Concrete dielectric properties investigation using ... - Springer Link

Materials and Structures (2013) 46:77–87 DOI 10.1617/s11527-012-9886-2

ORIGINAL ARTICLE

Concrete dielectric properties investigation using microwave nondestructive techniques M. Jamil • M. K. Hassan • H. M. A. Al-Mattarneh M. F. M. Zain

•

Received: 27 June 2011 / Accepted: 11 June 2012 / Published online: 3 July 2012 Ó RILEM 2012

Abstract The paper deals with dielectric properties of concrete and the effect of frequency, curing time and water to cement (w/c) ratio, concrete compression strength, steel fibre concrete and moisture content on the dielectric properties. The concrete dielectric properties such as transmission coefficients, reflection coefficients, dielectric constants and loss factors; were investigated using microwave nondestructive testing (MNDT) technique. The MNDT system consists of transmit and receive horn lens antennas, a vector network analyzer, mode transitions, and a printer. The horn lens antennas are used for minimizing diffraction effects due to the edges of the sample. Electromagnetic waves at microwave frequency range 7.0–13.0 GHz using free-space microwave method was used for measuring dielectric properties of concrete. Concrete specimens were prepared using different w/c ratios and different compressive strengths. Concrete dielectric properties were measured and correlated with compressive strength and M. Jamil M. F. M. Zain Sustainable Construction Materials & Building Systems (SUCOMBS), Department of Architecture, Faculty of Engineering & Built Environment, Universiti Kebangsaan Malaysia, Bangi, Malaysia M. K. Hassan (&) H. M. A. Al-Mattarneh Sustainable Construction Materials & Building Systems (SUCOMBS), Department of Civil and Structural Engineering, Faculty of Engineering & Built Environment, Universiti Kebangsaan Malaysia, Bangi, Malaysia e-mail: [email protected]

w/c ratio of concrete, curing time. The experimental results indicate that microwave non-destructive technique has correlated well with to determine the compressive strength and w/c ratios of concrete. The results also show the potential to be used to determine the percentage of fiber. This technique is easy to adapt for in situ measurements. Keywords Concrete Dielectric properties Microwave and nondestructive technique

1 Introduction Radio and microwave nondestructive testing of concrete materials is an important techniques for detection of flaws, cracks, defects, voids, inhomogenities, moisture content (MC), etc. [19]. These techniques have advantages over other non-destructive evaluation (NDE) methods (such as radiography, ultrasonic, eddy current) due to low cost, good penetration in nonmetallic materials, good resolution and contactless feature of the microwave sensor (antenna) [7, 13]. Microwave nondestructive testing (MNDT) techniques (such as ground probing radar, free-space microwave techniques) are increasing being used for quality control and condition assessment of concrete structures [2, 10]. MNDT methods are fast, contactless, accurate and continuous techniques for evaluation of MC, slope-of-grain (SOG), density of knots and specific gravity of the concrete. The MNDT system

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consists of transmit and receive horn lens antennas, a vector network analyzer, mode transitions, and a printer. The horn lens antennas were used for minimizing diffraction effects due to the edges of the sample. Redheffer [16] was the first researcher to suggest a simple free-space method for measurement of dielectric constant from the measured phase of transmission coefficients [13]. He reported that free-space methods are nondestructive and contactless techniques which are specially suited for dielectric measurement of materials. Bassett was the first researcher to measure complex permittivity in free-space using spot-focusing antennas at a frequency of 9.4 GHz [3]. He measured complex permittivities of fused silica as a function of temperature. In the last 20 years, a number of free-space methods were developed for measurement of dielectric properties [1, 9, 10, 12, 14]. Previous research work shows that there is potential in the use of electromagnetic properties of concrete to determine w/c ratio, strength and MC [4–6, 19]. The results show the potential use of reflection coefficients to determine compressive strength and w/c ratio. Also the variation of reflection coefficients was used to detect segregation of concrete and to predict aggregate size and content [5, 6]. To improve and extend this method there is a need to evaluate the effect of MC on dielectric properties of Portland cement concrete. Up to date, established research work directed toward the measurement of the electromagnetic properties of hardened concrete has been in a limited range of frequency and most of the available data is in the range of 0.25–0.70 GHz [8, 11, 15]. In addition, some researches were conducted on Portland cement paste and mortar [5, 6, 19]. Few research studies have been undertaken to determine the electromagnetic properties of concrete with limited aggregate size, i.e. less than 10 mm [4] or on a special concrete condition such as wet and dry [17]. Most of the available non-destructive technique (NDT) methods are coaxial transmission setup, ultrasonic pulse velocity and X-ray; showed feasible results for detecting the chloride contamination, determination of homogeneities and strength of concrete. However, although this method proved to be beneficial in characterizing concrete dielectric properties in the laboratory, it is not suitable for use in the field. These methods also need special sample preparation and sample size. Therefore, there is a need to develop a

