Beams, slabs and columns are the most important components of the concrete structures. Due to the large number of cracks or damages observed in these components of the structures, the repair and rehabilitation of such members assumes greater importance.
Techniques of repairs for beams, slabs and columns are discussed below:
Repair of Concrete Floor and Slab System:
For the change of occupancy or to increase the life spans of concrete slab system, generally their repair or renovation and up-gradation is needed. Provision of services below the floor surface may also require the replacement of floor surface. These may require complete structural up-gradation or the repair of visual damage only or provision of a thin surface topping to existing concrete to produce a wearing surface having a good resistance to abrasion and wear.
Before carrying out the repair of the concrete floor slabs, the causes of damage must be investigated and specifications of the repair must be drawn keeping in mind the future use of the floor. The floor should be surveyed for the defects and the existing levels of the floor should be recorded with respect to a known datum. The existing services in the floors should also be plotted and tested for their satisfactory condition.
In many cases only the bonding coat may be sufficient while in other cases the topping over the whole area may be required. Before carrying out the repair operation, the damaged area and the joints should be repaired, otherwise these defects may reflect up through the new topping.
Under certain circumstances, the slabs in concrete structures may deteriorate in selective locations exposing the reinforcement. Scaling may occur due to an inadequate internal air void system in the concrete. Considerable scaling may also occur in the presence of moisture and also due to freezing and thawing in cold regions.
The surface delamination of concrete in the slab may also occur due to the corrosion of reinforcement. The delamination of concrete above and around the reinforcement indicates deterioration which is dangerous for the long term structural integrity. To reduce the further corrosion of embedded reinforcement, repair should be done to prevent the moisture infiltration into the concrete.
For a large extensively damaged slab system, the use of a thin methyl methacrylate polymer concrete over lay has proved very useful, as usually polymer concrete does not absorb moisture.
Preparation of Surface:
Each repair area should be marked or delineated by a saw cut 3 mm wide and 6 mm deep groove around its perimeter upto at least 100 mm outside of the damaged area as shown in Fig. 27.22. The entire area in-between this boundary should be scarified by suitable machine or by hand to remove the concrete upto a level below the damage to get a sound and clean concrete suitable for repair. After the scarification of the damaged area, the delaminated surface should be located by using a small chipping gun.
The reinforcement in the delamination area should be exposed and chipping continued till all concrete within 12 mm of the entire exposed portion of reinforcing bars is removed. To ensure that all delaminated unsound concrete has been removed, the prepared surface should be resounded again. To remove all corrosion by-products from the reinforcement, it should be sand blasted. Finally the entire repair area should be blown off with the compressed air to remove any loose corrosion particles, concrete, blasting sand and dust etc.
In case of cement sand mortar repair, the prepared surface of reinforcement is coated with a cement paste layer. This cement paste layer will provide additional protection to the reinforcement. After the coated surface has dried or cured, the area to be repaired is kept thoroughly wet for 24 hours if possible. Before filling the cementitious material consisting of 1:3 cement sand mortar of suitable workability by hand, all water from the surface should be wiped off or removed. For small repair works, the proprietary materials may be used which are carefully batched and their quality controlled. Due to the higher workability of the repair mix they should be only hand tempted.
For other cement sand mixes vibrating hammer with square plate bottom are often used, but for a large area a short beam fitted with form vibrator may be used for compaction. The repair is finished off with hand trowel. It should be allowed to cure for 7 days by covering it with polythene sheets.
For high quality local repairs, dry cementitious mortar materials along with a polymer as styrene butadiene, rubber latex etc., can be used.
The exposed surfaces of concrete and reinforcement are coated with a primer compatible with the repair material system. The primer can be applied by rolling it on the surface with a paint roller and allowed to cure. The primer coat on the reinforcement provides it an additional protection against corrosion. After the primed surface has cured adequately, it becomes impervious to moisture and could remain protected from environmental effects.
In case of deep sections, an initial bedding layer of polymer concrete is placed around the exposed reinforcement to ensure that the exposed reinforcement is fully surrounded or encapsulated by the polymer concrete. In deeply removed areas the polymer concrete is spaded beneath the reinforcing bars to remove the air pockets. Then the chipped areas are back filled with sand loaded polymer concrete. Over the exposed reinforcement a thin coat of, neat polymer concrete followed by a 6 mm of polymer concrete layer is applied.
