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Ground penetrating radar wave
attenuation models for estimation of
moisture and chloride content in
concrete slab
Roszilah Hamid, Syahrul Fithry Senin
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2. Ground penetrating radar wave attenuation models for estimation
of moisture and chloride content in concrete slab
S.F. Senin a,b
, R. Hamid b,⇑
a
Faculty of Civil Engineering, Universiti Teknologi MARA Pulau Pinang, 13500 Permatang Pauh, Penang, Malaysia
b
Department of Civil and Structural Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
h i g h l i g h t s
GPR wave attenuation is measured on a concrete slab saturated with water and chloride.
The moisture content (MC) shows a strong linear correlation with the wave attenuation.
The chloride content (CC) attenuates radar waves at a higher rate than the MC.
Two models to estimate MC and CC in concrete slab are proposed.
The extent of these corrosion agents are useful in condition rating decisions.
a r t i c l e i n f o
Article history:
Received 20 March 2015
Received in revised form 30 November 2015
Accepted 22 December 2015
Keywords:
Ground penetrating radar
Moisture
Chloride
Amplitude attenuation
a b s t r a c t
The detection of moisture and chloride ingress through concrete cover is important for estimating the
extent of corrosion in reinforced concrete (RC) components. The concentrations of the substances are
monitored by observing the ground penetrating radar (GPR) amplitude attenuation in water and chloride
saturated concrete slab samples. The amplitude attenuation significantly correlates with the amount of
both substances. Two multiple nonlinear regression models were developed. The proposed models
demonstrate a strong correlation with the radar amplitude attenuation data as both substances are varied
in the concrete cover. The developed models can be employed to estimate the moisture and free chloride
content in – concrete cover for improved quantification of corrosion level.
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
The deterioration of reinforced concrete structures due to corro-
sion has received substantial attention from civil engineers due to
its common occurrence and high cost of repair. This impairment is
accelerated if concrete possesses high permeability due to impro-
per mixing and compaction during the concrete casting process
[1]. A high water–cement ratio in concrete facilitates the ingress
of water and chloride ions through the capillary pores in its cover
prior to the destruction of the rebar in subsequent stages.
The existence of moisture and chloride ions in a concrete cover
is a form of conditioning that can initiate the corrosion mechanism.
Chloride may also exist in a concrete surface via cracks in concrete
or the presence of contaminated sand or aggregates. Once the
chloride ion reaches the rebar level and exceeds the chloride
concentration threshold value, the protective thin passive layer
surrounding the rebar will gradually be destroyed, which will
cause the development of anodic and cathodic sites on the rebar.
Due to generations of electrical potential differences at anodic
and cathodic sites on the rebar, iron is oxidized at the anode and
dissolved in the concrete pore solution as ferrous ions (Fe2+
),
whereas electrons that move towards cathode and hydroxyl ions
(OH ) will be released. With sufficient oxygen at the anodic sites,
a rust component –Fe(OH)2– is formed when the hydroxyl ions
reacted with the ferrous ion. Further oxidization of Fe(OH)2 into
different corrosion products will produce a high volume of
corrosion products that comprise six times the original volume
[2]. With this volume increase, the interface between concrete
and steel will experience high tensile stresses that can cause
damage initiation (cracking, delamination and concrete spalling).
The rebar corrosion-damage process in concrete structures is a
time-consuming process that exhibits symptoms once the electro-
chemical reactions at the anode and cathode sites produce
corrosion-damage to concrete due to expanding rust on the
concrete-rebar interface.
http://dx.doi.org/10.1016/j.conbuildmat.2015.12.156
0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
⇑ Corresponding author.
E-mail address: roszilah@ukm.edu.my (R. Hamid).
Construction and Building Materials 106 (2016) 659–669
Contents lists available at ScienceDirect
Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat

3. The presence of moisture can caused several forms of attack on
concrete which leads to chemical or physical deteoriation. The
forms of attack on concrete can be caused by internal or external
source of moisture. One of the common form of attack in concrete
structures is due to the sulphate attack (SA). This traditional attack
is initiated by the chemical interaction of sulphate-rich soil or
water with high sulphate content within the cement paste. The
destructive nature of SA causes cement paste to disintegrate,
which eventually leads to a weaker cement matrix [3]. Several
attempts were done on how to assess concrete resistance to SA,
however, the experiments required a long term observation that
will take years as it involves the sulphate ingression by diffusion
mechanism in concrete, thus. An accelerated test has been
proposed to access the sulphate resistance on concrete made with
ordinary Portland cement and slag cement [4].
