Making and Setting-up precast concrete ribs
Posted on March 4, 2016 by
These concrete ribs were designed to give me more of an open space feeling without needing to build a wide and tall vault. For more about the design of the ribs or how I made the forms, you can read this earlier post.
This was really an epic part of the earth sheltered home build project, spanning (no pun intended) much more time than I would have liked. The costs for the forms and concrete were pretty minimal, the majority of the cost was actually in hiring the crane to move them around and set them up.
Side note: since I am so far behind… I will probably go and improve the section on the timeline next. Thanks to those of you who wrote to inquire if I died. Nope, just really busy and didn’t have time to put a video together. Speaking of which…
The Video
Lessons learned
- The Pump truck did not make things easier. Pump trucks are expensive and I needed it to come out for the floors anyway, so the first few rib pours needed to be coordinated with other jobs. This complicated the planning and made the pour days harder. It also slowed down the progress on the ribs by delaying the pours. And after all that extra planning and delay and expense, it was just plain harder to fill the ribs from the pipe than from the chute because it was really difficult to move it around. I guess my advice here would be to talk to the concrete guys and ask them how they would recommend you handle it. It was probably obvious to everyone except me.
- Originally, I used silicone caulk to seal the bottom edges of the forms against the floor. However, this was a pain to clean up later. For the 2nd set, we used play-dough that my wife got cheap on clearance somewhere. The play-dough came up easily enough at the end, but it was such a pain to roll out and put into place that we ended up going back to the silicone for the later ribs.
- The first set of ribs took about 4 hours per side to polish. I would start with a diamond cup wheel and then follow up with successive polishing pads at 50, 100, and 200 grit. I also used a special wheel to put a 3/4 inch round on the edges. For the second half of the ribs, I got a larger, more aggressive diamond cup wheel. It worked so well for the first step, that I quickly did all the other ribs. It was only when I got to the 50 grit pads that I noticed the diamond cup wheel had made deeper scratches than the previous one and it was much more difficult to remove them. I even went back to try the less aggressive cup wheel once I realize that the 50 grit was not working well. Overall, this mistake cost me several extra hours for each rib.
- The moving dollies were supposed to be able to handle 1000lbs each, but that was clearly an exaggeration. I used more than 10 for each 5000 lb rib and still we had crunching sounds as their ball bearings exploded out all over the floor. The tires on some of them shredded completely. Eventually, I learned that most of the damage happened as each wheel rotated into the correct position to roll forward. By the time I got to the 3rd set, I had learned to point the wheels all in the right direction before lowering the rib on to them. This increased the survival rate considerably. I also salvaged partially damaged dollies by consolidating the less damaged castor wheels on to other dollies.
- The rubber form liner molds were an interesting part of the build for me and I like the final look on the ribs, but again, not the best idea. More details below.
Liquid Rubber Form Liners
Originally, I planned to use the Styrofoam ceiling tiles directly, but after handling them a bit, I was worried that they were too fragile and wouldn’t last thru multiple uses. I also thought it would be a bit tricky to place them in the form so they would be centered because they were a bit narrower than ideal. However, if I used the ceiling tiles to form durable rubber form liners, I could get longer pieces that would be reusable and would be the inverse of the tiles. I could carefully center them on a board of the right width so the full depth forms could be easily placed, etc. I did some math to find the volume that I would need and found that I could get the PolyTek 75-75 ingredients for about 175$. That seemed reasonable enough to me at the time. The box of foam tiles was about $40. However, I soon discovered that mixing carefully was critical. My first few attempts were mostly good, but 95% isn’t good enough to cast concrete with. I only ended up with enough decent panels to do a small section of the first few ribs.
For the living room ribs, I decided to make the panels a little thicker (these were the ones I showed in the video). I would need to order more liquid rubber. This time, it was more like 225$, so I was in for roughly $400 worth of liquid rubber. That would have been enough to buy new Styrofoam tiles each time, so not the smartest move in hind sight.
I didn’t quite use up all my liquid rubber ingredients on the panels because I decided to try the Styrofoam ceiling tiles directly on each alternate rib. This way the pattern inverts, positive/negative for each rib.
Cost and timing
See the other page about costs for the forms, but they were just a few hundred dollars and were reused for all the ribs. So divided by 11 ribs, that is just about 30$ each.
The rebar was fairly affordable also. We used about 60$ worth of #5 and about 50$ worth of stirrups, so about 110$ per rib. There were also some steel plates that I built into the ribs and I think I paid about 10$ each for those from the scrap yard. Tie wire and welding costs are hard to guesstimate, but lets say it is less than 5$ per rib.
The concrete was about 1.3 yards per rib, which would be less than $185, even after some waste (Concrete costs about $100 per yard, delivered, but there are a couple other charges).
The molds and ceiling tiles were about 450$ total, so about 40$ each.
Then I bought about 20 of those little moving dollies and some other miscellaneous stuff for about 220$ total, so 20$/rib to move them out of the garage.
The grinder and all the pads were under 220$, but I still have the grinder many of the pads, so I am just going to leave that stuff out.
So lets say the total was $400 per rib. Not bad considering the quote to have it done by someone else was about $8,000 each.
Unfortunately, the crane and welding the ribs to the ring just about doubled that cost. The guys from RTC were great to work with and I appreciated their help, but I had not budgeted enough in that area.
Timing varied as I got better at each task, but here is the rough break down in man hours.
Form prep took about 2 hours per rib and includes cleaning the form segments, fixing any damage, re-assembling the form and attaching it to the floor, and then caulking the bottom edge.
Rebar was next and was taking about 8 hours on average, including getting all the rebar in, tying and welding.
After the rebar was in, I needed to call for an inspection, which didn’t take much time, but did delay the next step.
Next was the pour. It actually took less than half an hour to pour each pair of ribs, but then we would spend at least an hour or so troweling and finishing it off. Lets be generous and say 2 hours per rib. Then there was some delay (several days to a week) as we let the concrete cure.
Then we would spend about 6 hours (3 hours each) unpacking and moving the ribs out of the way. This included a lot of clean up.
And then the cycle could repeat.
After the ribs were out of the garage, I could polish them. The first half took about 4 hours per side to polish, so 8 hours per rib. But, thanks to an overly aggressive cup wheel and adverse weather conditions (hose freezing, etc), the second set took about twice that long. Lets put in an average of 12 hours per rib.
The flip and setup time wasn’t too bad, but lets write it down as an hour each there.
This brings the total hours per rib to about 28. I had 11 ribs to make, so about 308 hours total. If I had done those in 40 hour work weeks, it would have been nearly 8 weeks of constant work (yes, some of those hours or days were worked by other people like Sherri, Bonnie, Aaron, Dan, John and Mark). However, I already had a full time job, and also had other things to work on at the house, so I ended up spreading this part of the build over a whole year. The video on this article covers from July 2015 thru April 2016.
Needless to say, I am glad the ribs are all done.
Gallery
After a long and difficult project, looking over the pics feels pretty good.
