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Prestressed Precast Concrete
Types of pre-stressed
precast elements
Prestressed precast slabs
Slabs are used:
- for structural ceillings and roofs
- in office buildings, houses, carparks, bridges
By Ola Kompanowska & Kinga Małecka
Types of prestressed slabs:
- TT slabs
- hollow-core slabs
- solid slabs
Pre-stressed beams and girders
Beams and girders can be designed in various shapes and lengths.
Those slabs are designed for realization of floor under high loads and large spans, allowing for reduction of intermediate columns.
TT slabs have negative deflection.
The hollow core slabs allow for covering the spans up to 20 m. They are lighter than solid slabs and provide good fire resistance.
The solid core slabs are able to transfer significant operational loads.
Installation can be done without any scaffolds, shuttering, or temporary supports.
Most commonly, pre-stressed beams are designed as:
- rectangular sections
- I-sections
- IV-sections
- IT-sections
Applications of prestressed precast
beams and girders:
Why is pre-stressed concrete used?
- industrial buildings
- sports halls and stadiums
- shopping centres
- multi-storey housing
- multi-storey garages
- energetic constructions
- infrastructure and bridge construction
- lower construction cost
- savings in materials, labor, and construction time.
- enables lighter-weight building structures.
- reductions in structural sizes for structural elements such as columns, beams, slabs
- bigger spans can be obtained
Steel
what is
pre-stressed
concrete?
Dimensioning of beams
Recommendations for the selection of precast elements
During dimensioning procedure we have to take into account 2 situations of loading: constant and primary.
- The prestressing steel's strength is approximately 4.5 times higher than ordinary reinforcing steel.
- Producers use mainly the following types of prestressing tendons:
- • Y 1860 S7, Ø12,5mm = 93,0mm2, fpk = 1860 MPa
- • Y 1860 S7, Ø15,5mm = 141,5mm2, fpk = 1860 MPa
- • Y 1860 S7, Ø15,7mm = 150,0mm2 fpk = 1860 MPa
Pre-stressed
precast
elements
Materials Required for
Pre-stressed Concrete
- It is recommended to design halls with specific grid of columns: 12x12, 12x15, 12x18, 12x21, 12x24 i 6x6, 6x9, 6x12, 6x15, 6x18, 6x21, 6x24
- Spacing of columns should be as big as it is possible
- The design should take into account possibility of reconstruction
- Hall should be high enough for all of machines
- In case of uneven settlement of the ground we should make statically determinate system
Pre-cast elements in practice
Concrete
- steel: performed with special high-tensile steels costs only twice as much per unit weight as ordinary reinforcing steel.
- concrete: prestressing requires concrete of high strength at early stage, with comparatively higher tensile strength.
- Prestressing of beams and slabs is most economically applied in long-line precasting mass-production operations.
- The precast prestressed structural and non-structural concrete units are usually in production prior to and during site and foundation preparation work.
- precast elements usually have substantial curing periods and enough time for initial shrinkage of the concrete material to occur prior to installation
- Precast slabs for roofing are better than trapezoidal sheet only for green roofs, when on the roof there is a lot of heavy machines, we need smooth surface on the bottom of slab or when there is possibility of explosion
- The width of the girder should not be smaller than 1/60 of span.
- Except for load bearing the support has to provide stiffness against twisting moments
- When there is axial load the crossection of the column should be a square with each side of the length of 300 mm or more.
- for eccentrically loaded columns the proportion between sides should be between 1/1 and 1/2
- high resistance concrete is used for manufacturing of pre-stressed elements,
- class C50/60 is most commonly used for pre-stressed girders, beams and slabs
- concrete should be designed as resistant to significant creep and shrinkage
- good bond between tendons and concrete must be provided
Limitations
Pre-stressed concrete is a type of concrete used in construction which is "pre-stressed" by being placed under compression prior to supporting any loads beyond its own dead weight.
This compression is produced by the tensioning of high-strength tendons.
- Precast, especially pre-stressed elements allow for reaching spans too big or too expensive to be obtained for monolithic elements
- prestressed elements may reach spans of 20 m and more
- The most effective crossections for precast elements are rectangular and I-shaped
- In case of big spans pre-stressed slabs are most commonly used but we have to remember that tendors limit freedom of hole creation
- Precast elements should be self-bearing on every stage of construction
Some aspects need to be carefully addressed:
• Prestressing needs skilled technology.
• The use of high strength materials is costly.
• There is additional cost in auxiliary equipments.
• There is a need for rigorous quality control and inspection.
References
[1] Stowarzyszenie Producentów Betonów Warszawa, Prefabrykacja – jakość, trwałość, różnorodność, Warszawa 2014
[2] PN-EN 13225:2004+AC:2006 PREFABRYKATY Z BETONU- Prętowe elementy konstrukcyjne
[3] PN-EN 1168:2005+A3:2011 PREFABRYKATY Z BETONU- Płyty kanałowe
[4] PN-EN 13747:2005+A2:2010 PREFABRYKATY Z BETONU-Płyty stropowe do zespolonych systemów stropowych
[5] PN-EN 13224:2011 PREFABRYKATY Z BETONU- Żebrowe elementy stropowe
[6] pekabex.pl
[7] fabet.pl
[8] consolis.pl