Classification of Non Destructive Testing (NDT):
Various type of non destructive testing methods have been developed and are under development for examining different properties of concrete in addition to the necessary visual inspection. Tests are aimed at assessment of strength and other properties and to locate and obtain corresponding results showing permeable areas, cracks or laminations and areas of lower integrity than the rest. It is essential to highlight here, that it is not necessary to carry out all the tests in each case except the most relevant ones. The scope of work consisted of carrying out the following tests.
- Rebound Hammer Test
- Ultrasonic Pulse Velocity Test (Concrete)
- Covermeter Study
- Carbonation Test
- Concrete Core Strength
- Half Cell Potentiometer
- In-situ permeability test
- Pull out Test
- Windsor Probe Test
Non Destructive Rebound Hammer Testing:
Generally, Schmidt concrete test hammer is used for conducting the Rebound Hammer Test which is NDT of concrete. The test is a beneficial and quick procedure where we can get a clearer idea about the compressive strength of existing concrete of a particular structure. The main principle of the test based on the elastic mass rebound theory. The elastic mass rebound depends on the hardness of the existing concrete from where the mass impinges. A spring-loaded mass has a fixed amount of energy imparted to it by extending a spring to a fixed position. It is performed by pressing the plunger against the surface of the existing old concrete under test. Upon release, the mass rebounds from the plunger, still in contact with the concrete surface. The distance travelled by the mass, expressed as a percentage of the primary extension of the spring, is known as rebound number, It is shown by a rider moving along a graduated scale. The test surface is also prepared by rubbing the concrete surface with a carborundum stone. Approximately ten hammer readings are noted around the test points. The test is conducted as per IS:13311(Part 2)-1992.
The following figure represents the relationship between the Hammer Rebound No. and Strength of Concrete
Non Destructive Ultrasonic Pulse Velocity Testing:
The ultrasonic pulse velocity test on concrete is conducted by the. Portable Ultrasonic Non-Destructive Digital Indicating Tester (PUNDIT) is used for Non- Destructive Ultrasonic Pulse Velocity Test. PUNDIT is highly accurate pulse time recording system and has a good reputation all over the world. The ultrasonic pulse velocity method consists of measuring the time of travel (in microseconds) of an ultrasonic pulse passing through the concrete to be tested. Two transducers are used, one to transmit the pulse and the other to receive the pulse. The distance which the pulses travel in the concrete (i.e. the path length) is also measured. The pulse velocity is determined from the following relationship.
- Pulse velocity = (Path length) / (Transit time)
Depending upon the arrangement of transducers, the pulse velocity tests may be Direct, Semi Direct or Indirect. Direct transmission that is placing the transducers on opposite faces is the most accurate method. Accuracy of transit time measurement is dependent on
the good acoustic coupling between the transducer face and the prepared concrete surface. The test surface is prepared by rubbing the concrete surface with carborundum stone and cleaned subsequently. A light grease is applied as the coupling that helps the transmission of ultrasonic energy from the transducer into the test specimen/existing structure.
Pulse Velocity Ratings for Concrete:
As per table-2 of IS 13311 (Part 1): 1992 gives generalized Pulse Velocity Ratings for Concrete which are as follows:
Pulse Velocity (Km/Sec)
The above may be taken as guidelines for assessing the condition of the structure. Any weakness in the form of cracks, voids, weak concrete will result in lower pulse velocities.
Non Destructive Covermeter Testing on Concrete Structure:
The Covermeter tests is a non- destructive test which is conducted using the cover meter. The cover meters are magnetic devices, and the functions are based on the principle that the presence of the steel affects the field of an electromagnet. In a typical cover meter, the probe unit consists of a U-shaped magnetic core on which two coils are mounted. When an alternating current is passed through one of these coils, the current induced in other coil is measured. The induced current depends upon the mutual inductance of the coils and upon the nearness of the steel reinforcing bars.
