Concrete overlay and resurfacing field guide for decorative and repair work

What a concrete overlay is, and when you reach for one

A concrete overlay is a thin cementitious topping bonded over existing concrete to renew the surface, fix wear, or add a decorative finish. It is not a new slab. It is a wear-and-looks layer riding on the slab you already have, anywhere from a feather edge up to roughly 5/8 in thick depending on the system. Resurfacing is the same idea by another name: you keep the structure and replace the surface.

The real decision is resurface versus replace, and it turns on the slab underneath. If the slab is sound but the surface is worn, stained, dated, or lightly spalled, an overlay renews it for a fraction of what tear-out and re-pour cost, and you keep the slab in service faster. If the slab is failing on its own, an overlay buys you nothing but a delay before the same failure comes back through the new surface.

An overlay also does not fix bad original finishing. If you are placing new concrete and want the surface right the first time, that is a finishing problem, covered in the companion guide on the flatwork finishing sequence. If the goal is a hard, ground, no-coating floor on a sound slab, that is polishing, not overlay, and the companion guide on grinding and densifying covers it. Reach for an overlay when the surface is the problem and the slab is not.

Can you resurface concrete? When the slab can take an overlay

You can resurface concrete when the existing slab is structurally sound and bondable. Overlays renew the surface. They do not fix a slab that is heaving, settling, or cracked clean through its depth, and pretending otherwise is the most expensive mistake on this kind of work.

Assess the substrate before you quote, not after you have the topping on a pallet. Sound the slab for hollow, delaminated areas. Find the cracks and decide whether they are dormant or working, because a crack that opens and closes with the seasons is a moving joint whether you like it or not. Look at what caused the wear: a slab spalling from de-icing salt and a slab heaving from a frost line or a tree root are two different problems, and only one of them is a surface problem.

Here is the honest limit. An overlay is a surface, and a surface inherits everything the slab does. Put a topping over a slab that moves and the movement comes through the topping. Put it over a slab that is structurally done and you have spent good money to repackage a failure. When the slab is the problem, tear-out is the answer, and saying so up front is what separates a contractor from a salesperson.

Why surface prep decides whether the overlay lives or dies

Surface prep is the whole job. An overlay fails when the bond fails, and the bond fails at the prep almost every time, so the work you do before the topping touches the slab matters more than the topping itself. Skip the prep and you own the delamination.

Prep is four things in order. Clean the slab down to bare, sound concrete, removing oil, grease, paint, old sealers, curing compounds, and the weak surface laitance, because anything that sits between the topping and the slab is a bond-breaker. Profile the surface so the overlay has texture to grip. Repair the defects, the spalls and the unsound spots, so you are bonding to solid concrete. Then deal with moisture, which kills bonds from below after everything on top looks perfect.

The number-one factor in that list is profile, and it is the one rookies shortchange to save a day. A clean but glass-smooth slab still fails, because the overlay has nothing to mechanically key into. The grinding and blasting equipment, the dust control, and the way you read a finished profile carry straight over from polishing prep, and the companion guide on grinding and densifying covers that gear in depth. Get the prep wrong and nothing downstream saves you.

Concrete surface profile (CSP) and how to get it

Concrete surface profile is the measured roughness of the prepared slab, and the trade specifies it on the ICRI scale of CSP 1 through CSP 10, from nearly flat to very rough. CSP comes from ICRI Guideline 310.2R, which sets the profiles and the rule that ties them to the topping. The rule a foreman carries: the thicker the overlay, the more aggressive the profile it needs.

You make profile three common ways. Diamond grinding opens a tight, light profile and is the move for thin decorative work and for cleaning off coatings and sealers. Shot blasting throws steel shot to peen a uniform profile and is the workhorse for floors and larger areas because it covers ground fast and leaves a consistent texture. Scarifying cuts deeper and harder, for heavy removal and thicker repair toppings, and it leaves a coarse profile you would not want under a thin skim.

Match the method to the topping and verify it against the manufacturer's data sheet, because the system you bought specifies the profile it is tested on. A skim or microtopping commonly wants a light profile in the CSP 2 to 4 range. Self-leveling and polymer toppings commonly call for CSP 4 to 6. Repair and high-build toppings push toward the rough end. Confirm the number the product requires rather than guessing, and check the finished slab against ICRI profile chips instead of by eye.

