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The Formstress flat slab flooring system is a popular solution where an economic floor construction and a flat soffit is required. Precast flat slabs are cast in a 1200mm wide stressing bed allowing the manufacture of units up to a maximum of this width. A 75mm thick insitu concrete topping is cast over on site to produce a composite, versatile flooring solution.
Precast flat slabs are typically between 75 and 100mm thick. Special thicknesses (up to 175mm) can be provided on discussion with your Formstress engineer. All Formstress flat slabs are cast from 45 MPa (at 28 days) concrete. STC 55 rating can be achieved with a 75mm thick flat slab and 75mm thick topping. Fire ratings from 30 to 90 minutes can be designed into the unispan floor systems as required, depending on spans, continuity and floor thickness. Insitu topping concrete should be a minimum 75mm thick and achieve a minimum of 25 MPa at 28 days. Our Formstress engineers will produce the final design of your flat slabs, specifying the number and size of the prestressing strands based on your load case and the spans. Refer to NZS 3109:1997 Table 5.1 for precast size tolerances. Provide a minimum of 75mm end seating to flat slabs landing on concrete supports and 60mm on steelwork.
Before placing precast flat slabs, install any props to the required levels. This should be achieved to within ±3mm. The propping system (designed by others) must be adequate to carry all construction loads. Props should not be removed until the insitu topping has reached 75% of the specified concrete strength. Insitu concrete toppings should be water or membrane cured for at least 7 days. Floor systems with shorter spans may be installed without props on consultation with a Formstress engineer. Drilling holes through or cutting precast flat slabs should only be undertaken with the written approval of a Formstress engineer.
Contact Formstress about any specialist needs you may have. Formstress can in some cases improve fire ratings or arrange special concrete mixes to suit your situation.
Formstress will provide precast shop drawings of the project once a contract is signed. These must be signed off by the client, certifying that they are dimensionally and structurally correct before any manufacturing will commence. Formstress strongly recommends that your project architect and engineer review these documents as well.
All Formstress precast units are provided with lifting eyes. Chains and strops must be of the correct length and not more than 30 degrees of vertical.
For temporary storage of precast flat slabs provide bearers each end on level ground. Stack only units of equal length with bearers between layers vertically above each other.