Knowing the factors affecting durability of concrete is the key to define the life span of structures. Let’s discuss about the durability of concrete.
There are other factors such as concrete class, water-cement ratio, etc that need to consider when defining the durability. The structural classes clearly specify the durability periods of each type of structure.
Factors affecting durability of concrete listed below
- High humidity and Rain
- Ultraviolet resistance
- Chemical Resistance
- Seawater exposure
- Chloride resistance and steel corrosion
- Sulfate resistance
- Resistance to Alkali-silica reaction (ASR)
- Carbonation
- Abrasion Resistance
- Moderate to severe exposure condition for concrete
- Resistance to freezing and Thawing
- Cement Content
- Aggregate quality
- Water Quality
- Concrete compaction
- Placing time after batching and cold joint formation
- Curing period
- Permeability
- Temperature
- Construction defects (honeycombs, cracks, etc.)
High Humidity and Rain on Durability of Concrete
Moisture is essential for the concrete to get it to harden. The following factors can identify as key issues related to the humidity effect.
- Moisture is essential for concrete to get harden and retention of the humidity within the concrete support to the cement to reaction and increase the strength.
- However, excess water in the concrete could increase the gap between aggregates. Evaporation of the moisture in the concrete reduces the strength of the concrete as it has not completed adequately. The reduction of the strength of the concrete could reduce significantly and it could even drop by 40%.
- However, prolong retention causes issues. After the hydration process of the concrete, mid-range of moisture could lead to the carbonation of the concrete as shown in the following figure. Therefore, a very low or very high range of moisture could be acceptable in terms of durability.
- A high range of moisture with chloride will cause issues as chloride could penetrate into the concrete.
- The compressive strength of the concrete reduces with the high relative humidity resulting in durability issues.
- Higher moisture in the surface of the concrete leads to the growth of mold, mildew, and bacteria. Continuous growth could affect the strength of the concrete. Further, it could lead to health issues also.
- Relative humidity affects the pH levels of the concrete. It could affect the integrity of the concrete and any adhesive or coatings materials applied to it.
Rain will affect the concrete in two stages. Unharden concreting getting wet is a severe issue in terms of the durability of concrete and continuous periodic wetting and drying due to the rain is not considered as a very high-risk factor affecting durability of concrete.
- Rain effect the water cement ratio of the area exposed to wetting. It produces low strength concrete having less compressive strength.
- Further, it reduces the durability of the concrete and as a result, it could affect the reinforcement within the concrete.
Ultraviolet Resistance
Ultraviolet waves in solar radiation do not harm the concrete directly. However, it affects the applications on the concrete. The pigment in concrete retains its colour required for aesthetic appearance.
Further, application type water proofings exposed to ultraviolet leads to deterioration of the material.
Chemical Resistance
Concrete has a higher resistance to the chemicals that exist in the environment. In a natural environment, concrete most of the time survive without any damage.
However, there are harmful chemicals that can react with concrete with prolonged exposure. Chemicals such as chlorides, sulfates, bourbon, alkali-silica reaction, etc. can affect the durability of reinforced concrete and such actions of concrete are known as chemical attacks.
Chloride and sulfates attach to the reinforcement leas to the corrosion and as a result structure lift span reduces significantly. Carbonation is also a process of deterioration of the concrete and for more information, articles on this web could be referred.
Eventually, concrete gets cracked due to this issue and structures cannot be occupied. Attention shall be made at the design stages to minimize the effect on the durability of concrete.
Seawater Exposure on Durability of Concrete
Seawater is a definite corrosive environment that affects the durability structure. The structure could be close to the sea, submerged in the sea or part of the structure could be exposed to the tidal waves.
Due to the limited spaces and as per the requirement of the projects, the construction of marine structures is inevitable, and necessary precautions shall be taken at the stage of designing and constructing to protect the structures from probable risk.
Reduction of permeability, providing adequate cover, selecting a suitable grade of concrete based on the structural class and exposure conditions, etc. are to be considered at the design stage.
Properly design concrete mixtures tested for the above shall be used for the construction with a high level of quality control measures.
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Chloride Resistance and Reinforcement Corrosion
They are interrelated and more resistant to chloride attack, more the safety against the corrosion.
Chlorides are widely available in marine environments and areas close to the sea. Therefore, there is a high chance of exposure to chlorides.
permeability of the concrete is one of the key factors that allow the transportation of chloride irons to the reinforcements. Less the permeability, lesser the riks of corrosion of reinforcements.
Further, the curing of the concrete directly affects the permeability of the concrete. Quality control and quality assurance process in the construction stage directly influence the durability of the concrete structure.
Sulfate Resistance on Durability of Concrete
A sulfate attack is one of the deterioration methods of concrete. This process can be initiated without any influence on the environment or with the support of the environment. The increase in the volume of the concrete causes cracking.
- Generally, the sulfate related stresses are developed in the surface of the concrete referred to as external sulfate attack.
