Abstract

A study by making comparison between the accuracy of Impact-Echo Method and Ultrasonic Pulse Velocity Method in detecting distress in reinforced concrete has been made. Tests were undertaken to evaluate the viability of using Impact-Echo Method (IEM) and Ultrasonic Pulse Velocity Method (UPVM) in detecting distress and determining its depth during the early age concrete. Five reinforced concrete (RC) slabs of grade 25, 30, 40 and 50 specimens at day 3, 7, 14 and 28 with a fabricated distress namely 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 defects in specimens during the early age. However, IEM gave more accurate results in determining the depth of defect. Porosity has significant effect on the accuracy since lower porosity yield accurate determination of the depth of defect. 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 defects with the accuracy ranging from 51.81-99.8% from day 3-28 (full strength) respectively.

Keywords:
non-destructive testing, Impact-Echo Method, Ultrasonic Pulse Velocity Method, porosity, distress detection and accuracy

Introduction

Material distress is defined as the physical deterioration or breaking down of the concrete into small fragments or construction defect. The distress in concrete structure is a result of several degradation mechanisms that caused a decreased in the integrity of the structure such as acid attack, chemicals, chlorides or sulphates attack. Meanwhile honeycombing occurs as a result of construction defects due to improper construction technique or poor workmanship. The level of distress 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 distress detection. Distresses or defects are often introduced during casting and detection during in-service life is often too late to remedy the situation.

Previous work that have been done with both Impact-Echo Method (IEM) and Ultrasonic Pulse Velocity Method (UPVM) in relation to distress detection and depth of distress determination were 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 [1,2,6,7,9].

The purpose of this study is to compare the accuracy of these two methods by using the Germann Instruments’ DOCter Impact Echo System (IEM) [5] and ELE PUNDIT 6, Portable Ultrasonic Non-Destructive Digital Indicating Tester (UPVM) [4] for detecting the location and depth of distress. The main attention is to correlate concrete properties with age and how it affects the accuracy of defect detection.

Materials and methods.

Five reinforced concrete (RC) slabs of grade 25, 30, 40and 50 respectively with prerecorded location and depth of fabricated void namely the actual void depth (i.e. 37.5mm) were prepared. The proportions of the concrete mix are summarized in Table 1 2, 3 and 4.

Cement	Fine Aggregate Sand	Coarse Aggregate 20mm	Water
4.805 kg	14.35 kg	18.157kg	2.403 kg
1	2.99	3.78	0.5
Table 1: Grade 25 RC Slab (500 x 300 x 75).

Slump = 10-30 mm cured at room temperature

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 2: Grade 30 RC Slab (500 x 300 x 75).

Slump = 10-30 mm 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 3: Grade 40 RC Slab (500 x 300 x 75).

Slump = 10-30 mm cured at room temperature

Cement	Fine Aggregate Sand	Coarse Aggregate 20mm	Water
4.305 kg	7.07 kg	10.165kg	2.17 kg
1	1.64	2.36	0.5
Table 4: Grade 50 RC Slab (500 x 300 x 75).

Slump = 10-30 mm 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 [3]. Figure 1 shows the indirect method for detecting void. The void depth can be estimated using the following equation:

   (1)

   where v_d is the pulse velocity in the defect 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 5 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. 200mm as shown in Figure 2.

   Fig 1: Void Detections using the Indirect Method [4].

   DISTANCE (mm)	Transit Time (ms)
   	Day 3		Day 7		Day 14		Day 28
   	Grade 25	Grade 30	Grade 25	Grade 30	Grade 25	Grade 30	Grade 25	Grade 30
   100	12.7	13.0	12.8	14.6	13.7	12.9	17.5	16.1
   200	40.6	41.4	39.6	41.5	44.7	42.6	48.4	46.0
   300	65.8	67.8	59.7	66.1	68.9	70.4	64.8	68.8
   400	87.6	89.5	79.5	86.2	90.5	92.4	89.3	91.4
   Table 5: UPVM Test Data: Slab 1 (Grade 25), Slab 6 (Grade 30), Slab 11 (Grade 40) and Slab16 (Grade50).

