Mastercivilengineer

The durability of concrete is defined by its capability or the ability to resist weathering action, chemical attack, abrasion, or any other deterioration process and retain its original form, quality & serviceability when exposed to its environment. Different concretes require different degrees of durability on the exposure environment and properties desired.

To achieve good durability of concrete the following factors should be appropriately controlled:

• Structural design.
• Study of the environment in which the structure is being constructed. Temperature, humidity, and chemical conditions to be examined.
• Selection of all materials of concrete & good concrete mix design.
• Concrete specifications such as maximum water-to-cement ratio, maximum cement content, type of cement, and grade of concrete.
• Quality of concrete cover around the steel reinforcement and embedment. This includes the quality of concrete cover blocks as well.
• Workability and cohesiveness of concrete mix.
• Batching, Mixing, transporting, placing, compacting and most important curing. Concrete in plastic form should be uniform and care should be taken to prevent segregation.
• Maintenance and usage in service life. Concrete structures are often tampered with or modified. Structures are often overloaded without any consideration about its loads carrying capacity.

Factor Affecting Durability of Concrete

Deterioration of concrete can take place basically because of porosity.

Concrete has porosity of several types:

• Capillary pores
• Entrapped air
• Honeycombs
• Cracks

Porosity in Concrete

There can be micro or macropores present in concrete. Micro pores are in form of capillary pores in the cement gel. Macro pores can be due to entrapped air as a result of stiff workability or poor compaction. Honey-combing as a result of segregation or use of non-cohesive mix causes large voids. Leaching of excess lime also causes porosity.

Capillary Pores

Capillary pores in concrete can be as large as 5 µm in diameter. The number and size of pores depend on the water-cement ratio (W/C) used and the extent of chemical hydration that has taken place. The relation between the age of concrete at which capillary pores get blocked (concrete becomes almost impermeable) and W/C is given in the below table.

W/C	Age at which capillary pores become blocked
0.40	3 days
0.45	7 days
0.50	14 days
0.55	3 months
0.60	6 months
0.70	over 1 year
Over 0.70	infinity

Permeability of Concrete

Concrete produced with a low water-cement ratio (W/C) displays low permeability of concrete compared to concrete produced with a high water-cement ratio.

Sr. No.	W/C	Coefficient of Permeability (Valenta)
1	0.35	1.05 x 10-3
2	0.50	10.30 x 10-3
3	0.65	1000 x 10-3

Permeability & Porosity of Concrete made from Pozzolanic Cements

Permeability of pozzolanic cement pastes that is initially higher than OPC tends to become lower as the curing period proceeds. Even though pozzolanic pastes are always more porous than those made up of OPC, the permeability of pozzolanic cement pastes is identical to that of OPC after a lapse of time. For the first 7 to 15 days cement hydration only involves the clinker and gypsum fractions. Pozzolanic material or flyash will hydrate later on at a slower rate and within an already rigid structure. Some lime reaction products are formed mostly through the complex process of dissolution, transportation, and precipitation. Mass precipitation into the pores previously formed by hydration of the clinker fraction is not able to fill the large pores completely but blocks smaller capillaries connecting larger pores or, at least reduces their openings considerably. As a consequence, the porosity of pozzolanic cement pastes remains higher than or at the most becomes the same as Ordinary Portland Cement but the permeability becomes lower.

Concrete made up of 35% flyash containing cement has turned out to be 2 to 5 times less permeable than concrete manufactured with OPC or blast furnace slag cement. Concrete made using pozzolanic cement have a better flexural strength or compressive strength ratio and reduced tendency towards cracking than concrete made using OPC.

Leaching

If the surface of concrete comes in continuous contact with water or moisture, the free lime occurring in hardened concrete being easily soluble is the first compound to be attacked and will leach out. This lime extraction to the concrete surface increases both the porosity & permeability of concrete. The soluble calcium hydroxide Ca(OH)₂ leaches through the capillary pores of concrete & leaves a passage for other pollutants such as water, chlorides, and sulfates to enter. This also causes the alkalinity of concrete to drop initiating corrosion of steel within the concrete.

