6.3 and G.2.6.4)*.

 

Figures 5.11 and 5.12 – Strengthening system on the outer face of the vaulted roofs of the Church of the Padre Pio Giovane Convent – Serracapriola (Foggia – Italy)