CE 351 Concrete Materials Exam 2

SCC

Self Consolidating Concrete (SCC) can be placed without segregating paste or mortar from the CA and/or blocking of CA at highly restricted sections within the form (such as tightly spaced rebar)

in a WRA describe the dispersion action going on

adding the WRA you are adding hydrophilic particles, with polar chains with a negative end. the polar chain attaches to the cement particles and the negative end create a negative surface. The negative surface means the cement particles will repel each other, it will also repel neg. charged aggregates and air entrained bubbles.

retarder

an admixture that cause a decreases in the rate of hydration of the hydraulic cement and lengthens the time of setting

Effect of W/C ratio (C3A and deterioration)

as C3A content increases deterioration increases as W/C increases slope increase ( I think it is the slope)

WRA's effect (on hardened concrete)

as W/C ration decreases: 1. compressive strength increases 2. permeability decreases 3. chloride resistance increases 4. frost resistance improves 5. increases sulfate resistance 6. abrasion

natural pozzolans and ASR

as you increase the replacement % you decrease the expansion % significantly (i.e. 20% replaced with clacined shale 0% expansion) examples: calcinted shale and clay, diatomaceous earth, and metakaolin

How do we prevent steel corrosion

-non-chloride de-icers -membranes or sealers -low-permeability concrete -coatings on steel -non-corrosive metals (or non-ferrous reinforcement) -chemical corrosion inhibitors -cathodic protection

why does carbon-induced corrosion occur

-poor quality concrete -inadequate curing -low depths of cover -high volumes of fly ash or slag

air detrainers

decrease air content

sulfate attack mechanism

ettringite formation followed by water absorption, leads to expasion and cracking gypsum formation leads to loss of cohesion and "mushy" consistency of cement paste

alkali content of concrete

expansion unlikely if < 3 kg/m^3 (5 lb/yd^3)

accelerators applications

in cold weather: -construction not slowed -risk of damage due to early freezing is reduced in normal weather: -increase productivity -compensate for slow strength gain when pozzolans are used -increase early strength in repair materials

plasticizers and super plasticizers (on fresh concrete)

increase workability with the same water content (better placing but same strength and durability) 1.increase slump 2. improves : flow, placing, pumpability, finishability, formed surfaces

the time it takes the chloride to reach the steel depends on:

-the chloride concentration at the surface, Cs (the severity of the environment) -the diffusion coefficient, D (quality of concrete) -the depth of cover to the steel, Xd -the corrosion threshold, Ct (may be altered by chem. corr. inhibitors or corr. res. steel) -temperature (chlorides penetrate quicker in warmer climates)

measuring air content in fresh concrete

1. pressure method (not suitable for lightweight aggregate) 2. volumetric method (suitable for all types of aggregate)

Major reasons for admixtures

1. to reduce the cost of concrete construction 2. To achieve certain properties in concrete more effectively than by other means 3. to maintain the quality of concrete during the stages of mixing, transporting, placing, and curing in adverse weather conditions 4. to overcome certain emergencies during concreting operations

AAR

Alkali-Aggregate Reaction - chem. rxn in either mortar or concrete between alkalies from PC or other sources and certain constituents of some aggregates; under certain conditions, deleterious expansion of concrete may result the two types are ACR and ASR

ASR

Alkali-silica reaction - the rxn btw alkalies in PC and certain siliceous rocks or minerals. The products of the reaction may cause abnormal expansion and cracking of concrete in service

effects of relative humidity

little expansion if humidity < 80%

Set Control Admixtures

(accelerators and retarders) primarily work by slowing down or speeding up the dissolution of cement grains (by strengthening or weakening the diffusion barrier that forms around cement grains in early hydration)

rate of carbonation depends on

-CO2 in environment -WC ratio -SCM content (increases with fly ash and slag) -duration of moist curing -exposure condition of the concrete

Sources of Chloride in Concrete (internally)

-Cl in admixtures (CaCl2 is an accelerator) -NaCl found in some aggregates -mixing water may contain Chlorides

why does Fly Ash reduce sulfate resistance (especially class C/High CaO)

-Contributes C3A (also some sulfates and free lime) -Lowers Consumption of lime due to reduced pozzolanicity -presence of reactive calcium-aluminates in glass phase once you reach over 20% CaO you have a dramatic increase in C3A

alkali-aggregate reactivity inhibitors

reduce alkali-aggregate reactivity expansion

type b

retarding

fly ash and ASR

the effect depends on: -amount of fly ash used - fly ash composition -nature of reactive aggregate -amount of alkali present in the concrete (e.g. from cement) *Low Calcium Fly Ash is generally more efficient in controlling expansion than high calcium fly ash (^CaO ^expansion%)

role of pH in ASR

the higher the pH the higher the reactivity of the silica note the pH of Concrete is > 13

Why does steel start of "passive" in concrete

the highly-alkaline pore solution in concrete (pH>13.2) leads to the formation of a passive layer on rebar

Spacing factor

the maximum distance of any point in the cement paste from the periphery of an air void (L bar)

external sulfate attack can attack...

