Types of Concrete Cracks: Mechanisms, Patterns, Widths, and Timelines
Rockford Concrete Leveling Research · Last verified: · Dataset version 1.0.0
Rockford Concrete Leveling Research is the independent research and reference section of rockfordconcreteleveling.net.
What are the key concrete crack statistics?
Most guides to the types of concrete cracks hand you a single flat list. The list is the problem: it mixes why a crack formed, what it looks like, whether it is still moving, and separate deterioration that is not a crack at all. The clearest quantitative example comes from FHWA pavement-distress criteria: a transverse crack in jointed concrete pavement is rated high severity at 6 mm or wider, while a longitudinal crack in the same pavement system does not reach the high-width band until 13 mm.
Sixteen figures follow. Each is self-contained, dated, and traceable to the issuing organization named in the line.
- Under the FHWA Distress Identification Manual for the Long-Term Pavement Performance Program (FHWA-HRT-13-092, May 2014, with July 2019 errata), a transverse crack in jointed concrete pavement is rated high severity at 6 mm (about 1/4 in.) or wider.
- Under FHWA-HRT-13-092 (May 2014, with July 2019 errata), a longitudinal crack in jointed concrete pavement is rated high severity at 13 mm (about 1/2 in.) or wider. Based on width alone and with no other severity trigger, a 9 mm crack is high severity if transverse and moderate severity if longitudinal.
- Under FHWA-HRT-13-092 (May 2014, with July 2019 errata), the low-width band for both transverse and longitudinal cracks in jointed concrete pavement is less than 3 mm (about 1/8 in.), provided there is no spalling and no measurable faulting; well-sealed cracks with undetermined width are also assigned to the low band.
- FHWA's later durability tech brief, FHWA-HIF-16-033 (April 2016), gives 10–25 years as the appearance range for aggregate freeze-thaw deterioration, or D-cracking; FHWA-HIF-15-018 (June 2015) gives the narrower range 10–15 years.
- FHWA-HIF-16-033 gives 5–25 years for alkali-silica reactivity and 5–15 years for alkali-carbonate reactivity; the 2015 FHWA brief gives 5–15 years for both.
- FHWA-HIF-16-033 gives 1–10 years for freeze-thaw deterioration of hardened cement paste and 1–5 years for deicer scaling; the 2015 brief gives 1–5 years for both.
- FHWA-HIF-16-033 gives 3–20 years for corrosion of embedded steel to appear as cracking and spalling above or around the steel; the 2015 FHWA brief gives 3–10 years.
- ACI PRC-224-01 Table 4.1 (publication year 2002; reapproved 2008) gives 0.016 in. (0.41 mm) as a reasonable crack width for reinforced concrete under service loads in dry air, tightening to 0.007 in. (0.18 mm) where deicing chemicals are present and 0.004 in. (0.10 mm) for water-retaining structures.
- NRMCA CIP 5, Plastic Shrinkage Cracking (2014 revision), says plastic-shrinkage cracks are usually parallel, about 1–3 ft apart, mostly on horizontal surfaces, and generally do not intersect the slab perimeter.
- ACI CT-25, 2025 ACI Concrete Terminology, defines an active crack as one whose width changes with time and a dormant crack as one whose width does not change with time.
- NRMCA CIP 3, Crazing Concrete Surfaces (2021 revision), reports that craze cracks are rarely more than 1/8 in. (3 mm) deep, and the irregular cells they enclose are generally no more than 1.5 in. (40 mm) across.
- NRMCA CIP 3, Crazing Concrete Surfaces (2021 revision), says craze cracks are generally apparent the day after placement or by the end of the first week, and are most visible while a wetted surface is drying.
- FHWA-HRT-13-092 (May 2014, with July 2019 errata) lists 16 numbered jointed-concrete-pavement distress groups. Four are placed in its Cracking category; map cracking and scaling are placed separately under Surface Defects and have no defined severity levels.
- ACI CT-25 says critical saturation is usually taken as 91.7% because water expands about 9% when it freezes. FHWA pavement guidance uses differently framed values — approximately 87% in its 2015 brief and above approximately 85% in its 2016 brief — so these numbers should remain attached to their source and definition rather than collapsed into one universal field threshold.
- Rockford's 1991–2020 climate normals average 135.8 days per year with a low at or below 32 °F and 50.0 days with a high at or below 32 °F, according to the National Weather Service Chicago office.
- Using National Weather Service Chicago's 1991–2020 Rockford normals, subtracting the nested normal-day counts yields 85.8 air-temperature crossover days per year — days with a minimum at or below 32 °F and a maximum above 32 °F. This is an atmospheric exposure figure, not a measured count of freeze-thaw cycles inside concrete.
What are the different types of concrete cracks?
The word "type" is used for four genuinely different questions. Collapsing them into one list is what causes most of the disagreement between guides. This reference separates them explicitly.
| Level | Question it answers | Records in v1.0 | Examples |
|---|---|---|---|
| Mechanism | Why or when did it form? | 24 | Plastic shrinkage, drying shrinkage, D-cracking, corrosion |
| Visual descriptor | What does it look like? | 13 | Hairline, map, diagonal, transverse, crescent-shaped |
| Status / significance | Is it changing, and what is at stake? | 6 | Active, dormant, appearance-only, structural |
| Related condition | Is this actually a crack? | 5 | Scaling, spalling, popout, delamination, cold joint |
Source: Rockford Concrete Leveling Research, Concrete Crack Classification v1.0.0, compiled and verified July 24, 2026 from the issuing-organization sources listed below.
The rule that makes the rest of this page work: one observed crack can carry one label from several levels simultaneously. "Active, diagonal, corrosion-associated, durability-related" is a real description. "It is a structural crack" is one consequence label presented as if it were the whole answer.
