Why Is My Concrete Cracking – Causes, Prevention and Repair Tips

Is It Normal for Concrete to Crack?

Concrete is a brittle, cementitious material. It changes volume as it hydrates and dries, and that volume change often gets restrained by reinforcement, subgrade friction, adjoining slabs, or the concrete’s own geometry.

Restraint against volume change creates tensile stress inside the concrete. Concrete has limited tensile capacity compared to its compressive strength, so cracks can develop even in well-designed, properly built concrete.

How to Tell If a Concrete Crack Is Serious

Crack width alone doesn’t tell the whole story. Work through these factors together for a more complete picture:

• Width: measure it, and note whether it varies along its length
• Depth: does it appear to be a superficial surface crack, or does it seem to run through the slab?
• Vertical displacement: is one side of the crack higher than the other?
• Horizontal movement: has the crack visibly opened wider over time?
• Growth: is it getting longer or wider, or has it stayed the same?
• Location: is it in the middle of a slab, at a corner, along a control joint, near a wall, in a foundation, or around a column?
• Water: is water entering through the crack?
• Structural symptoms nearby: leaning walls, sticking doors, uneven floors, settlement, bulging, spalling, or exposed reinforcement

Looking at these factors together gives a far more reliable read on a crack than width by itself.

Concrete Crack Severity: A Practical Screening Guide

There is no arbitrary width, such as one-eighth of an inch, that universally separates cosmetic from structural cracking. Location, movement, and accompanying symptoms matter more than a single measurement.

What Does the Crack Pattern Tell You?

• Random hairline cracks: possible shrinkage
• Map-like surface cracking: possible crazing from surface drying or finishing issues
• Long straight crack: possible shrinkage, joint layout, or restraint
• Crack from a re-entrant corner: stress concentration at an inside corner or opening
• Diagonal crack: possible settlement, geometry, or stress concentration
• Offset slab sections: possible differential settlement or heave
• Crack parallel to an edge: possible edge or support issue
• Cracks around reinforcement: possible settlement or reinforcement-related issues
• Horizontal wall crack: possible lateral pressure or wall movement

The Main Causes of Concrete Cracking

FHWA’s crack-cause literature groups causes into plastic shrinkage, settlement, drying shrinkage, thermal stresses, chemical reactions, weathering, corrosion of reinforcement, poor construction practices, construction overloads, design and detailing errors, and externally applied loads. This guide walks through each of these in turn.

Plastic Shrinkage Cracks

Plastic shrinkage cracking occurs while concrete is still plastic, meaning it hasn’t reached final set. The surface loses water rapidly, and when the rate of surface evaporation exceeds the rate at which bleed water can replace it, the surface layer shrinks while the concrete beneath restrains it.

That restraint creates tensile stress in a weak, still-forming surface layer that has little bond strength yet, and cracking results. According to FHWA’s crack research, plastic shrinkage typically appears within roughly 30 minutes to 6 hours after placement, well before the concrete has hardened.

These cracks look shallow, appear early, and are typically random or roughly parallel, most commonly on slabs and pavements.

What Causes Plastic Shrinkage Cracking?

ACI’s cracking guidance identifies air and concrete temperature, relative humidity, and wind velocity at the concrete surface as the key factors driving the rate of evaporation. Several site conditions combine to increase risk:

• High evaporation rates driven by combined heat, wind, and low humidity
• Direct wind exposure across the fresh surface
• Low ambient or relative humidity
• Solar radiation heating the surface
• Elevated concrete temperature at placement
• Dry, absorptive subgrade drawing moisture from the mix
• Delayed start of curing after finishing

Don’t treat any single temperature or humidity number as a universal cutoff. Risk builds from the combination of conditions present at placement, not one factor alone.