Before applying the second layer of polymer concrete to build up the cover thickness in the area over the reinforcement, the first applied material should be allowed to cure. To provide a minimum thickness of 6 mm polymer concrete over the entire repair area, a final layer of polymer concrete is applied. Over the exposed reinforcement a minimum 12 mm polymer cover is desirable.
After curing the final layer of polymer concrete, the joint between the repair material and unrepaired concrete should be sealed with the same primer as placed on original prepared concrete surface.
In deeper patches, to control the shrinkage, the material should be applied in layers. At higher temperature above 21°C, the rapid evaporation of polymer liquid reduces the working time available before the material gains the initial set. Thus placing, screeding and troweling must be done quickly. In such situations working at night or early in the morning gives a better control for reducing ambient temperature.
Over Lays and Surface Treatments of Concrete Works :
In active or dormant cracks in structural and pavement slabs both can be repaired by laying bonded overlay or topping. Mostly cracks developed due to variation in loading, moisture and temperature are subjected to movement. These cracks will reflect through any bonded overlays and the crack repair will be ineffective, however drying shrinkage cracks can be repaired effectively by the use of overlays.
Fine dormant cracks of decks and slabs can be repaired by the use of overlay of polymer modified Portland cement concrete or mortar. In Highway Bridge a minimum thickness of 40 mm of the overlay should be used. For such repairs polymers such as latexes of styrene, butadiene acrylic, non-re-emulsifiable polyvinyl acetate and certain water compatible epoxy resin system can be used. The minimum quantity of resin solids should be 15% by mass of the port-land cement.
Before applying a topping or overlay, the old floor slab should be shot blasted to make it rough to develop good bond between the old surface and topping. After shot blasting the surface should be well cleaned and saturated with clear water. The localized depressions or damages should be repaired before applying cementitious topping. Usually two types of toppings may be used, namely bonded or un-bonded toppings. The thickness of the topping is governed by the strength and thickness of the old floor slab.
These toppings require bonding aids such as resins, polymers and cementitious grout. A cementitious grout of creamy consistency can be applied by brush on the floor slab immediately before placing the topping mix. The proportion of mix of 1:1:2 by mass of cement sand and 10 mm coarse aggregate has been found quite adequate for topping. The sand used should be medium grade (zone II) and the coarse aggregate should be clean and hard. Generally granite aggregates are used.
In addition to granite, flint or quartzite gravel, ballast and hard lime stone can also be used. The quantity of water to be added should be minimum to attain full compaction. The topping mix should be laid in 20 to 40 mm thick layers in bays such that the construction joints of old floor must reflect up through the topping.
The mix should be compacted on the old floor and troweled level at the intervals while topping is hardening. After final trowelling the topping should be left for curing by covering it with polythene for at least 7 days.
In case of polymer modified port-land cement topping, a bond coat consisting of broomed latex mortar or an epoxy adhesive should be applied immediately before placing the topping. The polymer modified topping should be mixed, placed and finished rapidly within 15 minutes in warm weather. 24 hours curing of such toppings is sufficient.
This is an additional slab laid over the old floor slab, hence no surface preparation is required. However the construction joints in the old floor must be reflected through the new slab. As a damp proof membrane properly lapped polythene sheets are laid over the base slab. Concrete of M30 grade having a cement content of 350 kg/m3 should be placed and compacted. The concrete should be laid in bays of sizes upto 15 m2.
The thickness of concrete layer may be 100 mm. Before hardening of this concrete a high strength topping of 10 to 15 mm thickness should be placed over it and compacted on the surface. The mix of the topping may be as discussed above. The trowelling during the hardening and curing should be done as in the case of bonded topping.
Reconstruction of Slabs:
In case of broken slabs, it is preferable to reconstruct it afresh. In case of ground floor slabs, the sub grade should be inspected, compacted and brought to correct level by using lean concrete about 150 mm thick or well graded crushed rock material. A polythene sheet should be placed over the top of the sub base to act as a damp proof layer before concreting. The concrete should be fully compacted with any suitable method, finished and cured for at least 7 days by covering it with polythene sheet. It should not be loaded for 28 days.
Repair of Beams:
In case of extensively damaged beams additional reinforcement at the bottom of the beam together with the new stirrups should be provided. The stirrups either can be anchored by expanding bolts set in the sides of the beam below the slab soffit or may be taken right round the beam through the holes drilled in the slab. To provide a good bond between the old and new concrete the old surface should be roughened. The new concrete may be placed by guniting. If required shear connectors may also be provided by expanding bolts etc.
