Ultrasonic Pulse Velocity Test for Concrete ||Non Destructive Testing Techniques || UPV Test

  Bridge structures have the most important key role in road network. Structural failure may huge impact on the transportation system and as well as humanitarian loss. Visually bridge inspection is not sufficient in case of distressed and critical staged Concrete structures (Buildings and bridges). We can find out some deficiencies/defects which are clearly visible during visual inspection. But assesment of a stucture with out knowledge of hidden honeycomb/ void/ cavity, depth of cracks, presence of hidden cracks, grade of concrete quality, depth of carbonation, condition of steel reinforcements inside concrete, depth of clear cover etc is not a expertise measure. Vissual bridge inspection report gives us rough and general ideas, which is help us for regular maintenance and minor repairing works. 
     So, we need to carry out some necessary Non Destructive Tests (NDT Tests) to evaluate the acual stage or condition of structures. The recomended NDT Tests are following 

   

1. Ultra Sonid Pulse Velocity Test (UPV Test),
2. Schmidt Rebound Hammer Test,
3. Cover Assessment,
4. Core Test,
5. Carbonation Test etc.

First of all we need to make a list, in which structures have to carry out Non Destructive Tests or NDT Tests. The locations of the points, where the NDT needed to be carried out should be marked. The marking should be carried out based on their locations. The details of above NDT Tests and their guiding principle are summarized below.

SITE INSPECTION AND TESTING

The site should be visited by a team, ,the team should be consisted of a Senior bridge enginner, one laboratory engineer, and two technicians. Other staff and casual labourers should be supported the team for the NDT Testing in field.

1. Ultrasonic Pulse Velocity Test || UPV Test:

Picture 1: Ultrasonic-Pulse-Velocity-Test.

Ultrasonic pulse velocity Test or UPV Test) is the most widely used technique for evaluating the homogeneity of concrete, integraty of concrete, presence honeycombing or void or cavity, measure the depth of crack; measure the thickness of delaminated section in concrete; strength estimation; and relative quality between members or within a concrete member. 

 This test consists of transmitting electro-acoustic pulse through the concrete from one point, receiving the signal from another point, and measuring the transit time. When the pulse is induced into the concrete from a transducer, it undergoes multiple reflections at the boundaries of different material phases within the concrete. The path length between the transmitting and receiving points, which is usually the thickness of a member, is measured and the pulse velocity is calculated by dividing the path length by the transit time. Comparatively higher velocity is obtained when concrete quality is good in terms of density, uniformity, homogeneity etc. In case of poor quality concrete, lower velocities are obtained. If there is a crack, void or flaw inside the concrete which comes in the way of transmission of the pulses, the pulse strength is attenuated and it passes around the discontinuity, thereby making the path length longer. Consequently, lower velocities are obtained. The actual pulse velocity obtained depends primarily upon the materials and mix proportions of concrete. Density and modulus of elasticity of aggregate also significantly affect the pulse velocity.

Depending on the measurement point locations, there can be three types of testing namely Direct testing method, semi-direct testing method and Indirect (or surface) testing method as shown in figure.

Picture-2: Direct-Method-UPV-Test

Picture-4: Semi-Direct-Method-UPV-Test

Picture-3: In-Direct-Method- UPV-Test

 The Direct method of taking UPV measurement wherein transmitting and receiving points are on the opposite faces of the structural member, is the most reliable from the point of view of transit time measurement, as maximum pulse energy is transmitted at right angles to the face of transmitter. Depending on the value of the pulse velocity, the quality of concrete is classified in the following four categories as per IS: 516 (Part 5 sec 1): 2018 for direct methods as shown in Table below:

Table 1.1 Concrete Quality Grading Based on Ultrasonic Pulse Velocity Measurements as per IS: 516 (part 5 sec 1):2018.

Average value of pulse velocity by cross probing (km/s)   Concrete quality grading 

      a) More than 4.40               Excellent

      b) 3.75 to 4.40                     Good

      c) 3.00 to 3.75                     Doubtfull*

      d) Less than 3.00               Poor

       * In case of doubtful quality, it may be necessary to carry out further tests.