The bond, the bonding agent, and saturated surface dry

The bond is mechanical first: the overlay keys into the profile you cut, and the profile is why the prep matters. On top of that mechanical grip, most systems want a bonding agent, a primer or a cementitious slurry brushed or broadcast into the surface so the topping and the slab knit together instead of sitting on each other.

Saturated surface dry, SSD, is the condition that trips people. A dry, thirsty slab pulls mixing water out of the bottom of the fresh overlay and starves the bond right where it matters. You prewet the slab so it is damp through the surface but holds no standing water, no shine, no puddles in the low spots. Damp, not wet. A slab with water sitting on it dilutes the bond coat and floats the topping, which is the same failure from the other direction.

Follow the manufacturer's system on the bond coat and the timing, because primers and slurries have a working window and some must stay tacky when the topping goes down. Where bonds let go, the tell is a hollow, drummy sound under a chain drag or a hammer, and it shows up at the edges and the high-traffic field first. Poor prep and a starved or skinned-over bond coat are the two causes, and both trace back to the same hour of work nobody wanted to slow down for.

Will slab moisture ruin a concrete overlay?

Yes, slab moisture will ruin an overlay if you ignore it, and it does the damage from below after the surface looks finished. An overlay is a relatively closed layer, so water vapor rising out of the slab gets trapped under it. When that vapor pressure climbs past the bond strength, it blisters and delaminates the topping, and you find it weeks or months later, not on placement day.

Test the slab before you commit. The two common methods are ASTM F2170, an in-situ relative humidity probe that reads moisture inside the slab, and ASTM F1869, the calcium chloride test that measures the moisture vapor emission rate at the surface in pounds per 1000 square feet over 24 hours. Many overlay manufacturers cap acceptance around 3 lbs per 1000 square feet per 24 hours, but the number that governs is the one on your product's data sheet, and note that calcium chloride is not allowed on lightweight concrete.

Where moisture is high, a slab on grade with no working vapor retarder under it is the usual culprit, and the fix is a moisture mitigation coating or vapor barrier system rated for your reading before the overlay goes on. The under-slab vapor retarder and how moisture moves through a slab are their own topic worth understanding. Skip the moisture test on a slab-on-grade overlay and you are gambling the whole job on a number you never checked.

The overlay types, and which job each one is for

Overlays split into a handful of types by how they go down and how thick they are, and picking the wrong one for the job is a common and avoidable failure. The thin decorative skims, the pourable self-levelers, the thicker stamped toppings, and the sprayed textures each solve a different problem. The next sections take them one at a time. Use the table to place the job, then read the section for the type you land on.

What is a microtopping?

A microtopping is a very thin, polymer-rich cementitious skim, commonly 1/16 to 1/8 in, troweled or squeegeed over a prepared slab in two or more coats to renew the surface and carry a decorative finish. It is the thinnest overlay in regular use, and it can feather down to almost nothing at an edge.

Because it is so thin, a microtopping hides nothing and telegraphs everything. Every gouge, ridge, and trowel line in the slab under it shows through, so it demands the tightest prep and the flattest substrate of any overlay. The skill is in the trowel work, building thin even coats and burnishing to the look the customer wants, smooth and burnished or lightly textured. It takes color well, by integral pigment in the mix or by stain and dye after.

The honest weakness of a microtopping is durability under abuse. At this thickness it is a finish, not a wear slab, so it lives under a sealer and it is at home on interior floors, accent areas, and vertical work more than under truck tires or snowplow blades. Push a feather edge too thin at a doorway or a slab edge and it chips off, because there is not enough material there to hold. That feather-edge failure is the classic microtopping callback.

Self-leveling overlay for flat floors

A self-leveling overlay is a pourable, high-flow cementitious topping that spreads out and finds its own level, used to flatten and renew interior floors that are out of plane or worn. You mix it loose, pour it, and move it with a gauge rake and a smoother, and it settles into a flat field with very few trowel marks. It is the type to reach for when flatness is the goal.

Self-leveler shines where a floor has dips, old adhesive ridges, or a surface too rough for finish flooring, and it is also used as an underlayment ahead of tile, wood, or resilient goods. It builds from a thin skim up to a half inch or so in a single pour depending on the product, and high-build versions go thicker. The flow that makes it flatten also makes it unforgiving of a dam left out or a low spot in a doorway, so you plan the pour and the perimeter before you mix.