Materials and Structures (2013) 46:77–87

reliable NDT method to evaluate and to test concrete structures and materials in the field. In addition there is a need to study and provide values of electromagnetic properties of hardened concrete over a wide range of frequency and concrete with different properties and conditions to be used for the nondestructive testing of existing concrete structure and construction facilities. In this paper, the dielectric properties of concrete were investigated using MNDT technique. Free-space microwave method can be used to measure transmission coefficients, reflection coefficients, dielectric constants and loss factors for evaluation of composite concrete materials. Electromagnetic waves at microwave frequency range 7.0–13.0 GHz was used for measuring reflection coefficients and dielectric properties of concrete. Concrete specimens were prepared using different w/c ratios and different compressive strengths. Dielectric constants and loss factors, transmission coefficient, reflection coefficients were measured and correlated with compressive strength and w/c ratio of concrete, curing time.

2 Concrete dielectric properties Concrete is considered as a dielectric material that stores energy when exposed to an electromagnetic signal. Therefore, the concrete can be characterized by its dielectric properties using electromagnetic (EM) field methods. The exposure of a dielectric material (not a perfect conductor) to an electromagnetic field results in change of the arrangements of its microscopic electric dipoles composed of positive and negative charges whose centers do not quite coincide. These are not free charges, and they cannot contribute to the conduction process. Rather, they are bound in place by atomic and molecular forces and can only shift position slightly in response to external fields. Free charges, on the other hand, are the ones that determine conductivity. Upon the exposure to EM field, a shift in the relative positions of the internal bound positive and negative charges against normal molecular and atomic forces results in a storage of an electrical energy in what is known as polarization. The latter is expressed by the real part of complex permittivity (or dielectric constant) of the material. While the real part (dielectric constant) reflects the amount of polarization of the material, the imaginary part (or loss factor) reflects the losses caused by


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conductivity (controlled by free charges) and the relaxation of the water dipole. This is why a perfect dry dielectric material (with no free charges) would have an imaginary part of zero. A dielectric material can be characterized by two independent electromagnetic properties namely, complex permittivity e* and complex permeability l*. However, most dielectric materials including wood and concrete are nonmagnetic, making the permeability very close to the permeability of free space. So the discussion is limited to the complex permittivity e* which is defined as: e ¼ e0 je00

ð1Þ

where e0 is the real part of the complex permittivity, e00 is the imaginary part of the complex permittivity and pffiffiffiffiffiffiffi j ¼ 1. Dividing Eq. 1 by the permittivity of free space e0 the property becomes dimensionless and relative to free space: er ¼

e ¼ e0r je00r e0

ð2Þ

where e0r is the real part of the relative permittivity called dielectric constant and e00r is the imaginary part of the relative permittivity called loss factor. The dielectric constant is a measure of how much energy from external electric field is stored in a material. The loss factor is a measure of how dissipative or lossy a material is to an external electric field due to current conduction [12]. The ratio of the energy lost to the energy stored in a material is given as loss tangent: tan d ¼

e00r e0r

ð3Þ

where tand is the loss tangent. These electromagnetic properties are not constant. They change with frequency, temperature, moisture, and mixture of the material, compression strength. Figure 5 show a planer sample of thickness d placed in free-space. For MNDT techniques, the measured parameters were reflection coefficients (S11), transmission coefficients (S21), dielectric constants (e0r ), loss factors (e00r ) as a function of frequency (microwaves) and temperature. These measured parameters can be related to material parameters of interest (e.g., flaws, inhomogeinities, MC, etc.) by suitable modeling and calibration. Reflection coefficients, transmission coefficients,

dielectric constant and loss Factor) were also measured to detect fiber distribution and concentration in steel fiber-reinforced concrete (SFRC). For concrete specimens, reflection (S11) and transmission (S21) coefficients were measured in the frequency range 8–12.5 GHz by using two microwave methods. The available these two microwave methods are as noncontact method such as free-space microwave measurement (FSMM) system and contact method such as open ended waveguide. Reflection and transmission coefficient were measured using the free-space microwave system to determine the dielectric constant and loss factors of concrete.

