### 8. 英文概要

Title: A Development Study on Reinforced Concrete Elements with Clinker-Free Concrete Consisting of Waste-concrete-slug Recycle Cement and By-product admixture Project number: 3K123015

Principal investigator: Tetsushi KANDA

Responsible organization: Kajima Corporation

For the reduction of carbon dioxide emission and energy consumption, in addition to the formation of recycle-based society and waste management, following three technical development strategies I, II and III are targeted in this project. These include development of sludge-recycled cement (SR cement) originated from returned concrete as a typical construction by-product, and versatile application of clinker-free concrete (R-CF concrete) to precast building constructions using high volume industrial by-products such as fly ash and the SR cement.

I. Recycling and reduction of the amount of disposal of returned concrete that is a major construction by-product in the construction industry.

II. Control of carbon dioxide emission and energy consumption in the construction industry.

III. Effective use and reduction of the amount of disposal of coal ash, which became important after the East Japan Great Earthquake.

Towards the goal, the following four studies have been performed. i) Development of SR cement originated from sludge-recycled binder often used as a soil improvement filler, ii) Examination of materials and mix proportion for structural R-CF concrete, iii) Durability evaluation of the R-CF concrete, and iv) Ensuring structural - durability performance and manufacturing method for precast R-CF concrete elements.

Development of SR cement was executed using full-scale model plant and manufacturing processes were improved according to the targeted quality. It was confirmed that strength more than 40 percent of the strength of ordinary portland cement was obtained by controlling the period for sludge dehydration even under production in summer where the quality may degrade due to the reduction in unhydrated component.

Method of mix design for R-CF concrete was studied through mixing experiments of mortar and concrete and a prospect of achieving a design strength more than 24N/mm $ ^{2} $ and favorable construction performance was obtained. Furthermore, a method estimating compressive strength from materials combination and water-binder ratio was established. Control of changes in fluidity with time was an issue of fresh state while addition of gypsum, which may be lost before SR cement production, was found to be effective for the improvement.

Durability evaluation of the R-CF concrete was studied in terms of (1) carbonation resistance estimation through accelerate carbonation test and durability estimation through long-term outdoor exposure test, (2) shrinkage characteristics and its control, and (3) cracking resistance evaluation through restrained cracking test using imitated precast members. In the study (1), the accelerated carbonation test results allowed mix design of R-CF concrete to be optimized for strength resulting in the carbonation resistance equivalent to that of blast furnace slag cement type B concrete. The test results were further led to an empirical formula of carbonation rate coefficient, which was then applied to a durability design capable of providing design values of cover depth and necessary compressive strength according to the design service life classes. Further, to confirm the carbonation resistance under working conditions, full-scale wall elements and cylinder specimens of $ \phi $ 100x200mm were subjected to long-term outdoor exposure test for 10 years. In the study (2), it was found that shrinkage strain of R-CF concrete was approx. 2x10-4 larger than that of normal concrete while it became smaller than that of normal concrete when gypsum was added. These findings allowed us to produce R-CF concrete with a shrinkage strain less than 10x10-4. In the study (3), it was observed that no cracking was found for imitated precast R-CF concrete specimen during the restrained cracking test, implying that excessive shrinkage cracking of precast R-CF concrete members was unlikely.

Ensuring structural-durability performance and manufacturing method for precast R-CF concrete