Water expansion in concrete pores during winter season is
another phenomena that can cause damage to the concrete in long
term duration. When water freezes, it will expands approximately
15% from its original volume [5]. As the water in moist concrete
freezes, it produces pressure inside the capillaries and concrete
pores. Once the tensile strength of concrete is exceeded, the pores
will dilate and rupture. Further successive freeze–thaw cycle will
caused significant expansion, cracking and crumbling of the
concrete. In view of this situation, few methods has been applied
on concrete to improve its freeze–thaw durability. The use of
air-entraining agent and the hydraulically pressed concrete were
the possible ways and its performance of this method can be found
elsewhere from other references [6,7].
Alkali-silika reaction (ASR), has been identified as one of the
deteorioration phenomena in concrete facilitated by the presence
of moisture. In this phenomena, some reactive aggregates react
with the akali hydroxide in concrete to produce gels; causing
expansion and cracking over a period of years. The typical indicator
of this reaction is the formation of random map cracking on areas
with frequent supply of moisture, i.e. near joints and close to the
waterlines. Moisture allows migration of alkali ions to the reaction
sites in concrete and the resulting gel absorb moisture, leading to
expansion on concrete. The expansive reaction can occur in
concrete having a relative humidity more than 80% [8].
Therefore, an assessment of the chloride and moisture content
in concrete cover during the early stage of corrosion is essential
for planning the maintenance of deteriorated concrete structure
with a relatively lower cost compared with maintenance after
severe damage over a certain period of time.
To assess the state of moisture and chloride ion content in a
concrete cover, a number of techniques have been developed to
measure both contaminants in concrete. The most prominent and
direct method for measuring moisture content in concrete is the
gravimetric method [9]. This method can be used to measure the
moisture content by observing the change in mass; however, it is
not a practical technique for quantifying moisture in large concrete
structures. The neutron hydroprobe is a non-invasive method that
relates the scattering of neutrons with the moisture content of con-
crete [10]. Unfortunately, this technique is only applicable in a lab-
oratory and may pose health risks to an operator. For quantifying
the chloride content in concrete, concrete dust samples are
obtained at different depths and titration method using silver
nitrate is used to determine the chloride content. However, this
method is time consuming to be applied to a large area of concrete
surface. Ground penetrating radar (GPR) has great potential in
enabling operators to measure the moisture and chloride content
using electromagnetic waves. This nondestructive method facili-
tates rapid data collection in large concrete areas, and only
requires a single-sided surface of concrete for inspection work.
Previous GPR researches have provided GPR wave attenuation
models based on one parameter: the effect of the water content
[11–13] or the effect of the chloride content [14]. The effect of
water content on GPR amplitude with 1.5 GHz antenna was stud-
ied using concrete sampels of 7 cm thickness [11]. They provide
a numerical simulation by modelling the amplitude variation with
degree of saturation and the results are correctly simulated with
the experimental values, except in the range of saturation laying
between 0% and 20%. The quantification of volumetric water con-
tent in 12 cm thickness concrete samples was conducted by Klysz
Balayssac, 2007 [12]. They analysed the direct waves of 1.5 GHz
antenna and successfully model the relationship between the nor-
malized amplitude with the saturation degree. The relationship
between the volumetric water content in fresh concrete mix and
its relative dielectric constant was obtained using GPR wave [13].
Microwave non-destructive testing has been used to evaluate the
separate effect of moisture and chloride contamination in concrete
and its relationship with relative dielectric constant and loss factor
were determined [14]. However, this current study will focussed
on studying and modelling the effect of both moisture and free
chloride content in concrete cover to the ground penetrating radar
amplitude; which has been identified to be the objective and the
novel aspect in this work. The first scope of the work is the collec-
tion of radar signals on concrete cover that was saturated with
varying moisture and free chloride content. The following scope
of work is to develop the attenuation model of radar amplitude
due to moisture and free chloride content in the concrete cover.
1.1. Ground penetrating radar
GPR is a method that is common for infrastructure inspection
work involving concrete structures. GPR can be used for subsurface
condition assessment and to monitor concrete infrastructures,
such as bridge decks [15–17] and building components [18]. It is
a nondestructive method that has been employed in geological
studies to map the embedded geological features [19], however,
with the advancement of new high-frequency GPR antenna and
better data processing software, the application of the method
has evolved from geological field applications to civil engineering
applications, such as the estimation of pavement thickness [20]
and the detection of defects in pavement [21].
In principle, this nondestructive method is dependent on mea-
suring the reflection of the reflected the electromagnetic wave that
propagates in certain lossy dielectric mediums, such as concrete,
after it impinges any embedded layer that possesses different elec-
trical properties of the propagated medium, i.e., the dielectric con-
stant and the conductivity. The degree of this wave reflection is
quantified by the reflection coefficient R, which is computed as
the ratio of the incident wave amplitude at an interface or object
to the reflected wave amplitude on the targeted layer or object,
as shown in Eq. (1) [22]:
R ¼
A
Ap
ð1Þ
where A is the reflection wave amplitude from the top surface of the
concrete structure and Ap is the wave reflection amplitude from the
metal plate placed at the concrete soffit.
