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# 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

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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