CT-3 @ Admixture and Corrosion

Dr. P. DINAKAR Department of Civil EngineeringPortland CementPortland CementConcrete TechnologyConcrete

**Department of Civil Engineering - IIT Bhubaneswar

Admixtures Definition & ClassificationMaterial other than water, aggregates, cement and reinforcing fibers that is used in concrete as an ingredient and added to the batch immediately before or during mixing.i. Air-entraining agents (ASTM C260)ii. Chemical admixtures (ASTM C494 and BS5075)iii. Mineral admixturesiv. Miscellaneous admixtures include:LatexesCorrosion inhibitorsExpansive admixtures


Admixtures Definition & ClassificationBeneficial effects of admixtures on concrete properties


Admixtures for ConcreteAdmixture in liquid form Admixture in solid form


Admixtures Water reducing admixturesWater-reducing admixture lowers the water required to attain a given workability.


Admixtures - How they workCement particles in the absence of a dispersing admixtureCement particles in the absence of a dispersing admixtureMechanism: Separate the cement particles Release the entrapped water


Cement particles in the presence of a dispersing admixture

Admixtures - How they work


Water reducing admixturesTwo kinds of water-reducing admixture:The normal range (WR):i. Reduce 5 10% of waterThe high range water reducing admixture (HRWR):i. Superplasticizerii. Reduce water in a range of 15-30%


Water reducing admixturesSuperplasticizerSuperplasticizers are used for two main purposes:i. To produce high strength concrete at w/c ratio in a range of 0.23 0.3 (60 150MPa)ii. To create flowing concrete with high slumps in the range of 175 to 225mm. Self compacting concrete: for beam-column joint and footing (heavy reinforced)Two formsi. Solid powerii. Liquid ---40% -60% of water


Water reducing admixturesSuperplasticizerDosage:Normal dosage of superplasticizer for concrete is 1%-2% by weight of cement.Other benefits on hardened concrete may be stated as: A lower w/c ratio would lead to better durability and lower creep and shrinkage.The major drawbacks of superplasticizer are:i. retarding of setting (especially at large amount addition)ii. causing more bleedingiii. entraining too much air.


Set Controlling AdmixturesMechanismsChange the rate of the crystallization of portland cement by adding certain soluble chemicals to influence the ion dissolution rate.Anions (silicate and aluminate)Cations (calcium)The setting will be speeded up when dissolution rates of cations and anions are higher. On the other hand, the setting will be slow down when dissolution rates of cations and anions are lower.


Set Controlling AdmixturesMechanisms (Contd)Accelerating admixture: must promote the dissolution of cations and anions.Retarding admixture must impede the dissolution of cations and anions


Set Controlling AdmixturesApplicationsi. Retarding admixtures:1. Offset fast setting caused by hot weather2. Setting control of large structural units3. Setting control of long distance transportii. Accelerators:1. Emergency repair: High way; Bridge2. Winter construction in cold regionE.g. use calcium chloride (CaCl2)


Set Controlling AdmixturesInfluence of calcium chloride in relation to setting period


Air Entraining AdmixturesAir entraining admixtures entrain air in the concreteSmall size air bubbles is introduced into concrete uniformlyThe air bubbles are enclosed, not connected


Two types of air bubbles introduced in concreteEntrained air:On purposeSize: 50 to 200 mEntrapped air:By chanceAs large as 3mm


Advantages of adding air entraining admixturesImproved workability ---air bubble as lubricantImproved ductility ---more deformation from small holeReduced permeability ---isolated air bubbleImproved impact resistance ---more deformationImproved durability ---freezing and thawing (release ice forming pressure)


Disadvantage of adding air entrainment admixture Strength loss of 10-20%


Mineral AdmixturesFinely divided siliceous materials which are added into concrete in relatively large amount.

Silica fumeFly ash


Pozzolans are siliceous or aluminous materials, which possess by themselves little or no cementitious properties, but in finely divided form react with calcium hydroxide in the presence of moisture at ordinary temperatures to form compounds possessing cementitious properties (definition according to ASTM C595).