   DISTANCE (mm)	Transit Time (ms)
   	Day 3		Day 7		Day 14		Day 28
   	Grade 40	Grade 50	Grade 40	Grade 50	Grade 40	Grade 50	Grade 40	Grade 50
   100	14.5	15.7	12.5	11.7	11.7	10.8	11.4	10.8
   200	44.0	47.6	36.9	32.8	36.1	34.6	38.7	35.9
   300	71.0	73.1	77.6	80.8	75.3	76.8	74.3	78.0
   400	99.7	99.7	98.0	99.1	96.4	98.9	95.3	97.5
   Table 5: UPVM Test Data: Slab 1 (Grade 25), Slab 6 (Grade 30), Slab 11 (Grade 40) and Slab16 (Grade50).

   All the depth detected was calculated using Equation 1 and the results were tabulated in Table 6. The detected depth was than compared with the actual void depth. From Table 6 a typical calculation for RC Slab 6 (Grade 30) at day 28 using Equation 1 is presented below.

   Accuracy = (Detected void depth/Actual void depth) x10

   = (36.39/37.5) x100 = 97.05%

   Fig 2: Void Depth Determination by the Indirect Method for RC Slab 6 (Grade 30) at Day 28.

   D A Y S	Xo (mm)		Vs (Km/s)		Vd (Km/s)		t Void Depth (mm)		Accuracy %
   	25	30	25	30	25	30	25	30	25	30
   3	200	200	5.345	5.271	4.956	4.829	19.43	20.91	51.81	55.75
   7	200	200	5.278	5.375	4.849	4.813	20.56	23.46	54.83	62.56
   14	200	200	5.456	5.584	4.872	4.695	23.78	29.41	63.41	78.69
   28	200	200	5.765	5.873	4.591	4.499	33.67	36.39	89.79	97.05
   Table 6: Ultrasonic Pulse Velocity Test Results: Slab 1 (Grade 25), Slab 6 (Grade30), Slab 11 (Grade 40) and Slab 16 (Grade 50).

   D A Y S	Xo (mm)		Vs (Km/s)		Vd (Km/s)		t Void Depth (mm)		Accuracy %
   	40	50	40	50	40	50	40	50	40	50
   3	200	200	5.172	5.073	4.545	4.456	25.40	25.45	67.73	67.86
   7	200	200	6.522	6.723	5.540	5.625	28.53	29.81	76.08	79.52
   14	200	200	6.732	6.845	5.547	5.698	31.07	30.23	82.84	80.63
   28	200	200	6.810	6.978	5.159	5.269	37.01	37.36	98.70	99.62

2. IEM
   Figure 3 showed the spectrum for RC Grade 30 Slab 6 at day 28 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 Slab 6 at day 28 using Equation 2 are presented.

   T=4344/2(56.30)=37.00mm

   Accuracy = (Detected void depth/Actual void depth) x100

   = (37.00/37.5) x100 =98.67%

   Fig 3: Spectrum for RC Grade 30 (Slab6) at day 28.

   CONCRETE AGE DAYS	fT (KHz)		C (m/s)		fv (KHz)		Void Depth (mm)		Accuracy %
   	G25	G30	G25	G30	G25	G30	G25	G30	G25	G30
   3	28.82	27.20	4150	4250	95.09	89.55	21.82	22.78	58.19	60.75
   7	27.72	26.11	4278	4078	83.16	71.81	25.72	27.26	68.59	72.69
   14	30.43	28.63	4369	4469	72.21	66.72	30.25	32.15	80.67	85.72
   28	29.45	27.81	4467	4344	62.83	56.30	35.55	37.00	94.80	98.67
   CONCRETE AGE DAYS	fT (KHz)		C (m/s)		fv (KHz)		Void Depth (mm)		Accuracy %
   	G40	G50	G40	G50	G40	G50	G40	G50	G40	G50
   3	26.73	25.29	4172	4018	73.65	71.01	27.19	28.29	72.50	75.44
   7	25.80	23.53	4031	4317	65.54	68.46	29.52	31.53	78.72	84.08
   14	27.21	26.66	4250	4417	60.61	63.72	33.66	34.66	89.77	92.43
   28	26.50	24.43	4140	4120	53.51	55.04	37.14	37.43	99.05	99.80
   Table 7: Impact Echo Test Results: Slab 1 (Grade 25), Slab 6 (Grade 30), Slab 11 (Grade 40) And Slab 16 (Grade 50).

