Repair and Rehabilitation of Structures

June 3, 2016 | Author: Saravana Chandhran | Category: Topics, Books - Fiction
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REPAIR AND REHABILITATION OF STRUCTURES C. Deepak – III Civil & R. Vivek – III Civil Annamalaiar College of Engineering Polur, Thiruvannamalai.

Abstract: All kinds of structures are susceptible to fire hazards. After a fire, detailed evaluation and assessment of the structure needs to be conducted by experienced structural engineers to assess the extent of damage. Then, rehabilitation of old masonry buildings (or) restoration of heritage structures presents a special case for repair. Here, in most cases, the construction materials used are unconventional like natural stones, wood and lime mortars and plasters as compared contemporary materials of today. Then, another special case of repair strategy, under water repair of structures. It needs much care and cost effective supervision. Here, in our paper, we have presented 360ο deep discussion about these three repair strategies for different (Fire damaged / Masonry & heritage / Underwater) structural types. We have given all steps to be done for a good repair system such as the initial procedure for evaluation of damaged structures, then, the preparation before carrying repair & various repair methods that can be done for the respective repair & problem.

1. Repair Systems for Fire damaged Structures 1.1 Material behavior affected by Fire The type of course aggregate has a significant effect on the damage to the concrete structural element. Based on studies done in the past, it is well established that “Siliceous aggregate” concrete retains approximately half its capacity @ 650οC (1200οF) while carbonate and light weight aggregate concretes exhibits near full capacity @ 650οC (1200οF) Reinforcing bars heated beyond 500οC (932οF) lose significant amounts of yield strength and ultimate strength. Both typical structural steel and high strength alloy steels used in the construction

industry, retain approximately 90% of their strength to nearly 315οC (600οF) as described in the National Codes and Standards Council of the Concrete and Masonry Industries (1994)

1.2 Process of Evaluation, Analysis & Repair Certain processes need to be followed by the investigating forensics engineers regardless of whether the structure damaged by fire is a building, stadium, bridge, industrial facility (or) a petrochemical plant.

Fig 1: 5 Step Procedure for Fire Investigation & Repair Generically the extent of damage and repairs will fit in one of the following categories. Extensive Damage: Economically not feasible to repair. Demolish the structure. Significant Damage: Total replacement required of several structural elements due to either severe distortion (or) degradation in strength; other areas repairable.

Repairable Damage: Various elements damaged, can be economically repaired. Superficial Damage: Minor repairs needed such as patching of concrete and replacing some bolts in steel structures. No Damage: No repair is needed. Structure can support the code required loads.

1.3 Various Repair Methods Several repair methods used in repair of fire damaged structures are briefly described below. In concrete structures, these ranges from re-bonding of cracked concrete, spall repair, adding additional concrete to the distressed section, externally bonded reinforcement to partial (or) complete replacement of the distressed elements.

1.3.1 Spall Repair Due to the non-uniformity of heat affected zones and variations in quality of construction, after a fire event, there may be several small areas of concrete that have spalled off from various structural elements at various locations. Areas as large as 1.0 m2 can be reasonably repaired using epoxy mortars (or) polymer cementitious mortars.

Fig 2: Repair of Concrete Spalls

Fig 3: Column Repair using Shotcrete

In spite of excellent bonding characteristics of epoxy mortars, such repair mortars should not be used for exterior applications due to a wide disparity in coefficients of thermal expansions between the parent concrete & epoxy repair mortars. A better and less expensive alternate is to use polymer cementitious mortars which have a coefficient of thermal expansion compatible with the parent concrete. The polymer in such polymer cementitious concrete repair mortars functions as plasticizers that improves the bonding characteristics while reducing the amount of water required to make a workable mix and also decreasing the permeability of hardened mortar.

