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slab thickness
My contractor has specified a 4" thick slab on grade for a new garage
floor. Is this the accepted thickness? I've read that driveways should be 5" thick and house foundations 8-10" thick. The plans *do* call out reinforcement. Comments? |
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wrote in message ups.com... My contractor has specified a 4" thick slab on grade for a new garage floor. Is this the accepted thickness? I've read that driveways should be 5" thick and house foundations 8-10" thick. The plans *do* call out reinforcement. Comments? Just what are you planning on driving on a 5 inch slab? A Fire Truck? 4 inches is standard, usually 3500 psi concrete. Some use rebar some use steel mats. Saw cut every 10 feet for expansion. Foundations are broken up into parts called footings and stem walls. The thickness is directly proportional to the height of the building. Where I live the footings are minimum of 12 deep x 18 wide about 18 inches below grade. That makes the stem walls 10-12 inches wide and 20-24 inches high. If you live in frost country, other considerations are necessary. |
SQLit wrote:
wrote in message ups.com... My contractor has specified a 4" thick slab on grade for a new garage floor. Is this the accepted thickness? I've read that driveways should be 5" thick and house foundations 8-10" thick. The plans *do* call out reinforcement. Comments? Just what are you planning on driving on a 5 inch slab? A Fire Truck? 4 inches is standard, usually 3500 psi concrete. Some use rebar some use steel mats. Saw cut every 10 feet for expansion. The OP said he read the info. Must have been a handout from the National Concrete Mfger's Association |
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In a previous post says...
My contractor has specified a 4" thick slab on grade for a new garage floor. Is this the accepted thickness? I've read that driveways should be 5" thick and house foundations 8-10" thick. The plans *do* call out reinforcement. Comments? For residential structures, I always recommend a 5-inch UNREINFORCED slab on a 6-inch compacted gravel base. Sawcut or tooled crack control joints every 150 square feet max. Unreinforced = no welded wire fabric, no rebar, no fibermesh No amount of reinforcing will prevent cracking. The only thing that will do that is pre-stressed or post-tensioned concrete. -- Bob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA |
Slab strength and thickness depends on extenuating circs, type of subsoils,
frost, use of salt on roads, etc. Reinforcing, if properly applied, greatly improves perform of crete, as does limiting the amount of H2O -- Remove the obvious to reply. Experienced and reliable Concrete Finishing and Synthetic Stucco application in the GTA. "Bob Morrison" wrote in message .net... In a previous post says... My contractor has specified a 4" thick slab on grade for a new garage floor. Is this the accepted thickness? I've read that driveways should be 5" thick and house foundations 8-10" thick. The plans *do* call out reinforcement. Comments? For residential structures, I always recommend a 5-inch UNREINFORCED slab on a 6-inch compacted gravel base. Sawcut or tooled crack control joints every 150 square feet max. Unreinforced = no welded wire fabric, no rebar, no fibermesh No amount of reinforcing will prevent cracking. The only thing that will do that is pre-stressed or post-tensioned concrete. -- Bob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA |
According to SQLit :
wrote in message ups.com... My contractor has specified a 4" thick slab on grade for a new garage floor. Is this the accepted thickness? I've read that driveways should be 5" thick and house foundations 8-10" thick. The plans *do* call out reinforcement. Comments? Just what are you planning on driving on a 5 inch slab? A Fire Truck? 4 inches is standard, usually 3500 psi concrete. Some use rebar some use steel mats. Saw cut every 10 feet for expansion. Some use fiberglass fiber reinforcement instead. I did ;-) Foundations are broken up into parts called footings and stem walls. The thickness is directly proportional to the height of the building. Where I live the footings are minimum of 12 deep x 18 wide about 18 inches below grade. That makes the stem walls 10-12 inches wide and 20-24 inches high. If you live in frost country, other considerations are necessary. Yeah, you generally need to get below the frost line. Here, it's at least 4'. -- Chris Lewis, Una confibula non set est It's not just anyone who gets a Starship Cruiser class named after them. |
In a previous post ConcreteFinishing&StuccoGuy says...
