ANALYSIS OF DURABILITY OF CONCRETE

Durable concrete can be defined as one that is designed, constructed and maintained to perform satisfactorily in the expected environment for the specified life of the structure without undue maintenance. The materials and mix proportions chosen should be such as to maintain the integrity of the concrete and to protect the embedded reinforcement.

The principal causes of deterioration of concrete have been identified as: Carbonation, corrosion of reinforcement, sulphate attack and alkali-aggregate reaction. Generally, the concrete suffers from more than one cause 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. Impermeability is dependent on the constituents and workmanship used in making the concrete.

IS 456:2000 identifies various factors influencing durability as:

1.            Environment
2.            Cover to the embedded steel
3.            Type and quality of constituent materials
4.            Cement content and water cement ratio
5.            Workmanship to obtain full compaction and efficient curing
6.            Shape and size of members.

IS 456:2000 classifies the general Environment in which the concrete will be exposed into five levels of severity--- mild, moderate, severe, very severe and extreme. The code has also specified the values of minimum and maximum cement content, maximum free water cement ratio and the grades of concrete for different exposure conditions.

These values are applicable for those mixes having 20 mm nominal size aggregate. For other sizes of aggregates, the values need to be changed as given in the table.

It is to be noted that the minimum specified grade for reinforced concrete is M20. Incidentally, the grades of concrete have been classified into three different categories in IS 456, namely,

1.            Ordinary concrete
2.            Standard concrete
3.            High strength concrete

Environmental Exposure Conditions : 
 (Reference IS 456:2000 Table 3)

Minimum Cement Content, Maximum Water-cement ratio and Minimum grade of concrete for Different Exposures with normal weight aggregates of 20mm nominal maximum size :
 (Reference IS 456:2000 Table 5)

NOTE:
Cement content prescribed in this table is irrespective of the grades of cement and it is inclusive of additions (Mineral Admixture). The additions such as fly ash or ground granulated blast furnace slag may be taken into account in the concrete composition with respect to the cement content and water-cement ratio if the suitability is established and as long as the maximum amounts taken into account do not exceed the limit of pozzolona (fly ash) and Slag (GGBS).

Limits of Mineral Admixtures to be used with cement : 

Maximum Cement Content

Cement content not including fly ash and ground granulated blast furnace slag in excess of 450 kg/m3 should not be used unless special consideration has been given in design to the increased risk of cracking due to drying shrinkage in thin sections, or to early thermal cracking and to the increased risk of damage due to alkali silica reactions.

Adjustments to Minimum Cement contents for Aggregates Other than 20mm
 Nominal Maximum size

Grades of Concrete
(Ref: IS 456:2000 Table 2)

Notes:

1.            In the designation of concrete mix, M refers to the mix and the number to the specified compressive strength of 150 mm size cube at 28 days, expressed in N/mm2.

2.            For concrete of compressive strength greater than M55, design parameters given in the standard may not be applicable and the values may be obtained from specialized literature and experimental result.

Carbonation and chlorides in concrete

Generally, impermeable concrete provides adequate protection to reinforcing steel. However, the atmospheric carbon dioxide can react with the products of hydration resulting in the process of carbonation, which on reaching the reinforcing steel makes it vulnerable to corrosion. This process may take a few years, or even decades, depending on a host of factors, such as depth of cover, its permeability, level of CO2, type of cement and/or additive used, etc.

Another major source of corrosion is the presence of chlorides in the concrete. The chlorides may get introduced into the concrete through the chlorides present in any of the ingredients, such as cement, aggregates, water, admixtures, etc, or through an external source into the hardened concrete.

IS 456:2000 lays down the limits of the chloride content (as Cl) in concrete at the time of placing.

Limits of Chloride content of concrete :
  (Ref: IS 456:2000 Table 7)

Sulphates in Concrete

Sulphate attack can originate from ground water, soils, sea water or industrial effluents. The reaction depends on the concentration of sulphate ions present in sulphate solutions (that is, sodium, potassium ammonium or magnesium), C3A content of the cement and the quality of the concrete. Sulphates convert the free lime in the hardened concrete to calcium sulphate, and the hydrates of calcium aluminates and ferrites to calcium sulphoaluminates or sulphoferrites.

These conversions occupy more than double the solid volume, which results in disruption, expansion and cracking of the concrete.

IS 456:2000 stipulates that the total water soluble sulphate content of the concrete mix, expressed as SO3 should not exceed 4 percent of the mass of cement in the mix. The standard also gives recommendations for the type of cement, maximum free water cement ratio, minimum cement content required at different sulphate concentrations in near neutral ground water having a pH of 6 to 9

Requirements for Concrete Exposed to Sulphate Attack : 
(Ref: IS 456:2000 Table 4)

Notes:

1. Cement content given in the table is irrespective of grades of concrete

2. Use of supersulphated cement is generally restricted where the prevailing temperature is above 40 degree centigrade.

3.            Supersulphated cement gives an acceptable life provided that the concrete is dense and prepared with a water cement ratio of 0.4 or less, in mineral acids, down to pH 3.5.

4. The cement content given in col.6 of this table is the minimum recommended. For SO3 contents near the upper limit of any class, cement contents above these minimum are advised.

5. For severe conditions, such as thin sections under hydrostatic pressure on one side only and sections partly immersed, considerations should be given to a further reduction of water cement ratio.

6. Portland Slag cement conforming to IS 455, with slag content more than 50 percent exhibits better sulphate resisting properties.

7. Where chloride is encountered along with sulphates in soil or ground water, ordinary Portland cement with C3A content from 5 to 8 percent shall be desirable to be used in concrete, instead of sulphate resisting cement. Alternatively, Portland slag cement conforming to IS 455 having more than 50 percent slag or a blend of OPC and slag may be used provided sufficient information is available on performance of such blended cements in these conditions.

Cover to reinforcement

It is observed that inadequate cover to the reinforcement is one of the major factors leading to early deterioration of reinforced and prestressed concrete structures. Provision of appropriate cover to all reinforcements and ensuring that the quality of the cover concrete including that of the cover blocks is same as that of the core concrete to go a long way in mitigating the problem of early deterioration. IS 456:2000 gives detailed guidelines on provision of cover.

The code defines nominal cover as the design depth of concrete cover to all reinforcement, including links. In order to meet durability requirements, the cover for normal weight concrete, including links as specified by the code is given in below table.

Nominal cover to meet durability requirements :

(Ref: IS 456:2000): Table 23

Notes:

1.            For main reinforcement up to 12 mm diameter bar for mild exposure conditions, normal cover may be reduced by 5 mm.

2.            For Exposure conditions of “severe” and “very severe”, cover may be reduced by 5 mm, where concrete grade is M35 and above.

3.            Unless specified otherwise, actual concrete cover should not deviate from the required nominal cover by +10 mm.

The code specifies that for longitudinal bar in a column nominal cover shall not be less than 40 mm, or less than the diameter of the bar. In those columns of minimum dimension of 200 mm or less, where the reinforcing bars do not exceed 12 mm diameter, a nominal cover of 25 mm may be used. For footings minimum cover shall be 50mm.

Cover for Fire resistance:

For the first time, IS 456 has specified cover for fire resistance. The nominal cover of normal weight aggregate concrete shall be provided to all reinforcement, including links to meet fire resistance as specified in Table.

Nominal cover to meet specified period of fire resistance: 
 (Ref: Table 16A of IS 456:2000)