DURABILITY OF CONCRETE STRUCTURES

During the recent past, the problem of early deterioration of concrete structures has assumed serious proportion all over the world. In India also, this problem is being witnessed in the past few years, especially in coastal and industrial belts as well as in other aggressive environments. Durable concrete can be defined as one that is designed, constructed and maintained to perform satisfactorily in the expected environment for the specified designed life. Some of the significant factors that govern the durability of concrete structures are mix design, structural design, reinforcement detailing, form work, concrete cover, quality of materials used, compaction, curing and supervision. Inadequate attention to these factors, and the presence of chlorides hasten corrosion of reinforcement, moisture, carbonation, sulphate attack and alkali aggregate reaction, leading to the deterioration of concrete structures.

Generally, concrete suffers from more than one causes of deterioration, which is generally seen in the form of cracking, spalling, loss of strength, etc. It is now accepted that the main factors influencing the durability of concrete is its impermeability to the ingress of oxygen, water, carbon dioxide, chlorides, sulphates, etc.

Concrete, under ideal conditions, protects embedded steel against atmospheric influences by denying access to aggressive elements, such as moisture, air, chlorides, sulphates and chemical fumes.

A detailed investigation of deteriorated structures is essential before planning its remedial measures. The investigations involve initial inspection, condition survey for cracks and other defects, sampling, measurement of concrete cover and assessing the material strength. The intensity of damage can be assessed on the data collected through various investigations including Non Destructive techniques.

Cracks

Before hardening	After hardening
Drying Plastic shrinkage Plastic  settlement	Physical Crazing  Aggregate shrinkage
Constructional Formwork movement Sub-grade movement	Chemical Carbonation of concrete Corrosion of steel Alkali - aggregate reaction
Physical Early frost damage	Thermal Early thermal contraction Insulation effects Freeze - thaw cycles
	Structural Design loads Creep Over loading

Behaviour of concrete

The behaviour of concrete depends on several processes, i.e. Physical, Chemical and Biological. These processes bring changes in material composition and performance due to transport of water and dissolved deleterious agents within the concrete. Moisture and its transport within the pores and cracks of concrete control the physical and chemical processes that lead to structural deterioration.

1. Physical process

Physical processes lead to gradual deterioration of concrete, and govern its long-term behaviour.

Cracking: Concrete cracks whenever tensile strains exceed its tensile strain capacity. Cracks may occur in green concrete due to plastic shrinkage, settlement of forms and support movements. The hardened concrete cracks due to loading, drying shrinkage, chemical and thermal effects. The reason for crac-king of concrete are given in the table.

Abrasion: The movements of person and traffic on concrete surfaces cause abrasive wear. Industrial floor and bridge deck slabs are subjected to abrasive wear. In the case of hydraulic structures, bridge piers and abutments, water flowing against surfaces causes wear due to suspended particles.

Frost & de-icing salts: The transition of water from liquid state to solid state due to icing involves an increase in volume by about 9%. In the porous concrete, (he freezing of water induces splitting forces. Several cycles of freezing and thawing of water may result the in spelling of concrete. The frost resistance of concrete depends upon several parameters, such as age of concrete, composition, aggregate type, pore size distribution, rate of cooling and drying between freeze-thaw cycles.

2. Chemical process

Chemical processes govern the rate of decomposition of concrete, and thus its durability. The reaction involves movement of reaction substances within concrete or from atmosphere to concrete. The process depends on the nature of chemicals, pore structure and ambient temperature as well as characteristic of concrete.

Acid attack: Acid attack involves conversion of calcium compound to calcium salts after attacking acid. The structure of the hardened concrete destroyed by acid attack, the rate of deterioration depends not only on the strength of the reactants but also upon the solubility of the resultant salts and their transport. The acids destroy concrete by converting hardened concrete, and its pore system. Impermeability of concrete is of little consequence in this case.

