Download
1 / 53
780 likes | 969 Vues
RETROFITTING AND STRENGTHENING OF STRUCTURES. Contents. Introduction Procedure for strengthening of structures Strengthening Techniques Section Enlargement Using Prestressing steel Using Fiber reinforced polymers Using Ferro cement Plate Bonding RCC Jacketing Fiber wrap technique
E N D
Contents • Introduction • Procedure for strengthening of structures • Strengthening Techniques • Section Enlargement • Using Prestressing steel • Using Fiber reinforced polymers • Using Ferro cement • Plate Bonding • RCC Jacketing • Fiber wrap technique • Strengthening of Columns, Beams and Slabs • Foundation Rehabilitation methods • Shoring • Underpinning
1. INTRODUCTION • Much of the recent development in the technology of patching mortars can be directly translated to the spraying technique, and some of these products have proved suitable for spraying without further modification. • The latest development in concrete repairs is the pouring of concrete behind a fixed shutter and appears to have brought technology full circle, but there are some dramatic differences. • This is concrete designed to be poured into thin (50–100mm), large area, vertical or overhead patches and to be completely self-compacting around congested reinforcement. • The repair materials, however, are so fluid that they run completely off the table, and so a flow trough of the type used for grouts is employed to monitor their performance. • The development of good diffusion barrier properties in cementitious repair compositions is now recognized as being of equal importance to their strength and bond.
2. PROCEDURE FOR STRENGTHENING OF STRUCTURES • Strengthening is carried out to enhance the ability of the structure elements to resist the internal forces that are generated due to any of the loading such as flexure, axial, shear or torsion. • Various techniques are available to strengthen, however the goal of the adopted technique must be to ensure a safe, durable and cost- effective means of upgrading the structure. • The following schematic diagram clearly describes the procedure for strengthening of structures that covers the activities starting from the conditional assessment of the structures to the execution of the strengthening technique
3. STRENGTHENING TECHNIQUES • SECTION ENLARGEMENT • One simple way to strengthen concrete members is to just add a new layer of reinforced or plain concrete, increasing the thickness of the member. • There are mainly two different approaches when strengthening members with concrete: • regular casting with moulds • with use of shotcrete
II. STRENGTHENING MEMBERS WITH PRESTRESSING STEEL • Prestressing is primarily suited for strengthening members with regard to the flexural capacity. • Prestressing of existing members functions in a similar manner as internal post tensioned tendons without bond, but with the tendons (or single strands) connected only to the exterior of an already existing structural member by end-anchors and deviators. • Deviators are used to change the angle of the tendon to acquire a better utilization of the prestressing. Since it is at the deviators and anchors that the forces are transferred to the concrete, it is not possible to follow the moment curve in the same way as with internal tendons.
Utilizing prestressing enhances the behavior of structural members by means of constraint forces. • The prestressing effect results in equilibrium of forces where the prestressing steel is in tension and the concrete is in compression. This creates a member with capacities to better resist tensile loading. • It should be noted that it is not possible to get any strain interaction between the concrete and the external tendons since the steel are not bonded to the concrete. • The effect can instead be calculated by introducing concentrated forces where the prestressing force is applied and also where the tendons change their direction. one possible way to apply external prestressing to a single span beam
The strain in the prestressing steel is not compatible with the strain at the same level in the concrete. • The elongation of the tendon is instead spread out over the length between the anchors due to the lack of interaction. Effect of external prestressing
The prestressing itself does not directly affect the ultimate load of the member. • It can easily be seen from the previous figure that vertical force at the deviator acts like an intermediate support. In this way, the moment in the mid-section of the member can be reduced. • One advantage with external prestressing is a better serviceability state behavior, since the deflection is reduced. • The cracking will also be delayed and any existing cracks can be pulled together. • Inspection, replacement and even re-tensioning of external tendons can be easily executed due to their accessibility. • Low frictional losses can also be expected, since no interaction between steel and concrete is present
III. STRENGTHENING WITH FIBER REINFORCED POLYMERS • Fibre reinforced polymers (FRP) is a composite material consisting of fibers surrounded by a polymer matrix. • The matrix is what keeps the fibres together and transfers the forces between the individual fibres. The matrix also acts as protection for the fibres. • Different materials can be used as matrix, but one that is epoxy based is most commonly used. • Carbon, glass, and aramid are however the three most commonly used fibres in civil engineering. All fibres behave elastically until a brittle failure and normally have a higher tensile strength than steel. • Carbon fibres are therefore the most common type used when strengthening buildings, creating so called carbon fibre reinforced polymers, CFRP.
At present, strengthening of concrete structures with FRP composites is not treated in the Euro codes or any other standards. If the member has suffered too extensive corrosion damages, other strengthening methods might be more appropriate. • Alternatively, a prior strengthening of the member, e.g. with section enlargement, can be followed by additional strengthening with CFRP • The following graph shows typical stress-strain relations for steel and different kinds of FRP. As illustrated steel has a ductile behaviour, while FRP behave almost elastically until a brittle failure is reached. • This means that the fibres will continue to carry loads even after the stress level at which the steel yields.
