Concrete Spalling: Causes, Detection, and Repair

Concrete spalling may remain hidden at reinforcement depth while corrosion progresses for years before concrete visibly breaks away. Building owners may use engineering evaluation to assess safety, durability, and repair options.

The National Institute of Standards and Technology (NIST) has described concrete repair as a multibillion-dollar industry. Durable repair strategies matter for parking structures and other concrete assets.

For building owners and property managers, the consequences may extend beyond repair invoices. Spalling in parking structures , plaza decks, balconies, and façades can create falling-fragment hazards and reduce load-carrying capacity. When deterioration surfaces without warning, it can also disrupt capital plans.

Spalling often signals structural conditions that extend well beyond the visible surface. The sections that follow outline how spalling develops, the inspection and evaluation methods used to confirm its severity, and the repair strategies available to building owners and property managers. These considerations can help asset owners plan inspections and capital budgets around structural risk rather than reacting after damage becomes visible.

Key takeaways about concrete spalling causes and repair

Concrete spalling can originate from five distinct mechanisms and take several forms, from cosmetic surface loss to deep breakouts that expose corroding reinforcement.

What matters most

• Corrosion of embedded reinforcement is the leading cause, though freeze-thaw cycling, ASR, and fire exposure can also produce spalling independently or together.
• Chloride content, carbonation depth, and half-cell potential testing help distinguish surface damage from active, ongoing corrosion.
• Patch repairs in chloride-contaminated concrete can fail prematurely through the incipient anode effect, sometimes requiring cathodic protection instead.

How to stay ahead of it

• Systematic condition assessments can catch delamination and rust staining before visible concrete loss occurs, when intervention costs are typically lower.
• Repair selection depends on damage extent and required service life; partial-depth, full-depth, and cathodic protection are not interchangeable.

Rimkus conducts condition assessments and develops repair specifications referencing ACI, ASTM, and ICRI standards; contact us to discuss a spalling investigation.

Defining concrete spalling

Concrete spalling is the breaking or detachment of fragments or layers from the surface or body of concrete. Depending on its depth and cause, spalling may expose coarse aggregate or reinforcing steel. Federal Highway Administration (FHWA) and NIST pavement-deterioration research describes cracking and spalling along joint edges as visible distress patterns. Scaling is a related but separate surface condition, generally evaluated by visual comparison against a standard rating scale rather than by the loss-of-cover mechanism that defines spalling. This distinction may help separate spalling from surface-level deterioration or linear cracking patterns, which may call for different analytical frameworks.

In the broadest sense, spalling describes fragments of material breaking or detaching from a larger surface; the term also applies to masonry, stone, and metal, though the mechanisms differ by material. In concrete, spalling refers to fragments or layers breaking away from the surface or body of the concrete. Reinforcement corrosion is a common cause of deeper spalling, but spalling can also occur through mechanisms such as freeze-thaw deterioration, impact, or thermal exposure. 

Explosive spalling in high-strength concrete typically involves loss of concrete cover protecting reinforcement. This loss of cover may affect structural capacity and may remove part of the physical barrier protecting steel from corrosion.

Types of concrete spalling

Concrete spalling manifests in several distinct forms, each with different causes, structural implications, and repair requirements.

The International Concrete Repair Institute (ICRI) defines a small spall as no more than 20 mm deep and 150 mm in any dimension. It defines a large spall as more than 20 mm deep and 150 mm in greatest dimension. These dimensions provide a consistent way to document damage. Significance also depends on location and reinforcement condition, along with whether deterioration remains active.

Surface spalling

Surface spalling affects the top layer of concrete without reaching reinforcement depth. Freeze-thaw cycling and exposure to deicing chemicals commonly contribute to this shallow deterioration, along with finishing defects during original construction. It is distinct from scaling, which is surface flaking or peeling from the same freeze-thaw and deicing-salt exposure rather than from reinforcement corrosion. While surface spalling may appear cosmetic, it can accelerate moisture infiltration and expose the concrete matrix to further degradation.

Subsurface spalling

Subsurface spalling originates from within the concrete mass, typically at reinforcement depth. Corrosion of embedded steel produces expansive iron oxide that may generate internal pressure, which can cause concrete to crack and break away in larger fragments. This type may pose a significant structural concern because it can indicate active reinforcement corrosion and potential progressive loss of load-bearing capacity. Rust staining often appears before visible concrete loss occurs.

