I am building a garage that will be attached to the house. The slab size is 24' X 31'. The slab will have a 14" X 18" footer on all 4 sides with rebar. My question: I have received 2 bids for the job, one with rebar in the 4" slab and one with no rebar in the 4" slab. Both bids have rebar in the footer. The bid without rebar uses 3500 concrete with HP fiber. Is 3500 concrete with HP fiber equivalent to
3500 with rebar? Should the grarage floor have re-bar regardless of type concrete? There is $1,100 difference in the two bids. Both bidders have provided good references. The re-bar bid is the higher one. I don't mind paying the difference if re-bar is the better option. The one with re-bar has the re-bar 18" o.c. Any comments will be appreciated.
A 4 inch slab with 3500 psi concrete with HP fiber is good basically for cracking. Normally minimum rebar is used in 4 inch slab with either welded wire fabric or #4 bars 18 inch on center each way. I would go with 4000 psi concrete with the HP fiber additive mainly for cost. I would also break up the slab with control joints into four pieces. For the footers, where are you located? Sounds like the footing is 14" wide and 18 inch deep. Do you have a frost level for footings? Some places call for bottom of footings at 18 inches below finish grade and that is OK. If frost level is not a problem you can just use dropped edges for footing. 8 inch deep 12" wide with a 45 degree angle toward the inside from the bottom of the footing to the slab. Put (2) #4 bars continuous in the dropped edge of the slab. Hope this helps.
If the sub-grade is prepared correctly - drainage issues addressed, organic soil removed, gravel and insulation added as required, compaction - the rebar adds little if any benefit. Concrete cracks - all concrete cracks. The rebar will not prevent cracking, but it will prevent gross movement between sections on either side of the crack. If there's enough movement between sections of the slab that the rebar is called upon to do work, the slab has already failed.
The fiberglass additive is designed to limit cracking on a small scale. As Chuck mentioned, creating control joints to induce the slab to crack where you want it to crack is a good idea. I'd space the control joints every eight or ten feet both ways.
BTW, unless that rebar is epoxy coated it's a bad idea to install rebar in a 4" slab. The IRC requires 3" of concrete coverage between the soil and the rebar.
The slab will be poured against the existing house slab. Should the new slab be "attached" to the existing slab in any way? For example, drilling holes in the existing slab and inserting re-bar. Neither bidder suggested "attaching" the new slab to the existing slab.
The footer will be 14" wide and 18" deep. I am located in central Oklahoma and normally 18" is sufficient for frost. The slab will be poured as a monolithic slab. Will these footers be sufficient?
Nothing heavier than a pick-up truck and car will be in the garage.
The slab will be cut into 4 sections. Both bidders indicated the slab would be cut the same day it is poured.
Chuck, from your comments it certainly appears I should go with the bid specifing #4 rebar 18 o.c. tied and on chairs. The slab will have a 4" sand base.
Thanks RicodJour for your comments. Although I am a little confused now.
RicodJour from your comments it appears a well constructed base is sufficient. Of course, with the cuts in the slab at an appropriate distance. I think I will get a third bid.
One other question: My water well will be in the new garage. Should the slab be isolated from the well's casing? I was thinking building a form around the casing and filling the void between the casing and concrete with pea gravel might be a good solution.
A third bid on two different techniques won't necessarily indicate the better construction method. In general you want to get three bids on the same thing so you can compare apples to apples.
A highly respected, very knowledgeable and long time poster on this newsgroup is out of commission at the moment. Here is a thread where you can read what he has to say on the matter:
formatting link
Bob is one of the few guys where I'll concede that his knowledge and experience outweigh my own. Don't tell him that - I don't want his head to explode. ;)
When you're searching for answers on, well, on almost anything, do a google and click on the More link at the top of the results page, then click on Groups in the drop down menu. That enables you to search newsgroups and is a highly effective tool. Most of these questions have been asked and answered before.
