The Concrete Underground's Guide to Floor Slabs

1320stang said:

Maybe I missed it, but sealer?

Pouring a flat 40x50 soon, 5" w/ #3 16 o.c. (it has footers w/ 4 #5 & stirups), I want a burnished finish, never plan on epoxy, no desire to reapply anything over time, what do I have put down and when? I'd like to minimize eventual staining as much as I can. I assume it gets sprayed on prior to final finishing? Worked into the top layer? In central Oklahoma, 2000 sqft shop.

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ConCretin said:

Cure and seal products are generally applied to the slab after finishing operations are complete. These products create a membrane on the surface to retain moisture for curing as well as some stain resistance.

You might however want to consider a penetrating sealer, which will provide far superior stain resistance and some can actually harden the surface. These products aren't typically applied for 28 days or so. If you go this route, skip the curing compound after placement since it will interfere with the subsequent application of the penetrating sealer unless you physically remove it. You could moist cure the floor or depending on weather conditions, skip curing entirely

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ConCretin said:

There have been many GJ discussions on garage slabs but I'm hoping a simple outline will be helpful to those who haven't done a lot of concrete work and are getting ready to place a slab. So let’s lay out the basics that will give the average guy a good shot at a great slab.

Base - What’s under your slab is just as important as the slab itself. Slabs on grade are thin, non-structural surfaces that can’t span very far over soft ground. If the ground under your slab can't support the weight you put on your slab, it will crack.

Start by removing all organics and top soil. It's a good idea to proof roll the sub grade by running a roller or compactor over it. If the soil is dry, stable and compact-able, you can go ahead and place your slab or if you need the bring the grade up, you can start adding base material. If the sub grade is soft, wet, contains silt or expansive clays, additional steps may be necessary.

Even if the native soils are fine, it's good practice to add a few inches of granular material to provide a capillary break and a surface that can withstand rain and construction traffic without getting muddy and rutted.

Base materials must be well compacted to provide uniform support to the slab. Your base should be a granular material such as crushed stone, sand or what we in the Northeast call gravel; a blended material with a range of aggregate sizes including fines that compacts without voids. A maximum aggregate size of 3/4" in the top lift will make it easier to fine grade.

Place and compact the material in 6" lifts. For a free-standing slab, extend the base far enough to ensure compaction and good support under the slab edges. Grade your base to 1/4” +/-to keep your slab thickness consistent.

Slab Thickness - Since you are building on a solid base, a 4" slab is more than adequate for most uses. While it's true that making a slab thicker makes it stronger, it's just not necessary since the base is carrying the load. Your slab would have to be several times thicker to be truly self-supporting so you aren't getting any real benefit from a few extra inches of concrete.

With that said, if you plan on installing a two post lift, I'd consider a 5" slab. While most manufacturers specify a minimum thickness of around 4", there are always variations in elevation in the base and top of concrete, which can result in thinner areas. An inch of concrete only adds .003 cy per sf and will ensure you have enough concrete when you drill your anchor holes.

Formwork - Use lumber that's a little narrower than your slab thickness avoid fighting high spots in the base and set the top to grade by nailing it to your stakes. Avoid anything sticking up so your finisher can screed and finish right over them. Backfill the bottom of your forms to fill any gaps and keep them from getting pushed down if stepped on. Your forms may move during the placement so it's a good idea to run some strings and check for straightness as the concrete goes in.

Grades - Many garage floors are level although some codes require the slab to be pitched to the door. Your slab should pitch to drains and away from the outside face of your garage door to keep water from running back underneath. If you want to minimize birdbaths on your sloped slab, you'll need an 1/8 to 1/4 inch per foot pitch depending on the skill of your finisher.

Vapor Barrier - This is a plastic sheet that goes between your slab and the ground to prevent water vapor from coming through the concrete. While it's always desirable to limit moisture intrusion, it's really only necessary if you intend to apply an adhered floor covering such as epoxy because moisture will interfere with the bond.

There is one potential downside to a vapor barrier that you should be aware of. You could get additional bleed water after concrete is placed, which can cause surface defects if not handled properly. More on that later.

A vapor barrier can be 6 or 10 mil poly but it often gets damaged during construction and degrades over time. A better choice is a 15-mil product such as Stegowrap which will stand a lot more abuse. Seams and penetrations need tone taped to provide a complete seal.

