Wednesday, June 25, 2014

Gateway (XII)

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.

Another view:


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:


Done:


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.

11 comments:

  1. Hi Chris,

    I recall a lengthy post on ICFs and concrete use in projects. In it, you concluded that concrete is an acceptable material for projects meant to last but that they required some amendments (fly ash, recycled content in the aggregate, fiber reinforcement, superplasticizers and low water content) and required a long curing period. Noticing that you opted to avoid these requirements for the gate footers, I'm curious why you chose to go the route of portland cement? Do the above requirements only apply to basements? To large buildings with greater overall mass?

    As always, your documentation of projects like this is greatly appreciated. It's incredible how much goes into something like this. A man of many hats, you are.

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    1. Hi Chris,

      thanks for the comment and questions.

      I did use super-plasticizing compound, aka water reducer, with the concrete. The main problem I ran into was obtaining materials to accomplish this work. The plasticizer was sourced from a company in New York. The epoxy rebar was hard to find and took many phone calls. As for fly ash, I guess I stopped looking after one building supply place after another - not to mention specialty masonry supply outfits - simply didn't have anything other than the run-of-the-mill products.

      I was under some time pressure to complete the concrete work (another contractor is waiting on my completion of the foundation and granite so they can do the granite paving), so that gave me less time than I might have liked to search out materials from afar. Also, I was bonding to existing portland cement based footings, and was concerned about getting the best adhesion I could, so a product that was as similar as possible to the existing concrete, and compatible to the bonding compound (which required portland cement) made the most sense.

      I think that starting from scratch with the foundation (that is, not bonding new concrete to old), and given more time, and what I have learned about the difficulty in obtaining the more specialized concrete materials, I would have incorporated the fly ash and more plasticizer and the fiber reinforcement. Nothing wrong with any of those options, just wish they were more conveniently obtainable. Hopefully the lessons I learned here will be of help to those wishing to employ non-standard materials in their concrete work.

      A low water content in the mix was certainly the goal, but the mixer problems and hot temperatures got in the way of that. I found mixing by hand in a trough yesterday enabled me to make a much stiffer mix with ease, so I am wishing, at least in that respect, that I had mixed all the concrete in the same manner. Not sure I would have enjoyed mixing 22 bags worth in the trough however!

      ~C

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  2. You've found some novel solutions to common construction challenges by pre-planning. When it comes to executing the plan in the real world, what strategies to you use to overcome the urge to make small compromises? What I'm referring to is the common urge to use the most expeditious methods once a project is in full swing.
    -Mike

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    1. Mike,

      it's an interesting question you posit. I think most of what constitutes 'urges to make compromises' revolves around how a person deals with stress and anxiety in particular. In most cases, the rushed solution is rarely a good one, nor is there really a need to have rushed so much in the first place. I'd rather do the job right and lose some financial reward than get it done regardless just to make a buck.

      I feel planning ahead as much as possible and being as prepared as you can be for contingencies is about all you can do. Knowing the minimum standard you will accept in your own work and sticking to that is also important. At the end of the day, the craftsperson knows what he has done and is the best judge of whether the work meets the standard or not. Integrity of workmanship is something only which is individually considered and to which is a person holds themselves as a commitment to their craft. If you care about your work, if you have a personal stake in it's quality, there is no other choice really. Better a late delivery than a poor one.

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  3. Thanks for the reply. I assumed there were factors I wasn't catching and bonding to pre-existing concrete definitely qualifies.

    I've been reading up on foundation work (super-plasticizers - as you mentioned, water reducer and dispersant, fly ash - strength increase due to resistivity and decreased permeability) but what type of fiber reinforcement would you use? I've seen reference to glass and fibre reinforced polymers, is there an ideal mix that minimizes environmental damage, maximizes strength while keeping cost down?

    There are also various classes of fly ash when I search, class C & F. Most architectural applications see up to 30% replacement of portland cement with fly ash but then one of the sites I read went on to state that footers and columns may contain much more.

    If you were planning a foundation for your own home, or perhaps your Magnum Opus, what mix would you use?

    Sorry to take this off track, as I'm sure most are interested in the woodworking involved here but there is so much information out there, I didn't know if you had settled on an "ideal" mix that meets cost requirements. No hurry for an answer, just something I've been thinking about in the event we build a home in the future.

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    1. Chris,

      your questions are large and i thank you for that but there really isn't room here in the comments to address your concerns adequately. I will be building a structure on a foundation for myself in the near future, and I can explore the topic of foundations at that time. I hope you can patiently wait until then- but sounds like you're doing some good research in the meantime.

      ~C

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    2. Hello again,

      Patience is subjective, the thought of seeing you build a home from the ground up with the attention and detail you give to your projects will be quite the show. I hope you'll have the time to document much of the build, that'll be a project that will draw a lot of attention. I don't know the criteria you'll be using for your home but I'll anxiously await that project.

      In the meantime, I'll enjoy the gate from afar. (As an aside, I'll be joining the Carpentry study group very soon, 1-2 weeks, I'm completing a stand for a jointer/planer then I'll be ready).

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    3. Planning an outbuilding, not a home, for the time being at least. You're very welcome to get involved with the CSG - drop me a line when you're ready to go.

      ~C

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  4. Chris;



    Ian`t concrete fun? Used to my self. Try looking up power plants in your area for fly ash. Looking God can`t wait for the wood! Also fiberglass fibers make good strength additive,maybe not eco friendly though. Keep up the good job! Really liked the chipmunk! Katie!
    J.T.

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  5. JT,

    good to hear from you and thanks for the comment.

    ~C

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    1. "Katie"? Not sure your meaning I'm afraid.

      ~C

      Delete

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