Cracks in Concrete

In modern concrete structures, not enough attention is being paid to the fundamental principles of concrete technology governing cracking. In general, cracks in concrete range in widths from 0.1mm to 1.0mm and are primarily caused due to the following:

• Temperature gradient including frost action.
• Humidity gradient (Drying Shrinkage)
• Rapid drying conditions (Plastic Shrinkage)
• Structural overloading, cyclic, or impact loading.
• Inadequate structural design and detailing.
• Chemical causes including corrosion of reinforcement.

Concrete starts cracking at an early age when it is still in the plastic stage. When freshly hardened concrete is exposed to temperature and humidity gradient. It experiences thermal and drying shrinkage strains. One of these two gradients will have a more dominating effect on concrete depending on the following:

• Temperature and humidity of the environment.
• Size of the structural element.
• The temperature of concrete.
• Physical and Chemical properties of concrete materials.
• Mix proportion of concrete materials.

Under the restraining conditions in hardened concrete, shrinkage strain causes tensile stress. The concrete material will develop cracks when this induced tensile stress exceeds the tensile strength of concrete. However, due to the viscoelastic behavior (creep) of concrete material, some of the stress is relieved and it is the residual stress, after the relaxation due to creep that will be responsible for cracking.

Cracks on concrete surfaces may or may not influence the strength. However, cracks on concrete surfaces will seriously affect the durability of concrete especially, when it is exposed to an aggressive environment and a number of cyclic loading conditions. Under such conditions cracks wider than 0.3mm seldom heal. Many standards recommend 0.15mm as the maximum crack width at the tensile face of a reinforced concrete structure subjected to alternate drying and wetting conditions or is located in the tidal zone and subjected to seawater sprays.

In concrete design and construction practice, crack widths are generally controlled by the proper deployment of the primary reinforcement and by the use of secondary reinforcement. However, it is well established that reinforcement steel does not prevent cracking or reduce cracking. It simply transforms a few cracks into many fine cracks and micro-cracks.

Entry of Chemicals

Chlorides, water, carbon dioxide, and sulfates are most harmful to concrete and steel within it. While chlorides and water enter through the pores and cracks in concrete and cause corrosion of steel the carbon dioxide and sulfates chemically react with concrete and cause deterioration and reduction of durability.

The chemical substances which need to be considered for their aggressive effect on concrete are water, chloride ions, and carbon dioxide and sulfate ions.

Water

While water required for cement hydration and later for curing of concrete, it is harmful to reinforced steel after the concrete has been taken into service.

Ingress of water can take place through capillary pores, cracks, or voids in the concrete surface. When the water penetration is beyond the concrete cover, and it reaches the reinforcement steel inside, it is dangerous. Water can cause corrosion of steel and subsequent disintegration. Water is the primary vehicle for the diffusion of other aggressive ions, such as chlorides and sulfates, into the concrete mass.

Corrosion

Before we discuss the problems of ingress of water and chlorides in concrete, it is important to understand the mechanism of corrosion of steel in concrete and the damage it causes to the concrete structure.

The mechanism of corrosion of steel is an electro-chemical process. The electro-chemical process starts when there is a potential difference caused due to the difference in concentration of dissolved ions such as Alkalies, chlorides, and oxygen, in the vicinity of steel. Rust appears on the anodic part as iron (ferrous) gets converted to ferrous oxide or ferrous hydroxide. For this chemical process, the presence of moisture and oxygen is necessary. The concrete act as an electrolyte and the electrochemical process takes place.

Depending on the state of oxidation, metal gets converted to rust which may occupy 6 to 8 times the original size of steel.

Preventive Measures: To avoid corrosion of steel following preventive measures are to be taken.

The concrete mix should be designed with a low water-cement ratio as possible depending on the environmental conditions in which the structure is proposed. Some guidelines as per BIS are given and they must be followed: 

• Concrete should be made in such a manner that voids due to entrapped air or segregation do not occur.
• Plastic and drying shrinkage cracking of concrete should be avoided by taking adequate care in designing concrete mixes and by proper construction practices, especially curing.
• The concrete mix should have good workability and cohesiveness and must be placed and compacted properly.
• Protective coating on steel can be considered as a second line of defense against corrosion.

Chloride

Harmful Effects of Chloride

The chloride ions in concrete can have harmful effects on concrete as well as on reinforcement. In the first case, chloride penetration brings about concrete swelling of 2 to 2.5 times larger than that observed with water penetration. This causes a slight reduction in concrete strengths as well as causes leaching of concrete, making it more porous and vulnerable. In the second case, the presence of chloride near the reinforcement steel is extremely dangerous. If the chloride to hydroxide ratio nears the reinforcement steel drops below 0.3, the passivation is destroyed and corrosion is inevitable. Chlorides have, therefore, to be prevented from entering concrete.