Per RIco's comments.....the merits (or lack thereof) of rebar vs fiber reinforced concrete vs thicker unreinforced concrete have been discussed & debated a number of times in the last year.
My bias is towards rebar but as Rico pointed out, 3" of concrete cover to soil is kinda hard to do in a 4" slab. :)
Realistically, you've got to go to at least a 5.5 or 6" slab to consider rebar
Additionally one normally settles on a design & then bids THAT design....having two designs & one bid per design really doesn't give you much info to work with.
I would suggest that you settle on a design and THEN bid it.
Here are some things (& numbers) to consider (my random slab on grade thoughts)
You've got about 700 lbs of rebar.
You could "spend" the rebar $'s & go with a thicker (adding 2" of concrete is about 4.5 yds in your pour)
Mix design, placement & curing will have major impact on concrete strength & future performance.
Scoring your slab into 4 pieces is optimistic...in the 31' direction that would be 15.5", that would be pushing it a little. For your slab the MAX distance between cuts is about 12'. I would suggest cutting
31' / 3 & 24' / 2; it's always a tradeoff between more cuts & the risk of uncontrolled cracks. :(
After the concrete cures, fill the crack with an elastomeric filler, protects the edge & keeps crap out of it.
Despite my bias towards rebar.....adding thickness to the slab will probably give you a better performing slab over rebar in a thinner (thin) slab at lower cost.
IMO thickening the slab will be less than 50% of the rebar cost (SWAG)
The $1100 delta between bids is not all due to the rebar cost, there is the contractor delta as well
Suggestion:
Ask the fiber mix guy;
cost increase to go to a true 6" (also check to see if the original 4" bid was a 4" guaranteed min) tell him the increase from 4" to 6" is 4.5 yds
My biased recommendation & guessed best performing solution (& lowest cost near second choice) ......
lose the fiber, go to 6", boost cement content, reduce water, add rebar, use the fiber contractor lose the fiber, go to 6", boost cement content, reduce water, no rebar, use the fiber contractor
Thanks everyone for taking the time to respond to this post. I really appreciate it.
RicodJour, I had read a number of posts by Bob Morisson and found him to be very helpful. I also read the post about his absence and I wish him the best. The main reason I posted these questions was to understand what makes a good slab. I find it difficult to tell a concrete contractor how to do his job, I know I don't appreciate that in area of expertise. The two bids were so different in how the slab would be poured is what confused me. And neither bid comes close to what's recommended on this forum. I understand that the best way to get bids is to have a design and have that design bid. I am not capable of writing a concrete slab design. Both bidders stated sand would be used for the base. Most expert posters on this site say the best method is "a 6" compacted crushed rock" for the base, with a 5" -
6" unreinforced slab cut where no area is greater than 150 Sq. Ft.
Question: Can you further defined crushed rock? What should the largest piece be in crushed rock for a base for a slab? What size sieve should the crushed rock be able to pass?
I have taken the time to visit a number of job sites with different builders, and have not seen crushed rock used in my area. The only base I have seen is river sand. I have not seen re-bar used in any garage slab. I am sure this varies based on the different soils and drainage. Is it reasonable to assume in my area (Central Oklahoma) that sand is a sufficient base?
Where are you in central Oklahoma? I'm in NW OKC. Fill sand is quite normal as the final pad to make concrete grade. If you are in a high water table area and being water proof is an important consideration, you can put down 4-6" of #57 stone (up to 1 1/2" concrete gravel) under the slab as a capillary break with vapor barrier directly under the slab. These efforts are not normally warranted under a garage slab, particularly if you have decent drainage away from your site. If you are in an old gummy wheat field, you may want to consider raising the pad area with red select - rotten sandstone that can be well compacted (you will see this quite normally on any commercial properties being brought up). Ask abut a vapor barrier under the slab if you plan to paint, epoxy, or tile the floor.
