Lec.2 materials

Analysis & Design of Reinforced Concrete Structures (1) Lecture.2 Materials

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Dr. Muthanna Adil Najm

Concrete:

In the design of concrete mixes, three principal requirements for concrete are of

importance:

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.

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.

Economical 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.

Concrete Mixing and Proportioning:

The influence of ingredients on properties of concrete.

Workability:

Workability measured by slump test :

1 2 3 4

300

mm

slump 300

mm

Materials


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1. Layer 1: Fill 1/3 full. 25 stokes

2. Layer 2: Fill 2/3 full. 25 stokes

3. Layer 3: Fill full. 25 stokes

4. Lift cone and measure slump (typically 5-15 cm.)

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 .

Admixtures:

Applications:

Improve workability

Accelerate or retard setting and hardening

Aid in curing

Improve durability

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.

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)

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.

Failure Mechanism of Concrete:


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1- Shrinkage Microcracks: are the initial shrinkage cracks due to carbonation

shrinkage, hydration shrinkage, and drying shrinkage.

2- 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.

3- 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%).

Concrete Properties:

1- Uniaxial Stress versus Strain Behavior in Compression:


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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:

1- Compressive Strength, f'c: Normally use 28-day strength for design strength

2- Poisson’s Ratio, n: n ≈ 0.15 to 0.20 Usually use n = 0.17

3- Modulus of Elasticity, Ec: Corresponds to secant modulus at 0.45 f’c

The ACI 318-05 (Sec. 8.5.1): use this equation for concrete modulus of elasticity

for unit concrete weight of 1440 kg/m3 < wc < 2480 kg/m3

ccc fwE 5.1

043.0 in MPa

For normal weight concrete Ec shall be permitted to be taken as:

cc fE 4700 in MPa

4- Concrete strain at max. compressive stress, εo

For typical ε curves in compression

εo varies between 0.0015-0.003

For normal strength concrete, εo ≈ 0.002

5- Maximum useable strain, εu

ACI Code: εu = 0.003

Used for flexural and axial compression

Typical Concrete Stress-Strain Curves in Compression

cε

cE

oε uε

c0.45f’

cf

cf’ 12”

6”


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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.

2- Tensile Strength:

Tensile strength ≈ 8 % to 15 % of f'c

Modulus of Rupture, fr

For deflection calculations, use:

cr ff 62.0 ACI Eq. 9-10 (ACI 318-05 sec. 9.5.2.3)


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Test:

Splitting Tensile Strength, fct

Split Cylinder Test (Not given in ACI Code)

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.

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

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 or (200 to 600 microstrain)

Creep:

Deformations (strains) under sustained loads.

P

Concrete Cylinder

Poisson’s

Effect ld

Pfct

2

P

rf

= P/2*a maxM

unreinforced

concrete beam

2

6

bh

M

I

Mcf r


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Like shrinkage, creep is not completely reversible.

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.

Steel Reinforcement:

1. General:

Standard Reinforcing Bar Markings

Most common types for non-prestressed members:


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hot-rolled deformed bars

welded wire fabric

Areas, Weights, Dimensions

Standard inch-pound

bars × in

8

1

Soft metric bars

mm

# 3

# 4

# 5

# 10

# 12

# 16

# 6

# 7

# 8

# 19

# 22

# 25


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# 9

# 10

# 11

# 29

# 32

# 36

# 14

# 18

# 43

# 57

2. Types:

ASTM A615 - Standard Specification for Deformed and Plain-Billet Steel Bars

Grade 60: fy = 60 ksi = 420 MPa, #3 to #18: Most common in buildings and

bridges

Grade 40: fy = 40 ksi = 280 MPa, #3 to #6: Most ductile

Grade 75: fy = 75 ksi = 520 MPa, #6 to #18 : Less used in building.

3. Stress versus Strain

Stress-Strain curve for various types of steel reinforcement bar.

Es = Initial tangent modulus = 29,000 ksi = 200 MPa(all grades)

Note: GR40 has a longer yield plateau

Education

Lec.2 materials