Concrete slab curling and warping control field guide

What slab curling is, and why a curled edge wrecks a floor

Curling is the edges and corners of a slab lifting off the base, leaving them sitting high and unsupported. It happens because the top of the slab dries and shrinks faster than the bottom, so the slab bends up at its free edges, the way a wet leaf cups as it dries. The center stays down where the base holds it. The perimeter and the corners are where it lifts, because that is where the concrete is free to move.

It matters because a curled edge is no longer carried by the base. A wheel that crosses it now bends an unsupported cantilever instead of a slab on the ground, and the stress that the soil used to share goes straight into the concrete at the joint. The edge cracks, the joint spalls, the slabs rock under traffic, and the floor that read dead flat green now telegraphs every joint to a forklift mast or a thin coating. None of that is a strength problem. It is a moisture and support problem, and you fix it at the mix, the cure, and the joints, not by ordering a stronger pour.

Two things in this guide live next door to it. The slab thickness, the subgrade, and the load come from the slab-on-grade design, and the joint spacing, depth, and load transfer come from the control-joint layout. Both are covered by topic in the related material, and both feed directly into how much a slab curls.

The gradient: why the top shrinks more than the bottom

Curling is a gradient problem. The top of the slab and the bottom of the slab end up at different moisture contents, or different temperatures, and the two faces want to be different sizes. The face that shrinks more pulls the slab toward itself, and the edges lift in the direction of the relative shortening.

The usual driver is moisture. The top surface is open to the air and dries from the day it is finished, while the bottom stays in contact with a damp base and barely dries at all. The dry top shrinks, the wet bottom does not, and the slab cups upward at the edges because the top is now shorter than the bottom across the panel. That difference top to bottom is the moisture gradient, and the bigger it is, the more the slab curls.

Temperature does the same thing by a different route. When the top of the slab is cooler than the bottom, the cool top contracts relative to the warm bottom and the edges lift, the same upward cup from a thermal gradient instead of a moisture one. Outdoors this reverses through the day: a slab heated by the sun on top can curl its edges down at midday and back up at night. Indoors, on a warehouse floor, the moisture gradient is the one that drives the permanent curl you measure months later.

Curling vs warping: the terminology

The two words get used for the same thing on most jobs, and that is fine in conversation, but there is a distinction worth knowing when you read a spec or a forensic report. Some references reserve curling for the edge lift driven by a temperature gradient, top cooler than bottom, and warping for the edge lift driven by a moisture gradient, top drier than bottom. By that split, the slow permanent edge lift on an interior floor is technically warping, since it is moisture, not heat, doing the work.

In the field, and in much of the published guidance, curling covers both. The widely cited concrete-industry summary on the subject is titled around curling and describes both the moisture and the temperature gradient under that one word. We use curling here for the whole behavior and call out moisture or thermal when the difference changes what you do about it.

The reason to keep them straight is the cause points to the fix. A thermal curl that comes and goes with the day is managed differently than a moisture curl that sets in and stays. If a report calls the floor's problem warping, it is telling you the gradient is moisture, which means the mix and the cure are where you look first.

How a curled slab shows up on the floor

The first tell is a joint that rocks. Stand on a slab corner near a joint and feel it move, or watch a forklift bounce as it crosses a line that should be flat, and you are standing on a curled, unsupported edge. The slab has lifted off the base there, so it deflects under your weight where the rest of the floor does not.

Tap it. A curled edge sounds drummy and hollow when you drag a chain or tap it with a hammer, because there is now an air void under the lifted concrete instead of solid base. That hollow band runs along the joint, an inch or two in from the edge, and it marks where the slab has come off the ground.

The damage follows the lift. Cracks form parallel to the joints, just inside them, where the unsupported edge bends under load. Joint edges spall as wheels pound the high, hard corner on every pass. And the flatness drops: a floor that read flat the day it was finished measures worse a week or a month later, because the edges have curled up since. On a defined-traffic floor that loss of flatness at the joints is what the lift-truck operator feels at speed, and it is the complaint that brings everyone back to the slab.

Why does a slab curl? Drying from the top only

A slab curls because it dries from one face. The top is open to the air and gives up moisture from the moment the bleed water leaves, while the bottom sits on a base that is often as wet as the ground under it and has nowhere to dry to. So the slab loses water from the top down, the top shrinks, the bottom holds its size, and the panel cups upward at the free edges.

