REPAIR AND RETROFITTING OF MASONRY STRUCTURES EFFORTS BY: ABHISHEK BHATT (0506) ANTARA MASHRUWALA (1706) DISHITA SHAH (3006) RAVI SONI (5207)
Objective: To study different repair materials and techniques to beadopted in various situation to repair defective structures of brickand stone masonry.Repairs: Process to replace or correct deteriorated/damagedmaterials and components of a building.Retrofitting: Upgrading of structural and non-structural systems toprovide a higher level of resistance than existed before .
HOW DAMAGE OCCURS IN BRICK-MASONRY STRUCTURES:Effect of Aging on Brick-Work: Rain Exposure Temperature Moisture Absorption Chemical Alteration Continuous Exposure to Chemicals Construction Error: Eccentrically Loaded Walls gives overturning effect (Decorative Panels on one side, Support of Working platforms on one side) Usage of Poor quality of materials Error due to saving in economy
Maintenance:Common maintenance to brick is to white wash or to paint them at regularintervals, preferably every 2 years.To maintain all sanitary installation in a good condition, any leakage noticeanywhere should be intended to immediately, since a small leakage from nani-trap would lead to a major damage in due course.Any cracking wherever noticed should be attended immediately either bysealing the joints with cement mortar or by strengthening the cracks withreinforced bands.
TYPES OF CRACKS IN MASONRY STRUCTURES:1.Vertical cracks in sidewalls at corner2.Vertical cracks around balconies3.Vertical cracks below the openings in line with the window jambs.4.Vertical cracks at the junction of RCC column and masonry wall.5. Vertical cracks in the top most stories at corners of structures having RCC slab.6.Horizontal cracks at window lintel or sill level in the top most story.7.Horizontal cracks in the top most story below the RCC slab level.8. Horizontal cracks at eaves level in the buildings having pitched roof with woodenjoists and purlins.9. Random cracks in all directions involving both external and internal walls.10. Diagonal cracks over RCC lintels spanning large openings.11. Shrinkage cracks.
GENERAL CAUSES TO DEVELOP THE CRACKS:Poor quality of bricks.For masonry work,use the porous stones.Absence of grading in before the use of fine aggregate fine mortar.If percentage of clay and silt in fine aggregate exceed 3 percent.Due to the excessive amount of soluble sulphate.Plumb alignment.The position of the element in the structure.Differential loading.Weak mortar.Insufficient bond.Improper curing.Entered dampness from ground, roof and exterior faces.High daily temperature variations.Atmospheric pollutions.Lack of strength at corner and at junction of walls.Improper binding of thick walls.
REPAIR TECHNIQUESSand / Cement Mortar:It is the most simplest and basic repair method. It is highly reliable but may failunder severe drying conditions.Thus cracks will appear as a result of shrinkage in the plastic state of mortar.Due to this shrinkage, debonding and delaminating frequently occurs.Due to high water cement, ratio strength decreases.High density of pores and capillaries allow water and acidic vapours to more easilypenetrate the repair and attack to the reinforcement bars.Polymer modified cementations mortars:Polymer modified cementitious mortars are factory controlled blend of speciallygraded fillers, cement and admixtures to control strength development in set time. Toreduce the shrinkage they contain polymers to improve the low permeability, qualityand to enhance adhesion.The thermal expansion properties are similar to normal concrete.Heat development is low so much larger volume repairs can be undertaken.Low impermeability can prevent ingress chlorides.
Epoxy resin mortars:Epoxy technology is based on the mixing of two liquid resins whichreact together to form a hard solid if aggregates are added the resin systemacts to bind them together to create mortar.Achieves compressive strength of 60-80 N/mm2 and very heavy highflexural strength.Due to good adhesion repair will never fail along correctly preparedbond line.More strength gain occurs in one day in normal atmospheric conditions.Properly compact mortar is impermeable to water.Most epoxy mortar requires primer.
Epoxy injection:Narrow cracks of 0.05 mm cracks are repaired by this method.High strength, highly viscous material can be achieved.In this, holes are drilled at closer interval repairs are done under external forcethrough pumps or greasing guns.Normal pressure should be of 1 kg/cm2.We should not apply too muchpressure which propagates the cracks.High degree of skills and executions are required to repair these types ofcracks.
Crack stitching:This method involves drilling holes on both sides of crack and grouting institching dogs.Stitching may be used when the tensile strength must be re-establishedacross major cracks.Stitching a crack tends to stiffen the structure, causing the concrete tocrack elsewhere. therefore it may be necessary to strengthen the adjacentsection with external reinforcement.
