Chloride attack is one of the most important factors affecting the durability of concrete and is a major cause of corrosion of steel reinforcement. Over 40% of structural failures are due to corrosion caused by chloride penetration through permeable concrete reaching the steel. For corrosion to occur, chlorides must penetrate the concrete along with water and oxygen to break down the protective oxide layer on the steel. Proper mix design and construction techniques are needed to produce impermeable concrete and prevent corrosion.
Mechanism of different chemical attacks in a concrete like chloride attack, sulfate attack , which causes corrosion and spalling. Other reactions are alkali aggregate reaction , alkali silica reaction in concrete etc.
Mechanism of different chemical attacks in a concrete like chloride attack, sulfate attack , which causes corrosion and spalling. Other reactions are alkali aggregate reaction , alkali silica reaction in concrete etc.
Corrosion of Reinforcement, Carbonation Of Concrete and It's Determination. ..Rushikesh Katkar
The corrosion of reinforcement inside concrete is one of the factors affecting the performance of the structure. Here the presentation will enlighten you about its mechanism and other aspects.
Parte 1_Corrosão em concreto armado: técnicas de avaliação, monitoramento, prevenção e reabilitação. Adriana Araújo, Instituto de Pesquisas Tecnológicas do Estado de São Paulo (Instituto de Pesquisas Tecnológicas do Estado de São Paulo).
When you design your product, you put a lot of money, energy and consideration into achieving the best possible function, good appearance and high reliability. Corrosion, wear and fatigue resistance may not be the first factors you consider.
However, material selection is crucial to obtain a long service life and to avoid damage such as operation failure – for instance due to inadequate mechanical strength - or unacceptable appearance due to corrosion products.
If you choose the correct materials for your product from the beginning, you can prevent damage deriving from corrosion, wear and mechanical impact. In many cases, you may completely avoid corrosion by choosing a resistant material for its actual application environment.
Corrosion of Reinforcement, Carbonation Of Concrete and It's Determination. ..Rushikesh Katkar
The corrosion of reinforcement inside concrete is one of the factors affecting the performance of the structure. Here the presentation will enlighten you about its mechanism and other aspects.
Parte 1_Corrosão em concreto armado: técnicas de avaliação, monitoramento, prevenção e reabilitação. Adriana Araújo, Instituto de Pesquisas Tecnológicas do Estado de São Paulo (Instituto de Pesquisas Tecnológicas do Estado de São Paulo).
When you design your product, you put a lot of money, energy and consideration into achieving the best possible function, good appearance and high reliability. Corrosion, wear and fatigue resistance may not be the first factors you consider.
However, material selection is crucial to obtain a long service life and to avoid damage such as operation failure – for instance due to inadequate mechanical strength - or unacceptable appearance due to corrosion products.
If you choose the correct materials for your product from the beginning, you can prevent damage deriving from corrosion, wear and mechanical impact. In many cases, you may completely avoid corrosion by choosing a resistant material for its actual application environment.
June 3, 2024 Anti-Semitism Letter Sent to MIT President Kornbluth and MIT Cor...Levi Shapiro
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The US House of Representatives is deeply concerned by ongoing and pervasive acts of antisemitic
harassment and intimidation at the Massachusetts Institute of Technology (MIT). Failing to act decisively to ensure a safe learning environment for all students would be a grave dereliction of your responsibilities as President of MIT and Chair of the MIT Corporation.
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Postsecondary education is a unique opportunity for students to learn and have their ideas and beliefs challenged. However, universities receiving hundreds of millions of federal funds annually have denied
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• The Committee on Education and the Workforce has been investigating your institution since December 7, 2023. The Committee has broad jurisdiction over postsecondary education, including its compliance with Title VI of the Civil Rights Act, campus safety concerns over disruptions to the learning environment, and the awarding of federal student aid under the Higher Education Act.
