Overview of Types and Causes
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Concrete Cracks: An Overview of Types of
Cracking/Deterioration and Their Implications
Table of Contents
Concrete Cracks: An Overview of Types of Cracking/Deterioration and Their Implications
Introduction
Causes of Cracking
Overloading
Corrosion
Freeze/Thaw
Alkali-Aggregate Reaction (AAR)
Shrinkage
Poor Workmanship
Types of Cracks
Crazing
Disintegration
Plastic Cracks
Hardened Cracks
Scaling
Delamination
Overloading Cracks
Spalling
Relevant Case Studies
Harbour Cay Condominium Collapse
Skyline Plaza Collapse
Conclusion
Annotated Bibliography:
Victoria Interval, B.A.E/M.A.E., The Pennsylvania State University
[Photo Credit: Victoria Interval]
Introduction
Concrete cracks. In fact, it is designed to crack to be able to fully engage the reinforcing steel. Concerns with
concrete cracking come up when owners and maintenance workers are unsure of what to look for or are
unaware of the implications of certain cracks. Some types of cracking indicate a structural issue, when others
do not indicate any type of issue other than normal weathering.
There are many different causes of cracks, which can lead to different types of cracking patterns. Each type of
cracking pattern can be associated with a likely cause. If this cause is recognized, it can be identified as
structurally vital or non-vital. It is of particular interest in discerning between these two so that the failure and
damage of these can be avoided or at least predetermined to minimize economic damage, future deterioration,
and in severe cases the loss of human life.
Causes of Cracking
Overloading
The cross section of concrete is designed with both calculated and estimated loads, determined from building
codes. Design includes such factors as the strength of the concrete, the number, sizing, and placement of
reinforcing bars, and size and shape of the concrete cross section. When a structure is overloaded to the extent
not covered in safety factors, concrete may be damaged or fail. Overloading may be in shear, flexure, or
tension, or may be a result of fatigue or cyclic loading. Each of these has a different cracking pattern to look for
(see Loading Cracks below).
Corrosion
Corrosion of the reinforcing steel in
concrete can be a major structural
issue. Under normal conditions, the
pH level of concrete is high (above
12.5). The high pH of concrete
allows an inactive layer of ferric
oxide to form around the
reinforcement, preventing rust
(Khan 2006, p. 14).
There are two major causes of
corrosion in the reinforcing steel: chloride
penetration and carbonation. Chloride
penetration reduces the pH level of the
Figure 2: Process of carbonation [Image Credit: Robert Pirro]
concrete when oxygen, chlorides, and
moisture all penetrate the concrete (Pirro
2012, p. 20). Chlorides can be found in
potable water, which should never be
used to mix concrete. They are also an
environmental factor that may add up
over the lifespan of a structure. For
instance, buildings exposed to salt water
or de-icing salts may experience faster
Figure 1: Process of chloride penetration [Image Credit: Robert Pirro]
chloride build up from the salts (Emmons
1993, p. 12). The chloride penetration process can be viewed in Figure 1.
Carbonation occurs when carbon dioxide and moisture infiltrate the concrete, reducing the pH level of the
concrete (Pirro 2012, p. 29). This process can be seen illustrated in Figure 2.
Both causes of corrosion end similarly. The pH level is the concrete’s last barrier against corrosion, so the
reinforcement begins to rust (Khan 2006, p. 14). Rust expands the steel to 10 times the volume, which can
cause major problems in the structure (see Spalling below).
Freeze/Thaw
Freezing and thawing cycles can be very detrimental to concrete over time. Unless a protective coating is
applied to the concrete, each cycle allows more moisture to penetrate into the concrete. The stress of the
moisture freezing inside the concrete causes larger defects with each cycle. Air-entrained concrete can be used
to help alleviate some of the expansive stresses of harsh temperature changes. However, not all freeze/thaw
effects can be assuaged in this way and many structures may succumb to cracking either caused or worsened
by these cycles. Manufacturers of crack repair kits suggest that cracks less than 1/16" in thickness can be
repaired without professional contractors ("Types" 2012). However, tolerable crack widths may be significantly
less than this (0.016" and less depending on the environment) because cracks may allow deteriorating
chemicals to damage the concrete in other ways (Emmons 1993, p. 13).
