The treatment of concrete structures is carried out in the two stages: 1. Preparation of Surface for Repairs 2. Application of Repair Materials
1. Preparation of Surface for Repairs:
Before the execution of any repair, one of the essential requirements common to all repair techniques is to remove the damaged or deteriorated concrete with the help of proper tools and equipments. While removing the damaged portion, care should be taken that no damage is caused to the un-affected portion.
For preparing the surface for repair following steps are adopted:
(a) Complete removal of damaged portion.
ADVERTISEMENTS:
(b) Under cutting along-with the formation of smooth edges.
(c) Removal of the cracks from the surface.
(d) Formation of well-defined cavity with rounded inside corners.
(e) Providing rough and uniform surface for repair.
ADVERTISEMENTS:
The surface so prepared should be clean, dry, strong and free of laitance:
(a) A clean surface means that there should be no foreign matter such as loose particles, dirt, grease or oil, paints, resin etc., on it.
(b) Dry surface means that no free water should be there on the surface.
(c) Free of laitance. Laitance is the very thin layer or skin of high water cement ratio gel which appears on the surface during concrete compaction. The laitance is quite weak and has poor adhesion with the parent concrete. Thus it should be removed before laying the repair material on the surface. Strong concrete below the surface refers to the ability of concrete to resist fractures due to the stresses developed on it by the repair material etc.
ADVERTISEMENTS:
Dust and other foreign matter present on the surface of the concrete usually can be removed by wire brushing. If this method is not found satisfactory, then grinding, sand or steel blasting and scarifying may be adopted, as these are the commonly adopted methods for dust-removing.
For cleaning a concrete surface, the sand or steel shot blasting is the most effective method. In some cases, low pressure water jet blasting has been found most effective. The presence of oily substance on the surface of concrete can be detected by sprinkling water on the surface. If the water stands in droplets without spreading out immediately, it indicates the presence of oily substance on the surface, which will interfere with the bonding of repair materials.
The grease, oil and animal fats etc. can be removed by scrubbing the surface with caustic-soda solution, detergents or tri-sodium phosphate. The process is known as chemical cleaning. During the washing procedure, a vigorous scrubbing action should be carried out with a stiff broom. To remove all traces of loosened oil and as well as the cleaning solution, the surface should be washed off thoroughly with a pressure hose.
If on scrapping the surface with a knife blade, fine powdery substance is observed on the surface, it indicates the presence of laitance on the surface, which may be removed by acid etching. In case acid etching method is adopted, then the surface should be pre-cleaned by removing any build-up of oil, grease, dirt etc. In case presence of chlorides in concrete is not objectionable, a 10% solution of HCl acid in water should be applied in the ratio 1:15 litre/m2 of the surface.
ADVERTISEMENTS:
A stiff broom or brush should be used to spread and vigorously scrub the acid solution uniformly on the surface. After the subsidence of foaming action, the surface should be washed off thoroughly with water and still scrubbing it with stiff broom. This is necessary to remove the salts which might have been formed by the reaction of the acid with the cement.
The presence of acid after washing should be checked with the litmus paper. When the presence of chloride is objectionable, a 15% solution of phosphoric acid can be used. The presence of acids from other sources may also be ascertained by placing a litmus paper in a thin film of water on the surface of concrete. A pH value below 4 indicates the high value of acidity of the concrete for the successful application of repair.
2. Application of Repair Materials:
The repair of damaged or cracked structure may be classified into two categories:
A. Ordinary procedures
ADVERTISEMENTS:
B. Special procedures
A. Ordinary Procedures:
The fine or superficial cracks are rectified by treating the surface with soft distempers, white wash or silicate cement paints etc. The methods and materials for the repair of patches of damaged concrete in the structure are discussed below.
The repair can be carried out in four steps as follows:
(i) Preparation of Surface:
The cracked and damaged areas are cut out from the solid concrete. The area to be cut out should be marked with a saw cut to a depth of about 5 mm to show neat edges. The edges should be cut out as straight as possible and right angle to the surface with corners rounded with in holes. The edges should be slightly under cut to provided keys at the edges of the patch.
