Precast concrete floor planks are used for a variety of building applications, residential, commercial, schools, car parks and hospitals. The advantage to the designer is that the precast floor can easily be used in conjunction with masonry, concrete or steel structures and offers rapid construction with consistent quality. Using fibre reinforced concrete to replace traditional steel mesh reinforcement, offers further time and cost savings in addition to improved health and safety benefits.
After the precast flooring planks have been located, fixed, and grouted, a concrete topping or screed is poured on to the planks acting as the finished surface for non-structural leveling or in some instances for structural purposes.
It is therefore important to clearly identify the design and purpose of topping to ensure that the appropriate fibre type and dosage is adopted, if used to replace traditional mesh reinforcement.
Where the topping or screed has no structural performance requirement, the fibres provide crack control only. All load is assumed to be taken by the precast floor system.
However, where the topping is designed as exhibiting structural performance then checks should be undertaken to ensure its suitability. “Toppings and Interface Shear in Hollowcore Composite Floors” published by the British Precast Flooring Federation highlights that the following should be considered:
- Compressive stress in the topping
- Horizontal (complementary) shear at the interface between the topping and the floor planks.
- Durability requirements of the concrete.
- Cracking of the topping due to shrinkage and thermal movement during and after construction.
Consideration of the above will allow a determination of the stress developed in the topping including any diaphragm action. Traditionally, a layer of mesh reinforcement would be included in the topping to provide the required moment capacity and or tensile restraint.
The lifting, placement, spacing and fixing of mesh reinforcement can prove difficult, particularly on congested sites and during distancing measures for operators, making construction of the topping with mesh a time consuming and onerous task.
Fibre Concrete Solutions (FCS) can review the design of the structural topping and advise on an alternative fibre solution. Any fibre alternative must meet the requirements of EN14889 “Fibres for Concrete” (either part 1 or part 2 – dependent on the proposed fibre solution). If the purpose of the fibres is to enhance the load bearing capacity of the topping, or provide tensile restraint, it is important that a structural fibre solution is adopted and that the minimum dosage of fibres is appropriate. This can easily be checked against the “Effect on Strength” for the minimum dosage rate on the CE label of the product (also a statutory requirement) and shown on an individual product’s Declaration of Performance (DoP).
By adopting a fibre reinforced concrete topping, as a replacement to traditional mesh reinforcement, the need for the delivery of mesh to site, followed by the lifting and difficult placement of mesh on multiple levels is removed. Concerns over the position and cover to the reinforcement during the placement of the concrete are also eliminated.
Fibre Concrete Solutions are experts in fibre reinforced concrete and can provide the necessary experience and technical knowledge to provide robust, cost saving solutions. For further information on how we can assist on the design of fibre reinforced topping for Precast Floors, please contact us.