The reflection of the wave in concrete is influenced by two main
electrical properties of the medium, i.e., the dielectric constant and
the conductivity, as concrete is a lossy material. The dielectric con-
stant describes a material’s ability to store and release electromag-
netic wave energy by electrical charge displacement and a
polarization process when an electrical field from the radar
antenna is applied [23]. The original energy of waves is subse-
quently converted to heat energy during the displacement and
polarization process, which causes amplitude attenuation to the
original wave amplitude. The moisture content in concrete has
660 S.F. Senin, R. Hamid / Construction and Building Materials 106 (2016) 659–669

4. been described in certain studies [24,25] as a substance that influ-
ences the dielectric constant in concrete; thus, it can characterize
the moisture content based on the radar signal changes due to
the dielectric variation in a concrete cover. The ability of concrete
to pass free electric charges under the influence of an applied elec-
tric or electrical conductivity of concrete also produced changes in
electromagnetic wave reflection. Dissolved cations such as chloride
ions (Cl ) in concrete pores will rapidly accelerate and randomly
collide with each other, which produces energy loss when exposed
to an electrical field from the radar. This situation causes
amplitude attenuation as the energy is converted to heat energy.
The degree of moisture and chloride content in concrete can be
estimated by analysing the direct and reflected waves in the time
domain. These two radar waves can be easily recognized and
identified by locating the wave peaks, as depicted in Fig. 1.
Direct waves (DW), which are usually termed as direct-
coupling, comprise the first radar wave energy to reach the
antenna receiver after propagating in an air medium and is
reflected by the top of the concrete. A direct wave is not a surface
wave, such as a Rayleigh wave, but is part of the radiating energy
that directly propagates along the air–concrete interface [26].
Depending on the radar frequency, this wave is only reflected by
the top surface of concrete to a few centimetres of its depth [27].
The remaining radar wave, which has less energy, propagates
through the concrete and is reflected by the concrete–air or con-
crete–metal interface with a contrast value in the electrical proper-
ties. According to reference [27], the depth of the object must be
greater than the antenna offset between the transmitter to the
receiver to prevent the inevitable ‘‘fuzzy zone” and an incorrect
propagation distance introduced by a very shallow object depth.
In this study, the proposed thickness of sample, D, is selected as
0.07 m (7 cm) to simulate the thin concrete cover dimension and
set to be greater than antenna radar offset distance (0.058 m) as
proposed by Klysz et al. [27]. Previous succesfull works had used
sample thickness ranging from 7 cm [28], 1 cm to 8 cm [29] and
8 cm [30] on relating the moisture content and chloride content
in concrete by GPR wave and they did not observed and reported
the overlapping of direct and reflected waves. Thus, based on these
research works, the minimum sample depth was selected as thin as
7 cm to ensure that correct and better signals can be captured
before analyses are conducted.
Both direct waves and reflected waves are displayed in a GPR
system as a two-dimensional plot of the image or waveform in real
time, which can be analysed to examine its amplitude and arrival
time. The time delay between the reflection and the direct wave
can be used to compute the dielectric constant, e of the concrete
and the wave velocity.
2. Materials and methods
2.1. Materials
Thirty-one unreinforced concrete slab samples with the dimensions of
0.25 m 0.25 m 0.07 m were prepared with a very low water-to-cement ratio
of 0.7 to simulate a high permeable concrete (i.e., deteriorated concrete). A mini-
mum depth of 70 mm is selected for the sample as this depth exceeds the antenna
offset of 58 mm. The mix design compositions, which are shown in Table 1, were
prepared based on reference [31]. The fine aggregates is obtained from natural river
sand and the coarse aggregate is crushed limestone (density of 2.7 g/cm3
) with sin-
gle size of 10 mm. Clean tap water was used for the samples mixture.
2.2. Sample preparation
In this study, seven samples were prepared to evaluate the effect of moisture
content on the radar amplitude, and twenty-four samples were prepared to inves-
tigate the effect of chloride content on radar amplitudes. All samples were initially
cured in the curing tank for 28 days. At the end of the curing period, the saturated
sample weights were recorded as wsat.
The effect of moisture content on the radar signals was evaluated using seven
samples. All samples, with the exception of the saturated sample, were dried in
an oven at 80 °C and weighted until a constant weight at the targeted degree of
moisture content, wdry was achieved. The percentage of water content in the sam-
ples x1 is determined using Eq. (2):
x1 ¼
wsat wdry
wsat
100 ð2Þ
In order to distribute homogenous moisture content across the slab cross sec-
tion, all samples were sealed by thin aluminium foils and dried in an oven at
80 °C for two months as shown in Fig. 2. The weight of each sample was checked
on a weekly basis to ensure no weight loss prior to the radar measurement.
