Physicochemical and mechanical changes of thermally damaged cement pastes and concrete for re-curing conditions

Cement and Concrete Research 125 (2019) 105831

Contents lists available at ScienceDirect

Cement and Concrete Research journal homepage: www.elsevier.com/locate/cemconres

Physicochemical and mechanical changes of thermally damaged cement pastes and concrete for re-curing conditions

T

Hong Jae Yima, Sun-Jong Parkb, Yubin Junc,

⁎

a

Department of Civil Engineering, Pusan National University, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea Department of Structural System and Site Evaluation, Korea Institute of Nuclear Safety, Daejeon 34142, Republic of Korea c Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, Daehak-ro 291, Yuseong-gu, Daejeon 34141, Republic of Korea b

ARTICLE INFO

ABSTRACT

Keywords: Thermally damaged concrete Post-heating curing condition Rehydration Nondestructive evaluation XRD

Thermally damaged concrete recovers its material properties under specific post-heating curing conditions. Among the previous studies to investigate the effects of post-heating curing, the nonlinear resonance vibration method was recently proposed to evaluate contact-type defects of concrete induced by high temperature although the use of physicochemical analysis is not reported to investigate the rehydration products of concrete on post-damage curing conditions. This study performed destructive and nondestructive tests, such as the measurement of tensile strength, dynamic elastic modulus, and hysteretic nonlinear parameter, to evaluate the degree of thermal damage of concrete and its restoration. Various types of concrete samples were prepared according to the different exposures of high temperature and relative humidity for re-curing. In addition, XRD analysis of heated cement pastes was performed to identify the formed and disappeared hydration products, and physicochemical characteristics of thermally damaged concrete was discussed with respect to various exposed temperature and re-curing conditions.

1. Introduction Although concrete is known as representative construction material with a superior fire resistance, it is easily damaged in fire accidents. An unexpected fire generates several types of damage including degradation in mechanical properties (such as compressive and tensile strengths and modulus of elasticity) with enhanced microcracks in interfacial zone and paste. The degree of the fire damage can be determined by exposed temperature, exposed time, or exposed area, etc. Previous studies [1–3] reported on the phenomena of thermally damaged concrete materials based on microstructural changes such as evaporation of free water in pores at approximately 100 °C; dehydration of chemically combined water in cement gel phases at approximately 180 °C; decomposition of Ca(OH)2 at approximately 500 °C; phase change of quartz (α-quartz to β-quartz) at approximately 570 °C; and decarbonation of CaCO3 and collapse of calcium silicate hydrate (C-S-H) at approximately 700 °C. With the microstructural damage, physical defects are also generated in interfacial zone due to different thermal expansion coefficients of constituent materials, and the phenomenon is identified as contact-type defects [4–9]. Prior to determining the reuse or repair of fire-damaged structure, it is necessary to evaluate the degree of fire damage in concrete, and thus, several nondestructive methods are specifically proposed using ultrasonic waves. In a previous experimental study, nonlinear ultrasonic measurements were verified as a useful method to sensitively represent the ⁎

occurrence of microcracks and contact-type defects by thermal damage in concrete materials [10–12]. Among nonlinear ultrasonic methods, the measurement of nonlinearity via impact modulation method and nonlinear vibration method is used to evaluate a contact-type defect introduced via fire damage. Yim et al. [6] reported that the measured nonlinearity by nondestructive method can correlate with the material properties obtained by destructive method, such as compressive and tensile strengths, and elastic modulus. Evaluation of the residual material properties is an important process in the post-heating state of the concrete structures, and it also required to continuously determine the reuse or repair of damaged concrete. Material properties including mechanical strength of thermally damaged concrete materials are recovered under a specific condition. It is known that recuring aids in the recovery of thermally damaged concrete and that the recuring degree depends on the grade of thermal damage (i.e., exposed temperature and time) and environmental condition of re-curing. It is reported that in the post-heating behavior of concrete structures, concrete regains its degraded material properties in the post-heating curing regimes [13–15]. In a previous study, a high humidity condition leads to the recovery of the material properties of thermally damaged concrete due to the rehydration of constituent materials in concrete [16]. Poon et al. reported that the compressive strength of thermally damaged concrete at 600 °C was recovered up to 93% under 56 days of water re-curing [14]. Under a sufficient moisture supplement, the C-S-H phase is

Corresponding author. E-mail address: [email protected] (Y. Jun).

https://doi.org/10.1016/j.cemconres.2019.105831 Received 12 December 2018; Received in revised form 3 June 2019; Accepted 24 July 2019 Available online 26 August 2019 0008-8846/ © 2019 Elsevier Ltd. All rights reserved.

Cement and Concrete Research 125 (2019) 105831

H.J. Yim, et al.

