E-Journal of Advanced Maintenance Vol.9-3 (2017) 164-166
Japan Society of Maintenology

Study on Material Properties of Concrete Structures Exposed
to Extremely High Temperature due to Severe Accident
Yuma Sato1, Do Thi-Mai-Dung1, Masahide Suzuki1
1Nagaoka University of Technology, 1603-1 Kamitomioka-cho, Nagaoka, Niigata 940-2188, Japan

ABSTRACTIn March 2011, the Fukushima Nuclear Power Plant suffered a severe accident because of the tsunami waves
when the Great East Japan Earthquake occurred and because of this accident the containment vessel was
exposed to high temperatures. However, studies related to concrete with extremely high temperature exposure
are very few. For this reason, the aim of this study is to understand the structural soundness of the concrete
structures exposed to high temperature by conducting heating experiments of concrete specimens which weremade to reproduce the concrete structure of the Nuclear Plant. From the test results, cracks were observed at thespecimen surface and they grew with the increase of temperature. Additionally, it was observed that concrete
crack volume grew to temperatures around 800C, but this growth saturated more than this temperature to1000C. It was observed that the amount of water content inside concrete decreased with temperature. Moreover,the chemical composition change of concrete was accompanied with the water content decrease. Results onother material properties are also to be presented.

KEYWORDS

ARTICLE INFORMATION

severe accident, containment vessel, concrete, high temperature, crack

Article history:

1.

Introduction

In case of a severe accident of nuclear
power plant, the structures surrounding thecontainment vessel like concrete structuressuffer significant damages due to exposure tohigh temperature which greatly exceeds thedesign condition. So, the strength of theconcrete structure should be drasticallydeteriorated. Although several studies on
non-destructive evaluation of structural
integrity of concrete structures have been
carried out, those related to the evaluation of
nuclear plant soundness after suffering severe
accident are not sufficient. Therefore, the aimof this study is to understand the micro/macro
structural properties of concrete at hightemperature, mechanical property change after
the high temperature heating and finally thepossibility of non-destructive evaluation
method through high temperature heating
experiments using concrete test specimens.

Received 7 November 2016
Accepted 29 May 2017

2.1. Preparation of the sample
The concrete samples were made fromordinary Portland cement according to themixture presented in Table 1. The size of thesample is 50 × 50 × 50 (mm3).
Table.1 Mixture table for concrete used.

2. Experiment outline
Fig.1 Heating condition for concrete specimens

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

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Y.Sato, et al./ Study on Material Properties of Concrete Structures Exposed
to Extremely High Temperature due to Severe Accident

Fig.2 The surface of the concrete test sample

Fig.3 Cracks on the surface of concrete test sample

furnace. The thermal history is shown in Fig.1.
Heating conditions of temperature rising at thespeed of 5℃/min, 2 hours holding at themaximum temperature, and natural cooling in
the furnace, were adopted.

2.3. Tests and Measurements
After the concrete samples were heated ateach target temperature, observation andmeasurements of surface cracks and crackvolumes, thermogravimetry (TG) analysis ,ultrasonic testing (UT) measurements, andSchmidt hammer test were carried out in orderto understand the concrete property changes
caused by heating. Additionally, for theobservation of surface cracks an inverted
metallurgical microscope was used, and theArchimedes' method was used to measurecrack volumes. Also, the strength of theconcrete was estimated by conducting Schmidt
hammer testing.

Fig.4 Ultrasonic sound velocity and strength ofthe concrete test sample before and after
heating.

Also, the curing of the concrete was conducted
for 28 days by poultice curing. The samples
prepared at the same time were used for all thetests.
2.2. Heating Tests
The samples were heated to 40℃, 600℃,800℃, and 1000℃ by using a small electric

3. Results and Discussion
3.1.

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The surface crack observation

Y.Sato, et al./ Study on Material Properties of Concrete Structures Exposed
to Extremely High Temperature due to Severe Accident

The surface of the concrete is shown in
Fig.2, and cracks on the surface is shown in
Fig.3. The observation was conducted before
and after heating using a metallurgicalmicroscope. As a result, the color of thesample after heating was changed to red.
Additionally, cracks became larger with theincrease of the heating temperature. The crack
width was 0.06 mm after 600℃, and 0.13 mm,

after 800℃, and finally 0.26 mm after 1000℃,
in the average values.
3.2. Ultrasonic test (UT) and Schmidt
hammer test
Ultrasonic sound velocity and compressive
strength of the concrete test sample are shownin Fig.4 before and after heating. About theDTATG

Fig.5 Crack volumes increase ratio vs. heating temperature

Fig.6 TG and DTA measurement results

temperature of 800℃, resulting in also thedrastic decrease of the concrete strength.
3.3. Crack volumes
The result of crack volumes measurement
are shown in the Fig.5. Here, crack volumes
increase ratio (CVIR) is expressed by thefollowing equation.