Which mechanism produced the crack, and when does each one show up?
Twenty-four mechanisms are grouped by the stage in which they act. Where an issuing organization publishes a time of appearance, it appears in the row; where the reviewed primary sources do not publish one, the table says so rather than manufacturing a window. For materials-related pavement distress, the table carries the broader ranges from FHWA's later 2016 tech brief and points to the 2015 comparison in Table 5.
| ID | Mechanism | Stage | What it looks like | Named in | Documented time of appearance |
|---|---|---|---|---|---|
| M01 | Plastic shrinkage cracking | Fresh/plastic concrete | Usually shallow, mostly on horizontal surfaces, commonly parallel and about 1–3 ft apart; generally does not intersect the slab perimeter. | NRMCA CIP 5; ACI broad cause family: plastic shrinkage | Hours, while concrete remains plastic |
| M02 | Plastic settlement cracking over reinforcement | Fresh/plastic concrete | Cracks may trace the line of reinforcing bars or embedded obstructions. | NRMCA CIP 5 | Before hardening |
| M03 | Formwork movement before hardening | Fresh concrete to early strength gain | Irregular cracking or settlement tied to moved form lines, unsupported areas, or displaced geometry. | NRMCA CIP 5; NRMCA CIP 7 | Before hardening or adequate self-supporting strength |
| M04 | Drying shrinkage | Hardened concrete | Random, transverse, re-entrant, or joint-related cracks depending on geometry and restraint. | NRMCA TIP 17; ACI 224R-01 Chapter 3 | Typically evident within days to several weeks; later movement can continue |
| M05 | Autogenous shrinkage | Very early age to hardened concrete | No unique visible pattern; appears as restrained shrinkage cracking. | NRMCA TIP 17; ACI CT-25 autogenous volume change | Most occurs during the first 24 hours; magnitude is small for most mixtures |
| M06 | Carbonation shrinkage | Exposed hardened concrete | No uniquely diagnostic pattern; may contribute to surface-zone shrinkage. | NRMCA TIP 17; ACI 224R-01 Chapter 3; ACI CT-25 | Long-term; no fixed appearance window in the reviewed sources |
| M07 | Thermal-gradient or restrained temperature cracking | Early age or later service | Random or patchwork surface cracks in thick members; slab and pavement cracks can resemble drying-shrinkage cracks and often run perpendicular to the long axis. | NRMCA CIP 42; ACI CT-25 temperature cracking | Often early age; slab and pavement cracks usually within the first year or first summer–winter cycle |
| M08 | Crazing | Early hardened surface | Small irregular cells; cracks rarely deeper than 1/8 in. (3 mm) and cells generally no more than 1.5 in. (40 mm) across. | NRMCA CIP 3; ACI CT-25 | Usually apparent the day after placement or by the end of the first week |
| M09 | Restraint from missing, late, or ineffective jointing | Hardened concrete | Random cracks where intended joints did not activate, or cracking where slabs are restrained by adjoining elements. | NRMCA CIP 4; NRMCA CIP 6; ACI 224R-01 Chapter 3 | No fixed appearance window; often becomes evident as drying or temperature movement develops |
| M10 | Re-entrant corner cracking | Hardened concrete | A crack runs diagonally outward from a window, door, opening, drain, or other re-entrant corner. | NRMCA CIP 7; NRMCA CIP 6 | No fixed appearance window in the reviewed sources |
| M11 | Differential support settlement | In service | Irregular cracking, faulting, rotation, or vertical offset across the crack. | NRMCA CIP 4; NRMCA CIP 7; FHWA LTPP faulting context | No fixed appearance window in the reviewed sources |
| M12 | Loss of support from pumping or subgrade erosion | In service | Water or fines at cracks and joints, staining, faulting, and cracking associated with support loss. | FHWA LTPP water bleeding and pumping; FHWA concrete pavement preservation guidance | No fixed appearance window in the reviewed sources |
| M13 | Frost heave or thaw-softened support movement | Seasonal service | Seasonal lifting, settlement, faulting, or cracking that changes with support conditions. | FHWA support-condition research | Seasonal; no universal year-of-appearance range |
| M14 | Expansive-soil movement | In service | Nonuniform lifting, settlement, rotation, or cracking tied to changing support. | NRMCA CIP 4; NRMCA CIP 7 | No fixed appearance window in the reviewed sources |
| M15 | Reinforcement-corrosion expansion cracking | Hardened reinforced concrete | Cracks or spalls aligned with reinforcing steel, rust staining, exposed reinforcement, or delamination above the steel. | ACI cracking cause family: corrosion; ACI CT-25; FHWA materials-related distress | FHWA 2016: 3–20 years; FHWA 2015: 3–10 years |
| M16 | Externally applied load | In service | Pattern depends on the element: corner breaks, flexural cracks, diagonal cracks, or cracking around openings or load points. | ACI broad cause family: externally applied loads; FHWA LTPP corner breaks | Under or after loading; no universal age window |
| M17 | Construction overload | Early age or construction stage | Permanent cracking tied to construction sequencing or temporary loading. | ACI broad cause family: construction overloads | During construction or early strength gain |
| M18 | Shear or diagonal-tension cracking | Structural service | Inclined cracks in a structural member; ACI notes approximately 45 degrees in a flexural-member context. | ACI CT-25 diagonal crack and diagonal tension | Under load; no universal age window |
| M19 | Freeze-thaw deterioration of hardened cement paste | Hardened concrete | Scaling, spalling, or map cracking, often initiating near joints or cracks in pavement contexts. | FHWA materials-related distress | FHWA 2016: 1–10 years; FHWA 2015: 1–5 years |
