Concrete
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Major Topics
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History
Uses
Materials Used To Make Concrete
Cement
 Aggregate
 Water
 Admixture
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Major Topics con’t
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Testing
Slump Test
 Compressive Strength Test
 Air Content Test
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Strength
Placing
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Major Topics con’t
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Transporting
Curing
Finishing
Reinforced Concrete
Pre-cast Concrete
Pre-Stressed Concrete
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Concrete History Facts
The History of Concrete: Textual
Noteworthy:
The Hoover Dam, outside Las Vegas, Nevada,
was built in 1936. 3 ¼ million cubic yards
of concrete were used to construct it.
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Concrete Resources
Concrete Admixtures - The Concrete Network
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Uses
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Foundations and Driveways
Architectural Details
CMU (Concrete Masonry Units)
Concrete Roofing (Arches &
Domes)
Columns, Piers, Caissons
Walls and Beams
Bridges
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Materials Used to Make Concrete
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Portland Cement – 5 types
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Should conform to ASTM C150
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Type 1 – standard; widely used; columns,
floor slabs, beams
Type 2 – has a lower heat of hydration;
used in massive pours; e.g. Dam
construction
Type 3 – high early strength; suitable for
cold weather
Type 4 – termed low heat; used in
massive pours to diminish cracking
Type 5 – sulfate resistant; used in sewage
treatment plants & concrete drainage
structures
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Air-Entraining Portland Cement
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Produces billions of tiny bubbles
Greatly reduce segregation of mix
Less water needed to produce a
“workable” mix
Has a better resistance to freezing
and thawing
Classified as Type 1A, 2A, 3A
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Aggregate
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2 classes
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Fine – sand; < 3/8 “ large
Coarse – gravel or crushed stone
Grading should conform to ASTM C33
Sieve analysis test (ASTM C136) and
analyses for organic impurities (ASTM
C40) often done
Represent 60-80% of the concrete
volume
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5 Aggregate Types
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Natural – sand and gravel
By-Product – blast-furnace slag or
cinders
Lightweight – materials heated and
forced to expand by the gas in them
Vermiculite – a type of mica that will
greatly expand
Perlite – a type of volcanic rock which
expands
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The Critical Role of Water in Mix
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Hydration – chemical reaction caused by
mixing the water with cement
Too much – prevents proper setting
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Laitance (bleeding) – white scum or light
streaks on the surface of concrete which
are very susceptible to failure
Too little – prevents complete “chemical
reaction” from occurring
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Proportioning of Mix
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1: 2: 4 – concrete consisting of :
1 volume of cement
 2 volumes of fine aggregate
 4 volumes of coarse aggregate
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Emphasis now on “Water-Cement”
ratio methods of proportioning
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Typical Design Mix (Yield: 1 cu.yd. of
3,000 psi of Concrete) ***
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517 lb. of cement (5 ½ sacks)
1,300 lb. of sand
1, 800 lb. of gravel
34 gal. of water (6.2 gal. per sack)
*** Data from Architectural
Graphics Standards, 2000
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Admixtures
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Materials added into the standard
concrete mixture for the purpose of
controlling, modifying, or impacting
some particular property of the
concrete mix.
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Properties affected may include:
 Retarding
or accelerating the time
of set
 Accelerating of early strength
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Admixtures con’t
 Increase
in durability to exposure
to the elements
 Reduction in permeability to liquids
 Improvement of workability
 Reduction of heat of hydration
 Antibacterial properties of cement
 Coloring of concrete
 Modification in rate of bleeding
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Testing of Concrete May Include
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Slump Test [ASTM C143]
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Compressive (Cylinder) Strength
[ASTM C192]
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Determines the consistency and
workability
Determines the “compressive unit
strength” of trial batches
Air Content
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Slump Test
**Concrete sample is placed into a 12”
sheet metal cone using 3 equal volumes.
**Each layer is tamped 25 times with a
bullet-nosed 5/8” by 24” rod.
**Last layer is leveled off with the top of the
cone.
**Cone is removed
**The vertical distance from the top of the
metal cone to the concrete is measured
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Compressive Strength Test
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Comply with ASTM C39
Basic steps:
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# of samples taken vary (no less than 3)
3 layers of concrete placed in a cardboard
cylinder 6” in diameter and 12” high.
