A Comparison Between Direct and Indirect Method of Ultrasonic Pulse Velocity in Detecting Concrete Defects

7/29/2019 A Comparison Between Direct and Indirect Method of Ultrasonic Pulse Velocity in Detecting Concrete Defects

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26/12 A COMPARISON BETWEEN DIRECT AND INDIRECT METHOD OF ULTRASONIC PULSE VELOCITY IN DET

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NDT.net May 2003, Vol. 8No.05

A COMPARISON BETWEEN DIRECT AND INDIRECT METHOD OF ULTRASONIC PULSE VE

DETECTING CONCRETE DEFECTS

N. Mohamed Sutan and M.Meganathan

Faculty of Engineering, Universiti Malaysia Sarawak,

94300 Kota Samarahan, Sarawak, Malaysia

Corresponding Author Contact:

Email: [email protected]

Abstract

Tests were performed to compare the accuracy between Direct Method and Indirect Method of Ultrasonic Pulse Velocity Method (UPV) in detecting the location of defects

during the early age concrete. Specimens of five reinforced concrete (RC) slabs of grade 40 with a fabricated void at a known location were used and tested at day 3, 7, 14 a

were compared to determine the accuracy of the two methods. While both methods were able to detect the location defects in specimens during the early age with accuracy of

was able to detect depth of defect (location inside specimens) with accuracy ranging from 60% -99%. Therefore, test results indicate that both methods can be used to assess

concrete or for quality control on site as soon as after the removal of formwork.

Keywords:

non destructive testing, reinforced concrete, Ultrasonic Pulse Velocity, Direct Method, Indirect Method, defects and accuracy

Introduction

Ultrasonic Pulse Velocity (UPV) is a non destructive technique involve measuring the speed of sound through materials in order to predict material strength, calculate low-strai

detect the presence of internal flaws such as cracking, voids, honeycomb, decay and other damage.The technique is applicable where intrusive (destructive) testing is not desirconcrete, ceramics, stone and timber. The main strength of the method is in finding general changes in condition such as areas of weak concrete in a generally sound structure.

should be treated with caution. At the same time, the UPV technique is not always practicable in testing sound concrete. Especially in investigation of crack depth, it is ineffecti

The performance is also often poor in very rough surfaces. Sometimes good contact requires the use of a coupling gel between the transducers and the structure. This may be

some structures. Figure 1 shows the leading portable UPV test instruments.

Fig 1: PUNDIT.

The development of ultrasonic techniques for non-metallic construction materials has lagged behind developments of sophisticated imaging systems in medicine and highly accu

systems for metals. A number of new systems are becoming available which utilize digital technology. Other related developed systems with more convenient features and adva

introduced. For example the UK1401 is a simple to operate hand-held meter, which measures the velocity of sound propagation through concrete or masonry without a need

coupling agent. This can be used to determine approximate strength, porosity and fracturing of rock while also able to search for near-surface defects. Another example of a m

frequency ultrasonic flaw detector. It is designed for inspecting concrete and masonry and can identify foreign inclusions cavities and cracks while able to determine approxima

of material.

Methodology

In achieving the required strength for concrete, it is needed to specify a proper mix design with appropriate mix proportions of water, cement, fine aggregate and coarse aggre

for the concrete in this context with strength grade 40, several mix designs need to be analyzed before coming up with a most suitable mix design. This is to configure the prop

concrete, which could also affect the pulse velocity. Description of the proportional materials in the concrete mix is as shown in Table 1.

Item Material Description1. Cement OPC

2. Fine Aggregate Uncrushed River Sand

3. Coarse Aggregate Crushed 20mm Size

4. Water Clean Tap Water

Table 1: Description of materials in concrete mix proportions.

Three cubes of grade 40 (with the dimension of 150mm x 150mm x 150mm) were casted in the specified dimension moulds on the same day in a concrete laboratory. In prior

carried out for every trial mixes to ensure the mix is with optimal moisture content. The cubes were then tested (crushed) on the 28 th day of casting to ensure the selected desi

grade (compressive strength). The results of the concrete cube tests for grade 40 of the mix design is as in Table 2. The mix design with proportional amount of water, cement,

was then confirmed for the concrete grade. Table 3 below is the summary of the concrete mix proportions for concrete grade 40.

Mix Design Weight, W (g) Load, F (kN) Compressive Strength (N/mm2)

Cube (G 40)7923.0

7895.0

884

866

39.3

38.5


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(Area = 22500mm2) 7948.0 922 41.0Table 2: The results of the cube tes ts for grade 40 of concrete mix design.

Cement Fine Aggregate (Sand) Coarse Aggregate (20mm) Water

5.0 kg 9.5 kg 13.0 kg 2.5 kg

1 1.9 2.6 0.5

Table 3: Concrete Mix Proportions For RC Grade 40 Slab (600x500x150)

After finalizing on the proportions of the desired concrete grade, concrete mix of grade 40 was then ordered from a selected and approved concrete plant by using the tabulate

plant. Cube tests were carried out as soon as the concrete was poured. A total of 12 cubes for concrete grade 40 were needed for casting so that three different cubes can b

28 respectively.