mostly rehydrated, and hydration of a few other components, such as unhydrated cement grains and lime, occurs [14,17]. The rehydrated or hydrated products refill the thermally induced contact-type defects in the interfacial zone between cement paste and aggregates, cement paste matrix, and aggregates themselves. The rehydration and process of thermally damaged concrete are validated by microscopic analysis using scanning electron microscopy (SEM), polarizing and fluorescent microscopy, and Xray computed tomography for three-dimensional investigation [14,16–18]. In addition, X-ray diffraction (XRD) is used to identify physicochemical, mineralogical, and morphological changes in the concrete after exposure to high temperatures [19,21], and microstructural changes of hardened cement paste under high temperatures is also investigated using XRD [20]. The rehydration process of thermally damaged concrete was also investigated using XRD with thermogravimetry [22]. Lin et al. [23] utilized the ultrasonic pulse velocity (UPV) method to evaluate the recovery of residual compressive strength in thermally damaged concrete and reported a relationship between residual strength and UPV during post-heating curing. Park et al. [8] investigated the tensile strength of thermally damaged concrete using nonlinear resonance vibration method and proposed a correlated relationship between the residual tensile strength and hysteretic nonlinearity parameter. Recovered mechanical strength due to self-healing of microcrack is also observed via changed ultrasonic velocity [24,25]. The nondestructive experimental results reflect a recovery of mechanical properties in thermally damaged concrete through re-curing process. It is potentially represented due to rehydrated minerals, but the reason for recovered mechanical properties and nonlinearity parameter is not investigated from a physicochemical aspect. The objective of this study is to investigate the recovery of thermally damaged concrete and cement paste with respect to re-curing conditions after exposure to elevated temperatures using destructive and nondestructive methods with physicochemical analysis. Thus, three postheating curing conditions were applied on concretes exposed at three different temperature conditions. Tensile strength, dynamic elastic modulus, and the nonlinearity parameter via the nonlinear resonance vibration method, a promising nondestructive method having sensitivity on thermally induced contact-type defects, were measured on thermally damaged and re-cured concretes samples to analyze the degree of thermal damage and its recovery. Additionally, the XRD analysis was performed for cement pastes with the same water to cement ratio as that of concrete, and changes in the hydration products were discussed with respect to exposed seven peak temperatures and different re-curing conditions. Experimental results with investigated physicochemical characteristics can provide scientific criteria to decide between repair and reuse of thermally damaged concrete.

Table 2 Chemical compositions of Portland cement (oxides in wt.%).

160

320

922

922

SO3

Fe2O3

MgO

Na2O

TiO2

MnO

Others

65.65

17.35

4.47

1.09

3.34

3.45

3.52

0.16

0.30

0.16

0.51

E ( , ) = E0 {1

[

+ (t ) sign ( ) +

]}

(1)

where, α denotes the hysteretic nonlinearity parameter (HNP), denotes the strain rate, ∆ε denotes the strain amplitude change over the previous period, and sign( ) = 1 if > 0 or sign( ) = −1 if < 0. The hysteretic nonlinearity of cement-based materials was used as a damage indicator based on the measured nonlinear characteristics such as energy attenuation, harmonic generation, and resonance frequency shift [27–29]. Specifically, the measured HNP from amplitude-dependent resonance frequency shift sensitively interacts with thermally damaged concrete including contact-type defects that occurred in all directions [7–9]. The resonance frequency shift with variation in the input amplitude is expressed as follows:

Table 1 Mix proportion of concrete sample (kg/m3). Coarse Aggregates

K2 O

2.2.1. Nonlinear resonance vibration method Nonlinear resonance vibration method is a nonlinear ultrasonic technique that measures the resonance frequency shift of a sample. Van Den Abeele et al. [26] proposed a hysteretic nonlinearity based on the phenomenological model that represents a degree of damage including randomly distributed microcracks and contact-type defects. The constitutive relationship by contact-type defects is expressed for degraded elastic modulus E as follows:

The mix proportion of used concrete sample is shown in Table 1. Waterto-cement ratio is 0.5, and fine aggregate to coarse aggregate ratio is 1. The fine aggregates corresponded to natural river sand with the maximum size of 4 mm, and the coarse aggregates corresponded to crushed gravel with a maximum size of 19 mm. Type I Portland cement was used in this study, where used cement is a 32.5 N type having normal strength class. The chemical composition of the cement is shown in Table 2 and is obtained from an X-ray fluorescence analysis. The fresh concrete was cast in a 100 × 200 mm cylindrical mold. Concrete samples were cured in water during 28 d to reach the design strength. After water curing, cylindrical samples were sliced as disc-type specimen of 25 mm-thick specimens, and

Fine Aggregates

Al2O3

2.2. Testing details

2.1. Experimental design and sample preparation

Cement

SiO2

subsequently all concrete disc were dried at 80 °C for 24 h to avoid explosive spalling. The dried specimens were exposed in an electric furnace for 1 h at three different high temperatures corresponding to 300 °C, 500 °C, and 700 °C, where electric furnace was heated in advance at target temperature before exposing specimens. The exposure time of 1 h was determined for sufficient thermal damage of the internal part of a sample [8]. After heating, the specimens were cooled in water for 5 min and dried for 24 h in a convection oven. The specimens were subsequently re-cured at a temperature of 20 °C with three different relative humidity (RH) conditions (10%, 60%, and 90%) for 7 d. Here, a thermos-hygrostat having recuring space of 1 × 1 × 1 m3 was used to maintain constant humidity and temperature. The re-curing conditions and experimental sequence for the concrete specimens are listed in Table 3. All concrete specimens were tested by nondestructive and destructive methods to obtain the nonlinearity parameter, dynamic elastic modulus, and residual tensile strength. The test results are reported as an average for the four duplicated samples. For the XRD experiment, additional samples exposed to high temperatures were fabricated. To identify an effect of rehydration through cement particles, cement paste with the same water-to-cement weight ratio as that of the concrete sample (Table 1) was prepared. All procedure for preparation of cement paste samples and its dimension were carried out in the same way of concrete samples. The experimental procedure for the cement paste sample was identical to that of the concrete sample with the exception of high temperature exposure. Table 4 lists the post-heating curing conditions and experimental sequence for the cement paste samples. As shown in Table 4, fabricated cement paste is exposed to an increased range of high temperatures (from 200 °C to 800 °C) than the exposure temperature conditions applied to concrete samples to identify a characteristic temperature range in thermal decomposition of the cementitious phase. The cement pastes samples were subsequently re-cured at a temperature of 20 °C with three different RH values (10%, 60%, and 90%) for 7 d.

2. Experimental details

Water

CaO

f0

f f0

=

(2)

where, f0 denotes the linear resonance frequency, f denotes the 2

Cement and Concrete Research 125 (2019) 105831

H.J. Yim, et al.

Table 3 Post-heating curing conditions and experimental sequence for concrete samples.

