Understanding the factors that affect the durability of concrete is essential to determining the useful life of structures. Let's talk about the durability of concrete.
When defining durability, other factors such as concrete quality, water-cement ratio, etc. are considered. must be taken into consideration. Structure Classes Clearly indicate the expiration date for each type of structure.
The factors that affect the durability of concrete are listed below
- High humidity and rain
- UV resistance
- Chemical resistance
- Exposure to sea water
- Chloride and corrosion resistance of steel
- Sulfate resistance
- Alkali-silica reaction resistance (AKR)
- Carbonation
- Abrasion resistance
- Moderate to Heavy Load Conditions for Concrete
- Frost and thaw resistance
- cement content
- Quality of aggregates
- Water quality
- Concrete Compaction
- Installation time after filling and cold joint formation
- Healing time
- permeability
- temperature
- Construction defects (honeycombs, cracks, etc.)
High humidity and rain affect the durability of concrete
Moisture is essential for concrete to harden. The following factors can be considered the main problems related to the effect of humidity.
- Moisture is essential for concrete to cure. Retaining moisture in concrete helps the cement react and increases strength.
- However, excess water in concrete can increase the distance between the aggregates. Evaporation of moisture in concrete reduces the strength of the concrete because it does not harden sufficiently. The strength of concrete can decrease significantly and even drop by 40%.
- However, prolonged retention can cause problems. After the concrete hydration process, a medium range of humidity Carbonation of the concrete may occur, as shown in the following figure. Therefore, a very low or very high humidity range may be acceptable in terms of durability.
- A high humidity range with chloride will cause problems as the chloride can leach into the concrete.
- The Compressive Strength Strength of concrete decreases with high relative humidity, leading to durability problems.
- Higher moisture levels on the concrete surface lead to the formation of mold, mildew and bacteria. Continuous growth can affect the strength of the concrete. Furthermore, it can also cause health problems.
- Relative humidity influences the pH value of concrete. It can compromise the integrity of the concrete and any adhesives or coating materials applied to it.
Rain affects concrete in two stages. Uncured concrete that gets wet poses a serious problem when it comes to the durability of concrete, and continued periodic wetting and drying due to rain is not considered a very risky factor affecting the durability of concrete.
- Rain affects the water-cement ratio of the wet area. This creates concrete with lower compressive strength.
- Furthermore, it reduces the durability of the concrete and consequently can affect the concrete reinforcement.
UV resistance
Ultraviolet waves from solar radiation do not directly damage concrete. However, they affect concrete applications. The concrete pigment maintains the color necessary for the aesthetic appearance.
Furthermore, the application of impregnations caused by ultraviolet radiation leads to deterioration of the material.
Chemical resistance
Concrete is more resistant to chemicals found in the environment. In a natural environment, concrete survives most of the time without damage.
However, there are harmful chemicals that can react with concrete after prolonged exposure. Chemicals such as chlorides, sulfates, bourbon, alkali-silica reactions, etc. can affect the durability of reinforced concrete. Such effects of concrete are called Chemical attacks .
Chlorides and sulfates adhere to the reinforcement, causing corrosion and, consequently, significantly reducing the strength of the structure. Carbonation It is also a process of decomposition of concrete. For more information, see the articles on this site.
At some point there will be cracked concrete. Due to this problem, the buildings cannot be used. At the design stage, care must be taken to minimize the impact on the durability of the concrete.
Exposure to seawater and concrete durability
Seawater is a corrosive environment that affects the durability of the structure. The structure may be close to the sea, submerged in the sea, or parts of the structure may be exposed to tidal waves.
Due to limited space and according to project requirements, the construction of marine structures is inevitable. During the planning and construction phases, the necessary care must be taken to protect structures from possible risks.
Reduce permeability, provide adequate coverage, select appropriate quality of concrete based on Structure Class and exposure conditions, etc. must be taken into account during planning.
Construction must use concrete mixtures appropriately designed and tested for the above points, respecting strict quality control measures.
Information about cement for use in seawater can be found on the website by clicking here.
Chloride and corrosion resistance of reinforcement
They are bonded and more resistant to chloride attacks, increasing safety against corrosion.
Chlorides are common in marine environments and near the sea. Therefore, there is a high risk of exposure to chloride.
The permeability of concrete is one of the main factors that allows the transport of iron chloride to the reinforcement. The lower the permeability, the lower the risk of corrosion of the reinforcement.
Furthermore, the hardening of concrete directly affects its permeability. Quality control and assurance during the construction phase have a direct impact on the durability of the concrete structure.
Influence of sulfate resistance on concrete durability
Sulfate attack is one of the methods of damaging concrete. This process can be initiated without the influence of the environment or with the support of the environment. Increasing the volume of concrete leads to cracking.
- Generally, sulfate-related stresses develop on the surface of the concrete, which is called external sulfate attack.
