Evaluation of Fire-Damaged Concrete: An Experimental Analysis based on Destructive and Nondestructive Methods
International Journal of Concrete Structures and Materials
Evaluation of Fire-Damaged Concrete: An Experimental Analysis based on Destructive and Nondestructive Methods
Hong Jae Yim
Fire damage to concrete causes contact-type defects that degrade its durability through impaired mechanical properties. Various nondestructive tests are used to evaluate defects induced by fire damage. Recently, nonlinear ultrasonic methods such as the nonlinear resonance vibration method and nonlinear modulation method have been introduced. These nonlinear methods are more sensitive to fire-induced contact-type defects than the linear ultrasonic method. This study involved an experimental analysis of the residual material properties of fire-damaged concrete, specifically, compressive strength, splitting tensile strength, and static elastic modulus. The residual material properties of 116 cylindrical concrete samples with various mix proportions and subjected to various heating temperatures were measured by a destructive method, and their nonlinearity parameters were measured by two nonlinear ultrasonic methods. Through regression analysis, correlated relationships that can facilitate the prediction of residual material properties of fire-damaged concrete using measured nonlinearity parameters were identified. In addition, the effect of fire damage on the mechanical strength of concrete was investigated by comparison with the relationships for undamaged concrete, and relationships for the evaluation of fire-damaged concrete were identified through regression analysis.
nonlinear ultrasonic method; fire-damaged concrete; mechanical property; correlation study
Although concrete is popular as a nonflammable material
with low thermal conductivity, thermophysical and
thermochemical alterations induced in concrete at high temperatures
can degrade its performance
(Bazant and Kaplan 1996)
the constituent materials of concrete have varying thermal
expansion coefficients, fire-damaged concrete has distributed
contact-type defects between the individual materials. These
defects manifest as openings and pores within the concrete,
and the nondestructive assessment of these defects can be
used to evaluate the extent of fire damage
(Yim et al. 2014)
Researchers have conducted experimental studies to identify
the effect of fire damage on the material properties of
concrete with various mix proportions under various fire
(Chang et al. 2006; Handoo et al. 2002; Lee et al.
2008; Tufail et al. 2016)
. It has been reported that high
temperatures degrade the material properties of concrete,
(Al-Nimry and Ghanem 2017; Dos Santos
et al. 2002; Li and Liu 2016)
. To evaluate the durability and
reusability of fire-damaged concrete, destructive and
nondestructive tests are performed in this study with various
An increase in contact-type defects within fire-damaged
concrete results in the degradation of mechanical properties
including stiffness and strength. Among the nondestructive
methods, ultrasonic measurement methods demonstrate high
potential and are applicable for evaluating damaged concrete
(Dilek 2007; Dilek and Leming 2007; Ham and Oh 2013)
These methods can be divided into two categories depending
on the measurement techniques and target defects: linear and
nonlinear ultrasonic methods. Linear ultrasonic methods,
which measure wave velocity, wave attenuation, and impact
echo, are conventional ultrasonic methods and have been
widely used to evaluate damaged concrete, particularly
(Chaix et al. 2003; Colombo and Felicetti
2007; Dilek and Leming 2007; Epasto et al. 2010; Kee and
Nam 2015; Yang et al. 2009)
. However, these linear methods
exhibit lower sensitivity to distributed defects and
contacttype defects at the micro-scale
(Jhang 2009; Park et al. 2015)
compared to nonlinear ultrasonic methods, which are more
sensitive to early-stage micro-scale defects as these defects
cause nonlinear behavior of an incident wave (Zaitsev et al.
2006). To investigate this phenomenon in concrete,
et al. (2010)
performed an experiment to characterize
microscale defects induced by an alkali-silica reaction by using a
nonlinear impact resonance acoustic spectroscopy technique.
Payan et al. (2007)
conducted nonlinear resonant
ultrasound spectroscopy to assess the damage to concrete
caused by exposure to high temperatures. Moreover,
recently, defects in fire-damaged concrete with various mix
proportions and under various temperature scenarios have
been evaluated by various nonlinear ultrasonic methods
(Park et al. 2015; Yim et al. 2014)
Park et al. (2015)
measured the hysteretic nonlinearity
parameter (HNP) of fire-damaged concrete with various mix
proportions and under various fire scenarios by using a
nonlinear resonance vibration method
(Van Den Abeele et al.
. The splitting tensile strengths of the samples were
also evaluated, and the correlation between both the
measurement results was reported. On the other hand, Yim et al.
