One of WECHull project end-users Ocean Harvesting is looking into alternative materials to steel and composite which are mostly used today for Wave Energy Converters (WECs) hulls. The choice of concrete was due to its low price and superior durability in the marine environment, leading to long service life.
RISE department Infrastructure and Concrete Technology has developed the initial high-performance concrete mix with lowered environmental footprint and very good durability in a marine environment based on Swedish supplementary cementitious materials (SCMs). To facilitate thin-walled, lightweight structure and easy, rapid manufacturing alternative reinforcement measures were investigated concerning the use of fibers of different types (carbon, aramid, glass, and steel), as well as polymer reinforcement in for of carbon-textile grids.
The main objective was to develop a sustainable high-performance concrete mix characterized by the following properties:
• Required compressive strength (>100 MPa)
• Required flexural strength (>15 MPa)
• Lower environmental impact (> 50 % cement replacement with supplementary cementitious materials, SCMs)
• High durability concerning: freeze-thaw, carbonation, chloride ingress ensuring service-life of at least 25 years
• Low material cost (< 300 EUR/m3)
• Self-compacting material (Slump flow class SF2 t500 class VS2)
WECHULL fulfilled the requirements by preparing a mix replacing more than 50 % of the binder with slag (waste from extraction of iron ore) and limestone, thus reducing the cement and CO2 footprint. One ton of cement clinker generates 700-900 kg of CO2, while one ton of slag is only around 20-40 kg[1].
The mix was also highly self-compacting, also when fibers were incorporated. The film below presents a typical flow test with a slump-cone.
WECHull performed a series of lab durability tests to ensure the required long service life. The most important parameter in the Swedish climate for slag-based binders is freeze-thaw. The WECHull mixes exhibited 4 times lower scaling than normal concrete after 56 cycles of freeze-thaw. Moreover, when the cement content from the basic WECHULL mix was further reduced by 50% the frost resistance remained excellent. The second mix had cement content lower by 50% compared to the values allowed by EN standards
.
The WECHull project achieved tensile strength by implementing steel fibers in the mix. The primary goal was to avoid as much as possible the traditional steel rebars placed at the construction stage to enable serial production in a short time. The addition of fibers
makes possible the transfer of loads between the cracks and therefore increases tensile strength. In result, the bent or tensioned elements may resist much higher loads without failure and additional rebars.
The film below presents a Digital Image Correlation analysis of WECHull concrete with steel fibers under standard bending test with a notch.
Soon more updates on the concrete material development will be shared. Stay tuned!
References:
[1] Merit - Swecem. Available at: https://swecem.se/Produkter/merit/