civil engineering laboratory testing equipment
Experimental Evaluation of Cracks in Concrete by Ultrasonic Pulse Velocity
T Sri Kalyan 1, a and J.M Chandra Kishen 2,b
Department of Civil Engineering, Indian Institute of Technology Kharagpur, Kharagpur, India
Department of Civil Engineering, Indian Institute of Science, Bangalore 560012, India
Abstract
With a sudden surge in demand for the usage of non-destructive testing of modern structures for their evaluation and health monitoring, various NDT techniques are developed to identify cracks that are potentially dangerous. In this present work the most widely used NDT technique namely ultrasonic pulse velocity is implemented for the evaluation of the cracks and their effects on the nature and strength of concrete. The ultrasonic equipment used in this study for the evaluation of the concrete properties was the portable ultrasonic non-destructive digital indicating tester (PUNDIT). Detailed analysis of the effect of cracks and their extent in deteriorating the elastic modulus and compressive strength of the concrete is studied. Various methods of probing are used for studying these effects in every possible direction of various types of samples. Evaluation of the depth of surface cracks using the ultrasonic pulse velocity method is studied which produced some interesting results. A comparison between the effect of the surface cracks and internal micro cracks on the compressive strength of concrete is also done. Directional effects of surface cracks on the experimental results are also analyzed.
1. Introduction
With a sudden pulse of increment in the construction of new infrastructure facilities all across the developing countries there is an increasing need to assess the condition of a structure to determine their safety and reliability [1]. The major problem faced in ensuring the safety of these structures is the presence of the defects in the materials which can act as cracks. To detect these faults various non-destructive techniques (NDT) are developed to detect potentially dangerous cracks developed in the critical sections of structures. This process is traditionally done by visual inspection and simple procedures such as the tap test. These defects can then either be repaired or the component can be replaced. Thus NDT can increase the reliability of the structures under consideration [3]. Of the various NDT techniques available, depending on the state of component to be tested the type of NDT is to be decided accordingly, as all the NDT techniques have their corresponding advantages and disadvantages. The four criteria that need to be taken into consideration while deciding the type of NDT to be used are: Material type, defect type, defect size, defect location.
In many studies it is indicated that ultrasonic waves were used mainly to predict and evaluate concrete strength and its properties [3]. However, this method can also be used to detect the internal defects of concrete such as cracks. Thus ultrasonic pulse velocity method is employed as the NDT technique in the following experimental setups since velocity of the pulse generated is almost independent of the geometry of the material through which they pass and depends only on its elastic properties [2]. Facaoaru [7] reported that ultrasonic wave velocity (UPV) into concrete is directly proportion to concrete strength and age. Mathematical models were also developed to predict Ultrasonic pulse velocity in concrete were developed based on experimental studies. The relationship between ultrasonic wave velocities measured using direct and indirect methods was More Info at Open Access Database www.ndt.net/?id=15207 evaluated by Yaman et al. [8]. The results indicated that the direct and indirect methods can be used interchangeably in evaluating the properties of the concrete.
2. Ultrasonic Pulse Velocity
As stated earlier this is one of the most popular techniques used for the detection of flaws in a specimen. The ultrasonic pulse is generated by an electro acoustical transducer. Now since ultrasonic waves do not travel through air or vacuum a couplant like grease is used to get the transducer in contact with the specimen [3]. When the pulse is induced into the concrete from a transducer, it undergoes multiple reflections at the boundaries of the different material phases within the concrete. A complex system of stress waves is developed which includes longitudinal (compressional), shear (transverse) and surface (Rayleigh) waves. The receiving transducer detects the onset of the longitudinal waves, which is the fastest. The material without any defects results in a higher velocity than that of the damaged ones. Thus by calculating this velocity one can assess the properties of the structural concrete.
3. Experimental Setup
As a part of the experimental setup a PUNDIT tester was used to evaluate ultrasound wave velocity in various types of concrete specimens prepared. Concrete specimens prepared have variability in the strength, surface notches and interior cracks so as to check the feasibility of using UPV to detect these cracks.
