Durability is an essential aspect to consider when designing a structure. Adequate durability can be achieved during the design phase through structural calculations, selection of materials and their proportions, designing details, and incorporating possible preventative measures. This would ensure the structure’s ability to withstand various forms of concrete deterioration, such as weathering actions, abrasion, and chemical attacks.
Concrete durability can also be ensured during the construction phase by taking preventative measures such as preparing, placing, compacting, and curing the concrete properly. These measures would help prevent issues such as chloride action, insufficient concrete cover, and the use of unsuitable construction materials like cement and additives.
Furthermore, preventative measures should be continued throughout the structure’s service life through programmed inspections, monitoring, and maintenance. These measures would help to identify any issues early and address them before they can cause significant damage to the structure.
In summary, adequate concrete durability is vital to ensure the longevity and safety of a structure. It can be achieved through design and construction measures as well as continuous preventative maintenance throughout the structure’s service life.
Design of Concrete Structures for Durability
Durability of structures is closely tied to their permeability, which refers to the ability of water and other substances to pass through their surface. To ensure the longevity of a structure, it is critical to regulate the factors that influence its permeability. This involves establishing acceptable ranges for key parameters such as the minimum amount of cement used, the maximum allowable ratio of water to cement, and the maximum permitted width of cracks in the structure. Another important consideration is the extent of cover to the reinforcement, which also affects the structure’s durability. Depending on the specific exposure conditions, it may be necessary to adjust these parameters to ensure that the structure remains stable and functional over time. By carefully controlling these variables, it is possible to build structures that are resilient and can withstand the elements for years to come.
1. Optimum Cementitious Materials Content and W/C Ratio
In order to ensure the durability of concrete, it is essential to use an optimal amount of cementitious materials. If the materials content is too low, it can compromise the strength of the structural elements, while using too much cementitious materials can increase the risk of shrinkage, which is a common cause of reduced concrete durability.
The water to cement (W/C) ratio also plays a crucial role in enhancing the durability of concrete. A lower W/C ratio is better since it reduces the permeability of concrete, which is a major factor that affects durability. To determine the appropriate minimum cementitious material content and maximum W/C ratio for different exposure conditions, Table 1 can be used as a reference. By following these recommendations, concrete structures can be constructed with greater durability and a longer lifespan.
Table 1 Minimum cementitious materials and maximum water-content ratio for PCC, RCC and Prestressed Concrete
| Exposure condition | PCC | PCC | PCC | RCC | RCC | RCC | PSC | PSC | PSC | |
| – | Min. grade of mix | Min. cementitious* material content, Kg/m^3 | Max w/c ratio | Min. grade of mix | Min. cementitious material content, Kg/m^3 | Max w/c ratio | Min. grade of mix | Min. cementitious material content, Kg/m^3 | Max w/c ratio | |
| Mild | M-20 | 300 | 0.55 | M-25 | 380 | 0.45 | M-35** | 400 | 0.40 | |
| Mod | M-25 | 350 | 0.50 | M-30 | 400 | 0.40 | M-35** | 400 | 0.40 | |
| Severe | M-25 | 380 | 0.45 | M-35 | 400 | 0.40 | M-45 | 430 | 0.40 | |
| Very severe | M-30 | 400 | 0.45 | M-40 | 430 | 0.38 | M-50 | 440 | 0.35 | |
| Extreme | M-30 | 400 | 0.40 | M-45 | 430 | 0.38 | M-50 | 440 | 0.35 | |
In concrete mix design, it is important to consider the maximum size of aggregate (MSA) that will be used in the mixture. For MSA of 20mm or less, a minimum amount of cementitious material content is required to ensure the durability and strength of the concrete. However, the specific minimum amount may vary depending on the other materials used in the mix.
If the MSA is 10mm, an additional 20 Kg/m3 of cementitious material should be added to the mix to achieve the desired strength and durability. On the other hand, if the MSA is 40mm, the amount of cementitious material can be reduced by 10 Kg/m3.
These adjustments are important because the size of the aggregate can affect the amount of cementitious material needed to fill the voids between the aggregates and ensure proper hydration of the mixture. By making these adjustments, the concrete mix can be tailored to the specific needs of the project and the materials being used.
Note:
The maximum amount of cementitious material that can be used per cubic meter is 500 kilograms. This means that if you are working on a project that requires the use of cement, you should not exceed this limit. Cementitious materials include cement, fly ash, slag, and other similar materials that are used to create concrete or other types of construction materials. It is important to follow this guideline to ensure the structural integrity and safety of the building or structure being constructed. Exceeding this limit could result in weaker construction materials that may not meet safety standards. Therefore, it is essential to be mindful of the maximum allowable content of cementitious materials and to adhere to it during construction projects.