Sample Label

I

II

III (Exposed Temp.)

IV

V

VI (Re-curing Condition)

REF

Curing in water for 28 d

Drying at 80 °C for 24 h

-

-

-

-

300 °C for 1 h

Cooling in water for 5 min

Drying in an oven for 24 h

-

C0

500 °C for 1 h 700 °C for 1 h 300 °C for 1 h

C1

20 °C, 10% RH for 7 d

500 °C for 1 h 700 °C for 1 h 300 °C for 1 h

C2

20 °C, 60% RH for 7 d

500 °C for 1 h 700 °C for 1 h 300 °C for 1 h

C3

20 °C, 90% RH for 7 d

500 °C for 1 h 700 °C for 1 h

Table 4 Post-heating curing conditions and experimental sequence for cement paste samples.

Sample Label

I

II

III (Exposed Temp.)

IV

V

VI (Re-curing Condition)

REF(P)

Curing in water for 28 d

Drying at 80 °C for 24 h

-

-

-

-

Cooling in water for 5 min

Drying in an oven for 24 h

-

I2 I3 I4 I5 I6 I7 I8 L2 L3 L4 L5 L6 L7 L8 M2 M3 M4 M5 M6 M7 M8 H2 H3 H4 H5 H6 H7 H8

200 °C for 1 h 300 °C for 1 h 400 °C for 1 h 500 °C for 1 h 600 °C for 1 h 700 °C for 1 h 800 °C for 1 h 200 °C for 1 h 300 °C for 1 h 400 °C for 1 h 500 °C for 1 h 600 °C for 1 h 700 °C for 1 h 800 °C for 1 h 200 °C for 1 h 300 °C for 1 h 400 °C for 1 h 500 °C for 1 h 600 °C for 1 h 700 °C for 1 h 800 °C for 1 h 200 °C for 1 h 300 °C for 1 h 400 °C for 1 h 500 °C for 1 h 600 °C for 1 h 700 °C for 1 h 800 °C for 1 h

measured resonance frequency according to the magnitude of the input amplitude, and HNP (α: obtained from the amplitude-dependent resonance frequency shift) is computed by measuring the resonance frequency shift and the strain amplitude in proportion to the maximum acceleration of the sample. This implied that the resonance frequency varies linearly with increases in the input amplitude and the degree of

20 °C, 10% RH for 7 d

20 °C, 60% RH for 7 d

20 °C, 90% RH for 7 d

shift increases with damage, and it evaluated the extent of thermal damage by contact-type as damage factor. Experimental setup for nonlinear resonance vibration method is shown in Fig. 1. For determining the nonlinear resonance characteristics of concrete samples, the concrete samples were placed on a support mat to minimize the effects of noise, and free vibration of each 3

Cement and Concrete Research 125 (2019) 105831

H.J. Yim, et al.

Fig. 1. Schematic of the experimental setup: impact resonance method.

concrete sample was induced by the free drop of a steel bead (with a diameter of 15 mm and mass of 13.8 g). It was measured by a piezoelectric shear accelerometer (PCB electronics, 353B15; PCB Piezotronics Inc.) that was attached to the center of the sample, and the signal was recorded by an analogue-to-digital converter (NI PXI 4472-B; National Instruments Corp.) and a control PC. In this study, the amplitude-dependent resonance frequency was measured to determine the extent of thermal damage, and thus the steal bead was separately dropped 20 times by varying the height of free fall. 2.2.2. Dynamic modulus of elasticity From the experimental setup for nonlinear resonance vibration method, dynamic elastic modulus of samples was also measured at sampling rates of 100 kS/s and a measuring duration of 50 ms. Hence, the resonance frequency was obtained from the frequency domain result that was converted by fast-Fourier transform (FFT). Prior to performing the FFT, the length of data was extended 99 times with zeros (zero-padded) to improve the frequency resolution. The measured resonance frequency of each sample influenced the generated strain amplitudes under hysteretic characteristics of concrete in the stress–strain relationship, and it also reflected the degree of thermal damage in cement-based materials. For more apparent measurement of dynamic elastic modulus, a previous study proposed the improved impact resonance method [9]. The identical resonance frequency of the sample was determined from the minimized experimental errors. Additionally, from the given resonance frequency of the thin disk-shaped concrete samples, the dynamic elastic modulus is evaluated as follows:

Ed = 2(1 + )

fd

2

(3)

where, Ed denotes the dynamic elastic modulus, ν is Poisson's ratio, ρ denotes the density of the sample, f denotes the fundamental circular natural frequency of sample, d denotes the sample diameter, and Ω denotes the dimensionless frequency parameter of fundamental vibration mode. Details of the measurement system are described in previous studies [9].

Fig. 2. Representative results of resonance frequency shifts depending on acceleration considered as a mean of input amplitude (REF, C0300 °C, C0500 °C, and C0700 °C represent the undamaged and damaged cases after exposure at 300 °C, 500 °C, and 700 °C, respectively). Table 5 Average values with standard deviation and decreasing ratios of the hysteretic nonlinearity parameter (HNP).

2.2.3. Tensile strength After measuring the HNP and dynamic elastic modulus, degradation in the mechanical properties of concrete sample due to thermal damage and re-gained properties were measured by the splitting tensile strength (Ts) test that was performed on each concrete sample in accordance with ASTM C 496 guidelines (ASTM, 2011) [30]. This is as follows:

Ts = 2F / ld

Sample Label REF C0 C1 C2 C3

(4)

where, F denotes the applied load on sample, l denotes the length of the sample, and d denotes the diameter of the sample.