Pozzolans


CH + Reactive SiO2 + H2O C-S-H Reaction is- Lime consuming- Pore refining- Interface refining (why?)- Slow (low heat of hydration)- Accelerated by alkalis and gypsumPozzolanic reaction


+Why mineral admixtures are key components for durability ? ++


Silica fumeSilica fume is a by-product of the induction arc furnaces in the silicon metal and ferrosilicon alloy industries.Silica fume has very fine particles20% below 0.05 micron70% below 0.10 micron95% below 0.20 micron99% below 0.50 micron


Silica fumeThe typical chemical composition of silica fume


Silica fume Powder form


Silica fume Liquid form


Effect of silica fumePhysical: PackingChemical: Pozzolanic reaction


As a bulk powder: Due to the low specific gravity of silica fume (~2.2), the bulk powder becomes very bulky and difficult to handle and transport.Dry-densified silica fume: Compaction by pressure is used to flocculate the silica fume particles. An efficient superplasticizer is required to deflocculate and cause a good dispersion of the silica fume in concrete. Slurry: 50% water + 47% silica fume + 3% chemical agent, that keeps the particles in suspension and prevents gelling. The slurry form is susceptible to gelling in cold climates. However, it is a very efficient way of dispensing silica fume. Also, storage space can also be reduced.

Silica fume available as..


Specific gravity: 2.2Typical fineness: 20000 m2/kgColour: light grey to dark grey (lighter implies purer)Cost: almost 10 times as much as PCTypically used at 5 15% replacement levelBenefits from silica fume are due to the pozzolanic reaction that produces additional C-S-H, as well as due to the particle packing (filler effect) of the fine silica fume particles

Properties


Silica fume


Because of its high fineness, the use of silica fume causes an increase in the water demand of concrete. Typically it is always used in conjunction with a superplasticizer.Silica fume causes the mix to be sticky and cohesive. Also, concrete mixes with silica fume are prone to slump loss problems. Because of its cohesiveness, a higher slump is needed to place silica fume concrete.Bleeding is reduced drastically. In fact, most silica fume mixes do not show any bleeding. In dry areas, if the evaporation rate exceeds the rate at which concrete sets, plastic shrinkage may occur.

Effects on fresh concrete


Creep and shrinkage are increased at high replacement levels (10 15%) because of an increase in the volume of the paste.Amount of air-entraining agent required for a particular volume of air is increased in silica fume concrete. Freeze-thaw resistance is reduced slightly compared to normal concrete, but damage is usually limited owing to the extremely low permeability of SFC.In most cases, silica fume concrete shows better resistance to chemical attack (exceptions being ammonium sulphate and magnesium sulphate attack), owing to the decreased permeability, as well as due to reduced CH in the paste.Expansions due to ASR are reduced in silica fume concrete. ..(contd.)


Corrosion rate is reduced with the use of silica fume. This is because of two reasons: the low permeability of SFC causes a lower availability of moisture and oxygen at the cathodic sites, and the high resistivity of SFC makes the flow of electrons difficult.Carbonation depth is generally lowered.SFC has very good abrasion and erosion resistance.Fire performance of SFC is not very good

..(contd.)


Applications of Silica fume concreteUltra High Strength Requirements High Abrasion Resistance Early-Age Strength Improvement Corrosion Protection Repair Applications


SourceBy-product obtained during combustion of coal in thermal power plantsThe quality and composition of fly ash depends on the type of coal being burnt

Fly Ash


Fly Ash


Need for fly ash utilizationNearly 73% of Indias total power generation is thermal (mostly using coal)100 million tons of fly ash being generated annuallyWorld Bank - by 2015, disposal of coal ash would require 1000 square kilometres or one square metre of land per person in India


Collection of fly ashDuring combustion of coal, 75 80% of the ash flies out with the flue gas, and is thus called fly ash. The ash that doesnt fly out is called bottom ash. This can be processed as aggregate, but is generally not used in concrete.The collection of fly ash is done using the following two types of precipitators:- Bag-house precipitator- Electrostatic precipitatorThe bag-house precipitator is found to be more efficient, and removes out very fine material