| M20 | Deicer scaling and deterioration | Hardened surface | Scaling or crazing of the slab surface, with staining at joints or cracks. | FHWA materials-related distress | 1–5 years in both FHWA 2015 and 2016 tables |
| M21 | Freeze-thaw deterioration of susceptible aggregate (D-cracking) | Hardened concrete | Closely spaced crescent-shaped hairline cracking beside joints, cracks, or free edges, commonly initiating at slab corners. | FHWA materials-related distress; FHWA LTPP JCP 2 | FHWA 2016: 10–25 years; FHWA 2015: 10–15 years |
| M22 | Alkali-silica reactivity (ASR) | Hardened concrete | Map cracking across the slab surface, joint closure, spalling, or blowups in pavement guidance. | FHWA materials-related distress | FHWA 2016: 5–25 years; FHWA 2015: 5–15 years |
| M23 | Alkali-carbonate reactivity (ACR) | Hardened concrete | Map cracking across the slab surface, spalling, or blowups in pavement guidance. | FHWA materials-related distress | 5–15 years in both FHWA 2015 and 2016 tables |
| M24 | Sulfate attack, external or internal | Hardened concrete | Fine cracking near joints or slab edges, or map cracking over the slab surface, depending on the mechanism variant. | FHWA materials-related distress; ACI CT-25 delayed ettringite formation | External: FHWA 2016 1–10 years vs. 2015 1–5 years; internal: 1–5 years in both |
Sources: ACI CT-25; ACI Cracking in Concrete topic page; NRMCA CIP 3, CIP 4, CIP 5, CIP 6, CIP 7, CIP 42, TIP 17; FHWA-HRT-13-092; FHWA-HIF-15-018; FHWA-HIF-16-033. Mechanisms M16–M24 derive from FHWA and ACI source families. Checked July 24, 2026.
How is a concrete crack rated low, moderate, or high severity?
Measured Low, Moderate, and High bands on this page come from a pavement-distress system, not from residential construction. FHWA-HRT-13-092 assigns severity using crack width, spalling width, faulting, loose material, and broken pieces, and the threshold changes with the named distress type. The July 2019 D-67 errata changed how sealant and joint spalling are identified and measured; it did not replace the numeric severity bands reproduced below.
| Distress type | Low | Moderate | High |
|---|---|---|---|
| JCP 4 — Transverse cracking | Width < 3 mm, no spalling, no measurable faulting; or well-sealed with undetermined width | Width ≥ 3 mm and < 6 mm; or spalling < 75 mm; or faulting up to 6 mm | Width ≥ 6 mm; or spalling ≥ 75 mm; or faulting ≥ 6 mm |
| JCP 3 — Longitudinal cracking | Width < 3 mm, no spalling, no measurable faulting; or well-sealed with undetermined width | Width ≥ 3 mm and < 13 mm; or spalling < 75 mm; or faulting up to 13 mm | Width ≥ 13 mm; or spalling ≥ 75 mm; or faulting ≥ 13 mm |
| JCP 1 — Corner breaks | Not spalled over more than 10% of crack length; no measurable faulting; corner piece intact, with no material loss or patching | Low-severity spalling over more than 10% of length; or faulting < 13 mm; corner piece intact | Moderate-to-high spalling over more than 10% of length; or faulting ≥ 13 mm; or piece broken into two or more pieces; or patch material present |
| JCP 2 — Durability cracking (D-cracking) | Cracks tight, no loose or missing pieces, no patching | Well-defined cracks; some small pieces loose or displaced | Well-developed pattern with significant loose or missing material; displaced pieces up to 0.1 m² may be patched |
| JCP 6 / 7 — Joint spalling | Spalls < 75 mm wide to the joint face | Spalls 75–150 mm wide with material loss | Spalls > 150 mm wide with material loss, or broken into two or more pieces, or containing patch material |
| JCP 8a — Map cracking | No severity levels defined. | ||
| JCP 8b — Scaling | No severity levels defined. Scaling is described as upper-surface deterioration, normally 3–13 mm deep. | ||
Source: Federal Highway Administration, Distress Identification Manual for the Long-Term Pavement Performance Program, FHWA-HRT-13-092, May 2014, Chapter 2, with LTPP Directive D-67 errata dated July 17, 2019. Thresholds and corrected measurement rules checked July 24, 2026.
Notice what does not get one universal width grade. D-cracking is graded by whether pieces are loose or missing. Map cracking and scaling receive no severity grade. Faulting — the vertical step across a joint or crack — is recorded in millimetres rather than converted into a separate severity label because FHWA says a complete record of measurements is more accurate and repeatable.
Is there a crack width that means a crack is dangerous?
No universal dangerous width appears in the primary sources reviewed for this page. The table most often reused for that purpose was written for reinforced concrete under service loads and changes with exposure — from 0.016 in. in dry air to 0.004 in. for water-retaining structures. It is an exposure-based design and evaluation reference, not a residential grading scale.
| Exposure condition | Reasonable crack width, in. | mm |
|---|---|---|
| Dry air or protective membrane | 0.016 | 0.41 |
| Humidity, moist air, soil | 0.012 | 0.30 |
| Deicing chemicals | 0.007 | 0.18 |
| Seawater and seawater spray, wetting and drying | 0.006 | 0.15 |
| Water-retaining structures, excluding non-pressure pipes | 0.004 | 0.10 |
Source: ACI PRC-224-01 Table 4.1 (publication year 2002; reapproved 2008), as reproduced in ACI SP-319. Table is for reinforced concrete under service loads. Checked July 24, 2026.
What determines whether a width matters is the element, exposure, reinforcement, movement, displacement, watertightness requirement, deterioration evidence, and governing standard. A pavement threshold written for a highway lane does not transfer to a garage floor, and neither transfers unchanged to a basement wall or structural beam.