⁎

2.2.4. X-ray diffraction (XRD) To observe the characteristics of hydration products without aggregates, cement paste samples that exhibit the same water-to-cement ratio as that of the concrete samples were subjected to an XRD test after exposure to elevated temperatures. Thermally damaged cement paste samples on Table 4 were finely powdered and examined by XRD analysis. The XRD patterns for cement pastes were collected using a Rigaku high power X-ray diffractometer. All data were collected as follows: 5–60° 2θ range with a step size of 0.01° and a collection time of 1 s. Specifically, Cu Kα radiation was used as the X-ray source. The acquired data were analyzed using X'pert HighScore Plus program [31] with the

⁎⁎

⁎

300 °C⁎⁎

500 °C⁎⁎

700 °C⁎⁎

HNP (×10-6)

Ratio

HNP (×10-6)

Ratio

HNP (×10-6)

Ratio

7.45 ± 0.98 6.46 ± 0.65 6.33 ± 0.88 5.94 ± 0.60

16.56 14.36 14.07 13.20

10.61 ± 1.62 9.80 ± 1.10 9.87 ± 1.04 8.34 ± 1.46

23.58 21.78 21.93 18.53

36.19 ± 5.74 35.29 ± 2.96 35.23 ± 21.12 30.63 ± 14.88

80.42 78.42 78.28 68.06

HNP for REF sample is 0.45 ± 0.07 (×10-6). Exposure temperature.

ICDD (International Center for Diffraction Data) PDF-2 database [32] and the ICSD (Inorganic Crystal Structure Database) [33]. 3. Results and discussion The HNP of concrete samples are computed based on the measured resonance frequency shifts. The resonance frequency shifts of REF and C0 samples are representatively shown in Fig. 2. From the experimental results, 4

Cement and Concrete Research 125 (2019) 105831

H.J. Yim, et al.

HNP is calculated based on Eq. (2) for each sample. The obtained HNP for each sample before (REF sample) and after exposure to elevated temperatures (C0 sample) and after re-curing (C1, C2, and C3 samples) are listed in Table 5. They correspond to the averaged result of four duplicated specimens with standard deviation, and the ratios listed in Table 5 represent the increased HNP of thermal damage and re-cured samples when compared with the HNP of the REF sample, which is (0.45 ± 0.07) × 10−6. For example, the HNP of the 300 °C, 500 °C and 700 °C exposed sample (C0) is 16.56 times, 23.58 times, and 80.42 times, respectively, that of the REF sample. Previous studies noted that increases in the HNP reflect a higher degree of opening and contact-type defects around the interfacial zone due to the thermal damage of concrete [5,18]. It is observed that after re-curing (temperature of 20 °C, RH of 10%, 60%, or 90% and duration of 7 d), the samples exhibit decreases in HNP when compared with that of the C0 sample for all exposed temperatures. For example, the HNP of C1300 °C, C1500 °C, and C1700 °C, which is re-cured in 10% RH, is 14.36 times, 21.78 times, and 78.42 times, respectively, that of the REF sample, and the results of C2 (re-cured in 60% RH) are similar with C1 for each exposure temperature (Table 5). This implies that the recovery of contact-type defects occurred in re-cured samples after thermal damage, and its recovered degree is increased with increase of moisture in the air around samples. In case of sample C3 (re-cured in 90% RH), the HNP of C3300 °C, C3500 °C, and C3700 °C is 13.20 times, 18.53 times, and 68.06 times, respectively, that of the REF sample. It can be seen that the HNP of C3 re-cured in 90% RH after exposure at 700 °C (30.63 ± 14.88 × 10−6) is lower about 20% than the HNP of C0 exposed at 700 °C (36.19 ± 5.74 × 10−6). Although the HNP of the re-cured sample does not completely recover back into the original HNP of the REF sample, it is observed that the higher RH (C1 → C2 → C3) leads to decreases in the HNP. Hence, it is concluded that high humidity in recuring condition can significantly aid in the recovery of sample, and the phenomenon is evaluated by the nonlinear resonance vibration method. Recovered mechanical properties of thermally damaged concrete including dynamic elastic modulus and tensile strength are compared based on the average values of five duplicated samples. Table 6 shows the obtained dynamic elastic modulus and relative ratios of thermally damaged or re-cured sample (C0, C1, C2, and C3) to those of the REF sample for dynamic elastic modulus. As shown in Table 6, dynamic elastic modulus of C0 sample after heating as 300 °C, 500 °C, and 700 °C is 11.39 ± 1.35 GPa, 2.56 ± 0.47 GPa, and 0.31 ± 0.05 GPa, respectively. This means that the dynamic elastic modulus decreases with increases in the exposed temperature when compared with that of the REF sample. Generally, the dynamic elastic modulus of the re-cured samples slightly recovers although the effect of re-curing is more pronounced than that for the sample exposed at 700 °C and re-cured in 90% RH. The dynamic elastic modulus of 700 °C exposed sample (C0700 °C) is 0.31 ± 0.05 GPa, and this mechanical property increased approximately 2 times after re-curing in 90% RH (C3700 °C) as 0.58 ± 0.17 GPa. Measured tensile strength results and relative ratios of post-heating or re-cured sample (C0, C1, C2, and C3) to REF sample for the strength are summarized in Table 7. As shown in the results, the residual tensile strength of the post-heating samples reduced with increases in the exposed temperature (decreased ratio was 0.17 under 700 °C). The recovery

Table 7 Average values with standard deviation and decreasing ratios of splitting tensile strength (Ts). Sample Label REF⁎ C0 C1 C2 C3 ⁎ ⁎⁎

300 °C⁎⁎

500 °C⁎⁎

700 °C⁎⁎

Ts (MPa)

Ratio

Ts (MPa)

Ratio

Ts (MPa)

Ratio

1.84 ± 0.27 1.64 ± 0.34 1.44 ± 0.15 1.79 ± 0.21

0.58 0.51 0.45 0.56

0.88 ± 0.25 0.89 ± 0.05 0.91 ± 0.19 1.06 ± 0.12

0.27 0.28 0.29 0.33

0.51 ± 0.13 0.47 ± 0.06 0.45 ± 0.10 0.48 ± 0.11

0.17 0.15 0.14 0.15

Ts for REF sample is 3.00 ± 0.21 (MPa). Exposure temperature.