Slab strength and thickness depends on extenuating circs, type of subsoils, frost, use of salt on roads, etc. Reinforcing, if properly applied, greatly improves perform of crete, as does limiting the amount of H2O CF&SG is correct in that rebar and lower water content will improve structural performance. However, only the latter will have any effect on crack control. Most cracks come from shrinkage. Shrinkage comes from having more water in the concrete that is necessary to make the chemical reaction happen. If left unspecified most batch plants will give you about 6 gallons of water per 94 lb. sack of cement. Only about 3 gallons is required for the chemical reaction. The rest is there so you handle and place the concrete. There are several ways to reduce the amount of water, including adding more air to the mix which isn't a bad idea for exterior slabs. The added air ("air entrainment") will also improve freeze-thaw resistance. -- Bob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA |
Bob Morrison wrote:
In a previous post ConcreteFinishing&StuccoGuy says... Slab strength and thickness depends on extenuating circs, type of subsoils, frost, use of salt on roads, etc. Reinforcing, if properly applied, greatly improves perform of crete, as does limiting the amount of H2O CF&SG is correct in that rebar and lower water content will improve structural performance. However, only the latter will have any effect on crack control. Most cracks come from shrinkage. Shrinkage comes from having more water in the concrete that is necessary to make the chemical reaction happen. If left unspecified most batch plants will give you about 6 gallons of water per 94 lb. sack of cement. Only about 3 gallons is required for the chemical reaction. The rest is there so you handle and place the concrete. Don't they measure or test that by measuring the "slump?" There are several ways to reduce the amount of water, including adding more air to the mix which isn't a bad idea for exterior slabs. The added air ("air entrainment") will also improve freeze-thaw resistance. -- Joseph Meehan Dia duit |
Don't they measure or test that by measuring the "slump?"
************************************************** *********** Yes they do but if the concrete measures a 6 slump, there is no way to take water out of the concrete. (the only way to stiffen up the concrete is to add more sand, aggregate and cement) -- JerryD(upstateNY) If left unspecified most batch plants will give you about 6 gallons of water per 94 lb. sack of cement. Only about 3 gallons is required for the chemical reaction. The rest is there so you handle and place the concrete. There are several ways to reduce the amount of water, including adding more air to the mix which isn't a bad idea for exterior slabs. The added air ("air entrainment") will also improve freeze-thaw resistance. -- Joseph Meehan Dia duit |
Joe wrote:
Don't they measure or test that by measuring the "slump?" ************************************************** *********** Yes they do but if the concrete measures a 6 slump, there is no way to take water out of the concrete. (the only way to stiffen up the concrete is to add more sand, aggregate and cement) And if it measures a 6" slump and you wanted a 4", then you send the truck back to the plant and ask for one that is to your specifications. You don't just take what they give you. -- Robert Allison Rimshot, Inc. Georgetown, TX |
.... and watch that the driver doesn't add water to move the mix down
the chute. The Navy had to rework a three story building because the inspector didn't pay attention. Engineer and lawyers spent a couple of months strugglng over that until a fired employee spilled the beans. TB |
"Robert Allison" wrote in message news:iWZre.5595$yw4.3168@trnddc09... Joe wrote: Don't they measure or test that by measuring the "slump?" ************************************************** *********** Yes they do but if the concrete measures a 6 slump, there is no way to take water out of the concrete. (the only way to stiffen up the concrete is to add more sand, aggregate and cement) And if it measures a 6" slump and you wanted a 4", then you send the truck back to the plant and ask for one that is to your specifications. You don't just take what they give you. It is a little more difficult to send back when the test cylinders don't meet compression days later :-) -- Robert Allison Rimshot, Inc. Georgetown, TX |
In a previous post P.Fritz says...