How long after the pour does each type of crack appear?
Age narrows the field. It does not close it. FHWA published two official tech briefs one year apart with different appearance ranges for several materials-related mechanisms, so the conflict belongs in the data rather than being averaged away or hidden.
| Mechanism | Surface manifestation in the FHWA tables | FHWA-HIF-15-018 (2015) | FHWA-HIF-16-033 (2016) |
|---|---|---|---|
| Freeze-thaw deterioration of hardened cement paste | Scaling, spalling, or map cracking, generally initiating near joints or cracks | 1–5 years | 1–10 years |
| Deicer scaling / deterioration | Scaling or crazing of the slab surface; staining at joints and cracks | 1–5 years | 1–5 years |
| External sulfate attack | Fine cracking near joints and slab edges, or map cracking over the slab | 1–5 years | 1–10 years |
| Internal sulfate attack | Fine cracking near joints and slab edges, or map cracking over the slab | 1–5 years | 1–5 years |
| Corrosion of embedded steel | Spalling, cracking, and deterioration above or around embedded steel | 3–10 years | 3–20 years |
| Alkali-silica reactivity (ASR) | Map cracking over the slab surface, with joint closure, spalling, or blowups | 5–15 years | 5–25 years |
| Alkali-carbonate reactivity (ACR) | Map cracking over the slab surface, with spalling or blowups | 5–15 years | 5–15 years |
| Freeze-thaw deterioration of aggregate, or D-cracking | Cracking parallel to joints and cracks, later spalling, sometimes staining | 10–15 years | 10–25 years |
Sources: Federal Highway Administration, FHWA-HIF-15-018, June 2015, and FHWA-HIF-16-033, April 2016. Both issuing-agency tables were checked July 24, 2026. Table 2 uses the later 2016 ranges and keeps the 2015 ranges in the same row so neither official value is silently discarded.
The difference is not a reason to invent a middle number. It is a reason to carry the publication number and year every time the range is quoted. The tables are pavement guidance, and the ranges are not predictions for an individual residential slab.
Early-age windows before the FHWA year-range tables begin
| Window | Mechanisms | Sourced detail |
|---|---|---|
| Hours, before set | Plastic shrinkage; plastic settlement; form or section-depth movement | Plastic-shrinkage cracks appear while concrete remains plastic and are commonly parallel and about 1–3 ft apart (NRMCA CIP 5). |
| Day 1 to end of week 1 | Crazing | Craze cracks are generally apparent the day after placement or by the end of the first week (NRMCA CIP 3). |
| First 24 hours | Autogenous shrinkage | NRMCA TIP 17 says most autogenous shrinkage occurs during the first 24 hours. |
| Days to several weeks | Drying-shrinkage cracking | NRMCA TIP 17 says drying-shrinkage-related cracking is typically evident within days to several weeks. |
| Days after form removal; usually within first annual cycle for slabs and pavements | Thermal cracking | NRMCA CIP 42 describes early thermal cracks after form removal and says slab or pavement temperature cracks usually occur within the first year or first summer–winter cycle. |
| Years to decades | The eight mechanisms in Table 5 | FHWA-HIF-15-018 and FHWA-HIF-16-033 publish the source-specific ranges above. |
Source: NRMCA CIP 3, CIP 5, CIP 42, and TIP 17; FHWA-HIF-15-018; FHWA-HIF-16-033. Checked July 24, 2026.
What does this reference show, and what does it not show?
It shows how ACI, FHWA, NRMCA, the National Weather Service, the City of Rockford, and IDOT name, measure, time, or contextualize concrete cracking. It does not diagnose a particular slab, wall, foundation, pavement, or structural member, and it does not establish whether a crack shown in a photograph is safe.
A photograph plus a crack name is screening information. Determining cause requires information the photograph does not contain: placement date and weather, mixture and curing information, element type and loading, what changed underneath it, whether there is displacement, and whether width changes with time. FHWA's materials guidance shows why: field patterns overlap, and separating mechanisms such as ASR and ACR can require petrographic and other material examination.
How was this classification built and checked?
The source documents were collected and rechecked on July 24, 2026. The classification uses issuing-organization material in this order: ACI terminology and committee material for definitions and broad cause framing; FHWA's LTPP manual for field-measurable pavement distress; FHWA materials guidance for mechanism and appearance ranges; NRMCA's Concrete in Practice and Technology in Practice sheets for early-age and flatwork behavior; and NWS, City of Rockford, and IDOT sources for the local evidence layer.
What was read directly. We read ACI CT-25, ACI's current cracking topic page, ACI's official Chapter 3 excerpt from ACI 224R-01, ACI's FAQ and ACI-published SP-319 reproduction of Table 4.1, FHWA-HRT-13-092, LTPP Directive D-67, FHWA-HIF-15-018, FHWA-HIF-16-033, the cited NRMCA sheets, the NWS Rockford normals table, Rockford's current code-adoption page and local-amendments PDF, and IDOT's current aggregate and concrete materials pages. ACI PRC-224.1-07 is used only for its verified document identity and scope; no unpublished paywalled number is carried into this page.
The classification rules. A record enters the dataset when a primary technical source establishes a distinct mechanism or recognized inspection term, or when an explicitly labeled editorial geometry descriptor prevents a common category error. Records are excluded when they are marketing labels, duplicate synonyms that add no distinct meaning, unsourced universal width rules, or repair-product categories. A visual term is never converted into a causal, structural, or safety conclusion.
The FHWA timing conflict. The 2015 and 2016 tech briefs are both official FHWA publications. The later document expands several ranges. Table 5 publishes both sets side by side, while Table 2 uses the later 2016 range and retains the 2015 value in the same cell.
