Prestress Concrete Technology

By prestressing reinforced-concrete elements, the optimum use of the basic structural materials is achieved. Compared to conventional precast elements where the Prestress Concrete Technology was not applied, this results in a potential savings of 10-20% in construction materials.

The basic principle of prestressing reinforced-concrete precast elements is to apply prestress on an elements, which leads to better mechanical properties. Thanks to the optimum use of structural materials in the prestressed precast element, it is thus possible to significantly reduce its structural height or the number of elements in the structure to be built.

Prestressing technology is used, for example, for hollow-core slabs, girders (I, T, TT-shaped), trusses, columns and wall and slab elements.

When producing reinforced-concrete precast elements from prestress concrete, high-strength prestressing steel cables, which also reinforce the element, are inserted into the moulds for concrete. In exceptional cases, non-metallic carbon fibre reinforcement is used.

The prestressing technology is placed at the start and end points of moulds for concrete for the production of precast elements.

The steel cables are tensioned by means of a special device (stressing jacks), which is controlled by pressure oil from the associated hydraulic unit.

High-strength certified anchors, which grip the steel cables and clamp onto the steel crossbars, are used for locking the stressed cables. The steel cables pass through steel crossbars.

All tensile elements created by the stressed cables are transferred through the anchors and crossbars to steel girders, called bracers.

The bracers are built into the foundation of the surrounding structures of precast production plants. A pair of bracers is always used – at the start and end of the prestressing line (in front of and behind the mould for concrete).

The bracer needs to be able to transfer all loads from the prestressed cables. Depending on the type of reinforced-concrete element being produced, tensile elements of up to 8000 kN are commonly…

The basic principle of prestressing reinforced-concrete precast elements is to apply prestress on an elements, which leads to better mechanical properties. Thanks to the optimum use of structural materials in the prestressed precast element, it is thus possible to significantly reduce its structural height or the number of elements in the structure to be built.
Prestressing technology is used, for example, for hollow-core slabs, girders (I, T, TT-shaped), trusses, columns and wall and slab elements.
When producing reinforced-concrete precast elements from prestress concrete, high-strength prestressing steel cables, which also reinforce the element, are inserted into the moulds for concrete. In exceptional cases, non-metallic carbon fibre reinforcement is used.
The prestressing technology is placed at the start and end points of moulds for concrete for the production of precast elements.
The steel cables are tensioned by means of a special device (stressing jacks), which is controlled by pressure oil from the associated hydraulic unit.
High-strength certified anchors, which grip the steel cables and clamp onto the steel crossbars, are used for locking the stressed cables. The steel cables pass through steel crossbars.
All tensile elements created by the stressed cables are transferred through the anchors and crossbars to steel girders, called bracers.
The bracers are built into the foundation of the surrounding structures of precast production plants. A pair of bracers is always used – at the start and end of the prestressing line (in front of and behind the mould for concrete).
The bracer needs to be able to transfer all loads from the prestressed cables. Depending on the type of reinforced-concrete element being produced, tensile elements of up to 8000 kN are commonly generated. This resulting force, depending on the height of the element being produced, produces a bending moment. The bending moment is transferred to the steel bracers into the foundations of the surrounding structures of precast production plants.
The stressed cables are encased in concrete and become part of the steel reinforcement of the precast element. After reaching the required strength of the concrete, the tempering process can begin.
Note: The tempering process takes place inside the mould. The reinforced-concrete precast element must not be moved while curing to obtain the required strength.
Due to the loosening (tempering) of the steel cables, the precast element absorbs the stress in the cables, which results in the required prestressing of the reinforced-concrete elements. Prestressing the element causes it to rise.
The tempering process is carried out by tempering hydraulic cylinders. These hydraulic cylinders allow transferring all tensile elements of the cables (usually 4000 kN per cylinder, depending on the application) into the steel bracers.
To eliminate residual stress in the cables, they need to be sufficiently loosened. The required cable loosening is performed by lifting/retracting the tempering cylinders.
Note: Before starting to stress the steel cables, the operator extends the rollers to the desired level and secures them with spreader bars. The spreader bars prevent the cylinders from retracting during the stressing. Before tempering, the spreader bars need to be removed.
The value of the required stroke of the tempering cylinders is calculated from the ratio of the length of the precast element to the free length of the cables. As a rough guide, we recommend 7 mm per 1000 mm of free steel cable length, by which the cylinders have to be retracted in order to stretch the cables.
Example: For a 50 m long prestressing line, where a 30 m long precast product will be produced, a cylinder stroke of at least 140 mm is required.
Included in our delivery is consultation before the actual production, complete engineering, production, assembly of steel units and the supply of all technology (stressing jacks, anchors, hydraulic aggregates, tempering cylinders, steel cable stands).
We can also offer our customers the production of moulds´for concrete in combination with the Prestress Concrete Technology, see Formwork system(proklik na naše stránky) .
The prestressing technology can be integrated into a self-supporting mould for concrete. Self-supporting moulds include all the above-described technological equipment for applying prestress on a precast product. Bracers (steel girders) are replaced with a rigid frame which is a part of the self-supporting mould. The mould is anchored into the surrounding structures of precast production plants without the need to make spreading foundations for bracers.
 
Steel cables
The high-strength seven-strand steel cables comply with the standard for steel reinforcement in reinforced concrete, EN 10138-3 Y177 or EN 10138-3 Y1860 S7. The nominal cable diameters are as follows:
- 12.5 mm or 12.9 mm – designation T13 or T13S (for a 12.9 mm cable)
- 15.3 mm or 15.7 mm – designation T15 or T15S (for a 15.7 mm cable)
The nominal tensile strength of prestressing cables is 1770 MPa or 1860 MPa. The values of the tensile force in the cables are 152-246 kN/cable, depending on the diameter and strength of the selected steel cable.

Prestress technology_prestressing cables 28x225kN

Prestress technology_hydraulic cylinders_2

Prestress technology_hydraulic cylinders_1

Prestress technology_Upper prestress technology

Technical specifications

tensioning line length: not limited in production (normally up to 100 m)
resulting axial force in the cables: normally 8000 kN (35 cables × 225 kN)
element height: approximately 100–2000 mm
element width: not limited
upper prestressing: YES

Concrete buckets

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Moulds for the production of structural elements

Moulds for the production of structural elements are a good solution especially when producing reinforced-concrete precast elements such as smooth and cantilevered columns, girders, load bearers, and trusses. With the appropriate structural adjustment made to the mould, the shape of the produced precast element can be easily and efficiently changed.

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Prestress Concrete Technology

Technical specifications

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