It is a little more difficult to send back when the test cylinders don't meet compression days later :-) That's when the trouble (and overtime for the engineer) really starts! I have had to require that the contractor tear out the bad stuff and replace with new. Finger pointing gets started in earnest at that point. -- Bob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA |
"Bob Morrison" wrote in message k.net... In a previous post P.Fritz says... It is a little more difficult to send back when the test cylinders don't meet compression days later :-) That's when the trouble (and overtime for the engineer) really starts! I have had to require that the contractor tear out the bad stuff and replace with new. Finger pointing gets started in earnest at that point. But it is always the contractor's fault :-) -- Bob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA |
wrote in message ups.com... My contractor has specified a 4" thick slab on grade for a new garage floor. Is this the accepted thickness? I've read that driveways should be 5" thick and house foundations 8-10" thick. The plans *do* call out reinforcement. Comments? Quoting from my recently purchased "Pocket Ref" book, the recommended thickness for driveways is 6 to 8 inches. Garage floors are 4-5 inches. Putting in a driveway is no small chore. Or expenditure. I tend to make things stronger and thicker than the minimum. A few hundred bucks spent now could save you some major work and expense later. And, making the forms a couple of inches thicker now is no big deal. Materials can be taken out, or the forms merely made a little higher. Think long term. DO IT ONCE. DO IT RIGHT. Steve |
"SQLit" wrote Just what are you planning on driving on a 5 inch slab? A Fire Truck? 4 inches is standard, usually 3500 psi concrete. My motorhome is 21,000 #. Now, I don't put it in the garage because it is too tall, but I do drive on the driveway. And when I go visit, I drive it on other people's driveways, too. Hope they have a good one, and didn't get the cheapo economy cut cost save a little money now and pay for it later kind of deal from their contractor. The way I think of it, I want to know if I ever wanted to drive a fire truck on it, I could. Steve |
Bob Morrison wrote:
In a previous post P.Fritz says... It is a little more difficult to send back when the test cylinders don't meet compression days later :-) That's when the trouble (and overtime for the engineer) really starts! I have had to require that the contractor tear out the bad stuff and replace with new. Finger pointing gets started in earnest at that point. And I have had to tear out concrete that didn't break at the spec'd pressure after 28 days. I didn't pay for it though, the cement plant did. They paid for the new concrete, the labor, the disposal of the old concrete, everything. When you do this to them a couple of times, they start paying close attention to the mix and their quality controls at the plant. Whenever I order concrete now, I always tell them that the testing lab will be there to take samples and do a slump test (whether they are or not). They are minding their Ps and Qs, then. We have had the odd batch not break at 3500 or 3000, but it is usually in an area where it doesn't matter, so it doesn't have to be torn out. They sometimes break at 2900 or 3400, but for a slab on grade, that is fine. When you get into retaining walls and other critical systems, the heat is on, and if you have all of your paperwork in order, it is on the supplier. Whenever I have to call the plant and tell them that their concrete is subpar, the first question I always get is How much water did you add? For that reason, I always have the drivers indicate on the delivery ticket if water was added and who requested it. Sometimes the drivers will add water on the way to the job to keep the mix turning. If it is a spec'd job with tests in place, that can come back to haunt them. Moral: Always keep your delivery tickets, indicate water added, and test, test, test. -- Robert Allison Rimshot, Inc. Georgetown, TX |
Robert Allison wrote:
.... Whenever I have to call the plant and tell them that their concrete is subpar, the first question I always get is How much water did you add? For that reason, I always have the drivers indicate on the delivery ticket if water was added and who requested it. Sometimes the drivers will add water on the way to the job to keep the mix turning. If it is a spec'd job with tests in place, that can come back to haunt them. Can you actually catch them if they don't "'fess up", though? When I drove one summer while in high school for a local mix service here (classmate's Dad owned the place) there would have been no way to prove it as they, at least, weren't monitoring onboard water consumption...I suppose w/ "real" construction, that's done now. (Note--Otto only let us kids deliver to the oil well sites where you couldn't mess anything up other than bury it to the axles in mud... :) ) |
