Testing of hardened concrete is one of the most important to be performed at the site to check the strength and durability of concrete. It plays an essential role in controlling and confirming the quality of concrete works.
Conventional concrete testing methods such as tensile strength, compressive strength, flexural strength etc. need strict supervision and experience. Apart from these methods, NDT (Non-Destructive Testing) is also considered as a powerful method for evaluating the strength and durability of existing concrete structures.
In NDT, test specimens are not loaded to failure instead some properties of concrete are measured (such as hardness, resistance to penetration of projectiles, rebound number, resonant frequency and ability to allow ultrasonic pulse velocity to propagate through it) to estimate durability, strength and elastic parameters.
The electrical properties of concrete (such as the ability to absorb scatter, transmit x-ray and gamma rays, response to nuclear activation) allows us to estimate its density, moisture content, cement content and thickness.
Various NDT methods are as follows :
1. Rebound Hammer
This method is developed by Scmidt’s in 1954 is used to measure surface hardness (uniformity of concrete). The test can be performed at any angle (i.e. vertical, horizontal or any intermediate angle) considering the fact that the rebound number will be different for the same concrete at each angle.
The results of this test is affected due to the following parameters :
(i) Surface smoothness
(ii) Shape size and rigidity of specimen
(iii) Specimen age
(iv) Type of coarse aggregate
(v) Internal moisture condition of concrete
(vi) Surface condition of concrete
2. Resonant Frequency Method
This method is performed to evaluate the deterioration of concrete for repeated cycles of freezing and thawing and deterioration due to acidic and alkali attack to concrete. The equipment used is known as Sonometer.
The method is performed in the laboratory on small specimens (i.e. limited to 150 mm x 300 mm cylinders or 75 mm x 75 mm x 300 mm prisms) rather than on structure members in field.
3. UltraSonic Pulse Velocity Method
This method involves measurement of travel time of electronically generated mechanical pulses through concrete. The pulse generator circuit consists of electronic circuit for generating pulses and a transducer for transforming these pulses into mechanical energy having vibration in the range of 15 to 50 kPa.
The device used in this method is PUNDIT (Portable Ultrasonic Non-destructive Digital Indicating Tester).
The applications are as follows :
(i) To establish uniformity in concrete (surface texture, density, availibility of concrete)
(ii) Determining modulus of elasticity
(iii) Estimate concrete strength (compressive and flexural strength)
(iv) Determine setting characteristics of concrete
(v) Durability of concrete exploration (freezing and thawing action, sulphate attacks)
4. Radioactive Method
This method uses X-rays and Gamma-rays for testing the properties of concrete. Since X-rays and gamma-rays are of high energy region, they penetrate concrete but undergo attenuation in the process. The values of the intensity of incident and emerging gamma-rays through the specimen are measured to calculate the density and thickness of the concrete structure.
5. Nuclear Method
This method is not used to find the strength of concrete. However, parameters such as moisture content (using Neutron Scattering Method) and cement content (using Neutron Activation Analysis) are determined.
6. Magnetic Method
This method is used to measure the depth of reinforcement cover and detect the position of reinforcement bars. The apparatus used is Cover Meter.
7. Electrical Method
This method is specifically used for rigid pavements where moisture permeation through concrete, moisture content, and thickness of pavement is determined. The method is based on the principle that the material offers resistance to the passage of an electric current. Thus electrical resistivity methods have been used to find pavement thickness.