of tensile strength is represented approximately 20% in 500 °C exposed sample with high humidity condition (C3) than C0500 °C although other samples exposed to 300 °C and 700 °C did not exhibit significant recovery occurrence in various humidity conditions. Although samples exhibit different recovery rates on the exposure temperature and RH of re-curing condition, it is observed that the recovery of mechanical properties in post-heating concrete samples occurs after recuring. This indicates that the physicochemical recovery of the thermally damaged sample underwent a change during re-curing. The results of the XRD analysis of thermally damaged cement pastes after re-curing are shown in Figs. 3 to 5. Fig. 3 shows the XRD pattern of the control sample (REF(P)) before exposure in high temperature. In the REF(P) sample, calcium hydroxide (Ca(OH)2), calcite (CaCO3), and C-S-H are identified and correspond to the main phases formed in the hydration reaction. Unhydrated cement phases of C3S, C2S, C4AF, and C3A remain, and quartz (which is the ingredient of unhydrated clinker [34]) appears in the pattern. Additionally, the peak characterized as oyelite [(CaO)x·SiO2·zH2O] [32] phase is identified in the sample.

Table 6 Average values with standard deviation and decreasing ratios of the dynamic elastic modulus (Ed). Sample Label REF⁎ C0 C1 C2 C3 ⁎ ⁎⁎

300 °C⁎⁎

500 °C⁎⁎

700 °C⁎⁎

Ed (GPa)

Ratio

Ed (GPa)

Ratio

Ed (GPa)

Ratio

11.39 ± 1.35 8.46 ± 2.19 10.86 ± 1.77 12.60 ± 1.57

0.39 0.29 0.37 0.43

2.56 ± 0.47 2.65 ± 0.91 3.56 ± 0.35 4.38 ± 1.50

0.087 0.09 0.12 0.15

0.31 ± 0.05 0.49 ± 0.09 0.56 ± 0.13 0.58 ± 0.17

0.011 0.017 0.019 0.02

Fig. 3. XRD patterns of the REF(P) sample. The numbers in parentheses are ICDD PDF or ICSD data numbers for identified phases. ▼: unidentified peak.

Ed for REF sample is 29.39 ± 1.29 (GPa). Exposure temperature. 5

Cement and Concrete Research 125 (2019) 105831

H.J. Yim, et al.

Fig. 4. XRD patterns of samples exposed to 200–800 °C and phase changes in samples. Specifically, I2, I3, I4, …, and I8 represent cement paste samples exposed at 200, 300, 400, …, and 800 °C, respectively. (a) Ca(OH)2 and CaO; (b) CaCO3 and CaO; (c) detailed XRD figure of samples I7 and I8 in 28–30° 2θ; (d) C2S; (e) C3S.

After exposing REF(P) at 200–800 °C, the XRD patterns of the samples are shown in Fig. 4. As shown in Fig. 4(a), at 500–800 °C (I5, I6, I7, and I8 samples), the peaks of Ca(OH)2 disappear, and the peak formation of CaO is observed. This indicates that Ca(OH)2 decomposes to CaO [Ca(OH)2 → CaO + H2O]. At 700 °C and 800 °C (samples I7 and I8), the calcite disappears when compared with the peaks at 35.9°, 39.4°, 43.1°, 47.4°, and

48.4° 2θ (Fig. 4(a)). However, the strongest peak of calcite indicates that the calcite does not completely disappear although it definitely diminishes above 700 °C. Increases in the temperature decrease the intensity of calcite (Fig. 4(b)). This implies that calcite decomposes above 700 °C. It is also observed that the peak intensities of CaO in samples I7 and I8 are stronger than those in samples I5 and I6. This is because CaO is generated from the 6

Cement and Concrete Research 125 (2019) 105831

H.J. Yim, et al.

Fig. 5. XRD patterns of thermally damaged and re-cured samples. I2, I3, I4, …, I8 represent cement paste samples exposed at 200 °C, 300 °C, 400 °C, …, 800 °C, respectively. Each sample is denoted with a specific letter, namely “L”, “M”, and “H” represent the re-curing conditions at 10% RH, 60% RH, and 90% RH, respectively, for 7 d. O: oyelite, B: C4AF, CH: Ca(OH)2, Q: quartz, CSH: C-S-H, L: C2S, C: CaCO3, CO: CaO, H: C3S, E: ettringite, CA: C4AH13, ▼: unidentified peak.

decomposition of Ca(OH)2 above 500 °C and the decomposition of calcite [CaCO3 → CaO + CO2] above 700 °C. The C2S and C3S remain unhydrated in the hardened sample while C2S and C3S present in the sample after exposure to high temperature indicate the occurrence of the dissociation of C-S-H because the compounds generate C-S-H gel during hydration. Temperature ranges for the decomposition of CS-H are reported in previous studies [1,2,16,35]. Henry et al. [16] reported that C-S-H gel decomposes due to its dehydration at approximately 550 °C. El-Gamal et al. [35] revealed that C-S-H transforms to C2S and C3S at

600 °C. Nijland and Larbi [1] noted that final dissociation of C-S-H occurs at 800–950 °C. Tantawy [2] reported that C-S-H phase begins to decompose at 450 °C and transforms to C2S and C3S at 600 °C and 750 °C, respectively. In this study, it was observed that C-S-H phase begins to decompose above 500 °C. As shown in Fig. 4(d), above 500 °C (I5 sample), the intensity of the C2S peak increases with increases in the temperature. The strongest peak of C2S (32.1° 2θ [32]) overlaps with other peaks (for e.g., C3S peak at 32.2° 2θ [33]), and thus the increases in the C2S peak is easily identified by the peak at 2θ of 32.927° (dash-dotted line and black arrow). Change are not 7

Cement and Concrete Research 125 (2019) 105831

H.J. Yim, et al.

