Here's an interesting post on why it's called concrete5, just thought I'd post for further reading.
http://www.concrete5.org/community/forums/chat/just-curious...-why-...

I discovered Concrete5 in 2009, only about a year after it went open source. One reason (of many) that I was sold on it from the start was because of the name, and I've been using it (almost) exclusively ever since.

When I hear or think "concrete", I think: solid, resolute, strong.

I would rather work with a solution that is solid, strong, and resolved in what it has to offer.

Also, you use concrete to build a foundation, right? It stand to reason that you'd use Concrete5 as a tool to build a solid online presence.

So, I think the phrase "We can build it together" works well because it takes a solid solution to get a solid result.