The pour is fast and the working time is short, so a self-leveler is a crew job with the mixing, the pouring, and the spreading all moving at once. Prep, primer, and moisture rules are the same hard rules as any overlay. The flow behavior and the underlayment side of self-leveling belong to the floor-prep trade, but the bond and moisture fundamentals here carry straight over.

Stamped and textured overlay

A stamped overlay is a thicker decorative topping, commonly 1/4 to 5/8 in, troweled on and then imprinted with stamping mats while it is still plastic to read as stone, brick, slate, or wood plank. It is how you put a stamped-concrete look over an existing slab without tearing it out, and it is the overlay people choose for patios, walkways, and pool surrounds that need to look like something other than concrete.

The texture comes from mats and skins pressed into the surface in a tight window, the same way stamping fresh concrete works, except you have less material and less time. You color it with integral pigment in the mix, a broadcast color hardener, and a release agent that both keeps the mats clean and leaves the secondary, antiqued color in the grout lines. Saw-cut or tooled lines define the field and double as the joints the topping needs.

Stamped overlays are thick enough to take real outdoor use, which is why they hold up on exterior flatwork better than a microtopping does. The detailed mechanics of stamping, the mat layout, the color steps, and the release work are a craft the decorative concrete trade trains for separately. The overlay-specific rules still rule: sound slab, right profile, bond coat, honored joints, and a sealer suited to the exposure.

Spray-down and knockdown texture, the pool-deck finish

A spray-down overlay is a thin, polymer-modified topping sprayed onto the slab with a hopper gun to build a textured, slip-resistant surface, most often on pool decks and patios. The gun throws the material in a splatter pattern, and that texture is the finish. It is the cheapest way to put a durable, grippy, decorative surface over a tired exterior slab.

Knockdown is the same spray with one more step. After the splatter sets up slightly, you drag a clean trowel flat across the peaks to knock them down, leaving rounded, flattened high spots and texture in the valleys. The result is grippy underfoot but smoother to sit and walk on barefoot, which is exactly why it became the standard pool-deck finish. A spray-down also runs cooler in sun than a dark, dense surface, another reason it lands around water.

The texture is functional, not just looks. The profile of a knockdown or splatter coat gives traction on a wet deck where a smooth overlay would be a slip hazard, and you can dial the aggressiveness by how much you knock down. Color goes in the mix or comes from a stain after, and the whole thing lives under an exterior sealer. Like every overlay, it bonds or it fails, so the prep underneath the spray is what carries it.

Why overlays are polymer-modified, not plain cement

Overlays are polymer-modified cement, not the plain sand-cement-water mix you would screed into a form. The polymer, usually an acrylic or a vinyl acetate co-polymer, is what lets the material bond hard to an existing surface, flex a little instead of cracking, and go down thin without falling apart. Try to feather plain cement paste over a slab and it powders off, because it has no bond and no tensile reach at that thickness.

The polymer earns its keep in three ways that matter in the field. It boosts adhesion so the topping grips the profiled slab and the bond coat. It adds flexibility and tensile strength so a thin layer survives the small movements and thermal cycling a rigid paste would crack over. And it adds water resistance, which is why the more flexible co-polymers are favored for exterior work in freeze-thaw climates. These are the properties a feather-edge overlay needs and plain cement does not have.

Treat the overlay as a manufacturer system, not a recipe. The polymer, the cement blend, the aggregate gradation, the bond coat, and the sealer are engineered to work together and tested as a set, so you do not bond Brand A topping with Brand B primer under Brand C sealer and expect the warranty or the performance to hold. Mix and match the system and you have built something nobody tested. The data sheet is the spec.

Thickness ranges, and why the type drives them

Overlay thickness runs from a feather edge up to roughly 5/8 in, and the type drives the number, not the other way around. A microtopping lives at 1/16 to 1/8 in because it is a finish. A stamped overlay needs 1/4 to 5/8 in because it has to hold an imprint and take outdoor use. A self-leveler covers a wide band depending on whether it is a skim or a high-build pour. Repair and industrial toppings go thicker still.

Thickness is not a free choice. Too thin for the type and you lose the imprint depth, the wear life, or the flow you needed. Too thick for a thin-set product and it cures wrong, cracks, or debonds because it was never formulated to be placed in a deep lift. The product is engineered for a thickness band, and placing it outside that band is the same category of error as using the wrong type.

