There are two possible cause of concrete impairment which is chemical procedure and physical procedures. Example of chemical mechanisms are leaching of paste constituent, carbonation, corrosion of embedded support saloon, alkali-aggregates reactions and exposure of concrete to aggressive chemical such as sulphates and acids. Most of the chemical impairments involve harm to the cement paste matrix. In physical impairment, it involves stop deading and dissolving, scratch, eroding and cavitation. The common component for the impairment to happen is the immersion of wet in the concrete. Without H2O and physical harm, there will no impairment mechanism go on. [ sulfate onslaught on concrete ]
For the undertaking of concrete span across the Hangzhou Bay, it exposed to severe marine environment every bit good as the climatic environment where both chemical and physical impairment occurs. This will significantly diminish the concrete strength and finally cut down the design life of the span.
Corrosion of steel in concrete [ corrosion of concrete in steel ]
Concrete is strong in compaction but weak in tenseness. With the steel embedded in concrete ensuing concrete can defy both compaction and tenseness. This is due to steel is strong in tenseness but weak in compaction. Generally steels eating with the presence of air ( O2 ) and H2O. Since concrete has high alkalinity degree, hence steel will non eat in concrete. This is due to concrete contain high concentration of soluble Ca, Na and K oxide. When these mineral reacts with H2O it will organize hydrated oxide and it is really alkalic. A inactive bed will organize at the surface of the steel likely from metal oxide/hydroxide and minerals from the cement. Two conditions which can interrupt down the alkaline environment without assailing the concrete are carbonation and chloride onslaught.
Corrosion procedure [ corrosion of concrete in steel ]
Corrosion occurs one time the inactive bed is break down and it starts to look on the steel surface. When the steel corrodes and fade out in H2O to give up negatrons, 2e- . This is known as the anodal reaction. In order to continue the electrical neutrality, these negatrons must be consumed at someplace else on the steel surface. If ferric ions, Fe2+ dissolved in pore H2O in the concrete at that place would non be any snap and spalling on the concrete surface. [ corrosion of concrete in steel ]
Fe i? Fe2+ +2e- ( anodal reaction )
The negatrons will be consumed if there is a chemical reaction. This chemical reaction required H2O and O. This reaction is known as chatodic reaction and bring forth hydroxyl ions, OH- . These ions will increase the alkalinity on the cathodic surface. Therefore it will beef up the inactive bed and keep off the effects of carbonation and chloride ions. [ corrosion of concrete in steel ]
2e- +H2O + ? O2 i? 2OH- ( cathodic reaction )
Both reactions are the early phase of making rust. Further reactions are needed for rust to happen. It can be expressed where ferric hydrated oxide become ferrous hydrated oxide and eventually hydrated ferrous oxide or rust. [ corrosion of concrete in steel ]
Fe2+ +2OH- i? Fe ( OH ) 2 ( Ferrous hydrated oxide )
4Fe ( OH ) 2 + O2 + 2H2O i? 4Fe ( OH ) 3 ( Ferric Hydroxide )
2Fe ( OH ) 3 i? Fe2O3.H2O + 2H2O ( Hydrated ferrous oxide or rust )
PICTURE..SCANE FROM CORROSION OF STEEL IN CONCRETE PAGE 8
When ferrous oxide, Fe2O3 is hydrated, it causes the steel to swell and go porous which consequences in an addition in volume of steel/concrete. The puffiness of the steels leads to checking 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 discoloration at the clefts. Other physical grounds of corrosion is the ruddy brown toffee, flakey rust on the saloon. [ corrosion of concrete in steel ]
Black rust is one of the unsafe type of corrosion because it give no indicant of corrosion due to there is no snap or spalling. It happens when anode and cathode is far separated which ensuing of famishment of O2 at the anode and the Ferric ions, Fe2+ is stay in solution. It normally go on under certain environmental status such as under damaged waterproof membranes, underwater and H2O saturated conditions. [ corrosion of concrete in steel ]
The major harm caused by corrosion is fall ining of strengthened concrete construction. It is non due to loss of steel but due to the growing of oxides which leads to concrete snap and spalling of concrete screen. Other jobs related to corrosion are the formation of black rust, corrosion is hard to observe in prestressed and post-tension constructions because the sinew is enclosed in canal.
