They are not talking about "concrete tubes". They are talking about the hard cardboard tubes that are used to form concrete pillars. You pour concrete into them, and when it dries, you cut the cardboard off, and you have a pillar or piling.
These tubes come in sizes from about 6" diameter to about 24" diameter, and they are thick hard cardboard. The most common size I see are 8" and 12". They are often used in DIY projects to make tuned "t-line" type enclosures for subwoofers. Unfortunately, the single diameter tube doesn't have the actual correct tapering for a true t-line, and winds up giving "one note bass". It's not really that bad, but tapered would be a lot better.
They could be used as bass traps, if you make them correctly.
These tubes come in sizes from about 6" diameter to about 24" diameter, and they are thick hard cardboard. The most common size I see are 8" and 12". They are often used in DIY projects to make tuned "t-line" type enclosures for subwoofers. Unfortunately, the single diameter tube doesn't have the actual correct tapering for a true t-line, and winds up giving "one note bass". It's not really that bad, but tapered would be a lot better.
They could be used as bass traps, if you make them correctly.