This is built into how a slab on grade sits. It is a thin plate drying against a damp base, which means a moisture gradient is the default condition, not a defect that something went wrong to create. Every interior slab is trying to curl to some degree. The job is to hold the gradient small enough that the curl stays under what the floor and the coating can tolerate.

Drier, windier, hotter conditions pull the top harder and steepen the gradient, which is the same evaporation pressure that drives plastic shrinkage cracking at the surface in the first hours. Here it works over weeks. The faster the top dries relative to the bottom, the more the slab curls, so anything that slows and evens the top-down drying buys you a flatter floor.

High water and high-shrinkage mixes

The more the concrete shrinks as it dries, the more it curls, so a high-shrinkage mix curls more for the same drying. Drying shrinkage is driven mostly by the water content and the paste fraction. Water you do not need leaves as the slab dries, and the space it leaves is what the concrete shrinks into. A wet, soupy mix shrinks more than a stiff one, full stop.

The single worst move is adding water at the truck to make the slab easier to finish. It raises the water-cement ratio, drops the strength, and feeds shrinkage and curling all at once. The slab finishes nicer and curls harder, and the crew that added the water is long gone by the time the edges lift.

Paste is the other lever. Concrete shrinks; aggregate does not. A mix heavy on paste and fines, light on coarse aggregate, has more of the material that actually shrinks and less of the material that restrains it, so it shrinks and curls more. A well-graded mix with the largest practical aggregate and no more water and cement than the job needs is a lower-shrinkage mix before any admixture goes in. The mix design and the water-cement ratio drive this, and that discipline is covered by topic in the related material.

Thin slabs and large panels curl more

A thin slab curls more than a thick one for the same gradient, because the same moisture difference top to bottom bends a thin plate further than a stiff one. The top-to-bottom shrinkage difference is what drives the curl, and a thinner section has less stiffness to resist the bending, so the edges lift higher.

Panel size is the other half. The further the free edge is from the center, the more lift accumulates at that edge, so a large panel curls more at its corners than a small one. Wide joint spacing means big panels means more curl at every joint. This is one of the quieter reasons the joint-spacing rule exists, and why pushing joints far apart to cut the joint count has a cost: bigger panels lift more at the edges even when they crack less in the field.

Thickness comes from the load, the subgrade, and the flexural strength in the slab-on-grade design, and joint spacing comes from the slab thickness and the shrinkage in the control-joint layout, both covered by topic in the related material. For curling, the takeaway is that the same two numbers that size the slab and lay out the joints also set how much the edges will lift, so they are a curling decision as much as a cracking one.

Poor and uneven curing

Curing is where a lot of curling is made, because curing is exactly the act of controlling how the top dries. Cure the slab badly, let the surface flash off while the bottom stays wet, and you have built the steepest possible moisture gradient on purpose. The slab cups because the top raced ahead of the bottom in drying, which is the thing curing is supposed to prevent.

Uneven curing is its own problem. A slab that gets covered with wet burlap in one bay and left bare in the next, or sheeted with plastic that lays flat in places and tents up in others, dries at different rates across the floor and curls unevenly to match. The patchwork shows up later as some joints lifted and others flat, with no pattern that makes sense until you remember how the cure went on.

Here is the hard limit worth stating plainly. You cannot cure the bottom of a slab on grade. It sits on the base and stays at whatever moisture the base gives it. All you can reach is the top, so the whole game is slowing and evening the top's drying to keep it closer to the bottom for as long as you can. That means a real cure, kept wet or sealed for the full duration the spec calls for, not a spray-and-walk. The curing method and duration are covered by topic in the related material.

The vapor retarder paradox

Here is the tension that runs through every moisture-sensitive floor. A vapor retarder directly under the slab keeps ground moisture out of the concrete and the finish above it, which any floor taking a coating or a moisture-sensitive covering needs. But that same sheet seals the bottom of the slab so it cannot dry downward, which holds the bottom wet while the top dries, steepens the moisture gradient, and makes the slab curl more. The thing that protects the floor from below is the thing that worsens the curl.

This is why the placement of the retarder was argued over for years. The old practice put a granular blotter layer on top of the retarder so the slab could lose water downward into the stone and curl less. ACI changed course around 2001, because that blotter takes on water from rain, curing, and sawcutting and then traps it under the slab with nowhere to go, which causes its own flooring failures. Current ACI 302 guidance places the retarder directly under the slab for floors getting moisture-sensitive coverings, and accepts the higher curling risk that comes with it.

