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There are two possible cause of concrete deterioration which is chemical process and physical processes. Example of chemical mechanisms are leaching of paste component, carbonation, corrosion of embedded reinforcement bar, alkali-aggregates reactions and exposure of concrete to aggressive chemical such as sulfates and acids. Most of the chemical deteriorations involve damage to the cement paste matrix. In physical deterioration, it involves freezing and thawing, abrasion, erosion and cavitation. The common element for the deterioration to occur is the ingress of moisture in the concrete. Without water and physical damage, there will no deterioration mechanism happen. [sulfate attack on concrete]
For the project of concrete bridge across the Hangzhou Bay, it exposed to severe marine environment as well as the climatic environment where both chemical and physical deterioration occurs. This will significantly decrease the concrete strength and eventually reduce the design life of the bridge.
Corrosion of steel in concrete [corrosion of concrete in steel]
Concrete is strong in compression but weak in tension. With the steel embedded in concrete resulting concrete can resists both compression and tension. This is due to steel is strong in tension but weak in compression. Generally steels corroding with the presence of air (O2) and water. Since concrete has high alkalinity level, therefore steel will not corrode in concrete. This is due to concrete contain high concentration of soluble calcium, sodium and potassium oxide. When these mineral reacts with water it will form hydroxide and it is very alkaline. A passive layer will form at the surface of the steel probably from metal oxide/hydroxide and minerals from the cement. Two conditions which can break down the alkaline environment without attacking the concrete are carbonation and chloride attack.
Corrosion process [corrosion of concrete in steel]
Corrosion occurs once the passive layer is break down and it starts to appear on the steel surface. When the steel corrodes and dissolve in water to give up electrons, 2e-. This is known as the anodic reaction. In order to preserve the electrical neutrality, these electrons must be consumed at somewhere else on the steel surface. If ferrous ions, Fe2+ dissolved in pore water in the concrete there would not be any cracking and spalling on the concrete surface. [corrosion of concrete in steel]
Fe ƒ Fe2+ +2e- (anodic reaction)
The electrons will be consumed if there is a chemical reaction. This chemical reaction required water and oxygen. This reaction is known as chatodic reaction and produce hydroxyl ions, OH-. These ions will increase the alkalinity on the cathodic surface. Therefore it will strengthen the passive layer and hold off the effects of carbonation and chloride ions. [corrosion of concrete in steel]
2e- +H2O + ½ O2 ƒ 2OH- (cathodic reaction)
Both reactions are the early stage of creating rust. Further reactions are needed for rust to occur. It can be expressed where ferrous hydroxide become ferric hydroxide and finally hydrated ferric oxide or rust. [corrosion of concrete in steel]
Fe2+ +2OH- ƒ Fe(OH)2 (Ferrous hydroxide)
4Fe(OH)2 + O2 + 2H2O ƒ 4Fe(OH)3 (Ferric Hydroxide)
2Fe(OH)3 ƒ Fe2O3.H2O + 2H2O (Hydrated ferric oxide or rust)
PICTURE..SCANE FROM CORROSION OF STEEL IN CONCRETE PAGE 8
When ferric oxide, Fe2O3 is hydrated, it causes the steel to swell and become porous which results in an increase in volume of steel/concrete. The swelling of the steels leads to cracking and spalling of concrete from the effects of the corrosion of steel in concrete. The corrosion can be visually detected where there is a rust stain at the cracks. Other physical evidence of corrosion is the reddish brown brittle, flaky rust on the bar. [corrosion of concrete in steel]
Black rust is one of the dangerous type of corrosion because it give no indication of corrosion due to there is no cracking or spalling. It happens when anode and cathode is far separated which resulting of starvation of O2 at the anode and the Ferrous ions, Fe2+ is stay in solution. It usually happen under certain environmental condition such as under damaged waterproof membranes, underwater and water saturated conditions. [corrosion of concrete in steel]
The major damage caused by corrosion is collapsing of reinforced concrete structure. It is not due to loss of steel but due to the growth of oxides which leads to concrete cracking and spalling of concrete cover. Other problems related to corrosion are the formation of black rust, corrosion is difficult to detect in prestressed and post-tension structures because the tendon is enclosed in duct.
