Precast Concrete: The Complete Guide - Converge

Precast concrete is a form of concrete prepared for casting, cast in a reusable mould and cured in a controlled environment away from its final destination.

Precast concrete units may travel a few metres when precasting on-site, or thousands of kilometres when the components are manufactured on a different continent.

Precast concrete elements can be joined together to build a complete structure. There are two types of connections: 

• Dry joint: the components are bolted together. This requires embedding steel connectors in the concrete during casting.
• Wet joint: reinforcement bars protrude from the precast concrete components. Additional reinforcement is installed fixing both elements, and cast-in-place concrete is poured to finish the connection.

Precast concrete is used to build all kinds of products (e.g., traffic barriers, sewage pipes, retaining walls) and structures (residential buildings, bridges, railway stations).

Origins of precast concrete

The history of the modern-day precast concrete industry began in the 1900s when John Alexander Brodie, an English civil engineer, discovered that precast concrete components could be joined together to build structures efficiently. He was the first to get a patent (no. 6,115), in 1901, for the process of creating precast concrete panelled buildings.

That same year Brodie started 18 months of trials in Cobbs Quarry, Everton. Casting concrete in vertical steel moulds, he built a prototype cottage in 1903 based on the patent drawings.

In 1905 Brodie built the first precast concrete tenements in Eldon Street (Liverpool): a three-story 12-apartment block of flats (demolished in December 1964).

He also exhibited a precast concrete cottage at the 1905 Cheap Cottages Exhibition in Letchworth Garden City. Now it is a Grade II* listed building located at 158 Wilbury Road, SG6 4JD.

In 1906 he built the corporation stables at Walton (demolished in 2002).

Brodie’s work influenced Grosvenor Atterbury, a New York architect who developed in 1908 a system of construction based on hollow-cored, precast concrete units with story-height wall panels. The components were factory-made and, like Brodie’s, transported to the site and hoisted into position by crane.

Atterbury built the houses at Forest Hills Gardens (New York City) with precast concrete panels between 1910 and 1918.

Benefits of using precast concrete

1. Speed of construction: Precast concrete elements can be manufactured off-site, allowing for faster construction times.
2. Quality control: Precast concrete units are manufactured in a factory environment, allowing for higher quality control than casting concrete on-site.
3. Economy on site: Using precast concrete components reduces the need for temporary works, requires fewer workers, and reduces the construction programme, saving costs.
4. Increased safety: Once precast concrete floors are installed, they provide a safe, non-combustible, and weather-durable working platform for site operatives. Offsite construction also reduces the amount of labour on site, helping to improve safety.
5. Better durability: Precast concrete elements are typically made with high-quality concrete and reinforcement, making them more durable than cast-in-place concrete.
6. Greater design flexibility: Precast concrete components can be made in various shapes and sizes. Repetition of elements can make even complex shapes more affordable.
7. Low maintenance: Precast concrete offers superior weathering and corrosion-resistant qualities.
8. Reduced site disruption: Because precast concrete elements are manufactured offsite, there is less disruption on the construction site.
9. Thermal and energy efficiency: The material’s high thermal mass can be improved further with insulation, helping to reduce energy costs.
10. Sustainability: Precast concrete fabrication uses materials and energy resources efficiently. It can also reduce waste on construction sites and decrease a project’s carbon footprint.

Disadvantages of using precast concrete

1. Initial cost: Precast concrete components can be more expensive than cast-in-place concrete but come as a finished unit, meaning installation on-site can be done with less labour and overall cost than casting the equivalent unit on-site.
2. Transportation costs: Precast concrete units must be transported from the manufacturing facility to the construction site, which can add to the project's overall cost.
3. Limited on-site customisation: On-site customisation may have limited opportunities because precast concrete elements are manufactured offsite.
4. Limited repair options: If a precast concrete unit is damaged, it may be difficult to repair it on-site.
5. Dependence on the manufacturer: The precast concrete elements' quality and timeliness can depend on the manufacturer's capabilities.
6. Risk of damage during transportation: Precast concrete units may deteriorate on the way to the construction site.
7. Limited ability to use on refurbished structures: It may be challenging to incorporate precast concrete units into existing structures.
8. Increased need for storage: Precast concrete components may need to be stored on site until they are ready to be installed, which can require additional space.
9. Specialised equipment needed: The handling and installation of precast concrete elements may require specialised equipment, which can add to the project's cost.
10. Incompatibility with certain soils: Precast concrete elements may not be suitable for use in certain soil conditions, such as expansive soils that can cause differential settlement.

Precast concrete products

Structural components

Beams

Precast concrete beams can be reinforced with steel reinforcement and prestressed with steel strands. They may work compositely with the floor or the columns.

Beam profiles are usually inverted T-beams or L-beams to support a precast concrete slab.

Columns

Precast columns are usually square, rectangular or circular, although other shapes are possible. It is good practice to apply 50 mm increments to the columns’ sides (or diameter).

Circular columns are typically cast vertically, reaching one-storey height. Square and rectangular columns can be cast horizontally, allowing heights up to 24 m.

The columns can have corbels or structural inserts to provide support for beams.

Walls

Precast concrete walls are used to build strong, durable and fire-resistant structures.