So you do not solve the curl by burying the retarder back under the base where it cannot do its job. You keep the retarder under the slab where it belongs and you manage the curl the other way: a lower-shrinkage mix and a proper, even cure. The vapor retarder material, its ASTM class, and its placement are covered by topic in the slab-on-grade material. The point here is that choosing it correctly commits you to controlling curl through the mix and the cure, because the easy drying path out the bottom is gone.

Prevention: a low-shrinkage mix

The most direct way to curl less is to shrink less, and that starts in the mix design before any concrete is on the truck. Less drying shrinkage means a smaller difference between the dry top and the wet bottom, which means a flatter slab. Four levers do most of the work.

Keep the water content down. A low water-cement ratio with the workability supplied by admixtures rather than water gives you a finishable slab that shrinks less, and it raises strength along the way. Use the largest practical maximum aggregate and a well-graded blend, so more of the volume is aggregate that does not shrink and less is paste that does. Hold the paste fraction down for the same reason. And on floors where curl cannot be tolerated, a shrinkage-reducing admixture cuts the drying shrinkage directly, while shrinkage-compensating concrete using an expansive cement goes a step further by growing slightly as it cures to offset the shrinkage that follows.

For the floors where flatness is the whole job, the heavy tools come out: shrinkage-compensating or post-tensioned designs, and vacuum dewatering to pull water out of the fresh slab before it ever has to dry out. Those are design and specialty calls, not a field substitution. The everyday version is the one that matters most. Do not add water at the truck, and do not let a high-paste, high-water mix onto a floor that has to stay flat. The mix design and water-cement ratio are covered by topic in the related material.

Prevention: cure well, and cure evenly

Curing is the field control that costs the least and gets skipped the most. The goal is narrow: slow the top's drying so it stays close to the bottom, and do it evenly across the whole floor, for the full duration. A common target is seven days of moist or sealed curing, longer in cool weather where the concrete gains strength slowly, but the spec and the conditions set the real number.

Even matters as much as long. Wet cure the whole floor or seal the whole floor, not a patchwork of methods that dry at different rates and curl unevenly. Water cure, wet coverings kept wet, or a curing compound applied at the right rate and coverage all work; what does not work is a thin mist that flashes off, or plastic that tents and leaves dry stripes. If you use a compound, put down the rate the manufacturer specifies and get full coverage, because a thin or skipped patch dries faster and curls.

Accept the limit and work within it. You cannot cure the bottom, so you cannot erase the gradient, only shrink it. A slab cured hard and even on top for the full duration still curls a little. A slab left to flash-dry curls a lot. The difference between those two outcomes is a few days of attention that nobody sees until the floor is flat or it is not.

Prevention: joint spacing and where the edges are

Curl concentrates at edges, and joints make edges, so the joint plan is also a curl plan. Closer joint spacing means smaller panels, and smaller panels lift less at each corner for the same gradient, because the free edge is closer to the held-down center. The slab still curls; it just curls less per joint when the panels are smaller.

There is a real tradeoff buried in this. The general drift on heavy floors is fewer, better joints, because every joint is a discontinuity that can spall and a maintenance item for the life of the building. But fewer joints means bigger panels, and bigger panels curl more at the edges. So the move on a flatness-critical floor is not simply to space joints wide or tight; it is to pair the spacing with the mix, the cure, and the load transfer so the panels you choose do not lift more than the floor can take.

Spacing, depth, timing, and panel aspect ratio are covered in full by topic in the control-joint layout material. For curling, carry one idea across: the joint spacing you pick sets how big the panels are, and panel size sets how much each edge curls. A wide-spaced floor with a high-shrinkage mix and a poor cure is a floor that will lift hard at every joint.

Dowels and load transfer at the joints

Dowels do not stop a slab from curling. What they do is carry the load across the joint and hold the two edges level, so a curled edge that has lifted off the base still hands its load to the panel next door instead of bending an unsupported cantilever and breaking. A doweled joint can curl and still survive traffic, because the dowel takes the shear the missing base support used to take.

Smooth round dowels are the ones that work here. They have to slide so the slab can still shorten along them as it shrinks, while transferring shear across the joint. Set them in baskets, aligned square to the joint and parallel to the slab in both plan and elevation. A dowel driven in crooked locks the joint instead of transferring shear, restrains the shrinkage, and cracks the slab right at the dowel, which is worse than no dowel at all.

