Durability of Concrete – Is 456-2000

The durability of concrete is very important for any concrete structure. It plays a crucial role in determining the structural longevity. Also, the Concrete Durability is one that performs satisfactorily in the working environment during its anticipated exposure conditions in service. The material and mix proportions (Cement, Sand, Aggregates) specified and used should maintain its integrity and, if applicable, protect embedded metal from corrosion.

One of the main characteristics influencing the durability of concrete is its permeability(IS 456-2000) (Water Flow or present in the concrete or W/c Ratio) (you can Read in detailed factors affectings on the concrete click here) 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, suitable low permeability is achieved by having an adequate content, sufficiently low free water-cement ratio, ensuring complete compaction of the concrete, and adequate curing.

Here are the Factors affecting durability –Is Code 456-2000

1. The environment
2. The cover to embedded steel
3. The type and quality of constituent materials
4. The cement content and water-cement ratio of the concrete
5. Workmanship, to achieve full compaction and efficient curing
6. The shape and size of the member

The degree of exposure anticipated for the concrete during its service life, along 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 into account compliance as described in this standard.

Shape and Size of Member

The shape and design details of exposed structures should facilitate effective water drainage, preventing stagnant pools and water run-down. Special attention should be paid to minimizing potential cracks that could collect or allow water transmission. Adequate curing is essential to prevent the harmful effects of early moisture loss.

Member profiles and their intersections with other structural elements should be designed and detailed to ensure easy concrete flow and proper compaction, particularly in partially immersed sections and at corners during concreting.

Concrete is more susceptible to deterioration from chemical or climatic influences when in thin sections or under hydrostatic pressure from one side, especially in partially immersed sections and at corners and edges. The longevity of the structure can be extended by providing additional cover to steel, using chamfering, and employing surface coatings to prevent or reduce the ingress of water, carbon dioxide, or aggressive chemicals.

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Exposure Conditions; IS 456-2000

General Environmental Conditions

The general environmental conditions to which the concrete will be exposed during its service life are detailed below as per IS Code  456-2000 ;

Level	Concrete Exposure Conditions
Mild	Concrete surfaces protected against weather and aggressive conditions, except those situated in coastal areas.
Moderate	Concrete surfaces sheltered from severe rain or freezing, while concrete is exposed to occasional condensation and rain.
	Concrete continuously under water or in salt air in coastal areas.
Severe	Concrete surface exposed to severe rain, alternate wetting and drying, occasional freezing while wet, or severe condensation.
	Concrete completely immersed in seawater.
	Concrete exposed to coastal environments.
Very Severe	Concrete surfaces exposed to seawater spray, corrosive fumes, or severe freezing conditions while wet.
	Concrete in contact with or buried under aggressive subsoil groundwater.
Extreme	Surfaces of members in tidal zones.
	Members in direct contact with liquid or solid aggressive chemicals.

 

Abrasive

Some specialists and literature are referred to for the durability requirements of concrete surfaces exposed to abrasive actions, such as those caused by machinery and metal tires.

Freezing and Thawing

In instances where freezing and thawing actions occur under wet conditions, enhanced durability can be achieved by using suitable air-entraining admixtures. When concrete of a grade lower than M50 is subjected to these conditions, the mean total air content by volume of the fresh concrete at the time of delivery into the construction site should be considered.

This ensures better resilience and endurance of the concrete in such challenging environmental conditions.

Nominal 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 to achieve the required strength

Exposure to Sulphate Attack – IS 456-2000

Table 4 provides recommendations for the types of cement, maximum free water-cement ratio, and minimum cement content necessary at different sulphate concentrations in near-neutral groundwater with a pH range of 6 to 9.

In cases of very high sulphate concentrations in Class 5 conditions, additional measures such as lining with materials like polyethylene or polychloroprene sheets or using surface coatings based on asphalt, chlorinated rubber, epoxy, or polyurethane should be employed to prevent sulphate solution penetration.

Requirement of Concrete Cover      

Protecting the steel in concrete against corrosion relies on an adequate thickness of good-quality concrete. The nominal cover for reinforcement should be provided as per 26.4.

Concrete Mix Proportions

The free water-cement ratio is a crucial factor governing concrete durability and should always be kept at the lowest feasible value. Appropriate values for minimum cement content and the maximum free water-cement ratio are detailed in Table 5 for various exposure conditions. The minimum cement content and maximum water-cement ratio apply to 20mm nominal maximum size aggregates. For other aggregate sizes, adjustments should be made as indicated in Table 6.