The quality of concrete in terms of uniformity, incidence or absence of internal flaws, cracks and segregation, etc (indicative of the level of workmanship employed) can be assessed using the guidelines given in Table 1. This table is only for concrete quality grading and shall not be used for estimating the concrete grades from ultrasonic pulse velocity values.

2. Rebound Hammer Test: 

Rebound hammer test is done to find out the likely compressive strength of concrete by using rebound hammer as per IS 516 (Part 5 sec 4): 2020. The underlying principle of the rebound hammer test is “the rebound of an elastic mass depends on the hardness of the surface against which its mass strikes”. When the plunger of the rebound hammer is pressed against the surface of the concrete, the spring-controlled mass rebounds and the extent of such a rebound depends upon the surface hardness of the concrete. The surface hardness and therefore, the rebound are taken to be related to the compressive strength of the concrete. The rebound value is read from a graduated scale and is designated as the rebound number or rebound index. The compressive strength can be read directly from the graph provided on the body of the hammer in case the quality of concrete assessed by ultrasonic pulse velocity method is 3.50 km/s for grades ≤ M25, and 3.75 km/s for above M25. Alternatively, the following table given in CPWD handbook may be used. 

Table 2.2 Concrete Quality Grading Based on Rebound Number as per Handbook on repair and rehabilitation of RCC buildings published by director general (works), central public works department, government of India, Nirman Bhawan.

 Average rebound     Quality of concrete

        >40                          Very good

       30-40                        Good

       20-30                        Fair

       <20                   Poor and/or delaminated

         0                            Very poor

3. Carbonation Test (CT): 

The measurement of carbonation depth in the cover zone of concrete is standard procedure in assessing reinforced concrete deterioration, particularly in respect of reinforcement corrosion problems. The carbonation test is carried out to determine the depth of concrete affected due to combined attack of atmospheric carbon dioxide and moisture causing a reduction in level of alkalinity of concrete. A spray of 1.0% solution of phenolphthalein is used as pH indicator of concrete. The change of colour of concrete to pink indicates that the concrete is in the good health, and where no changes in colour take place, it is suggestive of carbonation-affected concrete. The test is conducted by drilling a hole on the concrete surface to different depths upto cover concrete thickness, removing dust by air blowing, spraying phenolphthalein with physician’s injection syringe and needle on such freshly drilled broken concrete and observing change in colour. The depth of carbonation is estimated based on the change in colour profile. 

4. Half-Cell Potentiometer Test: 

The instrument measures the potential and the electrical resistance between the reinforcement and the surface to evaluate the corrosion activity as well as the actual condition of the cover layer during testing. The electrical activity of the steel reinforcement and the concrete leads them to be considered as one half of weak battery cell with the steel acting as one electrode and the concrete as the electrolyte. The name half-cell surveying derives from the fact that the one half of the battery cell is considered to be the steel reinforcing bar and the surrounding concrete. The electrical potential of a point on the surface of steel reinforcing bar can be measured comparing its potential with that of copper – copper sulphate reference electrode on the surface. Practically, this is achieved by connecting a wire from one terminal of a voltmeter to the reinforcement and another wire to the copper sulphate reference electrode. This method may be used to indicate the corrosion activity associated with steel embedded in concrete and can be applied to members regardless of their size or the depth of concrete cover. It should be clearly noted that the test does not actual corrosion rate or whether corrosion activity has already started, but it indicates the probability of the corrosion activity depending upon the actual surrounding conditions. The risk of corrosion is evaluated by means of the potential gradient obtained, the higher the gradient, the higher the risk of corrosion.

 The test results can be interpreted based on the following table.

Table 2.3 Corrosion risk by Half Cell Potentiometer

Half-cell potential (mv) relative to Cu-Cu sulphate Ref. electrode      % chance of corrosion activity

 Less than - 200                              < 10%

 Between – 200 to – 350                   50% (uncertain)

 Above – 350                                   > 90%