Factors Affecting the Durability of Concrete

1. Water-to-Cement Ratio

The water-cement ratio is the ratio of the weight of water to the weight of cement used in a concrete mix. The water-cement ratio has an inverse relationship with the strength of concrete, as the water-cement ratio increases, the strength decreases, and vice versa. It is directly related to workability because if the water-cement ratio increases then the concrete is wet and more consistent and can be placed easily. The water-to-cement ratio is a measure of concrete strength. The strength of cement paste increases as the amount of cement in the concrete grows but falls with air and water content. The water-to-cement ratio is inversely proportional to the strength of fully compacted concrete at a given age and thermal temperature. The strength vs. water-cement ratio graph shows that when concrete is vibrated to increase strength, a lower water-cement ratio can be used, whereas when concrete is hand compacted or manually compacted, a higher water-cement ratio is required. When the water-cement ratio falls below the practical limit in either situation, the strength of the concrete rapidly deteriorates due to the entrance of air spaces. Water-cement ratio of less than 0.38 is commonly considered very low because 38% of water is needed for cement to complete hydration.

Relationship between w/c ratio and strength of concrete

2. Permeability

When there is a potential of water penetrating the concrete, it loses its resilience. Water permeability in concrete causes it to increase in volume, resulting in the production of fissures and, eventually, disintegration. Small gel holes and capillary cavities are common in

concrete. Gel pores, on the other hand, because of their microscopic size, do not enable water to pass through. However, permeability is caused by capillary voids in concrete, which emerge as a result of a high-water-cement ratio. To avoid permeability, the smallest feasible water-cement ratio should be used. By filling capillary voids, pozzolanic materials can aid to lower permeability. Moisture movement combined with reactive chemicals in an aggressive atmosphere has an impact on the longevity of the product. Because of the concrete's porosity, it is possible for it to carbonate. With a higher permeability, the air has more room to travel. Carbon dioxide may move, and when water and carbon dioxide are present, carbonation occurs, resulting in reinforcing corrosion. Corrosion may also have an impact on the durability of concrete with a limited permeability. Because of the poor permeability, water absorbs and passes through the concrete portions, causing serviceability concerns such as paint discoloration, bacteria growth, and so on.

Capillary absorption in concrete block

3. Cement Content

Concrete's cement composition has a variety of effects on its long-term durability. The heat of hydration will increase as the cement content rises. Peak temperature, temperature gradient and temperature differential between the core and the surface will all rise as a result. Internal cracking and surface cracking have been seen as a result of temperature differences and larger temperature gradients. If the cement content utilized is less than what is necessary, the water-cement ratio decreases and the workability decreases as well. By adding more water to this mixture, capillary gaps emerge, making concrete a porous substance. If too much cement is applied, issues including drying shrinkage and alkali-silica reaction might develop, reducing the concrete's durability. The creation of delayed ettringite, which causes concrete cracking, may be triggered by a rise in core temperature over 70-75 degrees Celsius.

However, higher-grade concrete should be used since it is more durable than lower-grade concrete. Different ways may be used to reduce the rise in heat due to hydration, such as employing fly ash, controlling the formwork arrangement, and so on. Due to poor workability, lowering the cement content becomes a difficulty. Furthermore, the quality of the cement used in the mix will have an impact on the concrete's longevity. The quality of cement content is one of the factors that affect the durability of concrete, The mix must be designed to ensure cohesion and prevent segregation and bleeding. If cement is reduced, then at a fixed w/c ratio the workability will be reduced leading to inadequate compaction. However, if water is added to improve workability, the water/cement ratio increases and resulting in highly permeable material. If the cement content used is lower than the required, hen the water-cement ratio becomes reduced, and workability is also reduced. Adding more water to this mix results in the formation of capillary voids which will make concrete a permeable material. If excess cement content is used, problems like drying shrinkage, and alkali-silica reactions may occur which finally affects the durability of concrete.

In short, if the cement content is exceeded, shrinkage in concrete will happen on the other hand the lower cement content decreases the bondage between the concrete materials reducing the strength of concrete.

Cement and Fly ash

4. Aggregate Quality

The use of good-quality aggregates in concrete mix will increase the durability of hardened concrete. The shape of aggregate particles should be smooth and round. Flaky and elongated aggregates affect the workability of fresh concrete. For better bond development between ingredients, rough-textured angular aggregates are recommended but they require more cement content. Aggregate should be well-graded to achieve a dense concrete mix. Aggregates should be tested for their moisture content before use. Excess moisture in aggregate may lead to a highly workable mix. Selecting aggregates that are stable and durable by themselves is a primary requirement for durable concrete. Recent developments, however, have shown that sometimes aggregates that are durable by themselves can be deleteriously reactive with alkalis from the cement or elsewhere.

Aggregates quality should consist of naturally occurring (crushed or uncrushed) stones, gravel, sand, or a combination thereof. They should be hard, strong, dense, durable, clear, free from veins and adherent coating, and free from injurious amounts of disintegrated pieces, alkali, vegetable matter, and other deleterious substances. As for as possible, flaky and elongated pieces, pieces are avoided.

