8.1 General
A durable concrete is one that performs satisfactorily in the working environment during its anticipated exposure conditions during service. The materials and mix proportions specified and used should be such as to maintain its integrity and, if applicable, to protect embedded metal from corrosion.
8.1.1
One of the main characteristics influencing the durability of concrete is its permeability to the ingress of water, oxygen, carbon dioxide, chloride, sulphate and other potentially deleterious substances. Impermeability is governed by the constituents and workmanship used in making the concrete. With normal-weight aggregates
a suitably low permeability is achieved by having an adequate cement content. sufficiently low free water/ cement ratio, by ensuring complete compaction of the concrete. and by adequate curing.
The factors influencing durability include:
- a) the environment;
- b) the cover to embedded steel;
- c) the type and quality of constituent materials;
- d) the cement content and water/cement ratio of the concrete;
- e) workmanship, to obtain full compaction and efficient curing; and
- f) the shape and size of the member.
The degree of exposure anticipated for the concrete during its service life together with other relevant factors relating to mix composition, workmanship, design and detailing should be considered. The concrete mix to provide adequate durability under these conditions should be chosen taking account of the accuracy of current testing regimes for control and compliance as described in this standard.
8.2 Requirements for Durability
8.2.1 Shape and Size of Member
The shape or design details of exposed structures should be such as to promote good drainage of water and to avoid standing pools and rundown of water. Care should also be taken to minimize any cracks that may collect or transmit water. Adequate curing is essential to avoid the harmful effects of early loss of moisture (see 13.5).Member profiles and their intersections with other members shall be designed and detailed in a way to ensure easy flow of concrete and proper compaction during concreting.
Concrete is more vulnerable to deterioration due to chemical or climatic attack when it is in thin sections, in sections under hydrostatic pressure from one side only, in partially immersed sections and at corners and edges of elements. The life of the structure can be lengthened by providing extra cover to steel, by chamfering the corners or by using circular cross-sections or by using surface coatings which prevent or reduce the ingress of water, carbon dioxide or aggressive chemicals.
8.2.2 Exposure Conditions
8.2.2.1 General environment
The general environment to which the concrete will be exposed during its working life is classified into five levels of severity, that is, mild, moderate, severe, very severe and extreme as described in Table 3.
.
Table 3 Environmental Exposure Conditions
(Clauses 8.2.2.1 and 35.3.2)
| Sl No. (1) | Environment (2) | Exposure Conditions (3) |
|---|---|---|
| i) | Mild | Concrete surfaces protected against weather or aggressive conditions, except those situated in coastal area. |
| ii) | Moderate | Concrete surfaces sheltered from severe rain or freezing whilst wet Concrete exposed to condensation and rain Concrete continuously under water Concrete in contact or buried under non-aggressive soil/ground water Concrete surfaces sheltered from saturated salt air in coastal area |
| iii) | Severe | Concrete surfaces exposed to severe rain, alternate wetting and drying or occasional freezing whilst wet or severe condensation. Concrete completely immersed in seawater Concrete exposed to coastal environment |
| iv) | Very severe | Concrete surfaces exposed to sea water spray, corrosive fumes or severe freezing, conditions whilst wet Concrete in contact with or buried under aggressive sub-soil/ground water |
| v) | Extreme | Surface of members in tidal zone Members in direct contact with liquid/solid aggressive chemicals |
8.2.2.2 Abrasive
Specialist literatures may be referred to for durability requirements of concrete surfaces exposed to abrasive action, for example, in case of machinery and metal tyres.
8.2.2.3 Freezing and thawing
Where freezing and thawing actions under wet conditions exist, enhanced durability can be obtained by the use of suitable air entraining admixtures. When concrete lower than grade M 50 is used under these conditions, the mean total air content by volume of the fresh concrete at the time of delivery into the construction should be:
| Nominal Maximum Size Aggregate (mm) | Entrained Air Percentage |
|---|---|
| 20 | 5 ± 1 |
| 40 | 4 ± 1 |
Since air entrainment reduces the strength, suitable adjustments may be made in the mix design for achieving required strength.
