A ground slab is the bottom-level reinforced concrete element of a building structure that rests directly on the prepared subgrade and transfers vertical and horizontal loads from walls, columns, or partitions into the ground. It forms the lowest horizontal surface of a foundation system and, depending on design, also serves as the final floor finish substrate, a vapor and moisture barrier carrier, or part of a basement enclosure.
Function and Load Transfer
The ground slab distributes building loads over a defined bearing area so that soil pressure stays within the capacity of the subgrade. Unlike strip or pad footings, which concentrate loads at specific points, a ground slab spreads loads across its full surface, which makes it suitable for subgrades with limited bearing capacity or for structures without basements where a raft-type foundation is more economical than individual footings.
Construction Types
Three common configurations exist: the slab-on-grade, poured directly on a prepared base course without structural connection to perimeter footings; the raft or mat foundation, structurally integrated with foundation walls and designed to act as one continuous bearing element; and the insulated ground slab, which incorporates a rigid insulation layer either below or above the concrete to reduce thermal bridging at the building envelope. Selection depends on soil conditions, groundwater level, and whether the space below is heated or used as living area.
Waterproof Concrete Design
Where groundwater or capillary moisture is present, the ground slab is frequently executed as part of a watertight concrete structure, commonly referred to as a WU construction (Wasserundurchlässiger Beton). In this approach, the concrete mix itself, together with controlled crack widths and sealed joints, provides the water barrier instead of relying solely on applied membranes. This method is distinct from slabs combined with separate bituminous or plastic sheet waterproofing, which remain necessary where the WU concept cannot be applied consistently, for example at penetrations or where crack control cannot be guaranteed.
Subgrade Preparation
Before placement, the subgrade is leveled and compacted, and a capillary break layer of coarse granular material is typically installed to interrupt rising moisture. A blinding layer of lean concrete may follow to provide a clean, even working surface for reinforcement placement and formwork. Frost-susceptible soils require additional measures, such as frost protection layers, to prevent differential heave beneath the slab.
Reinforcement and Joints
Reinforcement is placed as one or two layers of welded mesh or rebar, sized according to the loads transferred and the structural system chosen. Movement and construction joints are positioned to control shrinkage cracking and to separate slab sections where differential settlement or thermal movement is expected, particularly at transitions to foundation walls, columns, or adjacent slab pours.
Interfaces with Other Building Elements
The ground slab connects to foundation walls, perimeter footings, and vertical waterproofing systems at its edges. These interfaces require careful detailing to maintain continuity of the water barrier and thermal insulation, since discontinuities at joints are common sources of moisture ingress or thermal bridging in finished structures.