(2014) measured the nonlinearity parameters of concrete
samples fabricated with mix proportions same as to those in
a previously mentioned study, using a nonlinear modulation
(Van Den Abeele et al. 2000b)
measurements of residual material properties such as
compressive strength, static elastic modulus, and peak strain were
also performed to obtain correlations with the nonlinear
parameter. Notwithstanding the numerous correlation studies
of fire-damaged concrete that have been reported to date,
several correlations remain to be analyzed. In this context,
the present study involves an experimental analysis to
evaluate the residual material properties of fire-damaged
concrete based on nonlinearity parameters measured using
two dissimilar nonlinear ultrasonic methods. To examine the
sensitivity of the two nonlinear ultrasound methods in
evaluating fire-damaged concrete of various mix proportions
at various temperatures, their results are compared, and
correlated relationships are proposed to assess residual
mechanical strengths using measured nonlinearity
parameters. In addition, the relationship between the compressive
strength and tensile strength of fire-damaged concrete is
proposed and compared with the corresponding relationships
for undamaged concrete.
2. Sample Preparation: Fire-Damaged
Four types of concrete samples were prepared for
destructive and nondestructive tests after fire damage. Each
type of concrete sample had a unique mix proportion
regulated by varying the water-to-cement weight ratios (w/cm)
and fine-to-coarse aggregate weight ratios for normal
strength concrete. Type I Portland cement was used to
produce all the samples with crushed gravel as coarse aggregate
(maximum size 19 mm) and fine aggregate (maximum size
4 mm). Additional admixtures or materials were not used in
any of the concrete samples. According to the various mix
proportions, the samples were labeled from C1 to C4, as
presented in Table 1. The concrete samples were cast into a
cylindrical shape with a height of 200 mm and diameter of
100 mm. Totally, 116 cylindrical concrete samples (29
samples for each mix) were then cast and cured for 28 days
prior to high temperature exposure.
For measurements using the nonlinear modulation method
and compressive strength test, 16 cylindrical samples,
including four reference specimens, were used for each mix.
On the other hand, for the nonlinear resonance vibration
method and splitting tensile strength test, 13 cylindrical
samples, including one reference specimen, were used for
each mix. Moreover, each cylindrical sample was divided
into five disk samples that were 25 mm in height.
Prior to exposure to high temperatures, the prepared
concrete samples were placed in a drying oven at 100 C for
approximately 24 h to avoid hygrothermal spalling, i.e.,
explosive spalling, during the fire experiment. The concrete
samples, apart from the reference samples, were then
exposed to various peak temperatures (200, 400, and
600 C). For the homogenization of temperature in the entire
volume of each sample, exposure to peak temperatures was
maintained for 2 h using an electric muffle furnace as it was
reported that heat conduction up to the center of a sample is
complete when it is subjected to 2 h of exposure
(Yim et al.
. After exposure, all the samples were cooled in water
at 20 C for 5 min to avoid unexpected recovery phenomena
during cooling, such as refilling of the fire-induced defects
due to rehydration under a high-humidity condition, and
were subsequently kept under an air-curing condition.
The degree of thermal damage in fire-damaged concrete is
influenced by several factors including exposure
temperature, exposure time, after-fire curing periods, and sample
sizes. Among these factors, it has been reported that the
after-fire curing period insignificantly affects the residual
material properties under an air-cured condition, while
firedamaged concrete under a completely saturated curing
condition appears to recover its mechanical strength
et al. 2015)
. Accordingly, for a correlation analysis, the
concrete samples in this study were air-cured after heating to
ignore the curing condition. Additionally, sample size
influences the degree of thermal damage; however, it has
been reported that varying the sample size negligibly
influences the measured nonlinearity parameters
(Yim et al.
. Therefore, this study attempts to correlate the
measured HNP of thin disk samples with the measured
nonlinearity parameter of the cylindrical samples.
