Cast in situ concrete, also known as cast in place concrete, is one of the most widely used structural methods in modern building design. Efficient for project timelines, cast-in-place concrete is poured and cured directly on site within formwork and around reinforcement of steel framework, forming key structural elements within a building.
From load-bearing walls and floor plates to balconies and roof parapets, cast-in-situ concrete meets the demand of any project. Its high conductivity rating makes it a leading source of thermal bridging, risking the energy efficiency of the building, allowing moisture to form and compromising structural integrity.
As sustainability legislation tightens and thermal performance becomes essential, understanding these effects should be built into structural building design, with solutions drawn into plans from the earliest stage.
What Cast in Place Concrete Is
Cast in situ concrete is concrete that is mixed, placed and cured directly at the construction site within the formwork. Unlike precast concrete, which is manufactured off-site in a controlled factory environment, before being transported to the site, cast in situ concrete is formed in its final position, conforming to the exact geometry of the structure being built.
The process is arguably more sustainable for the environment as it limits waste, by being formed on-site. It also ensures that all elements – walls, floors, columns and other connections are unified and free from leveling issues. The approach of cast in situ concrete also offers flexibility to construction workers, as it can be adapted to complex geometries, varying site conditions and evolving design techniques, which can be more difficult to achieve with factory-produced precast systems.
Cast in Situ Reinforced Concrete in Modern Construction
On its own, concrete is strong in compression but weak in tension. Combining concrete with embedded reinforcing steel (rebar), it combines concrete’s compressive strength with steel’s tensile strength. With that, cast-in-situ concrete appears throughout modern building design in a broad range of applications:
- Balconies and canopies
- The most common application of cast in situ concrete is balconies and canopies where typically these are cantilevered from the main floor slab, meaning they require continuous reinforcement running from the interior structure out to the exterior to withstand the load
- Parapets
- The walls that extend above a roof line at the building perimeter are frequently constructed in cast in situ concrete to directly bond to the building, ensuring superior structural stability, durability, and weather resistance.
- Slab edges
- Similarly to parapets, cast in situ concrete is used to maintain structural continuity at the building perimeter.
Cast in Place Concrete Walls and Thermal Performance
Modern building envelopes are designed around protecting the continuous insulation layer where possible. It is difficult to ensure continuous insulation stays intact as once windows, doors and other elements are installed this thermal layer is disrupted and reduces overall thermal efficiency.
Furthermore, as cast in situ concrete is a highly conductive material, any element made from this which interrupts the building envelope can create thermal bridges – weak spots that allow for the easy transfer of energy.
Managing Thermal Bridging in Cast in Situ Structures
With that in mind, insulation is a key element that must be carefully considered in order to minimize the risk of thermal bridging and to help improve the building’s thermal performance.
In some cases, it can be applied externally, wrapping the concrete face, reducing thermal bridging through the walls.
However, it can also occur in the steel framework, where it would be better to install solutions like thermal breaks to isolate these bridges and work toward improving the continuous insulation layer.
Frequently Asked Questions
Cast in situ concrete construction is the process of placing and curing concrete directly on site within the framework, forming structural elements in their final position rather than transporting factory-made precast units.
Cast in situ reinforced concrete is used wherever a project requires structural continuity, load transfer across complex connections and/or additional structural durability and stability. Common applications include balconies and canopies, parapets and slab edges. It is particularly prevalent in high-rise residential and commercial construction, where the flexibility of site-cast concrete allows structural engineers to adapt the design to varying conditions throughout the build.
Concrete’s relatively high thermal conductivity means that cast in situ elements which extend through the building envelope – such as balconies, parapets and slab edges – create thermal bridges that increase heat flow between interior and exterior environments. These bridges raise energy demand, lower internal temperatures (increasing the risk of condensation and mold) and can risk the integrity of the building. Addressing thermal bridging at cast in situ structural junctions improves the thermal performance of the building, whilst also helping the structure meet sustainability codes.
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