To prevent the edges from breaking under load, the thickness of the edges should not be less than 25 mm. All the loose and unsound concrete is removed. All the loose material should be cleaned and the surface should be washed off before actual patching work is started. Excess water from the cavity should be removed. To obtain a good bond, the surface of concrete should be coated with a thin layer of neat cement grout before placing patching concrete.
If the reinforcement has been corroded then the concrete should be removed from the considerable area to ensure that all corroded areas are exposed for cleaning with ease. Carbonated or chloride contaminated concrete in contact with reinforcement should be removed and in its place fresh concrete or impermeable resin compound is placed.
After cleaning the reinforcement corrosion, controlling or inhibiting chemicals such as phosphates may be applied on the exposed surface of the reinforcement. In case of cement based repairs, a slurry coating of polymer latex and cement can be used. Rasin based coatings are suitable for both cement based and resin based repair materials.
(ii) Selection of Materials:
The repair materials should be selected in such a way that the mechanical properties of the repair materials should be similar to those of structure being repaired. The cement based repairs can provide fire resistance while resins soften at relatively low temperature. For conventional repairs, the cement based materials to be used for patch work, may either be mortar or concrete depending upon the extent of repair.
(iii) Application of Materials:
Generally following methods may be used for filling the materials:
a. Dry packing
b. Concrete replacement
c. Mortar replacement
d. Grouting
e. Large volume prepacking of concrete
f. Guniting or shot creting
After preparing the surface, a bonding coat should be applied to all the cleaned exposed surfaces. It should be done with minimum delay. The bonding coat may be of cement slurry or an equal amount of cement and sand mixed with water to a fluid paste consistency.
Adequate preparation of surface and good work man-ship are the ingredients of efficient and economical repairs:
a. Dry Packing:
In this method the low water content mortar is placed on the prepared surface. To produce an intimate contact between the mortar and the existing surface, the mortar is well rammed. Due to the low water cement ratio of the paste, the shrinkage of the mortar is less and it provides a durable, strong and water tight patch.
For repair work, usually cement and sand mortar in the proportion of about 1:2.5 or 1:3 is used. Usually sand passing 1.18 mm IS sieve is used. However coarse concreting sand may also be used, but for a smooth finish of the surface and for the final layer finer sand should be used. The first layer of repair material should be applied immediately after the application of bonding coat, while latter is still wet. Provision of some mechanical anchorage for the patch work should be provided.
It may be done by providing dowels drilled and grouted into the surrounding concrete. The water content of the mix should be chosen carefully as excess water will increase the shrinkage, which may loosen the replaced material, whereas less water will not make a strong and sound solid pack. The water content in the mix should be such that the mortar could be moulded into a ball by hand, and at the same time it does not exclude water, but leave the hand damp only.
To minimise the shrinkage in place, the mortar should be left for 30 minutes after adding water and then remixed before it is used. The repair material should be filled in place properly and well compacted. The thickness of the compacted layer should be of about 10 mm. The next layer normally should be applied after the preceding layer has developed sufficient strength to support the next layer. In order to obtain a good bond, the preceding layer should be scratched before placing the next layer.
Each layer is compacted well over its entire surface with the help of 200 mm to 300 mm long hard wood stick and upto 25 mm diameter with a hammer. The last overflowing layer should be struck flush with the surface. There should be no time lags between the layers. In case there is a delay between the layers, a fresh bonding coat should be applied when the work is resumed.
The mortar may be finished by placing a flat hard wood against the mortar and strike it several times with a hammer. The patch work is cured by covering it with an absorbent material that keeps it damp, preferably it may be covered with polythene sheets sealed at edges. Shading from sun should be done. After the completion of the whole patch work curing membranes may be sprayed.
For the repair of inactive or dormant cracks, the portion of the crack adjacent to the surface is widened to form a slot of 25mm wide and 25mm deep with a power driven saw-tooth bit. The base of the slot should be slightly greater than the surface width. Now the slot is thoroughly cleaned and dried. After the slot is dry, the bonding coat is applied prior to placing the repair material. After finishing the surface it should be moist cured by tieing with wet burlap along the length of the crack.
b. Concrete Replacement:
Generally this method is used for large and deep patches as encountered in the repair of old and deteriorated portions of concrete structures where concrete is to be replaced to a minimum depth of about 15 cm. Usually this method is adopted for the repair of walls, piers, Kerbs, parapets and for refacing walls and relining channels. This method particularly has been found suitable for situations where the hole extends throughout the concrete section or where the surface area of the hole is at least 900cm2 or 0.09 m2 with a minimum depth of 10 cms for plain concrete and incase of RCC the minimum area of the hole should be 450 cm2 or 0.045m2 with a depth a little more than reinforcing steel bars.
