Half-precast “crossbar-slab-column” frame joint

Introduction. Half-precast frames embodied the positive properties of both precast and cast-in-situ structures. These structural systems are difficult to introduce due to their low level of knowledge, lack of experimental data, and an almost complete absence of a regulatory and technical framework for a design. This stimulates a need to improve and develop new joints with high operational reliability and manufacturability.

Aim. To identify the causes of the frame joint destruction.

Materials and methods. The frame joint is provided by the hidden cast-in-situ column cantilever in the grooves of crossbars and half-precast floor slab with the installation of additional longitudinal and transverse reinforcement. Precast and cast-in-situ elements interact mainly by reinforcement starter bars. The destruction of frame joints was considered using the example of a half-precast frame with the main spans of 9 and 12 m in a two-storey building according to the results of a full-scale survey during the construction period.

Results. At the joints of most crossbars with columns, inclined cracks were detected in the support zones along the lateral surfaces with an opening width of 0.1–5 mm. In addition, cracks were observed along the contact area of the support crossbar groove with the column grouting concrete. An opening revealed them spreading into the body of the groove grouting concrete along the column edge. No adhesion between the walls of a crossbar groove with the joint grouting concrete is observed. The crack formation pattern indicates the determined contour of the floor slab punching above the column.

Conclusion. The cause for the destruction of half-precast frame joints involves the insufficient bearing capacity of the section for the action of transverse forces due to the ductility of precast and cast-in-situ elements.

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