Carbonation occur when there is an interaction between carbon dioxide, CO2 in the atmosphere with alkaline hydroxide, Ca(OH)2. CO2 will form an acid when it dissolved in water. This acid did not deteriorate the cement paste, but it will neutralize the alkalies in the pore water. Therefore, the reaction will produce calcium carbonate, CaCO3. [corrosion of concrete in steel]
CO2 + H2O ƒ H2CO3 (carbonic acid)
H2CO3 + Ca(OH)2 ƒ CaCO3 + H2O
Calcium hydroxide, Ca(OH)2 is presents in the concrete pores which can dissolve in pore water. It helps maintaining the alkalinity level in the concrete, but when CO2 and Ca(OH)2 reacts and precipitate CaCO3. This reaction will reduce the pH to a level where steel can corrode roughly at pH 9. [corrosion of concrete in steel]
First step for carbonation is the diffusion of CO2 in to the concrete surface. It is happen due to the differences in the concentration between the atmosphere and the concrete pore. A few millimetres of carbonated layer appears below the concrete surface. The depth of further penetration is depends on the concrete permeability and the amount of Ca(OH)2. Over time, the depth of carbonation could reach the reinforcement embedded and causing depassivation. [fundamentals of durable reinforced concrete]
Factors influencing carbonation rate [fundamentals of durable reinforced concrete]
Diffusivity and permeability - carbonation is the diffusion of the CO2 from high concentration to the lower concentration. The definition of permeability is the flow which influence of by the pressure difference. Concrete has low permeability is greater resistance to the inwards infusion of CO2. Low permeability concrete can be achieved by low water cement ratio, proper compaction, adequate and proper curing. Therefore the rate of carbonation is inversely proportional to the concrete strength. This mean high permeability has low carbonation depths.
Reserve alkalinity - it is related the volume of the calcium hydroxide, Ca(OH)2 present in the concrete. Ca(OH)2 act as the barrier defence where the more Ca(OH)2 in the concrete the greater it resistance to CO2.
Environmental carbon dioxide concentration - the concentration of CO2 is depends on the location of the area. This mean, area located near to the industrial areas is prone to high CO2 concentration. Structures near to coastal environment has low rate of carbonation compared to the structures in the urban area. This is due to at the coastal environment has high humidity. High humidity will lower the rate of carbonation, but increase in ingress of chloride ions.
Exposure condition - moisture plays an important role and greatly influences the rate of carbonation. This is probably due to the effect of diffusion of moisture content in the pore structure. From the study show that (refer graph yg, scan page 85 of the fundamental of durable concrete. ) It show that concrete in the internal structure have high carbonation rate. This mean dryer concrete is susceptible to carbonation.
Sources of chloride
There are several sources of chloride. It can be either cast into the concrete or diffuse in from the outside of the concrete. The chloride cast into the concrete due to the addition of chloride as a set accelerator, using seawater in the mix and using contaminated aggregates. For the other source, it can be due to the sea salt spray or direct seawater wetting and from the deicing salts. The cast in chloride always became a problem when casting at marine environment. This is due to sea water contaminates the original concrete mix and diffuses into the hardened concrete. [corrosion of concrete in steel]
Transport of chloride into chloride
The rate of chloride ingress into the concrete due to chloride attack is the same as in carbonation. Initial mechanism for chloride ingress is through suction, when the concrete surface is dry. It is rapidly absorbed by dry concrete, follow by the capillary movement of the salt laden water through the pores followed by 'true' diffusion. There is also a slow down mechanism of chloride ingress which are chemical reaction to form chloroaluminates and absorption onto the pore surfaces. [corrosion of concrete in steel]
Chloride attack mechanism
Unlike carbonation, chloride ions ingress into the concrete and attack the passive layer of the steel without any overall drop in pH. It acts as a catalyst to the corrosion and not consumed in the process. When there is sufficient concentration of chloride ions to break down the passive layer and it allows the corrosion process to proceed quickly. Generally, it starts with the formation of pits. The number of pits increases and expands, finally join together to form a generalise corrosion. There is an electrochemical potential difference exist that attracts the chloride ions. Corrosion starts to develop and acids are formed. [corrosion of concrete in steel]
Picture scan from [corrosion of concrete in steel] page 23
Freezing and thawing [freezing and thawing of concrete - mechanisms and control]
Freezing and thawing of concrete is the physical deterioration which is easily seen and recognized. The most common damage from this deterioration is scaling and spalling of pavement which produce uneven surface, crumbling of cement paste and extensive cracking and popouts produce on the concrete surface. Freezing and thawing does not significantly affected the structural stability but it gives an impression that the structure is on the verge of disintegration. This is due to the exposure of the concrete aggregates exposed on the concrete surface.
