Class Day Nineteen

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29 Νοε 2013 (πριν από 3 χρόνια και 7 μήνες)

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Class Day Nineteen

Introduction to Reinforced Concrete



Concrete is a mixture of graded aggregates held
together by a matrix.



Concrete used in construction is a mixture of
graded aggregates:


Large aggregates:

Hard gravel ranging in
size approximately from ¼” to ¾”. Clean and
free of dust & small particles.


Small aggregates:

Clean washed sand free of
dust and small particles.


Aggregates form a mixture of larger stones,
with voids filled with smaller stones, with
voids filled with smaller stones, with voids
filled with sand particles, all coated and held
in place by a properly mixed paste made from
portland cement and water.

Concrete

STRUCTURAL CONCRETE


Ingredients


Portland Cement


Course Aggregate


Fine Aggregate


Water



Admixtures are not


standard

and are


used for special


purpose, such as


ease of placement or


protection from


freezing.


Even though concrete is “rocklike”, the final result
can appear pliable, and assume most any shape.


THE STRENGTH OF CONCRETE IS DEPENDENT
UPON FOUR CONDITIONS:



1

The hardness of the aggregates
. Obviously,
the hardness of the aggregate is important to
strength in compression, since concrete could fail
by forces that crush the aggregates.



2

The degree of proportion of distribution of the
aggregate sizes
. The variation of aggregate size is
important in that proper distribution is
instrumental in eliminating voids of hard material,
making the mix as homogeneous as possible.








3

The ratio of the amount of water to that of
cement
. The hardening of concrete is not a drying
process, but a chemical reaction called
HYDRATION
. It is the reaction between water and
portland cement, and a concrete mix requires only
a small amount of water for hydration to occur.




Too much water dilutes the hydration process.
Too little water limits the completeness of the
process.




If the water in concrete freezes before
hydration is complete, the process stops and the
resulting concrete is weak and poor in wearability.







4

The care given to the concrete after it has
been placed
. After concrete is placed, it must be
given the conditions to “cure” which is the
completeness of the hydration process.




Concrete placed where the hot sun evaporates
the water near the surface causes uneven
hydration and results in poor durability of the
outer surface. Concrete footings placed against
dry earth lose water due to absorption into the
dirt, causing uneven curing.



PORTLAND CEMENT




The manufacture of cement for concrete was
first developed in Portland, England in 1824. That
process has developed in all parts of the world
using the natural ingredients necessary for the
finished product.
The designation, ‘portland’ is
one of definition of ingredients rather than a
proper name.




Portland cement is a mixture, primarily of
ingredients that contain lime, iron, silica, and
alumina


all natural elements found in various
formations of limestone, marble, marl, seashells,
clay, or shale.



Portland cement is manufactured in seven types,
each containing admixtures for specific
requirements such as high early strength, air
entraining, low heat of hydration, and resistance
to sulfates.
The general use of cement for
concrete in construction is Type 1


normal
cement.



AS A DEVELOPING ARCHITECT
, or AS SOMEONE
WHO IS SUPPOSED TO POSESS INTELLIGENCE:




please do not ever refer to concrete items as
“cement walkways or cement footings or cement
driveways or cement roads.” Cement is an
ingredient in the manufacture of concrete


just as
flour is an ingredient in bread


or just as sugar is
an ingredient in ice cream.




MIXING AND PLACING CONCRETE



A

MIXING
: Concrete should be mixed, using a
reference of proportioning of aggregates by
weight. The condition of aggregates is important,
in that the amount of moisture present in gravel or
sand contributes to the amount of water for the
mix, and excessive water is detrimental to the
strength of the mix.



Concrete for construction is generally mixed at a
batching plant, set up for the scientific methods of
proportioning the mix. The ingredients are placed
in a large rotating drum on a truck, so the mix is
complete by the time it gets to the job site.


CONCRETE TRANSIT TRUCK




You as a project architect

will specify a
strength required for concrete. The contractor in
turn will order the concrete from a manufacturing
plant and hold them responsible for the strength
of the mix. You, the architect, will then get a copy
of the details of the concrete mix design.




Often the concrete batching manufacturer will
send a representative ( other than the truck driver)
to the job site to see that the concrete is placed
properly, and
that no additional amount of water is
added to the mix after it leaves the plant.




B

PLACEMENT OF CONCRETE




The depositing of concrete at the site is extremely
important to its strength.


If concrete is placed in excavations that can
absorb water, then part of the water needed
for hydration might be lost.



Concrete placed in forms and around
reinforcing steel must be worked and tamped
into place to eliminate voids in the final mix,
and to assure a complete bond between the
concrete and reinforcing steel.





If concrete is allowed to free
-
fall for a long
distance from the end of the chute, such as to
the bottom of a deep footing,
the aggregates
will separate and the mass will not have a
properly distributed mix of ingredients.





In large placements,
the deposited concrete
must be worked into place with vibrators to
fill voids

of cement paste around exposed
surfaces of the aggregates.




Several methods of depositing concrete are
employed:



Where the truck can have ready access to the
placement site,
the material is deposited from
a curved, adjustable, rotateable, metal chute
.



