Concrete

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Concrete
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Major Topics

• History
• Uses
• Materials Used To Make Concrete
• Cement
• Aggregate
• Water
• Admixture

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Major Topics cont

• Testing
• Slump Test
• Compressive Strength Test
• Air Content Test
• Strength
• Placing

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Major Topics cont

• 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

• 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

• Portland Cement 5 types
• Should conform to ASTM C150
• 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

• 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

• 2 classes
• Fine sand lt 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

• 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

• Hydration chemical reaction caused by mixing
  the water with cement
• Too much prevents proper setting
• 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

• 1 2 4 concrete consisting of
• 1 volume of cement
• 2 volumes of fine aggregate
• 4 volumes of coarse aggregate
• 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)

• 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

• Materials added into the standard concrete
  mixture for the purpose of controlling,
  modifying, or impacting some particular property
  of the concrete mix.
• Properties affected may include
• Retarding or accelerating the time of set
• Accelerating of early strength

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Admixtures cont

• 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

• Slump Test ASTM C143
• Determines the consistency and workability
• Compressive (Cylinder) Strength ASTM C192
• 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

• Comply with ASTM C39
• Basic steps
• 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

• 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

• Temperature
• Optimum temperature for curing is 73 degrees F
  may have problems curing if temperature below 40
  degrees F
• Forms
• 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 cont

• Free falling distance should not exceed 4-5 feet
  due to the threat of segregation of aggregates
  occurring

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Transporting Concrete

• Method selected depends on quantity, job layout,
  and equipment available
• Chutes
• Wheelbarrows/Buggies
• Buckets
• Conveyors
• Pumps

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Curing

• 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

• Cover with
• Wet burlap or mats
• Waterproof paper
• Plastic sheeting
• Spray with curing compound
• Leave concrete in forms longer

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Joints

• 3 types
• 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
• Rubber/plastic
• Vinyl, neoprene, polyurethane foams
• Metal/wood/cork strips

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Finishing

• 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 cont

• 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

• 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

• 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

• Placement, size, spacing, and number of bars used
  vary according to the specific project
• Markings on bars include
• 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

• 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

• 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

• Concrete which is subjected to compressive
  stresses by inducing tensile stresses in the
  reinforcement
• Attributes
• 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

• Smaller dimensions of members for the same
  loading conditions, which may increase clearances
  (longer spans) or reduce story heights
• Smaller deflections
• Crack-free members