Durability enhancements using fabric formwork

Abstract

By replacing conventional concrete moulds with flexible sheets of permeable fabric, the construction of optimised concrete elements that provide material savings of up to 40% when compared with an equivalent strength prismatic member is possible. This paper details the results of recent tests undertaken at the Building Research Establishment Centre for Innovative Construction Materials at the University of Bath that demonstrate significant additional durability advantages for fabric-cast concrete. Using accelerated test methods, 50% average reductions in both the non-steady state chloride diffusion coefficient and carbonation coefficients were found when comparing concrete samples cast against permeable and impermeable surfaces. Sorptivity, surface hardness and scanning electron microscopy tests demonstrate further beneficial changes in the fabric-cast concrete. The combined results demonstrate that fabric formwork may be used to create structures optimised for strength and durability. Notation B j coefficient of carbonation for formwork type j C a, j depth of carbonation for formwork type j at time a (mm) C i initial chloride content (% by mass of concrete) C i, j depth of carbonation for formwork type j at time i (mm) C r reference chloride concentration (0 . 05 mass%) C s calculated chloride content at exposed surface (% by mass of concrete) C x,t chloride content measured at depth x and exposure time t (% by mass of concrete) D nss non-steady state chloride diffusion coefficient (m 2 /s) K Cr chloride penetration parameter t a time of exposure in carbonation chamber (days) t i time of exposure to atmospheric conditions (days) ç correlation factor between accelerated test conditions and in-situ results (dimensionless)

Flexible formwork technologies – a state of the art review

“…Some investigations into creating customized materials have been undertaken [171] and many potential opportunities have been identified. The established benefits that permeable formwork has for concrete finish and durability can potentially reduce cover requirements and create longer-lasting structures [117,172], as described in section 3.2. At present there is no provision for this in design codes.…”

“…although it is possible to use non-woven membranes as a formwork material, woven fabrics are usually preferred, due to their availability, low cost, high strength and positive effect on surface finish [117]. A tough and durable material is desirable if the formwork is to be handled, prestressed and used multiple times.…”

“…There are examples of FRC used to create thin shell structures in combination with flexible formwork [9,116,125,127]. Fibres can be made from steel, glass, polymers or natural materials, and these can be used for either the partial or complete replacement of conven-ingress [117]. The evidence therefore shows that further material-savings could be made by decreasing cover requirements, although further investigation and standardization is required for this to become recognized practice.…”

Extraordinary possibilities for future concrete structures

“…Significant reductions in carbonation depth and chloride ingress have been recorded (Orr et al 2012a).…”

“…Chloride ingress testing was undertaken in a similar manner, using a sodium chloride solution with a chloride concentration 5.3 times that of Atlantic seawater. Full details of the tests and their results are provided by Orr et al (2012a). A summary of the improvements seen in this work is presented in Table 1.…”