The Rockford calculation. We copied the twelve monthly values from two rows in the NWS mean-number-of-days table and subtracted them month by month. Every day with a maximum at or below 32 °F necessarily also has a minimum at or below 32 °F, so the second count is nested inside the first. The monthly differences sum to 85.8, matching the annual subtraction of 135.8 − 50.0.
Dataset integrity. Version 1.0.0 contains exactly 48 records: 24 mechanisms, 13 visual descriptors, 6 status or significance labels, and 5 related conditions. The CSV and JSON are generated from the same record list used to build the visible tables. Adding, deleting, reclassifying, or materially rewriting a record increments the dataset version; a cosmetic edit does not reset the verification date.
Which cracks form before the concrete hardens?
Several mechanisms can act before set or during the transition into early strength gain. The three most useful surface distinctions are plastic shrinkage, plastic settlement around embedded items, and movement of forms or support; NRMCA CIP 5 separately warns that early thermal cracking and differential settlement at a thin-to-deep section can be mistaken for plastic shrinkage. Timing narrows the possibilities, but it does not replace placement records and geometry.
Plastic-shrinkage cracking happens when surface moisture evaporates faster than bleed water replaces it. NRMCA identifies wind velocity above 5 mph, low relative humidity, and high ambient or concrete temperature as conditions that increase risk. The cracks appear on fresh horizontal concrete, are usually parallel and about one to three feet apart, are relatively shallow, and generally do not intersect the slab perimeter.
One detail cuts against a common assumption: mixtures with reduced bleeding — because of high cementitious or fine-material content, lower water content, entrained air, high concrete temperature, or thinner sections — can be susceptible even when the apparent evaporation rate is not extreme. NRMCA says plastic-shrinkage cracks rarely impair strength, but they can permit ingress of aggressive chemicals and can become weak points for later cracking.
Plastic-settlement cracking forms when fresh concrete continues to consolidate but steel or another embedment restrains movement, leaving cracks that can trace the obstruction below. Form movement produces a different construction-history problem: the support moves before the concrete can carry itself. NRMCA CIP 7 also describes visible pour lines or cold joints where successive placements do not consolidate into one monolithic mass; that condition belongs in the related-condition table rather than being treated as a universal crack mechanism.
Source: NRMCA CIP 5, Plastic Shrinkage Cracking, and NRMCA CIP 7, Cracks in Residential Basement Walls. Checked July 24, 2026.
What is the difference between crazing and map cracking?
They are not synonyms across every source. NRMCA uses "shallow map or pattern cracking" as an alternate description for crazing, while ACI CT-25 defines map cracking as intersecting cracks extending below the surface of hardened concrete. FHWA uses "map cracking" both as a jointed-pavement surface-defect category and as a surface manifestation associated with several materials-related mechanisms.
| Term | NRMCA flatwork guidance | ACI CT-25 | FHWA LTPP distress manual | FHWA materials-distress guidance |
|---|---|---|---|---|
| Map cracking | Used in the phrase "shallow map or pattern cracking" for crazing | Intersecting cracks extending below the surface of hardened concrete; also called pattern cracking | JCP 8a: a series of cracks extending only into the upper surface; no severity levels | Listed as a surface manifestation associated with ASR, ACR, sulfate attack, and paste freeze-thaw |
| Crazing | Fine random surface network; rarely deeper than 1/8 in.; generally visible by the end of week 1 | Development or pattern of fine random craze cracks in a surface | Not a separately named JCP distress type | Listed as a manifestation of deicer scaling / deterioration |
| Scaling | A surface defect | Flaking or peeling away of the near-surface portion of hardened concrete or mortar | JCP 8b: upper-surface deterioration, normally 3–13 mm; no severity levels | A manifestation of paste freeze-thaw and deicer-related deterioration |
| Spalling | A surface defect or material loss | Development of fragments detached by impact, weather, pressure, or internal expansion | JCP 6 and JCP 7: joint distress graded by measured spall width and material condition | A manifestation associated with corrosion and later stages of aggregate freeze-thaw deterioration |
Sources: NRMCA CIP 3; ACI CT-25; FHWA-HRT-13-092 with Directive D-67; FHWA-HIF-15-018; FHWA-HIF-16-033. Checked July 24, 2026.
What does a crack's direction or shape actually tell you?
Direction and shape tell you what to describe and measure next. They do not, by themselves, establish cause or consequence. The FHWA pavement manual makes the distinction visible: longitudinal and transverse are geometric categories, and each receives its own width, spalling, and faulting bands.
