Abstract

Tests were performed to evaluate the feasibility of using Impact-Echo Method (IEM) and Ultrasonic Pulse Velocity Method (UPVM) in detecting discontinuity and determining its depth during the early age concrete. Two reinforced concrete (RC) slabs grade 30 and 40 specimens at day 3, 7, 14 and 28 with a fabricated void at a known location were used. The results obtained were compared to determine the accuracy of both methods hence the effectiveness of each method. Both methods detect discontinuities in specimens during the early age. However, IEM gave more accurate results in determining the depth of discontinuity. Porosity has significant effect on the accuracy since lower porosity yield accurate determination of the depth of discontinuity. Tests were also performed to examine the relationship between the porosity and strength of concrete at each stage with the accuracy of void depth detected. The test results indicate that both methods can be used to assess the in-situ properties of concrete or for quality control on site. Both methods showed better accuracy with stronger concrete detect discontinuities with the accuracy ranging from 55.75-99.05% from day 3-28 (full strength) respectively.

Keywords:
non-destructive testing, concrete, quality control, impact-echo testing, ultrasonic pulse velocity testing, discontinuity, porosity and accuracy

Introduction

The deterioration of concrete in a structure is a result of several degradation mechanisms that caused a decreased in the integrity of the structure. The state of deterioration is often invisible and is only evident when there is a significant reduction in the load carrying capacity. Ensuring better performance of concrete structures requires early discontinuities detection. Discontinuities are often introduced during casting and detection during in-service life is often too late to remedy the situation. In the past years great improvement has been made in the field of non-destructive testing in civil engineering (NDT-CE) and this development will certainly continue and even accelerate. [1][2][3][4] Words like quality control, monitoring, maintenance or replacement and building inspection emphasize this progress.

Base on the information obtained for both Impact-Echo Method (IEM) and Ultrasonic Pulse Velocity Method (UPVM), all the work that have been done in relation to discontinuity detection and depth of discontinuity determination are limited to in-service structures for case-study and on laboratory research specimens that are more than 28 days of age. None of them are done on early age structures or specimens except for the monitoring of strength development in concrete [5][6][7][8][9].

The scope of this study is to test and compare the accuracy of these two methods by using the Germann Instruments^’ DOCter Impact Echo System (IEM) [11] and ELE PUNDIT 6, Portable Ultrasonic Non-Destructive Digital Indicating Tester (UPVM) [12] for detecting the location and depth of discontinuities. The main interest is to correlate concrete properties with age and how it affects the accuracy of discontinuity detection.

Materials and methods:

Two reinforced concrete (RC) slabs grade 30 and 40 were prepared. The proportions of the concrete mix are summarized in Table 1 and 2. Another 2 RC slabs with prerecorded location and depth of fabricated void namely the actual void depth (i.e. 37.5mm) were then prepared with the same concrete mix.

Cement	Fine Aggregate Sand	Coarse Aggregate 20mm	Water
4.706 kg	13.858 kg	17.680kg	2.340 kg
1	2.94	3.76	0.5
Table 1: Grade 30 RC Slab 500mm x 300mm x 75mm (20in.x12in.x 3 in.)

Slump = 10-30 mm (0.4in – 1.2in) cured at room temperature

Cement	Fine Aggregate Sand	Coarse Aggregate 20mm	Water
4.500 kg	8.07 kg	12.105kg	2.25 kg
1	1.79	2.69	0.5
Table 2: Grade 40 RC Slab 500mm x 300mm x 75mm (20in.x12in.x 3 in.)

Slump = 10-30 mm (0.4in – 1.2in) cured at room temperature

All the specimens were tested from day 3, 7, 14 and 28 with both UPVM and IEM. The accuracy of the testing methods was determined by comparing the prerecorded location and depth voids. The methods of testing and determining the void location, void depth and porosity of concrete at different age are as follows.

1. UPVM
   The indirect method of testing is used since it is the best method to determine the effective path length [12]. Figure 1 shows the indirect method for detecting void. The void depth can be estimated using the following equation:

   (1)

   Fig 1: Void Detections using the Indirect Method [12].

   Where V_d is the pulse velocity in the anomalous concrete (km/s), V_s is the pulse velocity in the sound concrete (km/s) and t is the depth of the defect (mm), X₀ is the distance at which the change of slope occurs (mm). Table 3 showed the data obtained from the test. Figure 2 showed the transit time (ms) versus distance (mm) for the determination of void depth. A change of slope in the plot indicates the presence of void i.e. 300mm(12in.) as shown in Figure 2.

