Prep work for a concrete slab

[mastech08]

Hello All, I been lurking for a while and learned alot from this great site. Im not sure where to post this.Looking to get started on my garage build in the near future, I am going to build it myself to save $$. I have a few questions about the slab. I am going do all the prep work myself and hire a finisher to handle the pour.
The slab is going to be a monolithic type, I had the ground leveled out about a year ago so that it would have time to settle. I plan on putting down 3-4 inch of gravel and compact it, 8 mil plastic, wire mesh, and rebar around the edge of the slab, i will be installing a 2 post lift and will dig deeper under the post locations aprox 2-4 inch deeper than the rest of the slab. thickenss will be 4-5 in. slab, will use 4000 psi havent made up my mind on fiber mesh.

What are your thoughts? is the fiber mesh worth the extra $ and will it look as good finished then with out fiber mesh. I want it really smooth (slick)

If i go with fiber mesh would the wire mesh over kill??

 

[ForceFed70]

Fiber mesh and wire mesh is definately overkill. My concrete supplier only wanted $6/yard to add the fiber mesh, so you might do it anyway as it's cheap. You won't see the fiber when the floor has been finished.

Make sure your rebar goes though the entire slab and not just around the edges/footings.

 

[mastech08]

are you saying i should run rebar all the way across the slab in a square pattern? would that be over kill with the wire mesh and fiber? Thanks for you reply

 

[ForceFed70]

are you saying i should run rebar all the way across the slab in a square pattern? would that be over kill with the wire mesh and fiber? Thanks for you reply

I plan to run rebar through my whole slab with a 24" OC grid pattern. Most people do the same, especially if you are planning things like a lift.

All 3 would certainly be overkill. You should need to use either wire mesh or rebar, not both. Since the fiber mesh is cheap I would reccomend you go with rebar and fiber mesh but even then the fiber mesh is overkill.

 

[D.J.]

The fiber mesh has what looks like little hairs in the finish of the top coat of the cream if you want it really slick the fiber is not the way to go in my opinion. Just my $ .O2

 

[mastech08]

Does anyone have any pic of there finshed slab w/fiber mesh with a smooth finish. close up pics would be great

 

[mobetta]

fiber shows up on 99% of slabs. sometimes it can be really ugly.

if you want a polished finish, make sure you tell whoever you hire and make sure they have a power trowel. then use perimeter rebar and wire mesh, no fibers.

also an increase in the portland content will make a stronger slab.

 

[mastech08]

By increaseing the Portland mix do to me the psi, 4000-4500psi ?

 

[Northern Ninja]

The only thing fiber mesh(non structural) will help with is shrinkage cracking. If you plan on power troweling, fiber mesh is kind of a pain in the ***.

 

[rockchucker]

100% ^


The Fiber Mesh makes it tough for a really nice, clean, smooth Slab. If you over do it with the Rebar and Steel Grid you will not have any issues not using the Fiber. I still have Fiber coming up through my Garage Pad. They used a lot. The 22' x 22' Pad with out relief cuts cracked. Relief Cuts **** in a Shop though so I would just deal with the Slab cracking a little. If you wait a year after the pour and let it crack then you can fill the Crack, smooth it out and then Epoxy over it. Chances are that it will still show the crack but the Epoxy helps.


Make sure the guy running the Power Trowler gets on it and knows what the hell he is doing or you can end up with a real mess on your hands. This is where it pays to pay a little extra for a good Contractor with a good crew. Depending on how big the Pad is on the day of the pour buy them lunch and have it ready whenever they are ready to eat. Don't stand around and watch and have them supplied with Beverages of their choice if it is hot out. Sure it costs you 50-60 bucks out of pocket for Pizza and Soda the day of the pour but it will translate into you getting a better finished product.


Just my .o2


2 things you can always count on with Concrete. When it's dry or dark out, it's done. It will crack. =)

 

[KermitFrog]

I made the mistake of not doing plastic when I poured my slab. I thought I had someone who had a lifetime of knowledge that I payed good to help. It has been 3 years and I have a serious efflorescence problem. (hope I spelled that right)

 

[mastech08]

Thanks alot guys!! I am glad that i posted this, got some good info. looks like i will go with out the fibermesh b/c i want a smooth (slick) finish and use the extra $ and put more steel in it. may go alittle thicker 5-6 inch.

 

[mastech08]

I will also start my own build thread w/pictures when i start the build

 

[Red05GT]

I would thicken the slab more than 2-4 extra inches where the two post lift is to sit.
Make sure you are down to undisturbed soil. Are you going to epoxy finish the floor?
We have used fiber on our slabs for years and have had great results. But we will use
fiber, mesh and rebar in high traffic areas such as a business parking lot if subjected
to heavy truck traffic.

 

[mastech08]

Yes I plan on epoxy the floor later on down the road.
Some pictures would be great, of a finished slab with the mesh

 

[BLUBAYOU]

can't the exposed mesh fibers be burned off? thought I read that somewhere

 

[irishtom]

mastech,
What will be in the garage besides the lift? How heavy are your vehicles? The slab should be min. 4" thick, 5" for any light industrial use. Subgrade thickness should be a minimum 4", compacted.
At the lift posts the concrete should be thicker, not just the subgrade. And the thickness and area of the thickened slab depends on the rated load of your lift. If it were my garage, I'd pour a 12" slab under the posts, 24"x 24"
Are you planning on heating the garage? If so, nows the time to think about some insulation under the slab also.

