Post 12 in a continuing series on the design and construction of a new gate for the Tenshin garden at the Museum of Fine Art in Boston.
Driving to Boston takes me 2 hours each way, in the best of traffic conditions, so I am careful to plan my trip as carefully as possible in advance so that when I get to site I have all the bits I need. Despite this, I find it quite difficult to execute this plan to perfection. It seems very hard to be perfectly prepared, and I often find I am short some little item. Sometimes my helper Matt comes through, either having the piece we need in his van, or making a trip to obtain something we need on his way to site.
I just found out there is a building supply store a few miles from site, but this didn’t help yesterday – as far as I knew there was nothing nearby. This entire process (working out on site) would be a bit different if it was something I did all the time – I guess I would set up some sort of trailer with supplies well stocked for unforeseen eventualities – however this is the first job away from my shop in years so I am not totally set up for off-site work. I’m doing the best I can, but I do find it a bit stressful at times trying to anticipate every eventuality.
Yesterday was the day to pour concrete for the new foundation. I awoke in Greenfield to find overcast skies, and I thought conditions couldn’t be more perfect, however when I reached Boston, the skies were clear and the temperature over 80˚F, so conditions were a bit more challenging for concrete work. I arrived at site, with my helper a few minutes behind, and we spent the first while shlepping bags of concrete out to the garden and getting the mixer set up, etc.. Then we set up a string line between the two concrete walls where the main gatepost centers are to be located. The string line was affixed to a wooden cleat on the end of each wall, and the cleats were fastened to each wall using concrete screws.
Now, one of the walls was forgiving of placing Tapcon concrete screws, while the other was not, and we snapped a few screws off. I thought I had packed a box of Tapcons, however it was not to be found. Turns out it was in the trunk of my car, and I brought the truck to site, so there you go. Fortunately we managed to get the cleats attached to the wall, but it came down to the very last scrounged screw dug out of my helper’s van.
Here’s the set up, which is the same on the end of each concrete wall:
The cleat below fixes the threaded rod for the wall post at a precise offset from the wall, while the cleat above is for the string line anchor. You can see the inked centerline on the concrete that was used to align both cleats, checked with a spirit level before the fasteners were fully tightened.
With the string line in place, I could place an aluminum positioning jig, starting with the front rail alone, and later connecting up the rear rail. Here’s the jig completely set up:
I spent the weekend constructing the jig after having traveled down to Yarde Metals in Southington Connecticut to obtain the raw materials. The jig uses 2″x3″ box section extrusions, connected to one another with doubled 0.250″x2″ flat bar links. The jig employs a pair of cables to tension the works together – measuring the diagonals on the cables until they are identical ensures that the rear extrusion is centered to the front extrusion:
In the above photo, close to view is the main threaded rod which will be used to fasten the main post to the foundation. It’s a 1.125″ diameter high-strength stainless threaded rod. The rear post rods are 0.75″ stainless. A wooden ‘fork’ is screwed to the side of the forms on each side to firmly hold the extrusions in position. Many checks were made to obtain the correct alignment of the four threaded rods and the position of the rods relative to the opening in the wall, and getting everything level and plumb as required. You really can’t be fastidious enough in setting this sort of thing up prior to pouring concrete.
Not readily seen here are that the bolts holding the cables had been turned in a lathe by a local machine shop so as to leave a raised nubbin on the center of the bolts, making it much easier to check the diagonals with a measuring tape, bolt center to bolt center. So, how about a close up?:
Next, a view down the line – this gate will be precisely centered in the wall opening, which also means it will be centered to the stone paving:
The target depth for the threaded rods was that they be buried 11.5″ deep into the concrete, however existing concrete made this not possible at each location. At worst though, the rods are buried 10″ or more and I’ll have to trim a little off the top of the rods later on to bring them to target height. That’s plenty of depth into the concrete so the anchoring will be robust. The above photo also clearly shows one of the details about the flagstone paving, which is that it crests in height right in the middle of the opening for the gate.