METHOD FOR ADVANCE RETROFIT TECHNIQUE WITHHELICAL REINFORCEMENT Firstly slots are cut with cutter machine of length 500 mm on each side of crack.1. The depth of slot is 10 mm but it also depends on the plaster surfaces. Clear loose detritus from slots ensuring exposed brick surfaces are cleaned of2. mortar and flush thoroughly with water, or treated with a suitable primer.3. Now fill the slot with grout with pressure. Push Helical 6mm reinforcement crack stitching tie into grout to approximately4. two thirds of slot depth. Finish the surface and repair the crack on the wall with appropriate filler /grout5. chemical.
Tuck pointing: Defective joints can usually be repaired by tuck-pointing with Portland cement mortar, with the help of skilled stonemason In this method, all joints, vertical and horizontal, in the face of the wall should be tuck-pointed. This procedure requires removing and replacing all mortar to a depth of at least 16mm throughout every joint. each joint is raked to a depth not greater than 26mm unless the old mortar is so defective then it is necessary to remove the mortar up to greater depth. The depth of old mortar is removed first so that sound mortar acts as a base for new mortar. All dust and dirt within the raked joints should be washed out. The mortar is mixed at least 1 hour prior to use to ensure pre-hydration which stabilizes the plasticity and workability of mortar and minimizes any tendency to shrink after insertion into the joint opening. The sand/cement ratio should be about 3 to 1 by volume.
Grouting (cement mortar):Masonry structures can be sealed effectively with less cost by using groutprovided the cement-base stabilizing mixture contains an intrusion aid.The consistency of intrusion mixture is that of smooth slurry.Before the intrusion, grout is pumped, the holes are tested by pumpingwater to see the drilling is adequate and to determine correct consistencyfor the slurry repairs.Firstly plaster is removed near the cracks from inside and outside ofwalls and clean the cracks thoroughly with airjet and water.Wet the cracks ,if dry ,by sprinkling water.This mixture is pumped into holes previously drilled at various intervalto various depths without damaging the integrity of structure.Pumping the grout simultaneously into multiple series of drill holesensures good penetration into any fissures within the structure and formaking masonry structures watertight.
Grouting (chemical): Chemical grouts consist of solution of two or more chemicals (such as urethanes, sodium silicate) that react to form a gel. This grout opposed to cement grouts that consist of suspension of solid particles in a fluid. Advantages of chemical grout include applicability in moist environment and their ability to be applied in very fine facture. Disadvantage is high degree of skill needed for satisfactory use and their lack of strength. Crack as narrow as 0.05 mm can been filled with chemical grout.
Polymer coating: Polymer coating is use for the job condition and can be an effective protective coating if properly applied. Polymer coating should be selected from the material specifically for the intended application. Some formulation will adhere to damp surfaces and even under water but may required a completely dry surface. Mixing and applying polymers below 16 degree and above 32 degree Celsius will require special caution and procedure. Moisture passing through the sub grade backfill or from rainwater can accumulate under the coating, which will be interrupted by freezing and thawing.
REPAIR MATERIALSEpoxy resins Epoxy resins are excellent binding agents with high tensile strength. The epoxy components are mixed just prior to application. The product is of low viscosity and can be injected in small cracks too. The higher viscosity epoxy resin can be used for surface coating or filling larger cracks or holes.Epoxy mortar Epoxy resins when combined, with sand aggregate it forms epoxy mortar. Epoxy mortar mixture has higher compressive strength, higher tensile strength and a lower modulus of elasticity than Portland cement concrete.Quick-setting cement mortar It is a non-hydrous magnesium phosphate cement with two components, a liquid and a dry, which can be mixed in a manner similar to portland cement concrete.Gypsum cement mortar It has lowest strength at failure among these three materials. So it is used in minor repair works in structural application.Mechanical anchors Mechanical anchors provide both shear and tension resistance. Such anchors are manufactured to give sufficient strength.
RETROFIT: Post earthquake improvement of the seismic resistance of a structure including repair of damage . Upgrading of structural and non-structural systems to provide a higher level of resistance than existed before the earthquake.SURFACE TREATMENT:Surface treatment is a common method, which has largelydeveloped through experience.Surface treatment incorporates different techniques such asferrocement, reinforced plaster, and shotcrete.
FERROCEMENT:Ferrocement consists of closely spaced multiple layers of hardwaremesh of fine rods(Figure 1 (a)) with reinforcement ratio of 3-8% completely embeddedin a high strength (15-30 MPa) cement mortar layer (10- 50 mmthickness).The mortar is troweled on through the mesh with covering thicknessof 1-5 mm.Typical mortar mix consists of 1 part cement: 1.5-3 parts sand withapproximately 0.4 w/c ratio.
