Precast Floors
Safety, quality, and the transfer of the installation process to the manufacturer are factors driving the growing market share for precast floors. Precast concrete units have significant time-saving advantages over propped formwork flooring methods. Reducing the construction programme and presenting the building ahead of schedule for rental are important cost-saving factors.
Units are manufactured to exact standards in factory conditions and installed by Health & Safety trained operatives. In addition, openings can be left for services such as cabling and service hangers and fixings installed as part of the precast sub-contract package. Precast floor units are typically supported by concrete or steel beams or on solid block walls. Precast concrete floors are particularly valued for their excellent sound insulation and fireproofing qualities.
Selecting a Precast Floor System
There are three main precast floor unit types. The specific structural characteristics of each type must be taken into account in assessing the most suitable option for a particular application. In some applications the structural capabilities such as spanning ability will determine the choice. In short-span buildings, structural and material efficiency or proximity to site might be the determining factor. The most suitable choice is relative to the specific project requirements and conditions.
Precast floor systems have a number of advantages:
a) Reduction in Frame Size
Precast floors can be designed to act compositely with the structure of the building to reduce frame sizes: e.g. main support beams supporting solid slab and screed can be designed as T-beams.
b) Progressive Collapse
Prestressed composite floors can be tied-in to the main structure and are therefore particularly suited to buildings where progressive collapse is a consideration.
c) Diaphragm Action
Precast floor slabs together with the structural screed provide a structural deck with full diaphragm action where required in multi-storey buildings.
Hollowcore Floor.jpg "Hollowcore Floor Units")
Hollowcore slabs derive their name from the circular voids or cores which run from end to end of the slab. The cores can function as cable/services ducts and significantly reduce the self-weight of the slabs, maximising structural efficiency. Units are available in standard 1200mm widths and in depths of 150mm to 500mm.
Hollowcore slabs have excellent long-span capabilities, achieving a maximum capacity of 2.3 KN/sq.m over a 16 metre span. The long-span capability is ideal for office, retail or car park developments. Units are installed with or without a structural screed, depending on requirements. Slabs arrive on site with a smooth pre-finished soffit. In car parks and other open structures, pre-finished soffits offer a maintenance free solution.
Solid Wideslab Floors
Solid Wideslab is sometimes referred to as "plate flooring" which is generally used in residential developments. Wideslab flooring contains an internal mesh/strand which facilitates notching and the forming of opes.
There are two distinct types of wideslab:
a) In situ screeded slabs:.jpg "Apartments - Wideslab Floors")
2400mm wide and in depths of 65mm to 200mm deep with spans of up to 7.5m. These slabs are generally for upper floors and have smooth self-finishing soffits. Some manufacturers can apply a specialist painted finish to the soffit as part of the contract. Generally a 75mm structural screed is required. A triangular steel lattice girder reinforcement which projects into the screed can be used to increase the spanning capabilities of the wideslab unit and the overall robustness of the floor. The pre-camber of prestressed units should be discussed with the technical department of the selected precast company.
b) Pre-screeded slabs:
150mm to 200mm deep are delivered self-finished for ground floor applications. These slabs are particularly suited to poor ground conditions or where ëcut and fillí is required. Maximum spans of 7.5m can be achieved.
Double-Tee Floor Slabs.jpg "Double - Tee Floor Units")
Double-Tee floor units are produced in standard widths of 2400mm and in depths of between 250mm and 1000mm. The system offers greater structural capacity at longer spans than hollowcore or wideslabs, but often requires a deeper floor zone. The Double-Tee system is the only system which offers a solution for spans over 16m.
Note: For design advice, span/load details and material specifications, please contact the Technical Service Department of the relevant precast company.
Precast Floors with Composite Toppings
Structural toppings can be combined with precast units to produce Precast Composite Floors. The advantage of Precast Composite Floors is the increased bending and flexural stiffness. Structural toppings can also be specified to improve acoustic performance and reduce vibration. In situ toppings will also improve thermal performance.
Structural toppings are generally 75mm to 100mm thick and it is recommended that their thickness should not be less than 50mm.
Provided there is sufficient anchorage/bonding with the precast unit, the topping and unit can be considered as a monolithic.
Checking Calculations
The following items should be checked when designing precast floors:
Applied Loading
Assumed Loading Distribution
Design Assumptions (structural screed, temporary propping, etc.)
Section Size
Strand Pattern - Number and Location of Prestressing Strands
Fire Rating
Cover
Service Stresses - Allowable vs Actual
Ultimate Bending Moments - Allowable vs Actual
Ultimate Shear - Allowable vs Actual
Deflection
Workshop Drawings
It is important that design assumptions made about the slab are reflected in the production drawings used to manufacture the slabs. For example, the drawings should confirm the designed reinforcement strand pattern, whether open cores are required for forming stability tie details and if temporary propping is required while pouring the structural screed.
Slab Widths
All three flooring systems can be produced in narrower widths than the standard widths indicated. However, sq.m prices for off-standard widths are higher than for standard widths. Consequently, buildings designed on grids which are a multiple of 1200mm will maximise the cost benefit of precast floors.
Precast Stairs.jpg "Precast Stairs")
Precast stairs do not require the propping and fixing which is necessary for cast in situ stairs. Stair units are connected to floors and landings using steel angle joints. Other connections such as continuous halving joints and intermittent halving joints are sometimes used. Combined stairs and landing units are also available. Once installed, precast stairs allow immediate, safe access to upper floors for following trades.
Precast stairs are particularly cost effective when duplicated. The greater the number of identical units required, the lower the cost.
Precast Balconies.jpg "Apartments Precast Balconies")
Precast balconies are manufactured mainly for use in apartment complexes. Units have steel bars projecting from the back which tie-in with the steel mesh in the structural screed of the precast floor units. Balcony units are temporarily supported while the structural floor screed is being poured. When the screed has set, the supports are removed leaving the balcony unit cantilevered from the floor edge.
Precast balcony units typically have integral drainage slots to receive drainage outlets and an upstand to facilitate proper weatherproofing details at door threshholds.
