Strengthening of an existing reinforced concrete structure

Introduction

1.1: Introduction:

            As the whole fields of engineering are developing, civil engineering is developing too and our project is to present one of the most developments in the structural engineering field. This development is using FRP (Fiber Reinforced Polymer) in strengthening of existing structural elements instead of using the common strengthening methods. This development came parallel to the development of the most countries in the world (including UAE) in the field of construction and maintenance. The main objective of this project is to strength an existing building using FRP (Fiber Reinforced Polymer). This could be done by selecting an existing building that needs to be strengthen and performing structural analysis for its all structural elements.  In addition to that, experimental tests on this new material were conducted in the lab in order to investigate the local environmental effect UAE environment on the behavior of this material.

1.2 Problems facing reinforced concrete structure:

            Strengthening maybe required in structures when there are increase in loads, change in the use of structures and removal of individual supports and walls. This may lead to redistribution of internal forces, hence, more reinforcement in some elements are needed. In addition, structural strengthening may become necessary owing to wear and deterioration arising from normal usage or environmental factors.

Concrete structures need to be strengthened due to one or more of the following reasons:

1. Load increases due to higher live loads, increased wheel loads, installations of heavy machinery, or vibrations.

2. Damage to structural parts due to aging of construction materials or fire damage, corrosion of the steel reinforcement, and/or impact of vehicles.

3. Improvements in suitability for use due to limitation of deflections, reduction of stress in steel reinforcement and/or reduction of crack widths.

4. Modification of structural system due to the elimination of walls/columns and/or openings cut through slabs.

5. Errors in planning or construction due to insufficient design dimensions and/or insufficient reinforcing steel.

1.3 Strengthening methods:

When a concrete structure or member exhibits inadequate strength, behavior, or stability, it may be feasible to modify the structure using various stabilizations and strengthening techniques. The scope of this section provides conceptual discussion intended to foster the development of ideas for possible solutions.

1.3.1 Strengthening reinforced concrete structures by Concrete Jackets (Section Enlargement):

            Section Enlargement (Concrete jacketing) is a common strengthening method. Section enlargement can be defined as the placement of additional concrete and reinforcing steel on an existing structural member. It is important to mention that beams, slabs, columns, and walls, if necessary, can be enlarged to add stiffness or load-carrying capacity.

            In most cases, the enlargement must be bonded to the existing concrete to create a monolithic member for additional shear or flexural capacity. In this part, enlarging the cross section of an existing column will be discussed as an example of the structure elements.

            Enlarging the cross section of an existing column will strengthen the column by increasing its load carrying capacity. A column can be enlarged in various configurations. However, the drying shrinkage effects in the concrete used to enlarge the column must be considered. Drying shrinkage, if restrained, will induce tensile stresses in the new portion of the column.

            In the illustration, Method A (See Figure 1.1 below) will accomplish efficient load transfer if the new portion is cast with a bond breaker between the new and old concrete. After most of the drying shrinkage has occurred, the ties that link the old and new concrete can be installed. The gap between the new portion of the column and the existing member (to be partially supported by this column) can be filled with dry packing material. This will allow the new material to share its portion of the load.

            When Methods B and C are used (See Figure 1.1 below), extreme care should be exercised to select concrete mix designs with very low shrinkage rates. Pre placed aggregate concrete generally offers the lowest drying shrinkage; it is, therefore, an excellent material for column enlargements.

Figure 1.1: Methods of Enlarging a Column

1.3.2 Retrofitting reinforced concrete structures by bonding steel plates to their sides:   

Repair techniques often repair an attachment of steel to concrete. Many strengthening and stabilization techniques utilize steel to strengthen connections or provide additional tensile capacity. Attachment methods utilize mechanical connection and/or adhesives, permitting load transfer (shear, tension, or compression) between the steel and concrete. Use of adhesive as a connecting mechanism provides for uniform load transfer and corrosion protection at the concrete steel interface.

 For maximum bond, the steel requires a high level of abrasive cleaning (white metal, near white, commercial).The concrete surface requires roughening by mechanical or abrasive/liquid blast methods and removal of surface requires maintenance. Mechanical anchorage methods utilize different types of anchor devices. Where vibration exists in the connection, resin anchors or through-bolting should be utilized. In critical applications, a combination of adhesives and technical systems should be considered.