The effect of free chloride content in concrete on radar signals was evaluated
using twenty-four samples. As with the previous samples, this sample was oven
dried until a constant sample weight was achieved to evaporate the pore water
inside the concrete pores. The samples were directly immersed in the distilled
water, which contained different chloride concentrations of 10 g/L, 20 g/L, 30 g/L,
40 g/L and 50 g/L for one month duration to ensure that the entire concrete cover
is filled with chloride ions. All samples were partially dried at 80 °C to achieve cer-
tain targeted degree of moisture content. In order to obtain homogenous distribu-
tion of moisture in each samples, all sampels were sealed by thin aluminium foils
and oven-dried for two months. The weight of each sample was regularly checked
to ensure no weight loss during this process.
2.3. Ground penetrating radar (GPR) signal measurement
The GPR signal acquisition was performed on the top surface of all samples
using SIR-3000 system with ground-couple 1.6 GHz, monostatic antenna. All sam-
ples were tested at a marked point in the middle of the samples with a metal plate
inserted beneath each sample. One hundred measurements of radar signals were
performed, and the average of each signal amplitude was used to represent the
measured amplitude of the radar to minimize the signal-to-noise ratio as the num-
Fig. 1. Identification of direct and reflected waves from the GPR scan on concrete.
Table 1
Sample mix composition of the slab.
Materials Mix composition
weight (kgm 3
)
Ordinary Portland Cement 402
Coarse aggregates (10 mm size, crushed limestone) 939
Fine aggregates (river sand, density 2.7 g/cm3
) 1058
Water 286
S.F. Senin, R. Hamid / Construction and Building Materials 106 (2016) 659–669 661

5. ber of measurements was increased. The radar reflection on the metal plate can be
deemed as the real radar reflection from the top surface of the rebar embedded in
the concrete structures. Each signal was sampled at a sampling frequency of 85 GHz
to prevent an aliasing effect on the digitized signals [32]. In this study, the ampli-
tude is digitized using a 16 bit format as integer ( 32, 768 to +32, 768). The noises
in the signals were excluded from the original signal by a signal filtering process
that was performed prior to the analysis.
The plot of radar signals produced by both conditions was performed using
MATLAB. The arrival of the direct waves (DW) can be easily identified as the first
peak positive amplitude of both situations. As a metal plate has infinite values of
dielectric, it is capable of reversing the signal amplitude polarity; the arrival of
reflected waves (RW) is identified by locating the phase reversal of the signal pro-
duced by the sample with the metal plate [33]. The peak-to-peak amplitude of DW,
which are denoted as DWPP and RWPP, correspond to the peak-to-peak amplitude of
reflected waves, were determined from waveforms and normalized to peak-to-peak
amplitude of direct waves or reflected wave signal propagates in the control sample
App to obtain the radar wave attenuation per meter, a as shown in Eq. (3):
a ¼
20
D
log10
Ac
App
ð3Þ
where Ac is either the peak-to-peak amplitude of the direct wave (DWpp) or the
peak-to-peak amplitude of the reflected wave (RWpp) for certain moisture and
immersed chloride content and D is the sample thickness. As concrete is a low-
loss material [34], the velocity of the radar wave, v can be computed using Eq. (4)
and the sample dielectric constant, e can be computed using Eq. (5):
v ¼
2d
ðt3 t1Þ
ð4Þ
v ¼
c
ffiffiffi
e
p ð5Þ
Fig. 2. Samples are sealed by thin aluminium foils.
DW
arrival
me , t1
RW
arrival
me , t3
Mulple GPR reflecon
Fig. 3. Waveforms of GPR amplitudes with various moisture content.
Table 2
Peak-to-peak amplitude and amplitude attenuation.
Moisture
content (%)
Ad (mV) Ar (mV) aDW (dB/m) aRW (dB/m) Remark
10.1 3958.50 6816.30 99.543 130.424 Saturated
9.4 4304.70 7028.00 89.140 126.629 –
7.1 4501.10 7968.10 83.604 111.051 –
5 4661.60 9804.10 79.256 85.321 –
3.2 5411.30 12394.50 60.751 56.230 –
1.7 5922.10 14792.70 49.559 34.281 –
0 8829.40 19499.90 0.0000 0.0000 Dry
Fig. 4. Waveforms of GPR amplitudes with 3.2% of moisture content.
Fig. 5. DW amplitude attenuation at various moisture content.
662 S.F. Senin, R. Hamid / Construction and Building Materials 106 (2016) 659–669

6. where t3 and t1 are the arrival time of the peak positive amplitude of the reflected
wave and the arrival time of the peak positive amplitude of the direct wave, respec-
tively; c is the speed of light in free space (0.3 m/ns) and e is the sample dielectric
value; d is the actual propagation distance (0.07577 m) and e is the sample dielectric
value.
