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
- The environment
- The cover to embedded steel
- The type and quality of constituent materials
- The cement content and water-cement ratio of the concrete
- Workmanship, to achieve full compaction and efficient curing
- 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
- The cement content provided in this table is independent of cement grades.
- 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.
- 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.
- 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.
- 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:
- High moisture level within the concrete.
- Cement with high alkali content or other sources of alkali.
- 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:
- 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
FAQ’s
What are the different grades of concrete as per IS 456-2000?
IS 456-2000 specifies various grades of concrete depending on their specific strength, from M10 to M80. It classifies normal, standard and high strength concrete based on their specified specific compressive strength at 28 days in N/mm².
What is the significance of the ‘M’ in concrete designations such as M25 or M40?
The ‘M’ in concrete designations refers to the mix, and the number denotes the specified compressive strength of a 150 mm sized cube at 28 days in N/mm².
How do exposure conditions affect the minimum grade and composition of concrete?
: Exposure conditions directly affect the minimum grade of concrete, cement content and water-cement ratio. As per IS 456-2000, different exposure classes require different degrees of protection, which prescribe specific minimum grades and mix proportions.
What factors affect the durability of concrete?
The durability of concrete is affected by various elements such as environmental exposure, embedded steel casing, constituent materials, water-cement ratio, workmanship and size and shape of structural members.
How does the creep of concrete change with time and loading?
Creep stress in concrete depends on the age of the concrete at the time of loading and the duration of the load. As long as the strain does not exceed one third of its specific compressive strength, creep can be assumed to be proportional to the strain.
Pervious Year Gate/ESE Exms- Quest and Answer
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According to IS 456-2000, what are the different exposure conditions for concrete surfaces and how do they influence the minimum grade of concrete?
The exposure conditions in IS 456-2000 range from mild to extreme, affecting the minimum grade, cement content, and water-cement ratio of concrete. Mild exposure requires a minimum grade of M20 for reinforced concrete, while extreme exposure necessitates a minimum grade of M40.
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Explain the importance of the water-cement ratio in concrete durability as per IS 456-2000. How does it influence the strength and longevity of concrete structures?
The water-cement ratio significantly impacts the durability of concrete. Lower water-cement ratios lead to higher strength and reduced permeability, enhancing the concrete’s durability against environmental factors. Per IS 456-2000, a maximum water-cement ratio of 0.45 is recommended for reinforced concrete under severe exposure conditions.
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Discuss the measures to prevent alkali-aggregate reactions in concrete as recommended by IS 456-2000. What are the potential sources of these reactions and their impact on concrete structures?
IS 456-2000 suggests precautions such as using non-reactive aggregates, controlling the alkali content in cement, and reducing the degree of saturation of concrete during service to prevent alkali-aggregate reactions. These reactions occur due to the presence of high moisture, high alkali content in cement, and aggregates containing alkali-reactive constituents, leading to expansive cracking in concrete.
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Elaborate on the role of air-entrainment in concrete mix design according to IS 456-2000. How does air-entrainment impact concrete strength and what adjustments are necessary to maintain desired strength levels?
Air-entrainment in concrete mix design enhances durability by improving resistance against freezing and thawing. It impacts strength by reducing the overall strength of concrete. Adjustments in the mix design should be made to achieve the desired strength by modifying the cement content or adopting suitable admixtures while ensuring the mix’s workability and performance.
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