Anybody answer how thick and what PSI concrete one should have for a
driveway to support a 25K# truck or trailer? Also how pricey is it to have a testing person to do a slump test on site. In my neck of the woods (S WI) the big bear of a problem here is chert - for some reason in the winter the stuff explodes leaving divots in the concrete. Seems the cheap jobs and the city sidewalks all have these divots. |
SteveB wrote:
Think long term. DO IT ONCE. DO IT RIGHT. Steve Steve...what state do you work in? |
And if it needs to be 'pumped', I guess the slump is less, right ..so it can
flow thru the pump easier ? Don't they measure or test that by measuring the "slump?" Yes they do but if the concrete measures a 6 slump, there is no way to take water out of the concrete. (the only way to stiffen up the concrete is to add more sand, aggregate and cement) And if it measures a 6" slump and you wanted a 4", then you send the truck back to the plant and ask for one that is to your specifications. You don't just take what they give you. |
wrote in message
ups.com... My contractor has specified a 4" thick slab on grade for a new garage floor. Is this the accepted thickness? I've read that driveways should be 5" thick and house foundations 8-10" thick. The plans *do* call out reinforcement. Comments? Just an observation based on my poured driveway. Had a contractor who normally does commercial work do it, they were slow last winter. They used 3/8 rebar, tied in 16" squares. The perimeter was 6" thick. Foreman said the thicker perimeter serves two purposes. 1. Like a house concrete perimeter, serves as a beam. 2. Prevents water runoff wash from undermining the slab. The driveway itself is 3" thick. Simlar technique on the apron at the street. No frost line in the area, too warm a climate. |
Duane Bozarth wrote:
Robert Allison wrote: ... Whenever I have to call the plant and tell them that their concrete is subpar, the first question I always get is How much water did you add? For that reason, I always have the drivers indicate on the delivery ticket if water was added and who requested it. Sometimes the drivers will add water on the way to the job to keep the mix turning. If it is a spec'd job with tests in place, that can come back to haunt them. Can you actually catch them if they don't "'fess up", though? When I drove one summer while in high school for a local mix service here (classmate's Dad owned the place) there would have been no way to prove it as they, at least, weren't monitoring onboard water consumption...I suppose w/ "real" construction, that's done now. (Note--Otto only let us kids deliver to the oil well sites where you couldn't mess anything up other than bury it to the axles in mud... :) ) Well, if I have a delivery ticket that says no water was added, and the test breaks at 2800 for a 3500 PSI concrete, who is responsible? The plant! That is what I am talking about. If you order 3500 PSI mix and you don't get it, it is the cement plant that has to fix it. -- Robert Allison Rimshot, Inc. Georgetown, TX |
Robert Allison wrote:
.... Well, if I have a delivery ticket that says no water was added, and the test breaks at 2800 for a 3500 PSI concrete, who is responsible? The plant! That is what I am talking about. If you order 3500 PSI mix and you don't get it, it is the cement plant that has to fix it. That surely, is the "proof"...I was just curious during the finger-pointing phase if the driver tried the "not me!" trick whether with newer mix trucks water usage might be monitored... |
Rudy wrote:
And if it needs to be 'pumped', I guess the slump is less, right ..so it can flow thru the pump easier ? Slump would then be greater, not less... |
According to butch burton :
Anybody answer how thick and what PSI concrete one should have for a driveway to support a 25K# truck or trailer? I think this needs 6", plus a well compacted gravel base, and rebar/mesh. 4000PSI or better methinks. Also how pricey is it to have a testing person to do a slump test on site. It shouldn't be necessary - just stress to the supplier that it needs to be on-spec. In my neck of the woods (S WI) the big bear of a problem here is chert - for some reason in the winter the stuff explodes leaving divots in the concrete. Seems the cheap jobs and the city sidewalks all have these divots. Sounds like "spalling", rather than issues with chert. Usually caused by inadequate air entrainment. This is something you specify when you order. You're best off hiring a "real" concrete contractor to do the job or at least spec it out for you. A civil engineer could be used to spec the thing if you're going to contract it out yourself. -- Chris Lewis, Una confibula non set est It's not just anyone who gets a Starship Cruiser class named after them. |