Fig. 5. (continued)

observed in the C3S peaks until 800 °C (Fig. 4(e), dashed line), thereby implying that a part of the C-S-H gel did not transform to C3S. As shown in Fig. 4(c), the C-S-H gel still remains. It is noted in this study that the decomposition of C-S-H begins at 500 °C although the C-S-H did not completely dismantle until 800 °C. The physicochemical changes in concrete due to thermal damage are attributed to the destruction of C-S-H phase and loss of Ca(OH)2 and calcite, which results in microcracks and contact-type defects. The XRD results provide a guideline to predict the exposed temperature of thermally damaged concrete structure. Fig. 5 shows the XRD patterns of the re-cured samples in the curing conditions of 10% RH (denoted with L), 60% RH (denoted with M), or 90%

RH (denoted with H) for 7 d. The phase changes observed in Fig. 5 are listed in Table 8. As shown in Fig. 5(a)–(c), in the case of samples I2–I3, after re-curing, the intensity of Ca(OH)2 peak decreases while the CaCO3 peak increases with increases in the RH levels. It is noted that the carbonation of Ca(OH)2 [Ca(OH)2 + CO2 → CaCO3 + H2O] occurs in the samples, and it is slow at low RH [36]. However, in the case of sample I4, the carbonation level on Ca(OH)2 was similar in all re-curing conditions. From the results for samples I5–I8 (Fig. 4(a) and (b)), it is noted that Ca (OH)2 and CaCO3 are decomposed to CaO above 500 °C and 700 °C, respectively. When samples I5–I8 are re-cured, it is expected that CaO reacts with water to form Ca(OH)2 as its principal hydration product. However, 8

Cement and Concrete Research 125 (2019) 105831

H.J. Yim, et al.

Table 8 Phase change in the XRD pattern of Fig. 5.

C2S

C3S

Ca(OH)2

CaCO3

Oyelite o

I2

o

o

o

o

L2

↔

↔

↓

↑

M2

↔

↔

↓↓

↑↑

H2

C-S-H

New peaks

o

-

↔

Unidentified peak

↑

↔

Unidentified peak

↔

↓

↓↓↓

↑↑↑

↑

Ettringite, C4AH13, Unidentified peak

C2S

C3S

Ca(OH)2

CaCO3

Oyelite

C-S-H

New peaks

I3

o

o

o

o

o

o

-

L3

↔

↔

↓

↑

↔

↔

-

M3

↔

↔

↓↓

↑↑

↔

↔

-

H3

↔

↓

↓↓↓

↑↑↑

↔

↑

Ettringite, C4AH13

C2S

C3S

Ca(OH)2

CaCO3

Oyelite

C-S-H

New peaks

o

o

I4

o

o

L4

↔

↔

M4

↔

↔

H4

↓

↑

CaCO3

↔

↔

C2S

C3S

Ca(OH)2

o

o

-

↑↑

↔

-

↔

-

↑

↔ Oyelite

C-S-H

Ettringite, C4AH13 CaO

New peaks

I5

o

o

x

o

o

o

o

-

L5

↔

↔

x

↔

↔

↔

↔

-

M5

↔

↔

x

↔

↔

H5

↔

↔

x

↔

↔

↑

↓

Ettringite, C4AH13

C2S

C3S

Ca(OH)2

CaCO3

Oyelite

C-S-H

CaO

I6

o

o

x

o

o

o

o

-

L6

↔

↔

x

↔

↑↑

↔

↔

Unidentified peak

M6

↓

↔

x

H6

↓↓

↓

x

C2S

C3S

Ca(OH)2

↑ CaCO3

↑↑

↑

↔

↑↑

Oyelite

C-S-H

↓

New peaks

Unidentified peak Ettringite, C4AH13, Unidentified peak

CaO

New peaks -

I7

o

o

x

o

o

o

o

L7

↓

↔

x

↔

↔

↑

↔

M7

↓↓

↔

↑

↑

↔

↑↑

H7

↓↓↓

↔

↑↑

↑↑

↔

↑↑↑

C2S

C3S

Ca(OH)2

CaCO3

Oyelite

C-S-H

CaO

New peaks

I8

o

o

x

o

x

o

o

-

L8

↔

↔

x

↔

x

↔

↔

-

M8 H8

↓ ↓↓

↔ ↔

↑ ↑↑

↑ ↑↑

x x

↑ ↑↑

↓

Ca(OH)2 Ettringite, C4AH13, Ca(OH)2

↓

Ca(OH)2 Ettringite, C4AH13, Ca(OH)2

o: present of phase, x: not present, ↔: no significant change, ↓: reduced, ↓↓ and ↓↓↓: more reduced than for the above sample, ↑, ↑↑, and ↑↑↑: the opposite of ↓, ↓↓, and ↓↓↓.

