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Concrete Canoe 1997
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Concrete Mix Design


DESIGN OBJECTIVES

     The goal of the concrete mix design team was to transform the hull design into a smooth, lightweight, and durable concrete canoe. To this end, the concrete mix target properties considered were strength, unit weight, and workability. The preliminary hull design analysis yielded a maximum compressive stress of 1097 psi, and a maximum tensile stress of 168 psi. In addition, physical laboratory tests determined that an increase in concrete strength would coincide with an increase in overall unit weight. Therefore, a balance was sought whereby sufficient strength could be attained, while still maintaining a relatively light unit weight. With these criteria considered, Team Spartan aimed for a design mix having an allowable concrete strength of 2500 psi, with a unit weight less than 70 pounds per cubic feet (pcf).

BINDING MATERIALS

     The principle binding material of the canoe mix is Portland Type III cement, which makes up approximately 82 percent by weight of the binding materials. Type III was chosen because of its high-early strength properties. The remainder of the binding materials consisted of pozzolans (silica fume and fly ash). Silica fume was added to improve the strength and lower the permeability of the concrete mix. Fly ash was included to improve workability and lower the unit weight of the mix (by replacing the cement with a less dense material) with no significant loss in strength.

AGGREGATES

     Perlite was considered for use as the coarse aggregate because it was incorporated into last year’s canoe, the "Perlite Piranha ." Although perlite is commonly used as filler in creating lightweight concrete, it does not display the required amount of compressive strength. In contrast, test mixes utilizing varying sizes of ceramic spheres (both as fine and coarse aggregate) had higher strength-to-weight ratios, as Figure 3 illustrates. Therefore, mixes were designed using these aggregates to optimize the strength-to-weight relationship, and provide the best option for meeting the design goals.

MIXING

     The development and utilization of a spreadsheet program to calculate various batch portions simplified mix design proportioning. Mixes were designed with a low water/cementitious ratio to insure adequate high strength. Using the American Society for Testing Materials (ASTM C 128) testing method, the Saturated Surface Dry (SSD) of the combined aggregates were determined to obtain the correct batch weights, as well as control the net water content of the concrete mix. By pre-wetting the aggregate prior to mixing it with the correct amount of water, a controlled mix was insured where by the aggregate would neither absorb nor contribute water. Finally, in order to maximize workability and finish appearance, a high-range water reducer was added to the mix water.

TESTING & FINAL MIX DESIGN

     To test a large number of batches within a short period of time, 3" x 6" cylinders were tested at 7 and 14 days. Extended 21 and 28 day tests were performed on the more promising mixes. Compressive strength (ASTM C 39), and splitting tensile strength (ASTM C 496), which evaluates shear resistance, were determined in controlled laboratory settings. After testing, all values were entered into a database to quantify and develop trends for future testing. Mix quantities and ratios were incrementally increased or decreased by using the data from the spreadsheet until the mix design goals were achieved. After testing 40 mix batches, the final SpartAttack mix design was selected. The selected mix is workable and easy to trowel and place. It has a slump of one inch, making it workable and easy to trowel. Its 28 day strength of 2820 psi exceeded the minimum 1097 psi, and its unit weight of 60 pcf (which is less dense than water’s 62.4 pcf)produced a 85 pound canoe. Table 1 illustrates all relevant information pertaining to the proposed and final mix designs.

REINFORCEMENT

     After finalizing the concrete mix, reinforcing material was selected to provide rigidity and increased shear and tensile resistance for the concrete canoe. The two options considered for reinforcing material were steel mesh and plastic fibers. Plastic fibers are lightweight but do not provide the required strength. Furthermore, plastic fibers interfered with the workability, concrete placement, and finishing. Steel mesh was selected as the reinforcing material for this year’s canoe based on past experience using steel mesh on previous SJSU concrete canoes, as well as its superior strength, rigidity, and ease of shaping.

STEEL MESH SELECTION

     Considering the three standard galvanized steel mesh sizes tested (1"x1", ½"x½" and ¼"x¼" square openings), the ½"x½" mesh had the most promising strength-to-weight ratio and mesh opening size for ease of concrete placement and consolidation. Plate tests (12"x12" x varying thickness of 0.20" and 0.25") were conducted to determine the minimum allowable canoe thickness, as well as the number of layers of mesh required to adequately support an individual paddler weight of 200 pounds. Table 2 summarizes the results of the plate tests.

REINFORCEMENT LAYOUT

     The final design consideration for the concrete canoe was to determine the reinforcement system layout. Visual observation of the full-scale fiberglass prototype identified potential tensile stress areas directly beneath the paddlers. To alleviate this problem, both front and rear paddler seating areas were double reinforced with a second layer of steel mesh, (oriented 45 degrees from the original reinforcement grid), to provide sufficient support. Additionally, the prototype displayed flexural bending around the canoe bottom at maximum beam. To reduce this undesired reaction, two transverse ribs made from pre-fabricated strips of aluminum sheathing were secured with 28-gauge wire to the reinforcing steel mesh system. These ribs were positioned at 9.5 feet and 11.8 feet from the bow to optimize resistance to flexural bending. The final layout is shown in Figure 4.

 

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