Maximum Cement Content

Cement content, not including fly ash and ground granulated blast furnace slag, exceeding 450 kg/m3 should not be utilized unless special design considerations are given to the heightened risk of cracking due to drying shrinkage in thin sections, early thermal cracking, and increased vulnerability to damage due to alkali-silica reactions

Class	Concentration of sulphate Expressed as SO3			Types of Cement	Dense Fully Compacted concrete Made with 20mm nominal maximum Size Agregates Complaying with IS 383
	In Soil		In Gorund Water		Minimum Cement Content	Maximum  Cement Content
	Total So3	So3 in 2:1 Water soil Extract
	Percent	g/l	g/l
1	Traces (<0.2)	Less Then 1.0	less Then 0.3	Ordinary Portland cement or Portland slag cement or Portland pozzolana cement	280	0.55
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	0.5
				sulphated cement or Sulphate resisting Portland cement	310	0.5
3	0.5 to 0.1	1.9 to 3.1	1.2 to 2.5	Portland pozzolana cement or Portland slag cement	330	0.5
					350	0.45
4	1.0 to 2.0	3.1 to 5.0	2.5 to 5.0	Supersulphated cement or Sulphate resisting portland cement	370	0.45
5	More then 2.0	More then 5.0	More then 5.0	Sulphate resisting portland cement Supersulphated cement with protective coatings	400	0.4

Important Notes

1. The cement content provided in this table is independent of cement grades.
2. The use of super sulphated cement can ensure an acceptable service life, given that the concrete is dense and prepared with a water-cement ratio of 0.4 or less in mineral acids down to a pH of 3.5.
3. The cement content listed in column 6 of this table represents the minimum recommended for SO3 content near the upper limits of any cement class. Cement content above these minimums is advisable.
4. For severe conditions such as thin sections under hydrostatic pressure on one side only and partially immersed sections, considerations should be made for a further reduction in the water-cement ratio.
5. Portland slag cement, along with sulphates in soil or groundwater-resistant cement, can be used. Alternatively, Portland slag cement conforming to IS 455, having more than 50 percent slag, or a blend of ordinary Portland cement and slag may be utilized, provided sufficient information is available regarding the performance of such blended cements in these conditions

Chlorides in concrete`

Presence of chloride in concrete heightens the risk of corrosion. Upon subsequent exposure to warm, moist conditions, the risk of corrosion increases. All concrete constituents, including cement, aggregates, water, and admixtures, may contain chloride. To minimize the chance of concrete deterioration from harmful chemical salts, the level of harmful salts present should be limited. These harmful salts can originate from the external environment as well as diffuse from the concrete materials.

The total amount of chloride as Cl in the concrete at the time of placement shall be provided in Table 3 below.

The total acid-soluble chloride content should be calculated from the mix proportions and the measured chloride content of each constituent whenever possible. The total chloride content of the concrete should be determined

Sulphates in Concrete

Moist cements and certain aggregates contain sulphates. Excessive amounts of water-soluble sulphates from these or other mix constituents can lead to 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 the 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 supersulphate cement complying with IS 6909.

Alkali Aggregate Reaction:

Some aggregates, particularly those with specific varieties of silica, may be susceptible to attack by alkalis (Na2O and K2O) originating from cement or other sources. This interaction can cause an expansive reaction leading to cracking. This reaction usually occurs only when all the following conditions are present together:

1. High moisture level within the concrete.
2. Cement with high alkali content or other sources of alkali.
3. Aggregates containing alkali-reactive constituents.

When the service records are well established and do not include instances of cracking due to alkali aggregate reaction, precautions should be taken, such as:

1. Use of non-reactive aggregates from alternative sources

The cement content specified in this table is independent of cement grades and includes the additions mentioned in 5.2. Additions like fly ash or ground granulated blast furnace slag may be considered in the concrete composition concerning the cement content-water-cement ratio, provided their suitability is established. It is essential to ensure that the maximum amounts considered do not exceed the pozzolana and slag limits specified in IS 1489 Part 1 and IS 455, respectively.

The minimum grade of plain concrete under mild exposure conditions is not specified.

b) Use of low alkali ordinary Portland cement with a total alkali content not exceeding 0.6 percent as Na2O equivalent can be advantageous. Further benefits can be obtained by utilizing fly ash Grade 1 or blast furnace slag conforming to IS 12089 as a partial replacement of ordinary Portland cement, with a total alkali content not exceeding 0.6 percent. This is provided that the fly ash content is at least 20 percent or the slag content is at least 50 percent.

c) Measures to reduce the degree of saturation of the concrete during service, such as using impermeable membranes, are recommended.

d) Limiting the cement content in the concrete mix and thereby restricting the total alkali content in the concrete is advised