with respect to samples I5 and I6 re-cured in 60% and 90% RH, it is observed that the CaO peak decreases although the Ca(OH)2 peak is not present and the CaCO3 peak increases (Fig. 5(d) and (e)). This suggests that the carbonation of CaO [CaO + CO2 → CaCO3] occurs in the samples. As observed in samples I7 and I8, the CaO peak decreases, CaCO3 peaks increase, and Ca(OH)2 forms under the aforementioned curing conditions. Additionally, the intensity of Ca(OH)2 peak increases with increases in the RH level (Fig. 5(f) and (g)). It is observed that the rehydration and carbonation of CaO simultaneously occur, and it is fast at high RH. In samples I5–I8, the rehydration or carbonation of CaO did not occur under the recuring condition of 10% RH. It is observed in samples I2, I3, and I6 that the intensity of C3S peak decreases, thereby indicating the formation of C-S-H (Table 8). However, this does not imply that C3S generated from the decomposition of C-S-H is rehydrated. Changes in the C3S peaks are not observed in a heating event

until 800 °C. It is observed that unhydrated C3S that remains in samples was rehydrated. It is also observed that the unhydrated C3S is rehydrated only in the 90% RH condition (denoted with H) for the I2, I3 and I6 samples. As previously mentioned, C2S increases due to the decomposition of C-S-H at approximately 500 °C. Given the formation of C-S-H after recuring, the C2S in sample I5 did not change for all the re-curing conditions. The C-S-H formation due to the rehydration is observed in the re-curing conditions of 60% and 90% RH (denoted with M and H) in samples I6 and I8 and in all the re-curing conditions in sample I7. With respect to all the thermally exposed paste samples, it is observed that ettringite and C4AH13 are commonly formed due to the rehydration that only occurs in the re-curing condition of 90% RH. This result demonstrates that for concrete samples (i.e., C1, C2, and C3), C3 sample recured in 90% RH have the lowest HNP and the highest dynamic elastic modulus compared to C1 (re-cured in 10% RH) and C2 (re-cured in 60% 9

Cement and Concrete Research 125 (2019) 105831

H.J. Yim, et al.

RH) samples for each exposure temperature. When the rehydration products fill the microcracks that occur in the thermally damaged sample, it should be noted that high RH contributes to a significant improvement in the physicochemical form of the samples. For concrete samples, the sample exposed at 700 °C (C0700 °C) shows higher damage values (i.e., the increase in HNP and the decrease in dynamic elastic modulus) than that exposed at 300 °C (C0300 °C) and 500 °C (C0500 °C). This implies that hydration products are more decomposed in C0700 °C sample than the samples exposed at 300 °C and 500 °C. From the XRD result of paste sample, it can be mentioned that this is attributed to the decomposition of CaCO3 from 700 °C. The 700–800 °C exposed paste samples generally indicate that Ca(OH)2, CaCO3, C-S-H, ettringite, and C4AH13 are all rehydrated only in the recuring condition of 90% RH. The occurrence fills up microcracks including contact-type defects and leads to the recovery of material properties of the thermally damaged sample. The self-healing phenomenon is nondestructively evaluated by measuring the HNP and dynamic elastic modulus although the rehydrated products do not influence the recovery of mechanical tensile strength. This implies that generated products by rehydration improve the solid volume fraction of thermally damaged concrete although refilled products barely develop bonding strength with existing materials to recover the mechanical strength of concrete.

[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

4. Conclusions

[13] [14]

This study attempts to evaluate the contact-type defects and degree of thermally damaged concrete (200 °C, 300 °C, 400 °C, …, 800 °C) based on the measurement of HNP by nonlinear resonance vibration method, and the change in the measured HNP represented the recovery of material properties through various re-curing conditions (10%, 60%, and 90% RH) than that in other destructive and nondestructive methods, such as tensile strength measurement and dynamic elastic modulus measurement. Additionally, the XRD results with different exposed high temperature and re-curing conditions contributed to understanding the thermal damage with prediction of exposed temperature and its recovery due to physicochemical changes including collapse and generation of crystalline phases. In this study, it was confirmed that C-S-H phase in the samples after exposure to high temperatures began to decompose above 500 °C. Ca(OH)2 was not present at the exposure temperature of 500–800 °C. Calcite was decomposed at 700–800 °C, and it did not completely disappear although it definitely diminished above 700 °C. CaO was generated from the decomposition of Ca(OH)2 and calcite, and increased with increasing the exposed temperature. After re-curing, in some sample, the carbonation of Ca(OH)2 and CaO, and the rehydration of CaO and C2S occurred, leading to the formation of CaCO3, Ca(OH)2, and C-S-H phases. Ettringite and C4AH13 were commonly formed due to the rehydration only in the re-curing condition of 90% RH for all thermally exposed samples. Specific re-curing condition in high relative humidity mainly led to chemical changes in constituent materials, such as rehydration or carbonation, and it refilled the microcracks and contact-type defects generated by high temperature. The chemical recovery increased the solid phase and restored the HNP although it barely recovered the mechanical tensile strength of the concrete.

[15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]

Declaration of Competing Interest

[29]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[30]

Acknowledgment

[31] [32] [33]

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF2018R1D1A3B07044605).

[34] [35]

References

[36]

[1] T.G. Nijland, J.A. Larbi, Unraveling the temperature distribution in fire-damaged