Much of the civil engineering structures throughout the world is aging and deteriorating. A suitable technique for the rehabilitation of reinforced concrete beams is by bolting and gluing steel plates to their surfaces, as shown in the figure 1.2 below.

 Figure 1.2: Techniques of plating reinforced concrete beams being developed

Tests have shown that plating can substantially increase the strength, stiffness, ductility and stability of the reinforced concrete element but these tests have also shown that the plated structure is prone to premature debonding of the plate as shown in figure 1.3.

Figure1.3: Examples of debonding mechanisms.

1.3.3 Strengthening of reinforced concrete structures by using external prestressing:

General History:

External prestressing was revitalized and found a new field of application in the strengthening of prestressed concrete structures starting from the seventies. Since then, many important bridges and buildings all over the world have been constructed using external prestressing. The application of external tendons proved to be particularly suitable for long bridges in segmental construction with a tight schedule thus demanding a very quick production rate.

Post-Tensioning:

Externally post-tensioned structures are becoming increasingly popular, because of economic and aesthetic considerations in conjunction with modern construction methods; the possibility of monitoring the prestressing forces in the structure and if necessary modifying them; the ease of inspection; and the replace ability of external tendons. External tendons are also well suited for the strengthening of existing structures.

Post-tensioning is a technique used to prestress reinforced concrete. The tensioning provides the member with an immediate and active load-carrying capability.

Placement of the tension components can be achieved either internally within the member or externally to the member. Tension components are generally steel plates, rods, tendons or strands. The tension is imparted to the component by jacketing of, less commonly, by preheating. Post-tensioning enhances a member's ability to relieve overstressed conditions in tension, shear, bending and torsion. The post-tensioning technique can also be used to eliminate unwanted displacement in members and to turn discontinuous members into continuous members.(See figures 1.4 & 1.5)

Figure 1.4: External prestressing of the beams

Figure 1.5: External prestressing flat slabs

As a consequence of above, the following conclusions can be drawn:

·        The application of external prestressing is indeed not bound to the use with concrete, but it can be combined with any construction material as composite materials, steel, timber, steel and concrete combined and other modern plastic materials. This can considerably widen the scope of the post-tensioning applications.

·        As the tendons are outside of the structure, the tendons are more exposed to any environmental influences and the protection against these detrimental influences is therefore of special concern.

·        Due to the exposure and accessibility of the tendons, surveillance and maintenance measures are facilitated.

·        Due to the absence of bond, it is also possible to restress, destress and exchange any external prestressing cable, provided that the structural detailing allows for these actions.

As an overall result, better concrete quality can be obtained leading to a more durable structure and facilitating execution.

Typical Applications for External Prestressing:

Typical applications where external tendons are feasible, practical and economical are:

·        Repair work and strengthening of all kinds of structures.

·        Underslung structures.

·        Precast segmental construction, simple and continuous spans.

·        Incremental launching procedure, in particular centric prestressing.

1.3.4 Strengthening Reinforced Concrete Structures by Fiber Reinforced Polymer (FRP):

            Fiber Reinforced Polymer (FRP) is a new class of composite material for the development and repair of new and deteriorating structures in Civil Engineering. The importance of it that the Civil Engineers had been in search for alternatives to Steel and alloys to combat the high costs of repair and maintenance of structures damaged by corrosion and heavy use.

            In the last decade, the use of FRP composites to reinforce concrete members has emerged as one of the most promising technologies in materials/structural engineering. There is a wide range of applications of FRP reinforcement that covers new construction as well as rehabilitation of the existing structures. FRP systems for strengthening concrete structures are a practical alternative to traditional strengthening techniques such as steel plate bonding, section enlargement, and external post-tensioning. FRP systems are non-corrosive and have constructability advantages over traditional reinforcement.

            FRP composites consist of high strength fibers embedded in a polymeric resin. The fibers are the main load-carrying elements and have a wide range of strengths and stiffnesses with a linear stress-strain relationship up to failure. Fiber types typically used in the fabrication of FRP composites for construction are carbon, glass, and aramid. All these fibers are available commercially as continuous filaments. The polymeric resin surrounds and encapsulates the fibers to bind them together, protect them from damage, maintain their alignment, and allow distribution of load among them. Resins are available as thermosetting polymers (e.g. epoxy and vinyl ester) and thermoplastic polymers (e.g. nylon). A comparison among carbon FRP (CFRP), aramid FRP (AFRP), and glass FRP (GFRP) composites (based on fiber area only), and reinforcing steel Grade 60 in terms of stress strain relationship is illustrated in figure 1.6.