How to Prevent Plastic Shrinkage Cracking

• Monitor weather conditions, including temperature, humidity, and wind, before and during placement
• Take steps to reduce the surface evaporation rate when conditions are severe
• Plan placement timing to avoid the most extreme heat, wind, or dryness where practical
• Protect fresh concrete from wind and direct sun with windbreaks or sunshades where appropriate
• Use mixture adjustments suited to the conditions, as recommended by the concrete supplier or specifier
• Begin curing as soon as the surface can tolerate it without damage
• Use evaporation-control methods, such as evaporation retarders or fogging, when conditions warrant them

ACI’s hot-weather concreting guidance addresses concrete temperature, evaporation, placement, finishing, curing, and protection together as a connected system, rather than treating any one step in isolation.

Plastic Settlement Cracks

Plastic settlement cracks are a distinct mechanism from plastic shrinkage, even though both occur before the concrete hardens. As fresh concrete settles under its own weight, solid particles move downward while some of the mixture’s water rises to the surface as bleed water.

When that settlement is restrained locally, by reinforcement, forms, embedded objects, or a change in section thickness, the concrete above the restraint can’t settle as much as the surrounding material. This difference in settlement produces a crack directly over or beside the restraint.

ACI’s guidance associates settlement cracking with factors including reinforcement placement, slump, cover depth, consolidation, and form conditions. According to FHWA’s timing data, plastic settlement cracks typically appear roughly 10 minutes to 3 hours after placement, generally earlier than plastic shrinkage cracks.

Drying Shrinkage Cracks

Drying shrinkage is one of the most common causes of cracking in hardened concrete. As the cement paste loses moisture over time, the paste itself can shrink by as much as 1 percent in volume.

Aggregate within the mix provides internal restraint that reduces this effect substantially, down to roughly 0.06 percent volume change for typical concrete, according to ACI’s cracking guidance. Even that reduced shrinkage generates tensile stress wherever the concrete is restrained from moving freely, whether by subgrade friction, adjoining construction, or reinforcement.

When that stress exceeds the concrete’s tensile capacity, a crack forms. Drying shrinkage cracks typically don’t appear until weeks or months after placement, and they’re strongly influenced by mixture water content, cement paste volume, aggregate characteristics, slab geometry, restraint conditions, curing quality, and joint layout.

Why Concrete Shrinks as It Dries

Fresh concrete contains more water than is chemically needed for cement hydration. As the excess water evaporates from the hardened paste over time, the microscopic pore structure loses moisture and the paste contracts slightly.

This is a different process from curing, which refers to maintaining adequate moisture and temperature during early strength development. Drying shrinkage continues over a much longer timeframe, as the concrete gradually equilibrates with the surrounding environment’s humidity.

Thermal Cracking in Concrete

Concrete expands when heated and contracts when cooled, just like most materials. When that expansion or contraction is restrained, whether by adjoining structure, subgrade friction, or the concrete’s own mass, tensile stress develops and cracking can result.

Temperature gradients within a single concrete element, meaning one part is warmer than another, can also produce internal stress and cracking even without overall restraint. According to FHWA’s crack-timing research, thermal cracking related to heat of hydration and temperature gradients commonly appears from about 1 day to 2 to 3 weeks after placement.

Larger temperature swings, heavily restrained slabs, mass concrete elements that generate significant internal heat during curing, and rapid cooling all increase thermal cracking risk.

Freeze-Thaw Cracking and Surface Damage

When water within concrete’s pore structure freezes, it expands and creates internal pressure. Repeated freezing and thawing cycles can progressively deteriorate concrete that isn’t adequately protected, leading to cracking alongside surface scaling and spalling.

Air-entrained concrete, meaning concrete with intentionally incorporated microscopic air bubbles, is important for freeze-thaw durability. Those air voids give freezing water somewhere to expand into without generating damaging pressure in the surrounding paste, according to ACI’s guidance on freeze-thaw resistance.

