What Is Concrete Spalling, and What Causes It?

The chips of concrete on the floor of a parking garage, the rust-stained flaking on a bridge abutment, the rough, pitted surface of a loading dock slab that started looking fine five years ago: all of these are concrete spalling. And in every case, the visible surface damage is a symptom of something happening within or beneath the concrete itself.

Concrete spalling is one of the most common and consequential deterioration mechanisms affecting commercial and institutional structures.

What Is Concrete Spalling?

Concrete spalling refers to the breaking away, flaking, chipping, or delamination of concrete from a surface. It can range from shallow surface scaling affecting only the paste-rich top layer, to deep spalling that exposes or undercuts embedded reinforcing steel, to explosive spalling in fire-exposed elements, where internal steam pressure suddenly fractures large sections of concrete.

In engineering contexts, spalling refers to the separation of material from a surface into thin layers, plates, or irregular fragments. It is distinguished from:

• Cracking: Fractures through the concrete that may or may not result in material loss
• Delamination: Separation of a near-surface layer from the substrate, often without visible surface break initially
• Popouts: Localized conical voids caused by expansive aggregate particles near the surface
• Scaling: Progressive loss of the surface mortar layer, often in thin flakes, typically from freeze-thaw or chemical attack

In practice, multiple forms of deterioration often coexist. A parking deck may show corrosion-induced spalling at column bases, freeze-thaw scaling on the top deck, and delamination at waterproofing membrane interfaces, all on the same structure.

The Six Primary Causes of Concrete Spalling

Concrete does not spall randomly. Each mode of spalling has a specific mechanism, characteristic visual pattern, and implication for structural condition. The table below summarizes the primary causes and their distinguishing features. NOVA's construction materials testing and forensic engineering teams use this diagnostic framework when assessing deteriorating structures.

Cause	Mechanism	Typical Locations	Visual Indicators
Corrosion of Reinforcement	Water and chlorides reach embedded steel; rust expands and fractures concrete cover	Parking decks, coastal buildings, bridge structures	Delamination, map cracking, rust staining, cover loss
Freeze-Thaw Cycling	Water in concrete pores freezes, expands 9%, creates internal tensile stress	Northern climates; any structure exposed to moisture and temperature swings	Surface pitting, flaking, popouts; progressive layer loss
Alkali-Silica Reaction (ASR)	Reactive silica in aggregates reacts with alkalis in cement paste; gel absorbs water and expands	Long-term; common in older structures with reactive aggregates	Map cracking, gel deposits, large-scale surface pattern cracking
Fire Damage	Rapid thermal gradient causes explosive spalling; moisture converts to steam, fractures concrete	Any structure exposed to fire or extreme heat	Deep spalling, exposed rebar, loss of section; structural capacity reduction
Poor Construction Practice	Low w/c ratio, inadequate cover depth, improper curing, premature loading	Any age structure; often apparent within first years of service	Surface scaling, early delamination, inadequate strength at break
Chemical Attack	Acids, sulfates, chlorides, carbonation degrade the cement paste matrix	Industrial facilities, wastewater structures, coastal exposure	Progressive surface loss, soft or friable concrete, discoloration

1. Corrosion of Embedded Reinforcement

By volume, corrosion-induced spalling is the most consequential deterioration mechanism affecting reinforced concrete structures in commercial practice. When chlorides or carbonation penetrate the concrete cover and reach the embedded reinforcing steel, the steel begins to corrode. Rust products occupy three to six times the volume of the original steel, generating expansive pressure within the surrounding concrete. That pressure fractures the cover concrete, producing the characteristic delamination and spalling pattern seen on parking decks, coastal structures, and bridge elements.

The critical insight is that visible spalling represents a late stage of a process that began years or decades earlier. By the time concrete is falling, the steel beneath has already lost significant section area, and the structural capacity of the affected members may be meaningfully reduced. This is why a forensic condition assessment on a spalling parking structure is not just an aesthetic exercise; it is a structural engineering investigation.

2. Freeze-Thaw Cycling

Concrete is porous. Water that enters the pore network and freezes expands by approximately 9 percent, generating internal tensile stress that the concrete must resist. In climates with repeated freeze-thaw cycles, this cyclic expansion and contraction progressively fractures the cement paste-aggregate bond at the surface, producing the characteristic pitting, flaking, and pop-out pattern of freeze-thaw damage.

