Arisaema dracontium (green dragon, dragon root)
Arisaema dracontium 
(green dragon, dragon root)

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Overwintering troughs

Adirondack Chapter, North American Rock Garden Society

Concrete for Troughs:
It's not that hard

Follow-up report from the February 2005 meeting

True, he didn’t have any slides of flowers to help drive away the midwinter blues. But Dr. Ken Hover’s presentation on concrete still attracted a full house (and many new faces) to our first meeting of the year on February 19.

“Mr. Concrete,” as he’s known, didn’t disappoint. The Cornell professor of civil and environmental engineering gave a humorous, engaging and energetic – as well as informative – talk.

Over the years, I’ve built half a dozen or more troughs, following various recipes with mixed results. Hover demystified the process. He helped me understand why some of my previous attempts failed and will make my future efforts more successful.

When you make concrete, you are basically making artificial rock, Hover pointed out. The process involves taking aggregates (such as sand, gravel or crushed stone) and binding them together with an adhesive (most commonly portland cement).

In the case of hypertufa troughs, lighter aggregates such as peat, vermiculite or perlite replace some of the sand or gravel in the mix. There are also other binders and additives you can use to change the properties of your final product, including epoxies, polymers and latex.

Some key points that I took home include:

  • Curing is key. I never paid a lot of attention to curing. That’s a big mistake. While wet, the cement forms crystals that interlock, much like Velcro. That’s what holds the aggregates together. If the mix dries too fast, the final product will be weak. Cover your projects with moist burlap and/or plastic. Mist them often. You need the moisture to drive the cementing reaction. You don’t want the moisture blowing off in the breeze.
  • “Solid as concrete” is a misnomer. As the water reacts with the cement, it leaves tiny air pockets within the concrete. This porous structure relieves pressure and helps resist breakdown during freezing and thawing.
  • Jacket weather ideal. Warm temperatures speed up curing. But if curing goes too fast, the result can be weak concrete. On the other hand, freezing temperatures shut down curing. Free water in the mix can freeze and cause the concrete to fall apart. “Jacket weather” is good concrete weather.
  • Wear gloves. I usually start out with cheap disposable gloves that invariably fall apart before I’m done. That’s another big mistake. Most aggregates are abrasive and the concrete mix using portland cement is dangerously caustic (pH 13 or more). Next project, I’ll invest in some good rubber gloves.
  • Plywood forms work well. But you may need to paint them or spray them with vegetable oil to seal them so your concrete mix won’t stick to them.
  • Lighter aggregates. Perlite and vermiculite likely make longer-lasting hypertufa mixes than peat because their mineral nature resists breakdown. Peat, being an organic, is likely to degrade sooner, though it has a track record of working well in light-weight mixes for troughs and similar projects, Hover notes. Just don’t expect it them to last as long.

If you want to try your hand at concrete or hypertufa projects for your garden, I highly recommend any of several books by Sherri Warner Hunter, including Making Concrete Garden Ornaments and Creating with Concrete. An Internet search for “hypertufa” will turn up many sites with recipes and instructions.

Craig Cramer


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