The first Concrete Canoe race was a challenge between University of Illinois and Purdue University in 1971. Bit by bit this strange competition expanded among American universities up to 1988, when it became a national competition. The yearly competitions are organized by the association of the American engineers (ASCE) for the purpose of giving opportunity to university students to gain practical experience and leadership skills by working with concrete mix designs and project management. Actually, concrete canoe competitions are organized all over the world.
The Concrete Canoe Competition is a real challenge for all participants: students are asked to design and build a canoe made of concrete. It may look simple at first glance, but considering that the teams have to race their canoe it becomes more challenging to get a canoe that combines proper resistance, speed and maneuverability. In some cases, the team must also identify creative ways of raising money and gaining sponsors in order to fund the design and execution of the canoe. The team must be dedicated enough to overcome obstacles such as lack of funding or conflicts with their regular university workload, to practice the paddling technique and to outwit the competitors during the race.
The concrete canoe competitors are judged by the sum of four activities: an oral presentation of their project, the floating test, the final product and finally the race results. The evaluation of the Final Product is based on points attributed to the overall aesthetic of the canoe. Various deductions can be applied to the scoring for non-compliance to the rules, the inability to pass the flotation test at the first attempt or the use of tape as repair material for non-accident related damage.
Now, let’s have a look in the activities teams are involved in to build their Concrete Canoe.
1. HULL DESIGN
Engineers must design their canoe, thinking about dimensions, alignment, stress points into the canoe (depending on the number of paddlers). Project leaders are proved in experiencing computer design programs and modelling tools to define the best project. Structural analysis is conducted in order to determine the properties of the canoe such as load applied on it and deformation taking place after the load is applied.
2. MOLD PREPARATION
Preparation of the mold should start as soon as the hull design is optimized. The mold can be male or female and built using various materials such as polystyrene, wood, fiberglass, etc… Generally a male mold better suits the requirements since less materials is needed for this kind of mold compared to a female one. The male mold usually requires less sanding while maintaining the designed hull shape.
An example of a typical mold preparation is given by P. Leczovics and V. Sugár . In this project they chose an inner extruded polystyrene formwork, on which later build and stack the layers of concrete by hand. The required concrete quantity was not mixed all at once, rather bit by bit in smaller portions, while a part of the team continuously plastered the fresh concrete by hand onto the previously made formwork. The canoe was designed laminiform, therefore a fiberglass layer followed the first concrete layer. Totally the final body contained three layers of concrete and two layers of fiberglass.
3. MIX-DESIGN DEVELOPMENT
The organizers usually impose some rules (that can change year by year) referring to concrete mix-design’s specifications: among them, for example teams must use a minimum cementitious amount and a minimum percentage of secondary cementitious materials (fly ash, slags, metakaolin), a defined maximum water to cement ratio (usually 0.50), a minimum/maximum amount of a specific aggregate, a type of admixture that must be introduced into the concrete recipe, etc. These rules are intended to provide some restrictions and simulate the real life of an engineer, and this is why it is important to have an efficient method for concrete mixture development.
The hardened concrete should be lightweight, fatigue resistant, watertight, possibly crack free (when a canoe suffers extensive damage or cracking, water can flow into the canoe compromising its structural integrity). It has to possess considerable compressive strength (around 30 N/mm2 is desiderable for a concrete canoe, but realistically the compressive strength usually obtained is around 10 MPa, a value proven adequate over the years against accidental paddle impact), tensile (of 4 MPa or more, being this kind of stress concentrated around the paddlers’ knees – as example of this, check M. Morency and F. Paradis , where they developed mixtures reaching up to 11.96 MPa) and flexible strength, and with a modulus of elasticity of at least 6000 MPa.
Cementitious Materials. The application of CEM I 42.5 R Portland type cement is usually employed, partially replaced by supplementary cementitious materials (fly ash, metakaolin and silica fume at various percentages) to reduce concrete density without affecting much of its strength aspect. For creating a watertight concrete, metakaolin is a highly reactive fine aggregate very suitable for this purpose. Among the advantages of metakaolin, it is helpful in increasing tensile and compressive strengths and in decreasing shrinkage. The reaction rate of metakaolin is very rapid and helps to enhance compressive strength even at early stages with the enhanced production of CSH gel. The use of silica fume and fly ash in conjunction with metakaolin has a vital impact on the early strength development and increases the pozzolanic nature of the concrete mix.
