HOW TO REPAIR RCC BEAMS & COLUMNS DAMAGED BY STEEL CORROSION ? AND CORROSION OF CONCRETE REINFORCEMENT – CAUSES & REMEDIES
HOW TO REPAIR RCC BEAMS & COLUMNS DAMAGED BY STEEL CORROSION?
REPAIR OF CRACKS IN REINFORCED CEMENT CONCRETE BEAMS & COLUMNS
The primary cause of corrosion of steel in concrete is due to carbonation or due to chloride. Corrosion is electro-chemical process which results in formation of rust. The volume of rust formed due to steel corrosion is more than the volume of original steel. This increase in volume of steel inside the concrete causes cracks in concrete. Sometimes this causes falling down of concrete cover.
STEP-BY-STEP PROCEDURE
The following step by step procedure will guide you to repair a damage potion of RC beams & columns.
STEP-1 (PROVIDE SUPPORT)
Beams & columns carry heavy loads. Therefore the first step is to relieve part of its load. This is done by providing supports or prop up at beam column joints. Remove the damaged concrete until corroded steel reinforcement is visible.
STEP-2 (CAUSE ANALYSIS)
In this step, the steel is examined for the type of corrosion. This is done by phenolphthalein test. This test will help to find out whether it is carbon induced corrosion or chloride induced corrosion.
Note:
Here we are dealing with only carbon induced corrosion of steel. Because chloride induced corrosion is difficult to identify and this demands for replacement of whole concrete near steel.
STEP-3 (PROPER CHIPPING)
If the steel is fully corroded, then remove concrete from 15 mm to 25 mm around the steel. This will help to clean the steel well and all-round.
STEP-4 (CLEANING)
Using water or any rust removal solutions, clean the concrete and steel surface thoroughly.
STEP-5 (FIXING NEW STEEL)
Examine the percentage of corrosion of steel. If 15% or more portion of steel is affected by corrosion, then bind additional steel with the old steel. Instead of binding we may also weld it with old steel. Put the required shear reinforcement for beams and binders for columns.
STEP-6 (APPLY BONDING COAT)
After fixing additional steel with old steel, apply a proper bonding coat over the old concrete and steel surface. Bonding coat is used to bond the new concrete to the old concrete. 4 categories of bonding coat commonly used for repair works are as follow:
Epoxy based bonding coat
Acrylic based bonding coat
Styrene butadiene rubber (SBR) based bonding coat
Lingosulphate based bonding coat
STEP-7 (FORMWORK VS WIRE MESH)
If the volume of damaged portion of beam or column is very large, then it is suggested to assemble the formwork for columns and to use expanded wire mesh for beams.
STEP-8 (PLACE NEW CONCRETE)
Prepare a micro concrete mix made of cement, sand, coarse aggregate (of size 10 mm & below) and super-plasticizer. The water-cement ratio should not be more than 0.5. Place this concrete within the empty spaces in the member. Remember placing of concrete should be done before the bonding coat dries.
Note:
If the area to be repaired is small, then instead of using concrete, use polymer modified cement mortar.
In case of large area repair work, shotcrete/gunite method of concrete placing is suggested.
STEP-9 (APPLY FINISHING COAT)
Apply cement plaster of ratio 1:3 (cement : sand) as finishing coat. This should be done within 48 hours of concrete placing.
STEP-10 (CURING)
Cure the repaired work for at least 7 days.
STEP-11 (WATERPROOF COAT)
To protect the repaired member for further attack by water, apply a final coat of waterproof paint on th surface of the member.
Great care is to be taken to keep the repaired patches moist for several days. In general much amount of care is taken to cure material in patches than the whole structure. The reason is, being relatively small volume of repairs and the tendency of old concrete, around it, to absorb moisture from the new material. Curing must be started as early as possible after the patch is finished to prevent early drying. The various methods used for curing repaired surfaces are given below.
METHODS OF CURING REPAIRED CONCRETE WORKS
To keep repaired surfaces wet for the initial curing, water should be applied at regular intervals by a large brush or spraying device.
Damp hessian may be used or wet burlap pads can be used.
In some cases it may be difficult to hold damp hessian or wet burlap pads in position. In such a case a membrane curing compound is best suited. Best results are obtained by initial curing with water followed by sealing compound, when concrete surface is still damp.
