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I’m building a new detached garage and will be replacing the driveway (currently asphalt), a front walk way, concrete stairs/entranceway,and putting down a slab for a 2 car garage (26 x 26), as well as removing the old 12 x 22 slab, walk, stairs and drive. All of the estimates recommend different techniques and I’m unsure which is best. One estimate uses a 7 bag mix with 5″ concrete thickness reinforced with fiber mesh for all work, except 7″ depth for the approach and includes curing and sealing. The slab does not have a perimeter. Price – $6,600. Seemed to pay more attention to grading and base concerns than the 2nd estimate.
The 2nd estimate uses a 6 1/2 bag mix with 4″ thickness throughout except a 7″ thick approach. The slab includes a 12″ thick perimeter reinforced with rebar. The drive is also reinforced with rebar, while the slab is reinforced with wire mesh. Price – $7,900.
Any input would be greatly appreciated.
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Replies
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Michelle, what part of the world are you living in. Is this a residential drive only?
Maybe you should decide which specifications you are interested in and ask each contractor to re-bid using the same specifications. They probably could do that in a few minutes at home.
Choose the items from each bid that you are interested in. Specify the thicknesses, re-bar, mesh, base etc.
One or the other might refuse to re-bid using a set of specs that they might not feel comfortable with. Don't hold that against them. They need to do whatever they feel comfortable with to deliver a product that they can warrant.
Go to one of their current jobsites and watch the crew work.
Check some references.
blue
*Blue, I may be mistaken but I think Michelle is asking us what the difference (from her standpoint) between the two methods is and which she should be considering. I am not experienced enough in concrete to render an opinion, however I do know that geographical location is a big factor in the determination of pouring thickness, ground prep., and reinforcing methods. (soil conditions, climate, etc. Give us a little bit more detail Michelle and I bet there are some experts in here who can help you out.
*Mark, you're thinking exactly what I was thinking. I don't claim to know squat about concrete either, but I do know that more info was needed to make an intelligent discussion. I was kinda priming the info pump, and going to let the concrete guys tell her.blue
*The 12" thick perimeter you speak of is called the "beam" in concrete work. Not only should there be a beam along the outer perimeter of the garage, but there should be at least one in the middle too. 12" thick doesn't mean much if the frost line is below that. Code here is MINIMUM 12" on new work. The beam has a purpose. It will support the weight of the garage structure (outer walls - the ones holding up the roof). Get each of the contractors to explain WHY they are detailing the job the way they are. Also, go here to get some info about concrete work that you may not know. At least it will give you enough information to be able to ask some detailed questions of your contractors.Concrete InfoJames DuHamel
*Fiber mesh without any steel reinforcing will possibly result in cracking.Need more info on your location to evaluate this properly.Do not specify "bag mix", but rather your psi strength, slump and airentraining spec. Find yourself an engineer for a small fee to help you with this. Their advice is independent from the "installer".Their was no mention of how the base and sub base was to be compacted. With out a spec on that you will potentially have big problems.There was also no mention of location of control, isolation or construction joints? Without that you also may be inviting a project with problems down the road.
*you want at least 3000 psi concrete. The beam (footing) around the perimeter will vary in size depending on your locale and ground conditions. And, concrete will crack, regardless of rebar. it's a shrinkage thing. If you pour a slab / wall and don't get one, the gods were smiling on you that day!As others have said - do you have a bit more info??
*nothing less than 3,500 psi for exterior slabs and driveways. the cost difference should be minimal.
*I live in Madison, Wisconsin. The driveway (95' X 9') and slab (26' x 26') are for residential light traffic use. All of the contractors had logical explanations for their process and materials. What I read disagrees with the information I'm getting from the contractors. The books say that even for a detached garage, the foundation should be on footings below the frost line (48 " here). However, none of the contractors went with that recommendation and when I contacted the building inspector's office, they also said that a floating slab for a detached garage met code and was reasonable if it was well graded and a properly prepared gravel base was laid. Some of the sites say that rebar and wire mesh is the way to go. Others say that residential drives don't really need reinforcement. If I'm understanding what I'm reading correctly, fiber mesh seems to add strengthening when the concrete is laid, but does not add any actual reinforcement. All of this leaves me unsure as to what is needed and what I should go with, so any additional information would be great. Thanks much.
*There is two purpose of reinforcement. Concrete will shrink during the curing process, during this time cracks will appear on top coat. These cracks are not structural defect, just ugly. You control these cracks with wire mesh and fiber mesh, not rebar. Most of the time the wire mesh will be walk to the bottom and does not help in that area. I am a firm believer in fiber mesh due to saving in labor, material and it really does a excellent job in controlling hair cracks.Rebar is designed for the push and pulls of structual loads being applied to the slab as in dead load weights(bldg and static equipment) and live load( vehicle, equipment vibration, people walking). A concrete slab can take alot of load in compression but mot in tension. for every action there a reaction. So as a load applied from top the bottom of slab and footing are being curled on ends by weigth. This is where rebar comes in. It holds the ends together in tension. so the actions are balance. The beams mention is the footing. Around here the footingis below frost line( here 18 inches) minimum two #4 bars in footing, wire mesh or fiber in slab, 4 inch thickness, 3000 psi which is a 5 1/2 bag mix. 4 inches will hold up the world. Here in town a guy does driveways 100 ft x 10 ft 3000 psi $1500. that material,labor, turn key job.