Alkali-Aggregate Reaction (AAR)
AAR refers to chemical reactions taking place within the concrete mix. Certain aggregates inside the concrete
may react with alkalis, causing concrete expansion. The alkalis may be also be from within the concrete mix, or
may be from outside sources like sea or ground water, or deicing salts. Depending on the type of aggregate,
AAR also goes by other names. In siliceous aggregates, the reactions are called "alklali silica reactivity" (ASR).
In dolomitic carbonate rocks, the reactions are called "alkali-carbonate reactivity" (ACR) (Khan 2006, p.15).
When these types of ractions occur, they create a gel-like substance that swells when moisture reaches it. The
stresses from the swelling create internal tensile forces, which may crack the concrete from within (Khan 2006,
p.15).
Shrinkage
Concrete shrinkage may occur throughout a structure’s life cycle for different reasons with the majority occurring
within the first few months or years after casting. There are two primary categories of shrinkage: plastic (before
hardening), and drying (after hardening). Immediately after concrete is poured, there can be settlement
shrinkage, construction movement (e.g. formwork movement or removal), and drying shrinkage. After the
concrete has fully hardened, a structure will undergo temperature, volume and chemical changes throughout
the years (Winterbottom, p. 2). Each of these may also cause concrete shrinkage.
Shrinkage is an expected phenomenon in a concrete structure, and can often be controlled with stress-relieving
joints and properly placed reinforcing steel.
Poor Workmanship
Concrete itself is so variable that properly constructing a concrete structure can be difficult. Some issues related
to workmanship are as follows: over/under consolidated aggregates, improper location of rebar, over watering
for workability, finishing surface before bleeding occurs. Each of these may end up not mattering overall, or may
contribute to a structural failure.
Types of Cracks
Concrete cracking and defect patterns can often indicate its cause or causes and can help to define whether the
crack is architectural (affecting aesthetics only) or structural (may affect the load carrying capacity). Some of the
main types of cracking are described below.
Crazing
Crazing is a web-like series of fine cracks, usually at the surface of the concrete. These can be caused by
surface shrinkage, which can occur in low humidity, hot air or sun, and wind (PCA 2001, p. 3). Since these
cracks occur on the surface and do not penetrate deeper into the concrete, they do not indicate a deeper
structural issue. A general pattern of crazing can be seen below in Figure 3.
Figure 3: Crazing pattern [Image Credit: Victoria Interval]
Disintegration
Concrete disintegration can be a result of freeze/thaw cycles on the surface. Moisture enters concrete pores
and expands. The expansions can cause microcracking or they may force off a small amount of the surface.
Figures 4 and 5 depict disintegration on concrete surfaces. When tiny pieces of the surface come off, it is called
disintigration (Pirro 2012, p. 38).
Plastic Cracks
Figure 4: Concrete disintegration around column base [Photo Credit: Robert Pirro] Plastic cracks occur before
the concrete
has
hardened.
They are
caused by
rapid loss of
water during
curing or
settlement in
the concrete
itself (PCA
2001, p. 2).
Hot, dry air
and
excessive
water in the
mix may both
cause
cracking.
Figure 5: Sidewalk disintegration [Photo Credit: Victoria Interval]
Hairline
cracks may occur in as little as a few hours after a concrete pour, depending on the weather. The thin lines may
be misleading; although they may be very thin, these hairline cracks may extend through the entire thickness of
the slab (VandeWater 2012, p. 1). See Figure 6 below for an example of plastic cracking. This kind of cracking
mostly affects slabs and other large flat surfaces, whose surface area is high relative to the volume of the
concrete. This allows the water to evaporate quicker than it can bleed to the surface, causing the cracking
(Khan 2006, p. 9). These kinds of cracks may initiate other cracking issues because the plastic cracks
sometimes are initiation points for drying shrinkage (Emmons 1993, p. 68).