8.2.2.4 Exposure to sulphate attack
Table 4 gives recommendations for the type of cement, maximum free water/cement ratio and minimum cement content, which are required at different sulphate concentrations in near-neutral ground water having pH of 6 to 9.
For the very high sulphate concentrations in Class 5 conditions, some form of lining such as polyethylene or polychloroprene sheet; or surface coating based on asphalt, chlorinated rubber, epoxy; or polyurethane materials should also be used to prevent access by the sulphate solution.
8.2.3 Requirement of Concrete Cover
8.2.3.1
The protection of the steel in concrete against corrosion depends upon an adequate thickness of good quality concrete.
8.2.3.2
The nominal cover to the reinforcement shall be provided as per 26.4.
8.2.4 Concrete Mix Proportions
8.2.4.1 General
The free water-cement ratio is an important factor in governing the durability of concrete and should always be the lowest value. Appropriate values for minimum cement content and the maximum free water-cement ratio are given in Table 5 for different exposure conditions. The minimum cement content and maximum water-cement ratio apply to 20 mm nominal maximum size aggregate. For other sizes of aggregate they should be changed as given in Table 6.
8.2.4.2 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.
Table 4 Requirements for Concrete Exposed to Sulphate Attack
(Clauses 8.2.2.4 and 9.1.2)
| SI No. | Class | Concentration of Sulphates, Expressed as SO₃ | Type of Cement | Dense, Fully Compacted Concrete. Made with 20 mm Nominal Maximum Size Aggregates Complying with IS 383 | |||
|---|---|---|---|---|---|---|---|
| In Soil | In Ground Water | Minimum Cement Content kg/m³ | Maximum Face Water- Cement Ratio | ||||
| Total SO₃ | SO₃ in 2:1 Water: Soil Extract | ||||||
| Percent | g/l | g/l | |||||
| (1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) |
| i) | 1 | Traces (< 0.2) | Less than 1.0 | Less than 0.3 | Ordinary Portland cement or Portland slag cement or Portland pozzolana cement | 280 | 0.55 |
| ii) | 2 | 0.2 to 0.5 | 1.0 to 1.9 | 0.3 to 1.2 | Ordinary Portland cement or Portland slag cement or Portland pozzolana cement | 330 | 0.50 |
| Supersulphated cement or sulphate resisting Portland cement | 310 | 0.50 | |||||
| iii) | 3 | 0.5 to 1.0 | 1.9 to 3.1 | 1.2 to 2.5 | Supersulphated cement or sulphate resisting Portland cement | 330 | 0.50 |
| Portland pozzolana cement or Portland slag cement | 350 | 0.45 | |||||
| iv) | 4 | 1.0 to 2.0 | 3.1 to 5.0 | 2.5 to 5.0 | Supersulphated or sulphate resisting Portland cement | 370 | 0.45 |
| v) | 5 | More than 2.0 | More than 5.0 | More than 5.0 | Sulphate resisting. Portland cement or supersulphated cement with protective coatings | 400 | 0.40 |
Notes
- 1 Cement content given in this table is irrespective of grades of cement.
- 2 Use of supersulphated cement is generally restricted where the prevailing temperature is above 40 °C.
- 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 contents given in col 6 of this table are the minimum recommended. For SO, 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 C,A content from 5 to 8 percent shall be desimble to be used in concrete, instead of sulphate resisting cement. Alternatively, Portland sling cement conforming to IS 455 having more than 50 percent slag or a blend of ordinary Portland cement and slag may be used provided sufficient information is available on performance of such blended cements in these conditions.
8.2.5 Mix Constituents
8.2.5.1 General
For concrete to be durable, careful selection of the mix and materials is necessary, so that deleterious constituents do not exceed the limits.
