Concrete

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Concrete Man made stone
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constituents

• mixture 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 Cement

• Dry powder of very fine particles
• forms a paste when mixed with water
• chemical reaction-Hydration
• glue
• paste coats all the aggregates together
• hardens 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
• Good for preventing plastic shrinkage cracking
  and workability
• Bad for permeability, strength, durability.

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Aggregates

• cheap fillers
• hard material
• provide for volume stability
• reduce volume changes
• provide abrasion resistance

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Admixtures

• chemical
• set retarders
• set accelerators
• water reducing
• air entraining
• mineral
• fly ash
• silica fume
• slags

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Properties of fresh concrete

• Workability
• 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

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8
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Slump Test
slump cone
rod
concrete
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Slump test
Ruler
Slump
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Slump test results

• stiff 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 slump

• water 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 ratio

• if 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 gtgt 0.43

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Factors affecting slump- paste content

• constant 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 Content

• Add 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 content Harsh mix
High paste content Rich mix
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ball bearing effect-start
starting height
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ball bearing effect-end
slump
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Admixtures

• set retarding admixtures
• set accelerating admixtures
• water reducing admixtures
• superplasticizers
• air entraining admixtures

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Factors affecting slump

• Aggregates
• 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
block surface area 0.50.561.5
block surface area 116 6
volume 1 cubic in surface area 6 square inches
volume 1 cubic in surface area 1.58 12
square inches
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Break it up further
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Compute the surface area
surface area 0.250.2568824
0.5 in
0.25 in
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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 size
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Angularity and surface texture of aggregates
angular and rough aggregate
smooth aggregate river gravel
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Temperature
fresh concrete
aggregates
paste
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Bleeding
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Water accumulation on surface
Examine the concrete surface
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Interaction between bleeding and evaporation
Evaporation
surface water
Bleed water
Bleed water evaporation
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Too much evaporation leads to surface cracking
Evaporation
no surface water
drying
Bleed water lt Evaporation
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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 cracking
Parallel cracking perpendicular to the direction
of shrinkage
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Bleeding and its control

• causes
• lack of fines
• too much water content
• Remedies
• more fines
• adjust grading
• entrained air
• reduce water content

• Creates problems
• delays in finishing
• high w/c at the top
• poor bond between two layers

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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-Remedies

• Control 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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Curing

• The 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 around
  40 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 strong
  concrete, but durability might suffer.
• autoclave curing.