- It is created by the reaction of sulfates contained in groundwater or soil with the cement paste in concrete.
- Although wear begins at the contact surface, it progresses.
- The chemical reaction creates very high tensile stresses and leads to cracking and disintegration of the concrete.
- Porous concrete is generally attacked more severely than less permeable concrete.
- Continue sulfation until the sulfate can penetrate the concrete
- According to ACI 318, the following areas may be considered during design.
Irrelevant – If the sulfate content in the water is less than 150 ppm (mg/l), no restriction on the type of cement is necessary.
Moderate – If the sulfate content in the water is between 150 and 1500 ppm, type II cement with a pozzolan additive must be used.
Difficult – If the sulfate content in the water is between 1,500 and 10,000 ppm, type V cement and an aw/cm ratio lower than 0.45 must be used.
Very serious – If the sulfate content in the water exceeds 10,000 ppm, type V cement with pozzolan and an aw/cm value lower than 0.45 must be used. Additionally, the ACI Building Code recommends a minimum F'C of 29 MPa (4250 psi) (Mehta and Monteiro, 2006).
Some other sulfate sources could be connected
- Sea water
- Oxidation of sulfide minerals in clay adjacent to concrete
- Bacterial activity in sewage
- Sulphates are present in brick and can be released gradually over a long period of time, resulting in sulphate attacks in the mortar, particularly where sulphates are concentrated due to moisture movement.
Resistance to alkali-silica reaction
Alkaline-silica reactions are a concrete durability problem commonly observed in water-retaining structures. These cracks normally appear after a long period of construction.
The reaction occurs in the presence of alkali in the cement, reactive aggregate in the concrete, constant availability of moisture and after reaching the required heating level.
As a result of the reaction, cracks appear in the concrete and the crack pattern looks like the following image.
The reaction can be avoided or minimized by the following measures.
- The aggregate can be checked to determine reactivity. The use of a generator should be avoided.
- Use cement with low alkaline content
- Control the continued availability of water for the reaction. However, this is not possible for structures such as concrete dams, overflow pipes, water tanks, etc.
- Controlling the ideal temperature is one of the measures that can be taken to prevent the reaction. However, this is also very difficult to implement in real life.
Carbonation
Carbonation is a reaction of carbon dioxide from the environment with calcium hydroxide in concrete. This creates calcium carbonate and the pH value drops to 9. This causes the oxide layer around the reinforcement to break down and the reinforcement becomes susceptible to corrosion.
The presence of moisture is very important for the reaction. In dry environments, carbonation occurs slowly. Furthermore, carbon dioxide quickly penetrates the concrete pores if the concrete surface has many voids.
Carbonation can be avoided or minimized by the following measures
- The application of anticarbonate coatings prevents the penetration of carbon dioxide
- If carbonation occurs and the reinforcement is corroded, a repair procedure must be used.
- Anti-corrosion paint must be applied to the rebar before concrete repair can proceed.
Abrasion resistance
Abrasion resistance is the ability to resist wear caused by contact with another surface.
Concrete wear depends on the following factors.
- Concrete strength
- Furthermore, increasing cement content and decreasing water content improves abrasion resistance.
- The use of well-selected natural sand improves abrasion
- Coarse aggregate must be free of soft sandstone or soft limestone
If you consider the above points at the design stage, abrasion resistance will be improved.
Moderate to Severe Exposure Conditions for Concrete
There is a clear link between the durability and the loading conditions of the structure.
For this reason, we always take exposure class into account when selecting armor coverage.
A structure may be exposed to different environments, such as constant wetting and drying, infrequent wetting and drying, exposure to seawater, exposure to chemicals, etc.
Depending on the exposure class, factors must be considered to determine concrete durability requirements.
As loading conditions become more severe, they will have a greater impact on the durability of the concrete structure. Special care is required under medium to heavy load conditions.
Frost and thaw resistance
Freezing and thawing occur due to seasonal fluctuations in the world. The water molecules in the concrete freeze and the volume increases by 9%. This creates tensile stresses that cause cracks in the concrete.
The freezing and thawing effect can be controlled by controlling the air trapped in the concrete.
For more information on freezing and drawing, see the article Thermal Cracking in Concrete .
cement content
The cement content of concrete affects the durability of concrete in different ways.
- Increasing the cement content increases the heat of hydration.
- This increases the peak temperature, the temperature gradient and the temperature difference between the core and the surface.
- Due to temperature differences and higher temperature gradients, internal cracks and surface cracks can be observed.
- An increase in core temperature above 70-75 degrees Celsius can trigger the delayed formation of ettringite, which leads to cracking of concrete.
- However, higher quality concrete is used because it is more durable in terms of durability than lower quality concrete. The increase in heat caused by hydration can be controlled by several methods, such as the use of fly ash, control of the formwork layout, etc.
