A durable concrete is one that meets predetermined expectations under the working environment (exposure) conditions during its service life. 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.
7.1.1 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 Subsection.
7.2Requirements for Durability
7.2.1 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. 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.
7.2.2 Exposure Conditions
7.2.2.1 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 2.
7.2.2.2 Abrasion
Abrasion of concrete surfaces results from friction, which may cause a grinding action; or by repetitive impact and overloading, which causes local crushing.
Abrasion resistant surfaces primarily involve consideration of surface finishing technique curing regime, aggregate characteristics and cement content. Compressive strength of concrete may also provide a relative indication of resistance.
Concrete surfaces protected against weather or aggressive conditions, except those situated in coastal areas
ii)
Moderate
Concrete surfaces sheltered from severe rain or freezing whilst wetConcrete exposed to condensation and rainConcrete continuously under waterConcrete in contact or buried under non-aggressive soil/ground waterConcrete 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 condensationConcrete completely immersed in sea waterConcrete exposed to coastal environment
iv)
Very severe
Concrete surfaces exposed to sea water spray, corrosive fumes or severe freezing conditions whilst wetConcrete in contact with or buried under aggressive sub-soil/ground water
v)
Extreme
Surface of members in tidal zone and splash zoneMembers in direct contact with liquid/solid aggressive chemicals
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.
7.2.2.3 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 as follows:
Nominal Maximum Aggregate Size mm
Entrained Air Percentage
10
7 ± 1
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.
7.2.2.4 Exposure to sulphate attack
Table 3 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 actual 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.
7.2.3 Requirement of Concrete Cover
7.2.3.1 The protection of the steel in concrete against corrosion depends upon an adequate thickness of good quality concrete.
7.2.3.2 The nominal cover to the reinforcement shall be provided as per 25.4 of Part 6 ‘Structural Design, Section 5 Concrete, Subsection 5A Plain and Reinforced Concrete’ of the Code.
7.2.4 Concrete Mix Proportions
7.2.4.1 General
The free water cement ratio is an important factor in governing the durability of concrete and should always be the lowest value. Appropriate value for minimum cement content and the maximum free water-cement ratio are given in Table 4 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 5.
7.2.4.2 Maximum cement content
Cement content in excess of 450kg/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 thermal stresses in thicker sections.
7.2.5 Mix Constituents
7.2.5.1 General
For concrete to be durable, careful selection of the mix and materials is necessary, so that deleterious constituents do not exceed the limits.
7.2.5.2 Chlorides in concrete
Whenever there is chloride in concrete there is an increased risk of corrosion of embedded steel. 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
Table 3Requirements for Prestressed Concrete Exposed to Sulphate Attack (Clauses 7.2.2.4 and 8.1.2)
Sl 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 the Accepted Standard [6-5B(6)]
In Soil
In Ground Water
Total SO3
SO3 in 2:1 Water: Soil Extract
Percent
g/l
g/l
Minimum Cement Content kg/m3
Maximum Free Water-Cement Ratio
(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 - 0.5
1.0 - 1.9
0.3 - 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 - 1.0
1.9 - 3.1
1.2 - 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 - 2.0
3.1 - 5.0
2.5 - 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. Suitable adjustment may be made in the minimum cement content specified in case of use of aggregate other than 20 mm nominal maximum size.
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 7 of this table are the minimum recommended. For SO3contents 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 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.
acid soluble chloride content (as Cl) in the concrete at the time of placing shall be not more than 0.4kg/m3 of concrete.
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.
7.2.5.3 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 SO3 (where SO3=0.833SO4 ) 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.
7.2.5.4 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 occur only 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; and
c) Aggregate containing an alkali reactive constituent.
Table 4Minimum 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 7.2.4.1 and 8.1.2)
Sl No.
Exposure
Prestressed Concrete
Minimum Cement Content kg/m3
Maximum Free Water Cement Ratio
Minimum Grade of Concrete
(1)
(2)
(3)
(4)
(5)
i)
Mild
300
0.55
M 30
ii)
Moderate
300
0.50
M 30
iii)
Severe
320
0.45
M 30
iv)
Very severe
340
0.45
M 35
v)
Extreme
360
0.40
M 40
1 Cement content prescribed in this table is irrespective of the grades and types of cement and it is inclusive of additions mentioned in 4.2. 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 pozzolana as specified in the accepted standard [6-5B(11)] and that of slag as specified in the accepted standard [6-5B(12)] beyond which these additions, though permitted, shall not be considered for these purposes.
2 The minimum cement content, maximum free water-cement ratio and minimum grade of concrete are individually related to exposure.
Table 5Adjustments to Minimum Cement Contents for Aggregates Other than 20 mm Nominal Maximum Size (Clause 7.2.4.1)
Sl No.
Nominal Maximum Aggregate Size mm (2)
Adjustments to Minimum Cement Contents in Table 4 kg/m3 (3)
i)
10
+40
ii)
20
0
iii)
40
-30
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.
b) Use of low alkali ordinary Portland cement having total alkali content not more than 0.6 percent [as Na2O equivalent (expressed as Na2O+0.678K2O )].
Further advantage can be obtained by use of fly ash conforming to the accepted standard [6−5B(13)] or granulated blast furnace slag conforming to the accepted standard [6−5B(14)] 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.
7.2.6 Concrete in Aggressive Soils and Water
7.2.6.1 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 7.2.2.4.
7.2.6.2 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 impermeable barriers.
7.2.7 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 also provisions on curing under 13).