A durable concrete is one that performs satisfactorily in the working environment during its anticipated exposure conditions during 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. Maintenance plays an important role in sustained durability of concrete (see also 9.1). 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 Section.
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 minimise any cracks that may collect or transmit water. Adequate curing is essential to avoid the harmful effects of early loss of moisture (see 12.5). 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.
7.2.2Exposure 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 3.
PART 6 STRUCTURAL DESIGN — SECTION 5 CONCRETE: 5A PLAIN AND REINFORCED CONCRETE
Table 3Environmental Exposure Conditions (Clauses 7.2.2.1 and 34.3.2)
Sl No.
Environment
Exposure Conditions
(1)
(2)
(3)
i)
Mild
Concrete surfaces protected against weather or aggressive conditions, except those situated in coastal area
ii)
Moderate
a) Concrete surfaces sheltered from severe rain or freezing whilst wetb) Concrete exposed to condensation and rainc) Concrete continuously under waterd) Concrete in contact or buried under non-aggressive soil/ground watere) Concrete surfaces sheltered from saturated salt air in coastal area
iii)
Severe
a) Concrete surfaces exposed to severe rain, alternate wetting and drying or occasional freezing whilst wet or severe condensationb) Concrete completely immersed in sea waterc) Concrete exposed to coastal environment
iv)
Very severe
a) Concrete surfaces exposed to sea water spray, corrosive fumes or severe freezing conditions whilst wetb) Concrete in contact with or buried under aggressive sub-soil/ground water
v)
Extreme
a) Surface of members in tidal zoneb) Members in direct contact with liquid/solid aggressive chemicals
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.
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:
Nominal Maximum Size Aggregate 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 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.
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.
7.2.4Concrete 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 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.
7.2.4.2 Maximum cement content
Cement content not including fly ash and ground granulated blast furnace slag 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 early thermal cracking and to the increased risk of damage due to alkali silica reactions.
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 accepted standard [6-5A(21)] having more than 50 percent slag or a blend of ordinary Portland cement and slag may be used.
Table 4Requirements for 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 in Accordance with Accepted Standard [6-5A(12)]
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 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
iii)
3
0.5 to 1.0
1.9 to 3.1
1.2 to 2.5
Supersulphated cement or sulphate resisting Portland cement
310
0.50
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 7 of this table are the minimum recommended. For SO3 contents near the upper limit of any class, cement contents above these minimum are advised. For the purpose of calculation of SO3 content (if the test results are in the form of SO4), the following relation shall be used: SO3=0.833SO4.
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 accepted standard [6-5A(21)] with slag content more than 50 percent exhibits better sulphate resisting properties.