Because every product is different, hedge the exact thickness to the manufacturer's system rather than a rule of thumb. The data sheet gives the minimum and maximum lift, the number of coats, and the coverage rate, and those govern. Use the ranges here to scope the job and pick a type, then place to the number the bag and the data sheet call for.

Interior versus exterior: freeze-thaw and UV

Interior and exterior overlays face different enemies, and the product has to be rated for the one it will see. Inside, the overlay is a floor: it fights abrasion, point loads, and the slab moisture under it, and UV and frost are not in the picture. Outside, the overlay fights weather, and weather is harder on a thin topping than traffic ever is.

Exterior is where overlays get tested. Freeze-thaw cycling drives water into the surface, freezes it, and pops the topping apart over seasons if the material is not formulated and air-entrained for it, which is why the flexible, water-resistant polymers go outside. UV fades color, so the pigments and the sealer have to be UV-stable or the deck that looked right in June looks chalky by the next summer. Drainage matters too, because standing water on an exterior overlay finds every weak spot.

Choose the product for the exposure and do not move an interior-rated material outdoors to save a trip to the supplier. An interior self-leveler has no business on a freeze-thaw pool deck, and an aggressive exterior texture is wrong under office furniture. The companion guide on finishing covers why exterior flatwork wants air entrainment in the first place, and the same logic applies to the topping that goes over it.

Color, scoring, and stencils: the decorative finishes

The decorative side of overlays is color, pattern, and texture, layered to get the look a customer is paying for. Color comes four common ways: integral pigment mixed into the topping, a dry-shake color hardener broadcast onto the fresh surface, acid stain that reacts with the cement for a mottled, variegated tone, and water-based stain or dye that lays color on more uniformly and in a wider palette. Each reads differently, and they get combined.

Pattern comes from cutting and masking. Saw-cut or tooled score lines turn a plain field into tile, plank, or ashlar layouts, and on an overlay those score lines do double duty as joints, which matters for the next section. Stencils and adhesive tape mask off grout lines and borders before a color coat or a spray-down, so you peel them to reveal crisp lines and contrasting fields. Stamping mats, covered earlier, are the third pattern tool.

The decorative finish is where overlays earn their place over tear-out, because you can put a stained, scored, or stamped surface over a sound slab that would otherwise just get replaced gray. Integral color and the chemistry of stains and dyes run deeper than this overlay-focused guide goes. Whatever the finish, it rides on the same fundamentals: a bonded, sound topping with its joints honored, sealed for its exposure.

Do you have to honor the existing control joints?

Yes. You must carry the existing control joints and any moving cracks up through the overlay, lined up over the originals, or the topping cracks over them on its own schedule. This is the one rule on overlay work that does not bend. Bridge a control joint with a continuous topping and the slab keeps moving at that joint, and the movement has to go somewhere.

The reason is simple physics. A control joint exists because the slab will move there, by design, and that movement does not stop because you put a topping over it. If the overlay is continuous across the joint, the moving slab tears the topping at that line, then the edges grind and spall and delaminate along the crack. Honoring the joint means tooling, saw-cutting, or placing a joint in the overlay directly over the slab joint so the movement has a clean place to happen.

Lay out the joints before you place, not after. Find every control joint and every working crack in the slab, mark them, and plan the overlay's joints and score lines to sit over them. On decorative work you can often hide the required joint inside a score pattern so it looks intentional. What you cannot do is ignore it. The joint you skip is the crack you come back for.

Reflective cracking: why the base telegraphs through

Reflective cracking is when a crack or joint in the base slab telegraphs up through the overlay and reappears at the surface in the same pattern. It is the most common way a technically fine overlay still fails, and it is not a material defect in the topping. It is the slab moving underneath and the topping faithfully reporting it.

The mechanism is movement at the joint or crack. The two sides of a base crack shift, horizontally as they open and close with temperature and moisture, and vertically as loads pass over a joint with poor load transfer. That movement concentrates stress in the thin topping right above the line, and the topping cracks there because that is where the strain lands. Moisture then migrates through that crack and makes the spalling and efflorescence worse right along it.