Procedure:
As in other cases of repairs, the defective concrete is removed upto the sound surface and reinforcement is cleaned. In case, if it is not possible to place the repair material immediately, then a protective coating to reinforcement must be applied to protect it from corrosion.
In case of plain concrete, the defective area is prepared as explained above. In case of RCC a minimum clearance of about 25mm around the each exposed reinforcing bar should be provided. In case of wall repairs, the top of the hole should be cut to a fairly horizontal line with a 1 to 3 upward slope from back toward the face to prevent the formation of air pockets at the top during vibration. The bottom and sides of the hole should be cut sharp and approximately square as shown in Fig.26.9.
All interior corners should be rounded to a minimum radius of 25mm. For repairing the wall for more than 0.5m, the back form work is built in one piece and the front form work is made in horizontal sections to place concrete conveniently in about 30cm deep layers or lifts. All joints of the form work should be mortar tight. Before placing the front sections of the form work for each lift, the surface of old concrete should be coated with cement grout of the same water-cement ratio as that for the mortar in the replacement concrete.
To reduce the shrinkage, there should be a minimum interval of time of 30 minutes between the placement of two layers or lifts. The mix proportion and water-cement ratio of the replacement concrete should be the same as used in the structure. The water content should be used as low as possible. If possible, compaction of the repaired material should be done by internal vibration. In case external vibration is used, then care should be taken that the seals between the form work and the existing concrete are not damaged. The stripping period of the form work may vary from 20 to 48 hours depending upon the location and extent of repair.
c. Mortar Replacement:
This method is useful for cavities too wide for dry pack and two shallow for concrete replacement. Generally this method is used for shallow depressions not deeper than the thickness of side concrete to the nearest reinforcement bar near the surface. For replacement of deteriorated concrete this method is useful only for small patches. The mortar replacement can be done by hand or by using a small pressure gun.
It is preferable to pre-shrink the repair mortar by mixing it to a plastic consistency as long in advance as cement permits. The pre shrinking time ranges from 60 to 120 minutes. For hand placing the mortar should be of the same proportion as that of the mix of the structure. In case of pressure gun the mortar should have a cement sand proportion of 1:4.
In case the hole to be repaired is deeper than 25mm, then the mortar should be laid in layers not exceeding 15mm in thickness to avoid sagging and loss in the bond. The subsequent layers may be applied at an interval of 30 minutes or more. The final layer should be placed slightly overflowing the hole and struck off level with the surface.
d. Grouting:
The deep and wide cracks may be repaired by filling them with either neat cement grout or cement sand grout depending upon the width of the crack. The cement sand ratio may be kept 1:4. The water-cement ratio should be kept as low as practicable to maximise the strength and minimise the shrinkage. The water reducing agents may also be added to improve the properties of the grout.
Procedure:
The surface along the crack is cleaned first and then built up grout ports in the form of nipples or seals are provided at intervals. Afterwards, the crack is sealed between the ports with a cement paint etc. Before applying the grout, the seal is tested. After seal testing, the whole area is filled with grout mortar. After the crack is filled up, the pressure should be maintained for several minutes to ensure good penetration. This method particularly is useful for the repair of wide cracks in concrete walls and gravity dams etc.
For narrow cracks in concrete, chemical grouts consisting of solutions of two or more chemicals that can combine to form gel and a solid precipitate can be used advantageously. The chemical grouts are also useful in moist conditions and provide wide limits of control of gel time.
e. Large Volume Pre-Packed Concrete:
This method of repair is used for the repair of old works and usually is adopted in situations where placing of concrete by conventional methods is difficult. This method is used advantageously for the repair of larger jobs, under water placement, repair of tunnel lining, resurfacing of dams, spillways, retaining walls, piers etc.
Procedure:
Clean and well graded coarse aggregate is filled in the form work placed against the surface to be repaired. The aggregate is well compacted in the form work and wetted after compacting it. Now the grout of designed proportion of cement, fine sand and some pozzolanic material is pumped into the voids of the compacted mass of aggregate. The water content should be as low as practicable for the desired fluidity.
