This document discusses various types of cracks that can occur in concrete structures. It describes cracks that can form in fresh/plastic concrete like plastic shrinkage cracks due to rapid evaporation or placement on hot surfaces. It also discusses cracks that can form in hardened concrete like drying shrinkage cracks due to restraint of concrete shrinkage or thermal cracks due to temperature differences. The document provides details on the causes and prevention of different types of cracks like using curing to prevent plastic shrinkage cracks or limiting loads to prevent overload cracks. It also describes methods to evaluate existing cracks in structures.

1. Study of Cracks in Structural Concrete
K.Prasanna,#
Lakshminarayanan.B*
#
Assistant Professor, Department of Civil Engineering, SRM University
*
UG Scholar, Department of Civil Engineering, Valliammai Engineering College.
lprasanna.env@gmail.com
2lakshminarayanan303@gmail.com
Abstract— Cracks generally occur both in plastic and elastic state of concrete, plastic shrinkage cracks appears in the surface of the
freshly placed concrete exposed to hot, windy conditions often is prone to plastic shrinkage cracking. This type of cracking is
normally noticed on beams, slabs, pavements, and other flat concrete surfaces. Many factors affect plastic shrinkage cracking, in
particular the evaporation of water from the surface of freshly placed concrete. Other factors also influence the likelihood of plastic
shrinkage cracking such as admixtures, w/c ratio, fineness of cement, workmanship, and on site building practices. Evaporation itself
is a function of climatic variables such as temperature of air, relative humidity, climatic condition, and the wind flows.
The issues related to often observed in practice scratches and cracks of concrete structures arising just at the stage of their
construction. The main cause of these cracks are change of volume, temperature rise caused by exothermic hydration process of
cement and inhomogeneous. The paper discusses the origin of early cracks and their character in massive foundation slabs and
concrete walls. The development of cracks can be minimized if appropriate measures are taken prior to finishing of concrete, during
placing of concrete and after placing of concrete.
Keywords— cracks in concrete; Plastic shrinkage cracks; On dry subgrade or hot steel shutters; Plastic settlement; Temperature;
Evaporation; Cracking of hardened concrete; Evaluating cracking in concrete.
I. INTRODUCTION
Cracking are early indications of failure of structure. Cracks are often observed in concrete structure and it is very important
to understand that all cracks may have various causes and different effects on long term performance of structures. The cracks
in a concrete element is a problem when the size of cracks exceeds a critical value such that the serviceability, durability, and
appearance of the structure are impaired. Concrete is a brittle material with less capacity of deformation under the stress. Plastic
shrinkage cracks occurs mostly in horizontal surfaces, probably they are parallel to each other and relatively shallow. Loading
of material is the one of the reason for the early stage cracks on the slabs. When concrete becomes older cracks become causes
of leakages and seepages and give entree to the moisture, oxygen, chloride, carbon dioxide etc. and other aggressive chemicals
and gases into the concrete causing serious degradation of the structure and causing corrosion of steel and damage in the
concrete and at a same time causing structural failure of the member.
II. TYPES OF CRACKS
TABLE- 1
TYPES OF CRACKS
Cracks occurring before hardening Cracks occurring before hardening
Fresh Concrete Young Concrete Mature Concrete
Plastic Plastic Shrinkage
Volume Changes
Temperature
Variation due to
hydration process
Physical-chemical Cement
carbonation
On dry
subgrade/hot steel
shutters
Alkali aggregate
reaction
Plastic Settlement Freeze- Thawing
cycle
Construction
Movement
Sub grade
movement
Drying shrinkage Volume changes External seasonal
temperature
Formwork
movement
Drying shrinkage
Frost Damage Structural cracks Fatigue
Design or
accidental load
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2. III. TYPES OF CRACKS ON FRESH CONCRETE AND PREVENTIVE ACTION
A. Plastic shrinkage cracks.
Due to rapid evaporation under action of sun and wind causes plastic shrinkage cracking. when temperature of concrete is
much higher than ambient temperature the evaporation is increased. Under such circumstances plastic shrinkage can occur even
if relative humidity of the air is high. It occurs when rate of evaporation exceeds the rate at which the bleeding water rises to the
surface. Again cracks also form under a layer of water which may get accumulated as excess of bleeding water during finishing
stage. Such cracks become apparent on drying.