The behaviour of FRP can be customized. The carbon fiber reinforced polymers (CFRP) can be designed to have a high modulus of elasticity (HM) or high strength (HS). • The manufacturer normally offers two or three different levels of stiffness; low, medium and high. • The high tensile strength of CFRP makes them suitable for strengthening with regard to tensile forces. In compression the strength is significantly lower, since the fibres will behave similarly to the ones in timber and buckle away from each other. • CFRP have a very high tensile strength while also being very lightweight. It is used to retrofit structural members of concrete such as columns, beams and slabs, it can add significant capacity without adding notable weight that would further increase the load on foundations and other structural members
IV. FERROCEMENT • Ferro-cement is a thin wall type composite, having a total thickness ranging between 12 to 30 mm. It is composed of hydraulic cement mortar reinforced with a minimum two layers of continuous and relatively small diameter orthogonally woven wire mesh separated by 4 to 6 mm dia galvanized spacer wires. • The cement mortar is admixed with plasticizers and polymers for sealing pores. The wire mesh is mechanically connected to the parent surface by U-shaped nails fixed with suitable epoxy bonding system. • The mesh may be made of hot dip galvanised MS wire or some other metallic or suitable material. • Special technique for compacting ferro-cement layer is used with the help of orbital vibrators to ensure proper encapsulation of wire mesh in mortar.
It is a durable composite material, in which shrinkage cracks are distributed uniformly due to the presence of closely spaced, thin woven galvanized wire mesh. • It is coupled with excellent corrosion resistance and impermeability to the ingress of water. • This repair technique is used for providing protective reinforced membrane for rehabilitation of distressed RCC structures. • This acts as a protective layer against the vagaries of the environment. It is also used as a water proofing technique over reinforced concrete shell structures and RCC slabs as it provides impermeable thin membrane, which prevents seepage and leakage of water
V. PLATE BONDING • Plate bonding is an inexpensive, versatile and advanced technique for rehabilitation, up gradation of concrete structures by mechanically connecting MS plates by bolting and gluing to their surfaces with epoxy • Plate bonding can substantially increase strength, stiffness, ductility and stability of the reinforced concrete elements and can be used effectively for seismic retrofitting. • In this method the bolts, which are first used to hold the plates in position during construction, act as permanent shear connectors and integral restraints. • The bolts are also designed to resist interface forces assuming the epoxy glue used as non-existent assuming it as destroyed by fire, chemical break down, rusting or simply bad workmanship.
Since epoxy is prone to premature debonding, use of mechanical anchorage along with epoxy bonding is considered more reliable. • Since the steel plates are unobtrusive, with this technique original sizes of the structural members are not increased significantly. • This method is preferred where enlargement of the members is going to affect the headroom, existing windows, doors and other fixtures
VI. RCC JACKETING • Reinforced concrete jacketing increases the member size significantly. This has the advantage of increasing the member stiffness and is useful where deformations are to be controlled. • If columns in a building are found to be slender, RC jacketing provides a better solution for avoiding buckling problems. • As the new jacket is to behave compositely with the parent member, the new jacket can take additional loads only with the increase in the stresses & strains in the old one. • The problem arises if the; • Old concrete has reached limiting strain and is not likely to sustain any more significant strain • Old concrete is weak and porous and started deteriorating due to weathering action and corrosion of reinforcement.
It is however, necessary to ensure perfect bond also between the old and new concrete by providing shear keys and effective bond coat with the use of epoxy or polymer modified cement slurry giving strength not less than that of new concrete. • Plate bonding and RC jacketing are the common methods of strengthening RCC structures. • The cost difference between the two methods is not significant. A choice has to be made between the two methods based on actual needs and the suitability of each method with respect to the structural /architectural and other details of buildings
VII. FIBER WRAP TECHNIQUE • The fibre wrap technique, also known as Composite Fiber System is a non-intrusive structural strengthening technique that increases the load carrying capacity (shear, flexural, compressive) and ductility of reinforced concrete members without causing any destruction or distress to the existing concrete. • There are two systems followed in adopting this technique: • Bi-directional Woven Fabric • Uni-directional E-glass Fibers • Bi-directional Woven Fabric • This system comprises of woven fabric presoaked in specially formulated epoxy and applied over prepared surface after application of epoxy primer. • Woven fibre fabric is composed of bi-directional high strength fibers that are combined with specially formulated epoxy in a pre-determined proportion to form a composite-Material
This composite material is wrap applied onto the reinforced concrete or steel member requiring strengthening or protection and left to cure at ambient temperature. • The subsequent layer/s of unidirectional fiber fabric could be applied after giving the required overlap along the direction of fibers as per design requirements. 2. Uni-directional E-glass Fibers: • This system comprises of precut unidirectional E-glass fibrewrapped over epoxy primer applied prepared surface of member requiring structural strengthening and/or surface protection. • Subsequent to its wrapping, it is saturated with epoxy using rollers and stamping brushes manually to remove air bubbles, if any and left to cure at ambient temperature.