3. Nonlinear Ultrasonic Methods
3.1 Nonlinear Resonance Vibration
The nonlinear resonance vibration method is a nonlinear
ultrasonic technique based on the measurement of the
resonance frequency shift. This implies that it measures the
frequency variation between the input and output signals,
which is a function of the degree of damage. This can be
described by the phenomenological model for hysteretic
nonlinearity proposed by Van den Abeele et al. (2000a). The
constitutive relationship for the elastic modulus K can be
expressed as follows
(Van Den Abeele et al. 2000a)
where a is the HNP, e_ is the strain rate, De is the strain
amplitude variation over the previous period, and signðe_Þ ¼
1 if e_ [ 0 or signðe_Þ ¼ 1 if e_\0. The hysteretic
nonlinearity of cement-based materials can be measured by
the hysteretic nonlinear characteristics, namely nonlinear
attenuation, harmonic generation, and amplitude-dependent
resonance frequency shift, which are sensitive to
contacttype defects. Therefore, these characteristics have been used
as damage indicators
(Chen et al. 2011; Les´nicki et al. 2011;
Van Den Abeele et al. 2000a)
Park et al. (2015)
the HNP to evaluate the degree of fire damage by using the
amplitude-dependent resonance frequency shift. The
resonance frequency shift with variation in the input
amplitude can be expressed as follows
(Van Den Abeele
et al. 2000a)
where f0 is the linear resonance frequency, and f is the
measured resonance frequency according to the magnitude
of the input amplitude. Moreover, HNP (a: obtained from the
amplitude-dependent resonance frequency shift) can be used
as the damage factor. The resonance frequency varies
linearly with increasing input amplitude, and the degree of shift
increases with damage. Therefore, the extent of fire damage
can be evaluated through an analysis of the
amplitude-dependent resonance frequency shift; further details are
available in the study by Van Den Abeele et al. (2000a).
For an experiment based on this relationship, thin,
diskshaped concrete samples (25 mm in height) were prepared
and placed on a soft mat, which provides a soft boundary
condition that maximizes the capability of the disks for free
vibration and minimizes noise from the external
experimental conditions. The experimental setup is illustrated in
Fig. 1. To generate impact excitation in the sample, a steel
bead (diameter 15 mm, 13.8 g) was dropped 20 times from
various heights onto the center of the sample; the intensity of
impact of the steel bead is to be unique for each attempt to
obtain the amplitude-dependent resonance frequency shift.
The vibration response induced by an impact was measured
by a shear piezoelectric accelerometer (PCB 353B15, PCB
Piezotronics Inc.) attached on the opposite side of the
sample. Through an analog-to-digital converter (NI PXI 4472-B,
National Instruments Corp.), the analog response signal was
converted to a digital signal with a sampling rate of 100 kHz
and duration of 50 ms. The vibration signal was then
converted to the frequency domain via fast Fourier transform
(FFT); a representative vibration signal and the
corresponding FFT result are presented in Fig. 2. The linear
resonance frequency (f0) is determined by linear regression
analysis, as the x-axis intercept of the FFT result. This is
because the linear resonance frequency, which is dependent
on the amplitude, is challenging to obtain by an impact test.
The HNP is then determined by measuring the amount of
resonance frequency shift according to the intensity of
impact based on the linear resonance frequency; further
details regarding the experimental procedure are available in
the study by
Park et al. (2015)
3.2 Nonlinear Modulation Measurement
The constitutive law of concrete materials follows a
nonlinear behavior and is expressed up to second-order
nonlinearity in a straightforward manner as follows
Abeele 1996; Van Den Abeele et al. 2000b)
r ¼ E0eð1 þ beÞ
where r is the stress, E0 is the Young’s modulus, e is the
strain, and b is the second-order nonlinear coefficient.
Nonlinear ultrasonic phenomena such as higher harmonics
mode or mixed frequency response are represented by wave
propagation through a solid medium
(Payan et al. 2010; Van
Den Abeele et al. 2000b)
. The mixed frequency response can
be obtained using two types of generating waves with
unequal frequencies and nonlinear wave modulation
spectroscopy. A low-frequency vibration (fl) and a
high-frequency stress wave (fh) are generated by the shaker and
ultrasonic transducer, respectively. When simultaneously
applying both low and high frequencies to a sample, the
lowfrequency wave causes modulation of the high-frequency
wave at the contact-type defects in the sample. This results
in an additional spectrum of the modulated wave at the
sideband range in the frequency domain. The sideband is
generally located at the sum and difference of both
frequencies (fl fh), and the spectral component of the
modulated frequency can represent the nonlinear behavior
induced by contact-type defects.
The amplitude of the sideband is proportional to the
amplitude of the low- and high-frequency generated signals
and the area of contact-type defects in the sample
et al. 2001)
. This implies that the nonlinearity parameter (D)
can be evaluated by the energy relationship of both high (Ph)
and low frequencies (Pl) and the sideband component (Ps).