• The Committee on Oversight and Accountability is investigating the sources of funding and other support flowing to groups espousing pro-Hamas propaganda and engaged in antisemitic harassment and intimidation of students. The Committee on Oversight and Accountability is the principal oversight committee of the US House of Representatives and has broad authority to investigate “any matter” at “any time” under House Rule X.
• The Committee on Ways and Means has been investigating several universities since November 15, 2023, when the Committee held a hearing entitled From Ivory Towers to Dark Corners: Investigating the Nexus Between Antisemitism, Tax-Exempt Universities, and Terror Financing. The Committee followed the hearing with letters to those institutions on January 10, 202
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2. Chloride Attack
Chloride attack is one of the most important aspects while
dealing with durability of concrete. It primarily causes
corrosion of reinforcement. Statistics have indicated that over
40% of failure of structures is due to corrosion of steel.
Concrete and the PassiveLayer
The strongly alkaline nature of Ca(OH)2 (pH of about 13)
prevents the corrosion of the steel by the formation of a thin
protective film of iron oxide on the metal surface. This
protection is known as passivity.
If the concrete is permeable to such an extend that soluble
chlorides penetrate right up to the reinforcement and water &
oxygen is also present, then the corrosion of steel will take
place. This layer can also be lost due to carbonation.
3. Chloride enters the concrete from the cement, water,
aggregate and sometimes from admixtures. This can also
enter by diffusion from environment if concrete is
permeable.
The Bureau of Indian Standard had specified the maximum
chloride content in cementas 0.1%.
The amount of chloride required for initiating corrosion is
partly dependent on the pH value of the pore water in
concrete. At a pH value less than 11.5 corrosion may occur
without the presence ofchloride.
4. For corrosion to occur, these elements must be present:
~There must be at least two metals (or two locations on a single
metal) at different energy levels
~ an electrolyte
~ a metallic connection
In reinforced concrete, the rebar may have many separate
areas at different energy levels. Concrete acts as the
electrolyte, and the metallic connection is provided by wire
ties, chair supports, or the rebar itself.
5.
6. Anodic reactions
Fe Fe ++ + 2 e-
Fe++ + 2(OH)- Fe(OH)2 (FerrousHydroxide)
Fe(OH)2 + 2H20 + 02 Fe(OH)3 ( Ferrichydroxide)
Cathodic reaction
4e- +2H20 + O2 4(OH)-
It can be noticed that no corrosion takes place if the concrete is dry or
probably below relative humidity of 60% because enough water is not
there to promote corrosion. If concrete is fully submersed into water
corrosion does not take place because diffusion of oxygen does not
take place into the concrete. Probably the optimum relative humidity
for corrosion is 70 to 80%.
7. The passive layer can be lost by carbonation due to reduction in alkalinity
of the concrete.
In the presence of moisture and CO2 reacts as
CO2 +H2O H2CO3 (Dil. Carbonic acid)
H2CO3 +Ca(OH)2 CaCO3 +H2O
H2C03 reacts with Ca(OH)2 and carbonation of concrete takes place. This reduce
the alkalinity of concrete. When pH of concrete reduces below 8.3, then passive
layer destroyed and corrosion takes place.
The products of corrosion occupy a volume as much as six times the original
volume of steel. This exert thrust on cover concrete resulting in cracks, spalling
or delamination of concrete. (It is a result of water entering brick, concrete or
natural stone and forcing the surface to peel, pop out or flakeoff).
8.
9. Metallurgical Methods : Steel can be made more corrosion resistant by altering
its structure through metallurgical processes such as rapid quenching of the hot
bars by series of water jets or keeping the hot bars for a short time in a water
bath.
Corrosion inhibitors : Corrosion can be prevented or delayed by chemical
method by using certain corrosion inhibiting chemicals such as nitrites,
phosphates, benzonates etc. Calcium nitrite is generally added to concrete
during mixing of concrete.
Galvanising of reinforcement: Galvanising of reinforcement (dipping of steel
in molten zinc) is also effective. The zinc surface reacts with calcium hydroxide
in the concrete to form a passive layer and prevents corrosion.
