Lecture 5 Fundamentals

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Lecture 5 - Fundamentals

• September 11, 2001
• CVEN 444

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Lecture Goals

• Concrete Mixing and Proportioning
• Concrete Properties
• Steel Reinforcement

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A. Concrete Mixing and Proportioning

• Concrete Composite material composed of
  portland cement, fine aggregate (sand), coarse
  aggregate (gravel/stone), and water with or
  without other additives.
• Hydration Chemical process in which the cement
  powder reacts with water and then sets and
  hardens into a solid mass, bonding the aggregates
  together

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A. Concrete Mixing and Proportioning

• Heat of Hydration Heat is released during the
  hydration process.
• In large concrete masses heat is dissipated
  slowly temperature rises and
  volume expansion later cooling
  causes contraction. Use special measures to
  control cracking.

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A. Concrete Mixing and Proportioning

• 1. Proportioning Goal is to achieve mix with
• Adequate strength
• Proper workability for placement
• Low cost
• Low Cost
• Minimize amount of cement
• Good gradation of aggregates (decreases voids and
  cement paste required)

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A. Concrete Mixing and Proportioning

• Water-Cement Ratio (W/C)
• Increased W/C Improves plasticity and fluidity
  of the mix.
• Increased W/C Results in decreased strength due
  to larger volume of voids in cement paste due to
  free water.

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A. Concrete Mixing and Proportioning

• Water-Cement Ratio (W/C) (cont..)
• Complete hydration of cement requires W/C
  0.25.
• Need water to wet aggregate surfaces, provide
  mobility of water during hydration and to provide
  workability.
• Typical W/C 0.40-0.60

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A. Concrete Mixing and Proportioning

• Water/Concrete table

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A. Concrete Mixing and Proportioning

• Proportions have been given by volume or weight
  of cement to sand to gravel (ie. 124) with W/C
  specified separately
• Now customary to specify per 94 lb. Bag of
  cement wt. Of water, sand gravel
• Batch quantity wt. per cubic yard of each
  component

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A. Concrete Mixing and Proportioning

• 2. Aggregates
• 70-75 of volume of hardened concrete
• Remainder hardened cement paste, uncombined
  water, air voids
• More densely packed aggregate give better
• strength
• weather resistance (durability)
• Economy

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A. Concrete Mixing and Proportioning

• 2. Aggregates
• Fine aggregate sand (passes through a No. 4
  sieve 4 openings per inch)
• Coarse aggregate gravel
• Good gradation
• 2-3 size groups of sand
• Several size groups of gravel

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A. Concrete Mixing and Proportioning

• Maximum size of coarse aggregate in RC
  structures Must fit into forms and between
  reinforcing bars(318-99, 3.3.2)
• 1/5 narrowest form dimension
• 1/3 depth of slab
• 3/4 minimum distance between reinforcement bars

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A. Concrete Mixing and Proportioning

• Aggregate Strength
• Strong aggregates quartzite, felsite
• Weak aggregates sandstone, marble
• Intermediate strength limestone, granite

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A. Concrete Mixing and Proportioning
In the design of concrete mixes, three principal
requirements for concrete are of importance

• Quality
• Workability
• Economy

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A. Concrete Mixing and Proportioning

• Quality of concrete is measured by its strength
  and durability. The principal factors affecting
  the strength of concrete , assuming a sound
  aggregates, W/C ratio, and the extent to which
  hydration has progressed. Durability of concrete
  is the ability of the concrete to resist
  disintegration due to freezing and thawing and
  chemical attack.

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A. Concrete Mixing and Proportioning

• Workability of concrete may be defined as a
  composite characteristic indicative of the ease
  with which the mass of plastic material may
  deposited in its final place without segregation
  during placement, and its ability to conform to
  fine forming detail.

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A. Concrete Mixing and Proportioning

• Economy takes into account effective use of
  materials, effective operation, and ease of
  handling. The cost of producing good quality
  concrete is an important consideration in the
  overall cost of the construction project.

