- The primary function of concrete is of carry applied load, expressed in terms of strength.
- Strength is our index to other property performance including durability and permeability.
Types of Strength
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
- Constituent material
- Proportions of constituents
- Methods of preparation
- Curing procedures
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.
- 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
- 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%
- 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