3.1 Sample specifications and Probing method Four concrete cubes of dimensions 150mm*150mm*150mm with different concrete strengths were casted and a dog-bone specimen with dimensions (described later) is also cast. One of the concrete cubes is damaged by loading it to 45% of its ultimate strength and the other up to 65% of its ultimate strength resulting in the development of micro cracks. A normal undamaged cube of strength 25MPa is considered to be the standard reference case in our experiment. Three different types of probing methods are done to record the experimental values:
1) Direct or Cross Probing: The receiver and the transmitter are opposite to each other on either side of the cube.
2) Indirect or Surface Probing: The receiver and transmitter are on the same face of a cube.
3) Semi direct Probing: The transmitter and the receiver are on any two perpendicular faces of acube.
FIGURE 1: Pictorial representation of various positions of the transmitter and receiver probes [4].
3.2 Formulation
I) Elastic modulus of Concrete is calculated using the below stated formula in BIS 13311-
Indian Standard code for the non-destructive testing of concrete [5]
II) Formulas employed for evaluating the velocity of ultrasonic pulse to characterize the concrete properties [5]:
1) Calculated velocity =
2) Actual Distance travelled = (Average velocity of waves obtained from undamaged cube)* (Time recorded) (3) where,
E = Dynamic Young’s Modulus of elasticity of concrete in MPa
= Density in kg/m3, and
V = Pulse velocity in km/second.
Evaluation of Concrete material properties
Pulse velocity readings are recorded at different positions on the cube of size
150mm*150mm*150mm and Poisson ratio of 0.32 as shown in Fig 2(All dimensions in metres (m))
The representative numbers on the face of the cube indicate the different position of probes at which the pulse velocity readings were taken.
FIGURE 2: A 3-D view of the normal cube with probes positions numbered
The various cases under consideration are listed below:
- Case 1: Cube with compressive strength of 25MPa and without any Damage
- Case 2: Cube with compressive strength of 25MPa and damaged by loading it up to 45% of its ultimate strength
- Case 2: Cube with compressive strength of 25MPa and damaged by loading it up to 65% of its ultimate strength
In the cases 2 and 3 the cube is damaged so as to develop internal cracks to some extent. By this one will be able to assess the effect of cracks in deteriorating the strength of the concrete. Firstly, the ultimate strength is found using a material testing system and then the cube is loaded up to 45% and 65% of this ultimate strength. The ultimate strength is found to be 587kN. The estimated elastic modulus of Concrete and the recorded pulse velocity values are given in Table 1.
lso the compressive Strength of the samples is estimated from the plot represented in Fig 3. with the help of recorded velocity values.
FIGURE 3: A graph showing the variation of compressive strength (MPa) with calculated pulse velocity (km/s) from experimental data
- Evaluation of the depth of surface cracks
In order to evaluate the depth of surface cracks using ultrasonic pulse velocity method two cases were considered as mentioned below:
Case 4: Undamaged Cube with compressive strength of 25MPa with a single notch
Here, a notch of width 4mm and depth of 50mm is introduced into a cube for evaluating the effect of the surface cracks in the measurement of velocities using ultrasonic pulse velocity method.
Direct probing is employed on surface B and Surface probing is used on surface C to include the effect of notch. The observed readings are given in Table 2.
FIGURE 4: A 3-D view of the cube with a single notch and numbered probes positions.
TABLE 2: Summary of calculated velocities at different probing locations in a cube with a single notch
Case 5: A compact tension specimen with an inscribed notch of smaller width
This case is similar to previous one except that there is a change in the size of the specimen and the notch. The width of notch is 1mm and its depth is about 6mm. Thus one can compare these cases for the evaluation of the crack depth and the influence of sample size.
FIGURE 5: A 3-D view of the compact tension specimen along with probe positions.
- Theoretical evaluation
A mathematical expression is proposed [6] to theoretically predict the depth of surface cracks and it includes comparing the time-of-flight of an ultrasound wave through a sample without any cracks to the one around a crack and it is considered that the velocity of the longitudinal wave in the concrete is the same in both cases. Assuming the wave travel path presented in Fig. 6 [6], the crack penetration depth “h” can be evaluated by using Eq. 4 [6]
Here, represents the travel time around the crack; is the surface travel time in sound
concrete, and x is distance between the transducers and the crack, measured on the surface of the concrete and is the calculated depth of the notch by the given equation and h real is the real depth of the notch. The experimental values are compared with that of theoretical ones and presented in Table
FIGURE 6: Assumed path of travel by longitudinal wave in presence of a crack
TABLE 3. Comparison of theoretical and calculated
- Discussion
By comparing the obtained elastic modulus values of concrete we can see that the elastic modulus obtained in case where the cube is damaged up to 45% doesn’t show much difference from the standard undamaged sample whereas the elastic modulus of sample damaged up to 65% almost got reduced to half of the standard one. This implies that the internal cracks developed in case of loading up to 45% doesn’t affect the path in which the waves the travel to a large extent that is the cracks developed are not so significant that they can change the path of waves whereas in the other case where there is a reduction in the elastic modulus there are surface cracks developed which can have profound effect on the propagation of the waves through the material. Thus we can say that the ultrasonic pulse velocity method negates the effect of micro cracks in the measurement of elastic modulus of material. So a more sensitive method is to be preferred for the measurement of the compressive strength and the elastic modulus.
