The concrete strength test is a crucial step in determining whether the concrete has achieved the desired strength to perform its function in a structure. The most common method to test the strength of concrete is the compressive strength test, but there are several methods used based on the specific needs of the project. These tests help assess the quality, durability, and load-bearing capacity of the concrete.
1. Compressive Strength Test (Cube or Cylinder Test)
This is the most widely used test to determine the compressive strength of hardened concrete.
How It Works:
Concrete specimens are prepared by pouring fresh concrete into cubes (usually 150mm x 150mm x 150mm) or cylinders (typically 150mm in diameter and 300mm in height).
The specimens are cured for a specific time (usually 7, 14, or 28 days).
After curing, the specimens are placed in a compressive testing machine, which applies a gradual load until the concrete breaks.
The maximum load at which the specimen fails is recorded, and the compressive strength is calculated using the formula:
Compressive Strength (MPa)=Load at Failure (N)Cross-Sectional Area (mm²)\text{Compressive Strength (MPa)} = \frac{\text{Load at Failure (N)}}{\text{Cross-Sectional Area (mm²)}}Compressive Strength (MPa)=Cross-Sectional Area (mm²)Load at Failure (N)​
Why It's Used:
It helps ensure that the concrete meets the design strength specified for the project (e.g., M20, M25 concrete).
It provides an accurate measure of the concrete’s load-bearing capacity.
Common Test Durations:
7 Days: Early strength (65-70% of final strength).
28 Days: Full strength (100% of design strength).
2. Slump Test (Workability Test)
The slump test measures the workability or consistency of fresh concrete. Although not a direct measure of strength, the workability impacts how well the concrete will be placed and compacted, which influences the final strength.
How It Works:
Concrete is placed into a metal cone (300mm high) in three layers, with each layer being tamped down to remove air pockets.
The cone is lifted, and the concrete slumps or settles.
The difference between the height of the cone and the height of the settled concrete (slump) is measured.
Why It's Used:
Indicates how easily the concrete can be placed and compacted.
Ensures the mix is neither too stiff nor too fluid, affecting the final strength.
Types of Slumps:
True Slump: The concrete slumps evenly and symmetrically.
Shear Slump: Concrete slips or shears down one side.
Collapse Slump: Indicates too much water in the mix.
3. Split Tensile Strength Test
The split tensile test measures the tensile strength of concrete, which is important for understanding how concrete behaves under tension (important for beams and slabs).
How It Works:
A cylindrical concrete specimen (typically 150mm diameter, 300mm height) is placed on its side in a compressive testing machine.
The machine applies compression along the length of the cylinder, causing it to split in half.
The tensile strength is calculated based on the applied load at failure.
Tensile Strength (MPa)=2⋅Load at Failure (N)π⋅Diameter (mm)⋅Length (mm)\text{Tensile Strength (MPa)} = \frac{2 \cdot \text{Load at Failure (N)}}{\pi \cdot \text{Diameter (mm)} \cdot \text{Length (mm)}}Tensile Strength (MPa)=π⋅Diameter (mm)⋅Length (mm)2⋅Load at Failure (N)​
Why It's Used:
Concrete is weak in tension, and this test helps assess its ability to resist cracking under tensile forces.
4. Flexural Strength Test
The flexural strength test measures the bending strength of concrete, particularly important for pavements and other structures subjected to bending forces.
How It Works:
A rectangular beam of concrete (typically 150mm x 150mm x 700mm) is supported at both ends.
A load is applied at the mid-span of the beam until it fractures.
The flexural strength is calculated using the formula:
Flexural Strength (MPa)=Load at Failure (N)⋅Length (mm)Breadth (mm)⋅Depth² (mm)\text{Flexural Strength (MPa)} = \frac{\text{Load at Failure (N)} \cdot \text{Length (mm)}}{\text{Breadth (mm)} \cdot \text{Depth² (mm)}}Flexural Strength (MPa)=Breadth (mm)⋅Depth² (mm)Load at Failure (N)⋅Length (mm)​
Why It's Used:
It provides insight into how concrete will perform when subjected to bending or flexural stresses, such as in pavements, beams, and slabs.
5. Rebound Hammer Test (Non-Destructive Test)
The rebound hammer test is a non-destructive method used to estimate the surface hardness and compressive strength of in-place concrete without damaging it.
How It Works:
A spring-loaded hammer strikes the concrete surface, and the rebound distance is measured.
Higher rebound values indicate harder, stronger concrete.
Why It's Used:
Quick and non-destructive test.
Provides an indication of compressive strength without needing to break the concrete.
Useful for existing structures or quality control in construction.
Limitations:
Results can vary depending on surface condition (smooth or rough) and moisture content.
It provides only an estimate of compressive strength.
6. Ultrasonic Pulse Velocity (UPV) Test (Non-Destructive Test)
The ultrasonic pulse velocity test measures the quality and uniformity of concrete using high-frequency sound waves.
How It Works:
Ultrasonic waves are transmitted through the concrete, and the time taken for the waves to travel from one side to the other is measured.
Faster pulse velocity indicates denser and higher-quality concrete.
Why It's Used:
It’s a non-destructive way to check for cracks, voids, or defects inside the concrete.
Can assess concrete uniformity and strength estimation.
Limitations:
Cannot directly measure strength but provides a general indication of concrete quality.
Results can be affected by reinforcement and aggregate size.
7. Core Cutting Test (Destructive Test)
The core cutting test involves taking a core sample from hardened concrete and testing it for compressive strength.
How It Works:
A cylindrical core is drilled from an existing concrete structure.
The core is tested for compressive strength similarly to the compressive strength test for cubes or cylinders.
Why It's Used:
Directly assesses the in-place strength of concrete in existing structures.
Useful for quality assurance in critical projects or if there are concerns about the strength of the structure after construction.
Drawbacks:
It is destructive, as it requires removing part of the structure.
Core samples must be taken carefully to avoid compromising structural integrity.
Summary of Tests:
Compressive Strength Test: Best for determining the overall strength of concrete.
Slump Test: Measures the workability of fresh concrete.
Split Tensile Strength Test: Assesses tensile strength and cracking potential.
Flexural Strength Test: Measures bending resistance, especially important for pavements and beams.
Rebound Hammer Test: Non-destructive, provides an estimate of surface hardness and compressive strength.
Ultrasonic Pulse Velocity (UPV) Test: Non-destructive, checks for internal defects and uniformity.
Core Cutting Test: Destructive, direct measurement of in-place concrete strength.