3 Nondestructive testing techniques Radio and MNDT techniques are mainly used for nonmetallic materials. These techniques have advantages over other NDE methods (such as radiography, ultrasonic, eddy current) regarding low cost, good penetration in nonmetallic materials, good resolution and contactless feature of the microwave sensor (antenna). 3.1 Radio wave nondestructive testing technique The radio wave Nondestructive testing system used the radio frequency range of 1–100 kHz. Two different designs were developed. The first called parallel plate electrode system (PPES). This system is useful for laboratory and research testing as shown in Fig. 1. The second system called mobile dielectric probe system (MDPS). This system was designed as surface sensor. It is light weight and simple. The probe can slide on the surface of the material and provide information from different point. The MDPS fixed to a circular plate that allowed to rotate and to provide data at different angle by polarizing the electromagnetic signal on different direction as shown in Fig. 2. The plates are manufactured from metal sheet (e.g., copper or brass), which allows complete contact when placed on the concrete structural element. 3.2 Microwave nondestructive testing (MNDT) technique Microwave nondestructive testing (MNDT) techniques are an important electrical method in which

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Fig. 1 Parallel plate electrode system (PPES). a Complete set of parallel plate electrode system. b Close view of parallel electrode plate

Electrode plate

Laptop and Data analyzer

(a) Mobile dielectric probe

(b)

Laptop and Data analyzer

(a)

MDPS made of different shapes of plate

(b)

Fig. 2 Mobile dielectric probe systems (MDPS). a Mobile dielectric probe. b Different shapes of plate

microwave frequency are utilized. The term microwaves refer to alternating current signals/electromagnetic waves with frequencies between 300 MHz (3 9 108 Hz) and 300 GHz (3 9 1011 Hz). But in the microwave NDT system, the frequency of microwave generally used as 7–13 GHz. Since the penetration of microwaves in good conducting materials is very small, MNDT techniques are mainly used for non-metallic materials (dielectric materials). There are two types of MNDT methods which are (1) freespace microwave methods (FSMM) operating in the far-field region employing spot-focusing horn lens antennas and (2) waveguide methods operating in the near-field region which employ open-ended coaxial lines, rectangular waveguide, microstrip lines and cavity resonators as probes. This system is non-contact system because it excites the material under testing using horn antenna. The system is shown in Fig. 3.

Figure 4 shows a schematic diagram of the FSMM system which was used for microwave nondestructive evaluation of concrete materials. In generally the FSMM system is applied using two horn antennas consisting of open ended rectangular waveguides for radiating and receiving ray. Due to the frequency the distance of the horn is calculated from the sample. In the middle of the antennas a sample holder restrains the material under test. The position of antennas and sample holder is important to be aligned. The two antennas are symmetric in all directions to eliminate inequalities of incident and reflected waves. A pair of spot-focusing horn lens antennas (model no. 857012X-950/C) was used that was manufactured by Alpha Industries, Woburn, MA (USA). These antennas have two equal plano-convex lenses mounted back to back in a conical horn antenna. One plano-convex lens gives an electromagnetic plane wave and the other plano-convex lens

Materials and Structures (2013) 46:77–87 Fig. 3 Microwave NDT system (MNDTS): a Transmit and receive horn lens antennas with testing sample. b A vector network analyzer, computer and a printer

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Test Sample

Transmit and Receive horn lens antennas

(a) Vector network analyzer

Printer

Computer

(b)

focuses the electromagnetic radiation at the focus. This measurement set up covers a frequency range of 8–12.50 GHz. In addition, the same setup can be used in the frequency range of 7.5–40 GHz by appropriate change of mode transitions. The measurement system consists of a vector network analyzer (VNA) Wiltron 37269A and an open-ended waveguide sensor as shown in Fig. 4. The sensor has coaxial cables, a coaxial-to-rectangular waveguide transition and TeflonTM impedance transformer. The measurement system was calibrated up to the rectangular waveguide end of the coaxial-to-waveguide adapter by implementing LRL (line, reflect line) calibration technique. In LRL calibration, first line was a

Computer

Microwave Vector Network Analyzer Wiltron 37269B

Printer

Coaxial cables Spot focusing horn less antennas Antenna Mounts Sample Holder

through connection, the second line was a precision waveguide of length equal to quarter wavelength at midband. The reflect standard was a waveguide shorting plate. A computer program was implemented to calculate the complex permittivity of concrete from the measured and calculated values of complex reflection coefficients. Figure 5 shows a planer sample of thickness d placed in free-space in which reflection and transmission coefficient were measured using the freespace microwave system to determine the dielectric constant and loss factors of fiber reinforced concrete.