Sulphate Attack: Sulphate attack on only aluminate compounds, calcium and hydroxyl of hardened Portland cement forming ettringite and gypsum. In the presence of sufficient water, these reactions of delayed ettringite formation cause expansion of concrete leading to irregular cracking. The cracking of concrete provides further access to penetrating substances and to progressive deterioration. The effects of sulphate on concrete depend upon the severity of attack, accessibility (Permeability and Cracking), presence of water and susceptibility of cement- Concrete can be protected against sulphate attack by limiting the aluminates between 3 to 8%.

Blended cements perform better than Ordinary Portland Cement, when subjected to sulphate attack. Pozzolanaic materials such as fly ash, silica fume, rice husk ash provide moderate resistance.

Alkali attack: Alkalis react with silica containing aggregates and not with cement. The pore solution in concrete is lime-saturated and contains potassium and sodium ions. Free alkalis present in cement dissolve in the mixing water and forming a caustic solution, which attack the reactive silica in the aggregate. The alkali silica gel so formed swells in the presence of moisture, and exerts osmotic pressure on the concrete internally. This may result in pattern cracking and loss of strength, particularly in thin section. Besides alkali-silica reactivity, carbonate minerals may also cause deterioration of concrete due to alkali attack. However, alkali-carbonate reactivity is mil as common as alkali-silica reactivity.

3. Biological process

Plant roots penetrating cracks and other weak spots may cause mechanical deterioration of concrete; the resulting bursting forces may widen the existing cracks and cause spalling of concrete. In the case of sewers and biogas plants, the hydrogen sulphide produced in the anaerobic conditions may be oxidized in the aerobic conditions and form sulphuric acid, which attack concrete above the water level.

Environmental factors

The service life of the concrete structures depends on the environmental factors as well. The nature, intensity and timing of environmental influences affect the behaviour of materials. The permeability of concrete, concrete cover, structural form, type and location of reinforcement and nature of cement and aggregates determine the response of concrete to environmental influences.

1. Exposure conditions

There is no standard way of classifying climate to define the response of concrete and reinforcement. The general guideline for classification of exposure conditions are as given below:

Mild Conditions: The mild conditions, where the relative humidity does exceed 60% for most part of the year (not more than 3 months). Moderate Conditions : These conditions include interiors of building with high relative humidity, or subjected to corrosive vapors. Submerged structures or structures coming in contact with flowing water or regions of heavy rainfall without heavy condensation of aggressive gases come under moderate conditions.

Severe conditions: Exposure to slightly acidic liquids, saline or oxygenated water, corrosive gases and aggressive soils constitute severe conditions for concrete structures.

Very severe Conditions: Exposure to seawater spray, corrosive fumes, industrial atmospheric and severe freezing conditions can be categorized as severe conditions of exposure.

Extreme Conditions: These include tidal zone and direct contact of liquid or solid aggressive chemicals.

2. Temperature and humidity

The ambient temperature and humidity influence the rate of chemical reactions. An increase in temperature of 10°C. the rate of reaction is approximately doubled. The main parameters for determining the aggressiveness of atmosphere are moisture, ambient temperature and aggressive substances available in moisture. Carbonation of concrete lakes place rapidly, when the relative humidity is around 50-60%. The rate of corrosion is maximum, when relative humidity is 90-95%. The rate of corrosion is independent of humidity, in the presence of chloride.

Water: Water is essential for most of the processes leading to concrete deterioration. Constant wetting and drying is more detrimental to concrete than submerged conditions. The concentration of aggressive substances in the pore structures increases as a result of cyclic wetting and drying leading to corrosion. The splash zone and tidal zone of marine structures are more prone to corrosion than submerged zone.

Aggressive elements:

Aggressive elements in nature include water and air. The usual substance present in water and their actions detrimental to concrete are listed below.

• Oxygen dissolved in water is essential for corrosion of embedded steel
• Carbon dioxide leads to carbonation of concrete and subsequently reduction in its ability to protect embedded steel
• Chlorides cause corrosion of embedded steel
• Acids in water dissolve cement and change its pore structures leading to further deterioration
• Alkalis in water promote reactivity with silica aggregates
• Sulphates react with cements and cause its expansion
• Aggressive fumes from industrial processes may attack concrete.