Chlorides can be present in concrete materials and are termed as “domestic” chlorides or chlorides can be present in the environment around the concrete structure and are termed as “foreign” chlorides.

Limitation of Chlorides in Materials

The amount of chlorides permitted in concrete so far as corrosion of reinforcements is concerned is limited to acid-soluble chloride content of 0.15% by weight of cement, at the time of placing concrete. This is recommended in IS:456 Code of Practice for Plain and Reinforced Concrete.

Tricalcium Aluminate (C₃A)  

There has been a lot of discussion about the limits of chloride in concrete about “domestic” chloride versus “foreign” chloride. If there is a uniform distribution of chloride, corrosion may be minimal. Further, even if the chloride is initially uniformly distributed, a non-uniform distribution eventually may result, due to the movement of water containing chloride in solution. Some of the “domestic” chlorides can become chemically fixed by reactions with C₃A components of the Portland cement forming calcium chloroaluminate hydrates. This not only explains the good performance of Portland cement containing high amounts of calcium aluminates but also advocates such cement as a solution to the problem.

It is not advisable to use Sulphate Resistant cement in an environment where excessive chlorides are present as Sulphate Resistant cement has low C₃A content and therefore less able to form calcium chloroaluminate hydrates.

Chloride Diffusion

To protect the reinforcement from chlorides penetration, it is essential to produce impermeable concrete (concrete having low water to cement ratio) and give a thicker cover to reinforcement steel.

Studies have been undertaken and it is observed that with all mix parameters remaining the same, reduction of W/C reduces chloride ion penetration into the concrete to a considerable extent. The below table shows that chloride diffusion in concrete mixes reduces considerably as the W/C reduces.

Sr. No.	W/C	Chloride Diffusion x 10-8 Sq.crn IS
1	0.40	1.05 x 10-3
2	0.50	10.30 x 10-3
3	0.61	1000 x 10-3

Blended Cements in Chloride Environment

It is also recommended to use blended cement containing Pozzolanic materials or slag as the chloride diffusion through cement pastes of this cement is at a very slow rate than compared to OPC and sulfate resistant cement.

Below table shows chloride diffusion in various types of cement pastes having constant W/C = 0.5 at 25^oC.

Types of Cement	Chloride Diffusion Sq.cm IS x 108
OPC	4.47
Pozzolana Cement (70% OPC & 30% flyash)	1.47
Slag Cement	0.41
Sulphate Resistant cement	10.00

The above table clearly shows that cement with 65% slag is most suitable while sulfate-resistant cement is least suitable in a chloride environment. Pozzolanic material if present around 33% is considered to be very effective in the reduction of chloride diffusion into concrete. However, the percentage of Pozzolana being restricted to 25% in IS 1489 and slag being restricted to 65% in IS 455 concrete can be manufactured using Pozzolana or slag as mineral additives. Thus it’s possible to utilize a higher percentage of such materials in a very aggressive environment wherein high proportions of chloride are present.

Carbonation

Alkalinity of Concrete

Concrete is an alkaline substance & provides excellent protection to steel reinforcement embedded inside. The alkaline environment forms a protective oxide film around the reinforcement steel that passivates the reinforcement steel and protects it from corrosion. Concrete initially has a pH value of more than 13.

Because of leaching, carbonation, and defective construction practice, the pH value of concrete drops rapidly. Once the concrete pH value in the cover area drops below 10, corrosion of steel reinforcement is inevitable and therefore durability of concrete is at stake.

Dense concrete well-produced and placed without segregation and proper compaction will offer good protection to steel embedded in it. Concrete of higher strength have lower water to cement ratio and hence they are proffered.

Process of Carbonation

Concrete carbonation has been on the increase these days on account of the increase in levels of environmental pollutants especially in urban areas and industrial townships. As hydrated calcium silicates & aluminates are less stable than calcium carbonates, concrete carbonation can’t be avoided. The carbon dioxide (CO₂) in the atmosphere in presence of water reacts with the concrete surface and concrete gets carbonated or in other words, turns acidic. This chemical reaction starts at the surface and gradually goes within the concrete mass and is generally measured as the depth of carbonation.