Deliquescent salts such as calcium chloride may be used to hasten hardening and help to keep the patch in moist condition.
Horizontal surfaces can be cured by spraying method or by placing wet gunny bags.
CORROSION OF CONCRETE REINFORCEMENT – CAUSES & REMEDIES
CORROSION OF CONCRETE REINFORCEMENT
Corrosion is a chemical process of destruction of material because of its reaction with the environmental conditions. The most predominant among various factors of corrosion is the atmospheric corrosion which causes the rusting of steel. Appreciable corrosion only starts when the relative humidity of the air exceeds around 65%. In dry, pure air and below freezing point of water there is no danger of the corrosion.
Corrosion may be defined as the involuntary destruction of substances such as metal and mineral building materials by surrounding media.
Corrosion of steel results in reduction of cross-section area of steel and also causes cracks and splitting of cover concrete. Due to reduction of cross-section, the load carrying capacity is reduced, in addition to reduction of elongation properties and fatigue strength.
FACTORS INFLUENCING CORROSION OF REINFORCEMENT
In reinforced cement concrete construction the corrosion of reinforcement takes place due to the presence of chlorides and sulphates beyond a critical limit and when sufficient alkalies is not obtained within the concrete to maintain steel in a positive condition.
The following factors are responsible for corrosion of steel reinforcement in concrete structure.
1. QUALITY OF CONCRETE
Concrete consists of coarse aggregate, fine aggregate, cement and water. The right quality of materials with proper w/c ratio, correct mixing, adequate compaction by tamping or vibration and proper curing results in good quality concrete. If any of the above mentioned steps are not done in a specified manner, then that will result in a not so good concrete and there is a chance of corrosion of reinforcement.
High strength concrete, i.e. dense concrete is impervious to a large degree and generally resists the carrion of embedded steel.
2. COVER THICKNESS OF CONCRETE REINFORCEMENT
The reinforcement is protected by suitable concrete covering over it. The greater the cover thickness more is the degree of protection against the various climatic and other environmental conditions. For various structural members, the cover thickness should be different depending upon their importance and degree of exposure. Evenness of concrete cover over the reinforcement is also very important for its corrosion protection.
3. CONDITION OF REINFORCEMENT
The surface condition of the steel reinforcement, at the time of its placing in concrete, affects its corrosion rate. If the reinforcement is contaminated with salt or badly corroded, the corrosive action on reinforcement after placement in concrete is promoted rapidly.
4. EFFECT OF ENVIRONMENTAL AND OTHER CHEMICALS
Chemical either from environmental or from within the concrete making materials are the main source of deterioration process. Due to attack of chemicals, the concrete develops cracks, which is the first sign of deterioration. The effect of chemicals is mainly due to presence of salt, carbonation, chloride attack and reaction of sulphates with tricalcium aluminate (C3A) present in cement.
Concrete is an intimate mixture of cement, aggregate and water which in the green state is highly alkaline. The hydration of cement develops calcium hydroxide which increases the pH value up to 12.5. In such alkaline conditions, the reinforcing steel is covered with a film of oxide which protects the steel.
5. POROSITY OF CONCRETE
The penetration of aggressive chemicals is possible due to permeability or porosity of concrete. the porosity of concrete depends on size, distribution and continuity of capillary pores. This depends upon the w/c ratio for given degree of hydration. The porosity also depends upon other factors, such as
Age of concrete
Degree of compactness
The size and grading of aggregate
Type of cement
6. EFFECT OF HIGH THERMAL STRESS
Normal concrete can withstand temperature upto 1000C. Beyond this temperature the deterioration of concrete starts. The concrete in industrial plants and power stations required special protective measures otherwise the concrete may develop thermal cracks. Cracked concrete structures are consequently affected by the environmental chemical and the process of corrosion starts.
7. FREEZING AND THAWING CONDITION
In cold regions, the moisture in the pores of concrete freezes. The ice formation gives rise to volumetric expansion which may excess bursting pressure of surrounding concrete mass. This results in development of cracks in concrete and can lead to corrosion of reinforcement.
REMEDIAL MEASURES TO PROTECT REINFORCEMENT FROM CORROSION
The deterioration of concrete may be due to either corrosion of concrete / reinforcement steel or formation of expansive chemical compounds such as calcium silicates hydrate (C-S-H) in aggressive environments. The loss due to corrosion of steel is heavy. To produce durable concrete and resist the harmful effects of aggressive environment, the concrete should be produced with utmost care.