1. Preventive Action.
Complete prevention of evaporation immediately after casting eliminates plastic shrinkage cracks. This can be effected by
application of curing compound where specified, alternatively covering the concrete surface by plastic sheet or hessian cloth
after about 1 to 1.5 hours of finishing the surface when impression of finger on pressing does not appear. If minor cracks
develop during this period steel trowel finish should be done to remove them. Subsequent to covering of surface when green
concrete can take load of a person without impression sprinkling of water should be started followed by proper water curing as
specified.
B. Hot steel shutters and On dry sub grade.
Green concrete placed on dry sub grade of mud mat or on hot steel shutters is prone to losing some water which may induce
cracks at bottom extending to a considerable depth in some cases particularly when concrete is produced at low water cement
ratio.
1. Preventive Action
Dry sub grade shall be properly wetted before placing concrete or a separate polythene sheet shall be provided on top of sub
grade to avoid loss of water from green concrete. On hot steel shutter water shall be spread to wet and cool such surface before
placing concrete.
C. Plastic settlement cracking.
Plastic settlement cracking can occur also at normal temperatures but in hot weather plastic shrinkage cracking and plastic
settlement cracking are sometimes confused with one another. This type of cracking on surface of fresh concrete is caused by
differential settlement of fresh concrete due to some obstruction to settlement, such as large or oversize particles of aggregate or
reinforcement. In case top cover over reinforcement bars is inadequate and excess bleeding water is accumulated during
finishing reflection cracks at spacing of reinforcement bars below are commonly seen on top of finished concrete surface.
1. Preventive Action
Plastic settlement cracking can be avoided by use of a dry mix, good compaction, pacing in layers and by not allowing too
fast a rate of build up of concrete. Also top cover over reinforcement bars is to be maintained properly to avoid such cracks.
When concrete is still green such cracks can be made to disappear by application of steel trowel.
D. Formwork Movement
Relatively small movements of formwork in the early stages of hardening will cause cracks.
E. Sub-Grade Movement
Movement of the sub-grade, can crack concrete. This may be caused by changes in soil volume in response to changes in the
soil’s moisture content, or it may be caused by subsidence. Subsidence is settling that can have a number of causes. Sub-
surface mining, extraction of natural gas, the dissolution of limestone or conditions related to groundwater can all cause soil to
settle. An example is when groundwater dissolves the carbonate cement holding sandstone particles together, and then carries
away the particles, creating a void in the soil.
F. Temperature
Hydration of cement generates heat which causes a rise in temperature of concrete. Due to internal restraint when surface of
concrete loses heat to the atmosphere a temperature differential develops between cool exterior and hot core of concrete element
as heat is dissipated to the outside fast enough due to low thermal diffusivity of the concrete. Restraint of free expansion results
in stresses compressive in one part of the element and tensile in the other. If tensile stress at the surface of the element due to
expansion of the core exceeds tensile strength of concrete or if it results in tensile strain capacity exceeds then the surface
cracking will develop. Cracking will occur when such temperature differential exceeds 20 degree C. High ambient temperature
causes a high water demand of the concrete and increase in temperature of the fresh concrete. Subsequent cooling of the
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3. hardened concrete can introduce tensile stresses. Normal Portland cement produces more heat of hydration than blended cement.
High cement content per cum of concrete produces more heat of hydrations.
1 Preventive Action
In case of very thick temperature at various points should be monitored by installing thermocouples. To prevent a large heat
loss from top, the surface shall be adequately installed with polystyrene and evaporation prevented by use of plastic membrane
or application of the curing compound. Along sides of the raft, formwork shall be retained to act as insulation till temperature
differential has been reduce to 10 degree C. Placing concrete at a low temperature using pre cooled aggregates and water
alternatively using ice flakes would help in minimizing surface cracks. During concreting at high ambient temperature
evaporation of water after finishing shall be prevented by application of curing compound, alternatively covering the surface
with plastic sheet or hessian cloth shortly after finishing followed by sprinkling of water and regular water curing there after.