Insulation - In heated structures, particularly with in-floor radiant, a minimum of 2" of rigid board insulation is typically placed under the slab. You can use either XPS or EPS as long as it meets ASTM C578. In cold climates, rigid insulation should also be used vertically on frost walls to keep frost from getting under your slab.

You can protect unheated or floating slabs from frost movement by placing insulation under the slab and extending it out or down to frost depth around the perimeter. The insulation will retain the natural heat from the ground and keep the frost out.

Expansion Material - This is a compressible material that is installed on perimeter foundation walls and penetrations. It allows the concrete to move freely to help prevent cracking. I like to use a closed cell foam with a 1/2" deep perforated strip at the top you can remove and fill with sealant.

Shoot grades, chalk lines and attach the expansion material with spray adhesive. An added benefit is that your finisher can see where grade is, which will help keep your floor flat.

Reinforcing - Concrete is stronger in compression than it is in tension i.e. it's harder to crush concrete than it is to pull it apart. In structural members, reinforcing steel is used to compensate for this lack of tensile strength. Many people assume this means using reinforcing will keep their slab from cracking. Unfortunately, it doesn't work that way.

A slab on grade isn't thick enough to carry heavy loads so it 'bends' and transfers the load to the base below. If the base can't support the load, the concrete will crack regardless of how it's reinforced. Unlike the cables in pre- or post-tensioned concrete, rebar is passive and doesn't come under load until after the concrete has already cracked.

Another common cause of cracking is shrinkage and reinforcing won't prevent shrinkage cracks either. In fact, reinforcing can actually increase the likelihood of cracks by restraining the slab from contracting as shrinkage occurs.

What reinforcing will do is keep cracks from separating, which helps maintain the integrity and levelness of the slab. Think of steel reinforcing as crack restraint rather than crack prevention.

Slab reinforcing can be either rebar or welded wire fabric (WWF), often referred to as wire mesh. They perform the same function but since rebar is more rigid, it's easier to support while mesh is usually less expensive to install.

If using rebar, I’d suggest #3 bars at 18” o.c. supported every 3’ or #4’s at 24” o.c. supported every 4’. When using mesh, I'd recommend a heavier wire such as 6x6-W2.9xW2.9 supported every 3'. Don't let your finisher convince you he can pull the wire off the ground as he places as this rarely works consistently. If you are placing a load bearing monolithic slab, I'd recommend using rebar including a couple bars in the bottom of the thickened edge. Floating slabs have the potential for more movement and rebar will hold everything together.

You want to support the reinforcing as high as possible in the slab to keep cracks tight but you also need adequate concrete cover and clearance for cutting control joints. This generally places it close to the center of a 4" slab. There are many products available to support your reinforcing such as concrete dobies, metal slab bolster or plastic chairs. Make sure your supports can stand up to finishers walking on them and that they don't poke through your vapor barrier.

Overlap rebar splices a couple feet and tie your mat together well enough to keep it in place during placement. When using mesh, overlap the sheets one square and tie the sheets together. Stagger adjacent runs to minimize the number of sheets intersecting in one spot.

Another option is Fibermesh which consists of fiberglass strands that are mixed in with the concrete. Fiber is often discussed as an alternative to steel reinforcing, but it actually performs a different function. Fibermesh adds some tensile strength to concrete to minimize early age shrinkage cracks thereby reducing or eliminating the need for control joints. Fiber doesn't significantly increase the load bearing capacity of your slab and unlike steel reinforcing, it won't hold cracks together once they occur.

There are advanced types of fiber called macro fiber or steel fiber that not only limit cracks but like rebar, actually restrain cracks from getting wider. These products are expensive and have downsides of their own such as clumping and finishing issues. They have their uses but unless you know what to ask for and your finisher has experience using these products, I'd steer clear of them.

To summarize; If you want to minimize or eliminate control joints, use fiber. If you don't mind control joints and want the peace of mind that comes from steel holding everything together, use wire or rebar. If you have a solid, stable base you can even forego reinforcing entirely and save some money.

Concrete - Most concrete is what's called ready mix. Trucks are loaded at a batch plant with aggregates, cement, water and various chemicals called admixtures. The concrete gets mixed in the truck's drum on the way to the job site.

Concrete mixes are classified by their projected compressive strength at 28 days, which is determined by the ratio of water to cement. The more cement that is used relative to the mix water, the higher the strength will be. Since a certain amount of water is needed to make the concrete flow, more cement is needed to provide higher strengths.