Figure1.6: Stress-Strain Curves

              Fiber Reinforced Polymer (FRP) is also referred to as Advanced Composite Materials (ACM) because it consists of two main components:

·   Fibers.

·   Resin or Matrix.

FRP Laminate Structure:

            FRPs are typically organized in a laminate structure; such that each lamina (flat layer) contains an arrangement of unidirectional fibres or woven fibres fabrics embedded within a thin layer of light polymer matrix material.

The matrix, commonly made of polyester, Epoxy or Nylon, binds and protects the fibers from damage, and transfers the stresses between fibers.

It doesn't carry much force but enables the fibers to reach their full potential load carrying capacity.

            The strength and stiffness of FRP is provided by the fibers which reinforced the polymer matrix. The three main types of fibers used are:

·   Carbon.

·   Glass.

·   Aramid.

            The researches approve that the FRP is suitable for structural engineering uses because of its properties and advantages. That's make it ideal for wide spread applications in construction worldwide.

Materials: Glass, Armed and Carbon:

            Carbon fiber or glass fiber are used today in three main branches in the civil Engineering; first: carbon or glass fiber bars used as a replacement of the steel reinforcing bars in reinforced concrete structures, second, carbon or glass fiber sections used as a replacement of the steel sections in steel structures, third, carbon or glass fiber strips and sheets used in strengthening and repair of existing concrete and steel structures.

Applications of FRP in Structural Engineering:

            There are three broad divisions into which applications of FRP in civil engineering can be classified: applications for new construction, repair and rehabilitation applications, and architectural applications.                                                                .
FRPs have been used widely by civil engineers in the design of new construction. Structures such as bridges and columns built completely out of FRP composites have demonstrated exceptional durability, and effective resistance to effects of environmental exposure. Pre-stressing tendons, reinforcing bars, grid reinforcement and dowels are all examples of the many diverse applications of FRP in new structures.
One of the most common uses for FRP involves the repair and rehabilitation of damaged or deteriorating structures. Several companies across the world are beginning to wrap damaged bridge piers to prevent collapse and steel-reinforced columns to improve the structural integrity and to prevent buckling of the reinforcement.
Architects have also discovered the many applications for which FRP can be used. These include structures such as siding/cladding, roofing, flooring and partitions.

            FRP composites can be produced by different manufacturing methods in many shapes and forms; the most popular ones for concrete reinforcement are reinforcing bars (rebars), prestressing tendons, pre-cured laminates/shells, and fiber sheets for lay-up installation.

            Commonly used FRP rebars have various types of deformation systems, including externally wound fibers, sand coating, and separately formed deformations. Rebars are commonly used for internal or near surface mounted (NSM) concrete reinforcement. FRP tendons have been used as pre-tensioned as well as postensioned cables in prestressed concrete applications. FRP pre-cured laminates/shells and lay-up sheets are commonly used for external concrete reinforcement to upgrade existing structures.

            An important characteristic of FRP in repair and strengthening applications is the speed and ease of installation. Labor, shutdown costs, and site constraints typically offset the higher material cost of FRP. In general, concrete structures may need strengthening due to deterioration, design/ construction errors, a change in use or loading, or for a seismic upgrade. Bonded FRP essentially works as additional reinforcement to provide tensile strength.

            FRP reinforcement may be used on beams and slab soffits to provide flexural strength, or on the sides of beams to provide shear strength, or wrapped around columns to provide confinement, ductility (a primary concern in seismic upgrades), and shear strength. Quality control is crucial to the successful application of FRP systems. Since the late 80's, strengthening by externally bonded FRP composites has been studied worldwide. A sudden increase in the use of FRP composites was attained after the 1995 Hyogoken Nanbu Earthquake In Japan. By 1997, more than 1,500 concrete structures worldwide had been reinforced with externally bonded FRP composites. Co-Force has implemented FRP strengthening of structural elements such as columns, beams, slabs, walls, chimneys, tunnels, and silos.