Code will require a minimum of 2 #5 bars in an 18" deep footing. This footing can be and usually is poured monolithically with the floor. With all this rain, you may want to dig the footing, place the steel, and pour the footing quick before the next rain caves it in. You won't be digging this for while anyway. In the last 2 weeks, I've dumped well over 10" out of the rain gauge, and I'm pumping out my basement that hasn't been wet for over 10 years. More predicted through the fourth.
I agree with others, a well compacted sub grade is more important than rebar in a thin floor. Rebar is better than 6/6-10/10 mesh, some suggest that fiber is equivalent to mesh. I would never let any slab get larger than 12' in any direction without a saw joint and it is critical that those joints be cut the same day as the pour. You will help yourself a lot by curing the concrete. Pigmented cure (the reason is pigmented is so everyone can see that it was done) or clear is good - if you are planning a finish on the floor someday, be very cautious using a chemical cure as they are incompatible with almost all floor finishes. Wet burlap kept wet for 7 days is better. Build a dirt dam around the perimeter and keep it under water for 7 days is the best.
3500 # concrete with air. Slump maximum 4" ( don't let them add very much water at the truck). Wet down the subgrade just before the pour if you use fill sand. Make sure the bleed water has been absorbed before any toweling of any type is performed. Pray for an overcast day that is not too windy. The wind can cause alligator cracking (small surface cracks caused by the top drying too fast that are more cosmetic than structural).
Most any concrete man will promise two things:
The concrete will get hard. The concrete will crack. The reason for the saw joints is to make the cracks happen in a reasonably straight line.
Thanks DanG for the response, it was very helpful. I am in North OKC, Britton & Eastern area. If you are a concrete contractor I would be very interested in talking to you about the job. You are right, there will be no digging until the rain stops. The soil drains very well and I'm not in a high water table area. My place is fairly well protected from the Oklahoma Wind, lots of trees and I'm in a low area. It sounds like a well compacted sand base should work. I will take the suggestion by a previous responder and add 2" to the depth of the concrete slab (6" an additional 4.5 yds) and make sure 3 cuts are made in the 31' dirrection and 2 cuts in the 24 ft direction on the day of the poor. I will cover the slab with burlap and keep it wet for 7 days. I am not sure what a pigmented cure means, is that added to the concrete or applied after the pour?
I might mention there is an existing slab in the location that will be removed. This slab has no footings and no cuts. As a result it has several cracks. Based on the width of the cracks, no rebar was used which will make removal easier. This slab stays dry even with the rains of the last 3 months.
Curing compound ( I use Conspec brand) is an alternative to the labor intensive curing by water. It comes in both clear and pigmented. If you are doing a wet burlap cure, you won't need it.
I do concrete work within company, not as a contractor. I can give you a few names if you would like, though my Rolodex is at work so Monday at best. Some flat work men don't want to mess with the excavation of the footing. If I were younger, I would consider doing it with you . . . .
Call Wade Chickering and tell him Dan from Putnam City told you to call. He won't be cheap, but he's one of the best. He could tell you by phone if he wants to mess with the footings. He may recommend someone if he is busy.
This depends on the local authority regarding the slab on grade. The 3 inch clear space is for a footing or slab placed in soil without form work and directly in soil. I have experienced building departments that allow the 4 inch slab with rebar at the mid depth if the slab is placed on 4inch to 6 inch gravel sub base and vapor barrier. It depends on how the building codes is interpolated.
No doubt there will be substantial variance between building departments' interpretations of the code, and the code will also vary from place to place. I'm not arguing code requirements - arguing with a building inspector is often an exercise in futility. I just don't consider code - generally the minimum acceptable construction quality
- to be the be all and end all of construction.
Rebar in thin slabs has several fatal flaws. The standard practice is far from the recommended practice - typically rebar is placed without any attempt at raising it above the bottom of the slab, then when the concrete is placed, the workers pull up on the rebar in an attempt to position it in the middle of the slab. This is not a good practice. Other times it is raised up on brick pieces - also not a good practice.
