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I’m pouring a drive this fall and have a few of concerns.
1. When pouring slabs (driveway) in sections, is it preferable to pin together the sections at the cold joints or should those joints be separated only by expansion material.
2.Is there any advantage in using WWM and/or rebar in the slab? Concrete depth will be a min. of 5″ with 5″ crushed stone base.
3. What is the recommended spacing for control joints?
Thanks
JW
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Replies
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Hi JW,
#1 If you pin them together you defeat the purpose of the expansion joint.
#2 Yes it's better to use re-enforcing. WWM should be adequate for what you're doing. Make sure you have sufficient concrete chairs underneath the mesh to keep it midway into the concrete slab.
#3 10 feet should be fine. Control joints normally refer to 1" cuts in the concrete to encourge cracking at those locations first. Expansion joints are separations with a asphalt impregnated fibre board between them. I prefer expansion joints.
Gabe
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Gabe:
Perhaps I did not communicate my situation in adequate detail. I am forced to pour my drive in discrete pours (three of them) creating cold joints. My concern is whether I should pin the slabs (which as you correctly point out will eliminate the necessity of expansion joints) or, create expansion joints at the cold joints. My concern is that expansion joints may allow (unwanted) movement during the freeze-thaw cycle.
With regard to control of cracking within the slabs, do I understand that creating expansion joints is preferable to sawing or tooling joints?
Thanks,
JW
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Hi JW,
Can you tell me what size of slabs you intend on pouring each time?
You can do more to minimize (unwanted) movement by founding the slab on well drained, compacted stone.
Frost is more damaging in organic soil that holds water like a sponge.
If you inspect the sidewalks in your area, you will find that they are normally tool jointed at every 3 or 4 feet and have expansion joints every 3 or 4 sections. This is dependant on local codes and winter conditions.
With regards to pins, what I indicated was that it would render the expansion joint useless. If you have a pin connecting the two halves, the concrete will not be free to move and will be likely to crack at the pin location.
Gabe
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J.W. To add to what Gabe has already said....You only need expansion or isolation joints every 30' or so and where a slab butts into a surface that won't move (such as your garage floor slab). Control joints are usually cut at a distance of about 2 1/2 x the thickness of the slab, not to exceed 15'. What I do in situations like yours is cut 1 1/2" wide strips of 1" styrofoam and nail them to the inside of the form board to form a continuous strip that leaves a key joint when the form is removed. This keys the sections together so one does not rise above the other from frost heave, but also allows them to expand or contract if necessary as the joint can slip and still be keyed. Use 4000 psi concrete with additives if you are in a freezing climate. I like fibermesh and 5" minimum slabs. We often use steel mesh and fibermesh together.
*JW, #1; The cold joints (we call them Construction Joints) should be pinned or keyed in some manner. What you are trying to accomplish is to eliminate differential settlement between the two slabs. A piece of expansion joint material will not do the trick. A keyed joint or doweled joint with the dowels greased on one end allows horizontal movement but not vertical movement. Expansion joint material is more commonly used to isolate the slab from some vertical object such as a wall or column. If you are in a freeze / thaw climate your slab is going to move up and down. I believe the goal would be to keep the edges of the joint aligned.#2; The rebar would be used if you are trying to create a structural slab, but in this case WWM is probably sufficient. It's purpose is to control shrinkage as the concrete dries. It needs to be near the surface for this to work, generally at the bottom of the top 1/3 of the slab. Glass fiber mesh can also accomplish this if you prefer. Most important is to use proper curing procedures.#3; The American Concrete Institute recommends control joints to be spaced at no more than 15 to 20 feet. What is also important is that the panels should be nearly square as possible. The more control joints you have the less likely an errant shrinkage crack will occur.The American Concrete Institue has a good publication titled 'Concrete Craftsman Series - Slabs On Grade' for 17.50 that I have found useful. Please note that I am not affiliated with the ACI in any way.GW
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Morning Gregory, Arch.
#1 In exterior SOG pinning will not prevent damage as a result of frost heaving. To the contrary, the areas where the pins are located will normally pop. There is absolutely no substitute for a good, well compacted, properly drained base, containing no organic material in building a SOG that will endure frost cycles.