1.4 advantages and disadvantages of strengthening methods:

1.4.1 Concrete Jacketing:

Concrete Jacketing Advantages:

            Enlarging the cross section of an existing column will strengthen the column by increasing its load carrying capacity.

Concrete Jacketing disadvantages:

It is important to mention in this part that this method of strengthening has several disadvantages and some of them are listed below:

·   Increasing the size of the structure element, which make its usage very limited especially with services existence.

·   Difficult to construct in some active buildings such as hospitals, schools because of the noise of equipments.

·   Needs shuttering, formworks, reinforced steel, concrete, concrete pumps, vibrators, …etc.

1.4.2 Steel plate:

·        The tension elements respecting to prestressing tendons are placed on the outside of the physical cross-section (mostly in concrete) of the structure.

·        The forces exerted by the prestressing tendons are only transferred to the structure at the anchorages and at deviators.

·        No bond is present between the cable and the structure, unless at anchorages or deviators bond is intentionally created.

Steel plate disadvantages:

On the other hand the following disadvantages can be associated with external prestressing.

a) The tendons are more exposed to environmental influences (fire, vandalism, aggressive chemicals etc.).

b) The deviators and anchor plates have to be placed very accurately, which might sometimes be delicate.

c) As the tendons are not bonded to the concrete (or only at particular points), the ultimate strength can not be developed in ultimate design resulting in a higher prestressing steel consumption.

d) Usually the statical height of the cross-section can not be fully utilized, therefore requiring a greater depth or additional prestressing.

e) For certain cross-sections and construction procedures the handling of the tensioning devices may be more difficult.

 1.4.3 FRP:

FRP advantages:

            The advantages of FRP are the reason that the civil engineers select it in the design and maintenance and of Structures. The advantages could be mentioned briefly as the following:

·        High strength to weight ratios:

            A material's strength is governed by its ability to sustain a load without excessive deformation or failure. FRP responds linear-elastically to axial stress. High strength to weight ratios results in efficient use of FRP materials. FRP can have tensile strengths 8 times that of steel and weigh 1/5 of what an equal volume of steel would weigh. 

·   Corrosion Resistance:

            FRP is not subject to corrosion as is steel. This is an important advantage because it increases the building age. It also saves the cost for the repair and maintenance.

·        Lightweight:

            Compared to alternative materials, FRP has a lower density at equal or greater strength properties. Depending on the application, lighter weight can improve performance, reduce energy needs, simplify handling and speed installation.

·        Ease of installation:

            The procedures for FRP installation are easy and clear. The engineers can mostly install FRP anywhere they want in the structural elements. 

·        Less Finishing:

            Painting is not usually necessary, and an infinitely wide range of colours can be introduced into the laminate to provide a long-lasting appearance.

·   Less maintenance:

            FRP needs less maintenance because of corrosion resistance which Steel Structures are usually suffering from it.

·   Ductility of FRP wrapped members improves dramatically.

·   Their lightness eliminates the necessity of large supporting structures.

·   They are ideal for external application.

·   FRPs have high tensile strength (1700 - 3000 MPa).

·   FRPs have high ultimate strain (2-3%).

·   The strengthening can be tailored to the desired level.

·   They are extremely durable.

·   They are available in various forms: sheets, plates, fabric, etc.

·    They are available in long lengths; that eliminates joints   and splices.

·   FRPC retrofits do not increase the dimension or weight of the structure.

·   They cure within 24 hours.

·   Fire resistance.

·  Versatility.

·  Anti-seismic behavior.

·  Electrically nonconductive.

·  Thermally nonconductive.

·  Electromagnetic neutrality.

·  Excellent Fatigue behavior.

FRP Disadvantages:

            However, like most structural materials, FRP has a few disadvantages that would create some hesitancy in civil engineers to use it for all applications: high cost, brittle behavior, susceptibility to deformation under long-term loads, UV degradation, photo-degradation, temperature and moisture effects, lack of design codes, and most importantly, lack of awareness.

1.5 Decision:

            The comparison among the methods of strengthening explains the differences, as the following:

Table 1.1: Comparison between strengthening methods.

            For each Method of strengthening, the advantages and disadvantages were mentioned. The decision depends on the method that has more advantages and properties. According to the above comparison and our research it is clear that the FRP is suitable for structural engineering uses and it is ideal for wide spread applications in construction worldwide. So, it will be used in our project.

Home Page