Carbonation occur when there is an interaction between C dioxide, CO2 in the ambiance with alkalic hydrated oxide, Ca ( OH ) 2. CO2 will organize an acid when it dissolved in H2O. This acid did non deteriorate the cement paste, but it will neutralize the bases in the pore H2O. Therefore, the reaction will bring forth Ca carbonate, CaCO3. [ corrosion of concrete in steel ]
CO2 + H2O i? H2CO3 ( carbonaceous acid )
H2CO3 + Ca ( OH ) 2 i? CaCO3 + H2O
Calcium hydrated oxide, Ca ( OH ) 2 is nowadayss in the concrete pores which can fade out in pore H2O. It helps keeping the alkalinity degree in the concrete, but when CO2 and Ca ( OH ) 2 reacts and precipitate CaCO3. This reaction will cut down the pH to a degree where steel can eat approximately at pH 9. [ corrosion of concrete in steel ]
First measure for carbonation is the diffusion of CO2 in to the concrete surface. It is happen due to the differences in the concentration between the ambiance and the concrete pore. A few millimeters of carbonated bed appears below the concrete surface. The deepness of farther incursion is depends on the concrete permeableness and the sum of Ca ( OH ) 2. Over clip, the deepness of carbonation could make the support embedded and doing depassivation. [ basicss of lasting strengthened concrete ]
Factors act uponing carbonation rate [ basicss of lasting strengthened concrete ]
Diffusivity and permeableness – carbonation is the diffusion of the CO2 from high concentration to the lower concentration. The definition of permeableness is the flow which influence of by the force per unit area difference. Concrete has low permeableness is greater opposition to the inwards extract of CO2. Low permeableness concrete can be achieved by low H2O cement ratio, proper compression, equal and proper hardening. Therefore the rate of carbonation is reciprocally relative to the concrete strength. This average high permeableness has low carbonation deepnesss.
Reserve alkalinity – it is related the volume of the Ca hydrated oxide, Ca ( OH ) 2 nowadays in the concrete. Ca ( OH ) 2 act as the barrier defense mechanism where the more Ca ( OH ) 2 in the concrete the greater it opposition to CO2.
Environmental C dioxide concentration – the concentration of CO2 is depends on the location of the country. This mean, country located near to the industrial countries is prone to high CO2 concentration. Structures near to coastal environment has low rate of carbonation compared to the constructions in the urban country. This is due to at the coastal environment has high humidness. High humidness will take down the rate of carbonation, but addition in immersion of chloride ions.
Exposure status – wet plays an of import function and greatly influences the rate of carbonation. This is likely due to the consequence of diffusion of wet content in the pore construction. From the survey show that ( mention graph yg, scan page 85 of the fundamental of lasting concrete. ) It show that concrete in the internal construction have high carbonation rate. This average drier concrete is susceptible to carbonation.
Beginnings of chloride
There are several beginnings of chloride. It can be either project into the concrete or diffuse in from the exterior of the concrete. The chloride cast into the concrete due to the add-on of chloride as a set gas pedal, utilizing saltwater in the mix and utilizing contaminated sums. For the other beginning, it can be due to the sea salt spray or direct saltwater wetting and from the deicing salts. The dramatis personae in chloride ever became a job when casting at marine environment. This is due to sea H2O contaminates the original concrete mix and diffuses into the hard-boiled concrete. [ corrosion of concrete in steel ]
Conveyance of chloride into chloride
The rate of chloride immersion into the concrete due to chloride onslaught is the same as in carbonation. Initial mechanism for chloride immersion is through suction, when the concrete surface is dry. It is quickly absorbed by dry concrete, follow by the capillary motion of the salt loaded H2O through the pores followed by ‘true ‘ diffusion. There is besides a slow down mechanism of chloride immersion which are chemical reaction to organize chloroaluminates and soaking up onto the pore surfaces. [ corrosion of concrete in steel ]
Chloride onslaught mechanism
Unlike carbonation, chloride ions immersion into the concrete and assail the inactive bed of the steel without any overall bead in pH. It acts as a accelerator to the corrosion and non consumed in the procedure. When there is sufficient concentration of chloride ions to interrupt down the inactive bed and it allows the corrosion procedure to continue rapidly. By and large, it starts with the formation of cavities. The figure of cavities additions and expands, eventually fall in together to organize a generalise corrosion. There is an electrochemical potency 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 dissolving [ freeze and melt of concrete – mechanisms and control ]
Freezing and dissolving of concrete is the physical impairment which is easy seen and recognized. The most common harm from this impairment is scaling and spalling of paving which produce uneven surface, crumpling of cement paste and extended snap and popouts produce on the concrete surface. Freezing and dissolving does non significantly affected the structural stableness but it gives an feeling that the construction is on the brink of decomposition. This is due to the exposure of the concrete sums exposed on the concrete surface.
