Reinforcement of the flexural strength of a beam or slab

RELATED MATERIALS

MEGAPLATE Carbon plates for structural strengthening

EPOMAX-PL Two-component, epoxy adhesive paste for composite plates

MEGAWRAP-200 Carbon fabric for structural strengthening

EPOMAX-LD Two-component, epoxy impregnation adhesive for composite fabrics

EPOMAX-EK Two-component epoxy putty

MEGACRET-40 High-strength, fiber-reinforced repairing mortar

 

I. NATURE OF THE PROBLEM – REQUIREMENTS

The demand for increase of concrete beam’s or slab’s flexural strength may appear in the following cases:

  • Load increase or change of use.

  • Adjustment of old buildings to regulation amendments.

  • Ageing of construction materials, corrosion of reinforcement elements and/or construction defects.

  • Repairs in reinforced concrete elements after earthquakes.

 

II. SOLUTION

The increase of the flexural strength of concrete elements (beams, slabs, walls etc.) can be achieved by the external adhesion of composite materials to the tensible chord (side) of these elements. Carbon-fibre plates (MEGAPLATE), as well as single-directed carbon fibres (MEGAWRAP-200) are mainly used for this cause. These are adhered to the element, with their fibres aligned in the direction of the reinforcement’s stress.

With the above-mentioned reinforcement the following are
achieved:

  • Increase of flexural strength.

  • Increase of lexural rigidity (decrease of deformation).

  • Decrease of expected cracking.

  • Improvement of behavior in fatigue.

In relation to the conventional reinforcing systems, such as the adhesion of steel plates, MEGAPLATE carbon plates demonstrate the following advantages: 

  • They are lightweight, they come in rolls and they are easy to cut in the desired length at site. In general, their application is simple, fast and economical.

  • They are durable and resistant to fatigue, moisture and corrosion.

  • They are thin and easy to cover/paint.

 

III. APPLICATION

The substrate (concrete) must be free of loose parts, plaster, paint, oil or grease. After thorough cleaning, the surface is well rubbed with a hard brush.

The substrate should be as flat as possible. Surface imperfections are repaired using MEGACRET-40 fibre-reinforced cement mortar or EPOMAX-EK epoxy paste.

Then, the adhered tape is removed from one MEGAPLATE side and EPOMAX-PL epoxy adhesive is applied by trowel on that surface. Application of EPOMAX-PL on the plate should be done in a way that will force the excess paste to be concentrated along the plate axis and not close to it’s edges.

After that, the carbon plates are placed and pressed with a plastic roller on the dry and clean surface so that the paste overflows from the edges and the trapped air between the paste and the concrete surface is driven away. The total thickness of EPOMAX-PL after pressure exertion should be 0,5-2 mm.

After placing the plates on the substrate, a trapped air detection test is conducted with gentle taps on the plate (sound test).

Where there is a demand for improvement of the plate’s anchorage to the edges of the beam, this can be done by using carbon fiber strips (MEGAWRAP-200), which also functions as shear reinforcement (p.a).

 

IV. REMARKS

  • The elements that will be reinforced must be as discharged as possible, given that the action of the composite materials begins when the existing deformation increases.

  • The best possible adhesion of the plates (excellent substrate preparation), as well as their effective anchorage (beyond the area that needs flexural reinforcement) is a basic assumption for the effectiveness of the reinforcement.

  • It is recommended to avoid splicing of the plates, which is anyway not necessary since the plates are available in great lengths. Crossing of the carbon fibers or plates (by adhering the contact surfaces) is acceptable.

  • If the substrate’s strength has to be tested, this is done by the Pulloff method.

  • MEGAPLATE carbon plates bear an adhered (veneered) special tape on both sides. The tapes are removed just before application thus always assuring an absolutely clean and rough enough surface for perfect bonding of both the resin and the final cementitious coating layer, without cleaning or rubbing.

  • For further monitoring of the application efficiency, 1-2 more plates could be placed (not specified by the static sufficiency study), that will be tested with the Pulloff method right after hardening or periodically during the lifetime of the strengthening works.

  • Open time of epoxy systems is reduced by the increase of ambient temperature.

  • Since the development of high temperatures in the construction (e.g. in case of fire) decreases substantially the reinforcement’s efficiency, external protection of the carbon plates (by special coatings, gypsum boards etc.) is required. The repaired final surface must also be protected in cases of exposure to solar radiation.

  • The temperature of the concrete’s surface during F.R.P. applications must be at least +5oC.

 

 

 

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