Figure 6: Plastic cracks [Photo Credit: Robert Pirro]
Hardened Cracks
Hardened cracks occur after the concrete has hardened, and are generally caused by drying shrinkage,
settlement of the structure below grade, and thermal contraction effects. The cracks form because while the
concrete is drying, its volume is being reduced. The condition of the concrete is restrained, so instead of just
shortening the slab or member length, cracks form throughout to allow the reduction in volume (Emmons 1993,
p. 30). This kind of crack is depicted below in Figure 7. Drying shrinkage is the shrinking (or reduction in
volume) of the concrete due to loss of water (evaporation through the concrete surface) (Barth 2001, p. 2).
These kinds of cracks may indicate improperly spaced joints (PCA 2001, p. 2).
Figure 7: Hardened cracks [Photo Credit: Robert Pirro]
Scaling
Scaling appears as small divets in the concrete surface in which aggregate may be exposed. Scaling is often
caused by freeze/thaw cycles (PCA 2001, p. 10). Because scaling is a surface defect, it does not generally
indicate a more serious structural issue.
Delamination
Delamination occurs when the surface of a slab is finished prematurely. When concrete cures, it is necessary
for the excess water to escape to the surface (a process called “bleeding”). If a slab is finished before bleeding
has occurred, it can trap the water below the surface. When the water does escape, it leaves hollow patches
just below the surface. These patches may break open, resembling shattering, to expose the aggregate below
as seen in Figure 8 (PCA 2001, p. 12). This type of defect occurs near the surface, and does not indicate a
structural threat (unless over a cantilever, where the reinforcing steel is near top portion of the slab).
Figure 8: Delamination of concrete caused by premature finishing [Photo Credit: Robert Pirro]
Overloading Cracks
Overloading a concrete member may cause several types of cracks. Depending on the direction and location of
the crack (vertical, diagonal, top, bottom, etc), the type of loading stress can be identified. For example, vertical
cracks at the bottom of a simply supported beam and in the center indicate positive flexural cracks. Negative
flexural cracks show up over the supports on the top of the beam, also as vertical cracks (Pirro 2012, p. 47). It
should be noted that flexural cracks may be related to longitudinal splitting cracks. This relationship is based on
splitting cracks allowing moisture to reach the steel pieces in the concrete and corrode them, reducing their
ability to resist flexure cracks. Reduction in resistance may cause additional flexural cracks (Giuriani 1998, p. 1).
Shear cracks may appear as diagonal cracks at quarter points along the beam member (Pirro 2012, p. 47). See
the diagram below in Figure 9 for better understanding of locations of cracking. These cracks can indicate a
deeper structural issue if the crack width or lateral displacement exceeds 1/4" (CFA 2005, p. 3).
Figure 9: Diagram of locations and directions of overloading cracks [Diagram Credit: Robert Pirro]
Spalling
Spalling is primarily a result from the corrosion of the reinforcing steel and/or embedded objects such as clips,
chairs, anchors, etc. When the steel corrodes, the rust expands to 10 times the original volume, creating internal
tension forces in the concrete . Concrete is unable to handle the tension forces, and the pieces between the
corroded steel and the nearest surface will break off, called "spalling" (PCA 2001, p. 12).
Even just a small spall can indicate a much larger issue for two main reasons. First, a small spall can expose
the steel, leaving it ultra-vulnerable to more corrosive elements. This can been seen in Figure 10. If the steel
corrodes more, there will be more spalling, as seen in Figure 11. Second, a spall in one area may be the first
piece of a larger issue beneath the surface. It is likely that other rebar in the immediate area has also been
affected by the corrosive effects and will begin to spall soon. Small spalls are relatively simple and inexpensive
to fix, and repairing these early on can help to avoid large spalling areas.