You cannot always stop reflective cracking, but you control most of it the same way: honor the joints so the planned movement has somewhere to go, and treat working cracks before you overlay rather than burying them. For a slab with a lot of active cracking, some systems use a bond-breaker or a reinforcing layer over the worst lines, but those are partial measures. The durable answer is honoring the joints, and where a crack is truly working, expecting it to come back if you cover it solid.

Sealing and the wear surface

Sealing is what protects most overlays day to day, because the topping itself is a thin decorative layer and the sealer is the wear and stain barrier over it. A microtopping or a stained finish without a sealer wears through, stains, and dulls fast. The sealer takes the abuse so the color and the surface underneath last.

Match the sealer to the exposure and the finish, the same way you match the topping. Interior floors often take a film-forming sealer or a wax-style maintenance coat that can be stripped and recoated. Exterior decks want a UV-stable, breathable sealer that sheds water without trapping vapor and without turning slick when wet, which is why deck sealers carry a slip additive. The wrong sealer can blush, peel, or trap moisture, so this is a product call, not an afterthought.

Sealing is not once and done. A sealed overlay needs resealing on a cadence that depends on traffic and weather, and the maintenance plan is part of what you hand the customer. The sealer type, the recoat schedule, and slip resistance come down to the sealer manufacturer's system and the traffic the floor will see. The short version on the job: the overlay you place is only as durable as the sealer you maintain over it.

Repair and industrial overlays: resurfacing a worn floor

Repair overlays are the functional cousins of the decorative ones: a bonded cementitious topping placed to renew a spalled, worn, or chemically attacked floor and put it back in service. Here the goal is wear life and a sound surface, not looks, so the topping is chosen for compressive and abrasion strength and placed thicker than a decorative skim. This is resurfacing as repair rather than resurfacing as finish.

Draw the line between a structural and a decorative overlay early. A decorative topping renews appearance over a sound slab. A bonded high-build repair topping restores a worn wear surface and can carry real loads when it is designed and placed as a structural element bonded to the slab. They are placed thicker, over an aggressive profile toward the rough end of the CSP scale, because the bond has to carry forklift and pallet-jack traffic, not foot traffic.

Industrial and commercial floors are the common case: a warehouse slab spalling at the joints, a plant floor worn through its wear layer, a commercial space that needs a new surface without shutting down for a tear-out. Data center and other mission-critical floors often want the no-coating hardness of polished concrete instead, and the companion guide on grinding and densifying covers that path. When the slab is sound but the surface is spent, a bonded repair topping is how you reset the wear layer.

Curing a thin overlay before it dries out

Curing a thin overlay is about keeping water in long enough for the cement to hydrate, and that is harder than on a full slab because there is so little material. A thin topping over a thirsty slab in sun and wind flashes off fast, and a topping that dries before it cures is weak, dusty, and prone to debonding. The thinner the lift, the faster it gets away from you.

Follow the manufacturer's curing instructions, because overlay curing is not always the wet-cure routine you would use on flatwork. Many polymer-modified toppings cure with the polymer film helping hold moisture and want protection from fast drying, sun, and wind rather than ponding. Some decorative systems specifically do not want a long wet cure or a curing compound, because it interferes with color, staining, or the sealer bond that comes next. The system tells you which it is.

The practical moves are the same regardless: place out of direct sun and wind where you can, protect the fresh surface, and do not let the early surface flash white and dry while the body is still green. Then hold off on the sealer until the overlay has cured and dried to the moisture condition the sealer needs, because sealing a topping that is still wet traps moisture and blushes. Rushing the cure or the seal is how a good placement turns into a callback.

How do you test if an overlay is bonded?

You test an overlay bond two ways: sounding for hollow spots, and a pull-off test for a number. Sounding is the field screen and it is fast and free. Drag a chain or tap with a hammer or a steel rod across the surface, and listen. A bonded topping rings solid and tight. A debonded or hollow area sounds drummy and dull, and that change in tone maps the delamination without a single hole drilled.

The pull-off test gives the quantified answer, and ASTM C1583 is the method. You bond a steel disk to the overlay, core or score around it down into the substrate so only the area under the disk is loaded, and pull straight up with an adhesion tester until something lets go. Two things tell the story: the force at failure, and where it broke. Failure in the body of the overlay or in the slab is a strong bond. Failure clean at the bond line is a prep problem, and a low number on a torn bond line tells you the surface was not ready when the topping went down.

