A recent trend is the dense arrangement of rebar in concrete structures. This makes it difficult to collect cores with a diameter larger than three times the maximum size of the coarse aggregate from actual structures. For this reason, strength estimation of structural concrete has been performed using cores with diameters less than three times the maximum size of the coarse aggregate, which are not covered by JIS A 1107. In November 2022, the Japan Society for Non-Destructive Inspection established the standard NDIS 3439, “Method of sampling and testing for compressive strength of small size drilled cores of concrete” as a method for estimating the strength of structural concrete. This paper introduces the content of the studies conducted up to the establishment of the standard and gives an overview of the standard.

The quantitative evaluation of tensile ductility as a material property was studied, assuming the reflection in design of the superior ductility of steel fiber reinforced concrete with 0.5％ to 1.0％ by volume of steel fibers with hook geometry at both ends. From the experimental results on the effect of mix proportion conditions on tensile ductility, it was confirmed that tensile ductility can be generally evaluated based on the amount of steel fiber and compressive strength. Further, taking into account the effect of quality fluctuations in actual production, a quantitative evaluation method for tensile ductility assuming reflection in design was proposed.

The quality of joint treatment greatly affects the performance and durability of structures. Conventional joint treatment agents have the issue of spray timing variability and uneven spraying, resulting in variability of treatment effectiveness. To address this issue, we have developed a joint treatment agent that enables stable treatment effectiveness by being sprayable immediately after construction and prevents uneven spraying through improved visibility after spraying, resulting in higher quality of joints. Furthermore, the developed joint treatment agent shortens the waiting time until spraying and enables joint treatment even with days off in between, contributing to increased productivity. This paper describes the joint treatment mechanism and the development process of the joint treatment agent, and introduces examples of application to actual construction.