The following methods will help to protect concrete reinforcement from corrosion.
1. IMPROVING THE QUALITY OF CONCRETE
By adopting the rich mix: Higher cement content and lower w/c ratio give stronger and impermeable concrete
Adopting the best mix proportion: By designing the best suitable mix proportion the impermeable concrete can be produced
Efficient compaction during casting: This gives dense concrete with minimum voids
Leak proof formwork: This reduces the leakage of cement slurry during casting of concrete.
Adopting salt free sand: The salt content of mix can be reduced by washing the sand properly.
Using plasticizers: The use of plasticizers improves the workability without increasing the water content
Using sulphate resisting cement and Pozzolana cement
2. INCREASING DEPTH OF CONCRETE COVER TO REINFORCEMENT
Extra cover depth lengthens the time for ingress of corroding agents. Such a remedy increases weight due to additional concrete requiring changes in structural design. Increased cover thickness should be provided when surfaces of concrete members are exposed to the action of harmful chemicals, acids, vapors, saline atmosphere, sulphurous smoke etc.
As per observation, the increase in cover thickness may be between 15 mm and 40 mm, the total cover thickness should not exceed 50 mm. concrete cover more than 50 mm is not recommended as it may give rise to increase crack widths which may further allow direct ingress of deleterious materials to the reinforcement.
3. CONCRETE COATING AND SEALERS
When untreated reinforcing bar is used, the best method is to apply protective coatings to concrete surface to seal entry of moisture, carbon dioxide and chlorides.
The dry concrete surface should be roughened by chiseling. Then, a workable mixture of 1:3 cement sand mortar should be applied on the concrete surface after watering over the surface properly by trowelling to a thickness of 6 mm. The surface should be finished with neat cement slurry consisting of water and cement in the ratio 2:1.
4. GALVANIZING
In this type of treatment, Zinc itself becomes a sacrificial anode and then protects the bar from corrosion for five years approximately. This method is used when no superior treatment is available.
5. FUSION BONDED EPOXY COATING (FBEC)
Today the world over, fusion bonded epoxy coating (FBEC) has proved to be most effective, reliable and long-term economical method of anti corrosive treatment for reinforcing bar.
It is applied directly on the reinforcing steel which prevents corrosion by isolating and insulating the steel from the corrosive environment. These coatings protect against external and internal corrosive agents.
6. COATING OF REBARS
The corrosion of rebars can be prevented by applying proper coating to rebars. The coating can be one of the following:
Paint
Chemical compound
Metallic epoxy coating
Fusion bonded epoxy
7. PROPER STORAGE & STACKING OF REINFORCING STEEL
Steel reinforcement should be stored in such a way as to avoid distortion and to prevent deterioration and corrosion. It is desirable to coat reinforcement with cement wash before stacking to prevent scaling and rusting.
In case of long storage, reinforcement bars should be stacked above ground level by at least 150 mm. Also in coastal area or in case of long storage; a coat of cement wash shall be given to prevent scaling and rusting.
WHAT ARE THE EFFECTS ON CONCRETE DUE TO SEGREGATION?
Following are the effects on concrete due to segregation.
Segregation in Concrete
EFFECTS ON CONCRETE
A segregated concrete is very weak in strength
Due to segregation after placing, the concrete in the lower part of a pour of any significant depth can be stronger than that in the upper part.
Segregated concrete does not give a homogeneous mass throughout the structure
Rock pockets, sand streaks and porous layers in hardened concrete are the result of segregation.
Due to segregation excess mortar comes to the top of the surface, which causes plastic shrinkage cracks.
A segregated concrete is difficult to compact properly
Structural failures in concrete structures, a part of human failures, occur as long as man and structure exists. In the past man’s comprehension of the structural response was simple and straight; but in today’ complexity of the man and his structure, the structural failure phenomenon is of a multi-dimensional and multi-disciplinary character.
Failure is often stated as the stepping stone to success, but there is a high price to pay in terms of energy, time and money. Nobody wants a failure but yet they occur. Lessons from failures are everlasting, revealing and often shocking.
We define failure as the absence of a derived function, goal or objective, mission, task or purpose; failure is the opposite to success and there is no easy way to define each of them.
