Abstract

Tests have been carried out to examine the relationship between the porosity and strength of concrete at each stage with the accuracy of void depth detected by using Ultrasonic Pulse Velocity Method (UPVM). Five reinforced concrete (RC) slabs with fabricated void at a known location of grade 30 and 40 respectively were tested at day 3, 7, 14 and 28. The accuracy of detection obtained was compared with the porosity of the specimens as they mature. The test results indicate that porosity has significant effect on the accuracy since lower porosity yield more accurate location and depth of defect.

Keywords:
non-destructive testing, concrete, Ultrasonic Pulse Velocity Method, porosity, defects detection and accuracy

Introduction

The presence of capillary pores and air voids influence concrete permeability to a large extent. The ingress of aggressive agents into the pore structure is responsible for various durability problems in concrete structure. 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 defects detection. Defects are often introduced during casting and detection during in-service life is often too late to remedy the situation.

Base on the information obtained for Ultrasonic Pulse Velocity Method (UPVM), all the work that have been done in relation to defect detection and depth of defect 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 [1,2,5].

The scope of this study is to test and establish the relationship between the accuracy of the using the ELE PUNDIT 6, Portable Ultrasonic Non-Destructive Digital Indicating Tester (UPVM) [4] for detecting the location and depth of defects and the porosity of specimens. The main interest is to correlate concrete porosity with how it affects the accuracy of defect detection at early age concrete.

Materials and methods:

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

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 (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 2: Grade 40 RC Slab 500mm x 300mm x 75mm (20in.x12in.x 3 in.)

Slump = 10-30 mm cured at room temperature

All the specimens were tested from day 3, 7, 14 and 28 with UPVM and A specially designed ‘permeability unit’ is used to test a series of specimens for porosity. Water is forced under pressure through cured specimens and the passage of water measured. The resulting porosity is expressed in percentage [6]. The accuracy of the testing method was determined by comparing the prerecorded location and depth voids. The method of testing and determining the void location, void depth and porosity of concrete at different age are as follows.

Ultrasonic Pulse Velocity Method (UPVM)
The indirect method of testing is used since it is the best method to determine the effective path length [3,4]. 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 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 as shown in Figure 2.

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

DISTANCE (mm)	Transit Time (m s)
	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: Slab 1 (Grade 30) and Slab 6 (Grade 40)

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 3 a typical calculation for RC Slab 1 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%

Fig 2: Void Depth Determination by the Indirect Method: RC Slab 1 GD 30 at day 14.

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: Slab 1 (Grade 30) and Slab 6 (Grade 40).

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.

Ultrasonic pulse velocity 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 Figures 3 and 4 showed the correlation between percentage of accuracy and porosity of UPVM RC Grade 30 and 40 with concrete age respectively.

Fig 3: Correlation between Accuracy and Porosity (GD30).

Fig 4: Correlation between Accuracy and Porosity (GD40).

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. 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 the method 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.

Conclusions

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. 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. Neville A.M, "Concrete Technology", Longman Group UK Limited, pp282, 631-633, (1987).
6. 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, American Concrete Institute, Detroit, pp. 201-227, (1984).

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