Reinforcement of concrete columns and beams using FRP

What is the meaning of strengthening a concrete column?

Proper design of reinforced concrete columns and beams is very important to create structural strength in a building, especially against earthquakes and other natural disasters.

Reinforced concrete column is a structural member designed to withstand compressive loads. Concrete is used with an integrated steel frame for reinforcement. For design purposes, columns are divided into two categories: short columns and narrow columns.

The strength of short columns is controlled by the strength of the material and the geometry of the cross section. Narrow columns are suitable when their cross section is small relative to their length. Unlike short columns, narrow columns are limited by their geometry, and bend before the concrete or steel reinforcement can have an impact.

Bending behavior of FRP reinforced beam

Strengthening concrete columns is an important issue in which a variety of methods are expressed in this content and implemented by the builders of Pekran Sazan.

Reinforced concrete beams can be examined under bending as well as the changes that occur at different loading stages in the stress and strain values of the sections under the influence of concentrated loads. The force-displacement curve of these beams has three different slopes, and the first part of the curve is related to the tensile concrete cracking limit, where the beam stiffness is high (similar to an unreinforced beam). To the crushing extent of tensile concrete, when the rebar flows, the slope of the plan decreases, but its stiffness is still greater than that of the unreinforced beam, and the third part is related to the yield strength of the rebar to the rupture of FRP or the separation of FRP from the concrete surface.

Due to the high elastic modulus of FRP material, gluing it to the beam increases the rigidity and reduces the beam. Also, FRP materials exhibit linear stress–strain behavior up to the moment of rupture, and without sufficient rupture energy and lack of a yield zone similar to that of steel production, they rupture.

Therefore, its reinforcement and installation on the beam reduces the ductility and the amount of energy absorption. It should be noted that the decrease in absorbed energy is due to the local surrender of the long rebars in the rupture range of FRP.  Because in unreinforced beams, the beam cracks in a wide range and the rebars yield, while in beams Reinforced, the reinforcement is only in the rupture stage, the FRPS deforms and absorbs energy.

In general, the ductility of the package is reduced by increasing the FRP layers. This, due to its fact that the final residence of the FRP club is very high compared to the volume, when these materials are attached to the tension surface by the APICE for strengthening and promoting the curvature, before the FRP company begins to withstand the high loads, so steel that reaches its surrender.

Therefore, the beam stiffness or its resulting load cannot be increased without increasing the cross section of FRP, to further cooperate in the load of the beam before the steel yields. In general, strengthening and strengthening the beam with FRP system increases the final cross-sectional capacity.

Reinforcement of types of concrete columns is done by the builders of Bhikran Sazan Shamal. For a free consultation and price inquiries, call 88797928-021

Shear behavior of FRP reinforced beam

If the concrete beam is weak in shear resistance or after bending reinforcement, its shear capacity is weaker in shear resistance than the bending capacity, then shear reinforcement should also be considered. Shear strengthening, in most cases, is an essential and fundamental step in the effective strengthening of concrete beams.

The use of FRP substrate for flexural strengthening of reinforced concrete beams has little effect on increasing the shear strength, so the flexural strengthening effect of beams is neglected when designing shear reinforcement.

It should be emphasized here that although fibers glued to the side surfaces along the length of the beam do not contribute significantly to the shear capacity of the beam, fibers glued to these faces at other angles contribute to increasing the shear strength of the beams. Most angles for attaching fibers to the sides, other than the angle parallel to shear cracks, are useful and effective in restricting and reducing the width of cracks.

Different designs have been proposed for using FRP materials in shear reinforcement. These designs include gluing FRP to the side surfaces of the package, using a U-shaped cover for the side surfaces (U-JACKET) and the bottom surface of the package, as well as wrapping the cross section with FRP fibers and strips.

Strengthening and increasing the shear strength of the beam using a U-shaped coating for the side surfaces (U-JACKET)

Increase the deformation capacity and impact of the case on the FRP reinforced column

When stress approaches the compressive strength of concrete, cracks form within the concrete and the concrete opens in a direction perpendicular to the direction of the applied force. In this case, if we prevent the transverse movement of concrete by transverse rebar or any other device, the strength and flexibility of concrete will increase.