Some admixtures to the mixing water may be added to increase the fluidity of grout and to control early stiffening of the grout and mixing water. The form work must be a closed type, open only at the top to avoid trapping of air pockets. The form work may be of conventional rigid type that either encloses the member to be repaired or is sealed to it at its edges. To monitor the progress of aggregate grouting, sometimes transparent panels are provided.
The grout lines are attached to inlets or injection points fixed to the form. The pumping of grout should be started from the lowest point as shown in Fig.26.10 and proceed upwards in order to prevent the formation of the air pockets. In case complete filling from bottom requires very high injection pressure, more than one injection point, may be built into the form work at different levels.
The spacing between grout lines should not be more than 1.5 m centre to centre and the grout level in the mass of aggregate should increase uniformly, as determined by the observations of grout levels in the grout pipes. In the grout pipes above the level of the out lets a positive head of at least 1 to 2 m should be maintained. The injection of the grout should be smooth, uninterrupted operation. After the forms have been filled, positive head should be maintained till the grout has set.
f. Shot Creting or Guniting:
It is a mortar or concrete conveyed through pressure hose and applied pneumatically at high velocity on the surface. The application of this material has been found useful in several major repair works due to ease with which it can be applied on vertical, horizontal or overhead surfaces. Mostly it is used for tunnel lining and for protective covering of soft rocks, for repair or laying new canal and reservoir lining, for stabilizing of rock slopes and repair of old and damaged concrete work.
The objective of this type of repair is to replace the lost or removed concrete and to increase the effective cover to the steel reinforcement or to protect against the future damage by adding additional concrete.
Preparation of the Surface:
All the damaged or affected concrete should be removed upto the sound and strong surface. The undamaged surface also must be prepared by thoroughly roughening it, to remove all original cement laitance surface deposits and impurities. For the development of good bond, the prepared surface must be rough, sound and homogeneous.
When a large portion of the structure is defective, the whole of the accessible external surface should be encased with a specified minimum thickness of sprayed concrete, which should be brought out to uniform profile and eye able lines by additional infilling of the area from where damaged concrete has been removed. This can be achieved by applying sprayed concrete over the areas of un-damaged concrete also. The surface should be prepared as discussed above.
In case sprayed concrete is without polymer admixtures, generally a welded steel fabric or steel fibres are incorporated in it to minimize the risk of developing cracks of sufficient size, which may allow penetration of air and water. The incorporation of steel fabric encourages the development of a large number of very fine and insignificant cracks. A typical fabric for this purpose may be in the form of a mesh made of 3 to 4 mm diameter high tensile bars welded at a spacing of 75 to 100 mm.
With the use of such a fabric the minimum thickness of sprayed concrete should be 50 mm which will provide adequate cover to the fabric itself. The fabric reinforcement should be securely fixed by nails driven into the plugs set in parent concrete and bent over to grip the reinforcement. The spacers should be fixed to hold the reinforcement at 12 mm from the surface. The fixing points should be close enough so that the mesh does not bulge out during the concrete spraying. The thickness of sprayed concrete may be reduced if non corroding steel fabric is used. The minimum thickness of the sprayed concrete may vary from 30 mm to 50 mm.
Holes or chases of at least 20 mm wide and 20 mm deep should be cut at the perimeter of the area to be sprayed with the help of pneumatic tools. Into these holes the edges of the steel fabric will tuck to provide sound finish at that point. To check the correct thickness light timber profiles should be fixed securely at correct positions.
Specifications:
The maximum size of aggregate to be used in sprayed concrete should be 10 mm. It should be clean and well graded from 10 mm to 1.18 mm i.e. fines but there should be no excess of fine particles. It should be free from dust, silt and clay. The typical cement aggregate ratio may vary from 1:3.5 to 1:4. The strength of concrete at 28 days should be about 30 MPa.
Where ever possible the construction joints should be formed at a slope, not at right angle as in the case of cast concrete. Normally the face of the sprayed concrete is carried forward at a slope to form the construction joint sloping. To get a good bond at the joint the interface of the joint must be cleaned of rebound, overshoot and laitance etc.