D-line cracking - it occurs approximately parallel to the joints or the edge of the concrete surface. The parallel cracks are further developing from the joints as the integration progress. It is caused by water freezes in the cement paste voids and aggregates. When the force is significantly great, it loose the tensile strength of the cement paste allowing D-line cracks to develop. As the disintegration progress, more water is visible through the cracks.
Scaling - concrete mortar will crumble away as continuous freezing and thawing. This happen when producing low quality concrete which gradually exposed the coarse aggregates. There are several processes for scalling to develop such as pressure develops due to the force of water from saturated aggregates or changes of moisture to ice crystals in capillary voids below the surface. Scalling occurs depends on the degree of compaction of the concrete surface, the number of capillaries beneath the concrete surface and availability of moisture.
Crumbling of cement pate - crumbling of cement paste will exposed the aggregates. This process progress into the interior of the concrete. It is usually occurs when concrete is saturated above the critical saturation points and exposed to freezing and thawing cycles.
Freezing and thawing mechanism [freezing and thawing of concrete - mechanisms and control]
Concrete freezing and thawing deterioration mechanism occurs in the pore structure of cement paste and aggregates. There are three classification of voids which are gel pore (very small), capillary pores and bubbles of entrained air. When concrete is exposed to concrete it will fills the gel pores and capillary cavities. Ice crystal will appear in the largest capillary cavities when the temperature falls to freezing points. When the water changes to ices, the volume of water exceeded the original capacity of the cavity. Therefore, excess water must be expelled from the cavities. The only ways for the water to escape is to nearest air voids. The growing ice in the capillary cavities will act as a pump forcing the water to move to the air bubble boundary. Therefore, it generates pressure. Generally, during freezing, hydraulic pressure exists throughout the paste. The further away the escape boundary, the higher the pressure required. This pressure will cause the stress to the surrounding gel beyond its tensile strength. Therefore it will produce permanent damage.
Gambar from [freezing and thawing of concrete - mechanisms and control] page 28
Another form of chemical attack is sulfate attack. It is a series of chemical reaction between sulafte ions and the cement paste in hardened concrete. This reaction is due to the exposure of the concrete to sulfates and moisture. Sulfate can be in form of gaseous or liquid and is potentially harmful to the concrete. [sulfate attack to concrete]
Sulfate attack is associated to the durability failure of the structure. This is due to the concrete structure experiencing expansion of concrete when it is in contact with groundwater or soil containing sulfate. However, sulfate attack can be considered into three categories depending to their chemical or chronological characteristics:[fundamentals of durable reinforced concrete]
'classical' sulfate attack
Thaumasite sulfate attack
Delayed ettringite formation
The sources of sulfate can be from the internal source or external source. Internal source is usually from the constituent of the cement. One of the constituent in cement is calcium sulfate. It was added to clinker during cement grinding to enable control of cement setting characteristics. Aggregates containing sulfate used for mixing concrete is also consider the source of sulfate. External sources of sulfate are natural sulfates of calcium, magnesium, sodium and potassium. It is present in soil or ground water. industrial waste exposed to the ground water. [fundamentals of durable reinforced concrete]
'classical' sulfate attack
'classical' sulfate attack is associated with concrete buried in the soil or groundwater containing soluble sulfates. Two mechanism of sulfate attack are formation of gypsum and formation of ettringite. Both reactions products cause concrete damages.
