Today I built a form to make a concrete arch bridge (mass production style). I’ll post photos later this week of the method I used, which allows the user to vary the length and height of the form in a matter of seconds and doesn’t require any screwing.
Anyway, tomorrow, I wanted to pour the concrete in the form (it’s too cold and dark right now). However, the temperatures have been in the low 30s and up in 40s in the afternoon. I happened to be building a concrete table and after 2 days, the struck the mold and the table top broke apart, aparently still wet and not cured. I don’t want this to happen to the archway tomorrow.
Is there a minimum temperature for concrete? I know that cold weather takes longer. Maybe I should have just waited a few days longer before stryking the mold?
Portland cement sets by absorbing CO2 from the air. This is a chemical reaction and as in all things -the rate of reaction is proportional to the change in temperature. As a rough guide for every 10 deg Kelvin the temperature raises the reaction rate doubles, conversely for each 10 deg Kelvin fall the reaction rate halves.
If you can see if you can obtain low temperature high alumina portland cement mix -which is what we use for northern scandinavian climes. If you cannot “warm” your concrete mix then I would suggest you leave at least FOUR times as long as you would normally. The reaction will produce “some” heating that will cure your concrete mix that way.
Alternative methods of fast cure include putting black plastic sheeting over the cast and using solar heat absorbtion to warm the mass and a few “tea light” candles with the flames on the ends of the casting bars to keep it warm through out the night -thus providing both heat and CO2(!)
<>I don’t mind waiting 4 x longer; but I do mind if the ice forming in the concrete expands and weakens the mixture. I just checked on the piece that had crumbled from the first table and they are still moist and crumbly (I’m keeping track of the failed pieces and won’t strike the mold on the second batch until the failed pieces have adequately set up) a good way to test when to strike!@
Toad,
The link was VERy helpful and seems to confirm my fears about cold. It got into theupper 20s last night and there’s thick ice everywhere I was using the hose to mix the concrete.
Lesson learned. I may have to wait a while before going forward with more concrete projects and the weather forcast is notoriously bad in my area.
Obvisously the warmer the better. You can start by mixing your concrete with hot water not cold. I have kept concrete pours very warm overnight by covering them with a blue fiberglass tarp- tented. With the center higher than the rest and then placing a light with a 100 watt light bulb in it. This generates a lot of heat when comnfined. Be sure to coat your forms with some sort of mold release before you pour them. In construction whey use a special ‘oil’. You probably won’t want to use that. I have heard of people using WD-40 and Pam food spray. If it was me I would go with Pam as it should wash off if there is a residue after you remove the forms. Post pictures when you are finished please.
Professor Ralph would you be so kind as to re-state that reaction rate formula using Fahrenheit as the units of temperature? I looked but just cannot find a thermometer around the house that uses a Kelvin scale…[%-)]
Comming from a SI background -I forget that people still use Imperial.
For each 18 degrees Fahrenheit raise in temperature the reaction rate doubles and for each 18 degrees Fahrenheit fall in temperaure the reaction rate halves.
As I have customers who live north of the permafrost line it makes the maths a lot easier to work everything out in Kelvin!!!
Zero Kelvin is -273 deg Centigrade or -414 deg Fahrenheit.
regards
ralph
PS I have just checked with the St Paul Office and they work everything out in Kelvin too.
Here’s the base and pedestal of the bar table (for near pool table) that I was working on when the weather was warm. It’s got rebar in it and will be overlayed with tiles. The base and pedestal alone weigh about 200 pounds.
Due to the cardboard form that held the concrete, the pillar takes on the look of a giant toilet paper cardboard.
Here’s procedure for pouring the top of the table:
Notice that I wish to use the larger circumference of the top of the bucket for the table top so fill with crusher fines, then place a 45 lb weight plate (smooth side up, not side that is in picture), so the surface will be level and then plastic over the weight plate so concrete won’t adhere to weight plate. Then pour concrete (you can’t see it but chicken wire is emeshed in the cement)
when I removed this “table top” it cracked last week b/c it had not properly set. On Saturday, I poured a fresh batch and am waiting longer to see what transpires. I have some broken pieces from the first batch I’m using to judge the set time.
Arch form made with wood and vinyl siding. This arch form would be placed on its side over a thin sheet of Styrofoam and walls would be wood. (I want to create a series of arch bridges for trains to traverse. I want an old, weathered look to them, with some crumbling. Inci
As for the pillar they make a coarse stone on a handle you can buy at Home Depot and would think other stores that sands the coon creet down. Then fill in the holes if you choose to with a thin set. [2c]
Seeing how the observer is doing a great job she needs a scooby snack. [:D]
The piller and small holes will be covered with thinset then tile; I just thought it looked kinda neat. I’ve been collecting towell and toilet paper rolls incidentally, to model some building pillers. Maybe this will give some ideas to some of you.
The hound was bored to tears by all this so I took her rabbit hunting. That perked her up.
The table is starting to take shape. Tiles still need to go on the sides of the top and bottom and of course it all needs grouting. Each tile was buttered individually. The bottom and top tiles were hand cut with wet saw. The top was bolted in place in a recessed hole and thin set was used to cover the bolt and attach the top. I struck the mold for the top after a week and it was still green but held together (slow cure b/c of cold weather outside).
Here’s my mold for doing an arch bridge. Note the stones embedded at the bottom of the mold and in the last photo, they are placed in the wet cement for the other side. To prevent possible rust expansion from Rebar, I used corner bead instead, which cannot rust according to my experiments.
This particular mold uses wood forms and the arch is covered with house siding (screws taped over so they don’t stick). The house siding should allow easy release. I may strike the mold on Thanksgiving (so I can create another mold; I need about 15 of these), but won’t move the mold until the weekend as it likely isn’t cured.
The arch is designed to have a primitive and aged look of stone and concrete, not a neat and tidy new look like you will see on other model arches. The arch will be mortared to footings.
I’ll provide photos of the construction as it progresses. The arch, incidentally, took 1 bag of cement so it was $3. It would have cured faster down in Texas where weather is warmer.
This week I’m going to begin digging an open pit mine which will feature tracks spiriling down. It has never been done in a big way by any model railroader (just 1 or 2 levels is the most I’ve ever seen). I’ve also got a massive tunnel I’m emplacing with large diamter sewer pipe on a different part of the layout.
Doing stuff like this is more fun that running the trains.
Last night I struck the mold, although it was still somewhat green (lucky it didn’t break!). In 30 minutes I mixed another batch and this weekend I’ll strike another mold. Using 1 bag of 80 lb concrete plus all the rubble inside and rocks out and reinforcing metal, each arch is probably 150 lb
thanks! I do a lot of deadlifts in the gym. It is a painful hobby (weightlifting) compared with the leisures of garden railroading. Also at times rely on a dolly or homemade ramp.
Hope you got a way to stake them down, hate for BB to be crushed by one that fell over on her! Would be bad, it would crush something.
But good news its your hired for down here in Spring time.
Hoping to get another shop built ASAP but my way of building is different than these slack jokers I been speaking with. Even Morgan buildings don’t pass. I am looking for quality and it seems not many around to do a job and yes I am helping in the building to (have construction back ground).
bridges will be mortared down to concrete pilings and to each other. Right now, pretty hard to push them over (depending on terrain they are sitting on); but good point.
shoddy construction yes!
I used screws to build basement but builders used nails upstairs and nail pops are everywhere. Also, the 2 air condition units on the ground tilted and started creeping downhill; also burst pipes and so on and not tamping front concrete porch, which is good for BB b/c small animals burrow there and BB the beagle gets happy when she smells them
where weather is warmer.