lecture3混凝土结构设计原理-英文课件

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.
o u

c
Concrete Properties

Modulus of Elasticity, Ec Corresponds to secant modulus at 0.45 f’c ACI 318-02 (Sec. 8.5.1):
1.5
Ec ( psi) = 33 w
f 'c ( psi)
fc f’c 0.45f’c Ec
o
u
Concrete Properties

Maximum useable strain, u ACI Code: u = 0.003 Used for flexural and axial compression
fc Ec
f’c
0.45f’c
o
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 .
Concrete Mixing and Proportioning
The influence of ingredients on properties of concrete.
Concrete Mixing and Proportioning
3. Workability

Workability measured by slump test 12” slump
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%).

Biblioteka Baidu
chemically releasing water from fine aggregates.
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)
1. 2. 3. 4.
1 2 3 4 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.)
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.
Concrete Properties
1. Uniaxial Stress versus Strain Behavior in Compression fc
f’c 0.45f’c Ec
12”
6”
o
u

c
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:
Concrete Properties


Compressive Strength, f’c Normally use 28-day strength for design strength fc Ec f’c Poisson’s Ratio, n n ~ 0.15 to 0.20 0.45f’c Usually use n = 0.17
where w = unit weight (pcf) 90 pcf < wc <155 pcf
Ec ( psi) = 57,000
f 'c ( psi)
For normal weight concrete (wc 145 pcf)
Concrete Properties
Compute Ec for f’c = 4500 psi for normal weight (145 pcf) concrete using both ACI equations:
Concrete Mixing and Proportioning
Failure Mechanism of Concrete
Shrinkage Microcracks are the initial shrinkage cracks due to carbonation shrinkage, hydration shrinkage, and drying shrinkage.
u
Concrete Properties
Typical Concrete Stress-Strain Curves in Compression
Concrete Properties
Types of compression failure
There are three modes of failure. [a] Under axial compression concrete fails in shear.
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.
[b] the separation of the specimen into columnar pieces by what is known as splitting or columnar fracture. [c] Combination of shear and splitting failure.
Concrete Mixing and Proportioning
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
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
Ec ( psi) = 33 w1.5
f 'c ( psi)
Ec ( psi) = 57,000
f 'c ( psi)
Concrete Properties

Concrete strain at max. compressive stress, o For typical curves in compression o varies between 0.0015-0.003 For normal strength concrete, o ~ 0.002
Concrete Mixing and Proportioning
4. Admixtures

Applications: Improve workability Accelerate or retard setting and hardening Aid in curing Improve durability
Lecture 3 - Fundamentals
June 6, 2003 CVEN 444
Lecture Goals
Concrete Properties Steel Properties
Concrete Mixing and Proportioning
In the design of concrete mixes, three principal requirements for concrete are of importance:
Quality Workability Economical
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.