Improved natural hydraulic lime mortar slab for compatible retrofit of wooden floors in historical buildings

Stiffening of wooden floors is often required in the rehabilitation of historical buildings to reduce their deformability in service conditions. Among various techniques available in the literature, the application of an extrados reinforced concrete overlay is often selected because cost/structurally effective and relatively simple to place. Nevertheless, Ordinary Portland Cement (OPC) concrete is considered poorly compatible with historical structures and materials and is, therefore, not well received, especially in case of valuable heritage and monumental constructions. A new retrofit solution based on the replacement of OPC with improved natural hydraulic lime (NHL) is proposed. Like the traditional OPC concrete overlay, the novel solution with improved NHL allows the conservation of the existing planks, and, thus, preserves the integrity of the wooden ceiling. The structural effectiveness of the proposed technique is investigated through experimental tests on full scale beams with different geometries. Full scale tests are supported by material tests and tests on the wood-mortar stud connection, a key aspect of the beam design. Based on the literature in the field of OPC concrete-wood composites, analytical models are developed and applied to predict the behavior of the NHL mortar-wood stud connection and composite beams. Results in terms of both stiffness and strength are encouraging and set the path for the application of NHL mortar overlays in the rehabilitation of heritage wooden floors.