The reactions of sulfate with hardened cement past are as follow;
Sodium sulfate attack, Ca(OH)2
Ca(OH)2 + Na2SO4.10H2O ƒ CaSO4.2H2O + 2NaOH + 8H2O
Ca(OH)2 can leach out in the flowing water. Equilibrium of Ca(OH)2 is reached when NaoH accumulate. Therefore, parts of the sulphur trioxide SO3 are being deposited as gypsum.
The reaction with calcium silicate hydrate (C-S-H)
2(3CaO.Al2O3.12H2O) + 3(Na2SO4.10H2O) ƒ 3CaO.Al2O3.3CaSO4.32H2O + 2Al(OH)3 + 6NaOH + 17H2O
Calcium sulfoaluminate (3CaO.Al2O3.3CaSO4.32H2O) also known as ettringite is form when calcium sulfate attacks calcium aluminate hydrates.
C-S-H, Ca(OH)2 and calcium aluminate hydrate also attack by magnesium sulfate. The reaction is as follow:
3CaO.2SiO2.aq +3MgSO4.7H2O ƒ 3CaSO4.2H2O + 3Mg(OH)2 + 2SiO2.aq. + xH2O
A further reaction is possible between 3Mg(OH)2 and silica gel is and may cause deterioration. But, the most deterioration is the destruction of C-S-H due to magnesium attack.
Thaumasite sulfate attack
Thaumasite sulfate attack is describes as a complex sulfate-bearing mineral. It has a similar crystal structure to ettringite, but ettringite has silica-bearing phase as opposed to an aluminate. The formation of thaumasite be happened in two ways. Firstly is as a result from a reaction between sulfates which is obtained from external sources, calcium silicate present in cement and calcium carbonate is from limestone aggregates. The other reaction is from the reaction involving ettringite formation, calcium silicate hydrates and calcium carbonates. Thaumasite will replace the hardened cement paste which bond the concrete into a white and incohesive mush. Formation of thaumasite must be at low temperature, less than 15oC, pH level above 10.5 and internal relative humidity is greater than 95%. [fundamentals of durable reinforced concrete] [sulfate attack to concrete]
The reaction in formation of thaumssite:
3Ca2+ + Sio32- + CO32- + SO42- + 15H2O ƒ 3CaO.SiO2.CO2.SO3.15H2O
Delayed enttringite formation (DEF) (http://www.ce.berkeley.edu/~paulmont/241/Sulfate_attack.pdf)
It is a form of internal sulfate attack. During hydration process in plastic concrete, ettringite is formed. Ettringite does not precipitate when the cement paste temperature is exceeding 70oC. This is happen when pouring a massive amount of concrete or curing temperature exceeding the limit. This situation caused for the formation of monosulfoaluminate. The sulfates will absorbed by calcium silicate hydrate. Once the concrete cools down and reach to ambient temperature, the monosuifoaluminate transformed into poorly crystalline ettringite when it is exposed to high humidity. Therefore, it causes an expansion and microcracking in the matrix and generates a gap around the aggregates. Through time, larger crystals of ettringite will develops from the smaller ettringite with the presence of moisture.