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A. Concrete Mixing and Proportioning

The influence of ingredients on properties of
concrete.
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A. Concrete Mixing and Proportioning

• 3. Workability
• Workability measured by slump test

• Layer 1 Fill 1/3 full. 25 stokes
• Layer 2 Fill 2/3 full. 25 stokes
• Layer 3 Fill full. 25 stokes
• Lift cone and measure slump (typically 2-6 in.)

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A. Concrete Mixing and Proportioning
Slump test - The measurement of the consistency
of the mix is done with the slump-cone test. The
recommend consistency for various classes of
concrete structures .
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A. Concrete Mixing and Proportioning

• 4. Admixtures
• Applications
• Improve workability
• Accelerate or retard setting and hardening
• Aid in curing
• Improve durability

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A. Concrete Mixing and Proportioning

• 4. Admixtures
• Air-Entrainment Add air voids with bubbles
• Help with freeze/thaw cycles, workability, etc.
• Decreases density reduces strength, but also
  decreases W/C
• Superplasticizers increase workability by
  chemically releasing water from fine aggregates.

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A. Concrete Mixing and Proportioning

• 5. Types of Cement
• Type I General Purpose
• Type II Lower heat of hydration than Type I
• Type III High Early Strength
• Higher heat of hydration quicker
  strength (7 days vs. 28 days for Type I)

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A. Concrete Mixing and Proportioning

• 5. Types of Cement
• Type IV Low Heat of Hydration
• Gradually heats up, less distortion (massive
  structures)
• Type V Sulfate Resisting
• For footings, basements, sewers, etc. exposed to
  soils with sulfates

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A. Concrete Mixing and Proportioning
Failure Mechanism of Concrete
Shrinkage Microcracks are the initial shrinkage
cracks due to carbonation shrinkage, hydration
shrinkage, and drying shrinkage.
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A. Concrete Mixing and Proportioning
Failure Mechanism of Concrete
Bond Microcracks are extensions of shrinkage
microcracks, as the compression stress field
increases, the shrinkage microcracks widen but do
not propagates into the matrix. Occur at 15-20
ultimate strength of concrete.
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A. Concrete Mixing and Proportioning
Failure Mechanism of Concrete
Matrix Microcracks - are microcracks that occur
in the matrix. The propagate from 20 fc. Occur
up to 30-45 ultimate strength of concrete.
Matrix microcracks start bridge one another at
75. Aggregate microcracks occur just before
failure (90).
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B. Concrete Properties

• 1. Uniaxial Stress versus Strain Behavior in
  Compression

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B. Concrete Properties
The standard strength test generally uses a
cylindrical sample. It is tested after 28 days
to test for strength, fc. The concrete will
continue to harden with time and for a normal
Portland cement will increase with time as
follows
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B. Concrete Properties

• Compressive Strength, fc
• Normally use 28-day strength for design strength
• Poissons Ratio, n
• n 0.15 to 0.20
• Usually use n 0.17

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B. Concrete Properties

• Modulus of Elasticity, Ec
• Corresponds to secant modulus at 0.45 fc
• ACI 318-99 (Sec. 8.5.1)
• where wunit weight (pcf)
• 90 pcf lt wc lt155 pcf
• For normal weight concrete
• (wc ? 145 pcf)

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B. Concrete Properties

• In-Class Exercise
• Compute Ec for fc 4500 psi for normal weight
  (145 pcf) concrete using both ACI equations

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B. Concrete Properties

• Concrete strain at max. compressive stress, ?o
• See Fig. 3.17 for typical ? curves in
  compression
• ?o varies between 0.0015-0.003
• For normal strength concrete, ?o 0.002
• Maximum useable strain, ?u
• ACI Code ?u 0.003
• Used for flexural and axial compression