Classification of Fly ash (ASTM)Classified according to the coal burnedClass F: Anthracite or bituminous coalClass C: Lignite or sub bituminouscoalClass C is more active than Class F


ASTM ClassificationType C: This is also called High Calcium (CaO) fly ash, and possesses both cementitious and pozzolanic properties. 10 15% of the material has a particle size greater than 45 m, and the fineness (Blaine) is 300 400 m2/kg. The particles are primarily solid spheres with a smooth texture. The average particle size is less than 20 m.Type F: This is also called Low Calcium (CaO) fly ash, and is a normally pozzolanic material. 15 20% of the material is larger than 45 m, and the fineness is 200 300 m2/kg. Particles are solid spheres with a smooth texture, and the average particle size is 20 m.


Fly ash - ConstructionConcrete production25% to 30% replacementCan improve durability Good for long term strengthBetter to be used with other mineral admixtures


Fly ashAdvantages of incorporation of fly ashLow cost ---$200-300/Ton (cement $500/Ton)Low energy demand ---Industry by productLow hydration heat ---pozzolanic reactionDisadvantages of incorporation of fly ashLow early age strengthLonger initial setting timeSolution: Alkali activator (1% NaOH)


Effects on fresh concreteThe setting time is increased when fly ash is used.Workability and flow of concrete are increased due to the spherical shape of the fly ash particles, which lends a ball-bearing type effect on the concrete mixture.Bleeding and segregation are usually reduced for well-proportioned fly ash concrete.The paste volume is increased when mass replacement of cement by fly ash is done.


Effects on hardened concreteStrength gain of fly ash concrete is slower than normal concrete, as stated in the previous sections. Ultimate strengths are usually improved when fly ash is used.Creep and shrinkage of fly ash concrete are typically higher than normal concrete, because of the increased amount of paste in the concrete (when mass replacement is done).More air-entraining admixture is needed to entrain air in fly-ash concrete.The results on the effects of fly ash on sulphate resistance are inconclusive. Expansions during alkali aggregate reaction are reduced by the use of fly ash, because of the dilution of Portland cement (implying there are lesser alkalis available).


In high strength concrete, as an additional cementitious material.In roller-compacted concrete. Fly ash is good for bonding in-between the layers of this concrete.In controlled low-strength materials (CLSM), which are flowable mortars used as backfillAs a synthetic aggregateFor manufacture of bricks

Specialized applications


Silica Fume Fly Ash Cement

SiO2 Content85- 97 35 - 48 20 -25

Surface Area m2/kg17,000 - 30,000 400 - 700 300 - 500

Pozzolanic Activity(with cement, %)120 - 210 85 - 110 n/a

Pozzolanic Activity(with lime, psi) (MPa)1,200 - 1,660 800 - 1,000 n/a (8.3 - 11.4) (5.5 - 6.9) Comparison of Chemical and Physical Characteristics -- Silica Fume, Fly Ash and Cement


Concrete Hardened ConcreteHarden concrete is the concrete which gains sufficient strength and ready to carry out the external load. Its properties have to meet the requirement of end users.


Main Properties of Hardened ConcreteStrengthCompressive strengthFlexural strengthElastic modulusDurability of concrete


Concrete StrengthThe strength of a material can be defined as its ability to resist to stress without failure The strength of concrete is a very important property, normally used as a parameter for concrete specification, mix design and quality control Why is strength specified in construction design and quality control?It is relatively easy to be measuredOther properties are related to the strength


Concrete StrengthIn concrete the strength is normally measured by compression (compressive strength) or flexure (flexural strength) The compressive strength of concrete is always higher than its flexural strength, and it is clearly shown by the results of the tests performed in lab. Although there is no direct relationship between these two parameters, normally the compressive strength of a concrete is 8 to 10 times higher than its flexural strength


Influence of Concrete Constituents on its StrengthThe strength of concrete is determined by:Strength of the aggregatesStrength of cement pasteStrength of the transition zone (interfacial zone between aggregate and cement paste)


Compressive strength test Failure mechanisma. Random microcrackb. Stably growth of microcracksc. Microcrack localizationd. Failure (major crack in vertical direction)