Advantages and Disadvantages of Carbonation

Carbonation of concrete improves several characteristics of ordinary concrete but can also catastrophically affect the durability of reinforced concrete. If the concrete is well compacted & dense, carbonation reduces the total porosity, specific surface of cement pastes as well as water permeability which in turn increases resistance to sulfate and aggressive ion penetration.

The alkalinity of concrete on carbonation loses the pH value from around 13.5 to 8.3. Therefore steel is no longer passivates by the alkaline concrete around it. Oxidation of steel reinforcement, therefore, occurs in presence of moisture and oxygen (O₂), and rusting occurs. The rust increases the volume of steel and results in the cracking and spalling of concrete.

Rate of Carbonation

The rate of carbonation depends on various factors which can be subdivided into three groups.

• Concrete Quality.
• Environmental Conditions.
• Types of cement used.

The influence of the first two groups is very clearly understood. The concrete at the surface should be very dense, well compacted, and well cured.

Studies have indicated that carbonation depths are much less in high-strength concrete than in low-strength concrete, all other parameters remaining the same. The below table gives the carbonation depths for various concrete grades based on some accelerated studies.

Sr. No.	Estimated 20 Yrs. Depth (mm)	28 days compressive strength (N/mm2)
1	6mm	58.00
2	14mm	41.50
3	22mm	31.50
4	33mm	21.00

The above studies clearly indicate that if low strength reinforced concretes such as N 15 and N 20 are used then the carbonation depths will be very high resulting in carbonation of concrete and loss of passivation offered by concrete to steel embedded in it. It is also established that where there is excessive pollution and the presence of carbon dioxide, the concrete carbonates very fast and to a greater depth.

Sulphates

Sulfates are generally found in groundwater and subsoil. Sulfates can be naturally occurring or could be a consequence of industrial water.

Factor Causing Deterioration

The degree of deterioration will depend upon the following:

• Concentration and type of sulfates present in the environment
• Characteristics of concrete.
• Type of cement used.

Calcium sulfate reacts with calcium aluminates in cement hydrates forming an expansive ettringite. Sodium sulfate (Na₂SO₄) reacts with calcium hydroxide (CaOH₂) and forms expansive gypsum in presence of aluminates and may in turn lead to the formation of ettringite. Magnesium sulfates react with cement compounds thus decomposing cement itself and subsequently producing gypsum and ettringite.

Selection of Cement

For minimizing the danger of sulfate attack low C₃A content cement is recommended. Sulfate-resistant cement with very low C₃A content is most suitable. Although, if chlorides are also present in the groundwater and subsoil in addition to sulfates, then it isn’t recommended for use in view of the vulnerability of low C₃A cement pastes to chloride ion diffusion. Blended cement is most preferred when both sulfates and chlorides are together present in the environment.

Blended cement has low C₃A content and also enables the production of plates containing a small amount of calcium hydroxide. The Pozzolana cement has also shown great sulfate resistance which probably due to the composition and the structure of the pores in the hydrated paste.

Aggressive Attacks on Concrete

Concrete in marine environment faces a simultaneous physical, chemical and mechanical deterioration process.

The concrete structure is generally divided into three zones when it is placed or constructed in the marine environment. Each zone is subjected to different types of attack as shown below in the table.

Sr. No.	Zone	Type of Attack
1	Atmosphere – The part of structure above the highest high tide level or splash zone	Chemical & Physical
2	Tidal – The zone between the highest high tide and lowest low tide	Chemical & Physical & Mechanical
3	Submerged – The zone always submerged in sea water	Chemical & Physical

From the above table, it is evident that Tidal Zone faces the most aggressive conditions. Besides physical and chemical reactions, it also faces mechanical forces and therefore deterioration on any marine or offshore structure is generally observed to be more severe, on the portion which is in the tidal zone. Besides, the tidal zone faces alternate wetting and drying cycles which accelerates the chemical action of slats and water on reinforcement steel and concrete around it.

Chemical Attack 

Seawater has a very high salt content (around 3.5%). Amongst these slats, chloride and sulfates are most predominant and cause aggressive reactions greatly affecting the durability and strength of the structure.

