Why Investigation of Reinforced Concrete Structures for Repair and Maintenance Required?
Concrete is one of the most versatile man made construction materials of our times. Concrete by its flowability, in most complicated forms while wet, and its strength, development and durability characteristics when hardened, has gained a reputation as the most widely suitable material for most types of present day constructions.
It is well known that the embedded reinforcement in the concrete remains protected due to high alkalinity of concrete and in turn provides durability to the reinforced cement concrete. Concrete should be designed for durability.
But due to numerous specifications and multiple requirements to be complied with and coupled with adverse climatic conditions, the engineer at site faces many practical problems which reflect later the long-term durability of concrete.
Therefore, it is a myth that even good concrete is maintenance free; it does require periodical inspection, maintenance and timely repairs to ensure the designed durability.
The lack of awareness of the owner about the maintenance and repair of the structures leads to the neglect of timely remedial measures and the consequences are the same problems which manifest themselves in greater magnitudes after some period, where the durability and stability are not only impaired but also economically the cost of rehabilitation is changed multiple times.
A simple rust stain on the exterior, if neglected leads to the cracking and spalling of the protective cover and if still not attended to can lead to corrosion of reinforcement, even leading to impairing the structural stability of the concrete.
Moreover, it should be noted that repair or rehabilitation of distressed or damaged structures is quite different from the design of new structures, where the unknowns are either known or assumed. The reason for the distress has to be accurately identified before taking subsequent steps for repairs.
The latest techniques for diagnosis and developments in the fields of materials for such repairs should be resorted to. This in turn calls for absolute expertise in investigating as well as in the material science. An early diagnosis and immediate remedial measures can provide by far cheaper solutions than the neglect of the same.
Visual Signs and Type of Repairs for Reinforced Concrete Structures
When the structure is distressed or damaged the normal visual signs are: cracks different patterns and sizes; rust stains or rust spots; peeling of plasters etc.; spalling of concrete; and, rusted reinforcement if exposed.
It is the primary task to determine whether the damage is structural or non-structural. Structural repairs are undertaken to restore the structural stability of the structure to carry the present stresses under the service conditions. Non-structural repairs are undertaken to restore the long term durability but do not increase the load bearing capacity of the structure in question. A nonstructural repair or cosmetic repair if not conducted at appropriate time can lead to structural distress.
Steps to be Taken
When the structure shows signs of damage or distress, the following steps should be taken in principle:
(i) Preliminary investigation, detailed investigation
(iii) Laying out specifications for repairs
(iv) Selection of materials
(v) Surface preparations
(vi) Actual repairs
(vii) Periodical maintenance
Possible Causes of Structural Concrete Deterioration
Before getting to the investigations about a distress to the structure it is necessary to understand the factors controlling the deterioration of concrete. a knowledge of why, and in-depth investigation can only lead to proper diagnosis. There are two stages at which the deterioration of concrete can start.
To a great extent, through proper measures it is possible for us to arrest or minimize the deterioration of concrete in the first stage, that is before or during construction. However in the second case, most of the factors affecting the durability of concrete are beyond our control. At this stage, we can only take steps to maintain and take immediate action to mitigate the effects.
Cracks in Concrete
Concrete is subjected to tensile stresses in structures. However the concrete cracks when these tensile stresses exceed its tensile strength. This happens often and cracks tend to become one of the inherent defects. The reasons for cracking in concrete are many.
The Causes of Cracks in Concrete
It is essential to know about the cracks. It can be stated that the existence of cracks does not mean that there are defects needing repairs. First it should be established whether the cracks are: (a) structural, or (b) non-structural.
Structural cracks are mostly due to:
(i) Errors in design of stresses
(ii) Errors during construction
(iii) Excess loadings in service conditions vis-a-vis design loads
(v) Unforeseen physical damage like fire and explosions
(vi) Lowering of section of reinforcement in the second stage of corrosion.
Non-structural cracks are mostly due to:
(i) Plastic shrinkage cracking — rapid evaporation of water
(ii) Drying shrinkage cracking
(iii) Plastic settlement cracking — settlement of concrete in formwork
(iv) Thermal contraction cracking — evolution of heat of hydration
(v) Cracking due to bad workmanship
(vi) Alkali aggregate reaction
A careful observation of cracks provides very valuable information not only leading to the reason why the crack appeared but also as a major guideline for the selection of material.
Important things to be noted in the cracks:
(i) Pattern of cracks — Vertical, horizontal or diagonal
(ii) Width of the crack
(iii) Movements in the crack — Thermal variations. Dynamic or moving loads
(iv) Moisture in crack
(v) Dirt in crack.
Cracking due to reinforcement corrosion tends to follow the line or reinforcement.
Preliminary Investigation and Detailed Investigation of RCC Structures
The aim of investigation is primarily to determine the extent of damage or distress, whether the damage is structural or non-structural and to know the reasons for such a determination.
As unless the cause of distress is established, the remedial measures shall have no meaning as it is the cause that is to be rectified rather than the surface appearance of the damaged structure.
The investigation of concrete structures is carried to ascertain:
(a) Whether the concrete is carbonated
(b) Whether the chloride levels are high
(c) Depth of carbonation and cover depth
(d) Degree of corrosion
(e) Present load carrying capacity of structure
(f) Whether the defects are localized or total area
(g) Appearance of cracks and types of cracks
(h) Whether designed loads and service loads are same.
Important information should be obtained about the age of structure, type of construction, structural design assumptions, present loading conditions, whether already the repairs were carried out and their durability.
