The Difference Between Pre-tensioning and Post-tensioning Construction

the difference between pre tensioning and post tensioning construction
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The application of prestressing technology has greatly improved the bearing capacity and durability of bridges. According to the different sequences of prestressing tendons and beam concrete pouring during construction, it can be divided into two methods: pre-tensioning and post-tensioning.

Pre-tensioning and post-tensioning methods each have their unique advantages and limitations, and their main differences are as follows.

1. Technical Principle

The pre-tensioning method is to stress the prestressed tendons to the design value before pouring concrete, and then release the tension force when the concrete reaches the design strength so that the prestressed tendons and concrete can bear the load together. Its prestressing transmission principle mainly relies on the bonding force between concrete and prestressed tendons to prevent the elastic rebound of tensioning, so that the cross-section concrete structure obtains pre-compression stress to increase the load bearing limit.

Post-tensioning is a method of tensioning the prestressed tendons in the reserved channels in the concrete structure after the concrete is poured and reaches a certain strength, and transferring the tensioning force to the concrete through anchors. The principle of prestressing transfer is that when the prestressed tendons are tensioned, the tensioning force is transferred to the concrete structure through anchors, causing compressive stress in the concrete.

2. Construction Equipment

The main construction equipment for pre-tensioning method is:

1) Grooved pedestal: used to support prestressed concrete structure, usually the length and width are customized according to the project needs.

2) Beam and anchor plate: used to fix prestressed tendons to ensure stability during tensioning.

3) Pedestal jack: used to tension prestressed tendons to achieve the application of prestress.

pre tensioning construction

The main construction equipment of post-tensioning method is:

1) Steel bar cold drawing device: used for cold drawing of prestressed bars to improve their strength and toughness.

2) Jack and high-pressure oil pump: used for tensioning prestressed strands.

3) Grouting equipment: including mortar mixer, grouting pump, etc., which are used for grouting into the channel after tensioning to transfer prestress and fill the channel space.

3. Construction Technology

The construction of the pre-tensioning method mainly includes construction preparation → formwork installation → prestressed tendon laying → prestressed tendon tensioning to the design value → concrete pouring → prestressed tendon gradual relaxation.

The construction process of the pre-tensioning method is relatively simple, the stress loss of the prestressed tendon is small, and the transmission efficiency is high. However, it is necessary to provide fixed equipment such as pedestals or steel molds, which has high equipment requirements and occupies a large space.

The construction of the post-tensioning method mainly includes construction preparation → formwork installation → reserved hole installation → concrete pouring → insertion of prestressed tendons → prestressed tendon tensioning → anchoring prestressed tendons → tensioning and grouting.

The post-tensioning method has high construction flexibility, allowing prestressing to be applied after the concrete hardens, which is convenient for handling complex-shaped components and on-site construction conditions, and can more accurately control the prestressing force. However, the construction process is relatively complicated, requiring precise tensioning and anchoring technology, and strict requirements for concrete protection and duct grouting.

4. Applicable Scenarios

Pre-tensioning is usually applicable to the prefabrication of small and medium-sized prestressed concrete components produced in factories, especially in fixed prefabrication plants, which facilitates mass production of components, ensures quality and efficiency, and reduces the complexity and risk of on-site construction.

Post-tensioning is mainly applicable to on-site construction of large components and cast-in-place components, especially in bridges, high-rise buildings and other large projects where there is no fixed tensioning pedestal or a greater flexibility is required.

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