10

concrete by means of PFM microscopy: outline of the approach and review of potentially useful reactions, HERON 46 (4) (2001) 253–264. M.A. Tantawy, Effect of high temperatures on the microstructure of cement paste, J. Mater. Sci. Chem. Eng. 5 (11) (2017) 33–48. A.H. Akca, N. Özyurt, Effects of re-curing on residual mechanical properties of concrete after high temperature exposure, Constr. Build. Mater. 159 (2018) 540–552. Z.P. Bazant, M.F. Kaplan, Concrete at High Temperatures: Material Properties and Mathematical Models, Addison-Wesley, London, 1996. H.J. Yim, J.H. Kim, S.-J. Park, H.-G. Kwak, Characterization of thermally damaged concrete using a nonlinear ultrasonic method, Cem. Concr. Res. 42 (11) (2012) 1438–1446. H.J. Yim, S.-J. Park, J.H. Kim, H.-G. Kwak, Nonlinear ultrasonic method to evaluate residual mechanical properties of thermally damaged concrete, ACI Mater. J. 111 (4) (2014) 399–410. S.-J. Park, H.J. Yim, H.-G. Kwak, Nonlinear resonance vibration method to estimate the damage level on heat-exposed concrete, Fire Saf. J. 69 (2014) 36–42. S.-J. Park, G.-K. Park, H.J. Yim, H.-G. Kwak, Evaluation of residual tensile strength of fire-damaged concrete using a non-linear resonance vibration method, Mag. Concr. Res. 67 (5) (2015) 235–246. S.-J. Park, H.J. Yim, Evaluation of residual mechanical properties of concrete after exposure to high temperatures using impact resonance method, Constr. Build. Mater. 129 (2016) 89–97. M.Y.L. Chew, The assessment of fire damaged concrete, Build. Environ. 28 (1) (1993) 97–102. P. Cioni, P. Croce, W. Salvatore, Assessing fire damage to r.c. elements, Fire Saf. J. 36 (2) (2001) 181–199. M.C.R. Felicetti, New NDT techniques for the assessment of fire-damaged concrete structures, Fire Saf. J. 42 (6) (2007) 461–472. T. Harada, J. Takeda, S. Yamane, F. Furumura, Strength, Elasticity and Thermal Properties of Concrete Subjected to Elevated Temperatures, ACI Spec. Publ, (1972), p. 34. C.-S. Poon, S. Azhar, M. Anson, Y.-L. Wong, Strength and durability recovery of firedamaged concrete after post-fire-curing, Cem. Concr. Res. 31 (9) (2001) 1307–1318. R. Sarshar, G.A. Khoury, Material and environmental factors influencing the compressive strength of unsealed cement paste and concrete at high temperatures, Mag. Concr. Res. 45 (162) (1993) 51–61. M. Henry, I.S. Darma, T. Sugiyama, Analysis of the effect of heating and re-curing on the microstructure of high-strength concrete using X-ray CT, Constr. Build. Mater. 67 (2014) 37–46. M. Henry, M. Suzuki, Y. Kato, Behavior of fire-damaged mortar under variable recuring conditions, ACI Mater. J. 108 (3) (2011) 281–289. W.-M. Lin, T.D. Lin, L.J. Powers-Couche, Microstructures of fire-damaged concrete, ACI Mater. J. 93 (3) (1996) 199–205. S.K. Handoo, S. Agarwal, S.K. Agarwal, Physicochemical, mineralogical, and morphological characteristics of concrete exposed to elevated temperatures, Cem. Concr. Res. 32 (7) (2002) 1009–1018. G.-F. Peng, Z.-S. Huang, Change in microstructure of hardened cement paste subjected to elevated temperatures, Constr. Build. Mater. 22 (4) (2008) 593–599. E. Annerel, L. Taerwe, Revealing the temperature history in concrete after fire exposure by microscopic analysis, Cem. Concr. Res. 39 (12) (2009) 1239–1249. C. Alonso, L. Fernandez, Dehydration and rehydration processes of cement paste exposed to high temperature environments, J. Mater. Sci. 39 (9) (2004) 3015–3024. Y. Lin, C. Hsiao, H. Yang, Y.-F. Lin, The effect of post-fire-curing on strength–velocity relationship for nondestructive assessment of fire-damaged concrete strength, Fire Saf. J. 46 (4) (2011) 178–185. S. Liu, Z.B. Bundur, J. Zhu, R.D. Ferron, Evaluation of self-healing of internal cracks in biomimetic mortar using coda wave interferometry, Cem. Concr. Res. 83 (2016) 70–78. E. Tsangouri, G. Karaiskos, D.G. Aggelis, A. Deraemaeker, D.V. Hemelrijck, Crack sealing and damage recovery monitoring of a concrete healing system using embedded piezoelectric transducers, Struct. Health Monit. 14 (5) (2015) 462–474. K.E.-A. Van Den Abeele, P.A. Johnson, A. Sutin, Nonlinear elastic wave spectroscopy (NEWS) techniques to discern material damage, part I: nonlinear wave modulation spectroscopy (NWMS), Res. Nondestruct. Eval. 12 (1) (2000) 17–30. J. Chen, J.-Y. Kim, K.E. urtis, L.J. Jacobs, Theoretical and experimental study of the nonlinear resonance vibration of cementitious materials with an application to damage characterization, J. Acoust. Soc. Am. 130 (5) (2011) 2728–2737. K.J. Leśnicki, J.-Y. Kim, K.E. Kurtis, L.J. Jacobs, Characterization of ASR damage in concrete using nonlinear impact resonance acoustic spectroscopy technique, NDT& E Int. 44 (8) (2011) 721–727. K.E.-A. Van Den Abeele, J. Carmeliet, J.A. Ten Cate, P.A. Johnson, Nonlinear elastic wave spectroscopy (NEWS) techniques to discern material damage, part II: single-mode nonlinear resonance acoustic spectroscopy, Res. Nondestruct. Eval. 12 (1) (2000) 31–42. ASTM C496-11, Standard Test Method for Splitting Tensile Strength of Cylindrical Concrete Specimens, ASTM International, West Conshohocken, Pennsylvania, USA, 2011. X'Pert HighScore Plus, 3 ed., PANalytical, Almelo, Netherlands, 2012. PDF-2 database, Data ICDD, 2000 editor. Newton Square, PA. R. Allmann, R. Hinek, The introduction of structure types into the inorganic crystal structure database ICSD, Acta Crystallogr. A 63 (5) (2007) 412–417. Q. Zhang, G. Ye, Dehydration kinetics of Portland cement paste at high temperature, J. Therm. Anal. Calorim. 110 (1) (2012) 153–158. S.M.A. El-Gamal, F.S. Hashem, M.S. Amin, Thermal resistance of hardened cement pastes containing vermiculite and expanded vermiculite, J. Therm. Anal. Calorim. 109 (1) (2012) 217–226. O. Cazalla, C. Rodriguez-Navarro, E. Sebastian, G. Cultrone, M.J. De la Torre, Aging of lime putty: effects on traditional lime mortar carbonation, J. Am. Ceram. Soc. 83 (5) (2000) 1070–1076.