Accordingly, the nonlinearity parameter that reflects the
degree of contact-type defects in the sample can be obtained
by the following expression
(Van Den Abeele et al. 2000b;
Warnemuende and Wu 2004)
Yim et al. (2014)
proposed an impact-modulation method
to identify the degree of thermal damage to concrete. The
experimental setup of this method is illustrated in Fig. 3. A
low-frequency vibration (f0), instead of a low-frequency
ultrasonic longitudinal wave, was generated using an impact
hammer with a soft tip (086C03; PCB Piezotronics, Inc.) to
create a resonance vibration mode in the sample
et al. 2001; Warnemuende and Wu 2004)
sinusoidal signals were generated using a function generator
(NI PXI-5421; National Instruments Corp.) with a sample
rate of 100 MS/s, and a power amplifier (BA4825; NF
Corp.) was used to amplify the generated signal. Two
longitudinal narrow-band transducers (Panametrics X1019;
Olympus NDT, Inc.) with a center frequency of 46.1 kHz
were used for transmitting and receiving the high-frequency
ultrasonic wave passing through the sample, and a tri-axial
accelerometer (356A33; PCB Piezotronics, Inc.) placed
opposite to the impact region measured the impact vibration
(low frequency) in the three orthogonal directions. The
measured high-frequency signal was digitized (NI
PXI5105; National Instruments Corp.) with a 60 MS/s sampling
rate (12 bit resolution), and the dynamic vibration was
measured (NI PXI-4472B; National Instruments Corp.) with
a sampling rate of 102.4 kS/s (24 bit resolution).
The spectral energy of the impact vibration (El) was
determined by the integrated power spectral density of the
measured signal of frequency up to 5 kHz. The spectral
energy of the sideband was obtained by integrating the
power spectral density of the ultrasonic signal of frequency
between 41.1 and 51.5 kHz, excluding the range of the
highfrequency waves (46.1 kHz). The nonlinearity parameter
was then obtained from the slope of the relationship between
the modulated energy (Es=Eh) and the impact vibration (El).
Thirty measurements per test for each sample were
conducted at an identical position to obtain a linear curve, and
ten independent tests were performed for reproducibility at
other arbitrary locations of impact and accelerometer
placement. The representative results of frequency
modulation achieved by simultaneously applying the low and high
frequency waves are presented in Fig. 4. The nonlinearity
parameter can then be obtained from the average of the
values from the 10 tests. It has been reported that the ratio of
increase of the nonlinearity parameter represents
contacttype defects developed because of thermal damage to the
concrete. A detailed description of the phenomena related to
this experiment is available in the study by
Yim et al. (2014)
4. Mechanical Strength Measurements
After obtaining the fire damage and nondestructive
measurements of the fabricated concrete samples, two types of
mechanical strength measurements were performed. The
thin-disk samples were used to obtain the splitting tensile
strength according to the degree of fire damage. Based on
the measured dimensions and mass of the samples, the
splitting tensile strength test was performed following the
procedure of ASTM C 496 (2011). As illustrated in Fig. 5,
the concrete sample was placed in the direction of the
diameter at the center of the bearing plates, and two wood
straps were placed between the bearing plates and the sample
to avoid interface cracking and to apply a distributed load
along the length of the disks. The splitting tensile strength
can then be obtained by the following expression:
where Ts is the splitting tensile strength, F is the maximum
applied load until failure of each sample, and l and d are the
length and diameter of the sample, respectively.
Park et al.
reported that the measured tensile strength decreased
with increasing fire damage to the concrete sample.
Following ASTM C39 (2001), the compressive strength was
also obtained using cylindrical concrete samples subjected to
varying degrees of fire damage, and the axial deformation
and uniaxial stress–strain curve were measured using
displacement transducers attached on the sides of the cylinder.
From this experiment,
Yim et al. (2014)
residual mechanical properties of fire-damaged concrete,
such as compressive strength, static elastic modulus
(calculated as the secant elastic modulus), and peak strain, with
respect to the degree of thermal damage. The reported results
indicated that increasing the fire damage to concrete
Ts ¼ pld
degrades the mechanical properties and induces a linear
5. Results and Discussion
Based on previous studies
(Park et al. 2015; Yim et al.
, this study performed an experimental analysis using
selected parameters of mechanical strength (compressive and
tensile strength) and measured nonlinearity parameters (HNP
and nonlinearity parameter); the initial and post-fire-damage
material properties are presented in Table 2. Figure 6
presents the increased ratios of the nonlinearity parameters as
measured by the two methods with various mix proportions
and under various heating temperatures; the results were
calculated as the ratio of the increase to the reference result
(20 C). To ascertain their experimental adequacy, the
sensitivity of the two nonlinearity parameters, as measured by
the nonlinear resonance vibration method and nonlinear
modulation method, were analyzed by comparing the results.