| ID | Descriptor | What it describes | What it does not establish |
|---|---|---|---|
| V01 | Hairline | A concrete surface crack with a width so small as to be barely perceptible. | Cause, depth, activity, and structural or durability significance. |
| V02 | Checking | Shallow cracks at closely spaced but irregular intervals on a concrete, mortar, cement-paste, or plaster surface. | Whether cracking extends below the surface or which mechanism caused it. |
| V03 | Craze pattern | Fine random cracks or fissures in a surface; crazing is the development or existing pattern of those craze cracks. | That every network pattern is shallow or cosmetic. |
| V04 | Map or pattern cracking | Intersecting cracks extending below the surface of hardened concrete, varying from fine and barely visible to open and well-defined. | Which mechanism produced the network; several mechanisms can create similar patterns. |
| V05 | Random | Cracks running in varying directions without a regular orientation. | Whether shrinkage, restraint, thermal effects, support movement, or another mechanism dominated. |
| V06 | Parallel or regularly spaced | Cracks with broadly consistent direction and spacing. | Cause by itself; timing and context are still required. |
| V07 | Vertical | A crack whose visible path is predominantly vertical in the chosen reference frame. | Cause, activity, displacement, or consequence. |
| V08 | Horizontal | A crack whose visible path is predominantly horizontal in the chosen reference frame. | Whether lateral load, restraint, corrosion, or another cause is involved. |
| V09 | Diagonal | In a flexural member, an inclined crack associated with shear stress; in a slab, a crack not parallel to lateral or longitudinal directions. | That every diagonal crack is structural or shear-related. |
| V10 | Longitudinal | A crack generally parallel to a member's length or pavement centerline. | Severity; FHWA severity also depends on width, spalling, and faulting within a pavement-specific system. |
| V11 | Transverse | A crack crossing the longer dimension or running predominantly perpendicular to a pavement centerline. | Severity outside the named application and measurement system. |
| V12 | Re-entrant corner | A crack beginning at or extending from an inward-pointing corner or opening. | Whether jointing, shrinkage, support movement, or loading was the dominant mechanism. |
| V13 | Crescent-shaped at joints and edges | A closely spaced crescent-shaped hairline pattern adjacent to joints, cracks, or free edges. | Confirmation of aggregate freeze-thaw deterioration without material evidence. |
Sources: ACI CT-25; FHWA-HRT-13-092; NRMCA CIP 3, CIP 4, CIP 5, CIP 6, CIP 7, and CIP 42. Rows V05, V07, and V08 are explicitly editorial geometry labels grounded in the cited primary-source usage; they are not presented as ACI causal classes.
Related conditions often mislabeled as crack types
| ID | Related condition | What it is | What the label does not establish |
|---|---|---|---|
| C01 | Delamination | A planar separation roughly parallel to the material surface. | A crack orientation or mechanism by itself. |
| C02 | Scaling | Flaking or peeling away of the near-surface portion of hardened concrete or mortar. | A crack mechanism; scaling can accompany several exposure conditions. |
| C03 | Spalling | Development of fragments detached by impact, weather, pressure, or expansion within the larger mass. | The initiating mechanism; corrosion, freeze-thaw, loading, and other causes can all produce spalls. |
| C04 | Popout | A small localized portion of a concrete surface breaks away, commonly leaving a shallow conical depression. | A crack type or evidence of a broad structural failure. |
| C05 | Cold joint or pour line | A visible interface between successive placements; a cold joint forms when the earlier placement has hardened enough that the two placements do not consolidate into a monolithic mass. | That the interface is open, active, leaking, or structurally deficient. |
Sources: ACI CT-25; FHWA-HRT-13-092 with Directive D-67; NRMCA CIP 3, CIP 7. Checked July 24, 2026.
When does a crack stop being cosmetic?
When the evidence shows a consequence beyond appearance. ACI separates appearance, structural distress, and lack of durability as different possible implications of cracking; ACI CT-25 separately defines activity by whether width changes with time. One photograph cannot establish activity because activity is a comparison across observations.
| ID | Label | What the label asserts |
|---|---|---|
| S01 | Active | A crack whose width changes with time. |
| S02 | Dormant | A crack whose width does not change with time. |
| S03 | Appearance-only | Appearance is affected without evidence in that assessment of structural distress or durability loss. |
| S04 | Durability-related | The crack accompanies deterioration or creates a pathway relevant to moisture, chlorides, freezing, corrosion, or another exposure mechanism. |
| S05 | Structural | The crack is associated with strength, stability, load response, or the load path of the element. |
| S06 | Serviceability-related | The crack affects intended use, leakage control, moisture resistance, deflection, or another service function. |
Sources: ACI CT-25; ACI's Cracking in Concrete topic page; ACI 224R-01 Chapter 3; NRMCA CIP 7. "Appearance-only" and "serviceability-related" are editorial labels that make ACI's consequence framework explicit; they are not represented as verbatim ACI defined terms. Checked July 24, 2026.
Conditions that change the evaluation context include documented width change, repeated opening and closing, vertical offset or faulting, cracking in a load-bearing member, exposed or corroding reinforcement, leakage where watertightness is required, loose or falling material, and cracking that followed a known support or loading change. Those facts do not automatically produce one universal verdict; they show why the question has moved beyond visual naming.
How does Rockford's climate frame freeze-thaw exposure?
Rockford's 1991–2020 climate normals record 135.8 days per year with a low at or below 32 °F and 50.0 days with a high at or below 32 °F. Subtracting the nested counts yields 85.8 days per year with a minimum at or below freezing and a maximum above freezing. That is about 23.5% of a 365-day year, but it remains an air-temperature exposure metric rather than a concrete damage count.
| Month | Mean days, low ≤ 32 °F | Mean days, high ≤ 32 °F | Derived crossover days |
|---|---|---|---|
| January | 29.2 | 18.5 | 10.7 |
| February | 25.8 | 12.6 | 13.2 |
| March | 21.9 | 3.7 | 18.2 |
| April | 7.4 | 0.1 | 7.3 |
| May | 0.6 | 0.0 | 0.6 |
| June | 0.0 | 0.0 | 0.0 |
| July | 0.0 | 0.0 | 0.0 |
| August | 0.0 | 0.0 | 0.0 |
| September | 0.3 | 0.0 | 0.3 |
| October | 5.3 | 0.0 | 5.3 |
| November | 18.0 | 2.8 | 15.2 |
| December | 27.3 | 12.3 | 15.0 |
| Annual | 135.8 | 50.0 | 85.8 |
Source: National Weather Service Chicago, Rockford 1991–2020 Climate Normals, "Rockford Mean Number of Days" table. Inputs copied and arithmetic performed by Rockford Concrete Leveling Research on July 24, 2026. March, not January, produces the largest crossover count because many January days remain at or below freezing all day and therefore stay in both nested source counts.
What this figure is not. It is not 85.8 concrete freeze-thaw cycles. It does not measure concrete temperature, internal pore saturation, number of freezing events within the material, air-void protection, aggregate susceptibility, deicer exposure, or actual damage. For that context, see the Service Area page and FHWA-HIF-16-033.
