Confinement of Columns

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Confinement of reinforced concrete members with FRP is commonly used for static strengthening and seismic upgrading. Adequate confinement of reinforced concrete columns may determine an improvement in both the ultimate compressive strength and ductility of a structural member. Reinforced concrete members may be confined by winding FRP fabric around them to form a continuous external bandage (covered) or by staggering the fabric (banding). The slope of the fibres with respect to the longitudinal axis of the member may vary from 0° to 180° (ref. CNR DT 200/2004 par. 4.5). The design of FRP strengthening to increase the balanced design compressive strength or slightly eccentric design compressive strength of a reinforced concrete pillar must guarantee that the design value of the axial capacity of a FRP-confined concrete member NRcc,d is such that the following conditions are satisfied:

NRcc,d ≥ NSd

 

Confinement of reinforced concrete columns

This type of strengthening may be achieved by applying one or more layers of uniaxial fabric, such as WRAP C UNI-AX or WRAP C UNI-AX HM, to increase compressive strength and ductility. Confinement is carried out using WRAP G UNI-AX and WRAP B UNI-AX if only ductility needs to be increased.

 

I FRP FORMULA – DESIGN OF CONFINEMENT OF CONCRETE COLUMNS

The objective of the installation is to increase the compressive strength of a reinforced concrete pillar.

Characteristics of the materials used in the calculation

Depth of member: H = 450 mm;

Width of member: b = 450 mm;

Area of longitudinal reinforcement: 6 Ø 16 mm ≈ 1205.76 mm²

Steel: FeB38k (fyk ≥ 375 N/mm²

Concrete class: C 16/20

 

Factored axial force acting on the section:

Nsd, = 2800 kN*m

 

SELECTING THE APPROPRIATE STRENGTHENING

 

TYPE OF STRENGTHENING ➠ WRAP C UNI-AX 600 fabric

The axial capacity of FRP-confined concrete member on the section after strengthening:

NRcc,d (POST-STRENGTHENING) = 2895.9 kN*m ➠ Nsd < Nrcc,d

 

Figure 6.1 – Confinement of a reinforced concrete panel using uniaxial carbon fibre (WRAP C)

Figure 6.2 – Confinement of reinforced columns and cladding around beam-column hinge zones

 

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. 4.5.2.1.2).

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. Using the same product, fill and round off the corners so they have a radius of at least 20 mm.

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. Apply the fabric so that the sheets overlap each other by 50 mm on the vertical sides and 200 mm on the horizontal sides to guarantee efficient confinement.

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.1.5 and technical specifications G.1.5.1 and G.1.5.2 and procedure G.1.6 and technical specifications G.1.6.1 and G.1.6.2)*.

 

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

Procedure

1) Prepare the substrate.

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. 4.5.2.1.2).

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. Using the same product, fill and round off the corners so they have a radius of at least 20 mm.

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

7) Position the WRAP fabric immediately after applying the resin, making sure that it is spread on evenly without creases. Wear protective rubber gloves for this operation. Apply the fabric so that the sheets overlap each other by 50 mm on the vertical sides and 200 mm on the horizontal sides to guarantee efficient confinement.

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 any other product is to be applied on the surface of the fabric, we recommend dusting the WRAP 31 with sand while it is still wet.

(ref. “Design Guide” procedure G.1.5 and technical specifications G.1.5.1 and G.1.5.2 and procedure G.1.6 and technical specifications G.1.6.1 and G.1.6.2)*.