As floors typically account for the largest share of a building embodied carbon footprint, new strategies to build low-carbon floors must be developed and implemented. Among these strategies, the reuse of existing components is a promising circular approach in which components are carefully extracted from buildings undergoing transformation or demolition and reused into new buildings. Recent research reviewed built examples that mostly reuse prefabricated reinforced-concrete (RC) components but also revealed a lack of design explorations for floor systems that reuse pieces extracted from cast-in-place (CIP) RC structures. This paper presents two new floor systems reclaiming pieces cut from typical CIP structures. In the first one, cut pieces are as long as the new span. In the second one, cut pieces are smaller and supported by girders as long as the new span. The environmental footprint of each new system is compared to conventional ones through a life-cycle assessment (LCA) and for multiple design parameters. In addition, a procedure is introduced to estimate the maximum cutting length that allows the reuse of pieces as simply-supported slabs. Results show that these lengths correspond to those of the donor slab if it spans up to 5.6 meters, depending on the slab thickness, construction period, and design uses. LCA shows radical carbon-footprint reductions that average 85 % for all simulations. Overall, results show that discarded concrete is a valuable material source for building innovative, efficient, low-carbon floor systems combining existing (de-)construction tools.