Sodium and potassium chlorides are generally present in seawater in high proportions. However, they are not that dangerous so as to cause serious durability problems as compared to magnesium chlorides and sulfates present in seawater in smaller proportions. Magnesium salts present in seawater causes most of the chemical attacks thus seawater is classified as highly aggressive.

Magnesium hydroxide is formed as a result of chemical reactions. This compound is insoluble precipitates. In dense concrete, the magnesium hydroxide (brucite) tends to seal the pores and thereby reduces and prevents penetration of chloride and sulfates into the concrete mass. It also reduces leaching.

The other aggressive action on concrete comes from carbon dioxide which is dissolved in seawater. The dissolved carbon dioxide or carbonic acid leaches away the calcium from hydrated cement paste, destroying the passivation action of concrete on reinforcement steel. The leaching action also causes a reduction of the concrete mass. The reaction between carbon dioxide (CO₂) and calcium ions occurs in the surface layer of the permanently submerged area of concrete. Calcium carbonate is formed and precipitation of this compound in the cement paste pores acts as a seal thereby protecting the concrete from ingress of other harmful chemicals.

While CO₂ and Mg ions are stopped by the first layers of concrete due to precipitation, chlorides and sulfates penetrate slowly into the concrete where their concentration decreases from the surface inwards. The chemical attack will therefore greatly depend on concrete porosity and permeability to seawater and the aggressive chemicals within it.

As blended cement pastes have clearly shown their ability to block chloride ion penetration better than other types of cement, they are generally preferred for improving the durability of concrete. The reaction of CO₂ and Mg ions on the concrete surface and the precipitation of magnesium hydroxide and calcium carbonate within the pores of hydrated pastes helps in waterproofing the concrete surface and therefore the ingress of chloride and sulfate ions into the concrete mass further inside is greatly reduced. Even though appreciable amounts of aggressive chemicals are present in seawater the damage is relatively less as compared to the damage that can be caused to the structure exposed to surface water having the same concentration of sulfates and chlorides. As pozzolanic or slag cement have more compact pastes they hinder the sulfate penetration besides their low C₃A content and low calcium hydroxide content help in further reduction of aggressive action of sulfates on concrete.

Vulnerable Concrete in Tidal Zone

The zones in tidal and atmospheric locations are more vulnerable to the aggressive action of the sea than the zone fully submerged in water. This is on account of four main reasons given below.

• The rate of corrosion of embedded steel dependent on the availability of oxygen. Dissolved oxygen in seawater is very less and hence corrosion of the reinforcement steel seldom takes place in the totally submerged areas. In the atmospheric and tidal zones, where oxygen (O₂) is present adequate quantity corrosion of steel is much faster.
• In the portion of the concrete structure above sea level, the seawater rises upwards by capillary action. The water evaporates leaving behind crystals of dissolved salts. With the progressive wetting & drying cycles, this crystalline growth gradually increases causing tensile stresses. When these tensile stresses exceed that of concrete, the disintegration of the concrete surface takes place.
• Due to fluctuation of seawater level, the leached slats and corroded concrete fragments get washed away and erosion of concrete takes place, resulting in loss of mass.
• The mechanical impact of sea waves is active in the tidal zone only. This therefore continuously increases the wear and tear of concrete in this zone.

Alkali Aggregate Reaction

Various harmful chemical reactions between aggregates and Ordinary Portland Cement (OPC) have been reported. The most common reaction is the one between certain types of silica occurring in aggregates and Alkalies present in cement. The types of silica which are alkali reactive are opal, chalcedony, and tridymite.

Due to this reaction, a gel made up of alkaline and alkaline-earth silicate is formed. This gel has a tendency to absorb water & swell. The swelling causes internal stress & when this stress exceeds the tensile strength of the pastes, cracking of concrete can happen.

This problem can’t be always solved by changing the aggregates. Thus, cement of appropriate chemical composition has to be used. It is believed that expansive reaction does not occur with Portland Cements containing Na20 equivalent (Na20 + 0.658K20) not exceeding 0.6%. However, tolerable limits of Alkalis per cubic meter of cement are also suggested by some specifications. Alkali content less than 1.8 kg per cum of concrete is considered a safe limit. While concrete containing 3.8 to 4 kg per cum of Alkalis is potentially dangerous.

Using blast furnace slag cement and pozzolanic cement is yet another solution. However, some Pozzolanas contain excessive Alkalies and hence quantity and quantity of blended material will also influence the alkali-silica reaction.