Toshimichi Ichinomiya

Observation of fracture process zone by laser speckle technique and governing mechanism in fracture of concrete

Fracture tests with a wedge-loading device are carried out on mortar and concrete specimens so as to have stable crack growth. Using laser speckle technique the length of macrocrack and the distribution of crack opening displacement are measured. Results are compared with those obtained by the boundary element method (BEM) analysis for a Dugdale-Barenblatt-type model of a fracture process zone. The governing mechanism in fracture of concrete and the mechanism which is represented by the model are discussed with special attention to the microcracking zone. It is deduced that a Dugdale-Barenblatt-type model does not represent the microcracking zone, thus implying that the microcracking zone and the bridging zone correspond to the pre-peak nonlinear part of the stress-strain curve in a uniaxial tension test and the post-peak tension-softening curve, respectively. It is concluded that the effect of microcracking on the maximum load is less significant than that of bridging. Possible models which include the effect of microcracking in addition to that of bridging are proposed.

Basic Study on Pretensioned Members with Ultra-High Strength Concrete

Ultra-high-strength concrete (UHSC) with compressive strength of 150N/mm2 makes it possible to reduce the weight of prestressed concrete structures. Particularly in case that UHSC is applied to pretensioned members, high bond strength can also be expected to reduce the transfer length. Considerable autogenous shrinkage of UHSC, however, reduces effective prestress and causes cracking due to the restraint of reinforcing steel. A new type UHSC with less autogenous shrinkage was developed, and its basic characteristics were investigated. Prestressing tests parameterizing the type of prestressing steel and the amount of introduced stress were also conducted to determine transfer length and to verify the effect of reduced autogenous shrinkage. Furthermore the predictability of effective prestress considering shrinkage and creep strain was studied.