concrete and temperature question

[Anonymous]

Due to the cardboard form that held the concrete, the pillar takes on the look of a giant toilet paper cardboard.



observer

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.

Seeing how the observer is doing a great job she needs a scooby snack.

Toady

[FJ and G]

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. Incidentally, I read that in real bridge design, designers coat the rebar with zinc or epoxy or galvanize it sometimes to avoid rust expansion).



Experiment putting slate and thin rocks over concrete. Plan would be to make the concrete arch, coat with thinset, then overlay these thin stones. Sounds arduous but sort of fun. Incidentally, this piece of experimental concrete came from one of the concrete bags I'd purchased, apparently, the bag had gotten wet and part of the concrete solidified. I didn't feel like returning the bag, however, as it would have cost more in time and money returning it than what I paid ($3).



OR, I may do this procedure. Experiment putting aggregates on Styrofoam and pouring concrete over that. Considering using this for the arch bridge wall but worried water may get between aggregates, freeze and expand.



observer


(one other problem I have in my garden is that we didn’t get pears b/c of the killer snowstorm in mid-April. Now, due to the heat wave this year, the tree is full of pears that should be ripe in another month. Perhaps I need to prune?).

So many problems to contend with!

[cabbage]

Sorry...

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.

[gbbari]

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...

 Al

[rpc7271]

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.

[FJ and G]

Ralph,

<>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. 

<>

[cabbage]

(The Boring Science bit)...

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(!)

regards

ralph

[Anonymous]

You have a electric blanket you can raise temp. over the whole.

Here some info I found intersting.

http://www.precast.org/publications/mc/2005_septoct/techspeak.htm

Toad

[FJ and G]

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?

 

thanks