A layer of material which adheres to the surface and forms a continuous membrane or film is known as coating. Generally coatings adhere to the concrete and form membrane after their application. Concrete basically is a permeable or porous material. The porosity of concrete is developed due to the evaporation of about 60% water added to it for workability. After placing the concrete it retains about 25% water as water of crystallization and 15% water as gel water during its curing period.
The evaporation of water forms capillary pores which allow ingress or diffusion of carbon dioxide and other gases into the concrete. These gases and carbon dioxide dissolve in the pore water to form acidic solution, which further react with the ingredients of concrete and damage it. The porosity of concrete also allows ingress of water containing harmful reagents in solution, which are a potential source of damage to the concrete.
Concrete is strongly alkaline and is susceptible to attack from acidic reagents. Thus for the protection of the structural concrete, coatings of suitable materials are applied.
Types of Coatings:
1. Anti-Carbonation Coatings:
The coatings applied to the concrete surface to check the process of carbonation are called anti-carbonation coatings. The carbonation is the process in which carbon dioxide reacts with pore water forming carbonic acid, which reacts with calcium hydroxide and forms Calcium carbonate CaCO3 In the process other cement compounds are also decomposed damaging the concrete.
The anti-carbonation coatings are based on chlorinated rubber, polyurethane resin or acrylic emulsions. These coatings may be effectively used to resist carbonation and general atmospheric deterioration of reinforced concrete. In situations where the spalling and corrosion are more wide spread, the use of anti-carbonation coatings has not been found satisfactory.
2. Coatings to Resist the Acidic Effects:
In industrial areas, the concrete structures occasionally are subjected to abnormally acidic environment due to the release of sulphur dioxide from steel plants, oil refineries and power stations etc. into the atmosphere. These gases readily dissolve in atmospheric moisture and rain water forming sulphurous acids. Concrete in constant contact with such waters disintegrates easily. Thus to protect the concrete from such environment, coatings of highly chemical resistant materials should be provided. Under most circumstances two parts of polyurethane coating has been found most suitable.
3. Coatings to Protect Cracked Concrete:
The very fine cracks not considered structurally significant may be protected by applying coatings over the cracks locally. These are known as conventional coatings. The coatings should have flexibility and ability to bridge the cracks. Epoxy polyurethane and high build polyurethane formulations have been found very successful to protect the cracked concrete.
Leak Sealing in Concrete Structures:
The leakage of water in concrete structures is an inevitable source of damage to the reinforcement. The construction joints, shrinkage and restraint cracks form the leak paths. The amounts of leaking water vary from damp patches to running leaks. In case of damp patches the water evaporates from the patches so formed and in case of running leaks, pool of water is formed on un-drained surfaces.
Honeycombed concrete, expansion and contraction joints are also the common routes for heavy leakage. Damp patches may also be formed due to the passage of water through voids formed due to plastic settlement along the reinforcing bars.
In case of water retaining structures, the extent of leakage may be determined by monitoring the loss of water from the structure. According to B.S. 5337 the structure may be taken as water tight if total drop in surface level does not exceed about 1.5 mm in 24 hours.
For effective leak sealing it is essential to identify the sources and routes of leakage. Due consideration must be given to the likely cause of leakage and the behaviour of the structure during the service period.
Techniques of Leak Sealing:
Leak scaling is very expensive it should be taken up when it is necessary.
Leak sealing methods may be classified as follows:
1. Conventional leak sealing methods.
2. Leak sealing by injection method.
1. Conventional Methods:
Minor sources of leakage may dry up by autogenous healing, which is the accumulation of calcium salts along the path of leak. The accumulation of salts will obstruct the passage of water and reduce the leakage to negligible proportions. After identifying the leak spots, the remedial measure may involve the local or total surface seal in the form of a coating system.
Following sequence of action may be adopted:
(a) Surface preparation.
(b) Filling of surface dents or depressions etc. known as imperfections of surface with-resin based grouts.
(c) Application of primer.
(d) Finally application of two coats of high builds paint.
The suspected joints and random shrinkage cracks are filled with the injection of a low viscosity resin. Short patches of honeycombed concrete should be filled with a resin based mortar of putty. Laitance and surface contaminants should be removed by sand blasting and power wire brush. Thus the predatory work is quite extensive.
