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The residual strength tests after cyclic loadings on concrete cylinder specimens and stud anchor specimens were performed to obtain data regarding effects of cyclic loadings on concrete and stud anchor specimens for proposing a new method capable of evaluating the soundness of equipment support using the S-N curves. More specifically, cyclic loadings may lead to a reduction in the strength and stiffness of concrete and also lead to the pullout and a reduction of the strength of stud anchor sections, which may reduce the rigidity of the equipment support, and this situation is not preferable from the viewpoint of long-term soundness evaluation. Then, in order to investigate whether or not cyclic loadings reduce the strength and the rigidity of the equipment support, cyclic compressive loadings with constant amplitude were applied to cylinder specimens, then static compression tests were performed. Also, cyclic tensile loadings with constant amplitude were applied to stud anchor specimens, then static pullout tests were performed. As a result, it was found that the cyclic loading would not lead to the pullout of stud anchor and a reduction of the strength of stud anchor if the maximum stress ratio is less than the long-term allowable stress. 1. Background of research The degradation factors considered in the evaluation of long-term soundness of structures in nuclear facilities are heat (high temperature), irradiation, carbonation, chloride penetration, alkali silica reaction, and machine vibration (AIJ 2015). Since the degradation state due to machine vibration cannot be identified until cracks are visually identified as degradation phenomena, the soundness evaluation of the equipment support against machine vibration is seemed to be very rudimentary compared with the evaluation against other factors. On the other hand, although the number of load cycles due to vibration equipment, such as turbines, is very high in the normal operation of a nuclear power plant, since the stress ratio acting as the cyclic load is very low, it is considered that the fatigue caused by machine vibration would never damage concrete. In the cooperative research performed by electric industries for the common purpose, a method capable of evaluating soundness is investigated using S-N curves in the stage before cracks due to machine vibration can be visually confirmed. This paper reports the results of residual static strength tests of concrete cylinder specimens and headed stud anchor specimens conducted to obtain basic data for the purpose of establishing an evaluation method. In this paper, the equipment support means the reinforced concrete section with steel anchorage embedded for installing and holding equipment. A headed stud anchor was selected as steel anchorage because a large volume of its experimental data is available. Plain concrete specimens undergo changes of material properties with increasing number of load cycles. Plain concrete specimens exhibit increases in residual compressive strength and stiffness after cyclic loading. As the number of load cycles is getting closed to fatigue life, the residual strength and stiffness are decreased and become less than those without cyclic loadings. Bennett and Muir (1967) measured the residual strength after cyclic loadings of the plain concrete using 8×3×3 inch prism specimens with compressive strength of 40MPa to 64MPa. The tests were performed only on the 11 specimens which survived one million load cycles with speed of 4Hz to 8Hz at the maximum stress ratios of 0.67 and the minimum ratios varying 0.15 and 0.2 of the of static strength of prism specimens without cyclic loadings. The increase in the residual strength varied between 1% and 23% with average of 11%. Awad and Hilsdorf (1974) performed fatigue tests and creep tests using the 12×4×4 inch prism specimens with compressive strength of 28MPa. The maximum stress ratios were quite high at 0.90 and 0.95 of the static strength without cyclic loadings. There was an increase of 5% in the residual strength after cyclic loading or susGeneral Manager, Nuclear Power Department, Kajima Corporation, Tokyo, Japan. *Corresponding author, E-mail: kontani@kajima.com Deputy Manager, Nuclear Nuclear Power Department, Kajima Corporation, Tokyo, Japan. Assistant Manager, Power Generation Division, The Chugoku Electric Power Co., Inc., Hiroshima, Japan. Associate Manager, Power Generation Division, The Chugoku Electric Power Co., Inc., Hiroshima, Japan. Senior Manager, Civil and Architectural Engineering Department, Nuclear Power Division, Chubu Electric Power Co., Inc. Nagoya, Japan O. Kontani, N. Ishitobi, J. Kawada, N. Taogoshi, M. Koge and Y.o Umeki / Journal of Advanced Concrete Technology