3. Results and discussion
Six samples and one control sample were employed in this
study to observe the effect of moisture content on the peak-to-
peak radar amplitude; the remaining twenty-four samples were
analysed to determine the effect of free chloride on the peak-to-
peak radar amplitude.
3.1. Effect of moisture content on amplitude attenuation
3.1.1. Control sample
A dry control slab sample with no chloride contamination was
prepared in this study to serve as the reference sample. The control
sample in this study is a sample that exhibits GPR wave attenua-
tion as electromagnetic waves travel through the sample without
moisture and without free chloride content. The measured peak-
to-peak amplitudes of direct and reflected radar waves by the
GPR antenna is employed as the reference radar amplitude App to
be normalized to the measured peak-to-peak amplitude of the
remaining samples with varying moisture content and chloride
content.
3.1.2. Radar amplitude and peak-to-peak amplitude attenuation
The amplitudes of the radar signals of the samples with varying
moisture content were depicted as a series of waveforms in Fig. 3.
These waveform amplitudes comprised the average amplitude of
100 radar scans, which were conducted at the middle point of
the top slab surface. The control sample amplitude measurement
is represented by a black line waveform in Fig. 3. The peak-to-
peak amplitudes of the direct waves Ad and the reflected waves
Ar of each waveform and the amplitude attenuations are listed in
Table 2.
To compute the values of Ad and Ar in Table 2, a waveform with
a moisture content of 3.2% is employed; the calculation is shown in
Fig. 4. As shown in Fig. 3, the peak-to-peak amplitudes of the direct
waves and reflected waves are attenuated by 55.2% and 65%,
respectively, from the amplitude of its control sample as the mois-
ture content has increased from 0% (dry state) to 10.1% (saturated
state). This amplitude attenuation can be explained by the electro-
magnetic wave energy loss of the radiated waved from the trans-
mitter antenna. The moisture or free water that exists in
concrete pores absorb the energy in the waves and convert it to
heat energy.
The attenuation of the amplitude by moisture is analysed by
computing the radar wave attenuation using Eq. (3), as shown in
Figs. 5 and 6. Both plots demonstrate an acceptable linear relation-
ship (R2
= 0.82 and 0.97) with respect to the water content of the
samples; these findings are consistent with the findings obtained
by reference [35]. The direct wave amplitude attenuations were
Fig. 6. RW amplitude attenuation at various moisture content.
Fig. 7. Dielectric constant of the current work and others researchers.
Table 3
Wave velocity for different values of moisture content.
Moisture (%) Wave velocity (m/ns)
10.1 0.095
9.4 0.095
7.1 0.101
5 0.108
3.2 0.118
1.7 0.128
0 0.150
0
1
2
3
4
5
6
7
8
9
10
1 1 3 5 7 9 11 13
Amplitude
aenuaon
(dB),y
Water content (%), x
DW
Sbartai
Linear (DW)
Linear (Sbartai)
y= 0.4306x + 4.5633
y= 0.556x + 1.7196
Fig. 8. Comparison of current DW amplitude attenuation with [24].
S.F. Senin, R. Hamid / Construction and Building Materials 106 (2016) 659–669 663

7. decreased by 7.94 dB/m, whereas the reflected wave amplitude
attenuations were decreased by 12.70 dB/m. These reductions in
the amplitude attenuation with the increased moisture content
causes an increase in the dielectric constant, which affects the
GPR energy absorption attenuation, as shown in Fig. 7. The radar
wave propagates through the samples with a different wave veloc-
ity, v as the moisture content varies from a dry state to a saturated
state; the results are tabulated in Table 3. The wave velocity range
is consistent with the findings in reference [36]. When the electro-
magnetic wave travel is transmitted by the radar’s antenna
through the sample without any water content, it travels at a
higher speed with a certain degree of moisture content. This result
is attributed to the increasing dielectric value of the samples as the
degree of polarization increases as the moisture content is
increased. The experimental dielectric values ranged between the
values obtained by Zhang’s equation [25] and the values obtained
by Chen’s equation [13], as depicted in Fig. 7, which are consistent
with findings by other researchers. The moisture variation in the
samples influences the wave velocity, the moisture content and
the amplitude attenuation.
To compare this study with the work of other researchers, the
existing amplitude attenuation per meter results are multiplied
y = 0.8887x + 0.8057
y = 0.6467x + 0.1871
1
1
3
5
7
9
11
1 1 3 5 7 9 11
Amplitude
aenuaon
(dB),
y
Moisture content (%) , x
RW
Sbartai
Linear (RW)
Fig. 9. Comparison of current RW with reference [18].