   Fig 2: Void Depth Determination by the Indirect Method for RC Grade 30 at day 14.

   DISTANCE (mm)	Transit Time (ms)
   	Day 3		Day 7		Day 14		Day 28
   	Grade 30	Grade 40	Grade 30	Grade 40	Grade 30	Grade 40	Grade 30	Grade 40
   100	13.0	14.5	14.6	12.5	12.9	11.7	16.1	11.4
   200	41.4	44.0	41.5	36.9	42.6	36.1	46.0	38.7
   300	67.8	71.0	66.1	77.6	70.4	75.3	68.8	74.3
   400	89.5	99.7	86.2	98.0	92.4	96.4	91.4	95.3
   Table 3: UPVM Test Data

   All the depth detected was calculated using Equation 1 and the results were tabulated in Table 4. The detected depth was than compared with the actual void depth. From Table 4 a typical calculation for RC Grade 30 at day 14 using Equation 1 is presented below.

   
   Accuracy = (Detected void depth/Actual void depth) x100
   = (29.41/37.5) x100 = 78.69%

   CONCRETE AGE DAYS	Xo (mm)		V s (Km/s)		V d (Km/s)		t Void Depth (mm)		Accuracy %
   	30	40	30	40	30	40	30	40	30	40
   3	200	200	5.271	5.172	4.829	4.545	20.91	25.40	55.75	67.73
   7	200	200	5.375	6.522	4.813	5.540	23.46	28.53	62.56	76.08
   14	200	200	5.584	6.732	4.695	5.547	29.41	31.07	78.69	82.84
   28	200	200	5.873	6.810	4.499	5.159	36.39	37.01	97.05	98.70
   Table 4: Ultrasonic Pulse Velocity Test Results

2. IEM
   Figure 3 showed the spectrum for RC Grade 30 at day 14 from which the void depth was determined. The void depth T was determined by using the following equation,

   T=C/2fv

   (2)

   where f_v is the void frequency in kHz, T is the void depth in mm, and C is the true wave speed in m/s. A typical calculation for determining the void depth and accuracy for RC Grade 30 at day 14 using Equation 1 are presented.

   T = 4469/2(66.72) =32.15 mm (1.3in.)
   Accuracy = (Detected void depth/Actual void depth) x100
   = (32.15/37.5) x100 =85.72%

   Fig 3: Spectrum for RC Grade 30 at day 14.

   CONCRETE AGE DAYS	fT (kHz)		C (m/s)		fv (kHz)		Void Depth (mm)		Accuracy %
   	G30	G40	G30	G40	G30	G40	G30	G40	G30	G40
   3	27.20	26.73	4250	4172	89.55	73.65	22.78	27.19	60.75	72.50
   7	26.11	25.80	4078	4031	71.81	65.54	27.26	29.52	72.69	78.72
   14	28.63	27.21	4469	4250	66.72	60.61	32.15	33.66	85.72	89.77
   28	27.81	26.50	4344	4140	56.30	53.51	37.00	37.14	98.67	99.05
   Table 5: Impact Echo Test Results.

Results and discussions

The use of stress wave propagation to monitor the development of early-age mechanical properties is not a new idea. In this study, two parameters namely void location and void depth are used to determine the accuracy of both methods. Changes in strength of concrete with age that are influenced by porosity are the significant factor affecting the accuracy of readings since all other properties are similar for both specimens.

Fig 4: Accuracy vs Age for Rc Grade 30 and 40 using IEM and UPVM.

Figure 4 showed the accuracy versus age for RC Grade 30 and 40 using IEM and UPVM.

From Figure 4, the accuracy of both methods increased as the specimens matures. The results indicate that changes in aggregate, moisture and air void affects the readings of both methods as shown by the 2 different grade specimens. Grade 30 specimens yield less accuracy than grade 40 specimens. Ultrasonic pulse velocity and Impact Echo wave of reinforced concrete is affected by changes in the hardened cement paste. The changes in the water/cement ratio affect the modulus of elasticity of the hardened cement paste. Pulse travels faster through a water-filled void compared with an air-filled one. Therefore the moisture condition of concrete affects the pulse and wave reading. As the concrete age, the moisture content decreases and it can be observed from Figure 4 that as the concrete mature the detection of void is more accurate. The results also showed that IEM yield more accurate results than UPVM. This is due to the sensitivity of UPVM to air humidity as compared to IEM.[12] Air humidity does not have any significant effect on IEM[11].

Another reason is due to the mix design of both concrete specimens. Referring to Table 1 and 2 for both mixes, Grade 30 has more coarse aggregate than Grade 40 and this explained why Grade 40 specimen is more accurate in determining the depth of the void for both methods. Less homogeneous specimen yield less accurate reading since coarse aggregate can diffract the pulse or ultrasonic wave.