Though the underlying principle of the above two methods is more or less identical, but the application techniques and basic materials adopted are at slight variance. Each of the above systems has their own merits • Enhancement in lateral drift ductility and horizontal shear carrying capacities of a concrete member can also be obtained by confinement of the member by this method. • The flexural, shear and axial load carrying capacities of the structural members can be enhanced by appropriate orientation of primary fibers of the composites. • The resulting cured membrane not only strengthens the reinforced concrete member but also acts as an excellent barrier to corrosive agents, which are detrimental to concrete and the reinforcement. • Ingress of water, oxygen and carbon dioxide through the external surface of concrete member is prevented by the application of composite jacket.
The system is useful for its structural enhancement and protection capabilities under severe environmental conditions. • It can be used for retrofitting of a wide variety of structures that include bridges, flyovers, chimneys, water tanks, buildings, large diameter pipes, industrial plants, jetties, sea-front and underwater structures.
UNI-DIRECTIONAL E-GLASS FIBER BI-DIRECTIONAL WOVEN FABRIC
4. STRENGTHENING COLUMNS, BEAMS AND SLABS Strengthening of Columns The strengthening of columns may be required for the following: • Capacity: The load carrying capacity of the column can be enhanced by section enlargement. b)Ductility/confinement: The ductility of the column can be enhanced by providing additional tiles, steel plate bonding, and fibre wrap. c)Joints: The joints play crucial for resisting earthquake forces. The joints can be strengthening by enlargement, jacketing by steel collar and fibre wrap
Strengthening of Beams The strengthening of Beamsmay be required for the following: b) Shear Strength: The shear strength of the beam can be enhanced by any of the following: i. Section enlargement ii. Shear ties anchored in compression zone of beam. iii. Post tension strap around the section iv. Diagonally anchored bolts v. MS Steel plate bonding vi. Fiber wraps a) Flexural Strength: The flexural strength of the beam can be enhanced by i) Section enlargement in compression, ii) Additional reinforcement in the tension. iii) The provisioning for enhanced tensile strength if being undertaken, this should be accompanied with corresponding increase in compression as well. iv) MS plate bonding v) High Strength Fiber Fabric Wrap Technique (without section enlargement)
Strengthening of Slabs • The performance of the slab can be improved by providing overlays or underlay The addition of overlay/underlay will also increase the stiffness of the slabs and control the excessive deflections problems. • The slabs are generally safe in shear and as such no need is likely to occur for shear strengthening except flat slabs near column capital. • Cracks/Joints: • The concrete and masonry are weak in tension. The cracks indicate the tensile failure of the material. • The cause of cracking should be examined in detail and remedial measures taken accordingly. Inactive (i.e. non-moving) cracks in masonry can be repaired by stitching.
Grouting with non-shrink grouts also repairs these types of cracks. • The active cracks required for accommodating thermal movements shall be repaired by suitably locating the expansion joints and filling them with flexible materials like poly-sulphides, bituminous fillers etc B. Masonry: • The masonry may be required to be strengthened for resisting earthquake forces by external pre-stressing, splint and bandage methods • The techniques are explained in IS: 13935-1993.
5. FOUNDATION REHABILITATION METHODS 1 Shoring: • Before any shoring work is commenced, the building should be carefully surveyed & record of levels, cracks & tilts kept. • The observations should be continued throughout the period of shoring & under pinning and till the time when detectable measurements have ceased. The terminology used is: • Raking shores with the angle of shores generally 60o to 75o are usually used where external support is necessary. In case, the feet of raking shores are to be kept free, then flying shores can be provided which strut against another structure or wall. • Flying shores merely provide a restraint against building or tilting.
Dead shores are verified struts bearing on the ground at the required distance & supporting the vertical load of a wall wherever required in conjunction with flying shores or horizontal ties. • The level of raking shores & flying shores are so arranged as to bear on the wall at floor or ground with a firm bearing. • Folding wedges should be inserted at the foot of shores to take up yielding if any, of the ground & elastic shortening of the struts. Columns can be shored up individually by needle beams. • The needle system has to be properly designed to suite the particular requirements. Suitable placing of jacks for exerting upward pressure can also be planned & designed.
TYPES OF SHORES RAKING SHORES
2. Underpinning: • If underpinning is necessary to arrest settlement, it is essential that the underpinned foundation should meet the requirements of correct allowable bearing pressures. • Depending on the cause of settlement, shallow underpinning may be satisfactory in some cases, whereas in some cases the underpinning has to be taken down to a deeper & relatively incompressible stratum. • Underpinning material are metals in case of comparatively shallow underpinning. • Underpinning by piles or piers is suitable, only if the new bearing stratum is deep.