- It causes by the reaction of the sulfates contained in the groundwater or soil reacting with the cementitious paste in concrete.
- Though deterioration starts at the contact face it develops further.
- The chemical reaction creates very high tensile stresses and it causes cracking and disintegration of concrete
- Porous concrete attacked than the low permeable concrete in general.
- Sulfate attach continue until the sulfate can penetrate into the concrete
- According to the ACI 318 following ranges could be considered in the design.
Negligible – When sulfate content is less than 150 ppm (mg/l) in water, no restriction on cement type is necessary.
Moderate – When sulfate content is between 150 and 1500 ppm in water, Type II cement with a pozzolan additive should be used.
Severe – When the sulfate content is between 1500 and 10 000 ppm in water, Type V cement should be used, along with a w/cm ratio below 0.45.
Very severe – When the sulfate content exceeds 10 000 ppm in water, Type V cement should be used with a pozzolan and a w/cm of below 0.45. In addition, the ACI Building Code recommends a minimum f′c of 29 MPa (4250 psi) (Mehta and Monteiro, 2006).
Some other sources of sulfate could be attached
- Seawater
- Oxidation of sulfide minerals in clay adjacent to the concrete
- Bacterial action in sewers
- Sulfates present in bricks and can be gradually released over a long period of time, causing sulfate attack of mortar, especially where sulfates are concentrated due to moisture movement
Resistance to Alkali Silika Reaction
Alkali silica reaction is a durability issue of concrete that can widely observe in water retaining structures. Usually, these cracks appear after a long time of construction.
The reaction occurs in the presence of alkali in the cement, reactive aggregate in the concrete, continuous availability of moisture and having heated up the required level.
As a result of the reaction, concrete will crack and the pattern of the cracks will appear like the following image.
The reaction can be avoided or minimized by taking the following actions.
- Aggregate can be checked to identify the reactivity and the use of such aggregate shall be avoided.
- Use low alkali cement
- Control the continuous availability of water for the reaction. However, this could not be possible for structures such as concrete dams, spillways, water tanks, etc.
- Controlling the optimum temperature is one of the actions that can take to avoid the reaction. This is also very difficult to do in practice in real life.
Carbonation
Carbonation is a process of reacting to the carbon dioxide in the environment with the calcium hydroxide in the concrete. The process creates a calcium carbonate and lowers the pH values to 9. As a result, the oxide layer around the reinforcements breaks and reinforcements expose to corrosion.
The presence of the moisture is very important for the reaction and in dry environments, carbonation will be slow. Further, carbon dioxide penetrates through the pours in the concrete rapidly if there are more voids in the concrete surface.
Carbonation can be avoided or minimized taking following actions
- Application of anti-carbonation coatings prevent the ingress of carbon dioxide
- If carbonation has taken place and reinforcement is corroded, a repair method shall be used.
- Application of anticorrosive paint to the reinforcement shall be done and then proceed with the concrete repair.
Abrasion Resistance
Abrasion resistance is resistance to waring due to contact with another surface.
Wearing of the concrete depends on the following factors.
- Strength of the concrete
- Further, increase the cement content and reduce the water content improves the abrasion resistance.
- Use of well-graded natural sand improves the abrasion
- Coarse aggregate should be free from soft sandstone or soft limestone
Following above at the design stage will improve the abrasion resistance.
Moderate to Severe Exposure Condition for Concrete
There is a definite relationship between the durability and the exposure condition of the structure.
It is the reason we always consider the exposure class when selecting the cover to the reinforcements.
A structure can be exposed to different environments such as continuous wetting and drying, rarely wetting and drying, exposure to seawater, exposure to chemicals, etc.
Depending on the exposure class, there are factors to be considered to establish the durability requirement of concrete.
When the severity of the exposure condition increases its effect on the durability of the concrete structure becomes more severe. Much attention shall be made when the exposure condition is moderate to severe.
Resistance to Freezing and Thawing
Freezing and Thawing occurs due to the seasonal variation in the world. Water molecules in the concrete freeze and it increases the volume by 9%. As a result, tensile stresses are developed creating the cracks in the concrete.
Freezing and Thawing effect can be controlled by controlling the entrained air in the concrete.
Further reading on the Freezing and Drawings refer to the article Thermal Cracking of Concrete.
Cement Content
Cement content of the concrete affects the durability of concrete by different means.
- The increase in the cement content will increase the heat of hydration.
- As a result, peak temperature, temperature gradient and the temperature difference between the core and the surface will increase.
- Internal cracking, cracking in the surface could be observed due to the temperature different and due to the higher temperature gradients.
- Rise of the core temperature beyond the 70-75 celsius could initiate the delayed ettringite formation which causes the cracking of concrete.
- However, a higher grade of concrete use as it is more durable than the low-grade concrete in terms of durability. The increase of the heat due to the hydration can control by different methods such as using fly ash, control the formwork arrangement, etc.
- The reduction of the cement content also becomes an issue due to poor workability.