A large spall area in a slab may indicate immediate danger to a structure. If enough concrete has spalled off of
the bottom, exposing the reinforcing grid, then the concrete and steel are no longer working together to handle
the compressive and tension forces. Essentially, when the concrete reaches its tensile limit, it will fail. The steel
is not engaged by the concrete to take the excess tensile forces, and is only acting as a cage to hold up the
concrete. At this stage, repairs may be enormously expensive. Figure 12 shows a whole building spalling
failure.
Figure 10: Small spall area caused by corrosion of reinforcment [Photo Credit: Robert Pirro]
Relevant Case Studies
There are many examples of cases where concrete
cracking foreshadowed a structural failure. The two
Figure 11: Large spall area [Photo Credit: Robert Pirro] discussed here are cases were concrete cracking
warned about impending
failures.
Harbour Cay
Condominium
Collapse
click here to view the Wiki
page on this case study
In this case study, concrete
cracking in the floors was
Figure 12: Large spall area on all balconies of building [Photo Credit: Robert Pirro] noticed and brought up to
the engineer. The engineer
confirmed that the structure was strong enough, and the cracking was ignored. The structure collapsed on
March 27, 1981 (Kukorlo 2009).
Skyline Plaza Collapse
click here to view the Wiki page on this case study
The Skyline Plaza collapse occurred on March 2, 1973 during construction due to punching shear around the
columns. Had someone noticed the overloading cracks, the 14 deaths and 34 injuries may have been avoided.
(Perkins 2009).
Conclusion
With so many causes and types of cracks, it can be difficult to identify which cracks or defects indicate a more
serious structural issue and which are simply architectural. Many cracks are caused by either overloading,
corrosion, shrinkage, or poor workmanship. When looking at a specific cracking pattern or defect in concrete,
sometimes the cause can be attributed to a specific reason. Other times the pattern may have multiple causes
leading to its current state. In better understanding some of the causes of concrete cracks as well as different
cracking pattern types, engineers, construction managers, and others may be able to avoid major structural
catastrophes. If concrete is cracking when it should not be, it needs to be identified quickly and repaired before
a structural failure.
Annotated Bibliography:
1. Barth, Florian, et al (2001). "Control of Cracking in Concrete Structures." ACI Committee Report, Number
224R. American Concrete Institute, P. 1-8.
Technical Report: ACI Committee report goes through different causes of cracking due to shrinkage.
http://www.concrete.org/General/f224R(01)Chap3.pdf
2. Concrete Foundations Association (2005). “Concrete Cracking.” CFA:
<http://www.cfawalls.org/foundations/cracking.htm > (Oct. 1, 2012).
Website: CFA goes summarizes cracking causes for foundations. The website includes a flyer which is a
nice user’s guide for when to seek professional help with concrete cracks.
3. Emmons, P. (1993). Concrete Repair and Maintenance Illustrated, RS Means, Kingston, Massachusetts. P.
12-13, 30, 68.
Book: Emmons describes concrete problems and how to analyze each situation in case repair is
necessary.
4. Giuriani, and Plizzari, . (1998). “Interrelation of Splitting and Flexural Cracks in RC Beams.” J. Struct. Eng.,
124(9), 1032–1049. doi: 10.1061/(ASCE)0733-9445(1998)124:9(1032)
Technical Paper: Giuriani and Plizzari explore flexural and splitting cracks in reinforced concrete beams.
These expose large areas of reinforcing steel, putting the beams at high risk for corrosive effects. These
types of cracks will fall under the structurally corrupt category.
http://%E2%80%A2http://ascelibrary.org.ezaccess.libraries.psu.edu/doi/pdf/10.1061/%28ASCE%2907339445%281998%29124%3A9%281032%29
5. Khan, Mohammad S., et al (2006). Control of Cracking in Concrete. Transportation Research Circular,
Number E-C107. Transportation Research Board, Washington, D.C. P. 1-16.