Marine conditions: Marine conditions are more severe than those occurring on land. Seawater contains MgCI₂, MgS0₄, CaSO₄, KCI, K₂SO₄. The mean concentration of these salts is about 35 gm/L. Apart from these salts, sea water also contains' dissolved oxygen and carbon dioxide to add to corrosive process. The marine, environment may be classified in four zones according to exposure conditions:

• Marine Atmosphere Zone: In this zone, concrete is not exposed to sea water directly, but comes in contact with salt-laden mist.
• Splash Zone: This zone lies above high tides but is subjected to direct wetting by sea waves and spray.
• Tidal Zone: The zone between high and low tide is termed tidal zone.
• Submerged Zone: Concrete in the submerged zone or on the sea beds.

Causes of deterioration:

Concrete normally provides excellent corrosion protection to embedded reinforcement. The high alkalinity of concrete, i.e. above pH 12.5, results in the formation of protective oxide film on steel bars. However, unless concrete is well compacted and dense, it is susceptible to carbonation, and looses its capacity to protect reinforcement. Some of the causes for deterioration of concrete structures are discussed here.

Design and construction defects

Design of concrete structures, including detailing of reinforcement, governs the performance of structures to a considerable degree. Structures that are correctly designed and have good workmanship develop narrow cracks, as compared to poor design/workmanship.

The quality of form work also helps in quality of concrete. The beam-column junctions are particularly prone to defective concrete, if reinforcement detailing is improper or fabricated carelessly.

Concrete cover is also very important parameter, which help in protection of reinforcement from corrosion. It is essential to ensure adequate concrete cover, depending upon the aggressiveness of the environment. Cracks in reinforcement concrete structure can also result from design deficiencies.

Poor quality materials

The specified quality of materials should be ensured by frequent tests on cement, aggregates and water. Alkali-aggregate reaction and sulphate attack results early deterioration. Salinity in sand causes deterioration of concrete and reinforcement corrosion. Clayey material in fine aggregate weakens the mortar-aggregate bond, and reduces concrete strength.

Inadequate supervision

It is essential to ensure dm the minimum specification of concrete mix and construction practice are satisfied.

Environment

The root causes of deterioration in aggressive environment are the development of cracks and high porosity and permeability of concrete. The design of structures should consider environmental factors as well and not strength alone. 

Corrosion of reinforcement

Due to protection loss of concrete protection, steel bars embedded in concrete are also prone to electro-chemical effects. Corrosion affects structures in two ways. Firstly, the product of corrosion occupy a larger -volume than that of the steel destroyed and exert pressure on surrounding concrete causing cracking and spalling. Secondly, die area of effective steel reduces due to corrosion or migration of ions, and in course of time, area of steel may not be adequate to resist me imposed loads.

Inadequate understanding of materials

Concrete technology and structural design should not be separated, but unified in order to obtain durable structures of adequate safely margin. In most of me cases, ductile material with low Young's Modules is required in order to control early cracking of concrete. In the absence of such an ideal material, the use of surface coatings is recommended for durable structures.

Technological factors

The techniques of concrete manufacturing, handling and processing influence the quality of concrete significantly. The technological factors responsible for structural deterioration are given here.

• Characteristics of concrete making materials and the deleterious substances present in them
• Concrete mix proportions
• Water-Cement ratio
• Cement content of concrete
• Water content of the mix 
• Admixtures
• Workmanship in mixing, placing, compaction and curing of concrete

The right time measure to be taken to prevent die corrosion of reinforcement in concrete is during the design and construction stages of structures. The basic principle of prevention of corrosion is to maintain the passivity of the embedded steel;it is obvious mat the permeability of concrete is key to control me various process involved in the phenomenon.

Low permeability can be achieved by adopting tow water-cement ratio, adequate cement content, blended cements suitable admixtures, and proper control on size grading and quality of aggregates.

Proper compaction and curing of concrete are also essential. Some of these measures to be considered at the design and construction stages are discussed here briefly.

Concrete

The durability is governed by the quality of concrete. The manufacturing process of concrete plays a significant rote in assuring me structural durability.