Non Destructive Half-Cell Potentiometer Testing on Concrete:
Half-Cel Potentiometer survey is carried out using CANIN (Corrosion and Analysis Instrument). The corrosion of steel in concrete is an electrochemical process. It represents a galvanic element, similar to a battery producing an electric current and measurable as an electric field on the surface of the concrete. This potential field can be measured with an electrode known as a Half-Cell (copper/copper sulphate). By making measurements over the whole surface, a distinction can be made between corroding and non-corroding locations. A high impedance digital voltmeter is used to collect the data. An electrical correction is made to the steel either by exposing it or using already exposed steel. The foam rubber plugs saturated with water are pressed on the concrete surface at the pre-selected grid points. The readings in term of millivolt (mV) are recorded.
Potential in the Millivolt(mV) and Corrosion Condition Relation:
The ASTM criteria for corrosion of steel in concrete for copper/copper sulphate Half-Cell are as follows:
Low(10% risk of corrosion)
-200mV to – 350mV
Intermediate corrosion risk
High (more than 90% risk of corrosion)
Non Destructive Carbonation Testing:
Loss of alkaline protection against corrosion of the reinforcement due to carbonation of concrete on the existing concrete surface results serviceability of the structure. The atmospheric carbon dioxide reacts with hydrated cement compounds causing a reduction in alkalinity of concrete. The depth of the carbonation is measured with the help of spraying a solution containing 0.1% of phenolphthalein on the freshly broken concrete surface. After application, the concrete undergoes a distinct colour change from purple-red to colourless if any carbonation of concrete occurs. Thus the carbonation of concrete is checked at the particular depth of existing concrete.
NDT Concrete Core Cutting Method:
A portable core drilling machine is generally used for core cutting of existing concrete structure. It has a drill feed assembly which consists of a feed assembly, a screw assembly, flange assembly, drill shaft and water jacket with the diamond core bit. One portable electric hammer drill machine is used to rotate the drill feed assembly. The core drilling machine is fixed by means of clamping pliers at the selected test locations, after scanning the embedded reinforcements by the rebar locator instrument. The portable hammer drill is connected to the drill machine to start the drilling operation. Continuous water circulation has been maintained throughout the drilling operation for proper lubrication etc. Concrete cores of 50mm/68mm dia are collected, and simultaneously the carbonation of concrete are checked at that particular locations.
The samples are stored in water for two days prior to compressive strength testing(as per IS:516) and are tested in a moist condition. If required, the ends of specimens are flattened and capped with molten sulphur or high alumina cement to obtain a truly flat surface. Concrete in the member shall be considered acceptable if the average equivalent cube compressive strength of the cores sample is equal to at least 85% of the cube strength of the grade of concrete specified and no individual core has strength less than 75%.
In-situ permeability Test:
In-situ permeability test is a non destructive test of concrete. Because of the increasing instances of corrosion in reinforced/prestressed concrete structures, attention has been drawn on the significance of permeability in addition to compression strength in assessing the quality of concrete. The procedure consists of drilling a small hole in the concrete, sealing it with a rubber stopper and then evacuating the air from the hole with a vacuuming pump connected to a manometer. The rate at which the manometer recovers is a measure of the permeability of concrete. This method can be used to assess the resistance of concrete to carbonation, penetration of aggressive ions and quality of grout in post-tensioned ducts.
Pull out Test:
This Non- Destructive testing method may be used to assess the compressive strength of existing concrete. The test is considered to be superior to the rebound hammer test. But the disadvantage is that small damage to the concrete created in the test is required to be repaired.
Windsor Probe Test:
The above non- destructive test is generally conducted to find out the concrete hardness of the existing structure. With the Windsor Probe Test, we can also determine the relative strength of concrete in different structures. We have to remember the test does not give absolute values; further confirmation is required to assess the actual condition.
Based on the inspection report, maintenance scheme including remedial measures, shall be framed after detailed study, as mentioned. The studies should reflect the detailed analysis, cause of defects, deficiencies, construction faults or any supernatural effects etc. After the finalization of maintenance criteria, the maintenance team will prepare a necessary drawing, bill of quantities, tender document, complete maintenance report for maintenance work and will submit these to the higher authority for scrutinizing/comments. On approval of the maintenance scheme, the maintenance works shall be executed by the maintenance team or specialized agency appointed through tender.