*My vote would be for #1, but he must use a 12" thick reinforced perimeter on the garage. THE most important other thing to consider is the preparation of the sub base for these pours. Gabe Martel wrote an excellent article a few months ago for JLC magazine that you might want to read. Your sub soil conditions make a big difference in how your slabs will hold up. If your are pouring them on well drained compacted material and making sure the slab is very consistent in it's thickness , you should not have any trouble. Use joints every 10 or 12 feet to control cracking. Both mixes are rich and should hold up well to freeze / thaw cycles and road salt.
*Hi Michelle,Grading and base concerns are important! Especially when not using a frost wall. If runoff gets under your concrete and the frost follows then you've made a built in problem. Opting out of a frost wall is okay but my sugeestion is to use foam around the perimeter. This greatly reduces frost migration and its ensuing problems with heaving and cleaving. Regards,Dan-O..in the Northeast Kingdom of Vermont!
*Michelle-I'm about 70 miles north east of you. I've never heard of a garage being built in good construction without a 4' frost wall. Usually a concrete footing with 6 courses of 8" block, then a course of 4" on top. The garage is framed on top of the 4", and the slab sits on the 4" ledge which is the top of the 8" block. Normally the block foundation is backfilled on the inside at least with washed stone, as is the interior of the garage. Floors are typically poured of six bag mix, and reinforced with mesh. Drives are also poured with 6 bag, and wire mesh and re-bar are used almost always, and fiber mech is a common upgrade. I'm a fan of beams on the edges of the drive, reinforced with rebar. There's a great article in Fine homebuilding May 1996, which I am re-reading as I type, as I'm preparing to pour my own drive right now.. Slabs with beams are not commonly used (as far as I know)for garage foundations esp. in our area, as our soils have quite a bit of clay, and are thus pretty expansive, which moves things around during freeze thaw that don't have proper foundations. Best, Chris
*first recommendation - get another quote, try Mad Dog - he's in your area!I would not place these slabs without the 48" frost wall, myself. DanO has good advice tho if you choose to do it. I would also opt for the fibermesh to reduce shrinkage and assure consistency. It is the best option for thin slabs, IMNSHO. As others have said, the wire mesh will sink to the bottom unless they use bolsters, which, I am willing to bet, they won't do. Plus, wire mesh does not nelp out on the shrinkage issue.
*Michelle, Don't get carried away and spend a lot of money for nothing. ( I'm talking about frost walls 4' deep) The important thing is to make sure that the garage is sitting on coarse, well drained,compacted material. If the site has clay type soil, you will have to remove it and fill with sand or gravel and make sure that the water drains away under your building. Unless you are heating your garage, the soil beneath the floor is going to freeze in the winter anyway. If you don't have well drained soil underneath, the floor will frost heave. It doesn't matter if you have footings 8' deep. I have built at least a hundred garages with 12" reinforced perimeter footings and they are no problem at all if you allow for proper drainage under and around the building. If the soil beneath the building won't hold water, you will not have a problem if it freezes. If you are unsure of the soil conditions where you want to build, consult with a local expert before you build. The concrete guys may or may not be knowledgeable about this. I would ask them both the same questions and see if they agree with each other. In any event, you should have your garage walls sitting on a row of block or a formed ledge and not directly on the slab.
*This post has been very informative due to the differents in construction methods. Oue area of construction is miles from the gulf of mexico, so temperture in the 40's is rare. Gravel under slabs is unused, Concrete is typical 5 1/2 sack which yield about 3800 psi. footing are typical 18 inches deep. expansion clays, never saw them, everything here is sand. alot of double stry bldgs are on piling. Slabs are usually wire mesh, 4 inch slabs.
*Michelle,As Wolverine says, I work in your area. I am not an expert in concrete construction but believe what Chris Koehn says works best in our climate and soils, though soil type varies greatly from one area to the next. I'd like to take what G. LaLonde says and completely trust that to work here, but have seen too many seriously cracked garages/basements to take the chance. I poured a 6" slab in Northern Wisconsin 7 years ago, no cracks, all I used was wire mesh. It was poured on sand. What G. Lalonde said about drainage is right on. If the slab is poured on a properly drained bed, you shouldn't have frost heave. But what happens if you have a lot of clay surrounding the garage? Water takes the path of least resistance. When it courses through levels of subsoil, it can move laterally, and your well drained area under the garage may take on water because it can't penetrate the clay around it. If that becomes saturated and later frozen, it can force the slab upward. That's why we use a frostwall over footings 48" deep. At least in that respect if you get some frost heave, it will maybe crack the floor, but not affect your walls/structure very much.But as I said, slabs can and do work. Is your garage going to be heated or contain living space?
*Mad Dog,The garage will be unheated, no living space, but it will have a screened porch (6' x 16') down one side of the 26' x 26' area, with a double garage door in the remaining 20' of space.
*Thanks again for taking the time to explain relevant factors. It's been extremely helpful and I can go back to trying to make some decisions here feeling less like I'm driving with my eyes shut. Appreciate your willingness to share your experience!
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I'm building a new detached garage and will be replacing the driveway (currently asphalt), a front walk way, concrete stairs/entranceway,and putting down a slab for a 2 car garage (26 x 26), as well as removing the old 12 x 22 slab, walk, stairs and drive. All of the estimates recommend different techniques and I'm unsure which is best. One estimate uses a 7 bag mix with 5" concrete thickness reinforced with fiber mesh for all work, except 7" depth for the approach and includes curing and sealing. The slab does not have a perimeter. Price - $6,600. Seemed to pay more attention to grading and base concerns than the 2nd estimate.
The 2nd estimate uses a 6 1/2 bag mix with 4" thickness throughout except a 7" thick approach. The slab includes a 12" thick perimeter reinforced with rebar. The drive is also reinforced with rebar, while the slab is reinforced with wire mesh. Price - $7,900.
Any input would be greatly appreciated.