D-line checking – it occurs about parallel to the articulations or the border of the concrete surface. The parallel clefts are farther developing from the articulations as the integrating advancement. It is caused by H2O freezings in the cement paste nothingnesss and sums. When the force is significantly great, it loose the tensile strength of the cement paste leting D-line clefts to develop. As the decomposition advancement, more H2O is seeable through the clefts.
Scaling – concrete howitzer will crumple away as uninterrupted freeze and melt. This happen when bring forthing low quality concrete which bit by bit exposed the coarse sums. There are several procedures for scalling to develop such as force per unit area develops due to the force of H2O from saturated sums or alterations of wet to ice crystals in capillary nothingnesss below the surface. Scalling occurs depends on the grade of compression of the concrete surface, the figure of capillaries beneath the concrete surface and handiness of wet.
Crumbling of cement pate – crumbling of cement paste will exposed the sums. This procedure advancement into the inside of the concrete. It is normally occurs when concrete is saturated above the critical impregnation points and exposed to stop deading and dissolving rhythms.
Freezing and dissolving mechanism [ freeze and melt of concrete – mechanisms and control ]
Concrete freeze and dissolving impairment mechanism occurs in the pore construction of cement paste and sums. There are three categorization of nothingnesss which are gel pore ( really little ) , capillary pores and bubbles of entrained air. When concrete is exposed to concrete it will make full the gel pores and capillary pits. Ice crystal will look in the largest capillary pits when the temperature falls to stop deading points. When the H2O alterations to ices, the volume of H2O exceeded the original capacity of the pit. Therefore, extra H2O must be expelled from the pits. The lone ways for the H2O to get away is to nearest air nothingnesss. The turning ice in the capillary pits will move as a pump coercing the H2O to travel to the air bubble boundary. Therefore, it generates force per unit area. By and large, during freeze, hydraulic force per unit area exists throughout the paste. The farther off the flight boundary, the higher the force per unit area required. This force per unit area will do the emphasis to the environing gel beyond its tensile strength. Therefore it will bring forth lasting harm.
Gambar from [ freeze and melt of concrete – mechanisms and command ] page 28
Another signifier of chemical onslaught is sulfate onslaught. It is a series of chemical reaction between sulafte ions and the cement paste in hard-boiled concrete. This reaction is due to the exposure of the concrete to sulphates and wet. Sulfate can be in signifier of gaseous or liquid and is potentially harmful to the concrete. [ sulfate onslaught to concrete ]
Sulfate onslaught is associated to the lastingness failure of the construction. This is due to the concrete construction sing enlargement of concrete when it is in contact with groundwater or dirt incorporating sulphate. However, sulfate onslaught can be considered into three classs depending to their chemical or chronological features: [ basicss of lasting strengthened concrete ]
‘classical ‘ sulfate onslaught
Thaumasite sulphate onslaught
Delayed ettringite formation
The beginnings of sulphate can be from the internal beginning or external beginning. Internal beginning is normally from the component of the cement. One of the component in cement is calcium sulphate. It was added to clinker during cement crunching to enable control of cement puting features. Sums incorporating sulphate used for blending concrete is besides see the beginning of sulphate. External beginnings of sulphate are natural sulphates of Ca, Mg, Na and K. It is present in dirt or land H2O. industrial waste exposed to the land H2O. [ basicss of lasting strengthened concrete ]
‘classical ‘ sulfate onslaught
‘classical ‘ sulfate onslaught is associated with concrete buried in the dirt or groundwater incorporating soluble sulphates. Two mechanism of sulfate onslaught are formation of gypsum and formation of ettringite. Both reactions merchandises cause concrete amendss.