Portland cement with low rate of heat development and using blended cement in which part of cement is replaced by flyash can
reduce heat of hydration which in turn could minimize development of surface cracks. Whenever possible it is desirable to place
the concrete in the coolest part of the day and preferably at a time such that ambient temperature will rise following the setting
of the concrete
G. Shrinkage induced cracking.
Presence of clay in aggregates leads both to higher shrinkage and greater cracking. Increase in water cement ratio tends to
increase shrinkage and at the same time reduces strength of concrete. Increase in cement content also increases shrinkage and
therefore cracking tendency although effect on strength is positive. Carbonation produces shrinkage, but it reduces subsequent
moisture movement. When stress induced by free shrinkage strain is greater than tensile strength of concrete cracks would occur.
1. Preventive Action
Limited presence of silt or clay in aggregates within allowable percent reducing water cement ratio and cement content in
concrete can minimize cracks in concrete. Use of retarders may allow more shrinkage to be accommodated in the form of
plastic shrinkage, increase extensibility of concrete and there by reduce cracking.
Fig.1 Cracks in fresh concrete
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4. Fig.2 Causes of cracks in concrete
IV.TYPES OF CRACKS ON HARDENED CONCRETE AND PREVENTIVE ACTION
A. Climatic Condition
Concrete can be easily get damaged by freezing of water both in elastic stage and plastic stage. Freeze water inside concrete
result in increase in volume of concrete. The increased volume of concrete results in cracking of concrete. Climatic condition is
wear and tear of structures caused by freezing, drying and wetting of concrete.
1. Preventive Action
By use of the tough aggregate, low water cement ratio and adequate curing of concrete. Concrete can be protected against
weathering.
B. Chemical reactions ( Cement carbonation, Alkali aggregate reaction )
Chemical reaction inside concrete can be due to water filled inside water retaining structure, foundation that came in contact
with soil or due to air pollutant which react with concrete. Concrete get cracked when concrete react with aggregate containing
active-silica and alkalis resulting from cement hydration. When the alkalis in cement react with aggregate particles a reaction
film of alkali-silica gel is produced around the aggregate. If this gel is exposed to moisture it will expands causing an increase in
the volume of the concrete mass which finally results in cracking. Chemical reactions which occur due to reaction of concrete in
its firm state with materials used to make concrete or by materials that came in contact with it.
1. Preventive Action
Preventive measures include use of aggregates which do not take part in reaction. Certain carbonates rocks take part in
reactions with alkalis produce expansion and cracking. Sulfates from soil when react with cement paste Calcium Sulfo
aluminate is formed, which may be root cause in increase in volume of concrete. This increased in volume of concrete causes
development of closely spaced cracks and ultimately deterioration of the concrete. Sulfate- resistant cements are very beneficial
in reducing this problem. Using concrete with a low water cement ratio is important to have adequate protection against severe
sulfate attack.
C. Corrosion of Reinforcement
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5. Corrosion to reinforcement is signs rather than reason for concrete damage. Corrosion occurs due to electrochemical
oxidation of reinforcement bars in existence of moisture and electron flow inside metal. After corrosion the volumes of
reinforced bars get increased. Due to increase in volume of reinforced bars a bursting radial stresses are produced around bars
which result in local radial cracks around bars.
1. Preventive Action
Preventive technique comprises of epoxy coating of bars before fixing the reinforcement, use of richer grade of concrete and by
use of corrosion inhibitors admixtures.
D. Drying Shrinkage
Tensile stresses are developed within structure due to combination of shrinkage and restraint provided by another part of the
structure. Concrete has greater volume when it is in dried form and it volume decreases on drying decrease in volume is due to
loss of water. When decrease in volume of concrete is restrained by reinforcement bars then cracks is established called Plastic
shrinkage cracks. As we know that concrete are week in tension so when tensile stress which is developed during restraint
exceeds tensile strength of concrete then cracks started to develop. These cracks are detected at the surface which go deep later
on as time passes.
1. Preventive Action
Factors which affect drying shrinkage are type of aggregate and water cement ratio. Hardened aggregate offer more resistance
to shrinkage. Contraction joints and correct detailing of the reinforcement reduces shrinkage cracking.
E. Poor Construction Practices
When construction is not done correctly cracks started to develop due to wrong construction practice. Additional of water to
increase workability is the most common. Addition of water plays an important role in increasing drying shrinkage of concrete,
increasing concrete settlement and decreasing concrete strength. When less curing is done or curing is eliminated early stages
may cause develop of cracks in the structure.
