Concrete on moon, a flight of fancy!
Once I was a member of an interview panel to assess top performers with one of my previous employer. Amongst host of other aspects, we were to probe and comment upon the ability to thinking laterally by the candidates. I recall a particularly creative member of the interview panel had framed the following question. “If cricket were to be played on the moon, who would be the most disadvantaged, the batsmen, the bowlers or the fielders?” While it would take more than a few minutes, if not hours, to weigh all factors and conclude either way, the most creative answer to my mind in such situation should have been “the umpire”!!!.
It would be equally interesting to ask the question “How would concrete on the moon be different from the concrete on the earth?” The recent announcement of the discovery that water is available on the moon makes this question far closer in time to reality than one would have imagined a year ago. Concrete is second only to water when it comes to ranking the most used material on this planet. It would be in fitness of this statistic if concrete repeats this feat on the moon also.
The difference between concrete on earth and moon may well begin with a difference in the required properties. We all know that the pull of gravity on the moon is one sixth of that on the earth. This means that the dead loads and live loads on the moon will be one sixth of that on the earth for a structure designed for the same purpose/function and having similar physical dimensions. Will M10 concrete then be sufficient where M 60 is prescribed on the earth? Quite possible, assuming that there won’t be moonquakes and wind forces on the moon. However, moon is known to have been subjected to many impacts in the past. The force of impact be foreign bodies could well be the governing force in design of structures on the moon.
On the durability front also things may be far different on the moon. Devoid of any atmosphere, carbon dioxide will no more be a threat for carbonation and chlorides for corrosion of reinforcement! It obviously does not rain on the moon, so drying and wetting would also not be a cause for deterioration. What could matter is the temperature variation which is much more extreme on the moon! The maximum and minimum temperature is in the range of above 1000C and below -1000c respectively! The very large temperature variation would require considerable more attention to controlling thermal cracking at early age and also large expansion and contraction that would result throughout the service life of the structure.
Will we require RCC on the moon or will PCC suffice? Well, if all overturning forces and moments are generated by eccentricity of vertical loads and not external horizontal forces like wind or moonquake, it is likely that the structures would be far more stable by themselves. However, to make large span beams, slabs, cantilevers and bridge girders etc. RCC will be required. The normal / economic spans on the moon may however be 6 times that on earth!!! Isn’t it quite an incredible feeling to imagine beams that long and lightweight by themselves to support the loads?
While structural stability may be at an advantage, the same may not happen with ground support. What would happen to the active and passive earth pressure and the lateral stability at foundation level? My guess is that the embedment needed will be much higher as lateral resisting force from the earth, oops, moon will be smaller ( gamma being smaller!). However, this will matter only where the overturning loads are not gravity generated, like earth pressure, hydrostatic pressure etc.
Coming to the production of concrete, what problems can we foresee in such a low gravity scenario? For one, mixing may be less efficient in conventional rotating drum type mixers and we may have to use only paddle type / rotating blade type mixers. The slump of concrete of same workability may be one sixth and the concrete may not flow in the forms as easily in the absence of the strong gravitational pull! The force on formwork will be much smaller and the forms could be much lighter!
But the biggest problem could well be the periods of very low temperatures alternating with very high temperature on the moon. The sub zero temperature would mean that the cement as we know would cease to hydrate for a part of the time and may have accelerated hydration during other periods. All ingredients may have to be heated to produce concrete and curing would involve not only water, but also careful temperature control to facilitate hydration and strength gain. Perhaps, we will invent cement that hydrates at extremes of temperature!
Thinking about such imaginary issues, could sometimes clear some doubts regarding the scientific subjects that form the basis of Engineering and could help you think out-of-the-box. In the paragraphs above, I may have made some logical mistakes or erred regarding some of the facts. Readers are encouraged to point out flaws if any for the sake of better understanding the laws of physics and mechanics.