8.2.5.2 Chlorides in concrete
Whenever there is chloride in concrete there is an increased risk of corrosion of embedded metal. The higher the chloride content, or if subsequently exposed to warm moist conditions, the greater the risk of corrosion. All constituents may contain chlorides and concrete may be contaminated by chlorides from the external environment. To minimize the chances of deterioration of concrete from harmful chemical salts, the levels of such harmful salts in concrete coming from concrete materials, that is, cement, aggregates water and admixtures, as well as by diffusion from the environment should be limited. The total amount of chloride content (as Cl) in the concrete at the time of placing shall be as given in Table 7.
The total acid soluble chloride content should be calculated from the mix proportions and the measured chloride contents of each of the constituents. Wherever possible, the total chloride content of the concrete should be determined.
8.2.5.3 Sulphates in concrete
Sulphates are present in most cements and in some aggregates; excessive amounts of water-soluble sulphate from these or other mix constituents can cause
expansion and disruption of concrete. To prevent this, the total water-soluble sulphate content of the concrete mix, expressed as SO4, should not exceed 4 percent by mass of the cement in the mix. The sulphate content should be calculated as the total from the various constituents of the mix.
The 4 percent limit does not apply to concrete made with supersulphated cement complying with IS 6909.
8.2.5.4 Alkali-aggregate reaction
Some aggregates containing particular varieties of silica may be susceptible to attack by alkalis (Na₂O and K₂O) originating from cement or other sources, producing an expansive reaction which can cause cracking and disruption of concrete. Damage to concrete from this reaction will normally only occur when all the following are present together:
a) A high moisture level, within the concrete;
- b) A cement with high alkali content, or another source of alkali;
- c) Aggregate containing an alkali reactive constituent.
Where the service records of particular cement/aggregate combination are well established, and do not include any instances of cracking due to alkali-aggregate reaction, no further precautions should be necessary. When the materials are unfamiliar, precautions should take one or more of the following forms:
- a) Use of non-reactive aggregate from alternate sources.
Table 5 Minimum Cement Content, Maximum Water-Cement Ratio and Minimum Grade of Concrete for Different Exposures with Normal Weight Aggregates of 20 mm Nominal Maximum Size
(Clauses 6.1.2, 8.2.4.1 and 9.1.2)
| Sl No. | Exposure | Plain Concrete | Reinforced Concrete | ||||
|---|---|---|---|---|---|---|---|
| Minimum Cement Content kg/m³ | Maximum Free Water- Cement Ratio | Minimum Grade of Concrete | Minimum Cement Content kg/m³ | Maximum Free Water- Cement Ratio | Minimum Grade of Concrete | ||
| 1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) |
| i) | Mild | 220 | 0.60 | - | 300 | 0.55 | M 20 |
| ii) | Moderate | 240 | 0.60 | M 15 | 300 | 0.50 | M 25 |
| iii) | Severe | 250 | 0.50 | M 20 | 320 | 0.45 | M 30 |
| iv) | Very severe | 260 | 0.45 | M 20 | 340 | 0.45 | M 35 |
| v) | Extreme | 280 | 0.40 | M 25 | 360 | 0.40 | M 40 |
NOTES
1 Cement content prescribed in this table is irrespective of the grades of cement and it is inclusive of additions mentioned in 5.2. 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 and slag specified in IS 1489 (Part 1) and IS 455 respectively.
