Flexible formwork and reinforcement for concrete shells

The construction of concrete shells remains however a challenge in today's Western countries. The existing formwork methods, such as timber moulds and foam block, are often complex, labour intensive, costly or material wasting. A second issue hindering the construction of shells is the tensile reinforcement. Reinforcement is always needed in structural concrete elements, but the inflexibility of traditional steel reinforcement practically limits the curvature of structural surfaces.

There is a need for an inventive formwork solution that on the one hand provides the flexibility to obtain curved surfaces and on the other hand has a sufficient stiffness in casting stage, and that can moreover fulfil a reinforcing function after hardening of the concrete. We developed a new method which complies with these criteria by exploiting the properties of textile reinforced cement (TRC) composites.
Textile reinforced cement composites consist of continuous fibre systems, textiles, which are impregnated with a cement or fine grained mortar. They harden at ambient temperatures. TRC composites are flexible in the wet phase; hence curved shapes can easily be obtained. In hardened phase, they are stiff and strong in both tension and compression.

This research acts upon both the flexibility of the impregnated textiles to easily create curved surfaces as well as the good mechanical properties in the hardened phase.
To obtain a curved TRC surface, the TRC formwork can be shaped onto any (reusable) mould such as foam moulds, flexible formwork moulds, pneumatic formwork and prestressed membranes. The low weight of the TRC layer compared with concrete, which is normally poured on these moulds, is a great advantage. In other words, the TRC gives an added value to the existing systems.
The TRC formwork can also repeatedly be prefabricated on the same mould and transported to site to act as permanent formwork for the concrete shell thanks to its good mechanical properties.
Finally, after hardening of the concrete, a mixed TRC-concrete section is obtained. Thanks to the relatively high tensile capacities of TRC and the good bonding between the two materials a reinforcing function is expected.

During the research the TRC layer is extensively examined in its functions as formwork and tensile reinforcement. On the one hand numerical (material) models are developed, which describe the behaviour of the shells, and on the other hand experiments are performed to evaluate these models.