In case defects are in isolated areas, patch repair may be carried out, but the repair should extend at least upto 300 mm on the sound concrete at the perimeter of the defective area and should terminate in chases. Rectangular patches are preferable to irregular shapes.
The spraying should be carried out with the nozzle held at right angles to the interface and at such a distance that the concrete compacts effectively. Usually 1 m distance from the face is sufficient. To form an even layer on the surface the nozzle should be banned from side to side and up and down. If there is any unevenness in the spray of material, the nozzle should be turned away from the work till the spray becomes even again. The thickness of layer at a time should not be more than about 50 mm on vertical surfaces and 25mm on overhead surfaces. The surface of each layer may be slightly trimmed with the edge of a steel float and must be wetted again once set before applying the next layer.
Curing:
The curing of sprayed concrete is more important even than that of conventional cast concrete as water loss in thinner sections may take place easily and can cause more serious adverse effects on its properties. The curing may be done by fine water spray, by tieing wetted hession etc. Curing compounds may also be used.
Curing of Repair Work:
The curing of patch material needs much more care than that of a complete structure due to the tendency of old concrete to absorb moisture from the replacement material. Same curing methods may be adopted as used for curing of conventional concrete.
B. Special Procedures:
The Polymers are of recent origin. They have been introduced recently in concrete technology for multipurpose applications in the repair and maintenance of concrete buildings and other structures. For concrete repairs mainly following two different types of polymers are used.
Polymers are natural or artificial compounds made from large sized molecules:
(a) Polymers used as modifiers for cementitious systems.
(b) Reactive thermosetting resins.
For the repair of active as well as dormant cracks, epoxy (a resin) mortars consisting of an epoxy, hardener, and sand have been found effective to seal the cracks. The epoxy resins and polymer possess excellent adhesive and sealing properties, but the cost of this type of repair is very high.
(a) Polymer Modified Cementitious System:
Normally polymers used as admixtures for cementitious systems are latex (a milky white dispersion in water). These polymers are used to gauge the cementitious mortar as a whole or as a partial replacement of mixing water. The polymer latex forms a network of polymer strands inter penetrating the cement matrix and improves the structural properties and reduces permeability of the mortar. Such mortars provide the same alkaline passivation protection to steel as the conventional materials do. These mortars can be placed in a single layer of 12 to 16mm thickness and provide adequate protective cover.
Functions of Polymer Latex:
The functions of the polymer latex are as follows:
(i) It acts as a water reducing plasticizer.
(ii) It provides a good bond between repair mortar and concrete surface to be repaired.
(iii) It improves the tensile and flexural strength of the mortar.
(iv) It reduces the permeability of repair mortar to water.
Modifiers for Cementitious Systems:
Following type of polymer latexes have been used as modifiers for cementitious systems:
(i) Poly vinyl acetate (PVAC).
(ii) Styrene butadiene rubber (SBR).
(iii) Polyvinyldiene dichloride (PVDC).
(iv) Acrylics and modified acrylics.
Generally styrene acrylics, polyvinyl dichloride (PVDC) are not recommended for repair mortars for RCC works. Polyvinyl acetate (PVAC) latexes are widely used as general purpose bonding aids and admixtures for building industry for interior applications. In concrete repair mortars styrene butadiene rubber (SBR) acrylic and modified acrylic latexes most commonly are used as admixtures.
(b) Resin Based Materials:
The resin mortars are used for repair in locations where the area to be repaired comparatively is less and the cover is less than 12 mm. In case of resin mortars the protection of reinforcement depends upon the total permeability of the cover. In this case the surface of the reinforcement should be prepared to a very high standard. The resin mortars are made with reactive resins and carefully graded aggregates.
Usually in concrete repair works epoxy resin mortars are used. The polymer and acrylic resin based mortars are used for small area repairs where very rapid development of strength is required. In most of the repair situations the polymer based repair material is bonded directly to the concrete or other cementitious materials. Thus it is important that the mechanical and physical properties of polymer repair mortar and concrete should be similar. The polymer bonding aids may assist in achieving a reliable bond between green un cured concrete and cured concrete.
Polymer Based Repairs:
The polymer concrete includes composites prepared by one of the following methods:
(i) Polymer impregnated concrete (PIC)
(ii) Polymer cement concrete (PCC)
(iii) Polymer concrete