 

[rockchucker]

Agreed on the 24" x 24" x 12" under the Lift Posts. ^


Insulation would help or even Hydronic Heated Floors would be very cool.

 

[mastech08]

Irishtom
The biggest vehicle that would be in there is a f350, maybe a tractor from time to time. I will have 4 inch of compacted gravel.
I will def dig deeper under the post.
No heat I the floor

 

[darkk]

can't the exposed mesh fibers be burned off? thought I read that somewhere

Yes the (fiber mesh) hairs can be burned off. Get one of those Propane Ice melting rigs and just turn the flame/heat on a lower setting. It will burn the hairs off in a flash (no pun intended). The hairs are only to help prevent the surface from cracking while the cement is curing. Keeping the surface very very wet (run a sprinkler for the first 7 days) also helps prevent surface cracking while curing.

 

[Novicaine]

Some things that may or may not be relevent to you. I'm no expert, so take with grain of salt.

- If you ever think you'll want a loft or 2nd floor, think where you might want some columns or support posts or walls and dig some postholes down to solid ground/frost line for footings and fill with concrete.

- If doing a bathroom, run pex to the toilet for the water supply line. I forgot that. So did one other guy I know.

- Instead of putting in trowelled or cut seams in the concrete, put down that metal drywall corners where you want cracks to form. The concrete will be thinner there and so that's where cracks will form and you won't need the cuts. And you can put extra protection around things like pex lines where the joints will be, since you're picking where they will be prior to the pour.

- Use rebar or mesh supported off the ground before the pour. I had mesh laying on the ground and the concrete is basically poured on top of it... it'd be nicer/stronger in the middle. Just more work to get ready.

- run a small "trace" wire around any areas you need to mark off, with a wire going up out of concrete near the wall... for me, the area where I didn't put pex for the lift columns. Then I could locate it easier.

- If you're going to epoxy the floor, you don't need or want a smooth finish. Flat, yes, smooth, no.

- Run conduit for wires in places that could possibly need them someday.

That's all I can think of for now.

 

[rodm1]

grate information!

 

[Racecarl]

I did all the prep work for my 24X 30. I packed the subgrade with a plate ********. I then added gravel in 1" lifts, and packed it as I went. I have a good 5"-6" base of gravel. Since I am heating my floor, I have 2" 250 extruded polystyrene insulation surrounding the perimeter vertically (inside the foundation walls) and the first four feet inside the wall. I then laid down a 10 mil reinforced plastic vapor barrier. Every four feet is a 1/2" rebar with a 3/8" rebar between them. The rebars are held up with pieces of brick so they are in the center of the 6" slab. Tied to the rebar is 6X6 mesh and tied to the mesh is the PEX tubing for the floor heat. I put in 7 floor pulls and the rebar welded to these are tied into the rebar in the floor.

The foundation insulation was left 1.5" below floor level. On top of the wall insulation I installed 1.5 X 2" pressure treated board to act as a thermal block and protect the foam. This also gave the floor pouring crew a dead level surface to screed from, and they got the floor dead level and slick, just how I wanted it.

I thought about using fibermesh in the floor, but I decided against it after seeing it in the floor in the shop where I work. The fibers were clumped together and came to the surface. The crew that finished my employers floor was NOT the same crew I hired, but I didn't want to take a chance on the fibers making a mess. With all the steel and rebar in my slab, on top of a well-compacted base, I have no cracks anywhere and do not expect any.

 

[mastech08]

sounds good carl did you take any pictures of the work before or after the concrete was poured? no cracks that is great, how old is the slab?

 

[Cobra6]

On my 24' x 36' slab, it was a monolithic pour with rebar and wire mesh. I dug 2' footers with rebar - (see pic).

the slab is 5" - 6" thick and a couple of inches thicker on the bay where the lift will go - 4000 lb rated concrete with mesh and rebar. Got a crew to come in to pour and finish it with a concrete finisher - I am glad I did - this is a large area to get finished and level by yourself.

I also roughed in water, toilet, shower, sink, electrical. etc.

 

[Racecarl]

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The drawing shows how things were installed, and the picture shows how things worked out in 'real life'. At the top right corner of the picture you can see the wall insulation and the board used to protect the insulation.

The floor anchors are made from John Deere planter tool bar. The shaft in the square tubing is 1.25" and runs completely through the square tube. The round disk on the bottom is an opener disk from a John Deere planter.

BTW, I took these pictures before I got all the PEX laid out.

 

[Frank The Plumber]

You would use rebar or mesh in any case, the fibre crete is used when you increase the bag mix to achieve a greater load rating, the slab cures faster causing stress fractures, the fibre crete helps to slow that. In most cases the fibre crete is preferred for thick slab applications 6" or more, the thicker slabs are prone to more curing stress, most home center stores have a fibre in them, it can be polished out nice.

 

[chill430]

how do you determine the deminsion for the plumbing; toilet, sink. i was thinking about pouring a slab but I want a sink and bathroom.

 

[readhead]

Do a little searching. There are standard rough in dim's for everything. Will the fixtures be on an inside wall or an outside wall? Outside is easy. For inside you will need to place a string between the forms to show where the wall will be and rough to the string.