The other mechanism, aggregate interlock through the rough crack face below a tight sawcut, fades exactly when curling makes it matter. As a joint opens with shrinkage and the curled edge lifts, the faces lose contact and the interlock lets go, dropping load transfer right where the wheel is pounding. That is why heavy-traffic and wide-spaced joints get dowels and light close joints can lean on interlock. Load transfer detailing is covered by topic in the control-joint material; for curling, dowels are how you keep a curled edge from failing under the wheel.

Slab thickness and a thicker edge

A thicker slab curls less, because a stiffer section bends less under the same top-to-bottom shrinkage difference. Thickness is not chosen for curling, though. It comes from the load, the subgrade stiffness, and the flexural strength in the slab-on-grade design, and reducing curl is a side benefit of a slab thick enough for its load, not a reason to pour extra everywhere.

Where thickness earns its keep against curling is at the edges that take traffic. A free slab edge at a dock, an opening, or a construction joint sees far higher bending than the interior, and a curled free edge there is the worst case there is. Thickening that edge, or doweling and armoring it, gives the edge the bending capacity and the support to take the wheel even when it has lifted slightly.

Thickness, subgrade, and edge details are covered by topic in the slab-on-grade material. The curling angle on it is simple: a slab sized correctly for its load is already stiffer against curl than a thin slab pushed to its limit, and the edges that curl worst are the same edges that take the heaviest wheel loads, so spend the thickness and the load transfer where the traffic and the curl meet.

Reinforcement near the top to restrain the curl

Reinforcement can restrain curling, but only if it is in the right place. The curl pulls the top of the slab shorter than the bottom, so steel near the top of the slab restrains that shortening and holds the curl down, while steel near the bottom does little against curling because the bottom is not the face that is shrinking. This is the reverse of where you put steel to carry bending load, which sits low, so the curling job and the load job can want steel in different places.

How much it helps depends on how much steel and where. A heavy mat of steel high in the slab restrains curl meaningfully; light shrinkage-and-temperature wire does less, and wire laid on the ground does nothing at all, for curling or for crack control, because it is below the part of the slab that matters. The classic field failure is steel that was supposed to do a job lying flat on the base where it does no job.

Reinforcement type and placement, on chairs at the design height, are covered by topic in the slab-on-grade material. Treat top steel as one tool among several, hedged to the engineer's design, not a cure on its own. A mix that shrinks hard and a cure that flashes off will curl a slab past what any reasonable amount of steel will hold.

A stiff, uniform subbase leaves less room to curl

The base under the slab sets how much room the edges have to lift into. A stiff, well-compacted, uniform subbase supports the slab tightly and gives a curling edge less to move against, while a soft or uneven base lets the edges lift and rock more for the same gradient. The base does not stop curling, but it changes how much the curl shows up as a rocking, unsupported edge.

Uniformity is the part crews underrate. A base that is firm in one spot and soft in the next lets the slab curl and settle unevenly, and the rocking concentrates where the support drops away. The same consistent granular section that gives the slab the uniform support the design assumed also gives a curling edge a consistent surface to bear back down onto.

There is a real interaction with the vapor retarder. The retarder under the slab is what keeps the base from drying the slab from below, and it is what commits you to controlling curl through the mix and the cure. Subgrade and subbase preparation, compaction, and the retarder are covered by topic in the slab-on-grade material. For curling, a stiff uniform base is one more thing holding the edges down, and a soft uneven one is one more thing letting them lift.

Repairing a curled edge by grinding

Grinding takes the high, curled edge down to meet the floor, and it is the cosmetic and flatness fix. You grind the lifted concrete along the joint until the surface reads flat and a wheel rolls across without bouncing, restoring the ride and the F-numbers at that joint. For a lot of floors, where the complaint is the bounce and the lost flatness rather than a structural failure, grinding is the whole repair.

What grinding does not do is put the support back. The void under the curled edge is still there after you grind the top flush, so the edge is still unsupported, just no longer high. If the only issue was flatness, that can be fine. If wheels are pounding the joint and breaking it, grinding the high spot off without filling the void underneath leaves the edge still cantilevered over an air gap, and it can keep cracking and spalling under load.

The trap is treating grinding as the cause fix. Grind a floor flat while the mix is still drying and the slab is still curling and you will be back, because the edges keep lifting after you grind them. Grind once the slab has mostly finished its drying shrinkage, and on a floor that takes real traffic, pair the grind with filling the void so the edge is both flat and supported.

