hey all. I am trying to cast in place a concrete surround for a zero clearance firebox. The architect specified 3″ thick vertical concrete with a slightly larger cap. Has anybody had any experience with this? Cracking? Mix ratios, etc.?
Mal
Edited 2/15/2003 2:47:30 PM ET by mal
Replies
do you have to do cast-in-place ?
can't you build forms on the floor in front of the fireplace and then do a tilt-up ?
That would be the simpler way, no doubt. We are talking about 14' horizontally, stepping up from 2' to 5' to 8' high going left to right. I've pretty much figured out how to form and pour, I'm mostly worried about mix consistency and cracking later on. Any experience?
we have cast concrete counter tops... i like to use plastic trim for the forms if it's cast in place... small rebar... some wire... we used a ton of the metal eggcrate pallet rack shelf cut up... just because we had it... depending on the finish you want... you can use a lime mix with fine pool sand for the outer layer ...looks like limestone we have cast marbles... brass machine shavings... computer chips... pressed in leafs... got a cat to walk across the surface.... if you have time it's fun to work with and cheap as far as materials go
Can you be more specific about your mix? Do you think it will hold up in a vertical situation? 3" thick? It sounds like maybe you are parging with the limestone mix after the main slab is cast. The owner is worried about cracking, etc. After all it is concrete.
Did the arkytekt figure the weight of the final product? Work out the volume of the pour and figure the 'crete at 145 lbs per cubic foot. A lightweight mix runs about 108 lbs a foot.
I'd recommend a 7-sack mix.
Don't get too sloppy with too much water. Use a plasticizer in the mix, that'll allow it to flow much better while using less water. All that contributes to a stronger product and fewer drying cracks. Drying cracks are superficial, not structural. Even if you did get them they can be ground out.
Reinforcement? Rebar cage? Hardware cloth? Fibers? Which one(s) and how much depends on the design. That'll take care of the structural cracking.
If this is going vertical they'll be a fair amount of blow-out pressure at the bottom of the forms.
What are you building the forms out of? What kind of texture or surface finish are you looking for on the final product? Do you want a dead flat matte color or one wth a bit of depth to it?
Color? Integral pigment or dye, or post-cast acid stain?
Thanks Mongo for your detailed response. I figure about 24 cu. ft. I'm planning on using 3/4" MDO for the forms with 2"x4" wailers since I can only brace one side. 3/8" rebar horizontal and vertical @ 1' centers. 2 ea. #4 horizontally @ the bottom to create a "beam" . I checked w/ structural engineer to ascertain integrity of framing below. I will use pea gravel to facillitate flow of material in forms.
Client would like finish to have some depth. We are talking about lamp black in the intitial pour, then possibly acid stain after depending on the results.
I appreciate any further comments.
ooooh....words i hate to hear ..."one sided forms"..
can you anchor something behind the framing and bring tie wire thru the forms with a loop so you can make a spanish windlass at strategic locations ?
or run vertical whalers right up the face and brace to the floor ?
as big as you are describing.. scratch my tilt-up idea
Mike Smith Rhode Island : Design / Build / Repair / Restore
Yes I plan to use whalers, and yes I can anchor to floor. Wall behind is 3/8" plywood (welcome to California) w/ 1/2" sheetrock. The I've applied 15# felt as a water barrier while pouring and a bituthene membrane on the floor.
mal... i'm back to casting it on the floor in front of the fireplace.. cast some feet into it ( the bearing points ), put in your steel and your anchoring system.... get the mix from mongo for countertops.. or read up on precast countertops..
let it cure for 30 days before you tilt it upMike Smith Rhode Island : Design / Build / Repair / Restore
Now I'm really curious why nobody wants me to pour it standing up. Obviously securing the form is the critical factor. Do you see some problem with the curing process?
i can't envision you getting a good profile of those long narrow columns , getting good compaction and finish..
how deep , front to back, do you think this will be ? can you get reinforcemnt in place and still get good flow down into those legs ?
also.. if you do tilt-up , the top will be the back.. where it won't show.. you can cast detail into the face and the ultimate top..
if you do cast in place.. you will have to get your material all the way down both legs with no voids.. and the top will be a screed finish..
what am i missing ?Mike Smith Rhode Island : Design / Build / Repair / Restore
Good questions all. The slab is 3" thick. I am setting 3" SDS screws into the studs to secure the 3/8" rebar mat. (Which gives me good attachment to the wall). The whole slab is about 14' wide. 4'wx2'h to the left of the firebox, 5'wx5'h inc. firebox and 4'wx8'h to the right of the firebox. Obviously getting the material into the form with no voids is the trick. I plan on using some form of vibrator on the exterior of the 3/4" form panels to get out all air bubbles and settle the concrete. Disaster is certainly a possibility in which case there would have to be some application to the surface afterwards. It's certainly not what I would design on my own house, but the artichoke and the client both seem to want it so I feel I have to give it my best effort.
Mal,"I plan on using some form of vibrator on the exterior of the 3/4" form panels to get out all air bubbles and settle the concrete."Vibrating the formwork will pull the water and fines to the face of the concrete and give you a bad finish -- sandy, with water tracks.Far better to use a 'pencil' vibrator within the concrete or better yet, consider Mike's suggestion of pre-casting the whole thing.If you do decide to cast in-situ, make sure you fix the formwork down securely -- I can foresee that 8' form lifting as you pour -- and use a good release agent on the formwork, it'll give a better surface finish to the concrete. IanDG
" I checked w/ structural engineer to ascertain integrity of framing below."
Framing ? This wall slab is going to rest on a full masonry base, right ? I have a feeling the answer to this is 'No'. If it is no, then I would rethink the whole plan. If this is going on a wood floor system with an engineer's approval, see if he will guarantee a crack-free wall. Doubtful. This would be a good reason to consider precasting and setting slabs. There is a definite benefit to having joints in this wall whether you do it monolithic or in sections. If crack control is a major issue with you and your client, I think that control joints should be more of a concern than your mix ratio.
Ian had a good suggestion when he recommended running a pencil vibrator in the forms. Your problem here is that it won't fit. A pencil vibrator is about 1 to 1 1/2 " thick and you have a 3" thick wall with rebar in the center. You may need to thicken your wall. The pencil vibrator is the best way to ensure a quality, honeycomb-free finish. By the way, how high is your ceiling ? I see you have an 8' high pour, do you have a lot of space above to fill the forms and drop the vibrator in ?
carpenter in transition
I am going to see if I can convince the owner to allow for control joints. Certainly that will give it a better chance of minimizing visible cracking. I'm also toying witht the idea of duct taping a piece of rebar to the pencil vibrator to create a "vibrating rod". I am also worried about mix separating during placement. I am concerned about the wood movement underneath. Therefore the two #4 bars at the bottom to create a concrete beam. It is at the exterior wall of the house so load is delivered to the foundation. I am going to make a test form just to see how the concrete flows into the form and figure water ratio.
I can't see any reason to cast it inplace... all you are doing is "make'n stones" or slabs... maybe on the second or third try... you'll cast it then place it...
Client does not want seams or grooves. At three inches thick the concrete weighs in the neighborhood of 37 lbs./ sq. ft. One porton is 8' high by 4' wide. I worry more about it cracking from the stress of getting it into vertical position. I'm not worried about forming it . I'm more worried about the proper mix and getting it in the form as efficiently as possible. So mix and agitation methods are of prime concern. Thanks for your continued dialogue.
Can you give me more specifics on limestone slurry mix and method of application?