Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is it possible to pour concrete in a vacuum?

  1. Feb 20, 2018 #1
    One idea for a moon base would be reinforced concrete domes, but is pouring concrete in a total vacuum even possible? Wouldn't the water just evaporate, or is there another material we could use instead of water? Are the ingredients of concrete plentiful on the moon, or could we synthesize them from lunar materials?
     
  2. jcsd
  3. Feb 20, 2018 #2

    Orodruin

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member
    2017 Award

  4. Feb 21, 2018 #3
    In short - wouldn't work

    The water would instantly vaporize unless kept in basically some form pressure vessel during the curing phase. This would also change the construction timelines that are used on earth. Generally the time line is 1-3 days you can walk on it, 7 days its about 75% of its max strength and can be removed from forms, and 28 days it can be loaded to its design strength those times are dependent on the mix as well. The second the pressure vessel is removed the remaining water would quit reacting and vaporize so it'd have to remain formed for the full 28 days. On a side note water is pretty heavy and would be a valuable commodity on the moon so concrete's pretty much out. There are some interesting techniques that are developing with laser sintering but I don't know enough about the geology of the moon or the results to say its viable. Also remember that any design load on the moon would be about 1/6th of its earth equivalent so there will probably be entirely different construction techniques involved at all levels.

    edited for clarity, also out of curiosity I googled laser sintering regolith, it's actually being researched. Not sure if you consider the source to be trustworthy or not though =P -

    https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110000850.pdf
     
    Last edited: Feb 21, 2018
  5. Mar 7, 2018 #4
    Concrete on the moon looks like a non-starter. Concrete in the moon, either in existing caves or excavated tunnels may be a different story. Cover the walls, floor, and ceiling with a thin sealant, think plastic or spraying hot melt glue. Next pour concrete walls, floors, and even ceilings (using forms). Choose a low pressure, mostly water vapor, in the tunnel or cave until the concrete has hardened and the forms can be removed. Some water would remain in the concrete, mostly water you took out to form cement. The remaining water could be recovered and reused.

    Why go to all the trouble? If you are building a long tunnel, for transportation, habitation or both, concrete may be the best material for building through weak areas in otherwise solid rock. Metal, mined and smelted from moon rock will cost more per ton, and require labor intensive installation. (Think bolting or welding.) Plastics will require hydrogen, the expensive part of water on the moon (or Mars).

    Concrete/cinder/breeze blocks could be manufactured on the moon, then used for construction. The problem is they would need reinforcement (read rebar) if expected to handle side loads--and lifting loads--if they were expected to hold air in.
     
  6. Mar 9, 2018 #5

    Baluncore

    User Avatar
    Science Advisor

    Why do you need concrete on the Moon?

    On Earth we have about 100 kPa atmospheric pressure. On the Moon, with lower gravity and a change in gas mix, that might be reduced in a moon habitat to a pressure of 50 kPa. It would require a membrane capable of withstanding an internal pressure of 50 kPa which, for a large radius dome would require a very significant tensile strength. Concrete domes work well on Earth where there is no big pressure difference across the wall.

    The biggest challenge is countering the internal air pressure. A concrete dome will not solve the problem without high tensile fibres. The problem can be countered by a layer of ballast on the top of the dome, while the lower wall would still require high tensile material.

    This suggests, not a concrete dome, but a vertical axis cylindrical structure with low vertical walls and a flat roof. One possible static solution is a flat roofed area sunk into a convenient crater with the weight of ballast material on the flat film roof countering the internal pressure. The wall membrane is constrained by the rock structure, or maybe partly by the hydrostatic pressure of external material.

    Ignoring the problems of construction for the moment, we need some idea of how thick the roof ballast will need to be if it is cut from the floor of the selected crater, gathered from the surface, or from a quarry nearby. On Earth there is a buoyancy due to the atmosphere, that is missing on the Moon. We can ignore that buoyancy since it is only amounts to about 1.2 kg/m3.

