This stiffened slab is a structural panel that can be used to build floors or roofs. It’s essentially a concrete slab with rigid foam attached to the bottom and top for insulation. Made in layers, it consists of two sheets of plywood (or oriented strand board, OSB), each covered with a layer of cement and reinforced with steel rods (rebar).
Since its installation is faster than conventional foundations, you don’t have to pay for labour over time, instead, building costs are reduced upfront. It also uses fewer materials and is easier to transport, saving you money on both the purchase price and delivery cost.
What is a Stiffened Raft Slab?
This slab consists of a reinforced concrete slab that is supported by steel beams above and at the edges. The bottom of the slab rests on an unreinforced strip footing or mat foundation. This means that there are no cross-beams at the centre of the raft slab. Edge beam stiffening is necessary to provide lateral support and may be made of steel beams. The concrete layer is reinforced with rebars placed at regular intervals, which provides the slab with greater strength and stability.
The layers of wood and cement, as well as the positioning of the rebars, makes it possible to build it in any desired shape. It’s also possible to adjust the size of the panels, depending on how many are required to cover the floor surface.
Concrete is an extremely durable building material, which means that stiffened raft slabs are suitable for both residential and commercial construction projects. The high impact resistance of raft slabs makes them extremely suitable for any project that requires a floor in a basement.
The flexibility of raft slabs, along with its insulation properties, allows for very interesting design possibilities when it comes to both modern and traditional styles. The raft slabs can be cut into shapes that are more complex than conventional rectangular slabs, or they can be made to mimic elements that are traditional or historical features of your home’s design.
This raft slab is also very durable and fireproof, making them suitable for almost any type of project. They do not require the same amount of maintenance as other flooring materials because they may not rot, warp or become infested with pests.
What Type of Construction Method is Used for Construction?
The construction process used for building stiffened versions of a raft slab includes drilling piers down to stable soil and placing sections of heavy-duty unreinforced concrete between each pier. Then, cross members (which are often precast) are laid across these sections and are connected to each other with steel reinforcement bars. Finally, the concrete for the slab is poured onto the cross members.
How does It Support Itself?
The steel beams that frame the perimeter of the slab provide stability and help to distribute loads through the slab. The stiffness of this type of construction also reduces vibrations in high-rise structures, which provides added comfort to people who live or work in these buildings.
What are the Benefits?
The main benefit of building high-rises with stiffened raft slabs is that they have increased resistance to earthquakes compared with other types of foundation systems. They’re also lighter than cast-in-place concrete, which means they require fewer repairs over time.
What are the Disadvantages?
The steel beams required to frame the perimeter of stiffened raft slabs can increase construction costs. Businesses that own high-rises with this type of foundation system could regularly inspect the condition of their buildings for any evidence of structural damage or degradation.
Raft Slab
A raft slab is a type of concrete slab that is constructed by placing specially coated fabric between layers of concrete. The coating on the fabric provides a smooth surface for the raft slab finishing, which also impedes the ability of the concrete to absorb moisture. The fabric is then stiffened with a material such as metal or wood, which prevents horizontal cracking that would damage the structure.
A raft slab is typically thicker than a conventional concrete slab system, which means it can support heavier loads. The main features of a raft slab are that it has an impermeable and smooth surface, and it is stiffened on the bottom. Reinforced concrete footing can be used to support a raft slab, or steel beams can be poured between concrete piers.
Raft Slab Construction
Unlike other types of concrete slabs, which are constructed by placing a layer of wet concrete between two beams and allowing it to cure before adding another layer, the coating on the fabric used for a raft slab prevents the concrete from adhering to them. The coated fabric is placed upon a rigid platform and grooves are cut into its top edge. Reinforcing bars (one vertically and one horizontally) are then positioned into these grooves and encased in concrete. Once all the bars have been secured into place, another coat of concrete is poured over the top. This process is repeated until the desired thickness has been achieved.
What Type of Construction Method is Used for Construction?
Before pouring any concrete onto uncoated fabric, the fabric is stiffened to prevent it from shifting under load. There are three methods commonly used for stiffening: metal strips (either lead or steel), wood, and high-strength polymer fibres such as kevlar.