Each layer is rodded 25 times with a 5/8”
steel rod
Samples are cured under controlled
conditions
Test ages vary but usually done after 7, 14,
and 28 days
Sample removed from cardboard and placed
in testing apparatus which exerts force by
compressing the sample until it fails (breaks)
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Strength of Concrete:
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Stated as the minimum
compressive strength at 28 days of
age
Design strength:
Typical residential 2,500 – 4,000 psi
 Pre- or Post tensioned typically 5,000
– 7,000 psi
 10,000 – 12,000 psi used in columns
for high- rise buildings
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Placing Concrete
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Temperature
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Optimum temperature for curing is 73
degrees F; may have problems curing if
temperature below 40 degrees F
Forms
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Wood and metal commonly used
(reused)
Clean and sufficiently braced to withstand
the forces of the concrete being placed
Concrete weighs 135 – 165 pcf; if
lightweight then 85 – 115 pcf; often in
estimating the figure 150 pcf is used
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Placing Concrete con’t
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Free falling distance should not
exceed 4-5 feet due to the threat of
“segregation” of aggregates
occurring
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Transporting Concrete
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Method selected depends on
quantity, job layout, and equipment
available
Chutes
 Wheelbarrows/Buggies
 Buckets
 Conveyors
 Pumps
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Curing
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Proper curing is essential to obtain
design strength
Key factor: the longer the water is
retained in the mix – the longer the
reaction occurs – better strength
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Evaporation of Water Reduced
by:
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Cover with:
Wet burlap or mats
 Waterproof paper
 Plastic sheeting
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Spray with curing compound
Leave concrete in forms longer
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Joints
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3 types:
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Isolation (expansion) – allow movement
between slab and fixed parts of building
Contraction (control) – induce cracking at
pre-selected locations
Construction – provide stopping places
between pours
Materials used:
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Rubber/plastic
Vinyl, neoprene, polyurethane foams
Metal/wood/cork strips
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Finishing
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Screeds – used to level the
concrete placed in the forms
Consolidation – may be
accomplished by hand tamping
and rodding or using mechanical
vibration
Floating – done while mix still in
plastic state; provides a smooth
surface
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Finishing con’t
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Final stage may include:
Incorporation of materials for
toppings (adjust the “look”)
 Non-slip finish – use broom to
“rough-up” the surface
 Patterns – accomplished by
pressing form patterns into surface
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Reinforced Concrete
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Concrete has good compression
strength but little tensile strength
Steel excels in tensile strength and
also expands and contracts at
rates similar to concrete
Steel and concrete compliment
each other as a unit
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Reinforcing Steel [Rebar]
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Manufactured as round rods with raised
deformations for adhesion and
resistance to slip in the concrete
Sizes available from #3 to #18 –the size
is the diameter in eighths of an inch
Galvanized and epoxy coatings often
used in corrosive environments (parking
structures & bridge decks – where
deicing agents used)
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Reinforcing Bar
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Placement, size, spacing, and number of
bars used vary according to the specific
project
Markings on bars include:
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Symbol of producing mill
Bar size
Type steel used
Grades (yield & ultimate strength –
grades of 40, 50, 60, & 75 common)
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Welded Wire Reinforcing
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Also may be used as a
reinforcement in concrete
2 sets of wires are welded at
intersections to forms
squares/rectangles of a wire mesh
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Pre-Cast Concrete
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Individual concrete members of
various types cast in separate
forms before placement (may be at
job site or another location)
Walls and partitions are often
made of pre-cast units
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Pre-Stressed Concrete
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Concrete which is subjected to
compressive stresses by inducing tensile
stresses in the reinforcement
Attributes:
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Concrete strength is usually 5,000 psi at
28 days and at least 3,000 psi at the time
of pre-stressing.
Use hardrock aggregate or light weight
concrete
Low slump controlled mix is required to
reduce shrinkage
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Advantages of Pre-Stressed
Concrete
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Smaller dimensions of members
for the same loading conditions,
which may increase clearances
(longer spans) or reduce story
heights
Smaller deflections
Crack-free members
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