Why do sources disagree about what to call these cracks?
Each issuing organization publishes terms for a specific application. ACI CT-25 defines terms for the concrete construction industry broadly. FHWA's LTPP manual defines them for highway-pavement field inspection. NRMCA's CIPs address flatwork and residential construction. When a term travels from one context to another, its measurement system, severity criteria, and application scope travel with it — or they get left behind, which is where the confusion starts.
"Map cracking" illustrates this precisely. In NRMCA flatwork guidance, the phrase "shallow map or pattern cracking" describes crazing (a surface-only phenomenon). In ACI CT-25, map cracking intersects below the surface. In FHWA's LTPP manual, it is a separately numbered pavement distress (JCP 8a) with no severity levels. In FHWA's materials guidance, it is a surface manifestation of ASR, paste freeze-thaw, and sulfate attack. The same two words carry four distinct meanings depending on source and application context. See Table 6 for the full comparison.
What are the limits of this reference?
- It covers mechanisms, visual patterns, status terms, and related conditions. It does not cover mix design, repair methods, waterproofing systems, structural reinforcement, or property valuation.
- FHWA tables are pavement guidance. The severity thresholds, appearance ranges, and distress categories in Tables 3 and 5 do not directly transfer to residential slabs, basement walls, or structural members without professional evaluation inside the relevant element and application system.
- ACI PRC-224.1-07 (Causes, Evaluation, and Repair of Cracks in Concrete Structures) was reviewed only for document identity and scope; its full content is behind the ACI paywall and no paywalled figure is carried into this page.
- The dataset does not include proprietary repair-product categories, manufacturer-defined severity scales, or insurance-claim evaluation frameworks.
- The Rockford crossover-day calculation is a derived atmospheric metric from two publicly available NWS columns. It does not model concrete temperature, moisture history, or actual freeze-thaw cycles inside any slab.
- No local Rockford or Winnebago County concrete-crack incidence dataset was identified. None is estimated or substituted.
Which common crack claims were excluded after verification?
Several widely circulated claims did not survive comparison with the primary sources:
| Excluded claim | Why it was excluded |
|---|---|
| One universal "dangerous crack width" | FHWA and ACI publish application- and exposure-specific values, not one cross-application danger line. No universal threshold is published. |
| One definitive year range for D-cracking, ASR, paste freeze-thaw, external sulfate attack, or embedded-steel corrosion | Official 2015 and 2016 FHWA publications differ. Both ranges are published side by side instead of being collapsed. |
| A Rockford or Winnebago County concrete-crack incidence rate | No official local incidence dataset was identified in the sources reviewed. No modeled or invented rate is substituted. |
| Rockford's 85.8 crossover days as "85.8 concrete freeze-thaw cycles" | The derived number measures weather-station air-temperature days, not cycles or saturation inside concrete. The stronger claim is expressly rejected. |
Source: verification log for Concrete Crack Classification v1.0.0, completed July 24, 2026 against the primary sources listed below.
How should this page be cited?
The block below is neutral bibliographic information recording the publication identity, canonical URL, dataset version, and verification date.
Publication: Rockford Concrete Leveling Research Page title: Types of Concrete Cracks: Mechanisms, Patterns, Widths, and Timelines URL: https://rockfordconcreteleveling.net/research/types-of-concrete-cracks/ Dataset: Concrete Crack Classification v1.0.0 Last updated: July 24, 2026
APA: Rockford Concrete Leveling Research. (2026). Types of concrete cracks: Mechanisms, patterns, widths, and timelines. https://rockfordconcreteleveling.net/research/types-of-concrete-cracks/
Chicago: Rockford Concrete Leveling Research. "Types of Concrete Cracks: Mechanisms, Patterns, Widths, and Timelines." Last modified July 24, 2026. https://rockfordconcreteleveling.net/research/types-of-concrete-cracks/
Where can the dataset be downloaded?
Three files reproduce the visible research asset. The classification CSV and JSON contain the same 48 records used in Tables 1, 2, 7, 7A, and 8; the Rockford CSV contains the twelve monthly NWS inputs plus the annual row and the disclosed subtraction. The files are available without registration.
- types-of-concrete-cracks-dataset-v1.0.0-2026-07-24.csv — 48 rows; fields:
record_id, taxonomic level, family, name, stage, physical description, visual clues, standards designation, documented appearance time, interpretation limit, source IDs, source URLs, source editions, verification date. - types-of-concrete-cracks-dataset-v1.0.0-2026-07-24.json — the same 48 records in machine-readable form, with dataset metadata and a full source registry.
- rockford-air-temperature-crossover-days-2026-07-24.csv — 13 rows including the annual total, with the two NWS input series and the derived crossover value.
What questions do readers ask about concrete cracks?
These answers restate the most commonly confused parts of the classification in standalone form. Each keeps the application and source limitation attached so it remains accurate when read outside the surrounding section.
What are the main types of concrete cracks?
There is no single count, because "type" is used for four different things: the mechanism that produced the crack, the pattern it makes, whether it is still moving, and separate deterioration that is not cracking at all. This reference separates them into 24 mechanisms, 13 visual descriptors, 6 status or significance labels, and 5 related conditions. ACI’s top-level framework separates cracking in plastic concrete from cracking in hardened concrete and lists eleven broad cause families.
How wide does a crack have to be before it is rated high severity?
In jointed concrete pavement, FHWA-HRT-13-092 rates a transverse crack high severity at 6 mm or wider and a longitudinal crack high severity at 13 mm or wider. Those are pavement thresholds inside a named highway-distress system. They are not residential thresholds and do not transfer to driveways, garage floors, basement walls, or structural members.