4 Material consideration in experimental test Concrete specimens were made using ordinary Portland cement, fine aggregate (natural sand), coarse aggregate (granite), water and fiber. All the specimens were cast in duplicate. Since granite aggregate is more commonly used than limestone aggregate in Malaysia, therefore it was chosen for the majority of the mixes. Three water cement ratio of 0.40, 0.50 and 0.60 were selected. The results of presented in this paper are limited to w/c ratio 0.4, 0.5 and 0.6 as shown in Table 1.

Co axial cable To VNA

Table

Concrete Specimen Incident wave S11

Incident wave S12

Reflected Horn less antenna

Fig. 4 Free-space microwave measurement systems for NDT of fiber concrete

Fig. 5 Schematic diagram of planar concrete sample

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Table 1 Property of concrete samples Sample

Ratio of cement: sand:aggregate

w/c ratio

Compressive strength (N/mm2) at 28 days

G40

1:2:3

0.40

57.8

G50

1:2:3

0.50

37.6

G60

1:2:3

0.60

30.8

To study the effect of steel fiber concrete 4 mixes were prepared. The properties of these mixes are shown in Table 2. The steel fiber used in this study was Novotex IV manufactured by Novocon steel fiber, USA. The properties of novotex steel fiber are: length = 30 mm, the diameter = 0.7 mm, aspect ratio 43, tensile strength 1,150 MPa, flattened ends with round shaft, bright and clean wire. To evaluate the effect of concrete MC on it is dielectric properties. Samples from the 3 mixes listed in Table 1 were used. The dielectric properties of the specimens were measure at different MC ranging from saturated condition to oven dry conditions.

constant has a certain tendency to decrease with the increasing of frequency. This effect caused by the polarization phenomenon of the ions inside the concrete material which causes free ions and gases to be produced in the boundaries of the material [18]. It is also seen from Fig. 7 that the amount of loss factor varies at different frequencies. The result shows that the loss factor is frequency dependent. It is logical since the loss factor shows the amount of absorption of energy inside the concrete sample. This must change with different frequencies and the magnitude of energy transferred by electromagnetic waves is relative to its frequency. It is understood from the test results that the loss factor tends to decrease while the frequency of the propagation wave is increased. This means that the higher the frequency becomes the rate of absorption is less. This can be the effect of phenomenon that electromagnetic waves with higher frequencies have more power to penetrate inside objects and propagate through them with less energy absorbed by the materials itself. Consequently, at higher frequencies the amount of energy absorbed by concrete becomes less.

5 Results and discussion 5.1 Effect of frequency on dielectric properties of concrete The effects of frequency change on dielectric properties of concrete were measured in the microwave frequency range from 7 to 13 GHz. Figures 6 and 7 show the dielectric constant and loss factors of concrete specimens tested at frequency range from 7 to 13 GHz and w/c ratio 0.50. The results of effects of frequency change on dielectric properties such as dielectric constant and loss factors of concrete are shown in Figs. 6 and 7. From the results as shown in Fig. 6, it is observed that dielectric constant of concrete varies at different frequencies. The dielectric

5.2 Effect of curing time and w/c ratio on dielectric properties of concrete The electromagnetic properties (dielectric constant and loss factor) of water have higher value than all other concrete constituent (aggregate and cement). So, the amount and state of water is the most significant factor in determining the dielectric properties of concrete. The effects of curing time and water cement ratio on dielectric constant and loss factors of concrete specimens are shown in Fig. 8. As the curing time increases, the amount of free water in the concrete decreases due to the cement hydration and evaporation. The water changes from a free to an adsorbed state, which reduces ionic polarization and conductivity due to decreases in

Table 2 Properties of steel fiber concrete Fiber concrete properties

28 days Compressive strength (N/mm2) 28 days Flexural strength (N/mm2)