Lightweight concrete should be used: the lower the density of the concrete, the lighter the canoe will become, the faster top speed will be reached, the easier the canoe can be carried.
Expanded glass aggregates (PORAVER) are very useful for that purpose. The granules are basically manufactured from selectively collected and recycled glass garbage and waste. Among the advantages of this material, low body and agglomerate density, high compression strength, minimal hydration are useful for its use in concrete canoe mix-design. A typical maximum size for this kind of spheres is 4 mm (considering an ideal concrete thickness of 10 mm for the canoe). Hollow glass spheres (3M™ Glass Bubbles, composed by Soda-lime Borosilicate Glass), with a very fine particle size distributions (Dmax 80-120 μm), are also very common lightweight aggregates in concrete canoe recipes. An ideal bulk density of the concrete mixture is close or less than 1000 kg/m3, to allow the concrete to float by itself.
P.J. Hinson and C. Nichols  experimented a concrete layer containing 20% (of the total aggregate amount) of polystyrene granules as lightweight aggregate between two layers of lightweight concrete with no polystyrene, in order to avoid sanding problems once concrete had cured. This kind of aggregate did non interfere with the use of carbon fibers as reinforcement, that could be sandwiched between of two layers of this concrete polystyrene beads mix.
Since lightweight aggregates have less strength, they readily affect the characteristic strength of the concrete. In order to counteract reduction in strength, 10% of glass grit could be used in partial replacement of fine aggregates. Being amorphous and containing relatively large quantities of silicon and calcium, glass is, in theory, pozzolanic or even cementitious in nature when the particle size is less than 75 microns .
Instead of gravel as coarse aggregate, Lightweight Expanded Clay Aggregate (LECA - below 5 mm) has been used as a replacement (by volume). LECA is a special type of lightweight aggregate that has been pelletized and fired in a rotary kiln at a very high temperature. As it is fired, the organic compounds burn off forcing the pellets to expand and become honeycombed while the outside surface of each granule melts and is sintered. The resulting ceramic pellets are lightweight, porous and have a high crushing resistance and also played a vital role in the reduction of concrete density. The only drawback while using LECA is that it has high water absorption thereby increasing the water demand. Therefore, it has been used in saturated surface dry condition.
Reinforcement. The reinforcement material needs to be lightweight, to have a high tensile strength and a high modulus of elasticity. Another important consideration is adequate bonding between concrete and reinforcement to prevent any delamination. For instance, if the bond between concrete and reinforcement is weak, the composite resistance may be lower than the concrete resistance.
The use of polypropylene fibers or glass fibers allows concrete to raise both tensile and flexural strengths. It is possible to create thin (mandatory requirement for a concrete canoe, that is usually planned to be < 10 mm thin) but strong concrete pieces by using these fibers. Carbon micro fibers (length < 5 mm), dosed at 1% by volume, increase the ductility and the toughness, helping to prevent cracking, by limiting the propagation of microcracks which will lead to larger cracks. Carbon macro fibers (length > 5 mm) are less efficient to prevent cracking, but they help to maintain crack opening to a minimum. They also help to disperse fine cracks instead of one large crack which is good for aesthetics and also for the watertightening of the canoe even with the presence of cracks .
Chemical admixtures. With PCE superplasticizers, more fly ash or other supplementary cementitious materials can be used and they also improve adhesion between aggregate & mortar interface as well as new & old concrete interfaces. A viscosity-modifying admixture (VMA) as Welan Gum has been investigated to avoid segregation and maintaining workability to an acceptable level. M. Morency and F. Paradis  performed a series of tests around the optimal dosage for the VMA at 0.16% by mass of water, in combination with acetone (7.4% replacing water): a solvent like acetone improves the workability of the paste, even if a shrinkage increase by 20% was experimented by the authors.