Table 4
Peak-to-peak amplitude and amplitude attenuation.
Chloride content (g/L) Moisture content (%) Ad (mV) Ar (mV) aDW (dB/m) aRW (dB/m) Remark
0 0 8829.40 19499.90 0.00 0.00 Dry
10 10.1 3958.50 6816.30 99.54 130.42 Saturated
9.4 4304.70 7028.00 89.14 126.63
7.1 4501.10 7968.10 83.60 111.05
5 4661.60 9804.10 79.26 85.32
3.2 5411.30 12394.50 60.75 56.23
1.7 5922.10 14792.70 49.56 34.28
20 9.3 4961.50 5897.00 71.52 148.40 Saturated
4.9 5256.70 6345.00 64.35 139.30
4.7 5295.10 6581.90 63.45 134.77
4.4 5335.30 6652.30 62.51 133.45
3.2 6778.10 8838.10 32.81 98.19
0 8477.70 17078.10 5.04 98.19 Dry
30 12.1 3833.40 1941.60 101.92 286.25 Saturated
7.6 5135.60 3362.60 67.24 218.10
6.4 5508.80 4088.90 58.54 193.84
5.5 5114.45 5443.10 67.75 158.34
3.9 6516.30 7780.90 37.69 158.34
0 8985.20 20339.30 2.17 5.23 Dry
40 9 4077.90 1040.50 95.86 363.65 Saturated
5.7 4987.20 1598.80 70.88 310.35
5.2 5406.20 2300.20 60.87 265.22
4.4 6822.30 6860.90 32.00 129.62
2.9 6707.20 6439.20 34.11 137.49
0 8923.30 19066.10 1.31 2.79 Dry
50 10.2 3666.50 775.50 109.66 400.13 Saturated
5.9 4983.70 2198.40 70.97 270.84
10.2 3738.00 542.10 106.66 444.56
7.9 4132.90 497.70 94.19 455.16
7.4 4424.70 704.40 85.73 412.06
5.5 5180.80 1844.90 66.15 292.59
= 0.406x 87.275
R² = 0.314
107
102
97
92
87
82
77
72
0 10 20 30 40 50
Amplitude
aentuaon
(dB/m)
Free chloride content (g/L), x
Fig. 10. DW amplitude attenuation at various immersed free chloride content.
664 S.F. Senin, R. Hamid / Construction and Building Materials 106 (2016) 659–669

8. by the thickness of the sample as shown in Figs. 8 and 9. The exist-
ing DW and RW amplitude attenuation is similar to the findings in
reference [35]. The values of the DW amplitude attenuation in this
study are lower than the results in [35] with average of 66%,
whereas the RW results are higher than the values in [35] with
average of 41%. The lower value of amplitude attenuation pre-
dicted by the current linear relationship may be attributed to the
lower water content on the top surface of the concrete as com-
puted by Eq. (2). The lower water content on this surface will
absorb less radar energy and produce less DW attenuation. In the
case of the RW with a 1.6 GHz antenna, the absorption of radar
energy will exceed the absorption noted in reference [35], in which
a 1.5 GHz antenna was employed.
3.2. Effect of free chloride on amplitude attenuation
3.2.1. Radar amplitude and peak-to-peak amplitude
The amplitude values of the radar signals for the samples of var-
ious chloride content are listed in Table 4. Figs. 10 and 11 display a
plot of the amplitude attenuation of direct and reflected waves as
the immersed chloride content is varied from 0 g/L to 50 g/L for the
saturated samples. Both plots present linear relationships with sat-
isfactory determination coefficients for the reflected wave (R2
of
0.95), whereas the direct wave exhibited a very weak linear rela-
tionship (R2
of 0.31). This weak linear relationship may be attribu-
ted to the varying surface roughness conditions on the sample
surface which results in altering the GPR absorption rate character-
istics as compared with the very smooth surface sample condition.
The direct wave and reflected wave amplitude attenuation were
linearly decreased by 4.06 dB/m and 68.67 dB/m, respectively. This
wave attenuation can be explained by the increased mobility of the
free chloride ions in the moisture inside concrete pores, which
increases the ionic conduction and polarization. These findings
concluded that the attenuation of direct and reflected waves
increases as the chloride increases, which is consistent with the
findings by [37].
Figs. 12 and 13 show the comparison between the current DW
and RW amplitude attenuation with reference [35] after the cur-
rent amplitude attenuation values were multiplied by the thick-
ness of the sample D. The results indicate that both the DW and
the RW follow the same trend demonstrated by the moisture con-
tent case: the current DW amplitude attenuations were lower than
the values in [35] with average of 70% and the current RW ampli-
tude attenuations were higher than the values provided by [35]
with average of 18%. However, the slope value of the RW equation
(8.6735) is higher than the slope provided by [35]. Researchers
have hypothesized that the use of a higher antenna frequency will
contribute to higher radar energy absorption and produce a higher
slope value for RW.