- Further, the quality of the cement used in the mix will also affect the durability of the concrete
Aggregate Quality
- Having a thin layer of dust on the coarse aggregate and fine aggregates reduces the bonding strength
- As a result, the compressive strength of the concrete will reduce
- Further, dust could affect the chemical reaction between the water and the cement
- Aggregate shall inspect and check for these issues before using for concrete mixing
- Properties such as mechanical and physical directly affect the strength of the concrete
- The article, 30 construction material testing provide a comprehensive guide on selecting the material for construction.
Water Quality
- Higher the water content in a concrete mix, higher the water-cement ratio for the same cement content.
- Increase the moisture level within the concrete due to the excess water leads to reduce the compressive strength of the concrete and it will create durability issues.
- Similarly, it increases the content of impurities such as salt in the concrete with the increase of the water content.
- Polluted water affects the hydration process and necessary reactions could be slow down or obstruct.
- Polluted water may cause the corrosion of reinforcements.
- Further, the pH value of the water is also checked to known the quality of the water
Compaction of Concrete on Durability of Concrete
- Concrete is compacted when it poured for better properties. It directly related to the strength of the concrete which is directly a function of the durability of the concrete.
- It improves the abrasion resistance and durability
- Proper compaction decreases permeability and as a result, it helps to minimize the shrinkage and creep characteristics.
- The excess air trap in the concrete is removed by the compaction.
- Well compacted concrete minimizes the formation of honeycombs that causes cavities within and the surface of the concrete.
- Moisture and air can move through these defects and they can cause durability issues as discussed previously.
- Further, poorly compacted concrete could have more porosity than properly compacted concrete.
- Lack of compaction could lead to water leakages in water retaining structures.
Pacing Time of Concrete after Batching and Cold Joint Formation
Initial setting time of the concrete and final setting time of the concrete are key factors that need to be considered at the construction stage.
Any concrete shall be poured and compacted before the initial setting time allowing the start of the chemical reactions for hardening without any disturbances.
Any concrete poured after starting the initial setting time, it could lead to the formation of the cold joints. Therefore, it is very important to attend the placing time of the concrete.
In general, the initial setting time is about one hour for normal concrete. However, the use of admixtures in modern construction has increased or decreased the setting time depending on the requirement of the project.
A cold joint is a weaker plane in the sense of appearance but generally, it is not a structural problem. It could lead to durability issues due to the increase in the porosity at the joints. Due to the weak bond between the two pours, water leakages in water retaining structures, corrosion of reinforcements, etc. can be expected.
Curing Period on Durability of Concrete
The curing of the concrete is very important due to the following reasons.
- It is a process of controlling the moisture in the surface of the concrete
- Creates dense microstructure and Improve the permeability
- Prolong curing enhance the durability
- Proper curing improve the surface hardness and concrete can withstand surface wear and abrasion
- Improvement of the permeability due to the adequate curing avoids the entering water-borne chemicals. Increase the durability and life span of the concrete structure
- The above figure clearly evident that the curing will affect the strength of the concrete and eventually it affects the strength durability of concrete.
Permeability on Durability of Concrete
- Permeability directly affect the durability as it allows the movement of moisture
- Movement of moisture with reactive chemicals in an aggressive environment affect the durability
- Carbonation of the concrete could occur due to the porosity of the concrete. Higher permeability means it has more space to the air to travel. Carbon dioxide could travel and with the presence of water and carbon dioxide carbonation takes place creating a situation for reinforcement corrosion.
- Similarly, corrosion could also affect the durability of concrete with low permeability.
- Absorption of the water and passing through the concrete sections due to the low permeability creates serviceability issues such as discoloring of paints, formations of different bacterias, etc.
Temperature
The temperature of the concrete and the environment directly related and they are affecting the durability of the concrete structures from its existence. The followings are some of the key factors that can be highlighted.
- The durability of concrete connected to lots of properties of the concrete form its cast.
- Craking of the immature concrete due to the rise of the temperature causes severe durability issues.
- Rise of the core temperature of the concrete creat short term issues with crack appeared and it also creates long term issues such as the formation of delayed ettringite which causes the internal cracking of the concrete due to the internal reactions causes.
- The seasonal variation of the temperature increases the internal strains in the concrete. A structure could have not designed properly by taking into account the necessary factors and have not provided sufficient expansion joints, could crack with the time.
Construction Defects on Durability of Concrete
- It is the responsibility of the contractor to proceed with the construction without any defect and as per the approved quality assurance and quality control methods.
- The lack of attention of the contractors affects the durability of the concrete.
- Poor or inadequate curing will lead to cracking of the concrete
- The placing temperature of the concrete is influential in thick concrete as it increases the peat temperature rise.
- Placing concrete having the temperature than specified in the design could lead to durability issues such as cracking of concrete.
- Inadequate or in proper compaction of the concrete could lead to the formation of the honeycombs in the concrete. Further, it increases the trapped air that could cause durability issues.