Book: The beginning chapter of the book summarizes different causes of cracks in concrete, splitting
them into three main categories (mechanical loading, volumetric stability, and environmental loading and
durability).
http://onlinepubs.trb.org/onlinepubs/circulars/ec107.pdf
6. Kukorlo, J. (2009). “Harbour Cay Condominiums.” Failures Wikispace,
<https://failures.wikispaces.com/Harbour+Cay+Condominiums> (Oct. 2, 2012).
Website: Failures Wiki: This is a case study where concrete cracking was seen, brought to the
engineer’s attention, and ignored. This resulted in the collapse of the structure and the deaths of 23
people.
7. Perkins, S. (2009). "Skyline Plaza - Bailey's Crossroads." Failures Wikispace,
<http://failures.wikispaces.com/Bailey%27s+Crossroads+-+Skyline+Plaza> (Nov. 20, 2012).
Website: Failures Wiki: This case study explores the collapse of a concrete structure due to punching
shear. This resulted in the deaths of 14 people.
8. Pirro, R. (2012). "Concrete Evaluation and Repair Techniques." Professional lecture, Sept. 27, 2012.
Presentation: This presentation was given by R. Pirro, P.E. on concrete evaluation and repairs. It
describes why concrete problems occur and gives examples of each.
9. Portland Cement Association (2001). Concrete Slab Surface Defects: Causes, Prevention, Repair. Portland
Cement Association, Skokie, Illinois.
Booklet: PCA describes surface defects of concrete with various causes. These types of defects are not
considered structurally vital.
http://www.oboa.on.ca/events/2009/sessions/files/Slab%20Surface%20Prevention%20Repair.pdf
10. Rotunno, J. (2009). “Concrete System Collapses & Failures During Construction.” Failures Wikispace,
<https://failures.wikispaces.com/Concrete+System+Collapses+%26+Failures+During+Construction> (Oct. 2,
2012).
Website – Failures Wiki: This is a series of case studies in concrete failures. It describes causes of the
failures, which will shed light on which kinds of cracks the engineers should have seen before collapse.
11. “Types of Concrete Cracks.” Foundation Armor, <http://www.foundationarmor.com/article/types-of-concretecracks/ > (Oct. 2, 2012).
Website: This is a website selling concrete repair kits for homeowners. It suggests tolerance of crack
sizes (divided into categories of the concrete application) that are considered “fixable” with an amateur
remedy.
12. VandeWater, S. (2012). Why Concrete Cracks.
Technical Essay: Vandewater (a professional with 25+ years) summarizes civil concrete cracking
causes and effects. The essay focuses on sidewalks and small structures and covers cracking that, in its
context, is not particularly threatening, but could be on a larger scale.
http://indecorativeconcrete.com/idcn/wp-content/uploads/2012/02/Why-Concrete-Cracks.pdf
13. Wight, J. and MacGregor, J. (2009). Reinforced Concrete: Mechanics & Design, Pearson, Upper Saddle
River, New Jersey. P. 14, 41-44, 61-63, 236-240, 243-244, 256-257, 264, 320, 322, 418-427,844-850.
Book: The book touches on concrete cracks in various chapters. It touches on design cracks (necessary
to engage the reinforcement), and different types of cracks resulting from varied loads. The book also
looks at cracks caused by other internal (material) and external (weathering) effects.
14. Winterbottom, G. and Goodwin, F. Concrete Cracks: Causes, Correcting, and Coating, Degrussa
Construction Systems Americas, Shakopee, MN.
Technical Report: Scientists summarize causes of concrete cracks, splitting them into categories
(chemistry, plastic, hardened, shrinkage, expansion).
https://www.corrdefense.org/Academia%20Government%20and%20Industry/T-52.pdf
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