Iakov Iskhakov

Experimental Investigation of Full Scale Two-Layer Reinforced Concrete Beams

Two-layer beams (TLB), consisting of steel fibered high strength concrete (SFHSC) in the compression zone and normal strength concrete (NSC) in the tensile one, are studied experimentally. The current study is based on results of previous theoretical investigations and tests that showed high efficiency of such beams, carrying rather big bending moments. According to the previously developed concept, the fiber content for TLB should be calculated, corresponding to the required ductility level of an RC element. Following the results of a previous experimental study that were carried out to select the optimal fiber ratio, the current research is focused on testing full scale TLB. The study is aimed at experimental verification of the data related to interaction of concrete layers in TLB and proving the efficiency of two-layer bending element from beginning of loading and up to the ultimate load stages, including collapse. Development of Poisson deformations was studied to compare them with those obtained in the previous study for cylindrical specimens. The SFHSC layer was cast after the NSC hardening to study the influence of separate casting technology that is more convenient for TLB production in real construction. Three identical specimens with constant fiber content, corresponding to the proper ductility level, were prepared and tested by four-point loading. The results demonstrate the role of fibers in a high strength concrete layer and form a basis for development of TLB design provisions.

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 2 years 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 beams 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 beams cross sections were investigated, as this distribution has a direct influence on the sections 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.