Failure surface of a usual concrete


Failure surface of concrete containing weak coarse aggregate


Size of element (section, volume)Curing conditionChemical admixturesMain factors affecting early strength


Influence of Curing Conditions on Concrete StrengthThe curing process guarantees the cement hydration and, thus, the development of concrete strengthThe development of the strength of concrete can be accelerated by curing at high temperature


The strength up to 3 days of a cured and non-cured concrete are the same.But at 14 days, the strength of a cured concrete is around 2.5 times higher than the strength of the same concrete without curing

Influence of Curing Conditions on Concrete Strength


Elasticity Modulus of concreteThe elasticity modulus is the ratio between an applied tension (stress) and the correspondent deformation (strain) observed in the concreteThe elasticity modulus of the concrete increases with the increase of the:Elasticity modulus of the aggregateAggregate content Strength of cement pasteStrength of the bond between aggregate and cement paste


Elasticity ModulusThe elasticity modulus of the concrete also increases with the strength of concrete


Stress Strain curves of typical materials


Direct Tension testsDirect tension tests of concrete are seldom to carry out because it is difficult to control and because the specimen holding devices introduce secondary stress that cannot be ignored. Two frequently used methods are described below.


Direct Tension tests


In-direct Tension testBS 1881: Part 117:1983 150 x 300mm cylinder. Loading rate 0.02 to 0.04 MPa/sASTM C 496-71: 150 x 300mm cylinder. Loading rate 0.011 to 0.023 MPa/s


In-direct Tension testSplitting tensile strength fst = 2P/ DL


Flexural strengthBS 1881: Part 118: 1983Flexural test -150 x 150 x 750mm or 100 x 100 x 500mm (Max. size of aggregate is less than 25mm)


Flexural strength


Durability of concreteStrategies for concrete design have changed over the years

Strength-based design is being replaced by designs centered on theDURABILITY OF CONCRETE


What is durability of concrete ?Durability of hydraulic-cement concrete is defined as its ability to resist weathering action, chemical attack, abrasion, or any other process of deterioration (ACI)Durable concrete will retain its original form, quality, and serviceability when exposed to its environment


Common Durability Problems in ConcreteCorrosion of steel in reinforced concreteSulphate and other chemical attack of concreteAlkali-aggregate reactionFreezing and thawing damage


Durability and PermeabilityWater is common to all the durability problems in concrete. The presence of water, or its involvement in the reactions is necessary for the problems to occur. Thus, the durability of concrete is intrinsically related to its water-tightness, or permeability.


Concrete PermeabilityThe permeability of concrete is influenced by several factors. They are:Composition of cement paste: cement typeDegree of hydration of cement pasteWater/cement ratioAggregate contentCement content


Concrete PermeabilityInfluence of cement type, water/cement ratio and cement content on concrete permeability (*) According to ASTM C 1202

Amount of passing charge (Coulombs) Resistance of Chloride Penetration(*)> 4000High2000 4000Moderate1000 2000Low100 1000Very low< 100Negligible

Chart1

8672266

11312701

15736431

17207456

slag cement

OPC

water/cement ratio/ cement consumption

Chloride Diffusion (%)

Sheet1

slag cementOPC

0,45/3008672266slag cementOPC

0,45/350113127010,45/3008672266

0,65/200190835010,45/35011312701

0,65/250111770610,65/30015736431

0,65/300157364310,65/35017207456

0,65/35017207456

Sheet2

Sheet3


Permeability and PorosityPermeability of concrete is a function of the permeability of the cement paste, of the aggregate, and of the interfacial transition zone. The permeability of these components is in turn related to the porosity. Porosity and permeability need not be directly related. The interconnectivity of pores is generally responsible for a high permeability.


