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
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.