On Thu, 16 Jun 2005 10:54:04 GMT, "Lil' Dave"
scribbled this interesting note: wrote in message oups.com... My contractor has specified a 4" thick slab on grade for a new garage floor. Is this the accepted thickness? I've read that driveways should be 5" thick and house foundations 8-10" thick. The plans *do* call out reinforcement. Comments? Just an observation based on my poured driveway. Had a contractor who normally does commercial work do it, they were slow last winter. They used 3/8 rebar, tied in 16" squares. The perimeter was 6" thick. Foreman said the thicker perimeter serves two purposes. 1. Like a house concrete perimeter, serves as a beam. 2. Prevents water runoff wash from undermining the slab. The driveway itself is 3" thick. Simlar technique on the apron at the street. No frost line in the area, too warm a climate. That perimeter bean serves another function as well. A floating slab will do just that...float. That perimeter beam helps keep the monolithic slab exactly where it was placed and it no longer wants to float around on the surface of the ground. -- John Willis (Remove the Primes before e-mailing me) |
In a previous post butch burton says...
Anybody answer how thick and what PSI concrete one should have for a driveway to support a 25K# truck or trailer? I would go with a minimum of 6-inch thick on a minimum 6-inch compacted crushed rock (3/4" minus) base. You can help spread loads if you use a geotextile between the gravel and native material. Specify a minimum 3500 psi low-slump concrete with 6% air entrainment. Provide 1-1/4" deep crack control joints - either tooled or saw cut every 150 sq.ft. (maximum). For example, if the driveway is 12 feet wide then put a joint every 12 feet. Joints should be cut as soon the surface is hard enough to walk on. -- Bob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA |
Duane Bozarth wrote:
Robert Allison wrote: ... Well, if I have a delivery ticket that says no water was added, and the test breaks at 2800 for a 3500 PSI concrete, who is responsible? The plant! That is what I am talking about. If you order 3500 PSI mix and you don't get it, it is the cement plant that has to fix it. That surely, is the "proof"...I was just curious during the finger-pointing phase if the driver tried the "not me!" trick whether with newer mix trucks water usage might be monitored... One way to do that is to tell the plant that you want the water tanks full when they arrive at the jobsite. That leaves the driver the option of adding water and then stopping somewhere and refilling the tanks, but I don't think that they are that devious. Most drivers I have found to be pretty honest. I can't imagine a situation where one would want to ruin your mix just to be mean. Especially when they know that their jobs could be on the line for it. You can also check the level of the water tank on the truck when it arrives and when it leaves, keeping in mind that if the truck is not on level ground, the readings could be different if it moves. -- Robert Allison Rimshot, Inc. Georgetown, TX |
Duane Bozarth wrote:
Rudy wrote: And if it needs to be 'pumped', I guess the slump is less, right ..so it can flow thru the pump easier ? Slump would then be greater, not less... Exactly. And if you are pumping the mix, you have to let the plant know that in advance. It requires a certain size of aggregate and a 5-6" slump, which means that the plant has to adjust the mix so that when it cures, it is at the right PSI. You can't just order standard 3000PSI mix, then add water to get it through the pump. -- Robert Allison Rimshot, Inc. Georgetown, TX |
4" is fine if you use 3000lb concrete for slab on grade. 8-10" is for
foundation walls. wrote in message ups.com... My contractor has specified a 4" thick slab on grade for a new garage floor. Is this the accepted thickness? I've read that driveways should be 5" thick and house foundations 8-10" thick. The plans *do* call out reinforcement. Comments? |
Bob Morrison wrote:
In a previous post ConcreteFinishing&StuccoGuy says... Slab strength and thickness depends on extenuating circs, type of subsoils, frost, use of salt on roads, etc. Reinforcing, if properly applied, greatly improves perform of crete, as does limiting the amount of H2O CF&SG is correct in that rebar and lower water content will improve structural performance. However, only the latter will have any effect on crack control. Yes, but rebar will help prevent problems is there is a soft spot in the ground under the slab that settles a little. It can also prevent cracks from opening up or shifting vertically. Matt |
In a previous post Matt Whiting says...