Investigations for structural repairs should essentially include the semi-destructive and non-destructive tests. Test results should be cross-checked before condemning the concrete.
Diagnosis of Reinforced Concrete Structures
Diagnosis is interpretation of the results obtained from the investigations. The interpretation requires sound knowledge and experience in this field and should essentially be done by qualified engineers.
Specifications for Concrete Repair Methods
Since the field of repairs and maintenance is a specialized one, it is very important that proper specifications are laid out for carrying out the remedial measures. The specifications should include:
(a) Materials for repairs
(b) Calculations for extra reinforcement for structural repairs
(c) Materials for injecting the cracks
(d) Guidelines for surface preparations
(e) Steps for repairs
(f) Precautions to be taken while using the materials as well as the curing procedures etc.
(g) Supervision and quality control at sites
Selection of Materials for Concrete Repairs
The selection of material will be guided by the type of repairs in question.
While selecting a material, the following points should be taken as the basis for selection:
(a) Existing surface and bonding possibilities
(b) Strength of the repair materials required
(c) Non-shrinking characteristics of the material
(d) Temperature at application and service temperatures
Two separate types of materials may be required for total repairs — one type for filling of cracks and the second for the cosmetic repairs. Economic considerations shall also have to be considered while selecting the materials.
Surface Preparations of Damaged Concrete
This step is very important step but generally does not receive the attention it deserves. Many systems involving proper specifications and proper selection of materials fail because improper pretreatment and negligence of proper surface preparation.
Without going into specific details, in general, the concrete surface should be checked for cavities with hammer testing. All loose portions should be chiseled with a chisel and hammer. The reinforcement should be exposed about 50 mm extra from the rusted reinforcement either ways.
The steel should be cleaned with suitable mechanical or other methods. The surfaces should be firm and free from oils and fats, to ensure proper bonding of the subsequent coats.
Actual Repairs Methods for Concrete Structures
The actual repairs will be depending upon the type of damage to the structure that is, structural or non-structural. Steps in both the cases overlap and principally are the same. The basics of civil and structural engineering should be thoroughly adhered to, otherwise the same problems will manifest themselves in larger magnitudes.
In case of structural repairs the following steps must be followed:
(a) Injecting the structural cracks with suitable materials
(b) Strengthening the structure with guiniting
(c) Cosmetic repairs if required or in smaller areas
(d) Surface coatings
In case of non-structural repairs, the steps are:
(a) Sealing of cracks for stopping water leakages etc.
(b) Cosmetic repairs
In case of chloride contaminated concrete, the repairs can be conducted only in two ways:
(a) Providing a complete inert barrier around all reinforcement
(b) Resort to cathodic protection
Periodical Maintenance of Reinforced Concrete Structures
Periodical inspections are very important as they can detect the damages in very early stages and they can be repaired immediately. The periodical inspections should be done to not the following occurrences.
(i) Appearance of cracks
(ii) Rust staining
(iii) Spalling of concrete
(iv) Exposure of reinforcements
(v) Water leakages through body of concrete
(vi) Water leakages through sanitary fittings
(vii) Growth of moss, fungus and plants on wet surfaces
(viii) Peeling off the plasters and mortars
(ix) Degree of deterioration of paints and surface coatings.
A rectification of these defects in time can lead to savings and increasing the life of structures. Inspections can be carried out by qualified engineers once in about three years and documented. The owners should be made aware to report a damage or deterioration at its first occurrence.
Exterior paints are not only for decorative purposes but they also provide additional protection to concrete, seal fine cracks and prevent the ingress of water and oxygen into the body of concrete, thereby lowering the rate of corrosion.
Precautions for Repair, Maintenance and Rehabilitation of Concrete Structures
Though it is an acceptable fact that defects do creep in while construction, which adversely affects the durability of concrete, it is still possible to cast good concrete which can have a long-term durability with minimum maintenance if some basic and not so difficult precautions are observed.
(a) Proper detailing should be done at the design office to show the details of reinforcements etc. at the junctions, where there is congestion of reinforcements. Form work should be designed not only for alignment but also for settlements of props etc.
(b) At the site, proper shuttering must be made, the reinforcement should be aligned with properly designed constant covers by using bar spacers.
(c) Concrete should be of proper slump to enable the flowability in the form work. Water / cement ratio must be strictly adhered to. The mix should be a designed mix with optimum quantity of cement bearing in mind that cement quantity is required not only for strength but also for durability. Chloride free aggregates and potable water should be used for mixing the concrete.
(d) Optimum vibration for proper compaction is required. In case of doubt over vibration is better than under vibration.
(e) Form work should be removed after proper strengths are gained. Honeycombs and cracks should be filled immediately thereafter as delay can lead to filling up of joints etc. with dirt, oil etc.
(f) Proper curing is a must. Curing with membrane forming curing compounds protects the concrete even in the initial stages.
It is evident from the foregoing discussion that precautions are better than cure. Proper supervision at construction sites by qualified engineers is essential. Codes of practice should be strictly adhered to.
The diagnosis and the interpretation of the investigation data should be analyzed by experienced engineers. Selection of materials require not only sound structural knowledge but also of material science.
A knowledge of limitations or disadvantages of materials will be a better guide to the decision maker rather than the advantages and tall claims.
Repairs should be viewed as systems and the work should be allotted to experienced companies. Periodical inspections and timely repair of small defects are required. Short term economy should be forgone for long term implications.
Proper and timely maintenance is key for durability of concrete and longevity of structures.