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Concrete Man made stone. 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. Portland Cement. Dry powder of very fine particles

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slide1

Concrete

Man made stone

constituents
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
portland cement
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
water
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.
aggregates
Aggregates
  • cheap fillers
  • hard material
  • provide for volume stability
  • reduce volume changes
  • provide abrasion resistance
admixtures
Admixtures
  • chemical
    • set retarders
    • set accelerators
    • water reducing
    • air entraining
  • mineral
    • fly ash
    • silica fume
    • slags
properties of fresh concrete
Properties of fresh concrete
  • Workability
    • ease of placement
    • resistance to segregation
    • homogeneous mass
  • Consistency
    • ability to flow
slump test
Slump Test
  • Inverted cone
  • fill it up with three layers of equal volume
  • rod each layer 25 times
  • scrape off the surface

4”

12”

8”

slump test9
Slump Test

slump cone

rod

concrete

slump test10
Slump test

Ruler

Slump

slump test results
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
factors affecting slump
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

water cement ratio
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 >> 0.43

factors affecting slump paste content
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
factors affecting slump water content
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.
factors affecting slump paste content16
Factors affecting slump-paste content

Low paste content

Harsh mix

High paste content

Rich mix

admixtures19
Admixtures
  • set retarding admixtures
  • set accelerating admixtures
  • water reducing admixtures
  • superplasticizers
  • air entraining admixtures
factors affecting slump20
Factors affecting slump
  • Aggregates
    • grading the larger the particle size, the higher the slump for a given paste content
effect of aggregate size
effect of aggregate size

1”

1”

1”

Consider a single aggregate the size of 1”x1”x1”

compute the surface area as you break up the particles
Compute the surface area as you break up the particles

block surface area = 0.5*0.5*6=1.5

block surface area = 1*1*6= 6

volume = 1 cubic in

surface area = 6 square inches

volume = 1 cubic in

surface area = 1.5*8= 12 square inches

compute the surface area
Compute the surface area

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

0.5 in

0.25 in

angularity and surface texture of aggregates
Angularity and surface texture of aggregates

angular and rough

aggregate

smooth aggregate

river gravel

temperature
Temperature

fresh concrete

aggregates

paste

water accumulation on surface
Water accumulation on surface

Examine the concrete surface

interaction between bleeding and evaporation
Interaction between bleeding and evaporation

Evaporation

surface water

Bleed water

Bleed water = evaporation

too much evaporation leads to surface cracking
Too much evaporation leads to surface cracking

Evaporation

no surface water

drying

Bleed water < Evaporation

side diagram of surface contraction
Side diagram of surface contraction

Wants to shrink

Does not want to shrink

restrained shrinkage cracking
Restrained shrinkage cracking

Parallel cracking perpendicular

to the direction of shrinkage

bleeding and its control
Bleeding and its control
  • Creates 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
causes of plastic shrinkage cracking
Causes of Plastic Shrinkage Cracking
  • water evaporates faster than it can reach the top surface
  • drying while plastic
  • cracking
plastic shrinkage cracking remedies
Plastic Shrinkage Cracking-Remedies
  • Control the wind velocity
  • reduce the concrete’s temperature
    • use ice as mixing water
  • increase the humidity at the surface
    • fogging
    • cover w/polyethylene
    • curing compound
  • Fiber reinforcement
curing
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.
curing tips
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
temperature effects on curing
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.
temperature effects on curing44
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.