2 Minimum grade for plain concrete under mild exposure condition is not specified.
Table 6 Adjustments to Minimum Cement Contents for Aggregates Other Than 20 mm Nominal Maximum Size
(Clause 8.2.4.1)
| Sl No. | Nominal Maximum Aggregate Size mm | Adjustments to Minimum Cement Contents In Table 5 kg/m³ |
|---|---|---|
| (1) | (2) | (3) |
| i) | 10 | +40 |
| ii) | 20 | 0 |
| iii) | 40 | -30 |
| SI No. | Type or Use of Concrete | Maximum Total Acid Soluble Chloride Content Expressed as kg/m³ of Concrete |
|---|---|---|
| (1) | (2) | (3) |
| i) | Concrete containing metal and steam cured at elevated tempe-rature and pre-stressed concrete | 0.4 |
| ii) | Reinforced concrete or plain concrete containing embedded metal | 0.6 |
| iii) | Concrete not containing embedded metal or any material requiring protection from chloride | 3.0 |
- b) Use of low alkali ordinary Portland cement having total alkali content not more than 0.6 percent (as Na₂O equivalent).
- Further advantage can be obtained by use of fly ash (Grade 1) conforming to IS 3812 or granulated blastfurnace slag conforming to IS 12089 as part replacement of ordinary Portland cement (having total alkali content as Na₂O equivalent not more than 0.6 percent), provided fly ash content is at least 20 percent or slag content is at least 50 percent.
- c) Measures to reduce the degree of saturation of the concrete during service such as use of impermeable membranes.
- d) Limiting the cement content in the concrete mix and thereby limiting total alkali content in the concrete mix. For more guidance specialist literatures may be referred.
8.2.6 Concrete in Aggressive Soils and Water
8.2.6.1 General
The destructive action of aggressive waters on concrete is progressive. The rate of deterioration decreases as the concrete is made stronger and more impermeable, and increases as the salt content of the water increases. Where structures are only partially immersed or are in contact with aggressive soils or waters on one side only,
evaporation may cause serious concentrations of salts with subsequent deterioration, even where the original salt content of the soil or water is not high.
NOTE — Guidance regarding requirements for concrete exposed to sulphate attack is given in 8.2.2.4.
8.2.6.2 Drainage
At sites where alkali concentrations are high or may become very high, the ground water should be lowered by drainage so that it will not come into direct contact with the concrete.
Additional protection may be obtained by the use of chemically resistant stone facing or a layer of plaster of Paris covered with suitable fabric, such as jute thoroughly impregnated with bituminous material.
8.2.7 Compaction, Finishing and Curing
Adequate compaction without segregation should be ensured by providing suitable workability and by employing appropriate placing and compacting equipment and procedures. Full compaction is particularly important in the vicinity of construction and movement joints and of embedded water bars and reinforcement.
Good finishing practices are essential for durable concrete.
Overworking the surface and the addition of water/cement to aid in finishing should be avoided; the resulting laitance will have impaired strength and durability and will be particularly vulnerable to freezing and thawing under wet conditions.
It is essential to use proper and adequate curing techniques to reduce the permeability of the concrete and enhance its durability by extending the hydration of the cement, particularly in its surface zone (see 13.5).
8.2.8 Concrete in Sea-water
Concrete in sea-water or exposed directly along the sea-coast shall be at least M 20 Grade in the case of plain concrete and M 30 in case of reinforced concrete. The use of slag or pozzolana cement is advantageous under such conditions.
8.2.8.1
Special attention shall be given to the design of the mix to obtain the densest possible concrete; slag, broken brick, soft limestone, soft sandstone, or other porous or weak aggregates shall not be used.
8.2.8.2
As far as possible, preference shall be given to precast members unreinforced, well-cured and hardened, without sharp corners, and having trowel-smooth finished surfaces free from crazing, cracks or other defects; plastering should be avoided.
8.2.8.3
No construction joints shall be allowed within 600 mm below low water-level or within 600 mm of the upper and lower planes of wave action. Where
unusually severe conditions or abrasion are anticipated, such parts of the work shall be protected by bituminous or silico-fluoride coatings or stone facing bedded with bitumen.
8.2.8.4
In reinforced concrete structures, care shall be taken to protect the reinforcement from exposure to saline atmosphere during storage, fabrication and use. It may be achieved by treating the surface of reinforcement with cement wash or by suitable methods.