Types of chemical attackSulphate attackAcid attackChloride attackCarbonation


Sulphate attack: IntroductionSulphate attack: When the sulphate ions are from an external source react with the aluminates of cement to form expansive products leading to complete destruction of concrete over a period of time

Common chemicals carrying sulphate ions: Na2SO4, MgSO4, CaSO4, (NH4)2SO4 and FeSO4 these are present in various concentrations in seawater and groundwater


Sulphate attack reactionsHydrated cement phases react in an aqueous medium with the sulphate ions

Primary products formed are gypsum and ettringite

Other reactions result in a progressive loss of stability of the calcium silicate hydrate (CSH), which is the primary strength-giving compound of hydrated cement


Action on CH:CH + NS + 2H CSH2 + NHGypsum formationCH + MS + 2H CSH2 + MHGypsum and brucite!Gypsum causes softening and strength loss, and is also reported to cause expansion. However, the further reaction of gypsum with the aluminates C3A and its hydrates, and, monosulphate, leads to the formation of expansive ettringiteLoss of CH Reduction in pH of the systemFormation of brucite (MH) as a layer on the surface protects the concrete from further attack for a while; however, it does not help in the long run!Sulphate attack reactions


Modes of failureExpansions associated with gypsum and ettringite formation lead to cracking

Loss of strength and structural integrity due to a loss of the cementitious structure


Protection against sulphate attackUse of low C3A cements (sulphate resisting cements are proportioned based on this concept); low C3S would also helpUse of high alumina cementUse of supersulphated cementUse of pozzolanic materials and mineral admixturesLow w/c and good impermeability!!


Chloride attackSolutions bearing chloride ions can also react with cementitious compounds, although the products that form as a result do not cause any expansionsConsumption of cementitious phases could lead to increase in porosity, reduction of pH, etc.C3A can bind Cl- ions; thus, high C3A cements are good against chloride attack


Acid attackPrimarily a problem in sewer pipesTypically related to H2SO4Gypsum formation, and an associated softening and strength loss of the structure observedLoss of cementitious nature due to deterioration of CSH because of low pH


Seawater attackCombined effect of sulphates and chloridesIn addition to chemical attack, other complications are also involved:- Tidal zones subjected to drying and wetting (salt crystallization)- Splashing action of waves leads to mechanical degradation


CarbonationCarbon dioxide diffuses into the pores of concrete and reacts with calcium hydroxide; as a result, the alkalinity (pH) of the concrete is reduced

Reduction of pH causes the passivity of reinforcing steel (protective layer) to be destroyed


The corrosion of steel in reinforced concrete is a problem of mammoth proportions. It is estimated that 5% of a developed nations GDP is utilized for repair of corrosion-related damage. The yearly cost of repairs for reinforced concrete bridge decks in the US alone is estimated to be $ 50 200 million. Corrosion of steel in reinforced concrete


Reactions of corrosionCorrosion is an electrochemical problem. The overall mechanism can be broken up into the anode reaction and the cathode reaction


Rust formationThe ferrous and hydroxyl ions combine to form the rust products.2 Fe2+ + 4 OH- 2 Fe(OH)2 (greenish rust) The greenish rust, upon further reactions with O2 and OH-, can form Fe2O3 (red rust) and Fe3O4 (black rust). The rust often accumulates at places other than the reaction sites.


P. K. Mehta, P. J. M. Monteiro, Concrete, 2nd Edition, Prentice Hall, 1995Reactions of corrosion


Factors influencing rate of corrosionAvailability of dissolved oxygen and moisture at the cathodeResistivity of the medium (concrete and its pore solution)Passivation of steel: In an alkaline environment, the surface atoms of the steel get oxidized to form an thin oxide layer (thickness of about 10 nm) of FeOOH. This film is stable in the highly alkaline environment of concrete.


From Bentur, Diamond, and Berke, Steel Corrosion in Concrete, E&FN Spon, UK, 1997. Some factors influencing rate of corrosion


Depassivation of steelThe phenomenon of the formation of a protective oxide layer around the steel is called passivationDepassivation, or the breakdown of this layer, can occur by a number of mechanisms: (1) Consumption of OH- by carbonation and other reactions; when the pH falls below 11.5, the film is no longer stable; (2) Presence of a high concentration of Cl- (pitting corrosion)


Propagation of corrosionOnce depassivation occurs, the initiation process of corrosion is complete; the steel then corrodes at an almost constant rate