Corner and joint spalling

Corner spalling tends to occur at slab corners where two edges intersect. Impact and thermal movement can create stress concentrations at these locations, while inadequate support can increase vulnerability. Joint spalling develops adjacent to control joints, expansion joints, or construction joints. FHWA defines joint spalling as cracking, breaking, or chipping within 0.6 m of a joint. Improper joint construction or inadequate load transfer commonly contributes to both types. Freeze-thaw damage concentrated at discontinuities can also contribute.

Popouts

Popouts are conical fragments that break away from the concrete surface. They leave shallow depressions with a piece of coarse aggregate visible at the bottom. ASTM guidance associates popouts with finely porous aggregates near the concrete surface, and NIST describes alkali-silica reaction (ASR) as an expansive aggregate reaction that can contribute to cracking and concrete deterioration. While individual popouts may not affect structural capacity, widespread occurrence can indicate problematic aggregate sources or mix design issues requiring further investigation.

Delamination-induced spalling

Delamination, a horizontal separation plane within the concrete, often precedes visible spalling. When delaminated sections lose bond and break free under loading or impact, spalling can expose large areas of subsurface concrete. Inspectors may detect delamination by chain dragging or hammer sounding before visible damage occurs. Scheduled inspections may therefore target delamination.

Common causes of concrete spalling

Five primary mechanisms commonly cause concrete spalling in commercial and industrial structures. NIST describes reinforcing-steel corrosion as a leading cause of premature deterioration in reinforced concrete exposed to chlorides or carbonation and explains that expansive corrosion products can crack and spall the concrete cover.

Each mechanism affects concrete differently, but several may operate at the same time:

• Corrosion of reinforcing steel: Expansive corrosion products can create tensile stresses that may lead to cracking and spalling
• Freeze-thaw cycles: Water freezing within the concrete matrix can generate hydraulic pressure that may exceed tensile strength and contribute to progressive deterioration
• Alkali-silica reaction (ASR): ASR gels can absorb water and swell; this expansion can contribute to cracking, spalling, and pop-outs
• Fire damage and thermal stress: Elevated temperatures can cause volumetric expansion, differential expansion between concrete and steel, and strength loss
• Multi-mechanism deterioration: Corrosion cracking may accelerate freeze-thaw damage; ASR can create pathways for chloride penetration
  

These mechanisms can overlap, so evaluations typically consider more than one exposure. Chloride exposure from deicing salts or marine air is the most common trigger for reinforcement corrosion, but concrete can also lose its protective properties gradually through carbonation, a slower process driven by ordinary exposure to air rather than salt. 

Deicing chemicals can cause a related but distinct form of damage even in above-freezing weather. Testing methods exist to measure how far any of these processes has progressed, but interpreting them is an engineering task, not something an owner needs to do themselves.

Detecting concrete spalling

Visual surveys alone may not identify subsurface delamination. Professional concrete spalling detection therefore typically combines structured visual inspection with NDT methods covered by ASTM standards and NIST technical guidance. Findings that raise structural safety or durability concerns may also require engineering evaluation.

The three levels serve different purposes, from documenting visible damage to evaluating deterioration hidden below the surface:

1. Visual inspection: May include documenting surface scaling, visible signs of delamination, exposed aggregate or reinforcement, and pop-outs, with photographs, scales, and appropriate lighting as conditions require

2. Non-destructive testing: Impact-echo testing methods, including testing under ASTM C1383 for measuring P-wave speed and concrete-plate thickness; ground-penetrating radar (GPR) under ASTM D6087 for evaluating asphalt-covered concrete bridge decks; plus sounding, corrosion-potential, and carbonation surveys that reveal deterioration a visual review can’t 

3. Engineering engagement: Engineers may need to evaluate conditions when delamination affects load-bearing members, reinforcement exposure may affect structural integrity, or progressive deterioration indicates ongoing damage mechanisms

Together, these levels distinguish visible damage from hidden deterioration. Sounding, corrosion-potential, and carbonation testing all require trained personnel to run and interpret correctly, and where questions remain about material quality, petrographic examination under ASTM C856/C856M-25 can identify ASR or freeze-thaw damage directly in a lab sample.

Systematic building condition assessments may help property managers identify deterioration before it may progress toward structural compromise and may reduce repair costs and liability exposure.

Repairing concrete spalling

Repair selection depends on damage extent, corrosion activity, and required service life; partial-depth repair, full-depth replacement, and cathodic protection are not interchangeable, and costs vary too much by project to share a single defensible range.