1.3.2 Pneumatically applied concrete (or) Shotcrete Shotcrete is frequently used in repairing concrete structural elements damaged by fire. This is a process in which concrete is conveyed through a hose and pneumatically projected at a very high velocity onto the surface being repaired. The success of this process is heavily dependent on the skills of the nozzle man who has to ensure that the reinforcement will have full coverage of the pneumatically applied concrete all around. Dry mix Shotcrete involves placing the dry concrete mix ingredients into a hopper and then conveying them pneumatically through a hose to the nozzle. The mixing of water is controlled by nozzle man at the nozzle. Wet mix Shotcrete involves pumping of the previously prepared conventional concrete through the hose to the nozzle. Compressed air is introduced at the nozzle to propel the mixture onto the surface to be repaired. The wet mix process is preferred more due to less re-bound waste, dust comparatively. Multiple layers of Shotcrete will generally be required to accomplish the desired section profile. Approximately, 25mm (1.0”) to 50mm (2.0”) thickness of layers are usually placed for overhead applications and 100mm (4.0”) to 150mm (6.0”) for vertical surfaces to be repaired.

1.3.3 Externally bonded FRP Externally bonded composite fiber reinforced polymers (FRP) used in the construction industry are generally comprised of Carbon Fiber Reinforced Polymer (CFRP) or Glass Fiber Reinforced Polymer (GFRP) materials.

The resins in such composite FRP sheets provide the bond to the concrete surface to be repaired. The primary advantage of using FRP materials is their high tensile strength.

Fig 4: Strengthening of beam using FRP

Fig 5: Column strengthening using FRP

Tensile strength of CFRP ranges between 1720MP to 6200MP. Tensile strength of GFRP ranges between 1860MP to 4825MP. Another advantage os that the weight of FRP is approximately 1/3rd of steel and the thin sheets make it particularly easy to handle in the field. Flexural and shear capacities of concrete beams can be effectively repaired using such CFRP materials. Flexural capacity of both one way & two way slabs have also been successfully repaired using CFRP. Column axial load capacities can be restored to distressed sections by wrapping the columns around the perimeter to provide additional hoop restraint.

1.3.4 Externally bonded Steel Plates The use of externally bonded steel plates to restore the capacity of distressed concrete elements was pioneered in the 1960’s. Since then, the flexural and shear capacities of several concrete and pre stressed beams have been successfully repaired using epoxy steel plates in the UK & USA. Repair method is very similar to the use of FRP material where thin steel plates are bounded to the concrete surface. Plates used for such repairs vary in thickness from 1.5mm to 6mm. As in FRP, such repairs need to be protected from elevated temperatures since the epoxies do not perform well under elevated temperatures.

Fig 6: Epoxy bonded plates for shear

2. Repair Systems for Masonry & Heritage Structures 2.1 Evaluation of structures for Repair First and foremost, it should be checked that overall structure is structurally sound. This may not be a concern as most old buildings were normally designed with higher safety factors. Some of the key factors to be considered are: -

If columns have cracks and deteriorated bases and bottom spalling If iron oxide formations are observed, in case old iron part are embedded If there is flooding at the bottom of the structure due to level differences between the new roads and the old structures If dampness, rising dampness as well as growth of fungus and vegetation is observed in basement / plinth area. If there is dampness (or) condensation (or) deterioration associated with biological growth.

Fig 7: Spalling of plaster

Fig 8: Biological Growth

Fig 9: Efflorescence

2.2 Testing and Investigation Following testing should be conducted before any repair programme is undertaken. -

Visual observations and investigations Determine whether the damage is structural (or) non-structural Determining the degree of deterioration of natural stone as well as porosity NDT (or) coring can be conducted to determine existing strengths and chemical profile of structure. Once the decision has been made about the selection of cleaning, job site test should be conducted on small test area to determine the effectiveness.

2.3 Repair Systems -

Cleaning Surface Preparation Selection of appropriate repair materials Rising dampness barrier Water proofing Renders & Finishing

2.4 Non-Structural Repairs Clean and remove the existing biological growth in the structure. Coat the affected areas with appropriate fungicides (or) weedicides to avoid the recurrence of plant growth. Rising dampness should be treated by creating a chemical damp proofing course by using gelling / setting injection materials. On the surfaces where there is existing dampness, the plaster and overcoats should be removed and a coat of mineral based water proofing slurry should be applied.

Fig 10: Repair of damaged Masonry structure

Final step, would be to apply a water proof plaster / render. This material should ideally be a ready-made product. The bottoms of the pillars can be repaired using epoxy mortars if needed. In areas where major repairs are to be done, one can also resort to using polymer mortars by incorporating correct acrylic polymers (or) SBR based mortars / repair polymers. Roofs can be re-water proofed and protected by using polymer based flexible water proofing systems, which can be easily over coated with the required final finish.

