N. D. T (Concrete )
 

N.D.T. & Concrete

We carry out non destructive testing with rebound hammer based on IS 13311 (Part 2) : 1992 and ultrasonic pulse velocity tester based on IS 13311 (Part 1) : 1992 . In this method, the ultrasonic pulse is produced by the transducer which is held in contact with one surface of the concrete member under test. After traversing a known path length (L) in the concrete, the pulse of vibrations is converted into an electrical signal by the second transducer held in contact with the other surface of the concrete member and an electronic timing circuit enables the transit time (T) of the pulse to be measured.
Test : Method for determination of compressive strength by N.D.T by using Rebound Hammer.

A) APPARATUS: Rebound Hammer type ‘N’.

The concrete test hammer is a mechanical device used for performing rapid, non-destructive quality testing on concrete.

The device is to be used exclusively on the surfaces to be tested and on the testing anvil.

B) Standards
IS : 13311 (part 2) : 1992, Reaffirmed : 1999

C) Measurement

i) Measuring Principle

- The device measures the rebound value R. There is a specific relationship between this value and the compressive strength of the concrete.

- The following factors must be taken in to account when ascertaining rebound values R:

- Impact direction: horizontal, vertically upwards or downwards

- Age of the concrete

- Size and shape of the comparison sample (cube, cylinder)

ii) Measurement Limitations:

This Rebound Hammer shall be used for testing of:

- Compressive strength of concrete of 10 to 70 Mpa only.

- Concrete items 100 mm or more in thickness

- Concrete with a maximum particle size < 32 mm

iii) Environmental Conditions:

Preferably perform measurements at temperatures between 10° C and 50° C only.


iv) Measuring procedure

- Use the grindstone to smoothen the test surface.

• Position the concrete test hammer perpendicular to the test surface.
• Deploy the impact plunger by pushing the concrete test hammer towards the test surface until the pushbutton springs out.


Danger!
Always hold the concrete test hammer in both hands, as the impact plunger generates recoil when it deploys. Perpendicular to the test surface, before you trigger the impact!


Each test surface should be tested with at least 8 to 10 impacts.

The individual impact points must be spaced at least 20mm apart.

• Position the concrete test hammer perpendicular to and against the test surface. Push the concrete test hammer against the test surface at moderate speed until the impact is triggered.
• Press the pushbutton to lock the impact plunger after every impact. Then read off and note down the rebound value R indicated by the pointer on the scale.

D) Outputting and Evaluating Data

- Output
After every impact, the rebound value R is displayed by the pointer on the scale of the device.


ii) Evaluation

Take the average of the 8-10 rebound values R which you have measured.

Do not include values which are too high or too low (the lowest and highest values) in your calculation of the average value.

• Determine which conversion curve is appropriate for the selected body shape. Then, using the average rebound value Rm and the selected conversion curve, read off the average compressive strength.

Note the impact direction!

The average compressive strength is subject to a dispersion


TITLE : Guidelines for Testing of Hardened concrete by core testing.
Sampling : Ref. IS : 1199 – 1959 R.A. 2004.

i) The hardened concrete shall be minimum 14 days old before the specimen removed.
ii) Specimen that show abnormal defects or that have been damaged in removal shall not be used.
iii) Core drill shall be used for securing cylindrical core specimens.
iv) Dia of core shall be at least 10 cm for determination of pavement thickness.
v) Dia of core shall be at least 300% more than nominal size of aggregate and in no case be less than twice the nominal size of aggregate. The height (length) shall be twice of dia of core.
Core dia shall be taken to nearest mm, two at middle and two at quarter points of depth. Take average of six readings


TESTING : Refer clause 4 of IS : 516 – 1959 RA 2004.

i) Core of which , before capping, has a height of less that 95% of dia, or after capping, a height less than its dia shall not be used for testing.
ii) Capping shall be done where plane is not within 0.05 mm, with suitable material with a compressive strength not less than that of the core.
iii) Testing shall be done in compressive testing machine of class 1 or 2 and compressive strength shall be arrived as per the graph given herein below based on the height and diameter ratio of the core.



Limitation : Refer clause 17.4.1 of IS 456 – 2000

i) The core taken shall be representative of the hardened concrete work and in no case shall be less that three core specimen for each sample.

Interpretation : Refer clause 17.4.3 of IS : 456-2000

i) The concrete is acceptable if the average equivalent cube strength of the cores is equal to at least 85% of the grade of tested concrete for corresponding average and no individual strength shall be less than 75%.

Alternative : Refer clause 17.8 of IS : 456 – 2000

Non-destructive tests provide alternatives to core tests for estimating the strength of concrete in a structure.

Non-destructive tests: Non-destructive tests are used to obtain estimation of the properties of concrete in the structure. The methods adopted includes following.

ULTRASONIC PULSE VELOCITY (UPV) :Refer IS : 13311(part 1)1992 RA 2004

i) The estimated strength may vary from the actual strength by + 20 percent.

REBOUND HAMMER : Refer IS : 13311 Part 2 : 1992 RA 2004.

- The probable accuracy of prediction of concrete strength in a structure is + 25 percent. If the relationship between rebound index and compressive strength can be checked on various specimens the accuracy of result will be greatly increased.