Table 2 Description of Exposure Conditions
| Exposure condition | Description |
| Mild | Concrete surfaces protected against weather or aggressive conditions |
| Moderate | Concrete surfaces sheltered from severe rain or freezing while wet, concrete continuously underwater |
| Severe | Concrete surface exposed to severe rain, alternate wetting, and drying or occasional freezing or severe condensation. Concrete exposed to aggressive subsoil/groundwater or coastal environment |
| Very severe | Concrete surface exposed to seawater spray, corrosive fumes, severe freezing |
| Extreme | Concrete surface exposed to abrasive action. Surface of members in tidal zone |
2. Minimum Clear Concrete Cover
Adequate concrete cover thickness is crucial in maintaining the durability of concrete structures. This is because concrete cover is responsible for protecting steel bars from external hazards such as carbonation, chloride, fire, and other detrimental influences. To ensure sufficient protection, Table 3 presents the minimum recommended values for concrete cover for various structural elements in different exposure conditions.
It is also important to note that the concrete cover should not exceed 2.5 times the diameter of the reinforcing bar during the design phase. If the cover thickness exceeds this limit, chicken mesh may be provided in the concrete to keep it in position. By adhering to these guidelines, the integrity and longevity of the concrete structure can be preserved, and potential hazards can be avoided.
Table 3 Minimum clear cover for concrete structures under different environmental conditions
| Sr. No. | Structures RCC/PSC | Extreme environment | Very severe environment | Severe environment | Mild and moderate environment |
| 1 | Slabs | 50 | 50 | 25 | 25 |
| 2 | Beams | 60 | 50 | 40 | 35 |
| 3 | Columns | 75 | 75 | 50 | 50 |
| 4 | Well, piles, and footings | 75 | 75 | 75 | 50 |
| 5 | PSC girder | 50 | 50 | 50 | 50 |
| 6 | PSC girder for HTS cables | 75 | 75 | 75 | 50 |
3. Flexural Crack Width
To protect steel rebar against corrosion, it is crucial to control the width of cracks that may occur in concrete structures. One way to achieve this is by minimizing shrinkage and ensuring that reinforcement is distributed uniformly over the area of maximum concrete tension. This can be accomplished by using smaller diameter bars.
Moreover, it is recommended to use cement with a low heat of hydration and moderate fineness to further minimize the risk of cracks. To ensure that the design of the concrete structure meets the required specifications, it is necessary to consider the maximum allowable crack width values, which are provided in Table 4.
By adhering to these guidelines, the risk of corrosion in steel rebar due to cracking can be minimized. It is important to note that the consequences of not controlling the width of cracks can be severe and lead to the eventual failure of the structure. Therefore, careful attention should be given to the selection of materials and design considerations to ensure the longevity and safety of the structure.
Table 4 Maximum Crack Width to be Considered During Design
| Type of structure | Exposed to severe, very severe, and extreme environment | Unexposed to severe, very severe, and extreme environment | Exposed to mild and moderate environment |
| Bridges | 0.10mm | 0.20mm | 0.20mm |
| RCC beams, slabs | 0.20mm | 0.20mm | 0.30mm |
4. Concrete Placement
The key to achieving excellent durability for concrete is through proper placement. It is crucial to ensure that the concrete is placed in the right location and adequately consolidated. This is because voids or rock pockets that may form in the concrete can be eliminated with proper placement and consolidation.
If concrete is not placed correctly, it can result in weak points or flaws that could ultimately compromise its strength and durability. These weaknesses can result in cracks, which can then lead to further damage and potential failure. It is, therefore, essential to take great care when placing concrete and ensuring that it is done correctly.
Proper consolidation is also critical when placing concrete. Consolidation involves removing any voids or air pockets that may be present in the concrete mixture. This can be achieved through various methods, such as using vibrators or tamping the concrete. By adequately consolidating the concrete, it is possible to eliminate any potential weaknesses, which can help to improve its strength and durability.
Overall, achieving excellent durability for concrete is dependent on proper placement and adequate consolidation. Ensuring that concrete is placed correctly and any voids are removed can help to eliminate weaknesses and improve its overall strength and durability. It is, therefore, essential to take great care when placing concrete to ensure that it is done correctly and to avoid any potential issues that could compromise its integrity.
5. Proper Curing
Proper curing is crucial for ensuring the durability of concrete and achieving the desired strength. This is because the curing method employed has a direct impact on the quality and longevity of the concrete. Without adequate curing, the concrete may not reach its designated strength and may be prone to premature cracking and other forms of damage.
Moisture and humidity are particularly important during the curing period, as they help to maintain the necessary level of hydration of the cementitious materials in the concrete. This hydration process is essential for the concrete to develop the necessary strength and durability, and failure to maintain adequate moisture levels can result in shrinkage and subsequent cracking.
By employing an appropriate curing method and ensuring that the concrete is kept sufficiently moist and humid during the curing period, it is possible to avoid these issues and ensure that the concrete is strong and durable. This can help to extend the lifespan of the concrete and reduce the need for costly repairs and maintenance over time.