-calsium hydroxide -C-S-H -Monosulfate hydrate -other hydrates

Damage Caused by Corrosion

-cracking -spalling -delamination

minimizing risk of ASR

-use non-reactive aggregate -limit the alkali content of the the concrete -use supplementary cementing materials (SCM's) -use chemical admixtures

mitigation strategies

-use of sulfate-resisting cement -use of SCMs -combination of sulfate-resisting cement and SCMs -control of W/C ratio -reduce sulfate penetration (low permeability is KEY!) -modification of cement components (lower C3A with type 2 or 5 cements) -incorporation of SCMs

retarders applications

-used to offset effects of hot weather on hydration -enable longer haul times -keep concrete workable during placing -reduce temperature rise at early ages

11 Types of Admixtures

1. Air-Entraining 2. Water reducing 3. Plasticizer 4. Accelerator 5. Retarder 6. Hydration- Control 7. Corrosion Inhibitors 8. Shrinkage Reducers 9. Alkali - Silica Inhibitors 10. Coloring 11. Misc.

specific surface

the surface area of a quantity of air voids that have a volume of 1mm^3

The Stages of Corrosion

1. Corrosion Initiation 2. Corrosion products become visible 3. Cracking, spalling, or both 4. Structure loses load-carrying capacity

requirements for ASR

1. Reactive Silica 2. Sufficient Alkali 3. Sufficient Moisture

Three steps to hydration

1. dissolve 2. diffuse 3. precipitate

Silica Fume and Class C Fly Ash

work together well to control expansion with 5% SF you can use 30% FA instead of 50% FA it would need if it were alone

concrete alkali content calculation

=(cement content) X (%cement alkalies) X (1/100)

Ternary Blends and ASR

=PC + 2 SCMs ex: PC + SF + Slag ex: PC + SF + FA

Effect of Fly Ash on Sulfate Resistance

FA actually increases the Expansion %, the greater the %FA the greater the rate of expansion and therefore REDUCES SULFATE RESISTANCE

Phenolphthalein indicator

a convenient means of measuring the depth of carbonation as it changes from purple to colourless note: phenolphthalein is in ethanol

Water Reducing Admixtures (WRA's)

added to : 1. increase slump with same water content 2. same slump with decreased water content 3. decrease cement content 4. decrease water cement ratio and increase or maintain strength

preventing carbonation-induced corrosion

depth of carbonation linearly relates to constant k and the square root of t k increases as : -SCM increases -WC ratio increases -strength decreases -curing decreases

Sulfate Attack

deterioration of concrete through the actions of sulfate and/or acids, chemically or physically

Role of moisture in ASR

if sheltered from elements less ASR if not then more ASR

ASR Mechanism

the OH- attacks first followed by the K and Na which form a silica gel that forms around the silicaous particle. The gel is super hydrophilic and hydroscopic (absorbs tooonnnnn of water) then forms cracks in the concrete

Rapid Chloride Permeability Test - RCPT

- a way to test concrete durability -consists of monitoring the amount of electrical current passed through cylindrical specimens for a six hour duration

Sources of Chloride in Concrete (externally)

-Chloride in seawater -Chloride in groundwater -Chloride from deicing chemicals

What makes steel Corrode in Concrete?

-Chlorides from deicing salts or seawater penetrate through the concrete cover and reach the steel OR -CO2 from the atmosphere penetrates through the concrete cover and reaches the steel

Physical SALT Attack

-OCCURES WHEN SODIUM SULFATE SOLUTION ENTERS INTO CONCREETE AND INCREASES IN VOLUME AS THE TEMPERATURE CHANGES, LEADING TO TENSILE STRESSES THAT CRACK THE CONCRETE. -physical attack involving phase changes of salt soln as temp changes -no chem attack -similar to freeze-thaw -type V cement helps to control -lower w/c ratio helps resist physical salt attack BUT SCMs tend to make it worse

sources of alkali in concrete

-P.C. -Flyash/slag/silica fume -chem admixtures -wash water -aggregates -external sources (seawater, deicing chemicals)

Effect of SCM on Diffusion Coefficient

-Silica fume: @8% SF about 1/5 the D; SF low coulombs early on -Fly Ash: 15 times the reduction with FA; FA low coulombs eventually -Fly Ash Slag combo: even better -Silica Fume, FA/Slag combo: BEST

