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Strength

Strength

Significance:

  1. The primary function of concrete is of carry applied load, expressed in terms of strength.
  2. Strength is our index to other property performance including durability and permeability.

Types of Strength

  1. Compressive
  2. Tensile
  3. Flexural
  4. Shear
  5. Bonding

1. Compressive Strength

  • Most significant
  • Measured by cylinder test

2. Tensile Strength

  • Concrete has low tensile strength~10% of its compressive strength.
  • Test for tensile strength is typically done using the cylinder tensile splitting test or modulus of rupture.
  • As a rule we assume that tensile strength is zero and steel reinforcing is used to take up tensile stresses.

3. Flexural Strength

  • Due to bending of a member where in compression occurs on one side and tension on another.
  • The axial stress at any depth through the beam is     σ =My /I
  • Maximum tensile stress occurs at the bottom of the beam and is σt= Mc / I
  • Where c is the distance from the neutral axis to the bottom fiber, M moment of inertia of the section (bh3 / 12 for a rectangular section)

4. Shear Strength

  • Concrete seldom experience pure shear.
  • When subject to bending there is usually a shear component in the stress.

5. Bonding  Strength

  • Both cementations bond to aggregate and to re-bar
  • Bond strength increase with compressive strength
  • Bond strength is higher for deformed bar than for plain

Factors Affecting Strength

  1. Constituent material
  2. Proportions of constituents
  3. Aggregates
  4. Methods of preparation
  5. Curing procedures
  6. Moisture
  7. Temperature

1. Constituent Material

  • The finer the cement the more rapid the hydration, thus the faster the increase in strength.
  • This is reflected in the specific surface area which is the surface area of particles in a specific volume.
  • This is because the water content with cement particles can reach the core of the particle and hydrate the entire particle more rapidly.
  • Cement of the same type from different plants will have different strength, and in a given plant may have different strength at different times due to variability in raw materials

2. Proportions of Constituents

  • If increase w/c ratio then strength decrease
  • For a given w/c ratio, increased aggregate proportion (decrease cement proportion) decrease strength.
  • Increased air content will decrease strength.

3.  Aggregates

  • In aggregates the shape, texture and size are most important parameters.
  • Aggregates strength is generally less important.
  • Rough texture changes the shape of the σ-ε curve but has little effect on the ultimate strength
  • At low w/c ratio crushed rock will have higher strength because of improved bond and this effect disappears with increasing w/c ratio.
  • However, at the same workability (crushed needs more water) crushed and rounded aggregates given about the same strength.
  • Larger aggregates may given lower strength as it needs less cement to coat the particles, thus the mix will have less cement for a given workability.

4. Methods of Preparation

  • Batching must be done with a high degree of accuracy to ensure quality concrete
  • Mixing must be through but not excessive
  • Placement of concrete must not lead to segregation allowing excessive free fall and excessive vibration

5. Moisture

  • The moist curing prevents loss of moisture from concrete.
  • Loss of moisture will decrease strength.
  • If not moist curved, strength loss may approach 50%
  • Moist curing may be accomplished by covering concrete (wet burlap, plastic sheets) or apply curing compounds to prevents moisture loss.
  • ACI recommended 7 days moist curing for structural concrete in the field
  • In the lab, specimens are placed in moisture rooms with relative humanity ~ 100%

6. Temperature

  • Curing at cooler temperature will given lower short term strength but likely higher strength at longer times
  • Important that water in the concrete does not freeze until a strength of ~3.5 MPa is attained, (much of the water has hydrated with the cement) typically -24 hours of curing. The freezing point of the water may be depressed below  C0
  • Hydration may occur to ~ -10 C0
Strength

Strength

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About Mustafa

My name is Mustafa Awais, I am basically from Pakistan & Currently living in Saudia Arabia, I am Civil Engineer as well as Web Master.

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