“The value of tilt-up construction methods is often overlooked when structures are designed, and yet this method is one of the most cost-effective techniques available to architects and designers,” says Gary Theodosiou, an engineer and lecturer at the Concrete Institute.

He points out that concrete has distinct advantages over many other building materials in that elements can be purpose-built for specific applications.

“The tilt-up method means that elements can be cast on site and lifted into their final position, maximising many of the sustainable benefits and specific quality of concrete,” he says. “This not only reduces costs, but also provides for greater efficiency and lower wastage,” he adds.

Explaining the construction method, Theodosiou says elements are formed on a concrete slab – usually the building floor, but occasionally a temporary concrete casting surface – near where the element is to be used.

Once formed and cured, the elements are tilted off the horizontal, into the vertical position, using a crane and then braced in position until the remaining structural components are complete. “These components could be walls, intermediate floors or even roofs,” he says. “Once these elements are complete, the tilt-up structure is fixed and secured.”

Theodosiou is quick to emphasise that tilt-up panels are structural elements within the building that span from the base to the roof diaphragm, acting as beams that are capable of carrying and supporting roof and slab loads.

“They combine the building envelope or façade with the structure and provide a final finish in a single operation,” he adds.

The panels are set out according to the casting layout on the drawings. Ideally the panels are cast on the surface bed of the project or on the final concrete hardstand areas around the building.

Construction

Once the pad or casting surface has cured, forms for the concrete design are built on top of it, typically using a high quality plywood or fibre board with at least one smooth face.

“Sometimes aluminium or steel forms are used,” he says. Theodosiou says if the plywood or fibreboard forms are used, carpenters work strictly according to the engineered drawings provided for each panel or element that must be made on site.

“These panels incorporate all door and window openings, as well as any other architectural features or desired shapes that are moulded into the concrete itself,” he says.

“Studs, gussets and attachment plates are located within the form for embedding into the concrete to make assembly easier. The forms are usually anchored to the casting surface with masonry nails to prevent damage to the floor slab,” he says.

Next, a chemically reactive bond-breaker is sprayed onto the forms and casting surfaces to prevent the cast concrete from bonding with the slab.

“This allows the cast element to separate from the casting surface once it has cured – a critical step, as improper chemical selection or application will prevent the lifting of the panels, and may entail costly demolition and mean that the work done has to be redone,” he says.

A reinforcement grid is constructed inside the forms, after the form release is applied and spaced off the casting surface to the desired distance using plastic “chairs”.

Concrete is then placed, filling the desired thickness and covering the surrounding steel inserts, embedded features and reinforcement.

“The forms are removed when the concrete is cured. Then the rigging is attached and a crane tilts the panel or lifts the element into place, in one swift movement,” explains Theodosiou.

“In circumstances where space is at a premium, concrete elements can be cast one on top of the other, or stack cast. Quite often a separate casting pad is poured for this purpose and removed when the panels are erected,” he adds.

Cranes are used to tilt the concrete elements from the casting slab to a vertical position, and most often slabs are set onto a foundation and secured with braces until the structural steel and the roof diaphragm are in place.

“The tilt-up method does not sacrifice quality for cost, but rather provides a finished product that is superior to most other alternatives. It offers the strength and durability of reinforced concrete walls, while simultaneously being able to economically achieve aesthetic effects neither possible nor cost-effective using other construction methods,” he says.

“Invariably the end-result is both aesthetically pleasing and structurally sound,” he adds.

Concrete Institute
Tel: (011) 315 0300
Website: www.theconcreteinstitute.org.za.