Aggregates

Grades of Aggregates

5. Water Quality

Water is an important ingredient of concrete as it actively participates in the chemical reaction with cement. Since it helps to form the strength-giving cement gel; the quantity and quality of water required are considered. A popular suitability of water for mixing concrete is that if water is fit for drinking; it is fit for making concrete. This does not appear to be a true statement for all conditions. Some waters containing a small amount of sugar would be suitable for drinking but not for mixing concrete. Higher the water content in a concrete mix, the higher the water-cement ratio for the same cement content. In general, potable water is recommended for making concrete. The pH of water used should be in the range of 6 to 8. Water should be clean and free from oils, acids, alkalis, salts, sugar, organic materials, etc.

6. Concrete Compaction

Compaction of concrete is the process that expels entrapped air from freshly placed concrete and packs the aggregate particles together to increase the density of concrete. It significantly increases the ultimate strength of concrete and enhances the bond with reinforcement. It also increases the abrasion resistance and the general durability of the concrete decreases the permeability and helps to minimize its shrinkage-and-creep characteristics. The process of compaction of concrete consists essentially of the elimination of entrapped air from the concrete. The oldest means of achieving this aim is by punning with a rod or ramming, or by vibrating. Thus, the object of compacting concrete is to obtain a dense mass of concrete without voids, to get the concrete to surround all reinforcement, and to fill all corners.

Compaction of concrete influences its strength, durability, and impermeability of concrete to a great extent. Each one percent less compaction reduces the strength of concrete by about 5% on average. During the process of manufacturing concrete, a considerable amount of air is entrapped forming voids in it. The permeability of concrete will increase due to improper compaction and moisture can easily be entrapped to the steel and corrode the concrete element. Shrinkage cracks may appear on the concrete surface over time. Due to improper compaction, the honeycomb will be formed on concrete, and chances for reinforcement corrosion. The concrete ingredients will be segregated improperly due to improper concrete compaction, which results in strength reduction. The durability of concrete is reduced due to the cracks formed by inadequate compaction.

Compacting fresh concrete

Vibration of concrete

7. Curing Period

The curing of concrete has a significant impact on the durability of concrete constructions. Inadequate curing can result in a highly weak and porous material at the concrete's surface, making it exposed to the entry of numerous dangerous elements from the environment. Proper curing in the initial stages of concrete hardening results in good durability of concrete.

Improper curing leads to the formation of cracks due to plastic shrinkage, drying shrinkage, thermal effects, etc. thereby durability decreases. It's a method of regulating moisture on the concrete's surface. Produces a thick microstructure due to which permeability should be improved. Curing for a long time improves the durability of the product. Curing improves the surface hardness of concrete, allowing it to endure wear and abrasion. Improved permeability because proper curing prevents water-borne contaminants from entering. Increase the concrete structure's durability and lifespan.

• It will prevent concrete from thermal cracking.

• It elevates the durability of concrete.

• It will increase the moisture in concrete up to a satisfactory level.

• It also prevents shrinkage cracks.

• It enables the concrete to have abrasion resistance.

• It will enhance the stability of concrete.

• It will cause a reduction in permeability.

• It provides excellent performance of concrete.

• It provides quality control in the concrete.

• The concrete with proper curing will also protect steel from corrosion.

Curing of concrete

Drying shrinkage due to poor curing

How curing affects the strength of concrete

8. Construction Defects

It is the contractor's obligation to complete the construction without any defects and in accordance with the authorized quality assurance and quality control techniques. The contractors' lack of attention has an impact on the concrete's endurance. Concrete cracks because of poor or insufficient curing. The temperature at which the concrete is placed has an impact on the peat temperature rise in thick concrete. Placing concrete at a temperature other than that stipulated in the design might cause problems with durability, such as concrete cracking. The production of honeycombs in concrete can be caused by insufficient or improper concrete compaction. Furthermore, it increases the amount of trapped air, which may compromise the longevity of the product.

9. Cover

The concrete cover is the thickness of the concrete measured from the outermost reinforcement. The main reason for providing cover is to protect the reinforcement from the corrosive external environment. The cover of concrete varies depending on a variety of factors e.g. climate, temperature, soil conditions, water table, the importance of the structure, thickness of the concrete structure, etc.

What is the purpose of concrete cover?

• Covering is made to restrict the attack of corrosion to reinforcement. Reinforcement is prone to corrosion and fire due to atmospheric conditions.

• If a proper concrete cover is not made, then there is a high chance of rusting and hardened RCC cracks.

• The cover also provides protection against fire, it is also essential for developing adequate bond strength along the steel bar surface area.

• The concrete cover is essential for every building element i.e. beams, slabs, and footings. Where the reinforcement is present, we use concrete covers to maintain the proper arrangement of steel bars and provide a cover for bars.

Therefore to avoid rusting and hardened of reinforced cement concrete the thickness of The concrete cover must follow the limits set in the codes.