REINFORCED PLASTER:A thin layer of cement plaster applied over high strength steel reinforcementcan be used for retrofitting.The steel can be arranged as diagonal bars or as a vertical and horizontalmesh.In diagonal tension test and static cyclic tests, the technique was able toimprove the improvement in strength depends on the strengthening layerthickness, the cement mortar strength, the reinforcement quantity and themeans of its bonding with the retrofitted wall, and the degree of masonrydamage.
SHOTCRETE: Shotcrete overlays are sprayed onto the surface of a masonry wall over a mesh of reinforcing bars. In shotcrete the size of coarse aggregate is limited to 6mm. With thorough mixing of aggregates and adjustment of pressure at the nozzle should be controlled otherwise a large proportion of coarse aggregate is lost in rebound. Due to greater wear of nozzles during shotcrete there is a tendency to reduce the proportion or even eliminate coarse aggregates. Certain additives are available which can reduce wear and tear of nozzles. After shotcreting proper curing is essential. In situation where heavy construction machinery can not be moved or is not economically viable, manual shotcreting can also be resorted too. In manual method layers of concrete can not exceed 35-40 mm in thickness when laid. Mason should be trained properly to move the trowel applying vibrations and simultaneously for compacting effect on each layer. Non destructive testing of shotcrete repair work should be carried out.
GROUT AND EPOXY INJECTION: PROCESS : The crack or joint should be widen wherever the injection work is to be carried out (approximately 15 mm wide x 15 mm depth) Drill holes of about 13 mm dia upto 150 mm deep at 500 mm @ c/c. Use compressed air to clean the hole, as well as crack to remove dust, dirt, loose materials etc. Use 12 mm dia specially threaded injection nipples and fix them into the holes provided. Seal the entire surface around the nipple with putty. Seal the surface of the crack with putty so as to ensure that the injected grout does not leak from the gaps & from cracked surface. The surface now is ready for injection. Prepare the injection grout and load it into the injection gun.
EXTERNAL REINFORCEMENT:A steel plates or tubes can be used as external reinforcement for existingUnreinforced Masonry(URM) buildings.Steel system is attached directly to the existing diaphragm or wall.Two vertical members (via pin connections), which are placed next to theexisting wall (i.e. creating in-fill panel) can be used.
CONFINING URM USING R.C. COLUMNS: Confined masonry with R.C., weak frame represents one of the most widely used masonry construction system in Asia and Latin America. In China, they used such confinement in new masonry buildings as well as it is used as retrofitting for existing URM buildings. The basic feature of confined masonry structures is the vertical R.C. or reinforced masonry tie columns, which confine the walls at all corners and wall intersections as well as the vertical borders of doors and windows openings. In order to be effective, tie columns should connect with a tie beam along the walls at floors levels.
POST TENSIONING:Post-tensioning involves a compressive force applied to masonry wall; this forcecounteracts the tensile stresses resulting from lateral loads.There has been little application of this technique; post-tensioning is mainly usedto retrofit structures characterized as monuments.This is due in part to lack of knowledge about the behavior of post-tensioningmasonry.In addition, the codification of post-tensioning masonry has only begun recently.
(Contd…..)POST TENSIONING:Post-tensioning tendons are usually in the form of alloy steel thread bars.Bars typically show higher relaxation losses (2-3 times strand losses) and muchlower strength/weight ratio (VSL 1990); in addition, a major drawback for using ofsteel bars is corrosion. Fiber reinforced plastic presents a promising solution for thisproblemTendons are placed inside steel tube (duct) either within holes drilled along themidplane of the wall or along groves symmetrically cut on both surfaces of the wall.Holes are cement grouted and external grooves are filled with shotcrete.
GROUT OR EPOXY INJECTION INCRACKS1 - Plaster removed2 - Cracks sealed after cleaning3 - Grout portsStrengthening of existingmasonry1 - Brick or block wall2 - Injection holes3 - Grout mixture
STRENGTHENING WITH WIRE MESH AND MORTARSTRENGTHENING WITH WIRE MESHTwo steel meshes (welded wire fabric with an elementary mesh of approximately 50 x 50 mm)are placed on the two sides of the wall, they are connected by passing steel each 500 to 750 mmapart, A 20 to 40 mm thick cement mortar or micro concrete layer is then applied on the twonetworks thus giving rise to two interconnected vertical plates. This system can also be used toimprove connection of orthogonal walls. 1 - Welded wire mesh - 50 mm x 50 mm 2 - Mortar or micro-concrete rendering 3 - Concrete roof band 4 - Cross ties at 500 to 750 mm apart 5 - Corner bar diameter 8 mm 3 CORNER REINFORCEMENT
For Large cracks and crushed concrete For cracks wider than about 6 mm or for regions in which the concrete or masonry has crushed, atreatment other than injection is indicated. The following procedure may be adopted.The loose material is removed and replaced with any of the materials mentioned earlier, i.e., expansivecement mortar, quick setting cement or gypsum cement mortar. Strengthening of existing masonry 1 - Wire mesh on front face 2 – Clamps 3 - Wire mesh on back face 4 - Cement plaster 5 - Crack in wall
Splint and bandage strengthening technique The technique of covering the wall with steel mesh and mortar or micro- concrete may be used only on the outside surface of external walls but maintaining continuity of steel at the corners. This would strengthen the walls as well as bind them together. As a variation and for economy in the use of materials, the covering may be in the form of vertical splints between openings and horizontal bandages over spandrel walls at suitable number of points.