Industry codes govern the assessment, repair, and rehabilitation of existing concrete structures where a jurisdiction or contract has adopted them, covering how repairs are evaluated, specified, and inspected. Companion guidance from ASTM, AMPP, and ICRI supports comparing repair options and estimating budgets.

Partial-depth repair

Partial-depth repair may address localized deterioration where the remaining concrete and reinforcement are suitable for repair. Surface preparation can be specified using guidance such as  ICRI 310.2R, while repair-material selection depends on the application, exposure, required properties, and project specifications.

For spalling concrete pavement specifically, FHWA generally limits partial-depth repair to the upper one-third of the slab; deeper or more extensive deterioration typically requires full-depth removal, and that pavement-specific rule doesn’t transfer automatically to every structural member.

Full-depth repair or replacement

Full-depth repair or replacement may be warranted when cracking reduces load-bearing capacity, joint integrity is compromised, or deterioration is extensive. ACI PRC-546-23 provides guidance on concrete repair materials and methods. 

Why partial depth repairs sometimes fail

A partial-depth repair that looks fine for a year or two can still fail: fresh repair material placed next to older, chloride-contaminated concrete can set up a new corrosion cell right at the seam between old and new. Corrosion can then accelerate at that boundary, producing new cracking around the edge of a repair that otherwise looked successful, which is why engineers may recommend cathodic protection or chloride extraction alongside, or instead of, partial-depth repair when active corrosion extends beyond the immediate repair area.

Cathodic protection

Cathodic protection may be considered where active reinforcement corrosion is widespread, chloride contamination remains in the concrete, or conventional patch repairs alone are unlikely to provide the required durability. FHWA’s corrosion research program concluded it is the only rehabilitation technique proven to stop corrosion in salt-contaminated bridge decks regardless of chloride content, though that finding comes from bridge-deck research and doesn’t automatically transfer to every building. Performance in buildings depends on system design and ongoing maintenance of rectifiers and wiring.

For buildings, parking structures, and other atmospherically exposed concrete specifically, AMPP SP0290 (formerly published by NACE) and ISO 12696 provide standards for the design, operation, and monitoring of cathodic-protection systems.

Reducing concrete spalling risk

Crack control may be as important as cover depth in controlling spalling risk, since cracks give chloride and water a direct path to reinforcement that triggers the corrosion process behind spalling. FHWA found that cracks wider than 0.3 mm can provide chloride and water access to reinforcement regardless of nominal cover depth. Preventive maintenance programs that address joint sealants, open cracks, and drainage can slow chloride ingress in parking decks and other salt-exposed elements.

Systematic condition assessments tie these measures together. Catching open cracks, failed sealants, rust staining, hollow-sounding areas, and popouts before visible concrete loss occurs supports earlier and less costly intervention, informs repair prioritization, and provides records for compliance and capital planning. Internal deterioration can progress while a structure remains in service, so periodic assessment matters even when nothing looks wrong from the outside.

Rimkus engineers have experience conducting condition assessments and developing repair specifications and capital-planning analyses with reference to applicable ACI, ASTM, and ICRI frameworks. This work draws on 40+ years of experience and 900+ experts on staff. Building owners seeking project-specific evaluation may contact Rimkus regarding concrete condition assessments and spalling investigations.

Frequently asked questions about concrete spalling

Project conditions, exposure, and applicable requirements vary, so the answers provide general context.

What causes concrete spalling in parking structures and commercial buildings?

In parking structures, deicing salts and water can enter concrete through cracks, joints, and leaks, which may initiate reinforcement corrosion and intensify freeze-thaw deterioration. These exposures can interact and accelerate damage beyond what any single mechanism would produce.

What factors affect how often concrete structures are inspected for spalling?

A single interval may not be appropriate for all structures; frequency typically depends on environmental exposure, structure age, prior findings, and any inspection ordinances that apply in the local jurisdiction. Structures with previous delamination, active corrosion indicators, or heavy deicing-salt exposure may warrant more frequent evaluation.

When may concrete spalling warrant repair, and when might replacement be considered?

For concrete pavement slabs, partial-depth repair is generally considered only when deterioration remains in the upper one-third of the slab; representative coring or other evaluation can confirm repair selection, while building decisions typically rest on an engineering evaluation of deterioration depth, reinforcement condition, and active corrosion. Where chloride contamination remains in place, patching alone may not stop corrosion, and engineers may consider cathodic protection or replacement.