Why do transverse and longitudinal cracks have different thresholds?
The FHWA manual defines transverse and longitudinal cracking as separate pavement-distress categories and assigns different width, spalling, and faulting bands to each. The manual does not provide one universal explanation that can be carried into every concrete application. The defensible conclusion is that width is interpreted inside a named element, orientation, exposure, and measurement system — not by itself.
What is the difference between crazing and map cracking?
Crazing is a fine random surface network; NRMCA reports that craze cracks are rarely more than 1/8 in. deep and generally appear by the end of the first week. ACI CT-25 defines map cracking as intersecting cracks extending below the surface of hardened concrete. FHWA also uses "map cracking" as a pavement distress and as a surface manifestation associated with several different materials-related mechanisms, so the term must keep its source and context.
What is D-cracking?
D-cracking is freeze-thaw deterioration of susceptible aggregate. FHWA describes a closely spaced crescent-shaped hairline pattern beside joints, cracks, and free edges, commonly beginning at slab corners. FHWA's 2016 durability tech brief gives a 10–25-year appearance range, while its 2015 brief gives 10–15 years; both official ranges are preserved on this page.
Are hairline cracks always harmless?
No. ACI CT-25 defines a hairline crack only by its barely perceptible width. The term does not establish depth, cause, activity, displacement, durability consequence, or structural significance. Those are separate questions that require context and, for activity, observations over time.
How is the cause of a concrete crack actually determined?
Visible evidence is combined with information a photograph does not contain: placement date and conditions, element type and loading, support changes, exposure history, displacement, and movement over time. FHWA materials guidance also shows that overlapping field patterns can require petrographic or other material examination to separate mechanisms such as ASR and ACR.
Why does Rockford's weather matter to exterior concrete?
It establishes exposure context. Rockford's 1991–2020 normals produce 85.8 derived air-temperature crossover days per year — days with a minimum at or below 32 °F and a maximum above 32 °F. That number is not a count of freeze-thaw cycles inside concrete, because air temperature does not measure concrete temperature, pore saturation, air-void protection, aggregate susceptibility, deicer exposure, or material damage.
Which primary sources support this concrete-crack reference?
The list below contains the issuing organizations' own pages and documents used in the final verification pass. Each consequential figure on the page is traceable to one or more entries, and every listed source was checked on July 24, 2026.
- American Concrete Institute — Cracking in Concrete topic page. accessed 2026-07-24.
- American Concrete Institute — ACI CT-25: 2025 ACI Concrete Terminology. 2025 edition; accessed 2026-07-24.
- American Concrete Institute — ACI PRC-224-01, Control of Cracking in Concrete Structures. publication year 2002; reapproved 2008; accessed 2026-07-24.
- American Concrete Institute — ACI 224R-01, Chapter 3: Control of Cracking Due to Drying Shrinkage. 2001 edition; accessed 2026-07-24.
- American Concrete Institute — Frequently Asked Question: acceptable cracking. accessed 2026-07-24.
- American Concrete Institute — SP-319 paper reproducing ACI 224R-01 Table 4.1. ACI Special Publication 319; accessed 2026-07-24.
- American Concrete Institute — ACI PRC-224.1-07, Causes, Evaluation, and Repair of Cracks in Concrete Structures. March 2007; accessed 2026-07-24.
- National Ready Mixed Concrete Association — CIP 3, Crazing Concrete Surfaces. NRMCA-hosted edition; accessed 2026-07-24.
- National Ready Mixed Concrete Association — CIP 4, Cracking Concrete Surfaces. NRMCA-hosted edition; accessed 2026-07-24.
- National Ready Mixed Concrete Association — CIP 5, Plastic Shrinkage Cracking. NRMCA-hosted edition; accessed 2026-07-24.
- National Ready Mixed Concrete Association — CIP 6, Joints in Concrete Slabs on Grade. NRMCA-hosted edition; accessed 2026-07-24.
- National Ready Mixed Concrete Association — CIP 7, Cracks in Residential Basement Walls. 2014 revision; accessed 2026-07-24.
- National Ready Mixed Concrete Association — CIP 42, Thermal Cracking of Concrete. 2009 edition; accessed 2026-07-24.
- National Ready Mixed Concrete Association — TIP 17, Drying Shrinkage of Concrete. NRMCA-hosted edition; accessed 2026-07-24.
- Federal Highway Administration — Distress Identification Manual for the Long-Term Pavement Performance Program, Fifth Revised Edition. FHWA-HRT-13-092, May 2014. accessed 2026-07-24.
- Federal Highway Administration — LTPP Directive D-67, Errata to FHWA-HRT-13-092. July 17, 2019. accessed 2026-07-24.
- Federal Highway Administration — Tech Brief: Distress and Deterioration of Pavements — Materials-Related Distress. FHWA-HIF-15-018, June 2015. accessed 2026-07-24.
- Federal Highway Administration — Tech Brief: Concrete Pavement Durability. FHWA-HIF-16-033, April 2016. accessed 2026-07-24.
- National Weather Service, Chicago — Rockford 1991–2020 Climate Normals, Mean Number of Days table. accessed 2026-07-24.
- City of Rockford — Building Code Adoption and Local Amendments. accessed 2026-07-24.
- Illinois Department of Transportation — Aggregate and Concrete Materials. accessed 2026-07-24.
- American Concrete Institute — ACI 318-19 and ACI 318-14 referenced for load-path context only via ACI topic page. accessed 2026-07-24.
This research page is part of Rockford Concrete Leveling Research, the independent reference section of rockfordconcreteleveling.net. For concrete leveling service information in Rockford, IL, see the services and service area pages. Published and maintained by Rockford Concrete Leveling. Last verified: .