Non/Minor Destructive Methods for Investigating Concrete Durability of National Museum of Western Art, Japan

Transcription

1 Non/Minor Destructive Methods for Investigating Concrete Durability of National Museum of Western Art, Japan Kei-Ichi Imamoto 1 Akio Tanaka 2 Yuji Umezu 3 Manabu Kanematsu 4 Yoshiyuki Yamana 5 ABSTRACT The National Museum of Western Art (NMWA), designed by Le Corbusier and constructed in 1959, is an important cultural landmark of Japan. A working group (Chairman: Prof. Hiroyuki Suzuki, Aoyamagakuin Univ.) established under the Architectural Institute of Japan, assessed the condition of the structural concrete of this building, with the aim of devising a maintenance strategy in order to register the building as a national heritage site. This paper reports on the phenomenon responsible for the degradation and deterioration of the structural concrete members in the NMWA building, such as the excessive deflection of slab and corrosion risks due to carbonation. Both non-destructive test methods and those that inflict only minor damage to the structure (NDT/MDT) were used in the study. Non destructive test (NDT) methods were employed for measuring the air permeability of the concrete cover and the cover thickness in order to assess the steel corrosion risk due to carbonation. The results showed that half of the slabs exhibited excessive deflection. The carbonation depths varied from 5 to 35 mm. Furthermore, the results of the non destructive tests indicated that corrosion rate is 6% at present and will gradually increase. Half the steel bars will have corrode within 6 years. It is assumed that many historical reinforced concrete buildings worldwide suffer from such degradation. For such buildings whose authenticity must be maintained, non/minor destructive test methods (NDT/MDT), which do not damage the structure of the building, will be effective for assessing the condition of the building. KEYWORDS National Museum of Western Art, Concrete, Carbonation, Steel corrosion risk, Non destructive test 1 Faculty of Engineering, Tokyo University of Science, Tokyo, JAPAN, imamoto@rs.kagu.tus.ac.jp 2 Faculty of Engineering, Tokyo University of Science, Tokyo, JAPAN, at2862@yahoo.co.jp 3 Faculty of Engineering, Tokyo University of Science, Tokyo, JAPAN, yumezu@rs.kagu.tus.ac.jp 4 Faculty of Science and Engineering, Tokyo University of Science, Tokyo, JAPAN, manabu@rs.noda.tus.ac.jp 5 Faculty of Engineering, Tokyo University of Science, Tokyo, JAPAN, atelieryamana@hotmail.co.jp

2 K. Imamoto, A. Tanaka, M. Kanematsu, Y. Umezu and Y. Yamana 1 INTRODUCTION The National Museum of Western Art (NMWA), designed by Le Corbusier and constructed in 1959, is an important cultural landmark of Japan. A working group (Chairman: Prof. Hiroyuki Suzuki, Aoyamagakuin Univ.) established under the Architectural Institute of Japan, assessed the condition of the structural concrete of this building with the aim of devising a maintenance strategy in order to register the building as a national heritage site. This paper reports on the phenomenon responsible for the degradation and deterioration of the structural concrete members in the NMWA building, such as the excessive deflection of slab, presence of cracks, and corrosion risks due to carbonation. Both non-destructive test methods and those that inflict only minor damage to the structure (NDT/MDT) were used in the study. Non destructive test (NDT) methods were employed to determine the air permeability of a concrete cover in order to assess the steel corrosion risk due to carbonation. In this paper, the significant degradation that these historical reinforced concrete structures will face has been explained; thus, a reasonable repair method that will also maintain the authenticity of these strucures will be required in the near future. It is believed that many historical reinforced concrete buildings worldwide might suffer from different types of degradation. For such buildings, whose authenticity should be maintained, non/minor destructive test methods (NDT/MDT), which do not damage the structure, will be effective for assessing the condition of the building. NDTs should prove to be effective for determining the air permeability of concrete cover and the cover thickness in order to evaluate the steel corrosion risk faced by structural members in the near future. 2 THE NATIONAL MUSEUM OF WESTERN ART The NMWA (Photograph 1 and Figure 1) was constructed in 1959 with a concrete design strength of 18 MPa. Further details of the building and specifications of the concrete mixture are listed in Table 1 and 2, respectively. Round bars with a yield strength of 235 N/mm 2 were used for reinforcement. Photograph 1. Appearance of the NMWA Figure 1. Plan of 1 st floor Table 1. Details about NMWA Location Ueno Tokyo Completion 29 May 1959 Designer Le Corbusier Assistant designer Junzo SAKAKURA, Kunio MAEKAWA, and Takamasa YOSHIZAKA Structural design YOKOYAMA structural design office Contractor SHIMIZU Corporation Ground area 1,587 m 2 Total floor area 4,353 m 2 Height m Type of structure Reinforced concrete 3 stories Table 2. Concrete composition Materials Unit content Water - 24 kg/m 3 Cement Ordinary Portland 34 kg/m 3 cement Sand Sagami/Koito River 921 kg/m 3 Gravel Crushed stone (Futo) 829 kg/m 3 Max.2 mm Admixture Air entrained agent - 2 XII DBMC, Porto, PORTUGAL, 211

