After concrete canoes, the next unlikely medium to be conquered is space. NCEI discovers it is all a question of getting the right mix.
Visit the excellent website run by the University of Alabama at Huntsville's chapter of the American Society of Civil Engineers and click on 'launch video'. Watch as a typical amateur rocket enthusiast's solid fuelled model roars into the blue Alabama skies and returns safely to earth below a red parachute. Marvel at the punchline.
This is more than the typical small rocket that US enthusiasts fire off every weekend. It is in fact what is claimed to be the world's first concrete rocket - not quite as paradoxical as a lead Zeppelin, but almost. And the highly unusual flexible lightweight concrete that makes up the rocket's casing could soon be in outer space proper, replacing the current generation of epoxy based aerospace composites.
UAH professor of mechanical and aerospace engineering Dr John Gilbert says a cementitious composite has several key advantages. 'It should be far less vulnerable to corrosion, atomic oxygen erosion and radiation damage than the traditional alternatives. And while a cementitious structural section such as a space station wall or a telescope mount may be thicker than an epoxy composite, it may well be significantly lighter.'
In 1994 a UAH team sent a miniaturised concrete batching and mixing plant into orbit on board the Space Shuttle. Concrete was successfully mixed in zero gravity, but the hardened specimen, tested back on Earth, proved to be disappointingly normal, with only a higher density and a less random orientation of the hydrated cement crystals to mark it out from more terrestrial mixes.
Space was not the imperative that drove the development of the new composite, however.
UAH's chapter of the ASCE has a long and illustrious record in the US concrete canoe championships (see box). Over the last decade or so, the design teams have tested more than 200 different concrete mixes, striving for an ideal combination of weight, strength and ease of construction. By 1999 they had a potential winner.
A canoe skin is a testing application for any structural material. It has to be light, waterproof and capable of bending in both directions under the exertions of its crew. Impact resistance is important, as is the potential to be polished to a mirror smooth finish to minimise wetted drag.
Meeting these requirements with a 'concrete' needs lateral thinking.
Achieving low weight is relatively straightforward. The UAH team chose 105 micron diameter hollow glass microspheres as aggregate, making a lighter than water concrete density of 757kg/m 3possible. A high proportion of polymer added water resistance and flexibility.
The final touch is the use of three layers of non-impregnated carbon fibre mesh located symmetrically within the 7.4mm thick skin without spacers.
Survivor, the 2001 entry, is unique among all comparable watercraft. It has no internal stiffening, no ribs, no bulkheads and no thwarts. It is designed to flex and gives the crew something of a wild ride. Gilbert says that compared to rigid canoes, paddling Survivor was more like riding a camel than riding a bicycle. Exhaustive testing was needed to reassure the designers that the composite could handle this degree of flexure in the long term without damage.
Compressive strength of the concrete is almost irrelevant.
Tensile strength and modulus of elasticity are the key structural parameters. The objective, says Gilbert, is to produce a concrete flexible enough to enable stresses to be transferred from the matrix to the reinforcement so that the composite can absorb as much strain energy as possible.
The UAH mix Portland cement : 266kg/m 3(80% of binder content) Latex: 51.7kg/m 3(15% of binder content) Acrylic fortifier: 16.4kg/m 3(5% of binder content) Glass microspheres:
104.3kg/m 3Water: 318.4 litres W/C ratio: 1.02 Concrete canoechampionships To be eligible for the highly competitive US concrete canoe championships, sponsored by the American Society of Civil Engineers and admixture giant MBT, canoes have to comply with some very strict rules. Only open, Canadian style craft propelled by single bladed paddles are allowed. Kayaks and exotic designs like trimarans are banned. The definition of 'concrete' in this context is equally demanding.
Binders can contain any form of cementitious material, for example pozzolanas and slag, and resins and polymers in latex form. But at least 75% of the binder must be classic Portland cement, and there must be enough water for cement hydration to occur.
Any form of reinforcement may be used, but it must be solid. Hollow tubes are not permitted, nor reactive reinforcement that gains further strength once in place. And there must be no surreptitious strengthening of the hull by nonconcrete structural members, like gunwales.
The UAH design team was able to build on a long history of technological development when it sat down to plan the entry to the 2001 series of local and national events. The lightweight concrete mix and carbon fibre reinforcement was carried over from the 1999 and 2000 entries (see main story). For this year the main innovation would come in the hull form, a further development of the flexible, unbraced Ingenuity design of the 2000 entry This was 6.93m long and weighed just 35.8kg, with a nominal skin thickness of only 7.4mm. It was designed not so much to have a higher top speed than the opposition, but more to have a higher average speed - in other words, to slow down less between paddle strokes.
Allowing the hull to flex under the efforts of the two or four-strong crews would, the design team believed, store energy that could be released between strokes, driving the craft forwards.
Much to its dismay, success during the regional events could not be replicated during the national finals. A heavier and more conventional design from 1999 winner Clemson University eventually took the honours for 2000, a success the UAH team put down to a combination of a basically faster hull shape plus a slicker presentation to the judges. Race success counts for only 30% of competition marks, with the rest coming from success in the design, workmanship and display sections as well as for the quality of the team's oral presentation to the judges.
However, the 2001 Survivor team was still convinced the basic Ingenuity concept was the right way to go. Improving the hull shape and tuning it even more precisely to the natural 6Hz frequency of paddling was the chosen solution.
Compared to Ingenuity, Survivor is less stable, but faster and more manoeuvrable.
Building began with an accurate male mould from wood and glass fibre reinforced plastic. A sheet of plastic was draped over the mould, and a layer of graphite mesh laid on top. Then came the clever bit.
To eliminate the need for spacers between the three layers of reinforcement, spacers which would have created unwelcome stress concentrations, the team opted to string 2.8mm diameter wires at 76mm centres down the length of the mould on top of the layer of mesh. The mould was then plastered with the lightweight concrete mix up to just below the top surface of the wires.
After 12 hours the wires were carefully drawn out of the surface of the concrete, the resultant grooves filled with fresh mix and a new layer of mesh laid in place. The wires were restrung - carefully positioned so as not to be directly above their original location, and the operation repeated. A third cycle after another 12 hours completed the hull.
Hours of hand sanding and filling with a topping mix produced a slick finish. Once afloat, Survivor soon showed its potential, taking the key south east regional title before lining up with 23 other teams for the national championship at San Diego in June. UAH had promised to 'outsmart, outperform and outpaddle' the opposition, particularly arch rival Clemson, which was bidding for an unprecedented third consecutive victory.
UAH, however, was going for its own record - to be the first team to win five titles.
In the event, the team triumphed over Clemson by just five points, still losing narrowly out on the water but scoring heavily on technical merit. The 2002 entry is already on the drawing board.
Maximum tensile stress in the concrete was calculated to be 0.91N/mm 2, as opposed to a concrete tensile strength close to 1.8N/mm 2. Tests on cantilever specimens showed that failure typically occurred in compression, when the glass microspheres crushed, the carbon fibres buckled and the composite delaminated.
UAH's success and the unique technology involved has attracted lots of outside attention. As one of the training facilities for NASA engineers, UAH has strong links with the nearby Marshall Space Flight Centre and the US Army's Redstone Arsenal. Another, larger concrete rocket is due to be launched shortly. The concrete spaceship is close to reality.
INFOPLUS www. uah. edu/studentlife/organ isations/ASCE www. concretecanoe. org