Settlement Cracks from Soil or Base Movement

When the ground beneath a slab moves, the concrete above it often has no choice but to follow. Common causes include:

• Poorly compacted fill placed beneath the slab during construction
• Erosion of supporting soil, often from water movement
• Drainage problems that saturate or wash out the subgrade
• Expansive soils that shrink and swell with moisture changes
• Organic material left in the subgrade that decomposes over time
• Voids that develop beneath the slab from any of the above
• Changes in groundwater level
• Tree or root effects in some situations

How Drainage Causes Concrete Cracking

Water management around a slab is closely tied to its long-term crack performance. Common drainage-related contributors include:

• Downspouts discharging directly next to or under a slab edge
• Standing water that repeatedly saturates the subgrade
• Water flowing or seeping beneath the slab from an unrelated source
• Erosion of fine material from beneath the slab
• Chronically saturated soils that lose bearing capacity
• Freeze-thaw cycling in soil that stays wet
• Poor site grading that channels water toward the slab instead of away from it

For a complete discussion of preparing the ground beneath a slab, including drainage, see How to Prepare Soil for a Concrete Slab.

Cracking from Heavy Loads and Overloading

Concrete slabs are designed for a specific range of loads. Exceeding that range, even briefly, can crack a slab that would otherwise perform fine under its intended use.

Common overload sources include construction dumpsters, RVs and trailers parked in one spot for extended periods, heavy trucks, construction equipment, concentrated point loads from stacked materials, and equipment outriggers that focus enormous force on a small area.

Cracking from Poor Concrete Mix or Excess Water

Problems with the concrete mixture itself can increase cracking risk independent of site conditions. Contributing factors include excessive mixing water beyond the mix design, an inappropriate mixture for the application, poor-quality or poorly graded aggregate, improper admixture use or dosing, retempering with added water after initial mixing, and inadequate control of slump at delivery.

Avoid treating any specific number, such as a fixed strength loss per gallon of added water, as a universal rule. The actual effect depends on the mix design and how much water is added relative to the batch.

How Poor Finishing Causes Concrete Cracks

Finishing technique has a real, and often underappreciated, effect on cracking risk. Problems that commonly contribute include finishing while bleed water is still present on the surface, overworking the surface with excessive troweling, adding water to the surface to ease finishing, dusting dry cement onto the surface to absorb moisture, finishing too early before the concrete is ready, and other improper surface manipulation.

Each of these practices can weaken the surface layer, separate it from the concrete beneath, or trap excess water near the surface. That weakened layer is prone to crazing, reduced surface strength, scaling, and surface cracking that may not become visible until later.

How Poor Curing Causes Concrete Cracking

Curing, meaning maintaining adequate moisture and favorable temperature after placement, is essential to both strength development and crack resistance. Concrete that loses moisture too early, especially in hot, dry, or windy weather, is prone to plastic shrinkage cracking and reduced surface durability.

Cold weather curing brings its own risks, since low temperatures slow hydration and can leave concrete vulnerable to early-age damage. ACI’s concrete construction guidance treats curing and protection as essential components of both developing adequate concrete properties and preventing damage during the vulnerable early-age period.

For curing timing and methods across different conditions, see the Concrete Curing and Drying Time Guide and Concrete Curing Time Chart.

Control Joints: Why Concrete Cracks Where It Does

You cannot eliminate shrinkage in concrete. What you can do is give that shrinkage a planned, controlled location to occur, rather than letting it crack randomly across a slab.

A control or contraction joint is a deliberately weakened plane, typically created by a saw cut or tooled groove, that encourages the slab to crack along that line rather than elsewhere. ACI’s floor and slab construction guidance describes contraction joints as forming a weakened plane below which cracking is expected to occur, with joint spacing depending on multiple design and environmental factors rather than one fixed rule.

Joint depth, timing of the cut relative to placement, and overall panel geometry all affect how well the joint actually controls the crack.

Control Joints vs. Expansion, Isolation and Construction Joints

Corner Cracks and Re-Entrant Corners

Cracks frequently originate at inside corners, openings, drains, posts, columns, and any location where slab geometry changes abruptly. These re-entrant corners concentrate stress in a way that a simple rectangular slab area doesn’t experience.

This is a well-recognized geometric effect rather than evidence of a material defect. A crack radiating from an inside corner of an L-shaped patio, for example, is a common and largely predictable outcome of that geometry.

Cracks Around Rebar or Embedded Elements

Cracking that follows the line of reinforcement or other embedded elements can have more than one explanation. Possible causes include plastic settlement around the bar during placement, insufficient consolidation leaving voids near the reinforcement, insufficient concrete cover over the steel, restraint effects from the embedded element, or, in older concrete, reinforcement corrosion.

