Design of a Suitable Concrete Canoe Mix

Alexander Lamb

Eng 3060

Assignment #2

Design of a Suitable Concrete Canoe Mix

The project is to describe the process of designing a suitable concrete mix to be used for the hull of a concrete canoe. This mix design must be light enough to float on water and strong enough to withstand the forces imposed by 4 paddlers kneeling in the canoe. To begin the process description some background information on concrete and stress calculations are need to further define the technical aspects of the project.

The first step in the process is to determine the stresses the concrete must be able to resist in order to successfully complete the competition. To do this a model must be created. To construct the model several computer programs may be used that model the geometric shape of the canoe as thin shell plate elements. The readily available programs that are capable of performing this type of analysis are STAAD.Pro 2007, Rhinoceros 2.0 and Hypermesh 2.1. For simplicity the analysis process using STAAD.Pro 2007 will be described.

To produce the model using STAAD.Pro 2007 a model of the hull must be produced electronically using a CAD type program. Producing this model in a CAD type program has several advantages mostly pertaining to use of the coordinate system, determination of areas and volumes, and the ease with which it allowed conceptualization for some of the more complex geometries. Additionally it enabled the coordinate data to be generated in the form of a solid model for export into STAAD.Pro. Once the model in CAD is completed it may then be input into STAAD.Pro for what is called as a finite elements analysis. For the analysis the geometric shape is broken down into hundreds of thousands of infinitely thin plate elements. These plate elements must all be observed so that poorly shaped elements may be reshaped into appropriately small quadrilateral shapes. With this completed loads may be assigned to the analysis to begin producing stresses along sections of the hull. After the initial stress reading the model is run again but this time with the existing plate elements broken down into smaller elements. This produces a new stress which is closer to true stress that will be realized in each element. This process is repeated until the stress output numbers converge within 10-15% of each other. With this true stress determined through convergence the next step in the process may begin.

The next step in the process is to determine the material properties required of the concrete. For the competition it is required that the canoe full of water float. This essentially means that the concrete itself must have a unit weight less than that of water. This along with the stress numbers produced by the finite elements model allows for base materials for the concrete design to be selected. Knowing that the concrete must be ultra-lightweight Portland cement (the main ingredient in concrete) must be minimized. Also a lightweight aggregate source must be identified. For the aggregate a company known as Poraver is selected. They produce a lighter than water glass resin aggregate that is most suitable for the application. The next types of material needed for the design are pozolains. These types of materials are not cementitious by their own right but in the presence of lime and water will hydrate and produce chemical bonding depending on the level of surfactants used in the product. For the pozolains Fly Ash and Grade 100 Slag are selected because of their light weight characteristics (compared to Portland cement) and because of the strength and workability benefits that will be realized through their use.

The next step in the process is to determine the relative proportions of each of these materials that will produce a concrete matrix that is strong enough and light enough. To begin the mixing proportions of each material a phase diagram is produced. With this phase diagram trial mixes are able to be produce that certainly have a unit weight less than water. After multiple calculations using the phase diagrams, all possible combinations of the material ingredients that will produce unit weights less than water are known. After inspection it is found that more ingredients will be needed to produce this unit weight. A chemical known as AEA-92 was selected for testing. This chemical is known as an air-entraining admixture which will produce a system of air voids which in industry is used to provide the concrete with durability against repeated freeze-thaw cycles. For this application however the objective is to reduce unit weight.

To begin testing with the AEA-92 admixture a series of controlled tests were set up to determine the unit weight reductions per ounce of the AEA-92 as it corresponds to 100 pounds of concrete. The testing results in approximately 3oz of AEA-92 to produce 1% air void per 100 pounds of concrete. This relationship was used to formulate the next mixes. Using the previous mix numbers attained from the initial testing, the amount of air needed to achieve the necessary unit weight reductions was calculated (using the 3oz/100lb relationship). After these mixes are produced with the appropriate unit weight, strength is tested.

For the strength testing a few different methods are used. To test hydration as it relates to the maturity of the concrete standard 2”x2”x2” mortar cubes are used. This provides both qualitative and quantitative information about how well the tested mixes are hydrating which is directly related to the ability of the concrete to achieve strength later in it its life (after 28 days). The next type of strength testing required is a 3 point reinforced flexural test. For this test a type of reinforcement is used known as Carbon-Grid 38. This reinforcement is a series of woven carbon fiber threads that are impregnated with epoxy resign. This reinforcement is to be tested along the bottom edge of the flexural test specimen so that calculations may be performed that will be indicative the grids strength contributions in the concrete matrix. After testing is completed the reinforcement scheme is determined.

To determine the reinforcement scheme (how many layers and at what depths) a relationship known as the modular ratio is used. For this ratio the modulus of elasticity of the concrete is determined and same for the reinforcement. Using the modular ratio allows for the reinforcement to placed at certain depths to provide known strength increases. Testing shows that layers along the outside edges (near the extreme fibers of the material in flexure) will be required to produce the highest flexural strengths in the materials and a doubled up layer in the middle will help mitigate punching shear issues with the thin concrete section. The final product must be capable of withstanding the stresses determined by the elements analysis.

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