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B. Concrete Properties
Typical Concrete Stress-Strain Curves in
Compression
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B. Concrete Properties
Types of compression failure
There are three modes of failure. 1 Under axial
compression concrete fails in shear. 2 the
separation of the specimen into columnar pieces
by what is known as splitting or columnar
fracture. 3 Combination of shear and splitting
failure.
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B. Concrete Properties

• 2. Tensile Strength
• Tensile strength 8 to 15 of fc
• Modulus of Rupture, fr
• For deflection calculations, use
• Test

ACI Eq. 9-9
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B. Concrete Properties

• 2. Tensile Strength (cont.)
• Splitting Tensile Strength, fct
• Split Cylinder Test

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B. Concrete Properties

• 2. Tensile Strength (cont.)

(Not given in ACI Code)
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B. Concrete Properties

• 3. Shrinkage and Creep
• Shrinkage Due to water loss to atmosphere
  (volume loss).
• Plastic shrinkage occurs while concrete is still
  wet (hot day, flat work, etc.)
• Drying shrinkage occurs after concrete has set
• Most shrinkage occurs in first few months (80
  within one year).
• Cycles of shrinking and swelling may occur as
  environment changes.
• Reinforcement restrains the development of
  shrinkage.

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B. Concrete Properties
Shrinkage of an Unloaded Specimen
Fig. 3-21, MacGregor (1997)
80 of shrinkage occurs in first year
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B. Concrete Properties

• Shrinkage is a function of
• W/C ratio (high water content reduces amount of
  aggregate which restrains shrinkage)
• Aggregate type content (modulus of Elasticity)
• Volume/Surface Ratio

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B. Concrete Properties

• Shrinkage is a function of
• Type of cement (finely ground)
• Admixtures
• Relative humidity (largest for relative humidity
  of 40 or less).
• Typical magnitude of strain (200 to 600) 10-6
  
• (200 to 600 microstrain)

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B. Concrete Properties

• Creep
• Deformations (strains) under sustained loads.
• Like shrinkage, creep is not completely
  reversible.

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B. Concrete Properties

• Magnitude of creep strain is a function of all
  the above that affect shrinkage, plus
• magnitude of stress
• age at loading
• Creep strain develops over time
• Absorbed water layers tend to become thinner
  between gel particles that are transmitting
  compressive stresses
• Bonds form between gel particles in their
  deformed position.

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B. Concrete Properties

• Triaxial Compression
• Confined Cylinder
• Improved strength and ductility versus uniaxial
  compression
• Example spiral reinforced
  where,
• F1 longitudinal stress at failure
• F3 lateral pressure

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B. Concrete Properties

• Triaxial Compression

Fig. 3-15, MacGregor (1997)
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C. Steel Reinforcement

• 1. General
• Standard Reinforcing Bar Markings

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C. Steel Reinforcement

• 1. General
• Most common types for nonprestressed members
• hot-rolled deformed bars
• welded wire fabric

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C. Steel Reinforcement

• Areas, Weights, Dimensions

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C. Steel Reinforcement

• 2. Types
• ASTM A615 - Standard Specification for Deformed
  and Plain-Billet Steel Bars
• Grade 60 fy 60 ksi, 3 to 18
• most common in buildings and bridges
• Grade 40 fy 40 ksi, 3 to 6
• most ductile
• Grade 75 fy 75 ksi, 6 to 18

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C. Steel Reinforcement

• 2. Types
• ASTM A616 - Rail-Steel Bars
• ASTM A617 - Axle-Steel Bars
• ASTM A706 - Low-Alloy-Steel Bars
• more ductile GR60 steel
• min. length of yield plateau ?sh/?y 5

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C. Steel Reinforcement

• 3. Stress versus Strain
• Stress-Strain Curve for various types of steel
  reinforcement bar.

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C. Steel Reinforcement

• Es Initial tangent modulus 29,000 ksi
  (all grades)
• Note GR40 has a longer yield plateau