Precast and in-situ Construction – Importance and stage-wise Design approach

Posted by: Mr Bhavdip Shah

Designation: Marine Structural Engineer

Date: May 22, 2023

Objective: Precast and in-situ construction methods are commonly used in the construction industry. Precast refers to the manufacturing of components off-site, usually in a controlled factory environment, while in-situ (also known as cast-in-place or site-cast) construction involves pouring concrete on-site into formwork to create the desired structural elements.

Stage-wise design refers to the process of designing a structure in different stages or phases considering the construction sequence and the specific requirements of each stage. When combining precast and in-situ construction methods in a stage-wise design, the overall construction process can be optimized for efficiency and cost-effectiveness. For example, using pre-cast elements as permanent formwork for larger concrete structures.

Here’s an overview of how this approach can be implemented:

Preliminary Design: During the preliminary design stage, the overall structure is conceptualized, and its general requirements are determined. Factors such as loading, building codes, architectural considerations, and functional requirements are considered.
The decision to use precast or in-situ construction methods can be made after the preliminary design. It is based on factors like project schedule, cost, and site conditions. For marine structures, site conditions always favour more precast works with the main limitation being the ability to handle the weight of elements.

Structural Analysis and Design: Once the preliminary design is complete, the structure’s components have been analysed for structural stability and strength. The method of construction is then considered and the design can be refined by incorporating precast and in-situ elements as required for different stages of construction. Precast/prefabricated elements may include piles, columns, beams, wall panels, slabs, fender blocks and staircases, while in-situ elements could include topping of all elements to making structural cores and monolithic connections.

Construction Sequencing: Based on the design and construction requirements, a construction sequence is developed. This sequence identifies which elements can be prefabricated off-site and which must be cast in place. Typically, precast elements that are repetitive and standardized, such as decks, fender blocks, service trenches, wall panels and beams. Complex or unique elements may need to cast in place to accommodate site-specific conditions.

Precast Manufacturing: In the precast stage, the manufacturing of the precast components takes place off-site. Precast factories/yards have controlled environments that allow for quality control and efficient production. Precast elements are manufactured according to the design specifications, using specialized moulds, reinforcement, and concrete mixes. Quality control can be more accurately applied and higher standard achieved. Once manufactured, the precast elements are transported to the construction site. Stage-wise design at this stage also needs to be considered.

In-situ Construction: In the in-situ stage, the site is prepared for receiving the precast elements. Major precast elements, like beams, decks, blocks are put in place and then traditional formwork systems erected around them (and using them) to enable the in-situ concrete to be placed. Precast elements are installed and connected using various methods, such as welding, bolting, or grouting, in-situ monolithic connections etc. In-situ concrete is poured (or pumped) to complete the structure and ensure structural integrity.

Integration and Finishing: Once the precast and in-situ elements are in place, integration work begins. This includes joint sealing, grouting, and other connection details to ensure a monolithic structure. The finishing touches, such as architectural finishes, electrical and mechanical installations, and interior fitouts, are carried out to complete the construction process.

In conclusion, adopting a stage-wise design approach that combines precast and in-situ construction methods, several benefits can be achieved, including:

Accelerated construction schedule: Precast elements can be manufactured concurrently with site preparation and in-situ construction reducing overall construction time.

Enhanced quality control: Precast manufacturing in a controlled environment allows for consistent quality and precision.

Improved site safety: Prefabricating elements off-site reduces the need for extensive on-site construction activities minimizing risks to workers.

Cost savings: The efficient use of precast elements can lead to cost savings in terms of labour, equipment and material waste.

It’s important to note that the specific implementation of stage-wise design with precast and in-situ construction methods may vary depending on the project requirements, local regulations, and the capabilities of the construction team and precast manufacturers involved.

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