Air entrainment, achieved with air-entraining admixtures, provides microscopic voids within the concrete matrix that accommodate ice expansion and dramatically improve freeze-thaw resistance. Concrete placed without adequate air entrainment in freeze-thaw-exposed environments is significantly more susceptible to surface scaling. Poor curing, finishing too early while bleed water is present, and excessive water-to-cement ratio all increase freeze-thaw vulnerability.

3. Alkali-Silica Reaction (ASR)

Alkali-silica reaction is a chemical deterioration mechanism in which reactive silica minerals in certain aggregates react with alkalis in the cement paste to form a hygroscopic gel. This gel absorbs water from the surrounding concrete and expands, generating internal pressure that produces characteristic map cracking (also called crazing or alligator cracking) and, at advanced stages, surface spalling and extrusion of gel at crack faces.

ASR develops slowly, often over decades, and is most commonly observed in older structures. Diagnosis requires petrographic examination of concrete cores by a specialist laboratory; the map cracking pattern alone is insufficient for definitive identification, as other mechanisms can produce similar surface patterns.

4. Fire Damage

Concrete exposed to fire undergoes rapid deterioration through multiple mechanisms. The thermal gradient between the hot surface and cooler interior generates tensile stresses that fracture the outer layers. Moisture within the concrete rapidly converts to steam, and if the steam cannot escape quickly enough through the pore network, it builds pressure that can cause explosive spalling. High-strength, low-permeability concrete is particularly susceptible to explosive fire spalling because its dense pore structure traps steam more effectively than that of normal-strength concrete.

Post-fire forensic structural assessment of concrete members must evaluate not only surface spalling but also the depth to which thermal damage has altered the concrete's mechanical properties and the extent to which reinforcing steel has been heated above its yield strength. These assessments require sampling, laboratory testing, and structural analysis to determine whether fire-affected members retain adequate load capacity.

5. Poor Construction Practice

Construction deficiencies are a significant and frequently underappreciated cause of premature concrete spalling. The most common include:

• Excessive water-to-cement ratio: Adding water to improve workability reduces compressive strength, increases permeability, and leaves large capillary voids that accelerate deterioration
• Inadequate concrete cover: Insufficient cover depth over reinforcement reduces the time for chlorides and carbonation to reach the steel, accelerating corrosion initiation
• Premature finishing: Finishing while bleed water is still present traps water beneath the surface skin, creating a weak, permeable layer vulnerable to freeze-thaw and surface spalling
• Improper curing: Insufficient curing time or inadequate moisture retention produces a weak, permeable surface that deteriorates rapidly
• Inadequate air entrainment: Omitting or under-dosing air-entraining admixtures in freeze-thaw-exposed concrete significantly reduces surface durability

When spalling appears within the first few years of a structure's service life, construction deficiencies are often the primary cause. These cases frequently lead to construction defect investigations that require forensic engineering analysis to determine whether the deterioration resulted from design specification errors, material substitutions, or installation failures.

6. Chemical Attack

Certain chemical environments aggressively degrade the concrete matrix. Sulfate attack from soil or groundwater, acid attack from industrial processes or biological activity in wastewater structures, and chloride penetration in marine environments all progressively dissolve or disrupt the cement paste, weakening the concrete and creating conditions for surface spalling. The Southeast's coastal markets, particularly in Florida and along the Carolina coast, present particularly aggressive chemical exposure environments for concrete structures.

When Concrete Spalling Is a Structural Concern, Not Just a Cosmetic Issue

The most important judgment in evaluating spalling concrete is distinguishing between surface deterioration that is cosmetically objectionable and structurally benign, and deterioration that indicates loss of structural capacity or acceleration of a failure mechanism.

Surface scaling from freeze-thaw on a non-structural slab is a maintenance issue. Corrosion-induced spalling on a parking deck beam soffit, with exposed and section-reduced reinforcement, is a structural engineering concern that requires immediate evaluation and may require load restrictions, shoring, or emergency repair.

