CONCRETE Brief Description

7/31/2019 CONCRETE Brief Description

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Concrete

Man made stone


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constituentsconstituentsx mixture of aggregate and paste

x paste 30 to 40%

x portland cement 7% to 15% by Vol.x water 14% to 21% by Vol.

x Aggregates 60% to 70%

x coarse aggregates

x Fine aggregates

x Admixtures


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Portland CementPortland Cementx Dry powder of very fine particles

x forms a paste when mixed with water

x chemical reaction-Hydration

x glue

x

paste coats all the aggregates togetherx hardens and forms a solid mass


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WaterWaterx needed for two purposes:

x chemical reaction with cement

x workability

x only 1/3 of the water is needed for chemical reaction

x extra water remains in pores and holes

x results in porosity

x

Good for preventing plastic shrinkage cracking andworkability

x Bad for permeability, strength, durability.


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AggregatesAggregatesx cheap fillers

x hard material

xprovide for volume stability

x reduce volume changes

x

provide abrasion resistance


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AdmixturesAdmixturesx chemical

x set retarders

x set accelerators

xwater reducing

x air entraining

x mineralx fly ash

x silica fume

x slags


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Properties of fresh concreteProperties of fresh concretex Workability

x ease of placement

x resistance to segregation

x homogeneous mass

x Consistency

x ability to flow


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Slump TestSlump Testx Inverted cone

x fill it up with three layers

of equal volumex rod each layer 25 times

x scrape off the surface

8

4

12


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Slump TestSlump Testslump cone

rod

concrete


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Slump testSlump test

Slump

Ruler


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Slump test resultsSlump test resultsx stiff 0-2

xmassive sections, little reinforcement

x use vibration

x medium 2-5

x columns, beams, retaining walls

x Fluid 5-7

x heavily reinforced section, flowable concrete


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Factors affecting slumpFactors affecting slumpx water cement ratio

xw/c = weight of water / weight of cement

example:

weight of water mixed at the plant 292 lbs.

weight of cement 685 lbs./cu. yard

w/c = 292/685 = 0.43


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water cement ratiowater cement ratioif you add 10 gallons of water per cubic

yard at job site, then:

extra water

10 gallons/cubic yard * (3.8

liters/gallon) * (2.2 lbs./kg)*( 1kg/liter) = 83.77 lbs.

total water 282 + 83.77 = 365.77


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Factors affecting slump-Factors affecting slump-

paste contentpaste contentx constant water cement ratio

x increase paste content

x increase slump

xNO GOOD

x constant cement content

x increase water contentx increase slump

xNO GOOD


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Factors Affecting Slump-Factors Affecting Slump-

Water ContentWater Contentx Add water at the constant cement content,

w/c increases, slump increases.

x Add water at a constant water cement ratio,

have to increase cement as well, slump

increases.


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Factors affecting slump-pasteFactors affecting slump-paste

contentcontent

Low paste contentHarsh mix

High paste contentRich mix


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ball bearing effect-startball bearing effect-startstarting height


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ball bearing effect-endball bearing effect-end

slump


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AdmixturesAdmixturesx set retarding admixtures

x set accelerating admixtures

x water reducing admixtures

x superplasticizers

x

air entraining admixtures


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Factors affecting slumpFactors affecting slumpx Aggregates

x grading the larger the particle size, the higher the

slump for a given paste content


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effect of aggregate sizeeffect of aggregate size

1

1

1

Consider a single aggregate the size of 1x1x1


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Compute the surface area asCompute the surface area as

you break up the particlesyou break up the particles

volume = 1 cubic inurface area = 6 square inches volume = 1 cubic in

surface area = 1.5*8= 12 square in

block surface area = 0.5*0.5*6=1block surface area = 1*1*6= 6


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Break it up furtherBreak it up further


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Compute the surface areaCompute the surface area

0.5 in

0.25 in

surface area = 0.25*0.25*6*8*8=24


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Larger particles, less surface area,Larger particles, less surface area,

thicker coating, easy sliding of particlesthicker coating, easy sliding of particles


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Smaller particles, more surface area,Smaller particles, more surface area,

thinner coating, interlocking of particlesthinner coating, interlocking of particles


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Effect of aggregate sizeEffect of aggregate sizes ize # of partic le volume surface area

1" 1 1 cubic inc 6 square inches

.5" 8 1 cubic inc 12 square inche

0.25 64 1 cubic inc 24 square inche

0.125 512 1 cubic inc 48 square inche


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Angularity and surface textureAngularity and surface texture

of aggregatesof aggregates

angular and roughaggregate smooth aggregateriver gravel


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TemperatureTemperature

fresh concrete

aggregates paste


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BleedingBleeding


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Water accumulation on surfaceWater accumulation on surface

Examine the concrete surface


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Interaction between bleeding and evaporationInteraction between bleeding and evaporation

surface water

Evaporation

Bleed water

Bleed water = evaporation


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Too much evaporation leads to surface crackingToo much evaporation leads to surface cracking

no surface water

Evaporation

Bleed water < Evaporation

drying


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Side diagram of surface contractionSide diagram of surface contraction

Wants to shrink

Does not want to shrink


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Free Shrinkage,Free Shrinkage,

causes volume change, but no stressescauses volume change, but no stresses

before shrinkage After Shrinkage


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Restrained Shrinkage- createsRestrained Shrinkage- creates

stresses, which may cause crackingstresses, which may cause cracking


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Restrained shrinkage crackingRestrained shrinkage crackingParallel cracking perpendicular

to the direction of shrinkage


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Bleeding and its controlBleeding and its controlx Creates problems:

xpoor pumpability

x delays in finishing

x high w/c at the top

xpoor bond between

two layers

x causes

x lack of fines

x too much water content

x Remedies

xmore fines

x adjust grading

x entrained air

x reduce water content


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Causes of Plastic Shrinkage CrackingCauses of Plastic Shrinkage Cracking

x water evaporates faster than it can reach the

top surface

x drying while plastic

x cracking


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Plastic Shrinkage Cracking-Plastic Shrinkage Cracking-

RemediesRemediesx Control the wind velocity

x reduce the concretes temperature

x use ice as mixing water

x increase the humidity at the surface

x fogging

x cover w/polyethylene

x curing compound

x Fiber reinforcement


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CuringCuringx The time needed for the chemical reaction

of portland cement with water.

x Glue is being made.

x concrete after 14 days of curing has

completed only 40% of its potential.

x 70 % at 28 days.


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Curing tipsCuring tipsx ample water

x do not let it dry

x dry concrete = dead concrete, all reactions stopx can not revitalize concrete after it dries

x keep temperature at a moderate level

x concrete with flyash requires longer curing


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Temperature effects on curingTemperature effects on curingx The higher the temperature the faster the curing

x best temperature is room temperature

x strongest concrete is made at temperature around40 F.(not practical)

x If concrete freezes during the first 24 hrs., it may

never be able to attain its original properties.


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Temperature effects on curingTemperature effects on curingx real high temperatures above 120 F can cause

serious damage since cement may set too fast.

x

accelerated curing procedures produce strongconcrete, but durability might suffer.

x autoclave curing.


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Thank youThank you

CONCRETE Brief Description

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