A waffle slab foundation, also termed as ribbed slab foundation, is a structure that is plain at the top and has a grid-like system (called the ribs) at the bottom. The ribs are formed due to the pouring of concrete over a set of plastic waffles arranged in series on the ground. This type of foundation is more rigid and performs better on expansive or hydro collapsible soils.
The main features and construction details of the waffle slab foundation are described below.
Features of Waffle Slab Foundation
A waffle slab foundation is highly stiff and rigid in nature. It has the strength to resist any form of differential swelling caused due to the movement of underlying soil by flooding, surface drainage or any other reason.
This type of foundation is constructed above the ground and hence does not demand footings. It also avoids the need for pre-soaking of the underlying soil for the construction.
Mostly, the thickness of a waffle slab section ranges from 14 to 20 inches. Hence, the foundation does not require any additional barrier or coating to prevent capillary water or moisture penetration.
The waffles used to create ribs can be single-use plastic forms or expanded polystyrene blocks. These waffles are arranged in series as shown in figure-2. A post-tensioned or fiber reinforced or reinforced concrete slab is constructed monolithically with the waffles.
The figure-3 below shows the structural behavior of a waffle slab foundation compared with uniform thickness slab-on-ground under the action of uplift pressure from the ground.
When the underlying soil expands, it exerts pressure underside and pushes the slab upwards. This creates cracks in a uniform slab. In the case of a waffle slab foundation, it facilitates the soil movement, thus reducing the effect of uplift force. Waffle slab foundation, therefore, prevents cracks.
Also Read: Waffle Slabs and Ribbed Slabs
Construction Steps for Waffle Slab Foundation
The waffle slab foundation area is marked and leveled on the ground. Along the line, formwork is set up to employ waffle arrangement which is then followed by the concreting process. The steps involved in the construction of the waffle slab foundation are:
Step 1: Arrangement of Waffles within the formwork area.
Step 2: Waffles are connected together to keep them in place throughout the service period. Over the waffles, slab reinforcement is placed in a two-way direction.
Step 3: Concrete is poured throughout the formwork.
The figure above shows the 3D view of the final waffle slab foundation. Bottom of the foundation has waffles that help in reducing the upward pressure from the ground and the differential settlement of the structure.