Posted by Billy NY on December 10, 2012 at 10:52:17 from (24.29.79.122):
In Reply to: Concrete in December posted by G1050 on December 10, 2012 at 06:43:22:
I could have gone into this a bit further earlier but time was limited.
My experience with concrete is based on high rise and commercial building construction, I've performed the work myself as well as being supervisor/project manager, overseeing contractors perform the work to adhere to the old 03300 CSI specifications, including admixtures, placing concrete in cold and hot weather. I've done footings, column footings, beams, strap beams, columns, decks and flat work from floors to sidewalks. I've had to perform and supervise this work under stringent controlled inspections required by city of new york dept of buildings, that must have technical responsibility reports (TR-1's) signed off by a licensed professional engineer holding a NY license, per NYS dept of education or whatever you call it. Inspection firm, has an inspector at the plant, the day of the pour, places a field technician on the job to take test cylinders, review reinforcing/shop drawing, design drawings, inspecting work being done against same and 03300 or similar specifications as well and any ACI (american concrete institute criteria) Cylinders must prove the design compressive strength, tests are done at 7 days, 14 days, 21 days and 28 days. The work and the above must be approved and signed off on or you will not get a certificate of occupancy, and this is just one inspection, there will be a stack of these TR-1's at the end. A little background for you.
Now, on properly compacted undisturbed ground with a suitable compacted, 95% or better sub base, (building pad) of material that will compact per above with optimum moisture, placed and compacted in 12" lifts, each lift can be tested with a troxler nuclear density meter and or 3 point or modified proctor test, to insure each 12" layer is properly compacted from undisturbed, this is part of the TR-1's required in NYC FYI, they are very serious about these things. OK, so now you have a properly compacted pad, of a suitable well drained material that has the density required given the compaction proven to support the load. YOur design may have footings, knee walls, pad in the middle, depends on what you are building, I'll use just a slab for example and this discussion, same rules apply to the other structural components.
The undisturbed, sub base (pad) is unfrozen and you have ideal, above freezing conditions, it can be in high 30's and work. You can form and place the concrete in these conditions. YOu may want at least a 4000 PSI (compressive strength) or even 5000 PSI if you can afford and your building will have heavier point loads on it, ( of course reinforcing, metal deformed bars/rebar will need to be installed per a design to meet a strength, one that will exceed the highest point load imposed, say by a dump truck wheels crawler or heavy ag equipment, shop equipment etc. etc.) Cost will likely be an issue, 4000 PSI may suffice or even be more than you need, but for flat work, colder temps, I think a stronger design, with sufficient rebar, not 6"x6" woven wire mesh, (W.W.M.) is better in colder temperatures, just opinion, things vary, so do contractor methods and means.
The caveat is that in colder temperatures, you want that pour started and placed as early in the day as you can get it, DO NOT place in the afternoon, you SHOULD place it early, before or at sunrise. Admixtures for cold temperatures may change this, but this is my methodology for placing concrete in the winter or in colder temperatures. The temperatures will be on the rise, and depending on conditions you may be able to achieve enough hydration, cure or what you call in in your neighborhood to get the power trowel on it. The caveat is that you may not be able to get onto it to do the finish as soon as warmer temperatures, and there will be a window that will be ideal, one that I prefer not to be midnight or 3 am LOL ! An experienced contractor who knows this work in all conditions is a wise choice to consult with or have do the work. Its possible to also get the advice to wait until spring or warmer temperatures, things vary widely.
You do not want the concrete to freeze, it will spall like JD Seller desrcribes, it will also do the same in high heat, whereas it hydrates too rapidly unless burlap and sprinklers are used for the several days its vulnerable. The judgment call in deciding on to proceed or wait will depend on weather, again you have admixtures and similar, but straight up concrete, its almost like doing hay, plan ahead as best you can. I don't mind it in the mid 30's, but it has to be insulated well if its going to get colder, in the following days, if its close to 40 one day, you pour, then it drops to zero, blankets, straw sawdust, and whatever, you do take a chance, better to remain close to freezing, below certain temperatures, you have to ask yourself or supplier, will the admixture perform, can I get a finish on it in ample time, place the protection over it to prevent freezing without damaging the finish, and will it freeze, one mistake could cost you, contractors in your area will know the limitations, there is no better person or company paid than one that knows this work, and has a proven record of successful installations in the various conditions.
Planning is key, if you want to proceed, and your building design is where you have a footing and load bearing components for the structure, you want to keep going in cold temperatures, then build or design to build where you can enclose, provide temporary heat to protect the slab, as said below, thats how most buildings under construction are planned, get the core and shell + roof established and work inside during the winter, some are so large the construction may span more than one winter and they have no choice but to work in winter conditions.
To insure compressive strength, you order a certain PSI concrete, say 4000 PSI. Get yourself empty test cylinders, you can hire a testing outfit, but if you want to save some money, get the cylinders, 4 will suffice, on large pours every so many cubic yards cylinders are taken, at intervals to represent a composite sample of all the concrete placed.
When the concrete truck arrives, at some point during the pour, fill these cylinders, 1/3rd at a time, then take a 1/2" dia. steel rod and pack each 3rd, 25X, so fill the first 3rd, rod 25x repeat for the next 2/3rds, then evenly screed off the top of the cylinder, place where its safe, prevent freezing, rain or what have you, you can then take these to a testing lab and ahve them broken 7,14,21,28 days or all at 28 days, small job. I think on any reasonable size job its prudent, for yourself or when a contractor is involved, will keep them on their toes, knowing the material is being tested. Something does go wrong you can rule out the material if it meets compressive strength.
I've done flat work (slabs, decks and similar) in colder temperatures, it can be done but there are rules to follow, it can be risky so you have to be prepared for the site conditions/weather etc.
Again, speak with an experienced contractor or person who knows this work, in your area, should be good advice.
I hate seeing all that work and money go to waste or have the material fail, what you do up front is key to it. ALso do not forget any future use items, all stub ups, pipe, electrical conduit, drainage, anything that needs to penetrate the slab, even an extra conduit or pipe you may want for something later, so much harder once the concrete is placed.
Hopefully some help, there are lots of opinions on the use of this material as well as various experiences, better to research it a bit and decide based on good information than to guess.
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