The movement joints, as expansion and contraction joints may be sealed by filling a resin into them. On hardening or curing this resin will form a flexible sealant. The concrete joints must be thoroughly prepared and cleaned before the application of the sealant. If required an appropriate primer or bonding coat should also be applied.
2. Injection Sealing:
From pressure and liquid flow considerations the simplest and most cost effective way is to seal the leakage from the water retaining side of the structure. In case it is not possible to reach the wet side, the leakage can be tackled from the dry side which is considerably more difficult than wet side. To seal the leakage successfully, the water passage must be filled with grout (sealant) completely. As the working time of the typical repair material is very short the velocity of flow of the material should be kept very high to fill the water passage fully.
To seal the leakage, the first basic requirement is to restrict or confine the water flow to a tube through which the sealant may be introduced. Once the flow of water has been controlled, the connection between the tube and concrete must be made strong enough to withstand the injection pressure. During injection process, the concrete may be over stressed, hence it is preferable to maintain low pressure.
The grout may be injected by the following two methods:
(a) Direct method.
(b) Indirect method.
(a) Direct Method:
In this method the grout or material is injected against or up the pressure gradient from the downstream side. The direct methods are very slow as the grout or sealant is pumped very slowly through very narrow passages against pressure and the pressure cannot be maintained for long enough to achieve complete penetration. In many cases water may find another finer path way leading from the same source.
(b) Indirect Method:
In this case the grout or sealant is introduced on the pressure side so that the path ways are filled under the acting hydrostatic head. By this method the work of sealing the leakage can be completed quickly as surface seals are not required and mechanical anchorages may be used.
Under Water Repairs of Concrete Stuctures:
For under water repairs, same methods may be adopted as for above water i.e., dry repairs with modifications as per need. However the materials used in dry repair i.e. above water repair are often unsuitable for under water repairs.
To decide the use of material and technique for under water repairs laboratory trials should be carried out. Laboratory trials should also be carried out to identify the possible problem areas and to ensure smooth site operations. As usual, before undertaking the repair work, the damaged area should be cleaned of marine contaminants. This will help in detailed inspection for assessing the extent of damage.
In case of smaller areas this can be done by using mechanical wire brushes, needle guns or scrabbling tools etc. In case of larger areas a high pressure air jet may be used. After cleaning the surface the extent of cracked and sapling concrete should be ascertained either with the help of divers or remote operated vehicles to photograph the area.
Fire Damaged Concrete Structures and Its Repair:
The damage caused to a reinforced concrete structure by fire may be classified into the following two categories:
1. Completely Destroyed or Burnt:
In this case whole of the damaged portion has to be replaced during restoration operation of the structure.
2. Slightly Damaged or Deformed:
In this case only the repair and finishing of the damaged portion is sufficient.
The extent of damage caused to a R.C.C. structure during a fire depends on the duration of the fire and the temperature the structure experienced during the fire.
High temperature during a fire reduces the strength of reinforced concrete structures due to the following causes:
(a) The change in the strength and deformability of materials.
(b) Reduction in cross-sectional dimensions of the structural members.
(c) Weakening of bond between the reinforcement and the concrete, which determines the structural action under the load.
Normally the maximum temperature reached during a fire is estimated indirectly i.e., from the melting of metallic or other non-combustible articles. From statistics available on the damaged R.C.C. structures by fire, the duration and maximum temperature reached during the fire are shown in the following Table 27.1.
Thus the duration of fire and the maximum temperature reached vary over a wide range. Temperatures of 1000 to 1100°C in fires lasting from 1 to 2 hours have been observed more frequently than 1300°C.
An accurate estimation of the performance characteristics of structures damaged in a fire helps in taking effective measures of restoration. The performance characteristics take into account the physicochemical and mechanical properties of the materials burnt and that of heated concrete.
There is an accumulation of irreversible damages of mechanical and physicochemical factors. Under mechanical factors, creep, cracking, shrinkage and plastic deformations may be classified, while under physicochemical factors corrosion, absorption and degradation etc. may be classified.
These informations enhance the reliability of estimation of residual load carrying capacity of the structural members, resulting in a considerable saving in the cost of restoration of the structure. However from this information the determination of the physicochemical characteristics of materials and geometric dimensions of the structure is difficult.
