Placing Quality Concrete

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Placing Quality Concrete
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Control of Portland Cement Concrete
Pg 134

• Concrete can be designed and produced to give the
  desired strength under lab conditions, but under
  field conditions may fail.
• Factors which affect the quality of concrete in
  the field are
• Adequate support of foundations and forms
• Adequate field engineering to insure conformance
  with plans and designs
• Transportation and handling of concrete

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Factors for Placement
Pg 134

• Weather Conditions
• Methods of curing
• Foundations
• Any concrete structure must rest upon a
  foundation of uniform bearing firm enough to
  support the entire load.

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Foundations
Pg 135

• Footings for large structures are designed
  according to the foundation material on which
  they will be placed.
• For poor material in a great depth, the footing
  will be designed to rest on a piling.
• For medium bearing quality material such as clay,
  a spread footing may be used to distribute the
  load over a larger area.

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Foundations
Pg 135

• For good foundation material such as sound rock
  of sufficient depth, a smaller footing is used,
  and the rock will help support the load.
• To ensure the foundation is sound, the contractor
  is required to explore the foundation to a depth
  of at least five feet below the bottom of the
  footing.

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Foundations
Pg 135

• In concrete work such as pavements, curb and
  gutter, and paved ditches the foundation or
  subgrade should always be of uniform density.
• Before placing concrete on any foundation or
  subgrade, the subgrade should be moist to prevent
  rapid extraction of the moisture from the
  concrete.

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Field Engineering
Pg 136

• Concrete placed in a form may be dropped at a
  height of no more then five feet.
• Concrete forms must be constructed with enough
  rigidity to prevent distortion,
• as the concrete is being placed and cured.
• Reinforcing steel is used in concrete to give
  added strength to the concrete structure.

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Field Engineering
Pg 137

• Reinforcing steel is used for compressive and
  tensile strength.
• Compressive strength is defined as the resistance
  to mashing together. Tensile strength is the
  resistance to pulling apart.
• Reinforcing steel is placed according to design
  and within specified tolerance to achieve maximum
  resistance to tension and compression.

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Placing and Finishing Concrete
Pg 141

• The first thing the inspector checks is to verify
  that the subgrade has been properly moistened and
  the forms have been oiled or moistened.
• When placing concrete in forms, the concrete
  should be placed as near to the final position as
  possible.

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Placing and Finishing
Pg 144

• During and immediately following placement of the
  concrete into the forms, care must be taken that
  the concrete is consolidated.
• Specification requires consolidation be
  accomplished by mechanical vibrating internal to
  the concrete.
• Vibration should not be used to move concrete
  around in forms, as this can cause segregation.

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Time, Temp, Curing
Pg 152

• The length of curing time the concrete should be
  protected against loss of moisture is dependent
  on the type of cement, mix proportions, required
  strength, size of the concrete mass, weather, and
  future exposure conditions.

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Time, Temp, Curing
Pg 152

• All material, equipment, and labor necessary to
  promptly apply the curing compound should be on
  site before any concrete is placed.
• A curing day will be considered as any
  consecutive 24 hour period beginning when the
  manipulation of each separate mass has been
  completed and during which the air temperature
  adajacent to the mass does not fall below 40F.

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Plastic Shrinkage Cracks
Pg 154

• Cracking that occur on the surface of fresh
  concrete during setting is called plastic
  shrinkage cracking.
• Plastic shrinkage cracks appear when evaporation
  exceeds the rate at which bleed water rises to
  the surface of the concrete.
• When the rate of evaporation is around 0.2 pounds
  per square foot per hour, precautionary measures
  are necessary.

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Plastic Shrinkage Precautions
Pg 155

• Dampen subgrade and forms
• Dampen aggregates
• Erect windbreaks to reduce wind velocity over
  concrete surface
• Cool mixing water
• Protect concrete with wet coverings during any
  delay between placing and finishing

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Precautions
Pg 155

• Reduce time between placing and start of curing
• Protect concrete during the first few hours after
  placing to minimize evaporation
• Apply moisture to the surface using a fog sprayer
  nozzle

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Hot Weather Concreting
Pg 157

• High temperatures can accelerate the hardening of
  concrete and more mixing water is usually needed
  to get required consistency.
• Higher water contents mean greater shrinkage
  possibilities.
• Even concrete that is in the acceptable
  temperature range can have problems in hot
  weather.

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Hot Weather Concreting
Pg 158

• The amount of mixing water required to make a
  cubic yard of concrete increases as the
  temperature of fresh concrete increases.
• Increasing water content, without increasing the
  cement content, results in a higher water/cement
  ratio which will affect the strength of the
  concrete.
• The chart in figure 10 shows the effect of high
  concrete temp on compressive strength.

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Figure 10
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Effects of High Temperature

• At high concrete temperatures the concrete gets
  weaker as it gets older instead of gaining
  strength, losing strength as the temperature of
  the fresh concrete increases.

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Cooling Concrete Material
Pg 160

• Crushed or flaked ice is more effective then
  plain water in reducing concrete.
• One pound of ice as it melts absorbs 144 BTU.
• 1 Lb of ice heated from 32F to 73F absorbs a
  total of 185 BTU. 1 Lb of water absorbs only 40
  BTU when its temperature is raised from 33F to
  73F.

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Cooling Concrete Material

• If 75 Lbs of ice per cu yd were added to a
  concrete mix with a temperature of 90F the
  temperature of the mix would drop to 75.
• During extremely hot periods it is beneficial to
  place the concrete in the evening or at night.

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Table 1000-2 Elapsed time for
Placing Concrete
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Concrete can not be used which does not reach
its final position in the forms within the time
specified in table 1000-2.
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Questions?