Initial (20 C)
Initial (20 C)
Initial (20 C)
Initial (20 C)
Fig. 6 Comparison of ratio of increase of the nonlinearity parameters at various temperatures. a Nonlinear resonance vibration
method and b nonlinear modulation method.
The ratios of increase measured by both the methods are
similar at approximately 200 C for all the mix proportions;
however, from 200 to 600 C, the ratio of increase measured
by the modulation method is substantially higher than that
measured by the resonance method. These trends are
presented by comparing the results for all the mix proportions.
The ratio of increase as measured by the nonlinear
modulation method increases noticeably with temperature
compared to that for the nonlinear resonance vibration method.
Based on the comparison, it can be concluded that although
the results of the nonlinear modulation method are more
scattered than those of the nonlinear resonance vibration
method, the nonlinear modulation method can better reflect
fire damage (contact-type defects) at high heating
temperatures than the nonlinear resonance vibration method.
In addition, a comparative analysis of compressive
strength and splitting tensile strength was performed at
various temperatures, as illustrated in Fig. 7. The trends of
the ratios of decrease of the compressive strength and
splitting tensile strength according to temperature appear to
be highly similar regardless of mix proportions. The ratio of
decrease of compressive strength reduces to approximately
30% at 200 C, which exhibits a noticeable variation from
that of splitting tensile strength, except for the C4 mix
proportion; however, the ratio of decrease of both the
strength measurements are similar at 400 C (approximately
60% of undamaged strength). On the other hand, the ratio of
decrease of the splitting tensile strength reduces to
approximately 20% of the undamaged strength, at 600 C; the
degree of decrease is substantial when compared to the
decrease in compressive strength (approximately 50%). This
implies that the splitting tensile strength marginally
decreased compared to the compressive strength until
200 C and then reduced remarkably at 600 C. While the
compressive strength significantly decreased at a low
temperature (200 C), this decrease weakened with increasing
temperature. It is determined that reactions in concrete, such
as water evaporation in concrete and dehydration of the
cement gel, are initiated up to 200 C
(Bazant and Kaplan
. The experimental results that exhibited a relatively
significant reduction in compressive strength at a low
heating temperature indicate that these reactions in concrete have
a marginally higher effect on the compressive strength of the
concrete material than on is splitting tensile strength. On the
other hand, above 300 C, the cement paste undergoes
morphological alterations, and the skeleton does not appear
to be a continuous medium owing to its chemical
dehydration caused by the continuously increasing temperature
et al. 2012)
. These phenomena are likely to exert a higher
influence on the reduction of splitting tensile strength than
the reduction of compressive strength at a high heating
temperature. Accordingly, the various ratios of decrease of
mechanical strength can be used as a guideline to evaluate
the heating temperature during a fire.
The experimental results of previous studies have revealed
the relationship between the residual material properties and
the parameters measured by a linear ultrasonic method
et al. 2009)
and that between the residual material properties
and the nonlinearity parameters obtained from nonlinear
(Park et al. 2015; Yim et al. 2014)
nonlinearity parameters measured by both the nonlinear
methods were also correlated with the measured mechanical
strength of the fire-damaged concrete: the nonlinearity
parameter versus compressive strength; and the HNP versus
tensile strength. As a departure from previous studies, this
study presents new correlated relationships between the
residual material properties and the measured nonlinearity
parameters; this area is being addressed for the first time.
Figure 8 presents the correlation between the ratio of
decrease of the residual material properties, specifically
compressive strength, splitting tensile strength, and static
elastic modulus, and the ratio of increase of the nonlinearity
parameters as measured by each of the nonlinear ultrasonic
methods. These correlations are expressed after excluding
the effect of mix proportions and exposure times. The
dashed lines in Fig. 8 represent the relationships between the
two variables (nonlinearity parameter and mechanical
property) determined by regression analysis; the results
exhibit a consistent tendency notwithstanding the omission
of exposure time and mix proportion. The obtained
regression results are expressed as Eqs. (6)–(8), which
correspond to Fig. 8a–c, respectively:
where the subscript n corresponds to each experiment under
various thermal conditions, and the subscript 0 corresponds
to the experiment under the initial condition without
damage. Moreover, fc0 , ft, a, and D are the compressive strength,
splitting tensile strength, hysteretic nonlinearity parameter,
and nonlinearity parameter, respectively.