The strength and stiffness of concrete and steel decreases as the temperature of the member increases and dimensional changes take place. The changes in strength and stiffness of the concrete are influenced by the constituent elements of the concrete i.e. the type of cement and aggregate and water content.
The cracks or spalling develops in beams, columns and slabs due to the development of stresses caused by thermal strains. The development of cracks and spalling decreases the area available to resist the applied loads or forces.
The behaviour of different structural elements damaged in fire is discussed below:
Normally the failure of axially loaded columns takes place at mid height due to brittleness. The failure occurs due to the disintegration of concrete in the whole section accompanied by buckling of longitudinal bars. Due to the fire, a large variation in temperature between the concrete of periphery and centre of the section takes place.
The range of this temperature variation has been found 800°C or even more. This variation in temperature causes variation in the strength of concrete. The strength of concrete varies along the cross section. The central portion of the section keeps its original value of strength while it reduces to zero at the peripherical surface.
The temperature at which the crushing strength of the concrete is reduced to 50% of its initial value is known as critical temperature. The value of critical temperature depends upon the nature or type of aggregate used in the concrete. The value of critical temperature for concrete made with sand stone or granite aggregates is 550°C and for concrete made with lime stone is 700°C.
It has been observed that quartz, granites and gravels expand steadily upto about 573°C. At this temperature they go a sudden expansion of 0.85%. This expansion develops a disruptive action on the stability of concrete. At this temperature the fire resisting properties of the concrete are least if the quartz is the principal dominant mineral in the aggregate.
Amongst the igneous rock aggregate, the best fire resistant aggregates are dolerites and basalts. The lime stone expands steadily upto a temperature of about 900°C and then begins to contract due to decomposition and liberates carbon dioxides. As the decomposition of lime stone takes place at a very high temperature of 900°C, the dense lime stone aggregate has been found a good fire resistant aggregate. The blast furnace slag aggregate is regarded as the best fire resistant aggregate. Broken brick aggregate also has been found as a good fire resistant aggregate.
On the basis of extensive research work, it has been found that even the best fire resistant concrete will fail if it is exposed to a temperature more than 900°C for a considerable period. The serious reduction in strength takes place at a temperature of about 600°C. Upto a temperature of about 300°C Concrete does not show appreciable loss in strength, but at about 500°C the 50% strength of concrete is lost.
Due to the non-uniformity of temperature in the cross-section, the hottest layers of concrete and main reinforcement bars near the surface of the column are separated due to the thermal creep and loss of strength and also due to the contraction in concrete. This results in increased stresses in the centre of the section where the moderately hot concrete retains its strength and elasticity. The complete failure of column takes place when the stresses in the central portion of the cross-section become equal to the initial prism strength of the concrete and deformation approaches its limiting value of 0.0025 to 0.0030.
Rehabilitation of Fire Damaged Elements:
1. Ecentrically Loaded Columns:
The failure of ecentrically loaded columns takes place when the reinforcement bars in tension heat up. In such cases the fire resistance of the structural elements can be increased by increasing the thickness of the protective cover to the reinforcement.
2. R.C.C. Slabs:
The behaviour of R.C.C. slabs exposed to fire is governed by the temperature of bottom reinforcement and heat transmission in concrete. The reinforcing bars are assumed to retain 50% of their original strength. The carrying capacity of slabs can be increased by increasing their thickness.
3. R.C.C. Beams:
Due to heating up, the bond between the transverse reinforcement and concrete is weakened considerably. The weakening of bond between the concrete and transverse reinforcement reduces the residual shear load carrying capacity to a great extent. The depth and width of the beam can be increased. The required increase in the dimensions of beam, longitudinal and transverse reinforcements should be calculated by taking into account the change in compressive strength of concrete and modulii of elasticity of concrete and steel.
4. Axially Loaded Columns:
The carrying capacity of axially loaded columns depends upon the cross section of the column, coefficient of change in strength of concrete due to high temperature and corresponding critical temperature. The carrying capacity can be restored by increasing the cross section with suitable increase in the longitudinal reinforcement.
For restoring the damaged structure the reinforcing bars can be anchored conveniently into the existing concrete walls and foundations by drilling hole in the concrete somewhat larger than the diameter of the bar and epoxy gel coated bar is set into the hole.
In case of slabs also, the epoxy coated reinforcement may be used, but the thickness of epoxy coating should not be more than 0.25 mm.