If the placement site is not accessible by the
truck, concrete can be
pumped through a
flexible tube

to reach distant areas, or to
locations on upper stories of buildings.



Large quantities of concrete that must be
placed in remote areas can be transported in
steel buckets equipped with trap doors,
moved to the deposit site by overhead
cranes.










If concrete is to be placed in a deep footing such
as a drilled pier shaft, or if concrete is to be placed
underwater, the material may be conveyed through a
tube called a “TREMIE.”




This type of placement assures that the mix will
not be allowed to free
-
fall and separate the
ingredients, and in the case of underwater, will prevent
the mix from being diluted.


Concrete Placement Direct From the Transit Mixer


Placement with a Concrete Pump

Placement of concrete using a Crane & Bucket



SAMPLING CONCRETE FOR TESTING TO VERIFY
STRENGTH




Two procedures are done at the jobs site


and
are done each time for a specified quantity of
concrete that is placed.




You as the architect may direct samples to be
taken for a specified number of cubic yards of
concrete placed, and that each placement will be
documented as to specific location on the job.




These samples are taken by a representative of
a testing laboratory.



The Slump Test:




A cone shaped metal cylinder approximately
12” tall, 4” open top diameter, 8” open bottom
diameter, is placed on a clean, level surface.
Concrete is inserted into the top and tamped full
with a steel rod. The top surface is struck smooth.




The slump cone is then lifted upward, allowing
the concrete to remain. The concrete mass, still
being wet, will slump down due to its weight. The
distance from the top of the cone to the top is the
mass is measured. Too much slump indicates too
much water


a clue to see if the mix has inferior
strength. Slumps should be 6” or less.


Sample collected

Slump Measured

Cone Removed and Concrete

Allowed to ‘Slump’

Slump Cone Filled

THE
SLUMP
TEST


Concrete Test Cylinders:




You the architect will specify that three test
cylinders will be taken for each specific batch of
concrete placed.




A cardboard or plastic cylinder 6” in diameter,
12” tall is filled with concrete, tamped solid and
struck smooth at the top. Three of these will be
done for each batch.




The cylinders are allowed to “set”, then placed
in plastic bags, identified and sent to an
independent testing laboratory, where they will
remain for your specified length of time.







At age 7 days one cylinder will be tested in a
machine to determine its strength. It should attain
approximately ½ to 2/3 of the compressive
strength you specified.




At 21 days another cylinder will be tested for
strength. The results should be in the vicinity of
80% to 90% of specified strength.




At 28 days (the age of fully cured concrete) the
last cylinder will be tested to ascertain its matured
strength.




If strengths are high after the 7 day test, you
may choose to skip the 21 day test. But in any
case, the 28 day test should be made.





Failures are rare, but the consequences are left
to you, the architect’s specification. If a failure
occurs after 28 days, You may have required by
your specification that on
-
site samples of the
material be taken by the laboratory of the failed
batch, then tested for matured strength. These will
be samples, circular in shape, cut by a machine
made for the purpose.




If the cored samples do not test to the required
strength, the Contractor is required to remove all
the quantity of the failed batch and replace it with
new concrete. For these reasons, documentation
of batches of concrete placed is important.



Obviously, that is why the Contractor requires
the concrete manufacturer to be responsible for
the strength of the mix.


Test Cylinders Filled with a Sampling of the Concrete

Test Cylinders in

bags for curing

Preparation for breaking
a test cylinder


CONCRETE REINFORCING STEEL




Steel bars are made specifically to resist
tensile and shearing diagonal tension in concrete
structural members. They are made in several
strength grades, so identified, and have a
deformed surface to aid in bonding with concrete.



Steel bar size:




Eleven Standard Diameters




3, 4, 5, 6, 7, 8, 9, 10, 11, 14, 18




Number refers to eighths of an inch


Steel bar yield strength grades:

40,000 p.s.i.

50,000 p.s.i

60,000 p.s.i





Steel reinforcing is placed inside concrete
forms according to details of placement
requirements determined by structural
calculations, and secured in place with wire ties.




Sufficient room must be allowed between bars
and between bars and forms to allow the
installation and consolidation of the concrete mix.




Since steel is subject to weakening by fire, a
concrete cover of specific thickness on the
outside of reinforcement is required for
maintaining the fire integrity of concrete
structures.





Reinforcing supports placed inside forms for
the purpose of holding steel bars in position are
called ‘chairs’. They are made in numerous
configurations for specific conditions, constructed
of wire electronically welded together.




Reinforcing steel for underground concrete
footings is sometimes required to be suspended
from above in order to support the steel. A solid
connection such as a reinforcing steel bar stuck in
the soil as a support would leave a channel
through the concrete for moisture to enter and
deteriorate the reinforcing steel.






ORGANIZATIONS FOR REINFORCED CONCRETE
STANDARDS include:




CRSI

The Concrete Reinforcing Steel Institute

governs the development and standards for the
manufacture of steel reinforcing.



ACI


The American Concrete Institute

publishes the specifications for the design,
reinforcing, placing, and control of structural
concrete.


Improperly consolidated Concrete


Segregation at the bottom of the pour

(also note the trash at the bottom of the wall
)

Extensive Reinforcing
Can

Make Placement

and Consolidation Difficult

Reinforcing Supports