Results and discussions

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.

Figure 4-7 showed the accuracy versus age for RC Grade 25, 30, 40 and 50 using IEM and UPVM. It can be seen from these Figures, 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 4 different grade specimens. Grade 25 specimens yield less accuracy than higher grade 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 these Figures that as the concrete mature the detection of void is more accurate. The Figures 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.[9] Air humidity does not have any significant effect on IEM.[10]

Fig 4: Accuracy Versus Age for UPVM and IEM for RC GD 25.

Fig 5: Accuracy Versus Age for UPVM and IEM for RC GD 30.

Fig 6: Accuracy Versus Age for UPVM and IEM for RC GD 40.

Fig 7: Accuracy Versus Age for UPVM and IEM for RC GD 50.

Another reason is due to the mix design of concrete specimens. Referring to Table 1,2,3 and 4 for the four mixes, Grade 25 has the most coarse aggregate content than higher grade specimens and this explained why Grade 40 specimens obtained the highest accuracy in determining the depth of the void for both methods. Less homogeneous specimen yields least accuracy since coarse aggregate can diffract the pulse or ultrasonic wave.

Figures 8 and 9 showed the correlation between percentage of accuracy and porosity of UPVM and IEM for RC Grade 25, 30, 40 and 50 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. The porosity was determined from the relationship between compressive strength and porosity graph.[7, 11] 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 8: Accuracy of UPVM versusConcrete Age and Correlation with Porosity for RC GD 25(Slab 1-5) GD30(Slab 6-10) GD40(Slab 11-15) and GD40(Slab 16-20).

Fig 9: Accuracy of IEM vs Concrete Age and Correlation with Porosity for RC GD25(Slab 1-5), GD30(Slab 6-10), GD40(Slab 11-15) and GD50(Slab 16-20).

Conclusions

For the purpose of quality control on site, the use of either UPVM or IEM enables detecting distress in concrete structure as early as day 3. Based on the present study it is concluded that detection of distress location is possible as early as day 3 after the removal of formwork. The distress depth determination is possible with accuracy ranging from 58.19-99.8% and 51.81-99.62% 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 distress. 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. 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.,"The Validity of Ultrasonic Pulse Velocity Testing In-place Concrete for Strength, "N.D.T.International IPC Press, December pp. 296-300, (1980).
2. Bungey J.H.,"The Testing of Concrete in Structures," Surrey University Press pp. 35-59, (1982).
3. BS 4408: pt.5, "Non-destructive Methods of Test for Concrete-Measurement of the Ultrasonic Pulses Velocity in Concrete," British Standard Institution, London, (1970)
4. ELE PUNDIT 6, Portable Ultrasonic Non-Destructive Digital Indicating Tester, Operating Manual.
5. Germann Instruments: "Operation and Maintenance Manual for DOCter Impact-EchoTesting, Version 3.0, (1999).
6. Lin, Y., and Sansalone, M., "Detecting Flaws in Concrete Beams and Columns Using the Impact-Echo Method", Materials Journal of American Concrete Institute, 89, No. 4, (1992).
7. Neville A.M, "Concrete Technology", Longman Group UK Limited, pp282, 631-633, (1987).
8. Sansalone, M., and Carino, N.J., "Impact-Echo: A method for flaw Detection in Concrete Using Transient Stress Waves", NBSIR 86-3452, National Bureau of Standards, Gaithersburg, Maryland, pp.222 (1986).
9. Sansalone, M.J. & Street, W.B.:"Impact-Echo: Nondestructive evaluation of concrete and masonry", Bulbriers Press, Ithaca, N.Y., USA, ISBN No.: 0-96 12610-6-4, (1997)
10. Sansalone, M., and Carino, N.J., "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, (1998).
11. Sersale, R., Cioffi, R., Frigione. G. and Zenone, F., "Relationship between gypsum content, porosity and strength of cement", Cement and Concrete Research, 21, No.1, pp.120-126, (1991).

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