Protection against corrosionGalvanization: this process involves the plating or steel with Zinc. Zn, having a higher electrochemical potential compared to Fe (more negative), becomes the sacrificial anode, and Fe is protected as the cathodeCathodic protection: An external voltage or current is supplied to the steel to keep it cathodic and preventing oxidation from occurring


Protection (contd.)Use of stainless steel (very high Cr): Produces a stable passivating filmUse of epoxy coated steel: Very expensive!Use of corrosion inhibitors- Oxidizing or non-oxidizing passivators of steel- Oxygen scavengers- Film forming compounds (adsorption)- Cathodic effects: paste can be made hydrophobicSome typical corrosion inhibitors are:Inorganic: Calcium nitriteOrganic: Amines, esters, alkanolaminesThese compounds are usually added at high dosages, ~ 2% by weight of cement; $$ very expensive $$ !!


Protection (contd.)Adequate depth of coverGood quality concrete with low permeability


Alkali-Aggregate Reaction, What is it?Reaction of alkalis in concrete with certain specific types of aggregatesAlkali-silica reaction (ASR): when aggregates with reactive forms of silica are usedAlkalis could come from cement, admixtures, water, or from aggregates themselves!


Alkali-Silica Reaction (ASR)


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When the expansive pressure exceeds the tensile strength of the concrete, the concrete cracksAlkali-Silica Reaction (ASR)


When cracks reach the surface of the structure, map cracking resultsAlkali-Silica Reaction (ASR)


Manifestation of ASRASR is a very slow reaction. May take many years to show!Irregular cracks on surface with a map patternGel can also ooze out through the cracks to the surface


Freezing and ThawingThis is related to the failure of the paste. Parallel cracks form in the paste and proceed inward from the places where concrete first becomes highly saturated with water.


Protection against paste failureUse of air entraining agents: Creation of well-distributed air void system (Critical distance: 0.2 mm)Low w/c concrete very small pore size makes freezing difficult

**********************Les cristaux de silicates (ici C3S) donnent naissance 2 types dhydrates :- des silicates de calcium hydrats ou CSH- de la portlandite appele couramment chaux. Il sagit dhydroxyde de calcium Ca(OH)2. La portlandite est une base forte. Cest elle qui dtermine le pH lev (> 12.5) de la solution qui remplit la porosit de la pte de ciment. Les sulfates alcalins qui sont en solution participent laugmentation du pH de la solution. On peut estimer que le pH de la solution qui baigne les hydrates est suprieur 13 (lquivalent dune solution de soude caustique, ce qui explique les mains propres lorsque lon fait des travaux de maonnerie sans gants de protection).Sous les images, notation cimentire des principaux hydrates (H=H2O).***As-produced silica fume. This is silica fume as it comes from the bag house. While it can be used in concrete, this form is more difficult to handle and more expensive to transport because of its very low bulk density.

conc683*Slurried silica fume. Typically, the slurry consists of approximately 50 percent silica fume and 50 percent water, by mass. When first introduced to the market, slurried silica-fume products often contained chemical admixtures such as water reducers or high-range water reducers. Today, slurry is available without any such additions.

conc684****Schematic of the transition zone. Note that in the area adjacent to the surface of an aggregate particle, the relatively uniformly sized cement grains are not packed as densely as they are in the bulk zone. This wall effect is one of the reasons stated for the relative weakness and permeability of cement paste in the transition zone.

When silica fume is added to the concrete, better particle packing occurs in this area. The silica fume in this area can further refine the transition zone through its pozzolanic reaction.

This sketch is taken from H. H. Bache, Densified Cement/Ultra-Fine Particle-Based Materials, Presented at the Second International Conference on Superplasticizers in Concrete, 1981.

conc698.2****************This slide presents a comparison of some of the physical and chemical characteristics of silica fume, fly ash, and portland cement.

Note that the surface area measurements for silica fume are done using a nitrogen absorption method (BET) while those for fly ash and portland cement are typically done using an air permeability method (Blaine).

Note also that the pozzolanic activity testing was done using a constant water content for the silica fume mixtures rather than a constant flow.

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CT-3 @ Admixture and Corrosion

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