3.3. Comparison of amplitude wave attenuation between water and
free chloride content
The comparison of the GPR amplitude wave in the previous sec-
tion between water content and free chloride content showed sig-
nificant amplitude attenuation. A comparison of the GPR
y = 0.0284x + 6.1092
y = 0.0533x + 10.13
0
2
4
6
8
10
12
14
0 10 20 30 40 50
Amplitude
aenuaon
(dB),
y
Free Chloride content (g/L),x
DW
Sbartai
Linear (DW)
Fig. 11. Comparison of current DW results with reference [24].
α = -6.8665x - 123.91
R² = 0.9507
-445
-395
-345
-295
-245
-195
-145
0 10 20 30 40 50
Amplitude
aenuaon
(dB/m)
Free Chloride Content (g/L), x
Fig. 12. RW amplitude attenuation at various immersed free chloride content.
y = 0.4807x + 8.6735
y = 0.3437x + 7.7396
0
5
10
15
20
25
30
35
0 10 20 30 40 50
Amplitude
aenuaon
(dB),
y
Free Chloride content (g/L), x
RW
Sbartai
Linear (RW)
Linear (Sbartai)
Fig. 13. Comparison of current RW with reference [24].
S.F. Senin, R. Hamid / Construction and Building Materials 106 (2016) 659–669 665

9. amplitude attenuation by both corrosive agents is shown in Fig. 14.
The comparison of the effect of both corrosion agents on RW indi-
cated that the chloride content attenuates the GPR amplitude at a
higher rate than moisture content. The results differ for the DW as
the moisture content attenuates the GPR amplitude at a higher rate
than chloride content. As the linear relationship between the direct
wave and the chloride content is very weak, i.e., R2
is 0.314, the
GPR attenuation in this case is not considered in the comparison
with the RW case. The chloride content in the sample attenuates
the GPR amplitude at a higher rate than the moisture content.
3.4. Development of models
3.4.1. Modelling equations for the attenuation of direct and reflected
waves
The thirty one samples were analysed and the radar amplitude
attenuation for direct wave and reflected waves were computed
based on Eq. (2). The amplitude attenuation of the radar signal
can be directly linked to the energy loss from the energy absorp-
tion by water molecules and chloride ions in the concrete pores.
Using Table 4, the plots of the direct wave and the reflected wave
attenuation, respectively, as the moisture and immersed free chlo-
ride content was varied are denoted by red points in Figs. 15 and
16. A multiple nonlinear regression equation algorithm was used
80 60 40 20 0
DW
RW
Wave aenuaon (dB/m)
Chloride
Moisture
Fig. 14. Comparison of moisture and chloride on wave attenuation.
Fig. 15. DW attenuation model for varying moisture and free chloride content.
Fig. 16. RW attenuation model for varying moisture and free chloride content.
666 S.F. Senin, R. Hamid / Construction and Building Materials 106 (2016) 659–669

10. to correlate the amplitude attenuation per meter, a with the
immersed chloride content x and the moisture in concrete y based
on the general form of the following relationship in Eq. (6):
a ¼ y0 þ ax þ by þ cx2
þ dy
2
þ exy þ ð6Þ
where y0, a, b, c, d and e are the fitted coefficients of the nonlinear
multiple regression using an iterative least squares algorithm and
is the error term. The multiple regression equations plot of both
waves are shown in Figs. 15 and 16.
Reference [35] revealed a linear relationship if the amplitude
attenuation is related to either moisture or free chloride content;
however, the radar amplitude attenuations in this study were
observed to behave nonlinearly with variations in the moisture
and free chloride content in concrete. The coefficient of multiple
determination of the multiple nonlinear regression equation R2
indicates close agreement between the computed amplitude atten-
uation and the predicted value. The direct wave multiple regres-
sion model and reflected wave multiple regression models
exhibit coefficient of determination of 0.917 and 0.948 respectively
3.4.2. Accessing the quality of the modelling equations
The quality of the modelling equations can be evaluated by
studying randomness of errors, the trend of errors and the nature
of the errors distribution. The randomness of the errors is quanti-
fied by the average variability of errors, whereas the trend pattern
of the residual show the direction of each deviation about the
mean value.
3.4.2.1. Constant variance and trend pattern test on residuals on the
modelling equations. To access the quality of the developed mod-
elling equations, plots of the errors in both models were analysed.
The error in the model, is defined as
¼ ap am ð7Þ
where, ap and am represent the predicted wave attenuation per
meter by the developed model and the measured wave attenuation
per meter from this study, respectively.