When wooden pieces are used, they may be specially treated with preservatives to prevent them from rotting. Untreated lumber may absorb moisture and eventually break down. The polymer fibre method uses a synthetic material consisting of very thin fibres that provide the necessary stiffness without adding significant weight.
What Dictates the Spacing of the Edge Beam?
The width of a raft slab is dictated by the thickness of the slab. The spacing between edge and stiffening beams is determined by the size and shape of the construction site, as well as its proximity to other buildings or underground features such as subway tunnels.
What are the Benefits?
A major benefit of this type of construction system is that it can be used to construct slabs with openings for pipes and ducts without requiring separate castings. In addition, since there is no concrete on top of the fabric, finishing work (such as applying paint) after completion does not require heavy equipment to be driven onto the part, instead, painters can walk directly onto the job site and do their work from above or below the raft base.
If the fabric is left exposed, it can be reused in the future as a noise barrier.
What are the Disadvantages?
The primary disadvantage is that any construction requiring very large openings (such as those used to accommodate stereos or other multi-floor equipment) may be cast using separate pieces of fabric and steel beams.
Another possible problem is careful consideration may be given to how the cavernous underside may drain and ventilate.
The edge beam may dictate how far from the building’s edge you can go before you hit an obstacle such as a subway tunnel or underground feature. The edge beam is the most important part of the construction.
Waffle Slabs
A waffle slab is a type of precast, prestressed concrete flooring unit. It typically consists of a thin layer of concrete spread over a closely spaced grid of wire mesh. After the concrete has been allowed to cure, strands of a high-tensile steel cable are stretched across the grid and anchored into place at each intersection. When tensioned, these cables force the grid as well as the surrounding concrete together, creating an extremely dense and strong slab that resists cracking far better than other types of slabs.
How do Waffle Slabs Differ from Standard Slabs?
The main difference between a standard slab and waffle slabs is their ability to resist cracks. Waffle slabs can withstand greater tension without breaking than conventional slabs because they are made of closely packed cells that are forced together during the curing process. This means that the pieces of concrete which make up waffle slabs act as one unit rather than individual pieces, creating far more strength and rigidity. Perimeter footing edge beams used with waffle slabs can be thinner than edge beams used in conventional slab construction, which reduces the weight of the structure.
Internal Strip Footings
An internal strip footing is a type of continuous foundation that consists of two or more bearing pads linked together at one end by steel beams. Strip footings resist bending and shear forces while allowing movement perpendicular to the direction of the beams. The bearing pads may be cast integrally with the slab or may be added later when pouring the foundation.
Engineered Slab
One benefit of an engineered slab is that it can be fabricated offsite, then delivered to the construction site for installation. This reduces the amount of time required for concrete pouring because the product only needs to be installed once. Like standard slabs, engineered slabs can often incorporate trenches or holes to accommodate pipes and other utilities which would otherwise require cast-in-place concrete. The precast nature of this type of flooring makes it ideal for retrofitting existing structures whose footprints do not fall exactly over a room’s sub-slab voids.
Another benefit is that steel reinforcement bars are usually available in smaller gauges than those used with a traditional poured-in-place concrete slab system, saving on concrete and materials costs.
Disadvantages of engineered slabs include a decreased fire resistance when compared with that of traditional poured in place concrete. In addition, because the pieces are meant to be installed all at once rather than built up in sections over time, they may typically carry more weight than comparable poured-in-place slabs.
What does It Mean for an Exterior Beam to Be ‘Fully Stressed’?
Fully stressed edge beams are rigidly bonded to walls or columns at both ends, rather than simply resting atop them, this ensures greater structural stability. Shallow edge beams, which are not fully stressed, only rest upon the walls or columns.
Edge beams have a variety of architectural detailing options, including pre-cast panel walls and embedded steel beams.
Can You Use Cut-off Beams?
Yes. Cut-off beams can be used in areas where two chambers may connect but there isn’t enough room for full height panels to bridge the gap between them. They’re also useful in situations where you don’t want to build all the way to the walls, but you still want strength in the corners.
Conclusion – Stiffened Raft Concrete Slab Overview
You now know the different types of foundation systems, how they support a structure and which one is most commonly used. Structural engineers can provide further insight into this subject, however, you could always consult with your local building authorities before making any decisions about whether or not to use a raft slab for construction.
