Over the last month, concrete elements in an unusual shape of a hexagon were prepared and tested within the WECHULL project at RISE facilities in Borås (mechanical testing) and at the RISE marine test site in Kristineberg on the Swedish West Coast (biofouling tests). The hexagon shape is an effect of the concrete hull structural design performed by Ocean Harvesting Technologies. This part of the structure was identified as a critical section for the mechanical performance of the buoy.
Fig.1 The casting of CFRP-reinforced panel with WECHull sustainable concrete mix.
Two alternative solutions were tested:
1) Fibre-reinforced concrete (FCR) where dispersed fibers take over the function of reinforcement. This solution is preferable concerning the easy production of very slender elements (no need for reinforcement placing and self-compacting material including fibers)
2) CFRP-textile reinforced concrete - in this case, a carbon fiber-reinforced polymer textile was placed in the concrete similarly to traditional reinforcement grids. Unlike the traditional reinforcement, the textile nets are lightweight and very easy to bend and cut (can be done even with a knife) which makes the production much faster. The CFRP is also characterized with superior strength in the range o 3800 MPa.
The difference in the mechanical behavior of both materials was manifested by different ultimate strengths and stiffness, as well as in different failure modes. The panels with CFRP were punched in a brittle manner (see movie below), while the FCR panels failed in a pure bending regime with softening.
Fig.2 The punching failure of CFRP-reinforced panel
The final crack patterns present also the ability of fibers and textile reinforcement to redistribute the stresses after initial cracking.
Fig.3 Cracking patterns on the bottom of the tested panels (upper row CFRP-reinforced and bottom row FRC)
The mechanical tests will be continued with CFRP reinforcement with a larger thickness to achieve bending failure which will be the real case for the designed WEC, where the load will be evenly distributed.
RISE is also continuing the fatigue testing for the same two materials.
Last but not least important similar concrete hexagonal panels were deployed in the Kristineberg marine test site for biofouling testing of different coatings. More about biofouling testing soon!!!