These relationships for all the thermal damage cases are
adequately described by a negative power function. In the
initial phase, the ratios of the mechanical properties
exhibited a remarkable decrease until the nonlinearity parameter
ratios increased to 10 for the HNP (a) and 200 for the
nonlinearity parameter (D) (approximately before 400 C);
this is illustrated in Fig. 8. In the following phase, relatively
widespread values of the nonlinearity parameters are
apparent with marginal variations in the mechanical
properties. Generally, this widespread section can be attributed to
the fire-damage to the concrete when subjected to a
temperature of approximately 600 C. This implies that the
variations in the nonlinearity parameter ratios increase with
temperature to a significantly higher degree than the
variations in the strength ratios and static elastic modulus ratio. In
addition, it can be observed that the compressive strength
ratio, splitting tensile strength ratio, and static elastic
modulus ratio converged to approximately 0.5, 0.25, and 0.1,
respectively. This implies that among the three parameters,
the static elastic modulus is most sensitive to elevation in
temperature, followed by the splitting tensile strength and
It may also be feasible to correlate the measured
nonlinearity parameters with the mechanical properties regardless
of the mix proportions used in this study. Therefore, the
residual material properties of concrete induced by fire
damage can be evaluated using these criteria under the
assumption of the used mix proportions. For example, the
residual compressive strength of fire-damaged concrete can
be estimated by measuring the HNP via the nonlinear
resonance vibration method using thin concrete disks.
An additional correlation study based on the ratios of
decrease of the compressive strength and splitting tensile
strength (measured mechanical strengths) of the
fire-damaged concrete was performed, as illustrated in Fig. 9a. The
ratio of compressive strength is proportional to that of the
splitting tensile strength, and the optimized regression result
is expressed as follows:
Fig. 8 Correlation between ratio of decrease of the residual material properties and ratio of increase of the nonlinearity
fc0n ¼ 0:47
This study also investigated the effect of fire damage on
the relationship between the residual compressive strength
and splitting tensile strength of concrete (dashed line in
Fig. 9b) through a comparison with the relationship collated
and provided by
, which has been tested by
relationship between compressive strength and tensile strength
solid line in Fig. 9b); however, it was obtained through
regression analysis of measurements of undamaged concrete
samples, and the relationship for fire-damaged concrete
samples has not been reported yet. In the case of concrete
with low fire damage, the relationship follows the trend of
the result for undamaged concrete proposed by
. : On the other hand, as fire damage in the concrete
sample increase, the fire-damaged relationship represented a
steeper decline compared to undamaged relationship, which
yields the following expression:
ftn ¼ 3:78 lnðfc0nÞ
The regressed relationships illustrate that fire damage
degrades compressive strength more than it degrades tensile
strength. Furthermore, based on Eqs. (9) and (10), the
residual compressive strength of fire-damaged concrete can
be estimated from the measured value of splitting tensile
strength, and vice versa; thus, it is advantageous to select the
more efficient method based on the circumstances.
This study involved an experimental analysis of the
nonlinearity parameters measured by two nonlinear ultrasonic
methods. For the evaluation of fire-damaged concrete, the
nonlinearity parameter measured by the nonlinear
modulation method provided higher sensitivity than that measured
by the nonlinear resonance vibration method. The measured
nonlinearity parameter can be affected by experimental
conditions such as the sample size, the impact type and
location of impact that is generated, wave reflection, and
boundary condition of the sample. Accordingly, the
experimental results of this study, rather than representing general
relationships, can provide a guideline on the relationship
between the mechanical properties of fire-damaged concrete
and nonlinearity parameters measured by the two
nondestructive methods. Based on the results of these experimental
methods, correlated relationships were proposed to evaluate
the residual properties of fire-damaged concrete using the
measured nonlinearity parameter. From these relationships,
the residual compressive strength, tensile strength, and static
elastic modulus after being exposed to fire can be estimated
using the measured nonlinearity parameters without
considering the mix proportions of the concrete. In addition, the
effect of fire damage on the mechanical strength of concrete
was investigated by a comparison of undamaged
relationships, and relationships determined through
regression analysis were proposed for fire-damaged concrete.
This work was supported by the National Research
Foundation of Korea (NRF) grant funded by the Korea
government (MSIP) (No. 2015R1C1A1A01055474).
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