Water cement ratio 

Water cement ratio influences the permeability of concrete, and should be decreased with increasing environmental aggressivity. Cement content of concrete is of lesser significance than water-cement ratio for structural durability, provided the mix of adequate workability. The water-cement ratio should be lying tome range of 0.55 to 0.4. Depending on the aggressiveness of the environment.

Cement content

It is possible to obtain the required strength of concrete by adopting higher grades of cement. According to IS 456:1999, the minimum cement content for plain concrete must be 220 Kg/Cum for mild exposure, whereas 300 Kg/Cum reinforced concrete requires minimum cement content 300 Kg/ Cum for mild exposure conditions. For extreme environment condition, minimum cement content may go up to 375 Kg/Cum.

Curing

The strength and permeability of the cover-concrete can only be achieved if concrete is cured adequately. The exposed surfaces of concrete should be kept continuously wet for at least 7 days from the date of placing concrete for proper curing. However, longer curing periods, up to 28 days, are recommended for blended cement.

Steel

Steel is prone to corrosion when not protected adequately. Corrosion mechanism and process are governed by several parameters and require a multi directional approached to prevent deterioration of corrosion structures. Some of corrosion prevention methods are given below;

• Metallurgical methods
• Corrosion inhibitors
• Coating to reinforcement
• Cathodic protection
• Corrosion retardant steel
• Coating to concrete

Cover concrete

The concrete cover should be dense, strong, impermeable in order to resist the ingress of deleterious substances. The IS 456: 1999, specifies concrete cover 20 mm for mild exposure conditions increasing to 75 mm in extreme conditions.

Planning and construction details

Architectural planning and constructional details often determine the durability of structures. Attention to small and simple details of structural components prevents possible local deterioration of materials and subsequent effects on structure performance. It should be noted that, the exposed surface should be of simple profile to avoid local deterioration. Complex details often lead to maintenance problem later.

Drainage of water

It is important to note that water is essential to cause structural deterioration- Properly drained surfaces, with no possibility of water stagnating, enhance structural durability. The drained water should not How against the structure at the outlets.

Structural design 

Structural design Structural cracks, even if they are not detrimental to structural performance under loads, affect durability Sudden changes in cross section should be avoided. Differential settlement and thermal effects should be considered in the design to avoid inexplicable cracking.

Constructional aspects

During construction, proper attention should be made at the time of positioning the reinforcement, so that its usability is to its optimum level.

Accessibility and maintainability

The designer should consider accessibility of various structural components, their reparability and replaceability, and incorporate suitable measures. Lack of accessibility hampers inspection, and may lead to avoidable excessive repairs at a later date. Buried components of structures (footing and piles) cannot be reached or inspected after construction. Such inaccessible components require greater attention and care at construction stage itself from other components.

Replaceability

Structural components such as joints, seals, drainage system and water proofing treatments, can be replaced later on, if necessary. These components should be planned for easy replacement without damaging the adjacent structural component.

Conclusion and recommendation

Durability of concrete structures should be considered as a significant aspect of structural design. Concrete technology plays a significant role in ensuring durability of concrete structures. A designer should be aware of the constructional aspects of structures, as well as, in order to foresee durability problems due to any peculiarities of structural loads, layout as well as environment.

The following recommendations may be adopted to ensure safe and durable structures with trouble free long service life.

Concrete protects steel from corrosion only under controlled conditions. Good quality concrete mix with the lowest water cement ratio compatible with practical placement and finishing techniques should be used. Concrete should be properly placed, consolidated and cured. Over stressing of structures should be avoided.

Application of flexible surface coatings to avoid concrete surfaces, which can effectively control the ingress of chlorides, sulphates, carbon dioxide, oxygen and moisture, can be considered as an effective corrosion control measure. However, the coatings should be applied before structural deterioration occurs, and not afterwards, to be effective.

Exercising adequate care at every stage of planning, analysis, design and construction for the expected exposure conditions can effectively control corrosion of rebar. Corrosion retardant steels, coatings for concrete and cathodic protection enhance durability of structures. However, there is no substitute for well designed and well compacted concrete cover of adequate thickness Concrete, under Meal condition, protects embedded steel to ensure durable structures.