CThe measurement was conducted usingArchimedes method. From the results it wasshown that although crack volumes increased
with temperature, the crack volumes werealmost saturated after the temperature of800℃.
3.4.

TG/DTA measurement
The results of TG and DTA are shownin Fig.6. Water which is considered to exist as
bound water or free water decreased by theheating. All contained water seemed to be lostat around 800℃ because TG results showed
the saturation beyond 800℃. Marked change
was also seen at around 800℃ by the DTA
measurement.

Fig.7 XRD measurement results

UT results, the velocity was 3289 m/s after
40℃ and 1042 m/s after 800℃, resulting in
the decrease to about 1/3 of the initial value
after heating. The compression strength
estimated by Schmidt hammer test was 41.6N
for a non-heated sample and 24.7N after the166

Y.Sato, et al./ Study on Material Properties of Concrete Structures Exposed
to Extremely High Temperature due to Severe Accident

3.5.

XRD measurement
The results of X-ray diffraction (XRD)
are shown in Fig.7. Orange dashed lines
indicate the peaks which increased after
heating, while blue dashed lines indicate theones which reduced with heating to higher
temperature. The results indicated that SiO2 ,Ca2(SO3)(H2O), Ca(OH)2 , CaCO3 weredecreased or lost after the heating. Ca (OH)2
is lost at 600℃,and CaCO3 at 800℃,finally SiO2 was lost at 1000℃. On the otherhand, peaks of Ca2SiO4, Fe2O3 and CaO
appeared after the heating.

Additionally, the result of TG revealed thatwater inside the concrete was decreased withtemperature, and finally the total loss of water
occurred at temperature of about 800℃.
(3) UT results showed that ultrasonic soundvelocity decreased about 1/3 after hightemperature heating. AT the same time, thecompression strength estimated by Schmidt
hammer test showed the drastic decrease after800°C heating.
(4) From the results of XRD, CaCO3 , SiO2 etc.
were lost, and Ca2SiO4, CaO were generated.

References
3.6. Discussion
Because it was shown that water insideconcrete was fully lost at 800°C by TG, crack
generation appeared to be suppressed attemperatures higher than 800°C. Various big
changes were seen after the heating, i.e.,change in color and the formation of cracks
were clearly observed.
From the results of XRD, it was thoughtthat many chemical reactions occurred during
the heating. For instances, Ca2SiO4 wasconsidered to be produced from SiO2 andCa2(SO3) (H2O) . At the same time, it isthought that CaO was generated from CaCO3.
As the result of these micro/macro structuralchanges, strength of the concrete wasdrastically decreased, possibly due to crack
formation. At the same time, the crack
formation distinctively causes the ultrasonicsound velocity decrease. Therefore, thestrength is thought to be clearly measured
nondestructively using UT up to the heattemperature of 800℃.
As the result of
these micro/macro structural changes, strengthof the concrete was drastically decreased.

4.

[1] Omer Arioz: “Effects of elevatedtemperatures on properties of concrete”, Fire
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Japan[4] Toshimutsu Ishida, Norikazu Ooka, Taiji
Hoshiya, Hideo Kobayashi, Junichi Saito,Motoji Niimi, Hirokazu Tsuji: “Developmentof a non-destructive testing technique usingultrasonic wave for evaluation of irradiation
embrittlement in nuclear materials”, Journal of
Nuclear Materials 307-311(2002) 240-244,
JapanSummary(1) After the high temperature heating, thecolor of the concrete surface was changed. At
the same time, the cracks were increased bothin the amount and the size. The average size ofthe cracks observed was 0.06mm for 600℃,
0.13mm for 800℃ and, 0.26mm for 1000℃,
grew with the increase of heating temperature.
(2) The crack volumes increased withtemperature, the crack volumes were almost
saturated after the temperature of 800 ℃ .

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