Fig 5: Correlation between Accuracy and Porosity of UPVM and IEM for RC Grade 30.

Figures 5 and 6 showed the correlation between percentage of accuracy and porosity of UPVM and IEM for RC Grade 30 and 40 with concrete age respectively. As the concrete strengthened, the percentage of porosity decreased. Porosity is expressed as a fraction of volume of voids to the total volume of concrete. Porosity for particular day tested was determined by calculating the strength of concrete at that particular day. The porosity was determined from the relationship between compressive strength and porosity graph.[10, 13] It was observed that the decreased of porosity as the concrete matures increase the accuracy of both tests. The reason for this is based on the testing principle for both methods i.e. where the presence of void on the path will increase the path length as it goes around the void. Concrete with higher porosity acts like bigger voids and this will affect the readings of both methods. Since the changes in porosity with age are based on theoretical calculations, the relationships between the accuracy and porosity are recommended for further experimental investigations.

Fig 6: Correlation between Accuracy and Porosity of UPVM and IEM for RC Grade 40.

Conclusions

For the purpose of quality control on site, the use of either UPVM or IEM enable detecting defect in concrete structure as early as day 3. Based on the present study it is concluded that the location of discontinuity can possibly be detected as early as day 3 after the removal of formwork. The determination of discontinuity depth can be is possible with accuracy ranging from 60.75-99.05% and 55.75-98.70% for IEM and UPVM respectively. The IEM gives more accurate results in comparison to the UPVM in early age concrete. Stronger concrete gives better accuracy in determining the depth of defect. Theoretically, porosity of concrete has significant effect on the accuracy of the defect depth. It was observed that decrease of porosity with age increase the accuracy. The actual performance of in-situ concrete during early age is yet to be fully understood. Therefore, more studies and further research on actual bridge structure should be conducted. The effect of porosity should be done experimentally. Besides porosity, other effects that changes concrete properties during early age should also be taken into consideration for further research.

References:

1. (Bungey J.H., 1980),"The Validity of Ultrasonic Pulse Velocity Testing In-place Concrete for Strength, "N.D.T.International IPC Press, December pp. 296-300.
2. (Bungey J.H. 1982),"The Testing of Concrete in Structures," Surrey University Press pp. 35-59.
3. BS 4408: pt.5, "Non-destructive Methods of Test for Concrete-Measurement of the velocity Ultrasonic Pulses in Concrete," British Standard Institution, London.
4. "Non-destructive Testing of Concrete Structures," Proceeding of the Indo-US Workshop on NDT Indian Institute Uttar Pradesh, Roorkee, India, pp. 85 & 113, (1996).
5. (Strurrup, V. R.; Vecchio, F. J.; and Caratin, H, 1984) "Pulse Velocity as a Measure of Concrete Compressive Strength, " In-Situ Non-Destructive Testing of Concrete, 82, American Concrete Institute, Detroit, pp. 201-227.
6. (Sansalone, M.J. & Street, W.B, 1997):"Impact-Echo: Nondestructive evaluation of concrete and masonry", Bulbriers Press, Ithaca, N.Y., USA, ISBN No.: 0-96 12610-6-4.
7. (Sansalone, M., and Carino, N.J., 1986) "Impact-Echo: A method for flaw Detection in Concrete Using Transient Stress Waves", NBSIR 86-3452, National Bureau of Standards, Gaithersburg, Maryland, pp.222.
8. (Sansalone, M., and Carino, N.J., 1998) "Laboratory and Field Study of Impact-Echo Method for Flaw Detection in Concrete", Non-destructive Testing of Concrete, Special Publication of the American Concrete Institute, pp.1-20,
9. (Lin, Y., and Sansalone, M., 1992) "Detecting Flaws in Concrete Beams and Columns Using the Impact-Echo Method", Materials Journal of American Concrete Institute, 89, No. 4.
10. Neville A.M, "Concrete Technology", Longman Group UK Limited, pp282, 631-633, (1987).
11. Germann Instruments: "Operation and Maintenance Manual for DOCter Impact-Echo Testing, Version 3.0, (1999).
12. ELE PUNDIT 6, Portable Ultrasonic Non-Destructive Digital Indicating Tester, Operating Manual
13. (Sersale, R., Cioffi, R., Frigione. G. and Zenone, F., 1991) "Relationship between gypsum content, porosity and strength of cement", Cement and Concrete Research, 21, No.1, pp.120-126.

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