Reinforcement of the shear strength of a beam

RELATED MATERIALS

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 shear 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 adhesion of the fabric MEGAWRAP-200 in a crosswise direction to the beam offers:

  • Increase of shear strength.

  • Drastic decrease of the possibility of friability defects.

  • Significant improvement of the beam’s behavior to cyclic loads (seismic actions) and increase of its ductility.

The reinforcement can take place in the form of:

  • Continuous jacket (p.2a) or fabric ribbons placed with a space between them, (p.2b).

  • Closed jackets (totally encasing the beam) or open-type jackets (with their edges anchored at the compression zone of the beam).

Although closed jackets (p.1a)offer the best solution regarding their mechanical behavior, their application is not practical, in most of the cases, due to the presence of other structural elements supported by the beam (slabs etc.), that prevent wrapping of the fabric round the top side of the beam.

Therefore, in such cases, the reinforcement is usually achieved by placing the fabric in a U shape at the sides and the lower chord of the beam (p.1b).

In relation to the conventional reinforcement systems such as steel or concrete jackets, MEGAWRAP-200 jacket provides the following advantages:

  • Easy and fast work.

  • Increase of the strength of construction elements without changing their geometry or increasing their rigidity.

  • Resistance against time and protection of reinforcement against moisture and corrosion.

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.

Existing cracks should be repaired by resin injections.

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

The efficiently prepared surface is coated with EPOMAX-LD resin. The fabric is cut with a scissor in the desired dimensions, placed carefully well stretched on the wet surface and pressed well with a special plastic roller to achieve better contact with the substrate, thorough impregnation and removal of air bubbles. If, in spite of the careful pressing, there are still some dry spots on the fabric, these are additionally coated externally with EPOMAX-LD.

If more than one fabric layers are specified, the application process is repeated. In that case the previous layer should not be completely dry, or else good rubbing will be required once again before the new application.

The last fabric layer is also soaked on the outside with EPOMAX-LD and then quartz sand is broadcast on the still fresh resin coating in order to apply later a subsequent protective cement-based coating.

IV. REMARKS

  • In any case, the best possible adhesion of the jacket (excellent substrate preparation) as well as its effective anchorage (at its edges) is a basic assumption for the effectiveness of the reinforcement.

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

  • Special attention should be given during the fabric cutting process in order to prevent folding or crumpling of the fabric. Furthermore, the fabric’s surface must be clean before application.

  • Open time of epoxy systems decreases when ambient temperature rises.

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

  • In order to allow “breathing” of the structural elements (so that possible trapped humidity can easily come out) it is recommended to interrupt the longwise jacket’s continuity every 600 mm.

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

  • The use of protective equipment (gloves, goggles etc.) during application is essential.

skitso100

skitso200