Pre-tensioning Construction Technology

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1. The concept of the pre-tensioning method

The beam-making process of the pre-tensioning method is to stretch the pre-stressed steel bar before pouring the concrete, and temporarily anchor the pre-stressed bar on the tension stand. When the component concrete meets the strength and elastic modulus (or age) specified by the design, gradually relax the prestressed tendons. In this way, due to the elastic shrinkage of the prestressed tendons, the prestressing of the concrete is obtained through the bonding between the prestressed tendons and the concrete.

2. Application scope and advantages of the pre-tensioning method

(1) Scope of application: The pre-tensioning method is mostly used in the production of shaped small and medium-sized components in the prefabricated component factory, and is also commonly used in the production of prestressed bridge span structures.

(2) Advantages: fewer production procedures, simple process, easy to ensure construction quality, no permanent anchors on the components, and low production cost. On the long-line pedestal, multiple components can be produced at one time.

3. Equipment and machinery for pre-tensioning construction

(1) Pedestal

The pedestal is one of the main pieces of equipment produced by the pre-tensioning method, and it bears all the tension of the pre-stressed tendons. Therefore, the pedestal must have sufficient strength, rigidity, and stability, and at the same time meet the requirements of the production process. The pedestal method is generally used for the production of small and medium-sized components with steel wires as prestressed tendons.

The pedestal structure includes a pier pedestal, trough pedestal, and frame pedestal. The selection depends on the component type, tension tonnage, and construction conditions.

(2) Grips

The grip is a temporary anchoring device to maintain the tension of the prestressed tendons and fix them on the pedestal during the pre-tensioning method. The fixture can be removed and reused after the component is made.

Commonly used anchor grips include: 

a. Cone anchor: The cone anchor is used for the prestressed steel wire. It is suitable for anchoring cold-drawn low-carbon steel wire and carbon (scored) steel wire with a diameter of 3 to 5 mm. It can be used for both the stressing end and the dead end.

b. Single-hole three-piece anchor: The round three-piece anchor is used to anchor prestressed steel bars or wires, and consists of a single-hole anchor head and three-piece wedges.

c. Button-head anchor: Button-head anchor is used for anchoring the dead-end of prestressed tendons.

(3) Tensioning equipment

In the pre-tensioning method, the prestressed tendons can be tensioned individually or in groups. Commonly used tensioning equipment includes YDT series pedestal jacks, high-pressure oil pumps, and winches. The tensioning equipment should be used in conjunction with anchors, and the tensioning equipment and meters should be regularly maintained and checked.

4. Pre-tensioning construction technology

(1) Laying of prestressed tendons

The surface of the pedestal should be painted with a release agent before laying the prestressed tendons, and a non-oil-based template release agent should be used. The isolating agent shall not contaminate the prestressed tendons, so as not to affect the bonding between the prestressed tendons and concrete. If the prestressed tendons are contaminated, they should be cleaned with a suitable solvent. During the production process, rainwater should be prevented from washing off the release agent on the countertop.

(2) Tension of prestressed tendons

Before tensioning, the pedestal, beam, and various tensioning equipment should be checked in detail, and operations can be carried out only after meeting the requirements. The tension of prestressed tendons is divided into single tension and overall tension. In order to make the initial stress of the multiple tendons basically equal when the whole batch is stretched, the initial stress must be adjusted before the overall stretching, and the stress is generally 10% to 15% of the tensile stress.

(3) Pouring and curing of concrete

Concrete mixing must be proportioned according to the pre-determined mixing ratio, and its strength and elastic modulus must meet the design requirements. When mixing, master the best mixing time (2min～3min) and concrete to ensure the aggregate particle size and cleanliness, and the measurement must be accurate. The beam inner mold uses an inflatable rubber bag. After the beam completes the concrete of the bottom plate, it is inflated after passing through the airbag, and the pressure is maintained at 0.03MPa～0.05MPa, and then the beam is continued to be poured. The pouring interval of other parts of the upper and lower floors should be controlled within the initial setting time of the bottom concrete.

The concrete should be watered and cured immediately to ensure sufficient moisture, and temperature, and to prevent adverse effects caused by sunlight, strong wind, and impact. The curing time is not less than 7 days.

(4) Release of prestressed tendons

When the prestressed tendons of the pre-tensioning method are laid out, the strength of the prestressed concrete component must meet the design requirements. If the design is not specified, its strength shall not be less than 75% of the standard value of the designed concrete strength. For overlapping production components, the prestressed tendons can be released only after the strength of the last layer of components reaches 75% of the design strength. The premature release will cause greater loss of prestress.

5. Matters needing attention

(1) Before the construction of the project, organize relevant personnel to conduct safety training. Before each tensioning, a special person should be arranged to inspect the steel strand jacks, tensioning pedestals, beams, and other facilities, and if any problems are found, they should be dealt with in time.

(2) Personnel entering the venue must wear safety helmets.

(3) Before the stretching operation, warning signs should be set around, and a special person should be assigned to take care of site safety. The fan-shaped dangerous area radiates at an angle of 45 degrees between the two outer steel strands at both ends of the pedestal. Tensioning and anchoring operators must stand in a safe place on the side, and it is strictly forbidden for onlookers and miscellaneous personnel to enter the tensioning operation area to prevent the steel strand from breaking off the fixture and hurting people.

(4) When the concrete specimen reaches more than 80% of the design strength, it can be released. The load of the oil pump must be slow when unwinding, the oil standard reading of each oil pump should have little difference, and the jack unloading time should not be less than 10min.

(5) Tensioning operators should not be replaced frequently, and should maintain relatively stable and trained operations.

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