Many suppliers replace some cement with finely ground fly ash or blast furnace slag. These materials slow the set a bit but otherwise behave the same as cement and aren't a concern.

The amount of mix water in concrete is typically measured using a slump test. To measure slump, a 12" tall, tapered steel cone is filled with concrete and then slowly lifted off. The distance the concrete drops is referred to as it's "slump". The advertised compressive strength is usually based on the amount of water that will produce a 4" slump. Because of the correlation of water and viscosity, slump is commonly used to describe how wet or flowable the concrete is. For example, a 3" slump is stiff and hard to work with while an 9" slump flows easily.

It's advantageous to limit water but it's also important to have a workable mix to get a good slab. Dragging around a 4" slump is hard work. The best way to avoid killing your finisher or having him add water the second you turn around is to order a mid-range water reducer. This chemical admixture will give you a 6" slump with less water than normally required for a 4" slump.

While you don't need high compressive strength from a structural standpoint, you do want a durable surface. As I mentioned earlier, a higher strength mix will contain more cement which will help provide a stronger surface. Most residential concrete falls in the 3000 psi range. For a hard working garage slab, I'd go with at least 4000 psi.

3/4" aggregate is typical. The smaller the aggregate, the more shrinkage so stay away from 3/8" mixes. If you really want to minimize shrinkage, you could go with 1 1/2" aggregate but it is a little harder to work with.

Air entrained concrete contains a chemical admixture that forms millions of microscopic air bubbles that help the concrete resist damage from moisture in freeze/thaw conditions. Avoid using air entrained concrete with interior, steel troweled finish especially in warm weather. Air entraining admixture slows down bleed water and if the surface is sealed with steel blades too soon, it will pool under the surface and create voids. This leads to delamination as the thin layer of concrete above the voids spalls off. Unless your finisher is experienced using it, air entrained concrete is best left to exterior applications with a broom finish.

If you are buying the concrete, call the supplier's sales dept. ahead of time to get a quote, make payment arrangements and see how much notice they need when ordering. To order concrete you'll speak to their dispatcher. Your order is going to sound something like this; "I'd like __ yards of 4000, 3/4, no air with mid-range. I'm looking for a 5-6" slump. I'd like the first truck at __ o'clock and __ min apart after that".

You can hire a testing lab to test your concrete but in my experience, low breaks are extremely rare. Keep your slumps in the proper range and you'll be fine.

Finishing - The goals here are flatness, finish and durability. The techniques used by your finisher will determine whether you achieve these goals or not. There is no substitute for experience and craftsmanship, but you need to establish your expectations. There are three things I'd say to my finisher.

• "I want my floor flat. Can you walk me through your finishing process". He should have an accurate way to establish grades such as screeds, a laser to set wet pads or preset grade points. He'll also need a way to strike off the concrete such as a straightedge or vibra-screed. I'm not a fan of jitterbugs. A bull float should be used, preferably perpendicular to the direction of the screed. Finally, the floor needs to be uniformly floated with float blades or a pan. These steps will determine flatness before the finish is applied with steel blades.

• "I want a _______ finish". It's important to tell your finisher what finish you want but this can be challenging because the terminology varies. A flat or smooth troweled surface will be smooth but without much shine whereas a burnished or burnt finish will give you a glossy surface. If you are going to epoxy or tile the floor, you'll want the former. You definitely don’t want to see any float or trowel marks and it’s a good idea to mention that you expect some hand work around the edges and penetrations.

• "I want water kept to an absolute minimum". In addition to ensuring design strength is achieved and shrinkage is minimized, controlling mix water is vital to avoiding a range of common surface defects. As the fines settle in freshly placed concrete, excess mix water rises to the surface as bleed water. Water on the surface of a slab is never a good thing but excessive water from higher slumps can create serious problems including shrinkage cracks in the surface paste (crazing), a weak, chalky surface (dusting), blisters & de-lamination. Most concrete has some bleed water and finishing should not proceed until it has dissipated. Bleed water should never be worked back into the surface.

It's a good idea to mention to your finisher to make sure he has enough help to keep up with the finishing and that you don’t want to see water splashed on the slab because it got away from him. Water applied to the surface has the same effect as excess bleed water.