The figure below shows an example of the performance of fiberglass wrap (GFRP) in increasing the strength and ductility of cylindrical specimens. In these experiments conducted at Sharif University of Technology, we were able to double the ultimate strength by adding layers of fiberglass, but the most important effect of the fibers was to increase the deformation capacity of the concrete from about 6,000 microns to the concrete. Concrete: The capacity has increased to approximately 35,000 microlifts for reinforced concrete.

Therefore, if we strengthen the beams and columns adjacent to a concrete frame node in this way, we should expect their displacement capacity to be approximately 5 times greater. In other words, the acceptable limit of deformation of such structures in an earthquake increases the required risk level by five times.

Use the clamping feature to increase the compression force in the column

Why do you need columns?

Concrete to strengthen?

The structure will not show good resistance to lateral forces if the necessary points are not observed in the process of design, production or implementation of concrete columns or errors occur. In cases where the concrete is not of the required quality, the dimensions of the load-bearing member are smaller than the required standard, or due to change of use and lateral forces, the concrete column is twisted or moved beyond the main axis, it is necessary to make changes to increase the capacity of the concrete columns of the building.

What happens if structural columns are not strong?

If the columns of a structure or building are not strong, and are not strong, major problems can occur, some of which are listed below

Reducing the strength of columns
Column cracking (especially short columns)
Corrosion of steel reinforcement
Buckling of rebar

Reinforcing concrete columns with concrete jackets

Concrete jackets are one of the most common methods used to strengthen concrete columns. Concrete jackets are actually reinforced concrete coatings consisting of concrete layers, longitudinal reinforcement bars, and closed joints, and their application in structural structures increases the bending and shear strength of the column.

Concrete jackets greatly increase column ductility and are typically used in situations where the amount of damage to columns is high or the columns do not have sufficient capacity to withstand the lateral forces of an earthquake. Depending on project conditions, concrete jackets may be used all around (around) or on only one side of the concrete column.

If low thickness concrete jackets are used, the increase in stiffness in the concrete column will not be noticeable. This reinforced concrete cover also increases the dimensions of the column and the overall weight of the building.

When using the method of strengthening a concrete column with a concrete cover, if the goal is to increase the bending capacity of the column, the additional longitudinal reinforcement must be properly restrained in the foundation and must pass through the decks.

The longitudinal reinforcement added in the strengthening process is also generally placed at the four corners of the concrete column. These reinforcement bars must not connect to the main supports when passing through the roof. Another important point to note is that by increasing the loads in the concrete shell, we can see an increase in the shear strength of the concrete column. Therefore, there is no need to connect the used cover to the ceiling and beams.

Reinforce the concrete column with a steel jacket

Another common method used to strengthen concrete columns is the use of a steel or metal jacket.

To implement this method, concrete columns are covered with a layer of steel or metal, which increases the shear resistance of the concrete column. The steel shell method does not cause any significant changes in the structure and dimensions of structures compared to the concrete type, and the weight gain is negligible.

Welding is used to connect most parts of the steel cover, and the gap between the steel cover and the concrete column is filled with expanding mortar. By using rebar planting technology, it is possible to help better transfer the shear force between the steel cladding and the concrete column.

Reinforcement of types of concrete columns is done by the builders of Bhikran Sazan North. For a free consultation and price inquiries, call 88797928-021

This improves the overall performance of the steel jacket system. If we want to increase the capacity of axial forces in the column, the continuity of steel cladding in different categories must be considered.

If there is no connection between the steel cladding in different categories, the axial force capacity of the concrete column will not change. On the other hand, if we do not connect the steel sheet to the foundation and create a connection between the shell of different layers between the concrete slab, the bending strength of the concrete column will not increase.

Reinforcing concrete columns at the corner

One form of strengthening concrete columns using steel jackets is bending. In this method, the corner is attached to the column with the help of epoxy adhesive and is subjected to a certain pressure and heat at the same time. After the steel strips are welded together, the space between the components is filled with suitable mortar.