Build the QC into the job, not after a complaint. Sound the finished surface as part of closeout, and where a spec or a stakes-high floor calls for it, run pull-off tests to the acceptance number the spec or the manufacturer sets. The inspector on a commercial overlay will sound the floor and look hard at the joints and the edges, because that is where bonds fail first. Find the hollow spot with a chain now, or find it with a forklift later.

What to document

Nothing about an overlay can be checked once it is down and sealed, so the day it debonds the record is all anyone has to work from. The record is what answers the question a year out: was the slab right, was the prep right, and did the product get used the way it was tested. Write it down while you are doing it, because none of it is reconstructable after the topping is on.

Capture the substrate condition and any repairs, the moisture test method and reading, the profile achieved and how, the bonding agent and overlay product with batch and data-sheet references, the thickness placed, the joints honored, the curing and sealer used, and the bond test results if you ran them. The table below is the short list. Tie each overlay type to its thickness and use so the next person, or the next phase of the same job, knows what is on the floor.

Common mistakes

• Overlaying a slab that is failing, heaving, or cracked through, when the slab needs tear-out, not a new surface.
• Shortchanging the surface prep so the topping bonds to laitance, sealer, or a too-smooth slab and delaminates.
• Cutting the wrong profile for the topping, too light for a thick overlay or too coarse under a thin skim.
• Ignoring slab moisture and skipping the test, so vapor pressure blisters the overlay weeks later.
• Bridging the existing control joints with a continuous topping and getting reflective cracking over every one.
• Picking the wrong overlay type or thickness for the job, or placing a product outside its rated lift.
• Mixing products across manufacturer systems, bonding one brand of topping with another brand of primer.
• Skipping the bond test and closing out a floor with hollow, drummy areas nobody sounded for.
• Feathering a microtopping too thin at an edge or doorway, where there is not enough material to hold.

Field checklist

Standards and references

The standards that govern overlay work cluster around surface preparation, repair, and testing, and naming the right one for the point is what an inspector listens for. ICRI Guideline 310.2R sets the concrete surface profile system, CSP 1 through CSP 10, and the practice of selecting and specifying surface preparation for coatings, polymer overlays, and repair. When you specify or verify a profile, that is the document behind it.

ACI Committee 546 covers the repair of concrete and is the reference framework for bonded repair and overlay practice. The exact guideline numbers and editions shift over time, so cite the current ACI repair documents the project adopts rather than a number from memory. Above all of it sits the overlay manufacturer's system and data sheet, which specifies the profile, the bond coat, the thickness band, the moisture limit, and the cure for the exact product you are placing. The product spec controls.

For testing, ASTM C1583 is the pull-off method for the bond strength of an overlay and the near-surface tensile strength of the substrate. ASTM F2170 measures in-situ relative humidity in the slab, and ASTM F1869 measures the moisture vapor emission rate by calcium chloride, with F1869 not permitted on lightweight concrete. Hedge the thickness and coverage to the product, state the sound-substrate rule and the honor-the-joints rule firmly, and verify every standard number against the edition the project actually adopted.

Units, terms, and conversions

Overlay work mixes a few unit systems and a lot of trade shorthand, so the same idea reads differently across a data sheet, a spec, and a job ticket. Thickness runs in inches and fractions in the field, in mils for very thin coats, and in millimeters on metric sheets. Profile is the ICRI CSP number, 1 through 10. Moisture vapor emission is pounds per 1000 square feet per 24 hours by calcium chloride, while relative humidity is a percentage inside the slab.

The terms below are the ones that carry weight on an overlay job, where a misused word leads to a misbuilt floor.

Overlay / resurfacing

A thin cementitious topping bonded over existing sound concrete to renew or decorate the surface

CSP

Concrete surface profile, the ICRI 1 to 10 roughness scale that grades the prepared slab

SSD

Saturated surface dry, the slab damp through the surface with no standing water before the bond coat

Bonding agent / bond coat

The primer or cementitious slurry that helps the topping knit to the profiled slab

MVER

Moisture vapor emission rate, slab moisture in pounds per 1000 square feet per 24 hours

Microtopping

A very thin polymer-rich skim, around 1/16 to 1/8 in, troweled for a decorative finish

Reflective cracking

A base-slab crack or joint telegraphing up through the overlay in the same pattern

Delamination

The overlay letting go of the slab, sounding hollow under a chain or hammer

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