Finishers can be a crusty lot and may not appreciate being questioned but if you have any doubt that your guy is willing and able to do what you want, find one who can. No matter how payment is structured, you are very likely stuck with what you get on pour day. It’s nearly impossible to fix a bad slab and the reality is that very few get ripped out and replaced. You will probably only get one shot at this so find the right guy.

Crack control - There are basically two types of cracks; structural and shrinkage. Structural cracks are caused by overloading the slab/base and are easily prevented by proper design and construction but the most common cause of cracking is shrinkage, which is impossible to prevent.

Concrete shrinks as much as 1/2" in 100' as it sets. As your slab contracts, it is restrained by the ground, obstructions, reinforcing etc., which causes tension. Since this occurs when the concrete's inherent tensile strength is very low, shrinkage cracks often result.

There are things you can do to minimize shrinkage. The most important is to limit slump. More water means more volume will be lost when the concrete dries and the more it will shrink. Placing in cooler temps, fibermesh, increasing aggregate size and proper curing will help but there is a practical limit to how big a slab can be built before shrinkage cracks become inevitable

Control joints (CJs) prevent surface cracks you can see by creating a weak spot for the slab to crack where you can't. CJ's can be tooled into the slab while the concrete is plastic or saw cut after it hardens. Both should be cut at least 1/4 of the slab depth.

By definition, tooled joints are done early in the placement but it's vital that saw cuts be done in a timely manner too. Tension starts growing in your slab quickly as drying shrinkage occurs. If you don’t relieve this tension with CJs, the slab will do it for you in the form of random surface cracks.

If your finisher will be tooling the joints, make sure his groover is deep enough. He'll have to cut the joints with a pole mounted groover while the concrete is still plastic to move the aggregate aside and re-groove to ensure the joint doesn't fill back in.

If you are cutting your joints, use an early entry or Soff-Cut saw and make your cuts immediately after finishing. These saws have a thin blade that turns into the floor and a plate that presses down on the concrete, which keeps the blade from pulling up the surface.

Plan your CJ layout in advance and provide a sketch to your finisher. Recommendations for CJ spacing start at 8-12' but with some attention to overall crack control, you can spread them out twice that. Keep the panels created by the CJs as square as possible with a maximum ratio of length to width of 1.5 to 1. If you are going to install a lift, most manufactures require a min distance from CJ's to the posts so take this into account.

When it comes to cracks, think of it this way - use fiber to minimize them, control joints to hide them and/or reinforcing to hold them together.

With all that said, control joints aren't mandatory. A lot of people find a few random surface cracks less objectionable than a grid of dirt collecting saw cuts. The crack is the same in both cases. The only difference is the aesthetics and what you would prefer to look at. Many people think a visible crack is a sign of concrete failure not realizing that there are even more cracks that they can't see in the bottom of their control joints. There's nothing wrong with just letting the slab crack where it wants especially if you take steps to minimize the amount of shrinkage and use steel reinforcing installed as high as possible to keep the cracks tight.

One final note regarding what are called re-entrant corners. This is an inside corner that projects into your slab and is virtually guaranteed to produce a crack. As the concrete shrinks, tension is concentrated at the corner as the slab tries to move in two directions making diagonal crack from the corner almost inevitable. One obvious solution is to avoid square openings in your slab if possible - a round hole has no corners to crack. About the only other thing you can do is cut a diagonal control joint to the point of the reentrant corner to hide the crack.

Curing - The goal here is simply to prevent the mix water in the concrete from evaporating prematurely. Concrete gains strength as mix water reacts with cement in a chemical process called hydration. If the mix water is lost to evaporation, the process stops. It's always a good idea to protect your slab from drying out prematurely but it's most important in warm or windy weather.

One way to cure concrete is simply to keep it moist by spraying or ponding it with water and/or by retaining moisture with poly or a curing fabric such as ConKure. The concrete doesn't use the extra water - it just seals the surface. Avoid burlap since it will stain your concrete.

There are also chemical curing compounds that you spray or roll on. They are simple to use, reasonably effective and fast - just one application and you are done. Just be aware that these products can interfere with floor coverings you may want to put down later including penetrating sealers, which are typically applied 28 days after placement.

The important thing is to start curing as soon as possible after finishing and if you are moist curing, leave it in place for at least 7 days.