4. BUILDING TECHNIQUES
Shotcrete is one of the most popular concrete canoe’s building techniques. D. Burns and M. Hébert-Sabourin  of Université Laval introduced shotcrete to overcome a casting problem, that is to avoid the slide off of the concrete onto the surface, opting for a male mold to easier sand the exterior of the canoe than the inside curves. Another reason for the choice of shotcrete is to help in the compaction and mechanical properties, that were better for shotcrete than for the hand placed one. The use of shotcrete is also a much faster building technique, for example Laval team cut its construction time from 12 hours down to 2 hours. As a consequence, bond problems between different layers of reinforcement are reduced since the concrete does not have the time to set in between layers and the high velocity helps the encapsulation of the reinforcement with the concrete.
One of the difficulties of shotcrete technique to tackle is heterogeneity of the concrete mixtures: cement, lightweight aggregates, slags, fly ashes have different densities and it could be very difficult to have a homogenous mixture, especially using the dry-mix process. Another one is you have to choose the proper material for the mold to face the pressure at which concrete is shot: standard polystyrene is not enough, you must reinforce it or use wooden molds.
In pre-tensioning, the reinforcing tendons are stretched before the concrete is placed. After placement, the tendons are cut or released and the tensile stresses are transferred to the concrete through bond stresses. For the last several years, pre-tensioning the concrete in the critical areas of the concrete canoe has been the norm for most competitive teams. Universities that had used pre-tensioning for many years with Kevlar as the material of choice for the reinforcing tendons, never experienced a structural failure in the areas where the Kevlar was placed, however, hairline cracks would eventually appear, especially in the area of the gunwale.
Kevlar tendons have been used to try post-tensioning in the gunwale of a concrete canoe. Post-tensioning is a method of stressing concrete by tensioning the reinforcing tendons after the concrete has set. Kevlar is suitable for this purpose because of its high tensile strength (951 MPa). Tendons were firstly tensioned 7 days after concrete placement and re-tightened at 14 days due to relaxation in the Kevlar. Success was determined by the fact that not a single crack, hairline or otherwise, appeared in any canoe after using post-tensioning .
The final product is very important to present a nice and flawless concrete canoe in order to impress other teams and the judges.
To obtain a uniform shape and perfect surface finish, one of the key elements is sanding from a rough to a glazed surface. Most of the time, sanding is done by hand: tools that vibrate a lot could induce stresses in the hull which could lead to cracking. It might not be required to wait until 28 days of age before starting to sand the concrete canoe: sanding can start when the aggregates can be sanded without damaging the paste. The time before being able to start sanding depends on many factors: concrete properties, curing, concrete layer thickness and more.
1. P. Leczovics and V. Sugár, Concrete Canoe: A Complex Concrete Technology, YBL JOURNAL OF BUILT ENVIRONMENT, Vol. 1 Issue 2 (2013), pp. 43-55.
2. M. Morency and F. Paradis, Design of a High Tensile Strength Lightweight Concrete, Concrete Canoe Magazine (2010), pp. 22-28.
3. P.J. Hinson and C. Nichols, Polystyrene beads as coarse aggregate in concrete canoe mixes, Concrete Canoe Magazine (2006), pp. 13-16.
4. S. Jayakumar, A. Kurian, F.T. Zachariah, N. Philip, Construction of Concrete Canoe using Light Weight Aggregates, International Journal of Engineering Research & Technology ISSN: 2278-0181, Vol. 9 Issue 04, (2020), pp. 452-458.
5. F. Paradis and M. Morency, Optimizing a Concrete Canoe – a review, Concrete Canoe Magazine (2006), pp. 34 – 39.
6. D. Burns and M. Hébert-Sabourin, Shotcrete fundamentals for concrete canoes, Concrete Canoe Magazine (2007), pp. 6-12.
7. A. Delgadillo and S. Williamson, Post-tensioning as a means of reinforcing a concrete canoe, Concrete Canoe Magazine (2010), pp. 10 – 13.
Gunwale of post-tensioned concrete canoe after regional and national competitions and 6000-miles of travel
Hairline cracks in concrete canoe gunwale due to tensile forces (Spring 2010 / Concrete Canoe Magazine page 11)
Spring 2008 / Concrete Canoe Magazine, pag. 20
Florida’s “Gladigator” (Spring 2008 / Concrete Canoe Magazine, page 7)
Thanks to "Concrete Canoe Magazine" for the pictures