3 Non/minor destructive methods for investigating concrete durability 3 PREVIOUS SURVEYS In 1979, the first survey of the NMWA was conducted for investigating the cracks of the cantilever beams present on the second floor, which are supported by a Precast concrete panel (see photograph 2). In 1986, a second survey was conducted to study the spalling of the decorative stone from the PCa panel. In 1995, from the viewpoint of retrofitting the building, detailed surveys were conducted on the core compressive strength, carbonation progress of the cover thickness, cracks on the strucural concrete, and steel corrosion. The test results of the core compressive strength are listed in table 3. Table 3. Test result of core compressive strength BF1 BF1 1F F 2F 3F Photograph 2. Precast concrete panel of north-side wall 4 PRELIMINARY VISUAL INSPECTION In 29, preliminary visual investigations of the building were carried out; in these investigations, cracks were observed in several places. Photograph 3 shows cracks under the 1 st floor and photographs 4 and 5 show cracks on the walls along steel bars. The cracks shown in photograph 2 are probably caused by the excessive deflection of the slab; and those shown in photographs 4 and 5 by carbonation-induced steel bar corrosion. Photograph 3. Cracks under 1 st floor Photograph 4. Cracks on walls Photograph 5. Steel corrosion 4.1 Investigation areas According to the results of the preliminary investigations, a detailed investigation on the deflection of the slab and the carbonation progress of concrete was carried out. The National Museum of Western Art (NMWA) is an important cultural landmark of Japan. In order to maintain the authenticity of the building, a non/minor destructive test method (NDT/MDT) was performed. A combination of a small core sampling test and a non destructive on-site air permeability test [Torrent 1992] was performed to determine the carbonation progress of the concrete structures and also to minimize the impact on the concrete appearance. XII DBMC, Porto, PORTUGAL, 211 3

4 K. Imamoto, A. Tanaka, M. Kanematsu, Y. Umezu and Y. Yamana On-site air permeability test The method described here is known as the Torrent Permeability Tester and was developed in Switzerland. In this test, the pressures in the outer and inner chambers are always maintained at the same value. Thus, the outer chamber acts as a guard-ring, providing a controlled, unidirectional flow of air into the inner chamber (see Figure 2). The latest report by RILEM TC NEC [RILEM TC NEC 27], mentioned two advantages of this system: (1) The spurious air entering through preferential paths (e.g. microcracks or extremely porous outer skin) is absorbed by the outer chamber, and thus, it does not affect the measurements, and (2) because of the well-defined air-flow path, the coefficient of permeability kt [m 2 ] can be easily calculated by means of a theoretical model. Inner chamber Outer chamber Soft rubber Vacuum pump 25mm 5mm 25mm Unidirectional air flow Figure 2. Schematic description of Torrent permeability tester Small core sampling Small core specimens with a diameter of 2 mm were drilled in the structure in order to determine the relationship between the air permeability coefficient (kt) and the carbonation progress of concrete. The carbonation progress was measured, as shown in Photographs 6 and 7. After the core drilling, grout was injected into the hall and the sliced surface of the core was installed back into the concrete member structure (see Figure 3). This process enables the impact of the core drilling to be invisible as shown in photo 8. Core (diameter: 2 mm) Colour controlled paste with fly ash (width: 2 mm) Grouting Sliced surface of drilled core specimens Surface of concrete Photograph 6 Core drilling Figure.3. Schematic description of repair of drilled hole Carbonation zone Concrete surface Cored area (3 samples) Photograph 7 Carbonation of specimens Photograph 8 Impact of core drilling after repair (shown in Fig.3) 4 XII DBMC, Porto, PORTUGAL, 211