#2 WWM does not control shrinkage as the concrete dries. It is simply a minimum re-enforcing of the concrete. If a break should occur, it "helps" to keep the pieces together.
#3 You are correct in that sections should be in squares. I assumed that the driveway would be standard 8 to 10 feet in width, therefore I recommended that the joints occur at every 10 feet.
The "Concrete Craftsman Series" is good reading, but I suggest you spend a little more time with the craftsmen, instead.
Gabe
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You should not take G. Wyatts message so personal. There are different methods and degrees of construction to assure a desired outcome.
I think he was potentially correct in everything he pointed out; perhaps, there were a few technical omissions but they were, well, things taken for granted, like compacted sub soils and drainage beneath slabs.
Still, they were important but covered in the other posts.
First, once subsoil is compacted and gravel drainage base laid in place you can lay the concrete slab and have the cold joint. You can be reasonably assured that at cold joints w/ a suitable expansion joint the slab will behave adequately at the joint. However, if a person wanted to assure no vertical difference between the isolated slabs, then correctly pinning them will help in this endeavor. You are correct to point out that the pinning cannot happen in a vaccum - w/out subsoil preparation or base drainage but, correct pinning will please the fastidious person who doesn't want to feel a bump in that expensive entry drive. In this case, an inexpensive venture adding this (smooth) dowel at the cold joint can be justified. One edge of the smooth dowel at 12'oc gets an expansion cap and the same end gets lubricated, grease will work. Add expansion joint strip and joint sealer to the mix and it performs as intended.
Institutes like the ACI don't practice worthless research but, fact finding expeditions into what really works and what can better be done to remedy industry problems (and yes, failures). The hundreds of thousands of dollars they spend in research and practice to illicit more clientel from the marketplace for their products should not be confused with "pitching" their product in some frivolous manner. They do recommend this practice (pinning), at times; we have to understand the situations we face and act accordingly.
Reinforcement steel mesh fabric can do all that was described in the above posts including helping to control thermal stresses, shrinkage cracking, and slight differential movement in the soil bed. Yes, it was a fortuitous accident, this discovery that concrete and steel have relatively the same coeficient of thermal expansion but it nonetheless is now a worthwhile practice to use it for this reason. See, the American Concrete Institute's goal of educating the trades associated w/ concrete use IS paying off!
Chears.
*Oh, and I almost forgot: concrete as it hardens, does not "dry"; of course you already know, it cures. I just thought since we were getting so technical . . .
*Lawrence, Thankyou for standing with me on some issues. Your insight is correct. I was merely trying to provide some advice to the specific questions posted, not give an all inclusive lesson on preparing and pouring a concrete slab on grade. JW asked about pins so I referred to them in my post. My personal preference would be a keyed joint because I always have some scrap around that can be utilized. Pinning is a viable choice if done properly. The PA DOT uses many pinned joints in concrete roadways. Dowels are preassembled with greased sockets and mounted in chairs. Where an existing slab is encountered they have a rig for drilling holes into the edge of the slab.I used the word dries because that is what causes shrinkage cracks. Yes the concrete hardens chemically, but the water that is necessary for the chemical reaction to occur eventually evaporates. When the volume of the water leaves the slab (dries), shrinkage occurs. That is why concrete with low water/cement ratios will keep shrinkage to a minimum. This is the balance we seek. Just enough water that the concrete cures properly but not too much that will promote shrinkage cracks.Mr. Martel, I suspect you may have made judgement because of my title, unaware that I have spent many hours finishing concrete. I consider myself a craftsman much more than a designer.GW
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Hi Gregory,
No I wasn't influenced by your title, on the internet people call themselves all kinds of titles. It was the way your post was written that influenced me. You didn't talk the talk so I assumed that you couldn't walk the walk.
The last time I did pinning was for an airport apron and a hanger slab.
Hardly a residential slab.
If pinning is not done by a proffessional, during a failure, they do pop out chunks of concrete and they do nothing to prevent the lifting at the joint. The pin is designed to slip in and out.