CF&SG is correct in that rebar and lower water content will improve structural performance. However, only the latter will have any effect on crack control. Yes, but rebar will help prevent problems is there is a soft spot in the ground under the slab that settles a little. It can also prevent cracks from opening up or shifting vertically. Matt: Quite true. However, I tell the contractors to spend their time and effort on subgrade preparation. It's easier to do than correctly placing rebar, and with the current price of steel may even be cheaper. A good unreinforced slab on properly prepared subgrade with adequate crack control should not have any problems. -- Bob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA |
Bob Morrison wrote:
In a previous post Matt Whiting says... CF&SG is correct in that rebar and lower water content will improve structural performance. However, only the latter will have any effect on crack control. Yes, but rebar will help prevent problems is there is a soft spot in the ground under the slab that settles a little. It can also prevent cracks from opening up or shifting vertically. Quite true. However, I tell the contractors to spend their time and effort on subgrade preparation. It's easier to do than correctly placing rebar, and with the current price of steel may even be cheaper. A good unreinforced slab on properly prepared subgrade with adequate crack control should not have any problems. NY Code for concrete cover of reinforcing TABLE 1907.7.1 MINIMUM CONCRETE COVER MINIMUM COVER CONCRETE EXPOSURE inches 1. Concrete cast against and permanently exposed to earth 3 2. Concrete exposed to earth or weather No. 6 through No. 18 bar 2 No. 5 bar, W31 or D31 wire, and smaller 1-1/2 If I'm reading that right, and you can figure out a way to cast the driveway above grade and then lower it into place (!), you can go with as little as a 3" slab. Otherwise you need 6" if it's cast in place and reinforced. Weird thing. If you _reinforce_ it, code requires a thicker slab! Bob's got it right, the slab doesn't move unless something underneath it does. Preparation - drainage, compaction, etc. - is the key. R R R |
In a previous post RicodJour says...
If I'm reading that right, and you can figure out a way to cast the driveway above grade and then lower it into place (!), you can go with as little as a 3" slab. Otherwise you need 6" if it's cast in place and reinforced. Rico: Your math is almost correct: 1-1/2" top cover 1/2" (actually 5/8" OD) for #4 Longitudinal bar 1/2" (actually 5/8" OD) for #4 transverse bar 3" bottom cover Total = 5-3/4" Using #3 bards drops the total to 5-1/2" You would need at least that much concrete to get the steel to bond properly anyway. -- Bob Morrison, PE, SE R L Morrison Engineering Co Structural & Civil Engineering Poulsbo WA |
Bob Morrison wrote:
In a previous post RicodJour says... If I'm reading that right, and you can figure out a way to cast the driveway above grade and then lower it into place (!), you can go with as little as a 3" slab. Otherwise you need 6" if it's cast in place and reinforced. Rico: Your math is almost correct: 1-1/2" top cover 1/2" (actually 5/8" OD) for #4 Longitudinal bar 1/2" (actually 5/8" OD) for #4 transverse bar 3" bottom cover Total = 5-3/4" Using #3 bards drops the total to 5-1/2" You would need at least that much concrete to get the steel to bond properly anyway. Again with the math! You're such an engineer sometimes. ;) They allow some leeway with the coverage - I believe it's +/- 3/8". Oh, and coming from a woodworking background, I half-lap my rebar so it's only one layer thick. ;) R |
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