    It is going to be simpler to design the ballast for Earth's gravity of 9.8 m⋅s-2, then convert it to the Moon's lower gravity of 1.625 m⋅s-2, which is weaker by a factor of 9.8 / 1.625 = 6.03 The ballast layer calculated for Earth will need to be multiplied by 6.03 to correct it for the Moon.

    We have reduced the internal air pressure to 50 kPa which will require a force of 50k newton per square metre.
    On Earth, gravitational acceleration is 9.8 m⋅s-2. 50 kN would require 50 k / 9.8 = 5.1 tonne of ballast per square metre.
    Basalt rock ballast would have a density of about 2.8 tonne/m3 on Earth, so it would require about 5.1 / 2.8 = 1.82 metre of ballast to counter the 50 kPa internal pressure. Now we convert the thickness to allow for the Moon's lower gravity. 6.03 * 1.82 m = 10.97 m, call it 11 metres.

    That 11 metre layer of ballast will keep the roof membrane down. It will also act to thermally insulate the habitat from the month long cycle of lunar day/night. If the inhabitants are lucky, micrometeorites will be stopped before they perforate the inner membrane.
    Will gamma rays from the Sun be a problem, as when flying at high altitude, or will the ballast fix that problem too?

    Earthworks, an 11 metre thick layer will require a significant volume of cut and fill. If concrete was used instead of local rock ballast, then without tensile fibres in the concrete it would need to be a similar thickness. That is a massive amount of concrete.

    So how can we support an 11 metre thick layer of ballast before inflation or in the event of a major leak. Following a small leak such as a membrane perforation, the air pressure will remain constant while the roof gradually falls towards the floor. An internal patch would be pressed by air pressure onto the membrane. If the damage was too great, ballast would flow down into the air space until the remaining membrane was pushed upwards resulting in a massive blowout. There would need to be safe areas that were protected from the low roof during difficult times.

    If a tunnel was dug underground into a hill, then wall stability of the tunnel would be critical. A membrane lining would work 11 metres below the surface. Unless tunnels were cut in solid rock, deeper tunnels would need a wall under compression, probably sintered shells that lock together. Shallow tunnels would preferably be a smaller diameter, with a circumferential tensile fibre reinforced membrane to handle the hoop stress.

    Whatever is done, an airlock will be needed for access between the habitat airspace and the lunar surface.
     
  7. Mar 10, 2018 #6

    OmCheeto

    User Avatar
    Gold Member

    Thank you!
    I thought I'd lost my mind when I calculated it would take a layer 24 meters thick to counteract the full 1 earth atmospheric pressure.
    My idea was to "simply" o0) use some kind of solar powered, fancy schmansy 3D printer type robotic system to melt the lunar soil into a silica crystal casing.

    The material is all there, in just about the perfect ratio:
    composition of the moons crust
    % by weight
    42 oxygen
    21 silicon​

    A 24 meter thick roof.....

    Wow.

    Always good to do the maths, before investing in whack-a-doodle ideas.
     
  8. Mar 11, 2018 #7

    Baluncore

    User Avatar
    Science Advisor

    I did not believe it myself. It was only after writing post #5 above that I realised there was an easy way of estimating the order of magnitude.

    On Earth, 1 Atm = a 10 metre column of water. Multiply by Earth/Moon gravity ratio = 6, to get 60 metre depth of water on Moon.
    Divide by SG of material used, say the density of lunar cheese = 3, to get 60m / 3 = 20 metre.
    So I removed the line saying “please check my numbers”.

    It appears that it would be easier to dig a habitat into the lunar surface than it would be to support that volume of electrostatic moon dust.

    The fine component of Moon dust behaves in a similar way to photocopier toner. Dust is best handled on Earth by vacuum transfer systems and cyclone separators, which will not work on the Moon. Maybe a low ballistic trajectory could be used to move moon dust by throwing it to where it is needed. But how to stop it spreading due to electrostatic charge? Doing the work at night would preclude direct solar power for the plant.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted