Placing Quality Concrete

frizzflowerUrban and Civil

Nov 29, 2013 (3 years and 4 months ago)

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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 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


Pg 134

Factors for Placement


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 134

Foundations


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

Foundations


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.

Pg 135

Foundations


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.

Pg 135

Field Engineering


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.

Pg 136

Field Engineering


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.

Pg 137

Placing and Finishing Concrete


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.

Pg 141

Placing and Finishing


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.

Pg 144

Time, Temp, Curing


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.


Pg 152

Time, Temp, Curing


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 40
°
F.

Pg 152

Plastic Shrinkage Cracks


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.

Pg 154

Plastic Shrinkage Precautions


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

Precautions


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

Pg 155

Hot Weather Concreting


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.

Pg 157

Hot Weather Concreting


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.

Pg 158

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


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 32
°
F to 73
°
F
absorbs a total of 185 BTU. 1 Lb of water
absorbs only 40 BTU when its temperature
is raised from 33
°
F to 73
°
F.

Pg 160

Cooling Concrete Material


If 75 Lbs of ice per cu yd were added to a
concrete mix with a temperature of 90
°
F 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.

Air or Concrete
Temperature, whichever
is higher
Maximum Elapsed Time
No Retarding Retarding Admixture
Admixture Used
used
90

F or above
30 minutes
1 hr 15 minutes
80

F through 89

F
45 minutes
1 hr 30 minutes
79

F or below
60 minutes
1 hr 45 minutes
70

F through 79

F
60 minutes
1 hr 45 minutes
69

F or below
1 hr 30 minutes
2 hr 15 minutes

Table 1000
-
2

Elapsed time for Placing Concrete

Concrete can not be used which does not
reach it’s final position in the forms within
the time specified in table 1000
-
2.