5 Non/minor destructive methods for investigating concrete durability Radar test The cover depth and the steel bar arrangement was measured with a radar device shown in Photograph 9. This method utilizes the reflection of the radar onto a steel bar installed in a concrete member. The radar released from the device reflects on the steel bar and turn to concrete surface. The receiver measures the round trip time, from which the position of the steel bar can be detected based on the relationship between the velocity and time of radar in concrete. 5 RESULTS AND DISCUSSION Photograph 9 Radar test device 5.1 Deflection of slab The deflection of the slab was measured using level surveys. The results are shown in Figures 4 and 5. According to the guidelines for structural design proposed by the Architectural Institute of Japan, the deflection per short span of slab should be limited to less than 1/25 th of the span. Hence, a greater value of deflection per span implies excessive deflection. It can be seen that half of the slabs exceed this criterion, and thus, these slabs will require further strengthening in the near future Figure 4. Example of measured deflection of slab 5.2 Carbonation progress Frequency F Exhibition room xdeflection/ span Figure 5. Deflection of slab The carbonation progress was measured by conducting small core sampling tests at several places in the NMWA. Figure 6 shows the carbonation progress of concrete; carbonation gradually progresses with time and varies from 5 to 35 mm at the present age of 5 years (21). On the exterior wall, carbonation reaches up to 4 mm, and half of the steel bars face the risk of corrosion (the corrosion rate being 5%) 55 years hence; this rate is measured assuming that the cover thickness of the exterior wall is 4 mm. Figure 7 shows the relationship between the carbonation velocity and the air permeability coefficient (kt) of concrete. It can be seen that agreement is obtained and the air permeability tester can be considered to be a suitable tool for estimating the carbonation progress of concrete in a non destructive manner. Based on this finding, the carbonation depth of the north-side exterior wall was evaluated in detail (see Photograph 2). Figure 8 shows the distributions of carbonation depth, estimated through the air permeability test, and the cover thickness of the steel bar, measured through the radar test at 122 points. Assuming that the steel bar would corrode if the carbonation depth exceeds the cover thickness, the corrosion rate of the steel bars in the north-side exterior wall can be evaluated. Figure 9 shows the progress of the corrosion rate of the steel bars in the exterior wall. It can be observed that the present corrosion rate is 6% and that this rate is increasing gradually. Thus, XII DBMC, Porto, PORTUGAL, 211 5

6 K. Imamoto, A. Tanaka, M. Kanematsu, Y. Umezu and Y. Yamana the steel bars will have corroded by half within 6 years. This value almost agrees with the value obtained by the core sampling test and the square root time theory that is widely used to estimate the progress of concrete carbonation. Hence, a non destructive on-site air permeability test and radar test (to locate the steel bar position) will be useful for estimating the carbonation of the RC building at present. Carbonation depth (mm) Previous data Average of exterior wall BF1-1 BF1-2 Footing 1F Exterior wall RF Steel bar corrosionrate of 5%of exterior wall: Estimated carbonation progress curve according to square root time theory Cover thickness 4mm Age of building (Years) Target life time Figure 6. Carbonation progress of concrete by small core sampling test Carbonation velocity (mm/ years.5 ) B1(with finishing) B1(exposed concrete) Exterior wall y =.38 Ln(x) R 2 = Air permeability coefficient ( 1-16 m 2 ) Frequency Figure 7 Carbonation and air permeability Average 39.2mm ~29 3~34 35~39 4~44 45~49 5~ Cover thickness (mm) Figure 8 Distribution of cover thickness and carbonation depth Frequency Average 26.3mm ~19 2~24 25~29 3~34 Carbonation depth (mm) Corrosion rate (%) Present Age (Years) Figure 9. Progress of steel bar corrosion rate in exterior wall 6 XII DBMC, Porto, PORTUGAL, 211

7 Non/minor destructive methods for investigating concrete durability 6 CONCLUSION The National Museum of Western Art is an important cultural landmark of Japan, and it is over 5 years old. The deflections of the slab and the concrete carbonation depths of this building were investigated using non/minor destructive methods (NDT/MDT). It was found that deflections of the slab and concrete carbonation can lead to significant risk. A non destructive method, on-site air permeability test, and steel bar radar test were useful for assessing the risk of steel bar corrosion. The development of a suitable repair method will be required in the near future to maintain the authenticity of the building. REFERENCES Ministry of Construction: Report on Retrofitting of the National Museum of Western Art, RILEM Report 4: Non-Destructive Evaluation of the Penetrability and Thickness of the Concrete Cover, State of the Art Report of RILEM Technical Committee TC 189-NEC Non-Destructive Evaluation of the Concrete Cover, Edited by R. Torrent and L. Fernandez Luco, 27. Torrent, R.: A two-chamber vacuum cell for measuring the coefficient of permeability to air of the concrete cover on site, Mater. & Struct., Vol.25, No.15, pp , July XII DBMC, Porto, PORTUGAL, 211 7