The ferrocement/geopoly/basalt dicussion go go on based on theory for a long time. What we do know is that properly built concrete vessels have lasted a very long time, under exactly the tough conditions you worry about. As JL pointed out, it is a composite material, so it takes on characteristics of the base materials, but also exhibits unique characteristics specific to the composite’s construction technique.
I would not walk onto your theoretical flat .25" fiberglass span any more than the flat cement one. Or for that matter .25" steel. The sag in the middle would probably pull the ends loose before it would break, but either way… But I might very well walk onto one that had a shaped compression span that was that thickness.
As we speak, there’s a (poorly maintained) ferrocement sailboat parked three rows from my Carver 350 in the next moorage upriver. It was built in the 1980s and it doesn’t appear to have any problems staying afloat (which is more than I can say for many zombie sailboats in this area made of more popular materials). The lines are very nice, there is no rust or spalling apparent. It seems that the builder made sure that all armature was well-covered with cement. You have to look hard to know it’s ferro, but the weathering of the paint just looks different than fiberglass. I have thought about buying it as a fixer.
The biggest problems with ferro boats have been noted as:
- Builder doesn’t know much about boat design and makes a clumsy boat
- Builder doesn’t understand the amount of work involved in building a boat and never finishes (or takes shortcuts).
-These two points above are not unique to ferro boats.
- Builder doesn’t understand the importance of fully covering the steel armature
(Note a theme for the top 3 problems?)
-Up until recently, there simply hasn’t been a better armature material than steel rebar and wire mesh. Rebar takes a great deal of effort to shape, it’s moderately expensive, and great care has to be taken that it’s faired properly so as not to near the surface once plastered. Chicken mesh is not a good material for various reasons, but certain types of specialized metal mesh cloth have worked well. All of them require a great deal of hand work to shape and tie together (and some kind of support structure that holds it all up while waiting or and applying the cement). On the plus side, most of this armature work is not super time-critical like plastering is. It can be done slowly as resources permit.
- The density/weight benefits of steel outperform ferrocement for anything under about 27’-30’ lengths.
-Cement simply must be a bit thicker than steel for the same durability against load and impact. Given simple volume/surface area ratios the bigger boats benefit more from ferro than smaller. On the other hand, it is far more durable against chemical attack from the environment.
- The amount of labor to form the armature is intense.
- Plastering with OPC has to be done expeditiously and all in one go. The armature has to be complete, correct, and it takes a large number of people (unskilled is ok) to fill the armature and a small number of highly-skilled people to finish the plastering all in a short period of time before it starts to cure.
- OPC requires significant time to cure and must be managed (moisture/humidity) for some time.
- Steel can be welded into any shape and essentially becomes one piece of material (not entirely true)
So what variables can we change to make the calculation come out differently? With geopolymer and basalt:
Points 1 & 2 have nothing to do with materials, and everything to do with knowledge of boat design. So start with a decent design and make sure you have expert help with modifying it if necessary. Hartley’s in NZ has the most experience world-wide with ferrocement construction. They don’t have published guidance on Geopolymer, so keep that in mind and take their advice seriously, but with a grain of salt for the change in materials.
Point 3 is moot: basalt simply doesn’t corrode. You still need to fair the shape for reasons of performance and aesthetics, but the inner armature doesn’t need any minimum covering or distance from the surface to prevent corrosion.
Point 4: Basalt fiber (and even the rebar substitute) is lighter than steel, stronger than steel. You can put thinner armature elements in place for comparable performance. This may or may not translate into smaller boats under that 27-30’ trade-off sweet spot.
Point 5: The amount of work to form a basalt armature should be less. It is more flexible than rebar for larger structural elements. It is easier to shape. It is easier to cut. It can be epoxied together rather than tied. There is experimentation to be done on making sure that any tension elements point that tension in the right direction. In addition, basalt cloth can be used in ways that steel mesh never could. One of the biggest advantages of the basalt over steel as armature materials is that the basalt becomes part of the concrete. It chemically bonds with the cement in a way that steel simply does not. So the resistance to elongation of the cement past it’s ultimate tensile limit is applied at every molecule along the way, not at macro-scale mechanical protrusions. In combination with geopolymer’s long chain molecules (better flexural modulus better endurance under repetitive loading, better impact resistance) the geopoly and basalt have MUCH lower maintenance issues due to essentially zero corrosion vulnerability. If some basalt fiber gets too near, or even breaks the surface after plastering, simply grind it down smooth- you don’t even have to plaster over it.
Point 6: Unlike a cold joint formed when you put more OPC over previously-cured OPC, Geopolymer actually chemically adheres to base layers. So if your plastering is delayed or interrupted, it doesn’t necessarily ruin everything. It may be possible to add some efficiency to plastering with geopolymer and basalt by using power tools
Point 7: Geopolymer can cure very quickly with the right temperature conditions. Moisture levels are not as critical because water is incorporated into the molecule rather than acting as a catalyst that must be out-gassed.
Point 8: You don’t have to cut out enough to make a macro-sized mechanical joint for patching and filling cracks or holes with Geopolymer because it actually chemically bonds with stone or other Geopoly, not just a mechanical joint.
Other points to consider:
- Even steel has limits on scantling spacing and reinforcement. You can’t just leave an unsupported span or panel.
- Cement has better vibration-damping than steel
- Decks and bulkheads are opportunities for post-tensioning
- Keels typically need ballast. While cement isn’t as dense as steel or lead for this purpose, “pigs” of these materials are often cast in concrete in the bilge for weighting keels. Proper design of a keel/bilge ballast based on cement could add considerable structural strength to the hull by being a monolithic part of it.
- Managing geopolymer slump and mixture formula is more important than for OPC. Too much water or not enough changes the chemical composition and final strength. Water mixtures can affect final strength of OPC as well, but it is more forgiving.