Concrete Cracking from Reinforcement Corrosion

Reinforcement corrosion is a genuinely different mechanism from the shrinkage and settlement cracks discussed above, and it deserves separate treatment. Steel embedded in concrete is normally protected by a passive oxide layer, but that protection can break down through two primary pathways.

Carbonation occurs when atmospheric carbon dioxide reacts with alkaline compounds in the cement paste, gradually lowering the concrete’s pH until the steel’s passive layer is no longer stable. Chloride-induced corrosion occurs when chloride ions, commonly from deicing salts or marine exposure, penetrate the concrete and reach the steel surface, breaking down the passive layer even without a broad pH change.

Once active corrosion begins, the resulting iron oxide corrosion products occupy several times the volume of the original steel, generating expansive pressure around the bar. That pressure creates tensile hoop stresses in the surrounding concrete, and once those stresses exceed the concrete’s tensile capacity, cracking and eventually spalling occur along the line of the reinforcement.

This differs fundamentally from a shrinkage crack: it’s driven by a chemical and electrochemical process that progresses over time, and simply filling the visible crack does nothing to stop the ongoing corrosion beneath it.

Chemical and Environmental Causes of Concrete Cracking

Beyond corrosion, several other chemical and environmental mechanisms can produce cracking, particularly in older concrete:

• Alkali-silica reaction (ASR): a chemical reaction between certain reactive aggregates and alkalis in the cement paste, producing an expansive gel that can crack the surrounding concrete, sometimes visible as a gel exudate within the cracks
• Sulfate-related deterioration: chemical attack from sulfates in soil or groundwater that can expand and crack concrete over time
• Freeze-thaw deterioration: discussed earlier in this guide, a physical rather than chemical mechanism, but often grouped with weathering-related distress
• Corrosion of reinforcement: discussed in the previous section
• Aggressive environmental exposure: combinations of moisture, chemicals, and temperature cycling that accelerate other deterioration mechanisms

These mechanisms are distinct from ordinary shrinkage cracking, and they generally require a different diagnostic and repair approach since the underlying material itself is deteriorating, not simply responding to normal volume change.

How to Diagnose a Concrete Crack Yourself

1. Measure the width at a few points along its length.
2. Check whether both sides are level with each other, or if one side sits higher.
3. Check the depth, as best you can tell, superficial versus appearing to run through the slab.
4. Check the pattern: random, straight, diagonal, map-like, or following an edge or embedded element.
5. Check the location: mid-slab, corner, joint, wall, foundation, or around a column.
6. Look for water entering through or near the crack.
7. Look at drainage around the slab, including downspouts and grading.
8. Check whether it’s changing compared to when you first noticed it.
9. Check surrounding concrete for related cracks, spalling, or staining.
10. Photograph and document it, including a ruler or coin for scale.

How to Monitor a Concrete Crack

For cracks that don’t require immediate professional attention but warrant watching, a simple monitoring routine works well:

• Photograph the crack clearly, including something for scale
• Note the date
• Measure the width at a few marked points
• Mark the visible ends of the crack so you can tell if it extends further
• Re-measure periodically, and specifically after heavy rainfall or major seasonal temperature changes

For cracks that appear structurally significant, professional monitoring methods, such as crack gauges or survey-grade measurements, provide more reliable data than homeowner measurements alone.

When Should You Call a Professional?

There is no single crack width that automatically requires an engineer. These behavioral and locational signs are more reliable indicators than a measurement alone.

Can You Repair a Cracked Concrete Slab?

Often, yes, but the right approach depends on the cause, whether the crack is still actively moving, its depth, its location, whether it affects a structural element, and the overall condition of the slab. Potential options range widely:

• Leave it and monitor, for very minor stable cracks
• Seal it to prevent water intrusion and debris accumulation
• Use a flexible crack filler suited to cracks that may continue to move slightly
• Epoxy injection, where appropriate for the crack type and structural context
• Routing and sealing, widening the crack slightly to accept a proper sealant
• Slab lifting, when the underlying cause is loss of support rather than the concrete itself
• Partial replacement, for localized severe damage
• Full replacement, when the slab is too deteriorated to repair economically
• Structural repair, for cracks affecting load-bearing elements

For general crack repair procedures, see How to Fix Cracked Concrete. For large driveway-specific cracks, see How to Repair Large Cracks in a Concrete Driveway.

