What Does Waffle Slab Foundation Mean In Construction?
What Does Waffle Slab Foundation Mean In Construction?
Waffle slab foundation is an innovative above-ground mat foundation that provides load-bearing capacity on challenging soils such as expansive, rocky, compressible, or hydro collapsible soils.
This foundation system comprises a series of plastic forms or “Waffleboxes,” which are typically 8.5 or 12 inches (220 or 300 mm) in height and 19 by 19 inches (480 mm × 480 mm) in the plan.
These Waffleboxes are placed directly on the ground to create a waffle-like grid of ribs, which is then covered by a reinforced concrete slab that is post-tensioned to ensure greater strength and durability.
The design of the Wafflemat foundation system provides exceptional stiffness, with the cross-sectional shape of the ribs offering the necessary resistance against differential swelling that can occur due to landscaping practices, surface drainage, or flooding.
Unlike other foundation systems, the Wafflemat foundation does not require presoaking underlying soil pads or the installation of footings, which eliminates the need for earth spoils.
Moreover, the Wafflemat foundation is typically elevated 12.5 to 16 inches (320–410 mm) above ground level, which means it does not require a capillary break or moisture barrier.
The Wafflemat foundation system provides all the benefits of in-ground rib and uniform thickness slabs but with a superior performance due to its unique geometry and smaller contact area with the soil.
The system conforms to the International Building Code and PTI requirements, making it an attractive option for engineers and architects who need to design reliable and sturdy foundation systems.
By 2008, most states had adopted the changes to the 2006 IBC, which made on-grade mat foundation a more popular design choice. The Wafflemat foundation system is an engineered system that already meets the 2008 design recommendations, which further enhances its appeal.
In 2008, the Post-Tensioning Institute (PTI) approved two key specifications for waffle-slab foundations, which were incorporated into the PTI Manual. These specifications were frequently referenced by building codes for post-tensioned slab design.
Section 4.2.2.2 of the PTI manual was revised to allow rib spacing of less than 6 feet (1.8 m) and clarify the spacing to be used to compute moments and shears.
Section 4.2.2.1.2 of the PTI manual was also revised to limit rib widths to a range of 6 to 14 inches (150–360 mm), which is narrower than the previous range of 8 to 14 inches (200–360 mm).
In summary, the Wafflemat foundation system is an above-ground mat foundation that offers exceptional load-bearing capacity on challenging soils. Its unique design provides superior stiffness and resistance against differential swelling, making it an attractive option for engineers and architects.
The system conforms to the International Building Code and PTI requirements, and the recent revisions to the PTI manual have further enhanced its appeal.
Features Of Waffle Slab Foundation
The waffle slab foundation is known for its high stiffness and rigidity, making it capable of resisting differential swelling caused by movements in the underlying soil. This type of foundation is constructed above the ground, eliminating the need for footings and pre-soaking of the soil before construction.
The thickness of the waffle slab section typically ranges from 14 to 20 inches, which makes additional barriers or coatings to prevent water or moisture penetration unnecessary.
The waffle ribs that provide the foundation’s structure can be made from single-use plastic forms or expanded polystyrene blocks, and they are arranged in series. A post-tensioned or reinforced concrete slab, which may also be fiber reinforced, is constructed monolithically with the waffles. This construction technique offers several advantages, including cost savings and faster construction times.
Overall, the waffle slab foundation is a durable and reliable foundation system that can withstand various environmental factors, making it an ideal option for many construction projects.
Waffle Slab Construction Process
Waffle slab construction is a straightforward process that involves pouring concrete on top of void forms known as Waffleboxes or waffle pods. Since the foundation is constructed directly on-grade, there is no need for trenching or off-hauling of dirt.
Once the Waffleboxes are positioned, post-tensioned cables are installed in the waffle slab formwork to support the weight. The Wafflemat Foundation System uses patented brackets to guide the cables across the base, ensuring a precise fit every time.
Engineers prefer using Wafflemat because the waffle slab thickness in the ground always matches their waffle slab plan. Construction crews can complete the job in a matter of hours instead of days for other slab types, which can save time and money.
However, waffle slab foundations are limited to structures that can stand on level ground. Other foundation types, like crawlspace foundations, use deep in-ground footings to compensate for unlevel areas.