Undersealing, slab jacking, and stitching the joint

When the problem is the void under the curled edge, you fill it. Undersealing, also called subsealing or pressure grouting, restores support by injecting grout through holes cored along the joint to fill the gap under the lifted slab, so the edge bears on something solid again instead of cantilevering over air. On a slab-curl repair the common sequence is to core a line of holes along the control joints, pump grout to fill the void below the curled portion, and then grind the top flush, so the edge ends up both supported and flat.

Slab jacking is the same idea pushed further. Pumping grout or a structural polyurethane foam under the slab under pressure can lift a settled or curled edge back up to the elevation of the slab next door, raising the panel until the surface is flush. It is the move where the edge has dropped or the panels have faulted, not just curled, and it raises the slab without tearing it out.

Where the curled edge has already cracked the slab parallel to the joint, the crack itself gets stitched. Stitching sets U-shaped metal staples across the crack, embedded into the concrete on both sides, to tie the two sides together and keep the crack from widening under continued traffic. Stitching, undersealing, and grinding often go together on one joint: fill the void, lift or stitch as needed, then grind flat. The repair that lasts addresses the support and the crack, not just the high spot you can see.

From curled edge to joint spall under the wheel

Joint spalling on a traffic floor usually traces back to curl. The curled, unsupported joint edge sits slightly high and has an air void beneath it, so when a hard-wheeled forklift crosses it, the wheel slams the unsupported corner down on every pass and the concrete breaks away at the joint. The spall starts small, the gap widens, more of the edge loses support, and the deterioration feeds itself.

This is why the worst joint damage on a distribution floor is at the joints the lift trucks cross most, not the quiet aisles. Constant hard-wheeled traffic across a curled edge is the specific load case that turns a flatness defect into a structural one. The wheel is not breaking sound, supported concrete; it is breaking a lifted, hollow corner.

Two things fight it, and they line up with the prevention and the repair already covered. Control the curl at the source so the edges do not lift, through the mix, the cure, and reasonable panel sizes. Then protect the joints that take the traffic: fill them with a semi-rigid filler that supports the edges so the wheel rolls across the filler instead of the bare corner, dowel them so the load transfers across, and armor the heaviest joints with steel angle so the wheel hits steel, not concrete. Joint filling and armoring are covered by topic in the control-joint material. The chain to remember runs one way: curl makes the unsupported edge, the wheel finds it, the joint spalls.

How do you measure slab curling?

The quick field measure is a straightedge. Lay a long straightedge across a joint and measure the gap under it at the edge with a feeler gauge or a ruler, and you have the height the edge has lifted. Walk the joints and you can map where the floor has curled and how badly, which is enough to decide what needs grinding or undersealing.

The formal measure is the F-numbers, FF for flatness and FL for levelness, taken under ASTM E1155 with a profiling device that reads the surface in close steps. FF captures the bumpiness over short distances, the ride quality a wheel feels; FL captures the overall tilt. Higher numbers are flatter and more level, and on a defined-traffic floor the spec often calls out flatness in the wheel paths specifically.

Timing is the catch, and it is where curling and F-numbers collide. The flatness numbers are meant to be taken within about 72 hours of placement, before the slab curls, because curling lowers the numbers after the fact. A floor that passed flat green can read worse a week later purely from curl, with no change in the finishing. That is a real source of disputes: the floor met the flatness spec as finished, then curled, and the late reading fails. Measure inside the window to judge the finish, and understand that a worse late number is often the curl talking, not the trowel. The F-numbers and the tolerances tie back to ACI 117, and floor flatness is covered by topic in the slab-on-grade material.

Curling on warehouse, distribution, and data center floors

Curling is one of the most common complaints on industrial floors, and it is worst exactly where the floors work hardest. A distribution center runs material-handling equipment across the joints all day, and a curled, unsupported joint edge is what makes the equipment bounce, beats the joints into spalling, and throws off the flatness in the wheel paths that the operation depends on. The bigger the panels and the heavier the traffic, the more the curl costs.

These floors get the most engineering attention on the slab for that reason, and the answers line up with everything above. Low-shrinkage mixes to curl less, real even curing, panel sizes paired with the mix and the load transfer, dowels not just interlock at the traffic joints, semi-rigid filler to support the edges, and armored joints on the heaviest aisles. Flatness and joint condition are tied together, because a curled or spalled joint is a flatness defect a wheel feels at speed, and on a very-narrow-aisle floor a small lift at the wheels becomes a large sway at the top of a tall mast.

