NON-DESTRUCTIVE TESTING
Nondestructive testing methods are used to evaluate concrete properties by assessing the strength and other properties such as corrosion of reinforcement, permeability, cracking, and void structure. This type of testing is important for the evaluation of both new and old structures.
The Non-Destructive Testing done is based on random sampling and is considered to be a true representation of the condition of the structure. However small,the probability of inadequate random sampling does exist, and cannot be neglected.It is not feasible to carry out NDT on 100%of the structural elements/members.
Non-Destructive testing is a method by which the existing condition of the structure can be analysed without causing damages to the structure.
These methods are non-destructive as they do not impair the function of the structure and evaluate changes in properties with time. Based on the nature of distresses observed,the following non-destructive tests were suggested and carried out:-
Schmidt Rebound Hammer Test (Ref:IS:13311 Part II)
To check the approximate compressive strength of concrete >
Ultra-Sonic Pulse Velocity (Ref.IS:516 Part 5/Sec 1:2018)
To check the homogeneity of concrete
Half-cell potentiometer test (Ref.:ASTM/C876-80)
To check the probability of corrosion level in reinforcement >
Concrete core extraction for compressive strength
To acquire the actual compressive strength of the concrete in the structure
Chemical Analysis Test (Ref:BS:1881-Part 124:1998)
To check the pH,Chloride &Sulphate content in the concrete >
Carbonation test (Ref:BS:1881:Part 201:1986)
To check the depth of carbonation of the concrete
1. SCHMIDT REBOUND HAMMER TEST
A total of 16 impact readings obtained were taken at each location and an average of middle ten was calculated after discarding the top three and bottom three readings.In this manner,total 16 points were tested on the RCC members.
The probable accuracy of prediction of concrete strength by the Rebound Hammer is +25%as per the IS code 13311 (part-II). However, since the concrete under investigation is old and may be carbonated as also the result depends on the surface hardness at that particular location,the results obtained from the Rebound Hammer are not much dependable for quantitative assessment and shall be considered only for qualitative assessment.These results shall be compared with the compressive strength obtained by core extraction.
Safety First – Regular maintenance prevents structural failures, reducing risks of accidents and hazards.
2. ULTRASONIC PULSE VELOCITY TEST
This instrument works on the principle of passing high frequency sound waves through the body of the concrete &measuring the time taken. Distance of the path length divided by the time taken provides velocity of the waves through the concrete member being tested.
Depending on the velocity,the quality of concrete as regard to the homogeneity can be judged.Lower velocity (less than 3 km/sec)indicates one of the defects like honeycombing,cracks,voids,deboning etc.at the location of the test.
The concrete surface is thoroughly cleaned &dried.The instrument is calibrated before taking readings.Coupling medium such as grease is applied to the probes,and reading is taken for the pulse velocity at the location.Appropriate correction factors;wherever desired,are applied for the presence of steel.
Cost Savings– Timely repairs prevent small issues from becoming major, expensive problems.
3. HALF CELL POTENTIOMETER
Half-Cell Potentiometer works on the principle of measuring milli-voltage in the circuit of reinforcement &cover concrete using copper sulphate half–cell.This test measures the corrosion potential of the embedded steel in concrete with half–cell (Cu/CuSoa).This method essentially consists of measurement of the absolute potential at the concrete surface with reference to an electrode.It is necessary to have direct electrical connections to the embedded steel.The measured absolute potential with reference to the reference electrode is considered to be the best criterion for assessing the corrosion status of the embedded steel rods.The test is fairly indicative of corrosion response of the present and future.The negative terminal of the voltmeter is connected directly to a protruding end of the embedded steel by means of a “crocodile clamp”.The digital handhold operated milli-voltmeter will show the potential measurements.
Sustainability – Repairing instead of replacing minimizes waste and supports eco-friendly practices.
4. CHEMICAL ANALYSIS
It is of utmost concern to check the Sulphate, Chloride, pH value of concrete and cross verify the same as per British Standard. This test indicates chloride content in the concrete indicating the extent of corrosion of steel and pH value. If it is low, it indicates acidic’in nature. Low Sulphate content shows that there may not be any effect on the strength of concrete.High chloride content in cover concrete indicates the possibility of corrosion in the reinforcement bars. The higher percentage of chloride &Sulphate indicates the deterioration of concrete and the possibility of its disintegration. The samples are tested for Chloride, Sulphate and pH values. The pH of fresh concrete generally ranges between 12 &13.pH around 10 is generally considered low for concrete and is an indication of assured carbonation. A drop in pH increases the acidity of concrete which induces accelerated corrosion. The permissible limit for Chloride is 0.6 Kg/m³by weight of concrete. The permissible Sulphate content by percentage weight of cement is 4%.
Aesthetic Appeal – Well-maintained buildings look better and maintain property value.
5. CARBONATION TEST
This test is carried out to measure the depth of concrete from the external face up to which it has undergone carbonation. The test requires core samples of 25 mm or 50 mm diameter to be taken out from a depth of about 80 to 100 mm;higher diameter cores taken can also be used for this test. The core sample is sprayed by 2% Phenolphthalein solution starting from the exposed or external surface of concrete. If sprayed concrete turns Pink,it is considered as non- carbonated.The depth of carbonation is measured in milli-meters as “the depth from the external face of concrete to the point beyond which the phenolphthalein sprayed concrete turns pink in colour”.
If the core of concrete is not available,the test can be performed by suitably exposing the concrete by cutting or breaking with a chisel and performing the test on this freshly exposed surface as described above.
Energy Efficiency– Proper upkeep of insulation, plumbing, and electrical systems improves efficiency and reduces utility bills.
6. COVERMETER TEST
Covermeter test is a nondestructive test which is used to specify the location of reinforcement bars in concrete and determine the exact concrete cover needed. The magnetic rebar locator test plays a significant role in construction works because the information about the location of steel bars, concrete cover, and bar sizes is essential, directly or indirectly, in many field applications.
Legal Compliance– Regular inspections and maintenance ensure buildings meet safety regulations and codes.
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