My major objection to rebar in a thin slab is that it can't do what people think it is doing when they spec it and install it. Rebar does essentially no work when place in the middle of the slab - the neutral axis. Rebar is meant to take the tension that concrete can't. That requires the rebar to be near the tension side of the slab/beam, not in the center. Placed in the center of the slab rebar only does work if the slab has already failed. In other words, rebar in a thin slab is planning on having the slab fail. That's backwards.
Slab failures can be entirely prevented by proper sub-grade and site preparation. That preparation is a better way to spend the money than on rebar.
I was always understood that when you are using 'Ultimate Strength Design" the concrete has already cracked and the rebar is yielding. Isn't this considered as concrete failure???? The plastic neutral axis is not the middle of the slab and is just be low the top of the slab ( ' a'/.85 of the compression block ) and the location of rebar 'd' is located at mid depth. You are saying that it doesn't work even though compression equals tension and the moment capacity exceeds the factored moment required. This is the same way you would design a steel composite slab considering compression block in the slab portion to take the compression and the steel beam to take the tension. Are you saying this process is incorrect?
Even so, the slab on grade rebar is minimum at mid depth primarily temperature reinforcing to minimize cracking. The slab on grade needs to be placed on a sub base of gravel with vapor barriers if required.
Yes. That's why it's called Ultimate Strength Design. If the 4" slab experiences that ultimate load, you have problems.
That's the theoretical axis based on theoretically perfect rebar placement. I can tell you that is theoretically impossible to achieve in a 4" slab.
Sigh. No, Chuck, I am not reinventing structural engineering. How thick are the composite slabs you're designing? 4"? Didn't think so. How accurately do you think rebar in a very thin slab can be? Design location +/- 25% in depth on a thin slab would not be unusual at all.
A 4" slab with rebar is an exercise in futility. It's an insurance policy the owner pays for to hopefully take care of improper sub-grade preparation. Oh, and the owner owns the insurance company. Lose-lose situation.
Minimize and prevent are not the same. You won't eliminate cracking, you can only control the location and the severity. The first by control joints, the second by sub-grade preparation. If you believe the main purpose of the rebar is to control thermal expansion, wouldn't welded wire mesh make more sense? The loads are small, so the steel section can be far smaller than a #3 bar. The WWM also has a more closely spaced grid of steel which would be another benefit. Spending the time and money on rebar installation does not make structural or financial sense on such thin slabs. WWM makes more sense than rebar, but it has its own issues and won't replace adequate preparations.
Exactly. If you're designing the slab to be a beam, use rebar. If you're designing a slab on grade, take care of the grade first.
Another question here, and thanks for the scope of the answers so far, I have saved them to a file and will reference them in the future also. Now the question.
Would one want Control Joints placed in a basement slab? I can see in a garage or driveway but when the basement slab will be the main flooring would you want control joints across your floor visible? If you are covering the floor with carpet again wouldn't a control joint be felt walking on the floor when you encounter the joint?
A slab cracking is primarily an aesthetic concern, so if the slab is covered it's not really an issue. Some people might say that a basement slab cracking is more of a concern if there's a possibility of water infiltration through the cracks, but like I said before, the drainage issues should be addressed before the concrete is placed. Perimeter drainage and a suitable compacted gravel base will take care of almost all water concerns.
If the basement slab is the finished flooring the control joints can be filled in with caulking compound. You wouldn't feel the 3/16" saw cut control joint under foot unless you were barefoot. The caulking compound just keeps the joint from filling up with dirt and growing little plants. ;)
Oh, OK. As long as you're not asking the questions for a nefarious reason! ;)
Your choice whether you want it to crack randomly or in a straight line where you decide. Saw cuts are pretty discreet, but they can be filled. Exposed decorative concrete is all the rage right now, start looking for the saw cuts. You probably just never noticed them before.
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