Data center and heavy equipment floors push it further, because they carry concentrated rack and equipment loads on small footprints and demand tight flatness for the install moves and the equipment. The same curl that is a nuisance in a light warehouse is a real problem where a loaded rack rolls across a lifted joint during a build. The fix is the same chain of mix, cure, joints, and load transfer, sized to the load. The heavy-slab and flatness picture these floors sit inside is covered by topic in the slab-on-grade material.

What to document

Curling shows up months after the crew is gone, so the record is what tells you whether the slab was built to control it or set up to curl. When an edge lifts and a joint spalls, the question is always whether the mix, the cure, and the joints were right, and that question only gets answered if someone wrote down what happened at the pour.

Capture the things that drive the gradient and the lift: the mix and the water-cement ratio, whether water was added at the truck, the curing method and how long it ran, the vapor retarder and its placement, the slab thickness, the joint spacing and panel sizes, the load transfer at the joints, and the early flatness reading taken inside the measurement window. If the floor is later measured and the flatness dropped, that early number is what proves the slab was finished flat and curled afterward, which changes who owns the problem.

Common mistakes

• Running a high-shrinkage mix, or adding water at the truck, on a floor that has to stay flat.
• Curing poorly or unevenly, so the top flashes off and builds a steep moisture gradient.
• Pouring slabs too thin or laying out panels too large, so the edges lift hard at every joint.
• Leaving traffic and wide-spaced joints with no dowels, so the curled edge breaks under the wheel.
• Grinding the curled edge flat without filling the void or fixing the cause, so it lifts and spalls again.
• Burying the vapor retarder under the base to dodge curl, trading a flooring failure for a flatness one.
• Ignoring the moisture and thermal gradient entirely and blaming the curl on weak concrete.

Field checklist

Curling is decided across the whole pour, from the mix on the truck to the cure that runs for days after. Work the chain that controls the gradient and the edges, in order, and write down what you did so a late flatness reading has something to stand on.

Standards and references

The project specification and the structural engineer of record govern the mix, the cure, the joints, and the flatness tolerance. Everything below is the framework those documents are built on, and where the contract is stricter, it wins.

ACI 302, the guide for concrete floor and slab construction, covers floor construction, finishing, curing, and the vapor retarder placement guidance that moved the retarder directly under the slab. ACI 360, the guide for design of slabs on ground, covers the design that sets thickness, joint spacing, and load transfer, and both documents address the factors that cause curling and the steps to reduce it. ACI 117, the specification for tolerances, carries the flatness and levelness tolerances, and floor flatness and levelness are measured under ASTM E1155 by the FF and FL F-numbers. The mix design that controls shrinkage and the curing standard that controls the gradient sit alongside these, covered by topic in the related material.

The exact document numbers and their contents shift between editions, so confirm the editions the project and the jurisdiction have actually adopted before you cite a specific provision. Hold the rules of thumb in this guide as starting points, not mandates: the joint spacing, the shrinkage limits, and the flatness tolerances all defer to ACI and to the engineer's design. The constants worth carrying are the ones the physics enforces. The gradient causes the curl, a low-shrinkage mix and an even cure shrink the gradient, and dowels and joints keep the curled edge from failing under load.

Units, terms, and conversions

Curling work mixes a few naming systems across the forensic report, the mix submittal, and the flatness spec, so the same idea reads differently depending on the page.

Edge lift is measured in inches in the US and millimeters in metric, read off a straightedge with a feeler gauge. Drying shrinkage is given as a strain, often in microstrain or as a length change per length, and it tracks the water content and the paste fraction. Floor flatness and levelness carry no units; they are the dimensionless FF and FL F-numbers under ASTM E1155, tied to the tolerances in ACI 117. Slab thickness and joint spacing are inches and feet here, millimeters and meters in metric drawings, where a 6 in slab is about 150 mm.

Curling

Edges and corners of a slab lifting off the base from a moisture or temperature gradient top to bottom

Warping

Often used the same as curling; some references reserve it for the moisture-gradient edge lift

Moisture gradient

The difference in moisture content between the drier top and the wetter bottom that drives the curl

Drying shrinkage

The volume loss as concrete dries, driven by water content and paste, that the curl is built from

Vapor retarder

Sheet under the slab that blocks ground moisture but seals the bottom and can worsen curling

Dowel

A smooth steel bar transferring shear across a joint so a curled edge stays carried under load

FF / FL

Floor flatness and levelness F-numbers measured under ASTM E1155, lowered by curling after placement

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