Concrete Intro

8/10/2019 Concrete Intro

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Concrete

Man made stone


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constituentsmixture of aggregate and paste

paste 30 to 40%

portland cement 7% to 15% by Vol.

water 14% to 21% by Vol.

Aggregates 60% to 70%

coarse aggregates

Fine aggregates

Admixtures


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Portland CementDry powder of very fine particles

forms a paste when mixed with water

chemical reaction-Hydration

glue

paste coats all the aggregates togetherhardens and forms a solid mass


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Water needed for two purposes:

chemical reaction with cement

workability

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

extra water remains in pores and holes

results in porosity

Goodfor preventing plastic shrinkage cracking andworkability

Badfor permeability, strength, durability.


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Aggregatescheap fillers

hard material

provide for volume stability

reduce volume changes

provide abrasion resistance


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Admixtureschemical

set retarders

set accelerators

water reducing

air entraining

mineral

fly ash

silica fume

slags


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Properties of fresh concreteWorkability

ease of placement

resistance to segregation

homogeneous mass

Consistency

ability to flow


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Slump Test Inverted cone

fill it up with three layers

of equal volume rod each layer 25 times

scrape off the surface

8

4

12


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Slump Testslump cone

rod

concrete


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Slump test

Slump

Ruler


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Slump test resultsstiff 0-2

massive sections, little reinforcement

use vibration

medium 2-5

columns, beams, retaining walls

Fluid 5-7

heavily reinforced section, flowable concrete


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Factors affecting slumpwater cement ratio

w/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 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

new w/c = 365.77 / 685 = 0.534 >> 0.43


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Factors affecting slump-

paste contentconstant water cement ratio

increase paste content

increase slump

NO GOOD

constant cement content

increase water content increase slump

NO GOOD


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Factors Affecting Slump-

Water ContentAdd water at the constant cement content,

w/c increases, slump increases.

Add water at a constant water cement ratio,have to increase cement as well, slump

increases.


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Factors affecting slump-paste

content

Low paste contentHarsh mix

High paste contentRich mix


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ball bearing effect-startstarting height


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ball bearing effect-end

slump


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Admixturesset retarding admixtures

set accelerating admixtures

water reducing admixtures

superplasticizers

air entraining admixtures


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Factors affecting slumpAggregates

grading the larger the particle size, the higher

the slump for a given paste content


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effect of aggregate size

1

1

1

Consider a single aggregate the size of 1x1x1


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Compute the surface area as

you break up the particles

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

surface area = 1.5*8= 12 square inche

block surface area = 0.5*0.5*6=1.5block surface area = 1*1*6= 6


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Break it up further


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Compute 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,

thicker coating, easy sliding of particles


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Smaller particles, more surface area,

thinner coating, interlocking of particles


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Effect of aggregate sizesize # of particles volume surface area

1" 1 1 cubic inch 6 square inches

.5" 8 1 cubic inch 12 square inches

0.25 64 1 cubic inch 24 square inches

0.125 512 1 cubic inch 48 square inches


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Angularity and surface texture

of aggregates

angular and roughaggregate smooth aggregateriver gravel


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Temperature

fresh concrete

aggregates paste


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Bleeding


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Water accumulation on surfaceExamine the concrete surface


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Interaction between bleeding and evaporation

surface water

Evaporation

Bleed water

Bleed water = evaporation


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Too much evaporation leads to surface cracking

no surface water

Evaporation

Bleed water < Evaporation

drying


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Side diagram of surface contraction

Wants to shrink

Does not want to shrink


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Free Shrinkage,

causes volume change, but no stresses

before shrinkage After Shrinkage


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Restrained Shrinkage- creates

stresses, which may cause cracking


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Restrained shrinkage crackingParallel cracking perpendicular

to the direction of shrinkage


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Bleeding and its controlCreates problems:

poor pumpability

delays in finishing

high w/c at the top

poor bond between

two layers

causes

lack of fines

too much water content

Remedies

more fines

adjust grading

entrained air

reduce water content


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Causes of Plastic Shrinkage Cracking

water evaporates faster than it can reach the

top surface

drying while plastic

cracking


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Plastic Shrinkage Cracking-

RemediesControl the wind velocity

reduce the concretes temperature

use ice as mixing water

increase the humidity at the surface

fogging

cover w/polyethylene

curing compound

Fiber reinforcement


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CuringThe time needed for the chemical reaction

of portland cement with water.

Glue is being made.

concrete after 14 days of curing has

completed only 40% of its potential.

70 % at 28 days.


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Curing tips ample water

do not let it dry

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

keep temperature at a moderate level

concrete with flyash requires longer curing


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

best temperature is room temperature

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

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 curing real high temperatures above 120 F can cause

serious damage since cement may set too fast.

accelerated curing procedures produce strongconcrete, but durability might suffer.

autoclave curing.

Documents

Concrete Intro