How to Repair Hairline Concrete Cracks

For stable, nonstructural surface cracks, a straightforward approach generally works:

1. Clean the crack of debris, dust, and loose material.
2. Dry the area completely before applying any product.
3. Select a compatible product designed for the crack width and your surface type.
4. Apply according to the manufacturer’s instructions, not a generic online method.
5. Monitor the repair over time to confirm it holds and the crack doesn’t reopen or extend.

Don’t promise yourself, or a client, that a hairline crack repair is permanent. Even well-executed cosmetic repairs can be affected by future movement if the underlying cause wasn’t addressed.

How to Repair Wider or Moving Cracks

Rigid crack fillers can fail when the crack continues to move, since a rigid material can’t accommodate ongoing expansion, contraction, or settlement. For cracks that show signs of continued movement, consider a flexible sealant designed to accommodate some movement, routing and sealing to create a properly shaped joint for the sealant, and addressing any underlying drainage or settlement issue driving the movement.

If movement is significant or clearly structural in nature, professional evaluation should happen before any repair, since the repair method needs to match the actual cause, not just the crack’s current appearance.

When Concrete Crack Repair Is Not Enough

Concrete Resurfacing vs. Crack Repair vs. Replacement

Resurfacing only makes sense over a structurally sound substrate. Resurfacing over an unstable or actively cracking base simply telegraphs the same problems through the new surface.

How to Prevent Concrete Cracks Before Pouring

• Use a proper structural or slab design suited to the intended loads
• Select a suitable mixture for the application and exposure conditions
• Control water content to the mix design’s specified proportions
• Use quality, well-graded aggregate
• Prepare a sound, uniform subgrade
• Address drainage before placing concrete
• Use the correct slab thickness for the intended use
• Include appropriate reinforcement where the design calls for it
• Plan correct joint layout and spacing before placement
• Follow proper placement practices, avoiding segregation and excessive drop heights
• Finish at the correct time, not too early and not too late
• Cure properly for the specified duration and conditions
• Plan around weather conditions rather than ignoring them
• Plan for the actual loads the slab will experience, not just its nominal use

Concrete Crack Prevention by Cause

Common Concrete Cracking Mistakes

• Assuming every crack is a failure requiring immediate replacement
• Assuming every crack is harmless without checking for warning signs
• Repairing a crack without first diagnosing its cause
• Adding water to concrete during placement or finishing
• Skipping or shortening proper curing
• Poor joint layout or spacing at the design stage
• Cutting control joints too late after placement
• Treating expansion and contraction joints as interchangeable
• Ignoring drainage problems around the slab
• Pouring over poorly prepared or uncompacted support
• Assuming rebar prevents all cracking
• Assuming higher PSI concrete eliminates cracking risk
• Using rigid repair material on a crack that’s still actively moving
• Resurfacing a structurally unsound slab instead of addressing the underlying problem

Concrete Crack Repair Cost

Rather than a single nationwide figure, crack repair cost depends on several project-specific drivers: crack length, width and depth, the repair material selected, site accessibility, the underlying cause, whether slab lifting is required, whether replacement is more appropriate than repair, whether professional engineering evaluation is needed, and whether drainage correction is part of the scope.

For a project-specific estimate, use the Concrete Driveway Repair Cost Calculator.

Concrete Cracking: Quick Decision Guide

• Fine, stable, and level: usually monitor or seal
• Fine surface web pattern: evaluate the finishing and curing history
• Crack plus water intrusion: investigate drainage
• Crack plus settlement: fix the underlying support problem
• Crack plus visible movement: investigate the active cause
• Crack plus exposed steel: professional evaluation
• Foundation wall plus significant movement: professional evaluation

Concrete Cracking FAQs