A durable concrete is one that performs satisfactorily under expected exposure conditions for specified life of the structure. Various factors affecting the concrete durability are discussed below.
Factors Affecting Durability of Concrete
Following are the factors affecting durability of concrete.
- Cement Content
- Aggregate Quality
- Water Quality
- Concrete Compaction
- Curing Period
- Wetting and Drying Cycles
- Freezing and Thawing
- Alkali-Aggregate Reaction
- Sulfate Attack
- Organic Acids
1. Cement Content
- Quantity of cement used in concrete mix will also be a factor affecting durability of concrete.
- If cement content used is lower than the required, then water cement ratio becomes reduced and workability also reduced.
- Adding more water to this mix results in formation capillary voids which will make concrete as permeable material.
- If excess cement content is used, problems like drying shrinkage, alkali-silica reaction may occur which finally effects the durability of concrete.
2. Aggregate Quality
- Use of good quality aggregates in concrete mix will surely increase the durability of hardened concrete.
- The shape of aggregate particles should be smooth and round. Flaky and elongated aggregates effects 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 dense concrete mix.
- Aggregates should be tested for its moisture content before using. Excess moisture in aggregate may lead to highly workable mix.
3. Water Quality
- Quality of water used in concrete mixing also effects the durability of concrete. In general, potable water is recommended for making concrete.
- pH of water used shall be in the range of 6 to 8.
- Water should be clean and free from oils, acids, alkalies, salts, sugar, organic materials etc.
- Presence of these impurities will lead to corrosion of steel or deterioration of concrete by different chemical reactions
4. Concrete Compaction
- While placing concrete, it is important to compact the placed concrete without segregation.
- Improperly compacted concrete contain number of air voids in it which reduces the concrete strength and durability.
5. Curing Period
- Proper curing in initial stages of concrete hardening result in good durability of concrete.
- Improper curing leads to formation of cracks due to plastic shrinkage, drying shrinkage, thermal effects etc. thereby durability decreases.
- Concrete durability gets affected when there is a chance of penetrability of water into it.
- Permeability of water into concrete expand its volume and lead to formation of cracks and finally disintegration of concrete occurs.
- Generally concrete contains small gel pores and capillary cavities. However, gel pores do not allow penetration of water through them since they are of very small size.
- But, capillary cavities in concrete are responsible for permeability, which are formed due to high water cement ratio.
- To prevent permeability, lowest possible water cement ratio must be recommended.
- Use of pozzolanic materials also helps to reduce permeability by filling capillary cavities.
- Moisture present in the atmosphere will also affects durability of concrete structures.
- Efflorescence in concrete occurs due to moisture, which will converts salts into soluble solutions and when it evaporates salts become crystallized on the concrete surface.
- This will definitely damages the concrete structure and durability will be reduced.
8. Temperature effects
- Concrete is heterogeneous material, when fresh concrete is subjected to high temperature rate of hydration gets affected and strength and durability becomes reduced.
- Concrete ingredients have different thermal coefficients, so at higher temperatures, spalling and deterioration of concrete happens.
- Deterioration of concrete also occurs due to severe abrasion.
- When concrete is subjected to rapidly moving water, steel tires, floating ice continuously wearing of surface occurs and durability gets affected.
- Higher the compressive strength higher will be the abrasion resistance.
- When moist concrete is exposed to atmosphere, carbon dioxide present in atmosphere reacts with concrete and reduces pH of concrete.
- When pH of concrete reaches below 10, reinforcement present in the concrete starts corroding.
- Corrosion of reinforcement causes cracks in concrete and deterioration takes place.
11. Wetting and Drying Cycles
- When concrete is exposed to alternate wetting and drying conditions such as tidal waves from sea etc. secondary stresses are developed in concrete.
- Due to these stresses cracks will form and reinforcement is exposed to atmosphere.
- When chlorides or sulfates from sea water meets reinforcement corrosion occurs and durability of concrete is reduced.
- Use of low-permeable concrete, proper cover for reinforcement can prevent this type of problems.
12. Freezing and Thawing
- When fully saturated concrete is exposed to repeat cycles of freezing of thawing, it is deteriorated by the action of freezing and softening of water in it.
- It causes cracking on concrete surface in the form of maps which is called map cracking and effects durability of concrete.
- The coarse aggregate present the concrete also effected by freeze and thaw cycles, spalling of concrete may occurs.
- In this case, durability of concrete can be achieved by adding air-entraining admixtures to the mix and also reduce the maximum size of coarse aggregate.
13. Alkali Aggregate Reaction
- Alkali-aggregate reaction or alkali-silica reaction, takes place between alkali content of cement and silica content of aggregates is also a major factor effecting durability of concrete.
- Due to this reaction, Concrete expansion occurs which finally lead to severe cracking and concrete gets deteriorated.
- Use of cement with less alkali content, Non-reactive aggregates, pozzolanic materials like fly ash or slag cement, Lithium-based admixture in concrete will help to overcome this problem.
14. Sulfate Attack
- When concrete structures are attacked by sulfates like sodium sulfate, magnesium sulfate etc. concrete disintegration happens.
- This reaction is due to the chemical reaction between hydrated cement products and sulfate solutions.
- Sulfate attack generally happens when water used for concrete mix contains high sulfate content, Due to unwashed aggregates, when soil around the concrete structure contains sulfates in it etc.
- This can be prevented by using sulfate resisting cement, by adding slag cement, by decreasing permeability etc.
15. Organic acids
- When concrete surface is subjected to organic acids like acetic acid, lactic acid, butyric acid etc., concrete durability gets affected severely.
- Formic acid on concrete surfaces can lead to corrosion of concrete.