A shrub can lift and crack a slab, it doesn't take 3 foot deep frost to do it. If you were spec. writing, which is what your post indicated, you would be doing so for a tradesman, not a home owner to interpret.
The concrete delivered to most sites are of a quality that is second to none. The consistency from one jurisdiction to another is very good.
4000 psi in California or in Ontario is the same.
The soil is a different matter.
Regardless of the location, if the base is not to standards, the concrete will fail, pins or no pins.
Most failures are as a result of the base containing organic material or not being sound.
This is the one area we all agree on. Its only that I put more emphasis on this aspect because of what I know about the rest of the story.
By the way, never take my post personally, I don't.
Gabe
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JW
I'm an industrial carpenter for a local steel mill. We are always forming and pouring slabs of various shapes and sizes. Almost all of our slabs are "floating" slabs, and about 70% of these pours are located inside a building or under a roof. Therefore, not much movement. But in any case:
1. When we have a large slab that has to be poured in sections, we pin the sections with rebar. We sometimes use welded wire mesh, but only if the pour is 4 or 5 inches thick. Still, if we have the rebar on hand, we try to use it, nomatter how thick or thin the pour will be. We use 1/2 inch or 5/8 rebar for these situations; laid in a 12 inch grid pattern. The rebar is tied together with tye wire at every point where the bar criss-crosses along the perimeter of the forms and at every other criss-cross in the center of the form. The rebar is to be set atop chair rails or brick so as to be off the ground when the concrete is being poured. (If using chairs, place them down first and tie your first set of rebar to them. If using bricks, tie all of your rebar first and then go back and place the brick underneath in key places) Tying the rebar helps to keep the bar in place and prevents lift during the pour.
Drill holes 12 inches OC along the form board on the side where the next pour will continue onward. Insert rebar into these holes and tie them off to the other rebar inside the form. These "pins" will be your attachment points when come time to pour the next section. We don't use expansion material at these junctions, only where the slab would come into contact with another nearby surface such as a wall or building foundation of some sort.
I agree with Gabe that soil prep is "key" to having a good chance for a successful pour. Your soil should be rid of organics, along with adding compacted gravel fill. This is the right thing to do. Don't forget to lay out heavy plastic on top of the fill before putting in your rebar or mesh. The plastic helps ensure the concrete against rapid moisture lost during the pour and curing period.
As for control joints, I was always taught that the purpose of them was to help minimize cracking of the concrete and to "direct" such cracking to the control joint itself. Besides adding compression strength,the rebar you install helps to hold the concrete in place once it has cracked. Control joints approx. every 10 feet is the norm, but frankly, we seldom cut control joints into our slabs. If the slabs have room to move, yet is on solid ground with good drainage built into the site to direct runoff away from it, your cracking should be very minimal.
Since this is for a driveway,I would suggest going to a 6 inch thick slab rather than a 5 inch. the "6 bag" mix of 4000 psi is right-on for this sort of thing.
We have added fiberglass in with many of our pours as an extra step to safeguard against cracking. We have had mixed results with this. Many people say that if you use the fiber, then forget the rebar. Bull! Use the rebar first, and then if budget allows, add the fiber to the concrete mix.
We have never used keyways on slab pours or greased dowels. Whenever we decided to add an addition to an older existing slab, we drilled holes into the existing and chemically grouted rebar into these holes and then tied off our new rebar to these pins. The idea being to make the new slab tied-in with the old and to allow them to move together as one whole unit, rather than 2 seperate units.
Since I have no experience with keyways(I have formed many keyways, but not for this use) or greased dowels, I cannot say they will or won't work for your situation. If it was my driveway, I would do as I indicated earlier and probably cut the control joints in the slab at regular intervals.
Good luck,
Davo
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JWLong. Generally the advice given is good. However, just putting in WWF is useless. I did note that Davo304 calls for chairs which will guarantee the correct location of the WWF in the concrete.
Gabe. The concrete that arrives may be the best, but if watered-down by the floor finishers how good is it? Both the plant manager and the truck driver should be told to keep the water to cement ration(W/C) sat 0.41 or lower.If the mix is too stiff then add a mid range water reducer.