The scatter plots of the errors in direct wave attenuation model
and reflected wave attenuation model are shown in Figs. 17 and 18,
respectively. Both error terms fluctuated around zero values and
do not reveal a significant trend, which indicates that the errors
were not independent and random. The developed model satisfied
the assumptions of normality and equal variance and may be
employed for the prediction of radar wave attenuation [38].
3.4.2.2. Normal probability tests on residuals of the modelling
equations. The normal probability plots of residuals of direct wave
and reflected waves models were shown in Figs. 19 and 20 respec-
tively. Based on the normal probability plots, it appears that the
actual residual data points from both models lies almost to the the-
oretical normal probability lines. It was observed approximately
29% (9 points) and 13% (4 points) of the residuals plotted for direct
and reflected waves were not laying on the theoretical normal
probability lines. This is consistent with the coefficient of determi-
nation, R2
, from the previous section as direct wave models.
As a conclusion, the residuals data of direct wave and reflected
waves models behave randomly and satisfies the normally distri-
bution assumption.
3.4.3. The potential application of models on estimating moisture and
free chloride content in concrete cover
The potential assessment of the moisture and free chloride con-
tent in concrete cover can be estimated simultaneously using the
developed models shown in Figs. 15 and 16. In order to perform
the estimation of either moisture or free chloride content, the
developed modelling equation as shown in Fig. 15 will be
converted to contour plots as shown in Fig. 21. The radar signals
collection will firstly performed above the rebar location. The first
amplitude detected by the GPR antenna is analysed as the direct
Fig. 17. Residual plot of the DW residual. Fig. 18. Residuals plot of the RW residual.
Fig. 19. Normal probability plot of the direct wave residual.
S.F. Senin, R. Hamid / Construction and Building Materials 106 (2016) 659–669 667

11. wave whereas the signal that is reflected by the top rebar surface is
considered as the reflected wave.
The peak-to-peak amplitude each direct and reflected waves is
computed and divided to the peak-to-peak amplitude when the
GPR radar signal is measured in air as the normalized value of
direct wave, aDW, and reflected wave, aRW. The moisture content
of the concrete cover, x can be measured using the moisturemeter
or the free chloride content, y based on dust analysis. The intersec-
tion point between the aDW or aRW with either x or y value will be
marked on the contour plot and the estimate value of moisture or
chloride content can be determined by identifying the projected
value. The process on finding the estimate moisture or chloride
content is shown in Fig. 21 respectively. The same procedure can
be repeated on the direct wave to estimate the moisture or free
chloride content in the cover concrete.
4. Conclusions
The main contribution of this study is to provide a nondestruc-
tive evaluation of the moisture and chloride content in a concrete
structure, which is based on GPR direct and reflected wave ampli-
tudes. The results of this study are useful for monitoring the state
of chloride contaminant and moisture content in concrete struc-
tures that have been exposed to a chloride environment, based
on the measured radar amplitude attenuation. We highlight the
following results of this study:
1. The amplitude of GPR is significantly influenced by the amount
of moisture in concrete. The attenuation of the radar wave
amplitude exhibits a linear relationship between the direct
waves and the reflected waves measured in the moisture-
immersed slab samples.Reasonable correlation coefficients with
R2
= 0.82 and 0.97 were obtained. The reflected waves attenuate
more than direct waves for the varying moisture content in the
concrete slab samples.
2. Chloride content in concrete influenced the radar amplitude
attenuation higher than the moisture content in the samples.
This result is due to the free chloride ions that will conduct elec-
tricity produced by the GPR antenna’s electrical field and
increase the degree of the attenuation of the radar amplitude.
Linear relationships between the chloride content and the
amplitude attenuation of the direct and reflected waves were
observed. Correlation coefficients with R2
= 0.31 and 0.95 were
obtained. A weak correlation with the direct wave attenuation
was observed and may be attributed to the surface textures of
the sample.
3. Two nonlinear models to estimate the moisture and chloride
content in concrete were developed. The attenuation residuals
of the models are not dependent on both the MC and CC and
are random in nature, which satisfies the assumptions of nor-
mality and equal variance. The proposed models can be used
to estimate the moisture and chloride content in concrete
structures.
4. This field of research has the significant potential or impact on
monitoring and diagnosing the quality of concrete cover at site
by characterizing the amount of moisture and free chloride con-
tent using GPR. As concrete cover protects the structural ele-
ments and the reinforcement, the monitoring and evaluation
of moisture and chloride by using this method serve as the indi-
cator of structural quality. This quality evaluation helps the
engineers and contractors to decide the replacement program
on the concrete cover within its life cycle.
Acknowledgment
The authors acknowledge Universiti Kebangsaan Malaysia for
their financial support via the allocation grant underproject GUP-
2013-017 and DLP-2013-033.
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