NEGLIGENCE AFTER REPAIRS AND RETROFITTING No. Negligence Reasons 1 Hair cracks are found on lack of curing after 4 hours from the application the all ready strengthened of cement mortar. brick masonry walls. 2 Weld mesh are soon on the improper thickness of the cement mortar brick masonry walls after finishing coat. 3 Uneven plaster surface are works are going on without preparing proper found on the wall. level markers. 4 Different shades are found Improper mixing of cement with other on plastered surface. materials and improper curing
CASE STUDY : JABALPUR EARTHQUAKE• PHOTOGRAPHS OF DAMAGE DUE TO EARTHQUAKE• STRENGTHNING OF EXISTING CONSTRUCTION• STRENGTHNING OF NEW CONSTRUCTION• PHOTOGRAPHS OF NEW CONSTRUCTION AFTER EARTHQUAKE
JABALPUR EARTHQUAKE DAMAGED NUILDINGSPartial Building Collapse in the Faillure of Brick Masonry Walls in1997 Jabalpur Earthquake the 1997 Jabalpur Earthquake
STRENGTHENING OF EXISTING CONSTRUCTION Description of Seismic Strengthening provisions Seismic Deficiency usedLack of integrity (box-type action) Installation of seismic belt (bandage) at the lintel level; it consists of welded wire mesh installed above the lintel level and anchored to the wall. The mesh is covered with a thin cement plaster overlay (see Figure a) Cracks in the walls In case of small cracks, pressure injection of epoxy grout; in case of large cracks, filling the gaps with cement grout and jacketing with reinforced cement overlay.Inadequate wall resistance (shear and tensile) Reinforced concrete jacketing. Difficult to find skilled labor and materials for welded wire mesh in rural areas Flexible floor/roof diaphragm (Corrugated metal Installation of RC roof band (bond beam). Provision ofsheets/timber) roof band is expected to enhance the overall intergrity and improve torsional resistance of building Cracking/damage of wall corners (due to improper Corner strengthening of wall corners - installation ofinterlocking of cross walls) welded wire mesh anchored to the walls with steel dowels and covered with a thin cement plaster overlay , see Figure b.
STRENGTHENING OF NEW CONSTRUCTION Description of Seismic Seismic Deficiency Strengthening provisions used Reinforced concrete roof band; provision of roof band Roof results in an improved overall integrity and torsional resistance of the building. RCC lintel band; very effective, however skilled labour Wall and materials may not be available, see Figures 7D, 7E and 7F Improved quality of masonry (bricks and mortar) use of Wall better quality bricks will drastically improve the wall seismic resistance; use or richer cement/sand mortar will improve wall shear resistance. Provision of vertical reinforcement at wall corners and Wall intersections, see Figure 7G (Source: IAEE 1986)
7D-An Example of New Construction with 7E-Construction of RC lintel Seismic Features (note RC lintel band) band 7F-Construction of RC Lintel Band - Pouring of Concrete Completed
7G-Provision of VerticalReinforcement at WallCorners andIntersections
CONCLUSION Cost effective and safer approach will be to carry out appropriate repair and retrofitting of the building. It will offer the advantage of carrying out the work part by part or room by room so that the occupants may continue to live in the building during the repair and retrofitting work . By doing so the cost of repair and retrofitting came to only 8 % of the estimated cost of reconstruction.
REFERENCES: Civil Engineering & Construction Review (Vol.14 Dec.2001) Structural Consolidation & Strengthening of masonry (Department of Civil Engineering, Belgium) 13th International Brick & Block masonry Conference Deterioration & Damages in Buildings, Department of Civil Engineering, IIT, Kharagpur Jabalpur & Killari earthquake Damage reports & Retrofitting Method, Department of Civil engineering, IIT, Kanpur
QUESTION FROM OUR PRESENTATION Why cracks or deteriorations occurs in brick masonry structures? State any 6 techniques by which it can be repaired? Explain any one in detail with neat sketches.