Pouring the mix directly on the vapor diffusion retarder (VDR) is questionablle Yes it acts as a slip-sheet, but it also guarantees cracking.Furthermore if the w/c ratio is high excess mix water will pool between the bottom of the slab and the top of the VDR. Eventually it will move up through the slab and puddle on top of the slab.
It may be standard practice for the ready-mix plant to add air entrainment. I'd ask them about this. If they do not then have them add air entrainment. Given the slab's continual exposure the air entrainment will protect it in icy weather. GeneL.
*Gene,The original post was addressed to concrete experts, not concrete want to be writers.Tradesmen do not water down, the are for the main, responsible.Time for you to stop digging into the old posts to find things to judge. You don't have the experience in the field to grasp what is being discussed here. Go back to peddling your books.Putting plastic down dosn't guarranty anything, especially cracking, that's just stupid fear mongerring.Gabe
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To save time, you can skip the plastic and wet the subgrade down a bit just before the pour. You don't want standing water but just wet to prevent moisture wicking from the crete. The plastic isn't needed as a bond breaker between the slab and the subgrade. If the subgrade is uniform and compacted, the slab will slide. Dowels between slabs are called "Load Transfering Devices" by the NYSDOT. The help transfer the wheel loads of heavey vehicles from one slab to another as the wheel moves across the joint. I don't believe they are needed in residential construction and may actually cause the slab to crack in the vixcinity of the dowels. Highway slabs are generally 6 to 8 inches thick. Theres more concrete around the dowels to help confine them.
By far the most important step is the prep of the subgrade. Usually the most overlooked in residential construction too.
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I am about to finally put down the slab for my long delayed shop. I worked quite a bit with concrete in my younger days but it was heavy construction (dams and such) not slabs and anyway I have managed to forget more than I remember about it. However, I do remember in that the general advice given was to never use brick bits for chairs to hold up rebar - use wire chairs, broken cement chunks, or rocks. Or has that conventional wisdom changed?
My question is regarding the maximum size of the slab sections between joints. An APCA publication as well as one of the earlier posts indicates that joints should occur every 10 feet. However, I have had a number of two car garages with sizes around 24ft square and only one of them had a control joint in it (and that one managed to be a real pain when shuttling around under a car on a creeper). None of these garages had excessive cracking despite the lack of control or expansion joints. I am planning a 5" thick slab with a size of 24' X 48' in the inside of the shop (and a separate concrete apron outside). While it would be nice to have a continuous slab with no joints to catch rollers, lost screws, etc., I am assuming I should at least break it into 24' x 24' sections with an expansion joint. I am on the Columbia Gorge in Oregon and although it gets down to zero on occasion, I don't see much problem with frost heave here. What do the voices of experience say regarding the minimum number and types of joints I can get by with for the slab inside the shop (assuming that I do a decent job of preparing the base with a jumping jack before and after the gravel fill)?
*Hi Casey,All's well except for the control joints.I would still install an expansion joint at the half way mark so that you have two 24 x 24 foot sectionsthen I would further saw cut control joints and divide it into 12 x 12 foot squares.Here's the part your missing, FILL the joints with 2 part caulking so that the cut are filled flush with the concrete and you won't have any trouble with your wheels,Gabe
*Whats the reason for not using broken peices of brick for chairs ? Our finisher uses rebar on about a two foot grid instead of wire and raises it with small peices of brick , no problems so far . Are we just lucky ? Chuck
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I'm pouring a drive this fall and have a few of concerns.
1. When pouring slabs (driveway) in sections, is it preferable to pin together the sections at the cold joints or should those joints be separated only by expansion material.
2.Is there any advantage in using WWM and/or rebar in the slab? Concrete depth will be a min. of 5" with 5" crushed stone base.
3. What is the recommended spacing for control joints?
Thanks
JW
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Gabe, I must say that in my 30 years of pouring concrete, my experience with it mirrors yours. We both live in relatively harsh climates and have come to use or not use the same techniques. If you pour thousands of yards yearly, you will find out what works and what doesn't over the long term.