On analysing the results obtained in case 4 and case 5 it is found that the effect of the notch in case of direct probing measurements is negligible and there is only a slight change in the velocity whereas there is a drastic change in case of surface and semi direct probing. Thus for the detection and evaluation of surface cracks indirect probing is to be preferred. From the calculations of evaluation of crack depth it is observed that the calculated depth values and theoretical values mismatch with each other and there is a large difference in case of the cube with a single notch than that in compact tension specimen. From the large difference observed in the theoretical and experimental values in the cube with single notch case we can say that the path of waves cannot be approximated to a straight line without any constraints. Thus we can say that with increase in specimen size we can expect the calculated value of crack depth to be in proximity with theoretical value. The reason could be that with increased size of specimen the path of the waves can be nearly approximated to be a straight line while in case of smaller specimen this cannot be done since the distance is too small for valid approximation.
- Conclusions
Based on this study for evaluation of the effect of cracks on the properties of concrete using ultrasonic pulse velocity method the following findings were made:
1) Velocity of the waves increases as we go down the depth in the direction of casting. This can be attributed to the compaction differences and difference in aggregate settlements.
2) The effect of surface cracks in changing the direction of the path travelled by the waves is much more than that of internal cracks. The ultrasonic pulse velocity negates the effect of internal cracks and micro cracks in evaluating the elastic modulus and compressive strength of concrete.
3) Effect of the surface cracks in changing the velocity is negligible in case of direct probing.
Thus for the evaluation of their effect indirect or surface probing is to be implemented which shows a profound change in the velocity values.
4) With increase in the size of specimen or structure the path of the waves can be approximated to be a straight line and thus can be used for the evaluation of the depth of surface cracks.
Thus from these results we can say that ultrasonic pulse velocity can be employed for the detection of cracks and for evaluating their effects but this cannot be used solely and should be accompanied by other NDT techniques for better accuracy and identification of the cracks and evaluation of the properties of concrete.
- References
[1] Peter C. Chang, S. Chi Liu, “Recent Research in Nondestructive Evaluation of Civil Infrastructures,” Journal Of Materials In Civil Engineering, pp. pp. 298 – 304, 2003.
[2] Saad A. Abo-Qudais, “Effect of concrete mixing parameters on propagation of ultrasonic waves,” Construction and Building Materials, vol. 19, pp. pp. 257 – 263, 2005.
[3] Prashant Kumar, “Elements of Fracture Mechanics,” first ed., McGraw Hill companies, 2009.
[4] Tarun R. Naik, V. Mohan Malhotra, John S. Popovics, “The Ultrasonic Pulse Velocity Method,” CRC Press LLC, 2004.
[5] IS 13311 Part I “Standard Code of Practice for Non-Destructive Testing of Concrete: Part 1— Ultrasonic Pulse Velocity” Bureau of Indian Standards, New Delhi, 1992.
[6] PINTO, Roberto CA, Arthur MEDEIROS, J. PADARATZ Ivo, and Patrícia B. ANDRADE. “Use of Ultrasound to Estimate Depth of Surface Opening Cracks in Concrete Structures.”
[7] Facaoaru, I, “Non-destructive testing of concrete in Romania,” Proceedings of symposium on non-destructive testing of concrete and timber. London, UK: Institution of Civil Engineering; 1969
[8] Yaman, Ismail Ozgur, Gokhan Inci, Nazli Yesiller, and Haluk M. Aktan. “Ultrasonic pulse velocity in concrete using direct and indirect transmission.” ACI Materials, no. 6 pp. pp. 450 – 457, 2001.