The performance of structures should be monitored regularly from the stage of commissioning. Assessment of damage is the first step in a structural repair project. A successful program of damage assessment is often the key to cost effective repair system.

The response of the structural system to the changes due to repair must be understood for successful rehabilitation program. It is not possible to generalize rehabilitation schemes; each system has merits and demerits. Depending upon the type of structure and the nature of distress, various techniques in suitable combination may be adopted.

Trained supervision, workable specifications, speedy site decisions and enough working area should be made available for satisfactory, timely and efficient repairs.

For structure in non-corrosive environment, uniting is usually adequate for rehabilitation: surface coating may be necessary to protect concrete in corrosive environments.

A holistic approach should be adopted for structural systems, wherein structural design and durability are considered together rather than as separate entities.

Design for durability of concrete structures

Control of deflection

     The procedure for control of deflection is to control span to effective depth ratio .it assumes that the deflection of beam and slab will depend on the following factors.

1. The span/effective depth ratio

2. Type of supports as to whether simply supported , fixed or continuous

3. Percentage of tension steel or the stress level in the steel level at service loads if more than the necessary steel is provided at the section.

4. Percentage of compression steel provided.

    

      Span/effective depth ratio to be used for beams and slabs with span less than 10m are given in the table below

Design for limit state of deflection

       Excessive deflection of beams and slab is not only an eyesore  in itself but it can also cause cracking of partion .As given in IS 456(2000) the commonly accepted limits of allowable deflection are

 1. A final deflection of span/250 for  the deflection of  horizontal bending members like slabs and beam due to all load so as to be noticed by  the eye.

2. A deflection of span/350 or 20mm which is less for these members after the construction of the partitions and finishes etc,to prevent damages to finishes  and  partitions.

       The allowable crack width in concrete depends on the environmental conditions to which it is exposed. According to IS456(2000)the values are shown below

       The three aspects of cracking are of importance. They are

1. Effect of cracking on the appearance

2. Corrosion

3. Stiffness of the beam                     

      Structural cracking can be classified according to the cause of cracking.These cracks may belong to any one of the following categories.

1. Flexural cracks.

2. Diagonal tension cracks

3. Spitting cracks along with reinforcement due to bond and anchorage failure

4. Temperature and shrinkage cracks.

Method of limiting crack width

    Under service load the crack width in concrete should not be excessive. According to IS456 (2000), under normal condition crack width at the surface of concrete should not exceed 0.3mm for sake of appearance. In moderate exposure should be limited to 0.2mm and severe exposure to 0.2mm for corrosion resistance.

Table

The 0.3mm of crack width can be generally met by good practice of detailing reinforcement.

Method of crack control

      To control the crack width the important factors to be considered are the following

1. Maximum and minimum spacing of reinforcements

2. Maximum and minimum area of steel in the member

3. Curtilment of reinforcement bars

4. Anchorage of reinforcement bars          

5. Cover to reinforcement.

6. Maximum and minimum sizes of steel to be used for the various types of steel in the member.

Bar spacing rules for beams

       Considering factor 1.above namely bar spacing rules, the major parameters that affect crack width in concrete beams are as follows

1. The distance of crack from the nearest reinforcement bar spanning the crack

2. Distance from neutral axis of the cross section

3. Mean strain at the level of the section considered.

  These parameters can be related to the distribution of the reinforcement in the beam in terms of the following factors

1. Maximum horizontal bar spacing

2. Minimum vertical and horizontal bar spacing

3. Arrangement of the side reinforcements for members whose depth is larger than 750mm.

4. Corner distance to the nearest steel.

 Minimum bar spacing rule for beam

    The diameter of a round bar shall be its nominal diameters, and in the case of bars shall be its nominal diameters, and in the case of deformed bars or crimped bars, the diameter shall be taken as the diameter of a circle giving an equivalent effective area.

The bar spacing should not be less than the diameter of the largest bar and not less than the maximum size of aggregate plus 5mm