Concrete Canoes on the Neckar River

The Neckar river in Heilbronn, Germany, will be the venue for the 17th German concrete canoe regatta, held from 28 through 29 June with Master Builders Solutions as sponsor. In Munich, the team from that city’s Technical University is preparing for the big day: Mareike Thiedeitz, project leader and research assistant at the department of civil, geo and environmental engineering, and Elena Woydich, currently in the third semester of her civil engineering Masters course, talk about concrete canoe construction and about the support they receive from the Master Builders Solutions experts.

Concrete is considered a particularly durable and massive material. How can it be made to float?

Mareike Thiedeitz: Ah, yes, but you see – steel, for instance, is much more durable and much heavier. What is it that makes a fully loaded container ship float, then? The secret lies in the principle formulated by Archimedes 2,000 years ago: “Any object immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the object.”

When an object displaces water, buoyancy is created relative to the pressure exerted vertically and the displaced area where the buoyant force takes effect. When the overall weight of the object is less than that, the object stays afloat. Otherwise, it sinks. But of course, our canoe cannot get too heavy – it will have to support two people in addition to its own weight.

Compared to a traditional canoe, ours will be heavier, of course; also, with canoes in general, floatation depth isn’t very pronounced. We will have to prepare a buoyancy report to ascertain the amount of water that has to be displaced, relative to our overall weight, for our boat to stay afloat and to see whether the calculated floatation depth can be maintained while we are in motion. Hopefully, our calculations are correct – salvaging a concrete canoe isn’t that much fun.

How is the canoe constructed?

Mareike Thiedeitz: Typically, a concrete canoe is constructed using a kind of ‘scrape on’ technique. This is fairly straightforward and looks similar to tile-adhesive application. In order for this method to work, the concrete mix must be finely adjusted in terms of its consistency, neither too thin nor too stiff. Another option is to use sprayed concrete – in which case the concrete mix requires completely different ingredients and specialized equipment. And the overall approach to the design – particularly lightweight, for instance – must also be taken into account. All things considered, planning and development tend to take up more time than the actual construction process.

Elena Woydich: We have opted for a time-saving thin-wall design, and focus much of our attention on the concrete mix and the application method. We are experimenting with a sprayed-concrete technique, and any tests required are conducted working with the lab technicians at the Centre for Building Materials in Munich. This consists in preparing a concrete mix and spraying it onto a surface. The mix design is then adjusted step by step until we obtain the required result.

How do the experts from Master Builders Solutions support you?

Mareike Thiedeitz: Master Builders Solutions generously provides us with most of the less-conventional concrete admixtures we require. We are using a plasticizer from the Master Builders Solutions MasterGlenium range, for instance. At our institute, we have been working with admixtures from this range quite extensively – with the Trostberg Master Builders Solutions site so close –, and this is why we can usually pinpoint the admixtures we need fairly accurately. Throughout, the experts from Master Builders Solutions have been there for us with advice and practical support.

How important are concrete admixtures?

Elena Woydich: Admixtures help us to adjust the concrete to our requirements – that is, they make it possible for us to build a floating canoe in the first place. Using admixtures, we can determine both the consistency of the fresh concrete as well as the characteristics of the hardened material.

Mareike Thiedeitz: I don’t think that constructing a thin-walled structure of this kind, with its high fines-paste content, would be feasible without the use of concrete admixtures. We are using a lot of polycarboxylate-ether-based admixture for improving workability and application properties.

What is the effect you require from the admixtures you use?

Elena Woydich: For our canoe to actually stay afloat, the properties of the hardened concrete are of vital importance. This requires a concrete that is impermeable to water, and durable when exposed to water pressure. The concrete also requires low-porosity, high-strength and low-weight properties. The weight properties are obtained primarily by using lightweight reinforcement. Strength and porosity properties are mainly adjusted through admixtures. We use metakaolin as an admixture. Metakaolin has a puzzolanic effect (Editor’s note: Pozzolans are silicate-based materials that react with the calcium hydroxide generated by hydrating cement to form additional cementitious materials.). This means that, during the puzzolanic reaction, stabilizing crystalline phases are created in the concrete. This provides the concrete with high-strength properties and makes it resistant to water pressure.

The simplest way to obtain low porosity – or a concrete structure impermeable to water – is a low water/cement ratio. For the concrete not to get too rigid, we use the plasticizer mentioned before. Plasticizers maintain workability at a low water/cement ratio. In this way, we obtain a hardened concrete that has a low pore content and is highly impermeable to water.

How much time do you have to spend on the project?

Mareike Thiedeitz: An unusual boat with a complex design can well take up entire weekends. However, with a project such as this one, I wouldn’t count the hours but try and take away as much as I can – be it an increased knowledge of concrete as a construction material or new soft skills. I see the project as an opportunity to explore concrete as a material, playfully and ‘outside the box’. 

Elena Woydich: The construction process does take up a lot of time, yes. But we are highly motivated and have become quite good at managing our time and balancing uni duties and extracurricular activities. We get along very well as a team, and doing this together really is a lot of fun. Time, in fact, flies by and nobody really notices it when we do ‘overtime’ on the canoe.

Do you look at other boats for inspiration?

Elena Woydich: Since this is a first for all of us, we are of course curious to see what previous teams have come up with. Some designs were really funny and made us laugh loud and hard – and also got us thinking what we could possibly build. We are also following some of this year’s teams on their social media accounts. Anticipation rises with every new video and boat you see.

Are you planning to take the boat on a test trip?

Mareike Thiedeitz: None of use is an experienced canoeist. The minimum requirement, as I see it, is for everyone to have been in a canoe at least once without getting seasick. Personally, I would love to get in that canoe and take it down the Isar river as fast as it can go. I’m not sure the canoe agrees, though. If the canoes get ready in time, we will take them to the Oberschleißheim regatta course.

Do you want to win or is the construction process more important?

 Elena Woydich: In fact, what we are trying to do is put our knowledge of concrete as a construction material to the best practical use, while also learning in the process. If, in the end, we come up with the fastest or most-lightweight canoe, this would be great but isn’t our primary goal. We want to give our best while still having fun along the way.

Check out the second article by Elena and Mareike as they reflect on their progress so far and set new goals for the future. Click here.