Cathodic protection : This is extensively used in advanced countries. Due to
high cost and long term monitoring this method is not popular in India.
Coatings to reinforcement: The object of coating to steel bar is to provide a durable
barrier to aggressive material, such as chloride. Fusion bonded epoxy coating is one of
the effective method.
10. Most soils contain sulphate in the form of calcium, sodium,
potassium and magnesium. They occur in soils or ground
water. Ammonium sulphate is frequently present in
agricultural soil and water from use of fertilizers or from
sewage and industrial effluents.
Decay of organic matters in marshy lands, shallow lakes often
leads to formation of H2S which can be transformed into
sulphuric acid by bacterial action.
Therefore sulphate attack is common occurrence in natural
and industrial situations.
11. The main reactions of sulphate attack on concrete are as under;
1. Formation of sulphoaluminates (ettringite) by reaction of sulphatesalts
and the C3A- phase incement;
C3A+ 3 CS H2 + 26 H C3A(CS)3H32
Increase in volumeof reaction products causes expansion and spalling.
2.Formation of calcium sulphate (gypsum) in reaction with lime (CH)
formed by hydration of cement
NS + CH + 2H NH + CSH2
Increase in volumeof reaction products causes expansion and spalling.
3. Magnesium sulphate is more aggressive than sodium or calcium salts.
4. There are some other reactionsalso.
Sulphate Attack
15. Carbon dioxide in air or dissolved in water reacts with hydrated
cement systems.
The main concern is the reaction of carbon dioxide with the lime
(CH) - phase. It gives rise to calcium carbonate;
Ca(OH)2 + CO2 = CaCO3 + H2O.
In severe cases, the C-S-H phase, which gives strength, can also be
attacked.
In all such reactions, OH- is consumed, thus lowering down the pH
of concrete. If pH is lowered very much, protection to steel
reinforcement against corrosion may belost.
Carbonation is highest in pH between 50 to 80 percent.
Carbonation
17. •Chemical reactions between aggregate containing
certain reactive constituents and alkalis (sodium and
potassium salts) and hydroxyl ions released by the
hydration of cement can have deleterious effects on
concrete.
•Granite, granite gneiss and schist, quartzite and
sandstone, containing strained quartz are among
the reactive rocks found in India.
Alkali-Aggregate Reaction
20. The basic mechanism of corrosion of steel, is an electro-chemical
phenomenon, involving an anode process and a cathode process;
Anode: Fe 2e- + Fe2+
(Metallic iron)
-
Cathode: 1/2O + H O +2e 2(OH)-
2 2
In addition, the corrosion undergone by steel is due to combination
of iron and (OH-) ions;
Fe + 1/2O + H O Fe2+ + 2(OH) ironhydroxide
2 2
(rust)
Corrosion of Reinforcement
24. Main Reasons of
Corrosion
Presence of chloride ions – from water for mixing as
well as curing, from aggregates, other ingredients,
from the service environment.
Poor quality of concrete – high permeability,no
control on water/cement ratio.
Low pH value of concrete – mainly due to
carbonation,
Inadequate cover thickness.
25. Use of Sulphate resisting cement: Use cement with low C3A content is
most effective method. So use sulphate resisting cement which contains less
C3A.
Quality Concrete : A well designed, placed and compacted concrete
exhibit higher resistance to sulphateattack.
Use of air-entrainment: Use of air-entrainment to the extend of about
6% has beneficial effect on sulphate resisting quality. This is probably due to
reduction of segregation, improvement in workability, reduction in bleeding
and in general reduction in permeability of concrete.
Use of pozzolona : Use of pozzolanic materials reduce the permeability.
High Pressure Steam Curing: This improve the resistance of concrete to
sulphate attack.
Use of High Alumina Cement: Use of High Alumina Cement improves
the resistance of concrete to sulphate attack.