- Reducing cement content can also become a problem due to lower processability.
- Furthermore, the quality of the cement used in the mix also affects the durability of the concrete.
Aggregate quality
- A thin layer of dust on coarse and fine aggregate reduces bond strength
- This reduces the compressive strength of the concrete
- Additionally, dust can affect the chemical reaction between water and cement.
- Aggregate should be checked for these problems before it is used to mix concrete.
- Properties such as mechanical and physical have a direct impact on the strength of concrete
- The article, 30 Tests of Building Materials provide comprehensive instructions for selecting building materials.
Water quality
- The higher the water content of a concrete mix, the higher the water-cement ratio for the same cement content.
- An increase in moisture content in concrete due to excess water leads to a reduction in the compressive strength of concrete and causes durability problems.
- Likewise, as the water content increases, the content of impurities such as salt in concrete increases.
- Contaminated water impairs the hydration process and the necessary reactions can be delayed or hindered.
- Contaminated water can cause reinforcement corrosion.
- Additionally, the pH of the water is checked to determine the quality of the water.
Concrete Compaction on Concrete Durability
- Concrete is compacted when poured to obtain better properties. This is directly related to the strength of the concrete, which in turn is a direct function of the durability of the concrete.
- Improves abrasion resistance and durability
- Proper compaction reduces permeability and therefore minimizes shrinkage and creep behavior.
- Compaction removes excess air from the concrete.
- Well-compacted concrete minimizes the formation of honeycombs that cause voids in the concrete and its surface.
- These defects allow moisture and air to enter and, as previously mentioned, cause durability issues.
- Additionally, poorly compacted concrete may have greater porosity than properly compacted concrete.
- Lack of compaction can lead to water leaks from water retention structures.
Concrete setting time after mixing and cold joint formation
The initial and final setting time of the concrete are important factors that must be taken into consideration during the construction phase.
All concrete must be poured and compacted before the setting time has elapsed so that the chemical curing reactions can begin without interruption.
If concrete is poured after the initial setting period has begun, cold joints may form. Therefore, it is very important to pay close attention to the moment of concreting.
Generally, the initial setting time for normal concrete is about an hour. However, the use of additives in modern construction has increased or decreased setting time depending on project requirements.
A cold joint is a visually weaker plane, but generally does not represent a structural problem. It can lead to durability problems due to increased porosity in the joints. Due to the weak connection between the two castings, water leakage in water storage structures, corrosion of reinforcement, etc. may occur.
Curing time in concrete durability
Concrete curing is very important for the following reasons.
- It is a process of controlling moisture on the concrete surface.
- Creates dense microstructure and improves permeability
- Extended curing improves durability
- Proper curing improves surface hardness and allows concrete to resist wear and surface abrasion
- Improving permeability through adequate post-treatment prevents the penetration of chemicals from the water. Increases the durability and useful life of the concrete structure.
- From the figure above it is clear that curing affects the strength of concrete and ultimately its durability.
Permeability and durability of concrete
- Permeability directly impacts durability as it allows moisture to move
- Moisture movement with reactive chemicals in a harsh environment compromises durability
- Carbonation can occur due to the porosity of the concrete. Greater permeability means air has more room to move. Carbon dioxide can move and in the presence of water and carbon dioxide carbonation occurs, leading to corrosion of the reinforcement.
- Likewise, corrosion can also affect the durability of low permeability concrete.
- The absorption of water and its penetration into the concrete sections due to low permeability leads to maintenance problems, such as color discoloration, formation of various bacteria, etc.
temperature
The temperature of the concrete and the environment are directly related and influence the durability of concrete structures from the beginning. Below are some of the main factors that can be highlighted.
- The durability of concrete depends on numerous properties of the shape of the concrete and the casting.
- Increased temperature causes cracks to form in raw concrete, which leads to significant durability issues.
- An increase in the core temperature of the concrete leads to short-term problems such as cracking and also creates long-term problems such as the formation of delayed ettringite, which causes internal cracking in the concrete due to internal reactions.
- Seasonal temperature fluctuations increase internal stresses in concrete. A structure that is not properly designed with the necessary factors in mind and that does not have sufficient expansion joints can develop cracks over time.
Construction defects affect the durability of concrete
- It is the contractor's responsibility to carry out construction without defects and in accordance with recognized methods of quality assurance and control.
- Lack of attention from contractors harms the durability of concrete.
- Poor or insufficient curing leads to cracks in concrete
- For thick concrete, the installation temperature has a great influence as it accelerates the temperature rise of the peat.
- Placing concrete at a temperature higher than that specified in the design can result in durability problems, such as cracking in the concrete.
- Insufficient or incorrect compaction of concrete can lead to the formation of honeycombs in the concrete. This also increases the amount of trapped air, which can lead to durability issues.