Alkali-aggregate reactions [ the alkali-silica reaction in concrete]
Deterioration due to the alkali-aggregates reactions can cause effects on concrete structural elements. Generally, it is a chemical reaction involving alkali hydroxide from the cement with the reactive aggregates used. Alkali-aggregate reaction can be categorise into 3 types; alkali-silica reaction (most common), alkali-carbonate reaction and alkali-silicate reaction. [ the alkali-silica reaction in concrete]
In alkali-silica reaction, it is a reaction between alkali hydroxide and silicious material in the aggregates. This reaction will develop swelling pressure to crack and disrupt concrete. Unlike alkali-silica reaction, alkali-silicate reaction, it usually occurs in alkali-rich concrete which contain argillite rock type in aggregates. These minerals that presents in the aggregates cause it to expand and cause disruption to the concrete. Finally is alkali-carbonate reaction. This reaction can be classified as; 1) carbonate reactions with calcitic limestones, 2) reaction involving dolomitic limestones aggregates and 3) reactions involving fine-grained dolomitic limestone aggregates which contain calcite and clay. [ the alkali-silica reaction in concrete]
Alkali-silica reaction (ASR)
In ASR an alkali-silica gel is produce when reaction between the alkalis pore fluids in the concrete and silicious components of the aggregates particles. The gel cans absorb moisture and increase in volume and generating pressure. According to Vivian and others research, the chemical composition of the alkali-silica gel is variable and indefinite depending on the OH- concentration. This phenomenon will cause destruction to the concrete. The equation for ASR is as follow: [ the alkali-silica reaction in concrete]
4SiO2 + 2NaOH = Na2Si4O9 + H2O
SiO2 + 2NaOH = Na2Si3 + H2O
There are three primary factors influencing for the alkali-silica reaction to occur; an adequate supply of moisture, reactive silica and sufficient alkali present in the cement. These three factors nust be present simultaneously. [fundamentals of durable reinforced concrete]
Moisture - moisture play 2 important roles in ASR. Firstly, it act as a transport medium for the reactive ions and secondly, the absorption of moisture into the gel which leads for the expansion that cause the concrete damages.
Reactive silica - the degree of the reactivity of silica minerals is depends on the crystal structure. The most vulnerable is the minerals having disordered structure. This is due to disordered structure has greater surface area especially larger disordered structure for the reaction. Examples of vulnerable silica structure includes amorphous, glassy and microcrystalline.
Alkali level - the reaction will only proceed when it reached certain alkalinity level in the pore fluid. The alkalinity in the pore fluid is influence by the sodium and potassium alkali metals in the cement.
Efflorescence is cause by the capillary transport of calcium hydroxide to the surface. It will leach out and deposited on surface. It also can be transported by soluble salt that migrate to the surface zone or deposited by evaporation. After a certain period, a stain will appear on the concrete surface due to carbonation of calcium hydroxide. [fundamentals of durable reinforced concrete]
Efflorescence can be distinguished into three forms; lime bloom, lime weeping and salt crystallisation. Lime bloom occurs when calcium hydroxide migrates to the surface area. A thin film of calcium carbonate will develop if calcium hydroxide reacts. Lime weeping is deposition of thick white deposits that form from the points of water leakage such as cracks or joints. Sometimes lime weeping occurs at the water drips off a free edge of the bridge or retaining walls. Slat crystallisation on the concrete surface. It is the migration of soluble salts through the concrete to the surface zone. These salts may come from the seawater or contaminated mix constituents. The mechanism of salt crystallisation is through evaporation process where it leads for the deposition on the concrete surface. [fundamentals of durable reinforced concrete]
Other physical deteriorations [fundamentals of durable reinforced concrete]
Abrasion [fundamentals of durable reinforced concrete]
The definition of abrasion in concrete context is the results from friction. The friction may be due to the grinding action, by repetitive impact and overloading. The source of the friction is from vehicular traffic, materials dragged across the pavements and wind-borne particle impacting on the concrete surface. In this project, there is a definite physical deterioration due to abrasion.
Erosion [fundamentals of durable reinforced concrete]
The causes of erosion can be due to the suspended materials in the water across the concrete surface. It may be due to the particles such as sand, impact on the concrete surface by the wind. There are several factors influencing the rate of erosion such as the velocity of water flow, the quantity of the materials being transport as well as the size and shape.
Cavitation [fundamentals of durable reinforced concrete]
Cavitation is the sudden formation and collapse of vapour bubbles due to the pressure changes in flowing water. High flow of liquid velocity will develops a sub-atmospheric pressure zone, but there is a sudden change in velocity or direction at the downstream. Vapour bubbles is produced and collapsed by force on entering the next zone of high pressure. This situation produces an impact effect and pressure waves strike the concrete surface. It will give a local loosening of the concrete if the impact and effect is continuously happen.