The benefits of waffle or poly-raft slabs such as the original Firth RibRaft® floor system are well known. Firth has taken the RibRaft® floor one step further, through the introduction of the Firth RibRaft® X-Pod®. In addition to the environmental and user-friendliness of the RibRaft® X-Pod®, it is significantly stronger and stiffer, enabling it to be used for sites where the ground conditions may be challenging.
One example of challenging ground condition is the phenomenon of expansive soils. Expansive soils are soils that are likely to change in volume due to changes in moisture content. It can absorb large quantities of water due to rainfall or local site changes (such as water leaks, ponding) causing it to swell, or can shrink when dry, usually manifested by cracking of the ground. Such changes to the ground can result in loss of support to foundations and floor slabs.
Waffle slabs can be designed to provide sufficient strength and stiffness in order to mitigate the effects of expansive soils. Traditionally, this has largely been done based on the design approach presented in the Australian Standard AS 2870:2011. The New Zealand building code (B1/AS1) was updated in late 2019, where provisions for expansive soils were included and notably, the thresholds by which the different classes of expansive soils are classified were increased, when compared to the Australian Standard.
When designed correctly, waffle slabs should have increased levels of steel reinforcement, wider ribs, increased concrete strength or deeper floors (or a combination of any of the above), depending on the level of soil expansivity, geometry of the slab and building loads. The effects on the edges and the internal areas of the slab (as is often missed) need to be considered. Due to the tapering ribs and the ability to have closer spacing of the ribs, Firth RibRaft® X-Pod® is stronger and stiffer than typical waffle slabs, so it can be a cost-effective option, typically resulting in less steel reinforcement needed.
The same benefits can also be had for other soil conditions, such as the potential for liquefaction and softer ground resulting in uneven ground settlement. It is possible for RibRaft® X-Pod® to be designed for TC2 conditions in-lieu of slabs with additional steel reinforcement or steel fibres in concrete which is of particular benefit when concrete floors are to be exposed or polished.
In order to achieve the best result (a robust but efficient foundation) in any condition, an experienced structural engineer should be engaged to specifically design the slab to suit the building loads for the given ground conditions.