Several tests, such as the cube test, split tensile test, and so on, are done to evaluate the qualities of concrete. These tests are performed on a specifically constructed specimen that may or may not represent the structure. These examinations necessitated advance planning. In the procedure, the specimen is destroyed.
Non-destructive testing has been developed to address these issues. The fundamental advantage of these tests is that they are non-destructive. It enables retesting to track changes over time at the same or nearly the same area. It can be utilized in construction.
Among these qualities are hardness, rebound number, and the capacity to allow ultrasonic pulse velocity to propagate through concrete. It also aids in the investigation of crack depth, microcracks, and concrete deterioration.
Although these are essential methods, they require competent and experienced personnel to interpret and analyze test data.
What are these tests meant to do?
- Estimate the uniformity and homogeneity of the concrete.
- Detecting cracks and flaws in concrete.
- Calculate the in-situ compressive strength.
- Determine the elastic modulus.
- Determine the chloride and sulfate content.
- Examine changes in the structure of concrete for the effect of time.
Types of Non-Destructive Tests on Concrete
Based on the equipment used and the principle of working, Non-Destructive tests can be classified as follows:
Rebound Hammer Method
Ernst Schmidt (Zurich), a Swiss engineer, attempted to develop a test hammer to measure the hardness of concrete using the Rebound principle. Schmidt Rebound Hammer is another name for this hammer.
Principle:
The rebound of an elastic mass is determined by the hardness of the surface it strikes.
Procedure:
The hammer weighs approximately 1.8 kg and comprises a spring-controlled hammer mass that slides over a plunger within the tubular housing. The spring forces the hammer to the surface of the concrete structure, and the hammer's rebound distance is measured.
The test surface can be horizontal, vertical, or at any angle as long as the instrument is calibrated. Calibration is performed using cylinders made of the same cement and aggregate. Many readings are taken, and the average represents the cylinder's rebound number.
Advantages:
- Simple to use.
- Uniformity can be established.
- Inexpensive
- Used in the restoration of monuments.
Disadvantages:
- The results are not directly related to the properties of strength and deformation.
- The equipment must be cleaned regularly.
- Not very good at detecting flaws.
Penetration Method
The penetration method assesses concrete's uniformity, imperfections, and strength. It is carried out following the procedures and specifications of the Standard Test Method for Hardened Concrete Penetration Resistance (ASTM C 803/803M-97) or British Standard (BS 1881 Part 207).
Probe, measuring equipment (Vernier caliper, depth gauge), and positioning device
Procedure:
- Position a device on the concrete surface.
- Introduce the probe into the concrete (the probe is mounted in the driver unit properly)
- Take away the positioning device.
- Tap it with a hammer to ensure that the probe is securely embedded.
- Cover the probe with the measuring base plate.
Advantages:
- It allows for the rapid assessment of the quality and maturity of in-situ concrete.
- It also evaluates strength development after curing.
Disadvantages:
- Doesn't provide accurate values of concrete strength.
Pull-Out Test Method
Principle:
The force required to pull a steel disc embedded in fresh concrete is measured, and a relationship between concrete compressive strength and forces is established.
Method:
Steel rod-shaped equipment is cast into the concrete to a depth of 3 inches (7.62cm), which is then pulled out, and the force required to pull it out is recorded. The concrete remains in tension and shear simultaneously, but the force recorded correlates with the concrete's compressive strength.
Advantages:
- It provides a local measure of the concrete's strength.
- It provides some basic information about the maturity and development of concrete strength.
Disadvantages:
- This method requires pre-planning
Ultrasonic Pulse Velocity Method
It is a Dynamic Non-Destructive Test that measures concrete strength in situ. It records the time interval required by the Ultrasonic pulse while passing through the concrete.
Equipment: Pulse generator, Pulse receiver.
Method:
The concrete under test must have a smooth surface. To avoid errors, the path length required for an ultrasonic pulse to travel should be at least 12inches (30cm). A temperature between 5 and 30 degrees Celsius must be maintained. Steel and other pulse-affecting materials must be avoided in the pulse path.
Advantages:
- Perfect for determining uniformity.
- It is appropriate for testing under-construction structures.
- Cracks are detectable.
Disadvantages:
- Less used in determining strength.
Radioactive Method
These are commonly used to determine the location of reinforcement in concrete and to measure density. Not suitable for concrete thicknesses of up to 45cm and is possibly expensive in comparison.
Internal defects and variations, porosity, geometric variation, density changes, and misaligned parts are all measured.
Advantages:
- Records can be kept indefinitely.
- Portable.
Disadvantages:
- Radiation has a dangerous nature.
- Expensive.
- Skilled labor is required.