The reactions of sulphate with hard-boiled cement yesteryear are as follow ;
Sodium sulphate onslaught, Ca ( OH ) 2
Ca ( OH ) 2 + Na2SO4.10H2O i? CaSO4.2H2O + 2NaOH + 8H2O
Ca ( OH ) 2 can leach out in the streamlined H2O. Equilibrium of Ca ( OH ) 2 is reached when NaoH accumulate. Therefore, parts of the sulfur trioxide SO3 are being deposited as gypsum.
The reaction with Ca silicate hydrate ( C-S-H )
2 ( 3CaO.Al2O3.12H2O ) + 3 ( Na2SO4.10H2O ) i? 3CaO.Al2O3.3CaSO4.32H2O + 2Al ( OH ) 3 + 6NaOH + 17H2O
Calcium sulfoaluminate ( 3CaO.Al2O3.3CaSO4.32H2O ) besides known as ettringite is form when Ca sulphate onslaughts calcium aluminate hydrates.
C-S-H, Ca ( OH ) 2 and Ca aluminate hydrate besides attack by Mg sulphate. The reaction is as follow:
3CaO.2SiO2.aq +3MgSO4.7H2O i? 3CaSO4.2H2O + 3Mg ( OH ) 2 + 2SiO2.aq. + xH2O
A farther reaction is possible between 3Mg ( OH ) 2 and silica gel is and may do impairment. But, the most impairment is the devastation of C-S-H due to magnesium onslaught.
Thaumasite sulphate onslaught
Thaumasite sulphate onslaught is describes as a complex sulfate-bearing mineral. It has a similar crystal construction to ettringite, but ettringite has silica-bearing stage as opposed to an aluminate. The formation of thaumasite be happened in two ways. First is as a consequence from a reaction between sulphates which is obtained from external beginnings, Ca silicate nowadays in cement and Ca carbonate is from limestone sums. The other reaction is from the reaction affecting ettringite formation, Ca silicate hydrates and Ca carbonates. Thaumasite will replace the hard-boiled cement paste which bond the concrete into a white and incohesive pulp. Formation of thaumasite must be at low temperature, less than 15oC, pH degree above 10.5 and internal comparative humidness is greater than 95 % . [ basicss of lasting strengthened concrete ] [ sulfate onslaught to concrete ]
The reaction in formation of thaumssite:
3Ca2+ + Sio32- + CO32- + SO42- + 15H2O i? 3CaO.SiO2.CO2.SO3.15H2O
Delayed enttringite formation ( DEF ) ( hypertext transfer protocol: //www.ce.berkeley.edu/~paulmont/241/Sulfate_attack.pdf )
It is a signifier of internal sulphate onslaught. During hydration procedure in plastic concrete, ettringite is formed. Ettringite does non precipitate when the cement paste temperature is transcending 70oC. This is happen when pouring a monolithic sum of concrete or hardening temperature transcending the bound. This state of affairs caused for the formation of monosulfoaluminate. The sulphates will absorbed by Ca silicate hydrate. Once the concrete cools down and make to ambient temperature, the monosuifoaluminate transformed into ailing crystalline ettringite when it is exposed to high humidness. Therefore, it causes an enlargement and microcracking in the matrix and generates a spread around the sums. Through clip, larger crystals of ettringite will develops from the smaller ettringite with the presence of wet.
