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Placing Quality Concrete


Placing Quality Concrete Control of Portland Cement Concrete Concrete can be designed and produced to give the desired strength under lab conditions, but under field ... – PowerPoint PPT presentation

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Title: Placing Quality Concrete

Placing Quality Concrete
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

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.

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.

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

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

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.

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.

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

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.

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.

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.

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.

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

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

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
  • Even concrete that is in the acceptable
    temperature range can have problems in hot

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
  • The chart in figure 10 shows the effect of high
    concrete temp on compressive strength.

Figure 10
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.

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

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.

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