Heat transfer in concrete

[rotocor]

I need to know how much heat in BTUs I will require and how long it will take to raise the temperature of a concrete core or cylinder 18" in diameter and 12 inches high to a depth of 1" all around, from 32ºF to 50ºF and the same for the corresponding concrete hole of similar size. We plan to use hot air as a heat source at a temperature between 150º-200ºF and the optimum time would be 30 - 45 minutes. The application is required to heat the concrete so as to allow cementatious grout used to rebond the cylinder back into the concrete substrate to set up and gain strength (20-30 minutes) before surface frost returns under winter conditions. Thanks

 

[chicopee]

Before I do any calculation, I want to make sure that I understand the problem. You have a 18" long hollow concrete cylinder with an 18" O.D. and a 1" I.D. which is to be heated on the outside and on the inside to a depth of 1" on both sides. The concrete cylinder is initially at 32 deg. F and you want the temperature to be at 50 deg.F at a depth of 1" on both sides.
First question: is the heated air recirculated such as in an oven whereby the temperature under some velocity is somewhat constant let's at an average temp of 175 deg.F? Would the cylinder be exposed to such recirculation or would the air speed be created by a hot air blower whereby the heated air is blowing longitudinally? Is the concrete core already cure when the cylinder is to be heated?

 

[MortenA]

If this is not a homework question what will you do if theres an error in any answer provided? If its a critical calculation and you cant do it yourself: Get a consultant!

These forums are not well suited for "free consultancy services" because: How will you document your work? If documentation and accuracy is not require - why worry?

Best regards

Morten

 

[chicopee]

I wonder if you are right MortenA!!!By now I would have expected a reply from Rotocor.

 

[rotocor]

No guys this is not a homework question. I have been out of town without my computer or the last week. The application relates to a process used by utilities to access their underground plant. It invloves coring an 18" diameter hole through pavement and removing the cylindrical core. After the repair has been completed the core is reiserted back into the road and bonded to the remaining slab of pavement with a water soluble cementitious bonding agent. Everything works fine in warmer weather but when the temperature drops below freezing we either have to wait for spring or dig a massive trench with a backhoe -- a less safe and not very environemtally favourable alternative. We want to know if we can develop a portable heater that will drive the frost or cold back from the surface of the concrete in the hole and in the core and raise the surface temperature to a depth of approx. 1" of each to say 50ºF so that the fast setting bonding compound will have time to set (15 minutes) before the freezing temperature returns. We're working on the heater but we need to know how much heat we will need and for how long we will need to apply it to the surfaces to achieve this result. That's it in a nutshell.

 

[IRstuff]

Why not preheat the core and surroundings separately to, say, 70ºF, prior to re-insertion? Then, with the top insulated with some styrofoam or the like, the retained heat from within the core and the surrounding pavement should keep the interface warm for the duration of the cure time.

I think that makes more sense, since it allows you to ensure that the entire interface is controllably heated. You can still use the hot air blowers to do that.

TTFN

 

[abeltio]

so this is, after all, a real practical problem...

i do not know if i understood your description correctly...
i am not a theory guy... inclined to field fixes...
but if the following is correct:

____V__ 18in dia _______
| |
| |
| | 12in high
| |
-------------

pit in the concrete and a cylinder that fits in this hole...

i think the fastest way to heat hole and plug is with an immersion heater... in any case the grouting is water soluble.


fill the hole with water.
put immersion heater in.
use contact TC's to measure temp at the top (V) one inch away from the edge to detect the temp u need - connected to a thermostat that will control the immersion heater.
have a pit pump handy to empty the hole.
(in case you leave the water in the hole overnight... you will need to melt it with a flame, not the immersion heater)

build a bucket that will accomodate the plug, with some spikes at the bottom so the plug does not sit at the bottom
i would say that the ID of the bucket should be 30 in to have some water volume.

set the plug in the bucket
start the immersion heater
measure with contact tc's the temp 1" in away from the edge
the tc's should be connected to a thermostat in the immersion heater to keep the temperature at the desired level for as long as required.

add salt, pepper and serve with sprinkled parsley (it does sound like a food recipe, doesn't it?)


water has much better heat transmittion and retention characteristics than air.
to heat the pit, the air will be bouncing off the walls and bottom and the efficiency of the process cannot be very high. also the air will flow around the cylinder and most of the heat will dissipate into thin (and very cold) air.

as water retains heat very well, if there is a delay in the process it will keep the temperature right until the crew is ready to proceed. and the thermostats will help keep the electricity bill low...
i.e. remove the water from the hole and the cylinder from the water only when ready...

you will need all the typical electrical protections required per the applicable codes and standards.

the power required to heat up the volume of water in 45 min is approximately 1900 watt, i would say that a 2.5 KW immersion heater is quite enough for the pit

the bucket will need more power (more water) a 5 KW immersion heater should be plenty.

may be there are other considerations that will preclude using water as the heating medium... but if not i think this could be a good field solution...

what do you think?

suerte.








saludos.
a.

 

[rotocor]

Yes, IRstuff we plan to heat the hole and the core in advance to 70 degrees if possible. Thanks abeltio, we know that fluid heating is best but it is impractical for this application. We intend to use hot air through a portable propane driven blower that can generate as much as 85,000 BTUs. The framework that covers the hole is ducted to direct the hot air into the hole. The core which sits on top of the framework is surrounded by a metal hood and the hot air is expected to rise up from the hole to encompass the core and then exit through a vent in the top. We control the velocity of the air through a baffle that also compensates for backpressure. We are at the practical build it and try it out stage but we wanted to know if it would work in theory and thus our inquiry as to the thermodynamics of heat transfer in concrete.

 

[chicopee]

Specs used on concrete w/10%moisture are: 140lbs/cu ft , .156btu/lbm-deg F, 0.7btu/hr-sq ft-deg F

Temperature distribution at .1" increment and after one hour of heating the hole at 250 degF and 1000 degF. are:

250,151,101.2,67.7,48.5,38.8,34.5,32.8,32.3,32.0,32
1000,845.6,561.4,339.2,190.7,105.11,62.1,43.2,35.7,33.1,32.3

Above results shows concrete to be a good insulator even w/ 10% moisture content.
Heating the concrete w/ a torch will probably spall your concrete surface.