Steel fiber concrete G50 (0 kg/m3)

G50C1 (10 kg/m3)

G50C2 (20 kg/m3)

G50C3 (30 kg/m3)

35.6

37.0

36.0

33.9

3.18

3.49

3.55

3.87


83

16.0 14.0 12.0 10.0 8.0 6.0 7.0

8.0

9.0

10.0

11.0

12.0

13.0

Frequency (GHz) Fig. 6 Effect of frequency on the dielectric constants of concrete at 28 days of curing

7.50 7.30

Loss factor

7.10

water remaining in the concrete. Therefore, dielectric constants decrease with curing time. The results of dielectric constants presented in Fig. 8 indicate a rapid decrease in the values of dielectric constants during the first 7 days, particularly for the higher w/c ratio specimens. This is as expected due to bleeding and subsequent evaporation of free water from the surface of the concrete specimens. After 7 day the rate of decreasing is slow and after 21 days up to 28 days the dielectric properties become almost constant. After 21 days, most of the remaining water in the specimens is bound. This would indicate that the dielectric constant measurements of concrete taken after 21 days are an indication of hydration and therefore of w/c ratio and compressive strength. It is clearly observed that a trend of increasing values of dielectric constants after 21 days curing is with decreasing w/c ratio.

6.90 6.70

5.3 Effect of compression strength on dielectric properties of concrete

6.50 6.30 6.10 5.90 5.70 5.50 7.0

8.0

9.0

10.0

11.0

12.0

13.0

Frequency (GHz) Fig. 7 Effect of frequency on loss factors of concrete at 28 days of curing

Fig. 8 Effect of curing time and w/c ratio on concrete dielectric constant

Figures 9 and 10 show the results of concrete dielectric constants, loss factors and compressive strength. The results show a high correlation between dielectric constants, loss factors and compressive strength. Simple linear equations were established for these relationships. Figures 11 and 12 show the dielectric constant and loss factors of concrete specimens tested at frequency 12 GHz and w/c ratio 0.50. From the results, it is observed that both dielectric constant of concrete increase with increasing compression strength of concrete, but loss factors property of concrete decrease with increasing the compression strength of concrete. Compressive strength (N/mm2)

Dielectric Consatant

18.0

70 60 50 40 30 20 Compressive strength = - 79.135 + 24.888(Dielctric constant)

10

2

R = 0.9999

0 4

ion production. Also, the pore structure changes with curing time. The pore sizes become very small, thus making it difficult for the movement of free ionized

4.2

4.4

4.6

4.8

5

5.2

5.4

5.6

5.8

Dielectric constants Fig. 9 Relationship between compressive strength and dielectric constant at 28 days of curing

Compressive strength (N/mm2)

84

Materials and Structures (2013) 46:77–87 70 60 50 40 30 20

Compressive strength= + 69.798 - 2.9112(Loss factors) 2

R = 0.9798

10 0 0

2

4

6

8

10

12

14

16

Loss factors Fig. 10 Relationship between compressive strength and loss factor at 28 days of curing

5.4 Effect of steel fibre concrete on dielectric properties of concrete The effects of steel fibre concrete on the reflection and transmission coefficients of steel fibre are shown in Figures 11 and 12 respectively. From the result as shown in Fig. 11, it is also clear that the reflection coefficients of all fibre concrete samples decrease with increasing curing age which can be attributed to the loss of free water during curing time since part of the free water is lost by evaporation and the rest is bound in the cement paste. The relationship between the fiber content and the reflection coefficients and the fiber content and the transmission coefficients respectively were established at 2, 7, 11, 14, 21 and 28 days of curing. The result of the transmission coefficients shown in Fig. 12 increases with increasing curing time because with increasing curing time the free water decreases by evaporation process, this will increase

Fig. 11 Effect of fiber content on reflection coefficients with respect to curing time

Fig. 12 Effect of fiber content on transmission coefficients with respect to curing time

the penetration of the microwaves in steel fibre concrete which increases the transmission coefficients. Figure 12 shows that the higher fibre contents in the concrete indicate the lower transmission coefficients. 5.5 Effect of moisture content on dielectric properties of concrete The effect of MC on the electromagnetic properties of concrete has been studied. The amount of free water in concrete is expected to play a significant role in the electromagnetic properties of concrete because it has a dielectric constant of 81, which is greater than that of dry concrete. The measurement values of dielectric