Then came time to dance with the concrete. Anyone who has poured much concrete will know that it waits for no man and this went doubly so given the midday temperatures over 80˚F (27˚C). In order to keep the waste concrete from hardening inside the mixer barrel or just setting up too fast generally, we had to keep going pretty much continuously for three and a half hours and keep the mix slightly wetter than I might have liked. There was absolutely no time to breathe really. Trying to keep the mix on the dryer side seemed to lead to the mixer producing a bunch of rounded concrete balls inside the mixer so more water went in. It would have been good to have had some concrete retarder on hand but I hadn’t thought to obtain any. Trowels, shovels and buckets had to be continuously washed and rewashed to keep concrete residue from hardening and making a mess of things.
Adding to the excitement was the performance of the mixer itself. At the rentals yard, the fellow told me that though the mixer was supposedly rated for two bags worth of concrete, in reality it could apparently handle 1.5 bags. This turned out to be an exaggeration. The mixer could only really handle about half a bag of concrete at a time. Add more concrete, and the motor would bog down, which meant that the barrel had to be physically assisted to keep turning. Possibly the drive belt started to slip but I really didn’t have time to futz with it given the demands of the rapidly setting concrete. The drive gear on the mixer’s motor was partially stripped as well, and mounted in a carrier assembly which allowed it to the gear shaft to slip down too low, allowing the gear to have about a 1/3 engagement with the planetary gear on the back of the mixing barrel. So, periodically one had to push the back of the shaft with a block of wood to get the gear to mesh properly. Oh joy.
The fact that the mixer could only do half a bag at a time, or 2/3rds of a bag with physical assistance from yours truly, meant that the processing of the twenty bags of 80lb concrete – 1600 lbs of material, plus water – went fairly slowly. We added to the mix a bit of plasticizing agent, and some Portland cement. The existing concrete was primed using a mix of concrete bonding compound and Portland cement, applied as a slurry, followed by immediate application of fresh concrete. The only conveniently available size of Portland cement is a 94 lb bag, so I supplemented the ready-mix concrete with a half shovel of extra Portland cement with each bag so as to make use of the material.
Here’s one half done:
The other half at the end of the day:
I had made careful estimation of the concrete required, and had brought 2 extra bags, however it turned out that we were short by two bags, so the left side rear post was left unfilled at the end of the day.
I returned the mixer to the rentals yard this morning and explained the scintillating level of performance achieved by this fine precision unit, and they were kind enough not to charge me for the rental. Sometimes things you get from the rental yard work well, and other times…not so much. If I mixed concrete more frequently I would get my own mixer, but usually I’m hacking away at wood so….
Back to site today to finish up. I mixed the remaining two and a half bags using a trough and shovel:
This was four hours of driving and one hour of work, but that’s how it goes sometimes. Squishing the mix into the form, and starting to trowel it off:
Today it was over 90˚ in Boston. I sprayed the concrete down to cool it off a bit and then put a few layers of burlap on top which was further soaked:
Then I laid a layer of plastic on top of the burlap. This should keep the concrete moisture from evaporating too quickly. You want to keep it wet and cool for the first few days. Weather forecast calls for heavy rain tonight and tomorrow so that is a help.
The site as I left it:
It’s satisfying to be complete through this phase of the project. Whew! It was a bit of a slog. It’s kind of funny to hop from furniture making one moment to concrete work the next. I think I’ll take up lumberjack sports soon, or possibly mining.
Next up, in a couple of weeks, will be the stripping of the forms and back-filling, followed by placement of the granite footings a couple of weeks after that. I will be sending drawings of the various granite components required to the company in New Hampshire that will be fabricating the parts in a couple of days, so the ball will be rolling soon. I also need to cut the metal shoes off of the kiosk footings as these will be modified so as to raise the posts further away from the soil.
All for now, over and out. Thanks for visiting the Carpentry Way, and comments most welcome. On to post 13.