Crosswall is the most common form, where the panels can form structural load-bearing partition walls, external walls and floor slabs.

Architectural cladding

Precast concrete cladding provides low-maintenance facades. The panels can include windows and insulations to reduce construction time on site. They are divided into structurally load bearing and non-loadbearing.

Load bearing cladding systems have an inner structural panel, an insulation layer and an outer non-structural panel. The most common type is sandwich panels.

Non-loadbearing cladding systems, like the single-leaf precast cladding, have no structural function. They provide the required architectural finish to the building, along with a weather barrier.

Hollow core slabs

Hollow core slabs have voids throughout the component. The cores help reduce the slab’s self-weight, optimise the structural capacity, and provide room for service ducts.

Units are available in standard 1200 mm widths and depths from 110 mm to 400 mm. Other dimensions can be fabricated when required.

Double tees

Double-tee floor units are ribbed precast prestressed concrete units. Depths between 300 mm and 800 mm are common. Higher depths are also available if needed.

Double-tee floor units are produced in standard widths of 2400 mm. They offer greater structural capacity at longer spans than hollow cores.

Precast concrete manufacturing process

1. Create a detailed design for the precast component.
2. Prepare the mould to obtain the shape required.
3. Place steel reinforcement bars (and prestressed wires if needed) in the mould.
4. Pour concrete into the form.
5. If you don’t use self-consolidating concrete (SCC), vibrate the mix to remove air bubbles and increase the strength of the precast concrete.
6. Let the concrete cure.
7. Remove the precast element from the mould after reaching the required strength.
8. Do a quality check. If it’s satisfactory, move the component to the storage area.
9. Once the construction team orders the precast components, load them in a lorry considering the order of erection and take them to the site.
10. Additional quality controls are carried out on-site.
11. If the precast concrete components are accepted, use a crane or other heavy equipment to lift them into place.
12. Connect the elements after they are in the final position.

Precast concrete repair

Precast concrete components may require on-site repairs to satisfy the original requirements of the material. The most common repair methods are:

Patching

Patches replace damaged or deteriorated concrete with a cementitious material. They may be structural (for load transfer) or cosmetic (for visual appearance).

Crack repair

Not all cracks on precast concrete components require repair. A low-viscosity epoxy is a good solution to repair cracks. It can be installed by gravity flow or pressure injection. The cracks should be cleaned before performing the repair.

Connection repair

The connections may require repair if the components are misplaced or damaged or the embedment is missing.

Structural calculations are needed before choosing a repair method. Some available solutions are: increasing the dimensions of the connection material, installing a new connection close to the original, or adding stiffeners to the connection.

Precast concrete maintenance

Precast concrete elements usually require little maintenance except when subjected to harsh conditions, in which case they should be inspected periodically to detect and prevent long-term maintenance issues. 

Structures with precast concrete components

Residential buildings

Budenberg, Altrincham

The Lansdowne, Birmingham

Burlington Gate, London

Office buildings

Police Federation of England and Wales HQ, Leatherhead

Kings Cross P2, London

11 Baker St, London

Hotels

Ramada Encore Hotel, Warrington

Marriott Hotel, Manchester

Motel One and Staycity Aparthotel, Manchester

Educational facilities

Crown Woods School, Greenwich

Bishop of Rochester Academy

National School of Ballet, London

Public buildings

Victoria and Albert Museum Dundee

Roehampton Library

Parking structures

Poole Multi-Storey Car Park

Wakefield Multi Storey Car Park

Bridges

N22 Bridges over the Sullane and Laney Rivers, Macroom

Plymouth Road Bridge, Blackpool

Stations

Twickenham Station Development, London

London Bridge Station, London

Farringdon Station, London

Stadia

Kia Oval County Cricket Stadium, London

Olympic Copper Box Arena, London

Conclusion

Precast concrete allows you to build faster, safer and save costs. However, the process of casting and demoulding is critical to meet the project deadline.

If you demould too soon, the elements won't be ready, and you must cast new units. This could delay the whole project.

If you demould too late, the precast concrete will be stronger, but you are throwing time away.

ConcreteDNA Precast allows you to demould sooner without losing quality:

• It predicts concrete strength on real-time.
• It notifies in advance of the optimum time to demould.
• It keeps track of the precast factory production so no time is wasted.

Speak to sales to save time and money on your project.

References

Elliott, Kim S.: Precast concrete structures. Boca Raton (FL), CRC Press, 2018.

Manual for fabrication of precast prestressed concrete products. Springfield, Illinois Department of Transportation, 2020.

Moore, Richard: “An early system of large-panel building”, RIBA Journal, vol. 76, 1969, pp. 383-386.

Offsite concrete construction: The solutions. MPA The Concrete Centre, 2018

Offsite concrete construction: A guide to the design and construction of precast concrete in buildings. MPA The Concrete Centre, 2019.

PCI design handbook: precast and prestressed concrete, 8th Edition. Chicago, Precast/Prestressed Concrete Institute, 2017.

“Precast concrete”, Designing Buildings, Designing Buildings Ltd, https://www.designingbuildings.co.uk/wiki/Precast_concrete [Accessed on 22 January 2023]