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Domain: masterbuilders.com

Stories and comments across the archive that link to masterbuilders.com.

Comments · 6

  1. Concrete Canoes, etc by bjtuna · · Score: 2 · on The Huntsville Concrete Rocket

    The American Society of Civil Engineers runs a nationwide contest each year, where teams of college students build concrete canoes, race them against other schools in their area, and then meet for a national race. This year's race is in Wisconsin. I've also been told that some places build concrete tobaggans too, but not for a contest.

    My girlfriend is the captain of the Johns Hopkins University Concrete Canoe Team. The presentation was held just this past Sunday, against Catholic University in DC. Over the past 3 years of her doing this, I've picked up a few things. Concrete isn't just for cinder-blocks... depending on how you mix it, what you add to it, how you pour it and cure it, and what you pour it ON (example: mix it with microspheres or layer it between sheets of fiberglass mesh), you can make a very lightweight, low-density (read: floats) structure.

    Even though I know all this, the old jokes never get less funny. "Concrete canoe? You row, I'll watch."

  2. Re:Concrete Canoes by NVHall · · Score: 3, Informative · on The Huntsville Concrete Rocket

    The American Society of Civil Engineers holds and annual concrete canoe competition for student chapter/club programs. Check out the site for more information...

  3. Re:Concrete? by PopeAlien · · Score: 2 · on Cement Canoe With A Contrarian Approach

    You're right - I was curious about the composite ratio in the rules.. I've now found out - The official rules state that the mix must contain 75% portland cement, and the UAH pdf list their mix as containing 80% portland cement. In fact they are using the same mix as the team from the previous year. I must admit I am suprised that a composite with 80% Portland cement could be that flexible. br>

  4. People Clear on the Concept Unclear on the Concept by blair1q · · Score: 4 · on Cement Canoe With A Contrarian Approach
    From the Purpose, Canoe Design and Past Winners page at the canoe contest website

    "The trick is to create a concrete mix that is less dense or lighter than water. Regular concrete is 140 pounds per cubic foot, but the students create concrete that is as light as 41 pounds per cubic foot. Water is 62.4 pounds per cubic foot, so the less dense concrete will float. However, students can create heavier canoes that will float, as long as the canoes displace their own weight (and the weight of the paddlers) in water."


    The concrete doesn't ever need to be lighter than water. The boat plus passengers and gear needs to be lighter than water. Which means the maximum displacement of the boat (fully loaded volume including outside hull sectioned at a plane level with the waterline) needs to weigh less than the load plus the boat. The concrete itself can be far more dense than water.

    Having concrete that is lighter than water means you can make a raft out of concrete. It means your boat won't sink if it floods. It also means your boat has less inertia.

    It looks like a minor point, I know, but this is an engineering contest. It's all about minor points and the error bars on them. To put conflicting statements in the brochure is to sandbag the less-experienced contestants.

    The real trick is that normal building concrete is much heavier than water (and somewhat absorbent, which reduces its effective displacement in contact with water). But this contest years ago ceased to be about floating a hunk of sidewalk.

    --Blair
  5. People Clear on the Concept Unclear on the Concept by blair1q · · Score: 4 · on Cement Canoe With A Contrarian Approach
    From the Purpose, Canoe Design and Past Winners page at the canoe contest website

    "The trick is to create a concrete mix that is less dense or lighter than water. Regular concrete is 140 pounds per cubic foot, but the students create concrete that is as light as 41 pounds per cubic foot. Water is 62.4 pounds per cubic foot, so the less dense concrete will float. However, students can create heavier canoes that will float, as long as the canoes displace their own weight (and the weight of the paddlers) in water."


    The concrete doesn't ever need to be lighter than water. The boat plus passengers and gear needs to be lighter than water. Which means the maximum displacement of the boat (fully loaded volume including outside hull sectioned at a plane level with the waterline) needs to weigh less than the load plus the boat. The concrete itself can be far more dense than water.

    Having concrete that is lighter than water means you can make a raft out of concrete. It means your boat won't sink if it floods. It also means your boat has less inertia.

    It looks like a minor point, I know, but this is an engineering contest. It's all about minor points and the error bars on them. To put conflicting statements in the brochure is to sandbag the less-experienced contestants.

    The real trick is that normal building concrete is much heavier than water (and somewhat absorbent, which reduces its effective displacement in contact with water). But this contest years ago ceased to be about floating a hunk of sidewalk.

    --Blair
  6. Rules and Regs by Kozz · · Score: 3 · on Cement Canoe With A Contrarian Approach

    are posted here. They're both PDF files. The rules are quite complex, explaining the ratios of different types of cement and so forth. Interesting read.


    Quidquid latine dictum sit, altum viditur.