Effects of Chlorides

-The Cl ions become incorporated into the passive film (replacing some of the oxygen) increasing its solubility, permeability, and ionic conductivity; therefore the film loses its protective character -Cl ions are rarely distributed homogenously over the steel surfave therefore this leads to pitting corrosion (random spots)

mechanism of air-entrainment

-air is generated in concrete during mixing -the admixture is a surface active agent which concentrate at the air/water interface and reduce the surface tension and encouraging the formation of STABLE bubbles (in contrast to the unstable high surface tension bubbles that get removed during the compaction process) The admixtures hydrophobic side is attracted to the air bubbles and fill the surface of the bubbles relieving the surface tension and stabilizing the bubble. the hydrophilic end (-) makes the air bubbles repel each other but is also attracted to the cement and aggregate particles; this improves the cohesion of the mix and further stabilizes the bubbles.

mitigating Sulfate Attack

-determine susceptibility of sulfate attack -select mitigation strategies

Effect of increase in air (on hardened concrete)

-improved freeze-thaw, scaling, and sulfate resistance -reduced permeability and strength (5% loss in comp stength for every 1% air)

Effect of increase in air (on fresh concrete)

-increased workability -reduced water demand, segregation, bleeding, sand content -improved pumping

How do we produce low-permeability concrete

-low w.c. ratio -use a SCM (fly ash, slag, silica fume, nat. pozz., ternary blends) -good curing -minimize cracking

external sulfate attack can involve various sulfate species (ranked from most aggressive to least aggressive)

-magnesium sulfate -sodium sulfate -calcium sulfate

measuring air content in hardened concrete

1. linear-traverse method (in lab) - measuring the air voids under a microscope (compare distance from air voids and paste and aggregate) 2. modified point count method - faster

why are high CaO ashes (like Class C) less effective than low CaO ashes (like Class F)?

1. low CaO ashes reduce pore solution pH more effectively through pozzolanic rxn 2. low CaO ashes produce more C-S-H that binds significant amounts of alkalies, further reducing pore solution pH

Carbonation of Concrete

CO2 from the atmosphere reacts with cement hydration products (e.g. Ca(OH)2) and reduces the pH of concrete from above 13 to less than 9 when carbonation front reaches the steel, the passive layer is lost an the steel my corrode (if sufficient O2 and H2O are available)

External Sulfate Attack (Classical)

Caused by a source external to concrete, including sulfate from ground water, soil industry waste, fertilizers, atmospheric SO3 or liquid industry wastes.

what is DEF (delayed ettringite formation)?

Damage (expansion & cracking) of concrete due to the formation of ettringite after the concrete has hardened may undoubtedly occur as the result of excessive temperatures during curing which prevent the normal "early" formation of ettringite

effect of slag on sulfate resistance

INCREASES SULFATE RESISTANCE increase slag decrease expansion % rate slag slows the rate of expansion. 40%slag is plenty

air-entraining admixtures

Primarily used to produce concrete that is resistant to the effects of freezing and thawing, and to improve workability "improve durability in freeze-thaw, deicer, sulfate, and alkali-reactive env'ts. " typically made up of hydrophilic heads (-) and hydrophobic tails

Effect of CaO in Fly Ash

Recall: *Low Calcium Fly Ash is generally more efficient in controlling expansion than high calcium fly ash (^CaO ^expansion%) When CaO < 20% you can usually use ~ 15% fly ash once >20% requires double the fly ash (i.e. CaO= 23 %; FA=30% CaO=29%;FA=35%)

Chemical Sulfate Attack

Sulf Atk the result of chem rxns involving sulfate anion, SO4^2-, whcih forms ettringite from monosulfate and gypsum, and/or forms gypsum

Silica Fume and ASR

^ % SF decrease expansion % need significantly less SF than FA or Slag (10% SF vs 15 - 30% FA vs 25% Slag)

accelerator

accelerate settling and early strength development 1. causes an increase in rate of hydration of the hydraulic cement and thus shortens the time of setting 2. increases the rate of strength development 3. or Both

type c

accelerating

Slag and ASR

effects depend on: -amount of slag -nature of reactive aggregate -amount of alkali present in the concrete (e.g. from cement) [^% slag; decrease expansion % ]

in a WRA describe the steric mechanism going on

polycarboxylate consists of ether, which is made up of a polar chain (with neg end) and long hydrophilic side chains. The polar chains absorb onto the cement particles and the side changes physically separate the cement particles.

Internal Sulfate Attack

source of sulfate is internal to concrete, including excessive cement sulfate and delayed ettringite formation (DEF)

effect of air entrainment

stabilize tiny bubble generated in concrete to protect against freeze/thaw cycles

Physical Sulfate Attack

usually refers to (a) formation from the soln of sodium sulfate mirabilie, Na2SO4*10H2 then (b) its repeated recrystallization into thenardite, Na2SO4 and vise versa

type A

water reducer

type e

water reducing and accelerating

type d

water reducing and retarding

type g

water reducing, high rang, and retarding

type f

water reducing, high range

effects of alkalies on expansion

when less than .6 %cement alkalies, there isnt much expansion but after .6 % it is increase expansion to nearly .4%

Silica Fume and Slag

with 7% SF and 10% slag you can reach minimal expansion