Development of a basalt fiber-reinforced composite duct for post-tensioned functionally graded concrete structures

This paper presents the development of a mineral-based composite duct for the placement of unbonded tendons in functionally graded concrete slab systems. In this regard, a highly innovative field of research is addressed, as studies in this direction are limited, and the focus has been on ducts made of steel or polymers. The proposed composite profile is produced through a pultrusion process and consists of basalt fibers and a mineral matrix of chemically bonded phosphate ceramics (CBPC). Metrology studies as well as imaging via scanning electron microscope (SEM) have been conducted to verify production quality. The investigations carried out have shown that the targeted impregnation of single basalt filaments has been achieved and that the deviations between the produced and target geometry are small, accounting for approximately 10 % of the cross-sectional area. This demonstrates the suitability of the chosen production method. Additionally, characterization of mechanical properties has been carried out through experimental testing. Results have shown average tensile and flexural strengths of 124 MPa and 38 MPa, respectively. The concluding question, of whether the manufactured profile is suitable as a duct was investigated in a casting test with a high-flowable concrete. No leakages have been visually detected, which proves the applicability of the produced basalt fiber-reinforced CBPC composite duct, fostering a mineral-based prestressed and post-tensioned concrete construction with high recycling potential.