Hot or Cold Weather Concrete - Most slabs are going to be placed outside in the elements. It's a patient and wise man who waits for favorable weather because there is no substitute for a calm, cool day to produce a high-quality slab. Sometimes this isn't an option, so you need a understand the risks and have a plan for mitigating them.

In hot weather, particularly if it is windy, the primary risks are crusting and excessive shrinkage cracking. Crusting occurs when the surface sets before the concrete underneath can support the weight of finishers resulting in webs of fine cracks called crazing. Crazing can also be caused by too much water on the surface, which causes shrinkage cracks in the cement paste.

Build wind breaks and use an evaporation retarder such as Confilm or Day1, which will slow down the surface set and give the underlying concrete a chance to catch up. To minimize shrinkage cracks, cut your joints and get your curing in place as soon as possible after finishing.

In cold weather the primary risk is obviously freezing. Placing when temps are below freezing is risky and best left to those with experience. Concrete will set very slowly, which will delay finishing. An accelerating admixture is a must because you’ll need to get your slab finished and covered before the surface freezes.

Your fresh concrete must be protected against freezing until it gains adequate strength to resist damage. Insulated blankets can retain the heat of hydration to a point (maybe to 25 degrees) but below that you’ll need to provide additional heat and cover to protect the concrete. Avoid unvented heaters because they can cause chalking of the surface.

You’ll ideally want to cure a concrete slab at 55 degrees for at least 72 hours before exposing it to freezing temps. Use a probe type thermometer in a drilled hole near a corner to check concrete temp.

So there you have it. This is obviously just one man’s opinion in 20,000 characters or less. There are a lot of experienced guys on here who might have something to add or might even take issue with my statements. Feel free to weigh in but if the average garage builder follows these steps, I’m confident it will result an attractive and durable garage floor. Feel free to post or PM if you have questions.

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1320stang said:

Excellent, I kept my driveway wet for a few days when we poured it and its done great. This sounds like something I can apply myself? Is there a brand I should be looking at?

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MichaelP said:

But now, as a bonus, I need to learn how to fix a large crack in the slab. It started developing 1 or 2 years after the garage was built, and by now it's, probably, 1/4"-1/2" wide and travels parallel to the entrance about 4' away from it across whole garage width (35') from the left to the right wall . It looks like the front portion of the slab sunk a bit and caused the crack. The slab does have control joints. And there is no moisture in the crack.

What can you suggest as a fix (I realize it can be temporary if the movement continues)? Maybe some kind of elastic filling material? Or just any concrete mix?

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RMW1 said:

1.) Does the radical change in thickness at the step cause a stress point that leads to cracking problems in the future? Will running the rebar under the form, as seen in the picture, solve that?

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RMW1 said:

2.) Does the raised step cause difficulty in screeding the new floor to be extremely flat? Any crown or depression more than +/- 1/4" will be noticeable on the new hard surface of the vinyl flooring.

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RMW1 said:

3.) Is it necessary to dowel the slab into the stem walls? (The original floor did not, but I doubt that was the reason for cracking.)

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RMW1 said:

4.) Are there any of you contractors in the Ventura county, California, area who want to bid on this project?

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ConCretin said:

A swing machine with a nylon pad does a good job.

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ConCretin said:

If you have control joints in your slab, they are a perfect place to split it up.

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VietGnome said:

@ConCretin
What's your take on different types of stem walls for a slab on grade? I feel like block is more labor, but simpler to do, versus forming and mono pouring with a concrete stem wall?

Do you have a take on either or, and which is usually easier/better/cheaper?

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ConCretin said:

-snip-

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VietGnome said:

Thanks! I'm trying to DIY the build, but idk if I trust myself to pour a bad, especially that big. Thinking about doing all base prep, possibly forms, and getting a company to pour. I'll get quotes for with/without mono stem wall, and if it's too much I'll just DIY with blocks.

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Overboost44 said:

I think I mentioned it your other thread, but I am in the same boat as you. 26x32 5" 4k psi. I plan on doing base prep, maybe forms, vapor barrier, insulation, rebar and pex. Others will be doing the block work. Going with block due to cost or a form wall.

@ConCretin
This thread is a wealth of information. I will have a 2 post lift so need to plan out cuts carefully. Figuring, at 12' & 24' perpendicular to the 32' side but I don't know what to do on the 26' side as the lift will be in the middle at 13'. Can I off-set and have just one at 10'? Or would it better to do one at 8'6 and another at 17' perpendicular to the 26' side?

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