Petrographic observation and long-durability of concrete from sea fort No. 2 constructed more than 100 years ago

Abstract

Daini Kaiho is an artificial island constructed at the entrance of Tokyo Bay for the purpose of defending the capital city of Tokyo. Construction started with mounding the foundation in 1889 (Meiji 22), installation of cannons in 1900 (Meiji 33) and was completed in 1914 (Taisho 3). Present compressive strength of concretes taken from the north side concrete slope (constructed 1889 (Meiji 22)) on the foundation of the island, and the encircling wall of the battery cannon (constructed 1907 (Meiji 40)), yielded about 30 N/mm2. It was relatively high seeing that the designed compressive strength at that time was 15 N/mm2. Both concretes contained large irregular voids and large cement particles (maximum size > 200 μm) rich in II type belite, suggestive of a cement from a shaft kiln. Sand particles were fine-grained and round, occupying a large part of concrete section. Unhydrated cement particles were coarsely grained, about the same size as the fine aggregate. Hydration speed of those belite-rich cements was so slow that the compressive strength developed gradually over 100 years. Neutralization depth of concrete was also examined in this paper.