What is Structural Failures of Concrete Structures?
Structural failures refer to the absence of its desired / designed / intended performance, behavior, response under all expected environmental conditions (loads, forces, etc.).
There are tension, compression, shear, flexure and torsion failures, occurring singly or in a combined state.
The classical notions of factors of safety have undergone tremendous changes giving rise to partial safety factors and limit state factors. There is undoubtedly a great rationale in the stipulation of these factors to design but fabrication, erection and assembly factors to application are left entirely to the field conditions.
Material failures in structures are viewed as ductile or brittle failures, or sometime as transition ductile brittle failures. Soil and concrete media have their own unique failure mechanisms. Steel is largely governed by ductile failure.
Structural failures of concrete structures often imply large and unwanted deformations, severe honeycombing and cracking with spalling, relative displacement of supports and ultimate collapse.
In a damaged structure, the vexing issues that arise are:
What is the extent of damage and how to quantify the same as required in the strengthening calculations?
What has been the rate of decay of the material properties and what realistic values should be assessed for strength assessment at that point of time?
What is the mode of treatment to be adopted and what is the life span of such treatment?
What is the cost benefit ratio of salvaging a damaged system?
What should be the criterion for demolition and how to accomplish the same?
Buildings (dominant in civil engineering) do not normally fail; but they are always in a damaged state over a period, with faults, defects, cracking, decay, spalling, ground settlement etc., very pronounced with consequent changes in the structural soundness and psychological human perceptions to comfort and safety.
Structural control and testing are intimately interlinked. Testing need not be destructive (DT) while today Non-Destructive Tests (NDT) has become the routine rather than the fashion. Structural testing calls for a philosophy, technology and methodology and an ability to interpret what is observed and infer what is invisible.
Structural failures in Indian and Western environments have some striking differences; our perception and reaction to failure is highly subjective. They learn from past mistakes, while we repeat them.
We shudder to admit our faults, discrepancies and deficiencies and we push them under the carpet. Sometimes we pass the buck on to a weaker neighbor. For us failure means end of everything in life.
While we know that failure is a better teacher than success, in practice we are not prepared to pay the price to learn the art of success. The administrative, financial and legal overtones coupled with enormous delays in post mortem, have made us shudder to think of failure.
Money and time are in opposite senses and are a great premium to us, and we always prefer to “play safe”. Structural soundness and cost effectiveness rarely go together. This is in short, our story to structural failure.
Issues pertaining to failed structures, such as collection and compilation of data and evaluation of the most probable cause of the collapse, emerged as a special branch of civil engineering. The science of material chemistry, material testing and in situ strength assessment — all this put together gave rise to a very absorbing technique known as “forensic engineering”, which is now about three decades old.
Interestingly old structures are still more or less performing satisfactorily. The problem is observed with structures recently constructed in the early fifties or later. This paradox of better performance of old structures vis-a-vis recent structures, offers an interesting insight into the quality of old structures and why and where we are now lacking.
At this stage, it is necessary to analyze our activity, to identify grey areas, which we have inadvertently permitted to continue in our set up. Startlingly, in spite of advances, in technology, we have not changed our basic set up. This could be one of the major contributors to our handicap.
There have been rapid advancements in almost every technological field. Civil engineering is no exception. There has been remarkable progress in the use of new materials, new design techniques, improved provisions, codal awareness, construction methods and user needs etc.
This has been backed by computerized support in planning, design and construction management. However, despite all this back up of development and advancement, the construction industry’s performance has not been satisfactory.
There is a kind of awareness to maintain the interior of a structure but not the common exterior or the material as a whole. We are more interested in beautifying the interiors like fixing marble slabs and resorting to costly painting but are totally oblivious and least interested about the basic quality of the material.
We spent liberally on the interior decoration but are hesitant to spend more on the quality of concrete and reinforcement. Our basic attitude needs to be revised.
Malfunctioning of structure performance, is on the increase. Failures have become more common than we would like to accept. They are somehow ill-reported and not discussed due to involved complexity, and a sort of aversion by those involved.
It is important that failures are discussed threadbare. This will create necessary awareness about the extent of malfunctioning, leading to timely remedial action..
Thank you for sharing your thoughts and knowledge on this topic. This is really helpful and informative. I would love to see more updates from you.
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