Fig 2: Styrofoam (fabricated void).

Concrete was then casted in the prepared slab moulds (600 x 500 x 150 mm). A total of 5 reinforced concrete slabs of grade 40 specimens with a fabricated void were neede

every test at specified days. After the casting of the slabs, regular surface curing to prevent plastic shrinkage (excessive loss of surface moisture), which may cause pre-harden

i) Cube Test Procedures

Concrete were casted into cube moulds (with 150 x 150 x 150 mm dimension), where the internal surfaces were lubricated with oil in prior. Concrete was filled in three layer

with 35 blows. The cubes were then left for 24 hours to be hardened. The moulds were dismantled and the hardened cubes were then submerged into water filled tank. Three

tested at day 3, 7, 14, and 28. The cubes were then weighed and crushed in a compressive strength test equipment to get the compressive strength. The compressive strength

ensure it is appropriate with the concrete grade requirements.

ii) Slump Test Procedures

Concrete was filled into a slump test cone by three layers. Every layer was compacted with 35 blows. The cone was then slowly taken out by leaving the concrete slump a sid

built height with the original height of the cone was measured. The allowable range for the concrete grade 40 is between 75mm and 125mm.

iii) Ul trasonic Pulse Velocity testings

The measurement of the velocity of ultrasonic pulses as a means of testing materials was originally developed for assessing the quality and condition of concrete and the PUND

predominately for this purpose. Figure 1 shows the picture of the equipment. In most of the applications it is necessary to measure the pulse velocity to a high degree of accurachanges in pulse velocity usually reflect relatively large changes in the condition of the concrete. For this reason it is important that care be taken to obtain the highest possible a

time and the path length measurements since the pulse velocity measurement depends on both of these. Accuracy of transit time measurement can only be assured if good acou

transducer face and the concrete surface can be achieved. For a concrete surface formed by casting against steel or smooth timber shuttering, good coupling can readily be ob

from dust and grit and covered with a light or medium grease or suitable couplant. A wet surface presents no problem. If the surface is moderately rough, stiffer grease should b

surfaces require more elaborate preparation.

a) Direct Method

When an ultrasonic pulse traveling through concrete meets a concrete-air interface, there is a negligible transmission of energy across this interface so that any air-filled crack o

the transducers will obstruct the direct beam of ultrasound when the void has a projected area larger than the area of the transducer faces. The first pulse to arrive at the receivi

diffracted around the periphery of the defect and the transit time will be longer than in similar concrete with no defect.

The arrangement for direct method is as shown in Figure 3, where it requires access to two surfaces. The transmitting and receiving transducers are placed on opposite surface

will give maximum sensitivity and provide a well-defined path length.

Fig 3: Void Detections us ing the Direct Met hod.

b) Indirect Method

Performing UPV testing requires access to two surfaces, unless indirect (surface transmission) testing is to be done. Though indirect arrangement is least satisfactory upon sens

but it is more commonly used since direct method is not possible to use at most of the time. Figure 4 shows the indirect method for detecting void. The void depth can be estim

equation:

(1)


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Where Vdis the pulse velocity in the defect concrete (km/s),Vs is the pulse velocity in the sound concrete (km/s) and tis the depth of the defect (mm), x0 is the distance at wh

(mm).

Fig 4: Void Detections us ing the Indirect Method .

In a UPV test, a piezoceramic source is electrically pulsed to generate ultrasonic waves, which travel in the structural element, and are sensed by the matching receiver on the

member. The waveform at the receiver is recorded (including the signal transmission start time) by the PC-based system. Knowing the travel distance and travel time, the ultras

velocity is then calculated. After the receiver output is recorded by the PC data acquisition system, the data can be analyzed. Three parameters are used in the interpretation o

1. arrival of compression waves,

2. signal strength and

3. distortion of the transmitted signal.

All the depths detected are calculated using Equation 1 and the results are tabulated. The detected depth is than compared with the actual void depth. Figure 5 shows the exam

distance (mm) for the determination of void depth. A change of slope in the plot indicates the presence of void.

Fig 5: Example Void Depth Determination by the Indirect Method.

In defect areas, the compression wave velocity is slower than in sound areas. In some defect areas, such as honeycomb, the compression wave velocity may be almost the sam

distortion of the signal (filtering of high frequencies) may be used as an indication of honeycomb defect. In addition, defect areas such as honeycomb will generally result in low

Data and Analysis

i) Locations of void in slab specimens

a) Direct Method

UPV Test using Direct Method can easily identify the location of the void in slabs. The void can be detected when the travelling time of the Ultrasonic Pulse shows the highest

locations of the void in the slabs on different days are spotted with the underlined readings of the Ultrasonic Pulse travelling time. The detected location of the void using Dir

4.6 below.