Fig. 13 Effect of moisture content on dielectric of concrete


constant at different MC are presented in Fig. 13. At the same MCs, the dielectric constants increase with decreasing w/c ratio of concrete. In dry condition, dielectric constants can be used to indicate the compressive strength and w/c ratio of concrete. The experimental results shows slow increase of reflection coefficient and dielectric constant at lower water content and this is valid up to a MC of about 5 % by volume. After 5 % of MC, both the reflection coefficient and dielectric constant increases rapidly until full saturation. Therefore, this can be used to measure w/c ratio and compressive strength of concrete. It is important to notice that any measurement of w/c ratio or compressive strength based on electromagnetic properties should take the MC into consideration otherwise it will lead to false results. A statistical analysis using nonlinear regression analysis (NRA) was done to establish the relationship between dielectric constant and the MC of concrete shown in Fig. 13.

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Fig. 14 Effect of curing time on dielectric constants at different chloride contents

5.6 Effect of chloride content on dielectric properties of concrete To study the effect of chloride presence in concrete on its dielectric properties, two methods of chloride contamination were applied. In Method 1, chloride was added to the mixing water at 3 different content levels, 0, 1.5 and 3.0 kg/m3. NaCl was added at the ratio 58/35 of the intended chloride content. In method 2, after concrete specimens were cured for 28 days, they were immersed in seawater for 3 weeks. The dielectric properties of the specimens of the two contaminated methods were evaluated over curing time and at different MCs. The effects of curing time on the dielectric properties of concrete are shown in Figs. 14 and 15. These figures show that both dielectric constants and loss factors decrease with increasing curing time over all chloride contents. This phenomenon can be attributed to the following; (1) a reduction in the Portland cement paste conductivity may occur when water becomes absorbed from its initial bulk form, reducing the effect of dipole polarization, (2) a decrease in the overall pore diameter of the Portland cement paste pore system, restricting the movement of ionized water, may lead to reducing the effect of ionic polarization, (3) a reduction in the ion concentration in the capillary water, due

Fig. 15 Effect of curing time on dielectric constants at different chloride contents

to Portland cement hydration process, at early stages of curing will reduce the effects of ionic conduction.

6 Conclusion The concrete dielectric property is one of the most important properties that allow the detection of concrete deterioration, because the real part shows the ability of concrete to store energy as an electric charge, while the imaginary part shows the loss of energy due to molecular friction and conduction. Reflection coefficients, transmission coefficients, dielectric constant and loss factor were measured to detect fiber distribution and concentration in steel fiber-reinforced concrete. The conclusions of this study are as follows:

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(1) (2)

(3)

(4)

(5)

(6)


The dielectric constant and loss factors of concrete decreases with increasing frequency. The dielectric constant of concrete decrease with increasing curing age. The measurement values of dielectric constant after 28 days of curing indicate the w/c ratio and compressive strength of the concrete. The measurement system and open-ended rectangular waveguide used in this study can be used to measure dielectric properties of concrete and estimate the w/c ratio and compressive strength. Determination of w/c and compressive strength of concrete at early stage would be very valuable to the construction industry. Mainly, early prediction of the strength of concrete members will facilitate quick building construction. The reflection and transmission properties of concrete specimens were evaluated over a range curing age at different steel fibre content concrete. These properties also show the potential to be used to determine the percentage of fiber content in concrete. The results indicate that microwave non-destructive technique using open-ended rectangular waveguide has the ability to detect fiber distribution and concentration in steel fiber-reinforced concrete. This technique is easy to adapt for in situ measurements. The effect of MC on the electromagnetic properties of concrete has been studied. The amount of free water in concrete is expected to play a significant role in the electromagnetic properties of concrete. The experimental results shows slow increase of dielectric constant at lower water content and this is valid up to a MC of about 5 % by volume. This can be used to measure w/c ratio and compressive strength of concrete. The dielectric properties of concrete with the chloride contamination were evaluated over curing time and at different MCs. The results show that both dielectric constants and loss factors decrease with increasing curing time over all chloride contents.

Acknowledgments The Authors would like to thank Sustainable Construction Materials & Building Systems (SUCOMBS) Groups, Faculty of Engineering & Built Environment, Universiti Kebangsaan Malaysia (UKM) for supporting this research.

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