Laboratory tests of two-layer beams consisting of normal and fibered high strength concrete: Ductility and technological aspects

General concepts for the design of two-layer beams, consisting of fibered high strength concrete in a compressed zone and normal strength concrete without fibers in the tensile one, were developed about 2years ago. It was shown that such beams are effective when the reinforced concrete (RC) section carries rather big bending moments. Steel fibers have little effect on beams elastic deformations, but increase the ultimate ones, due to the additional energy dissipation potential of fibers. Changing the fibers content, a required ductility level can be achieved. Providing proper ductility is important for design of structures to seismic, wind, and other dynamic loadings. The idea of two-layer beams was further developed for pre-stressed beams that become, in this case, high performance concrete elements. It was demonstrated that calculation of fibers content for such elements is important, like that of reinforcing steel bars for usual RC beams. The current study is focused on finding optimal fiber content, yielding the highest Poisson coefficient, and consequently higher ductility of the beam's section. Fiber weight ratio is used as an alternative to fiber volume ratio as the first is a more accurate parameter for the definition of fiber content in the concrete mixture. Additionally, manufacturing technology and its influence on distribution of fibers in the beam's cross sections were investigated, as this distribution has a direct influence on the section's ductility. The experimental results, obtained in the frame of this study, form a basis for general technological provisions, related to manufacturing of two-layer beams.