Slab Location (mm) Location, Xo (mm)Day 3 Day 7 Day 14 Day 28

1 200 200 200 200 200

2 200 200 200 200 200

3 200 200 200 200 200

4 300 300 300 300 300

5 200 200 200 200 200

Table 4: Detected locations of the v oid using Direct Method

The detected locations of the void using Direct Method in Slab 1, 2, 3 and 5 are found at the distance of 200mm, while for Slab 4 it is at distance of300mm. The determined

compared with the actual defect location. It showed exact similarity with the actual one for all the slabs and days. Therefore the accuracy for determining the defect location usi

ii) Indirect Method

Unlike Direct Method, UPV Test using Indirect Method in identifying the location of the void is done by chart visualization. Charts below show the detected locations of the v

grade 40.


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Fig 6: Example of Detected Location, X0 of the Void using Indirect Method for Slab 1.

Slab 1G40 De pth= 53mm

Age Xo Vs Vd Depth Accuracy (%)

3 200 3.05 2.361275 35.67575 67.31127

7 200 3.67 2.604167 41.21607 77.77

14 200 4.125 2.695418 45.78246 86.38

28 200 5.02 3.025719 49.78641 93.94



3 200 2.625 2.076843 34.14431 68.29

7 200 3.56 2.590674 39.69846 79.40

14 200 4.14 2.853067 42.8987 85.80

28 200 5.4 3.267974 49.59495 99.19



3 200 3.91 2.484472 47.2156 70.47

7 200 4 2.832861 41.32948 61.69

14 200 3.85 3.284072 28.16513 42.08

28 200 5.24 4.040404 35.95291 53.66



3 300 2.58 2.197802 42.42494 70.71

7 300 3.375 2.73 48.86991 81.45

14 300 4.01 3.115265 53.15425 88.59

28 300 5.04 3.731343 57.93901 96.57



3 200 2.4 2.132196 24.30827 60.77

7 200 2.93 2.515723 27.58147 68.95

14 200 3.2 2.583979 32.63505 81.59

28 200 4.4 3.210273 39.53881 98.55

Table 5: Accuracy Of Concrete Slabs Grade 40

From the Table 5, the accuracy of Slab 3 is not consistent where it does not show any progress to the maturity period. This is definitely not of an appropriate data (could be du

during data recording). Therefore the rest four reliable slabs are only will be taken into consideration for further analysis.


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Fig 7: Accuracy of Slab Grade 40 with Concrete Age (Indirect Method).

During experimentation, there were several errors that were detected.

1) Inconsistent data output in ELE PUNDIT

Usually after using PUNDIT for around half an hour, the results offset a lot from the original results. For example, a normal reading of 20 ++ microseconds will offset to 60 - 1

same day. Besides that, very often the results will not remain constant. The results will lessen and drops until the point of zero.

2) Concrete not properly vibrated

Due to the unavailability of a concrete vibrator, most of the concrete slabs are not properly compacted and vibrated. This has to be done manually and upon removal of formw

detected on most of the slabs. To remedy the situation, the honeycombs are filled with cement mortar. In one of the case, two slabs had to be rejected due to many honeycomb

3) Rained during concreting

On the day of concreting, it rained. Though only a slight drizzle, it changes the amount of water/cement ratio in the concrete. This affects the strength of concrete to some exten

tests results of concrete grade were of satisfactory results.

Conclusions

The analysis shows that the accuracy of Ultrasonic Pulse Velocity Test does affected by the concrete age. Where as it matures, the accuracy of UPV Test increases. Apart fro

between Direct Method and Indirect Method, though direct method shows convenient and satisfactory upon sensitivity for determining the location of the defect but the ability

concrete slab is not possible and it is also not suitable to use at most of the time since it requires access to two surfaces. Therefore in determination of both depth and location

concrete slab, there is only Indirect Method would be possible though it is least satisfactory upon sensitivity and defined path length. In general, Ultrasonic Pulse Velocity Meth

ranging from 60% to 99% respectively to the ages from day 3 till day 28 (full strength).

Acknowledgements

The authors wish to extend their deepest gratitude to Universiti Malaysia Sarawak (Unimas) for the Short Term Grant in supporting this research.

References

1. Bungey J.H.,"The Validity of Ultrasonic Pulse Velocity Testing In-place Concrete for Strength, "N.D.T.International IPC Press, December pp. 296-300, (1980).

2. BS 4408: pt.5, "Non-destructive Methods of Test for Concrete-Measurement of the Ultrasonic Pulses Velocity in Concrete," British Standard Institution, London, (1970)

3. ELE PUNDIT 6, Portable Ultrasonic Non-Destructive Digital Indicating Tester, Operating Manual.

4. Neville A.M, "Concrete Technology", Longman Group UK Limited, pp282, 631-633, (1987).

5. Strurrup, V. R.; Vecchio, F. J.; and Caratin, H., "Pulse Velocity as a Measure of Concrete Compressive Strength, " In-Situ Non-Destructive Testing of Concrete, 82,Ameri

Detroit, pp. 201-227, (1984)

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A Comparison Between Direct and Indirect Method of Ultrasonic Pulse Velocity in Detecting Concrete Defects