Alkali-aggregate reactions [ the alkali-silica reaction in concrete ]
Deterioration due to the alkali-aggregates reactions can do effects on concrete structural elements. By and large, it is a chemical reaction affecting alkali hydrated oxide from the cement with the reactive sums 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 hydrated oxide and siliceous stuff in the sums. This reaction will develop swelling force per unit area to check and interrupt concrete. Unlike alkali-silica reaction, alkali-silicate reaction, it normally occurs in alkali-rich concrete which contain argillite stone type in sums. These minerals that nowadayss in the sums cause it to spread out and do break to the concrete. Finally is alkali-carbonate reaction. This reaction can be classified as ; 1 ) carbonate reactions with calcitic limestones, 2 ) reaction affecting dolomitic limestones sums and 3 ) reactions affecting powdered dolomitic limestone sums 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 base pore fluids in the concrete and siliceous constituents of the sums atoms. The gel cans absorb wet and increase in volume and generating force per unit area. Harmonizing to Vivian and others research, the chemical composing of the alkali-silica gel is variable and indefinite depending on the OH- concentration. This phenomenon will do devastation 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 act uponing for the alkali-silica reaction to happen ; an equal supply of wet, reactive silicon oxide and sufficient base nowadays in the cement. These three factors nust be present at the same time. [ basicss of lasting strengthened concrete ]
Moisture – wet play 2 of import functions in ASR. First, it act as a conveyance medium for the reactive ions and secondly, the soaking up of wet into the gel which leads for the enlargement that cause the concrete amendss.
Reactive silicon oxide – the grade of the responsiveness of silica minerals is depends on the crystal construction. The most vulnerable is the minerals holding disordered construction. This is due to broken construction has greater surface country particularly larger broken construction for the reaction. Examples of vulnerable silicon oxide construction includes formless, glassy and microcrystalline.
Alkali degree – the reaction will merely continue when it reached certain alkalinity degree in the pore fluid. The alkalinity in the pore fluid is influence by the Na and K base metals in the cement.
Flower is cause by the capillary conveyance of Ca hydrated oxide to the surface. It will leach out and deposited on surface. It besides can be transported by soluble salt that migrate to the surface zone or deposited by vaporization. After a certain period, a discoloration will look on the concrete surface due to carbonation of Ca hydrated oxide. [ basicss of lasting strengthened concrete ]
Flower can be distinguished into three signifiers ; calcium hydroxide bloom, lime crying and salt crystallization. Lime bloom occurs when Ca hydrated oxide migrates to the surface country. A thin movie of Ca carbonate will develop if calcium hydrated oxide reacts. Lime crying is deposition of thick white sedimentations that form from the points of H2O escape such as clefts or articulations. Sometimes lime crying occurs at the H2O drips off a free border of the span or retaining walls. Slat crystallization on the concrete surface. It is the migration of soluble salts through the concrete to the surface zone. These salts may come from the saltwater or contaminated mix components. The mechanism of salt crystallization is through vaporization procedure where it leads for the deposition on the concrete surface. [ basicss of lasting strengthened concrete ]
Other physical impairments [ basicss of lasting strengthened concrete ]
Abrasion [ basicss of lasting strengthened concrete ]
The definition of scratch in concrete context is the consequences from clash. The clash may be due to the crunching action, by insistent impact and overloading. The beginning of the clash is from vehicular traffic, stuffs dragged across the pavings and wind-borne atom impacting on the concrete surface. In this undertaking, there is a definite physical impairment due to scratch.
Erosion [ basicss of lasting strengthened concrete ]
The causes of eroding can be due to the suspended stuffs in the H2O across the concrete surface. It may be due to the atoms such as sand, impact on the concrete surface by the air current. There are several factors act uponing the rate of eroding such as the speed of H2O flow, the measure of the stuffs being conveyance every bit good as the size and form.
Cavitation [ basicss of lasting strengthened concrete ]
Cavitation is the sudden formation and prostration of vapor bubbles due to the force per unit area alterations in fluxing H2O. High flow of liquid speed will develops a sub-atmospheric force per unit area zone, but there is a sudden alteration in speed or way at the downstream. Vapour bubbles is produced and collapsed by force on come ining the following zone of high force per unit area. This state of affairs produces an impact consequence and force per unit area moving ridges strike the concrete surface. It will give a local relaxation of the concrete if the impact and consequence is continuously happen.