(green dragon, dragon root)
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.
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