Vol. 14, 634-642, 2016 635 tained loading with the maximum stress ratio of 0.9 at life ratio of 0.3 which means at 30% cycles of fatigue life or 30% of duration to cause the creep failure. Ballatore and Bocca (1997) measured the residual strength after cyclic loadings using the plain concrete core specimens of 45.5 mm diameter and 140 mm long with compressive strength of 19MPa and 59MPa. The maximum stress ratios were at 0.1 and 0.2 and the minimum ratio was 0.02 of the static strength without cyclic loadings. The cyclic loadings of at most 7200 cycles were applied with loading speed of 1Hz. The increases in the residual strength and stiffness were 0.11 and 0.87 respectively for concrete with 19MPa and were 0.14 and 0.22 respectively for concrete with 59MPa. Bocca and Crotti (2003) measured the residual strength after cyclic loadings using the plain concrete core specimens of 60 mm diameter and 120 mm long with compressive strength of 24MPa, which were similar tests performed by Ballatore and Bocca (1997). The maximum stress ratios were at 0.2, 0.6 and 0.8 of the static strength without cyclic loadings. The cyclic loadings of 3600 cycles were applied with loading speed of 1Hz. The increases in the residual strength and stiffness were 5% and 6% respectively for the maximum stress ratio of 0.2, were 7.6% and 36% respectively for the maximum stress ratio of 0.6, and were 11.4% and 36.1% respectively for the maximum stress ratio of 0.8. Ballatore and Bocca (1997) stated that dynamic loading reduces pore space to improve material compaction and strain hardening takes place all the same time as is reflected in an increase in strength and stiffness after cyclic loadings. Bocca and Crotti (2003) stated that the test pieces subjected to preloading yielded more homogenous results, with a reduction in standard deviation, compared to non-preloaded test pieces probably because of a decrease in the stress peaks occurring in the material and a redistribution of stresses due to cyclic load induced creep. For the headed stud anchor, only one datum of the residual tensile strength after cyclic loadings was found in Matsuzaki, et al. (1981). With the stud diameter of 19 mm, embedded depth of 80 mm and concrete strength of 23.5MPa, the residual tensile strength after two million of load cycles was increased by 27% compared with the static tensile strength without cyclic loadings. The maximum and minimum stress ratio was 0.75 and 0.11 respectively with loading speed of 10Hz. The residual bond strength between deformed bar and concrete after cyclic loadings was found in Rehm and Eligehausen (1979). With the rebar diameter of 14 mm, embedded length of 42 mm and concrete strength of 23.5MPa, the residual tensile strength after two million of load cycles was increased by 5% compared with the static tensile strength without cyclic loadings. The maximum and minimum stress ratio was 0.5 and 0.1 respectively with loading speed of around 1Hz. 2. Concrete cylinder specimen This section reports the residual strength test results of the concrete cylinder specimens in order to obtain the very basic information of concrete under cyclic compressive stresses. 2.1 Outline of tests Table 1 shows the mix proportion of concrete. The concrete was manufactured using gravel and sand produced in Shizuoka Prefecture and ordinary Portland cement with water to cement ratio of 0.59. Compressive strength of 35MPa was the target. The cylinder specimens were 10 cm in diameter and 20 cm long and were cast in steel molds. The specimens were demolded the next day, and were cured and sealed in aluminum bags at 30°C for 4 months to prevent further strength development during fatigue tests. After 4 months of curing, cylinder specimens were still kept in aluminum bags until those were submitted to tests. It took 3 months to finish all the tests. During cyclic loadings and residual strength tests, the specimens were exposed to an atmosphere in the testing laboratory. Table 2 shows the maximum stress ratios and number of load cycles before residual strength tests for each specimen. Three levels of the maximum stress ratios (0.75, 0.65 and 0.33) were considered in the fatigue tests. The minimum stress ratio was 0.05 and the loading speed was 5 Hz in all cases. Figure 1 shows test cases in terms of the number of load cycles and the maximum stress ratios in the S-N curves. During tests, the compressive strains were Table 1 Mix proportion for cylinder specimens. Unit Mass(kg/m) ID W/C(%) s/a(%) water cement sand gravel WC59 59 42 158 268 781 1095 Additives : air entraining water reducing agent (1% of cement weight) air entraining agent (0.003% of cement weight) Slump :15±2.5cm, Air

Residual Static Strength of Concrete Cylinder Specimen and Stud Anchor Specimen after Cyclic Loadings