2.5 Structural Repairs The cracks in structural elements, should be injected with two component low viscosity epoxies (or) the appropriate cement based polymer modified grout formulated for the purpose, depending upon the width of the crack. The bottom portion of the building should be grouted with polymer based grout (or) the correct type of structural resin groutes, to restore the structural integrity of the building. Verandah pillars and balustrades should be either replaced (or) repaired with appropriate epoxy (or) polymer modified mortars. RCC slabs should be repaired using the appropriate concrete repair systems. They should be replaced if found to be structurally inadequate.

3. Repair Systems for Under-water Structures 3.1 Special Features Above water (dry) repairs may also be used under water with only minor modifications. However, the materials specified for use in air are often unsuitable for under water applications. The special features of under-water repairs are: -

Due to high cost and complexity of under -water working, the repair operations need to be made as simple as possible. Adequate preparation of damaged area may require specially adopted techniques. The repair materials must be compatible with underwater applications both placing & curing Formwork and placement method adopted must minimize mixing between repair material and water Under-water supervision of repair operations is difficult and costly

3.2 Preparation of Damaged Area Before a repair is undertaken, it is necessary to clean the damaged area of marine entrustations (contaminants) to allow detailed inspection to assess the extent of damage. For large areas, a high pressure jet may provide a solution. Once the area has been cleaned, the extent of cracked and spalling concrete may be defined with the help of divers (or) remote operated vehicles to photograph the area. For the removal of cracked concrete, High pressure water jetting (200 atm – 1000 atm) is directed onto the concrete surface. Alternatively, the damaged concrete can be cut by Splitting techniques, where in hydraulic expanding cylinders are inserted into pre-drilled holes and pressurized until splitting of concrete occurs. Removal can also be done by Soft explosions. After the removal of damaged concrete, all broken (or) distorted reinforcement will have to be removed and replaced before reinstating the cover. The commonly used methods are, Oxygen-fuel gas cutting, Oxy-arc cutting and Mechanical cutting.

3.3 Application of Materials The generally used methods for the application of repair materials are as follows: -

Mortar Replacement Injection into Cracks Large scale Repairs

3.3.1 Injection into Cracks The general principles for injecting grouts in the cracks under-water are the same as for water jetting. Owing to the risk of washout of cement, non-conventional epoxy resin injection are normally used. Epoxy putty is used to seal the crack between injection points as shown below.

Fig 11: Injecting the crack with epoxy grout

Epoxy resin must be low viscosity solvent free underwater-grades in order that the water in the crack is replaced by a structural material. For small repairs, the use of hand-held cartridge injection guns is a satisfactory procedure.

3.3.2 Large Scale Repairs For bulk underwater, placement of repair material, an easy-to-erect formwork complete with inlet pipes and external vibrators and tolerant of variations in the existing structure, is required. Flexible seals are preferred as they ensure a leak-tight fit. For vertical repairs, positive attachments using steel straps (or) rock bolts drilled into the concrete to secure the form as shown below, with a thick layer of compressible gasket (e.g. neoprene rubber) to form final seal, may be employed. Finally the gaps due to unexpected variations in line (or) level are sealed.

Fig 12: Form work for under-water Repair The mix design for underwater repair may require certain modifications depending upon the nature of the work. The cement content is then raised by approximately 25%. Lean mixes of less than 350kg / m3 are not likely to be suitable due to washout of cement. Conclusion When complete demolition and total removal and replacement have been ruled out, various strategies are available to the engineer to resolve the integrity of fire damaged structures. The primary cause of masonry / heritage structures needing repairs, is moisture and salt transport through the masonry / load bearing elements, causing structural failure. This puts the role of construction chemicals only during the actual repairs, but the application and initial investigations are equally important.

References -

ACI 546R – 04 (2004): “Concrete Repair Guide”, American Concrete Institute, Farmington Hills, Michigan 48333, USA Neville ,A.M (1995): “Properties of Concrete”, 4th Edition, Pearson Education Limited. An Article of “Repair systems for Masonry and Heritage Structures “ by Sunny Surlaker – MC –Bachemie India Limited. “Concrete Technology” by M.L.Gambhir “Building Construction” by P.C.V arghese “Pre stressed Concrete Bridges” by N.KrishnaRaju

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