Non-destructive testing

Non-destructive testing is a modern day technology to access structural strength, stability, integrity of concrete. This technology helps to avoid any destruction of concrete/concrete structure under revaluation. Following tests methods are predominantly used by Structural Engineers world over:


1.Schmidt Rebound Hammer

The standard method of determining strength of hardened concrete consists of testing concrete cubes in compression. The quality of entire concrete of a structure cannot be fully assessed by testing a few concrete cubes. The results obtained in testing cubes do not always reflect the actual strength of concrete in construction. In a whole day, concreting work cubes are cast in a few batches, the differences (unintentional and intentional) in the composition are not uncommon, their compaction and their hardening conditions always differ more or less from those of the structure. In addition, the number of test cubes is generally so small that they can only be considered as random tests. Sometimes, in case of failure of cubes, doubtful concrete, cracks, deterioration of concrete, etc. it becomes necessary to assess the quality and strength of concrete of the structure. As far back as early thirties, the necessity was felt to develop instruments by which in-situ strength of concrete may be obtained. Various non-destructive methods of testing concrete have been developed, which include, Firing method, Skramtayev’s method, Polakov’s method, Magnitostroy method, Fizdel ball hammer, Einbeck pendulum hammer, Ball indentation hammer, Rebound hammer, Pull out techniques, Windsor probe, Ultrasonic pulse velocity methods, Radioactive and nuclear methods, Magnetic and electrical methods. In all these methods of tests, due to simplicity, rebound hammer test based on surface hardness becomes most popular in the world for non-destructive testing of in-situ concrete.

REBOUND HAMMER :

handy non-destructive testing instrument should be cheap, easy to operate and should have reproducibility for, fairly accurate results. In 1948, a Swiss Engineer, Ernst Schmidt developed a test hammer for measuring the hardness of concrete by the rebound principle.

OBJECT :

The rebound hammer method could be used for (IS: 13311 Part 2-1992):

  • Assessing the likely compressive strength of concrete with the help of suitable co-relations between rebound index and compressive strength.
  • Assessing the uniformity of concrete.
  • assessing the quality of the concrete in relation to standard requirements, and
  • assessing the quality of one element of concrete in relation to another.

2. Ultrasonic Pulse Velocity

Ultrasonic Pulse Velocity Test is conducted to assess the quality of concrete which is suspected to have low compaction, voids (porosity), delamination or damaged material in concrete under test.

Ultrasonic Pulse Velocity Test can also be used for the following applications:

  • Estimation of Strength of Concrete
  • Establishing Homogeneity of Concrete
  • Studies on the Hydration of Cement
  • Studies on Durability of Concrete
  • Analysis of Surface Crack Depth
  • Determination of Dynamic Modulus of Elasticity

3.Cover meter

The cover meter technique is the least complicated and expensive of all the NDT techniques (Fig. 2). The meter applies a low frequency alternating magnetic field into the concrete surface. The presence of steel in this magnetic field induces eddy currents and a voltage change that can be detected and displayed by the meter. The cover meter has the ability to locate reinforcing steel and other metal objects, measure the depth of concrete cover and estimate the size of the reinforcing bar. The accuracy of a cover meter can be affected by the amount of steel reinforcement present, particularly by the amount of congestion and the presence of multiple layers or other metallic inclusions. Its accuracy can be improved by calibrating against reinforced concrete with known dimensions and size of reinforcing bars. A cover meter is generally reliable to a depth of 100 mm to 125 mm and should be used by an experienced operator. Refer to BS1881-204-Testing concrete-Recommendations on the use of electromagnetic cover meters.


4.Concrete cores

While Rebound Hammer, CAPO/Pullout, Windsor probe and ultrasonic pulse velocity tests give indirect evidence of concrete quality, a more direct assessment on strength can be made by core sampling and testing. Cores are usually cut by means of a rotary cutting tool with diamond bits. In this manner, a cylindrical specimen is obtained usually with its ends being uneven, parallel and square and sometimes with embedded pieces of reinforcement. The cores are visually described and photographed, giving specific attention to compaction, distribution of aggregates, presence of steel etc. the core should then be soaked in water, capped with molten Sulphur to make its ends plane, parallel, at right angle and then tested in compression in a moist condition as per BS 1881: Part 4: 1970 or ASTM C 42-77.


5.Half Cell potentiomete

Electrochemical Half-cell Potentiometer test provides a relatively quick method of assessing reinforcement corrosion over a wide area without the need of wholesale removal of the concrete cover. Quantitative measurements are made so that a structure can be monitored over a period of time and deterioration can be noted. Areas of usage include marine structures, bridge decks, abutments and so on. Used in conjunction with other tests, it has been found helpful when investigating concrete contaminated by salts.


6.Bond Test of epoxy or other bonding materials

This test method covers the determination of the bond strength of epoxy-resin-base bonding systems for use with Portland-cement concrete. The test method covers bonding hardened concrete to hardened or freshly-mixed concrete. The bond strength is determined by using the epoxy system to bond together two equal sections of Portland-cement mortar cylinder. After suitable curing of the bonding agent, the test is performed by determining the compressive strength of the composite cylinder. Apparatus to mix Portland-cement mortar shall be as described, except for the sections on specimen molds, tamper, and testing machine. The molds shall be constructed in the form of right cylinders. A dummy section shall be machined of a hard material that is not attacked by Portland-cement mortar. The testing machine shall be as described. Laboratory conditions, materials, proportions, and procedures for mixing the Portland-cement mortar shall be tested to meet the requirements specified.


7.Slab / BEAM Load Test

In the event of pre-stressed concrete beams, a laboratory is consulted regarding fatigue tests being conducted on proposed product samples. The requirement is to determine the shear strength of the pre-stressed concrete beams. In general circumstances, a beam will be statically loaded, to about eighty percent of the ultimate flexible capacity. This will be followed by the test beams, being subjected to repetitive loads of varying magnitudes, for about 2,000,000 cycles, of between twenty and forty percent of the flexible capacity. It is an intensive part of the testing program relative to a pre-stressed concrete beam load test to failure.

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