The factors that elevate spalling from cosmetic to structural concern include:

• Exposed, section-reduced, or debonded reinforcing steel
• Spalling on load-bearing members: beams, columns, walls, slabs in spans
• Deep spalling that affects more than the cover depth of the concrete
• Active delamination with hollow sound response across large areas
• Rapid progression since the last inspection
• Evidence of structural distress concurrent with spalling: deflection, cracking at connections, misalignment

Any of these indicators warrants engagement of a licensed structural or forensic engineer for a professional condition assessment. NOVA's forensic engineering team performs structural condition assessments on spalling concrete structures across the Southeast, providing engineering-based findings that property owners, asset managers, and legal counsel can rely on for repair decisions, insurance claims, and litigation support.

Concrete Spalling in the Southeast: Regional Factors

The Southeast's climate and construction environment create specific drivers of concrete spalling that differ meaningfully from northern markets.

Chloride Exposure in Coastal Markets

Florida and the coastal markets of Georgia, South Carolina, and North Carolina expose concrete structures to airborne and direct chloride from seawater and salt air. Reinforced concrete in these environments faces significantly accelerated corrosion initiation compared to inland sites. Structures built before modern understanding of chloride-induced corrosion and cover depth requirements are particularly vulnerable, and the inventory of aging parking structures in markets like Miami, Fort Lauderdale, Charleston, and Wilmington reflects this exposure history.

High Humidity and Temperature Cycling

The Southeast's hot, humid climate accelerates carbonation of concrete surfaces, reducing the concrete's pH and removing the passive protection provided by embedded reinforcement. Combined with the warm temperatures that accelerate chemical reaction rates, carbonation-induced corrosion can progress faster in the Southeast than in more temperate climates.

Aggressive Soils and Groundwater

The Southeast includes significant areas of aggressive geochemistry, including sulfate-bearing soils and groundwater in parts of Georgia and Florida, and chloride-bearing groundwater in coastal areas. Geotechnical investigations that characterize soil and groundwater chemistry are an important input to the design of concrete foundations and below-grade structures in these areas, because inadequate sulfate resistance in the mix design can lead to deterioration that eventually manifests as spalling and structural degradation.

Older Parking Structure Inventory

Atlanta, Charlotte, and Florida's major markets have large inventories of reinforced concrete parking structures built in the 1970s through the 1990s, many of which are approaching or exceeding their design service life. These structures were frequently designed with cover depths and waterproofing systems that are less robust than current standards. Corrosion-induced spalling is endemic in this inventory, and condition assessments and repair programs are a significant and growing part of NOVA's forensic engineering practice across the Southeast.

The Role of Construction Materials Testing in Diagnosing Spalling

Visual observation tells you where spalling is occurring and what pattern it follows. Laboratory testing tells you why. NOVA's AASHTO-accredited construction materials testing laboratories provide the analytical support needed for accurate spalling diagnosis:

• Petrographic analysis: Microscopic examination of concrete cores by a petrographer can identify ASR, freeze-thaw damage, carbonation depth, and construction defects such as improper finishing or inadequate curing
• Chloride profile testing: Chloride content measured at multiple depths from the concrete surface allows engineers to calculate the rate of chloride ingress and estimate the time to corrosion initiation at the reinforcement depth
• Carbonation depth: Phenolphthalein indicator testing of freshly fractured cores reveals how deeply carbonation has penetrated and whether it has reached the reinforcement
• Half-cell potential survey: Electrochemical measurement of reinforcement potential mapped across a deck surface identifies areas of active corrosion before spalling is visible
• Compressive strength testing: Core compressive strengths compared to the specified design strength reveal whether the concrete met specification at placement and whether deterioration has reduced its load-carrying capacity
• Cover depth measurement: Ground-penetrating radar or covermeter surveys map reinforcement depth across a structure, identifying zones of inadequate cover that are most vulnerable to accelerated deterioration

Seeing Spalling on a Commercial Property? NOVA's forensic engineers and construction materials testing teams have assessed concrete spalling on parking structures, commercial buildings, and infrastructure projects across Georgia, Florida, North Carolina, and South Carolina since 1996. We can determine the cause, quantify the extent, and scope the right repair. Learn more: usanova.com/what-we-do/forensic-engineering