F. Design or Accidental load
Concrete gets damaged due to structural overload which are very easy to detect. Precast member like beam and are generally
subjected to this type of load. Most unfortunate things about cracks is due to structural overload are that cracks are detected at
early stages.
1. Preventive Action
These types of cracks can be prevented if designer limit the load on structure. And avoid overloading or accidental loading in
newly constructed building
G. Thermal Stress
Thermal cracks occur due to excessive temperature within a concrete structure. The temperature differences causes the
coolest portion to contract more than the warming portion, which restrains the contraction. Thermal stresses are produced when
there is normal expansion and contraction of concrete due to surrounding change in air temperature. When there is no provision
of thermal expansion concrete will crack.
1. Preventive Action
The key to minimize thermal cracking is to recognize when it is occur and take steps to minimize it. Reduce heat of hydration
by optimum utilization of cement and Jute bags is used to cover concrete and keep watering it, The frequency may be varies
according to the weathering condition.
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6. Fig.3 Cracks in hardened concrete due to impact load or over load
V. EVALUATING CRACKING IN CONCRETE
A. Direct and indirect observation
The exact location and breath of cracks should be marked on the drawing. A grid marked on the surface of the structure
can be useful to accurately locate cracks on the drawing. A small hand held microscope with a scale and lens closet is used to
view the cracked surface, Crack widths can be measured to an accuracy of about 0.025 mm. The movement of cracks can be
monitored with mechanical movement indicators. Sketches can be supplemented by photographs documenting the condition of
the structure at the time of investigation. However it is generally more convenient to estimate crack thicknesses using a clear
card which have lines of specified thickness marked on it. Observations such as reinforcement which exposed to environment,
surface wear and tear and rust mark on reinforcement bars should be noted down on the sketch. Internal conditions of the crack
at definite locations can be observed with the use of flexible shaft fiber- scopes or rigid bore scopes. In this method first we
note thickness of crack on a sketched of structure. Then grid are marked on the surface of the structure and crack widths are
measured by this instrument to an accuracy of about 0.025 mm .This instrument comprises of a small hand-held microscope
with a scale on the lens closest to the surface being viewed as shown in (Fig.4) below. However it is generally more convenient
to estimate crack thicknesses using a clear card which have lines of specified thickness marked on it, as shown in (Fig..5) below.
Any movement of the surface across the crack should also be documented. Observations such as reinforcement which
exposed to environment, surface wear and tear and rust mark on reinforcement bars should be noted down on the sketch.
Internal conditions of the crack at definite locations can be observed with the use of flexible shaft fiber- scopes or rigid bore
scopes.
Fig.4—Comparator for measuring crack thicknesses Fig.5—Card used to measure crack thickness
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7. B. Non – Destructive Testing.
Nondestructive tests can be used to determine the presence of internal cracks and voids and the depth of cracks visible at the
surface. Tapping the surface with a hammer is simple method to recognize laminar cracking near the surface. A hollow sound
indicates one or more cracks below and parallel to the surface. The presence of reinforcement can be determined using a
pachometer. Results of Pachometers are observed by use computer algorithms and magnetic fields to provide a visual picture of
the reinforcing bars layout in the scanned area. This device is very useful in detecting reinforcement bars, measure concrete
cover, and estimate the position and reinforcement size.In some cases, however, it may be necessary to remove the concrete
cover often by drilling or chipping to identify the bar, especially in areas of congested reinforcement. The most common
technique is through-transmission testing using commercially available equipment (Malhotra and Carino 1991; Knab et al.
1983). A mechanical pulse is transmitted to one face of the concrete member and received at the opposite face, as shown Fig.
2.4. The time taken for the pulse to pass through the member is measured electronically. If the distance between the transmitting
and receiving transducers is known, the pulse velocity can be calculated. When access is not available to opposite faces,
transducers may be located on the same face [Fig. 6]. While this technique is possible, the interpretation of results is not
straightforward. A significant change in measured pulse velocity can occur if an internal discontinuity results in an increase in
path length for the signal. Generally, the higher the pulse velocity, the higher the quality of the concrete. The interpretation of
pulse velocity test results is significantly improved with the use of an oscilloscope that provides a visual representation of the
received signal. If corrosion is a suspected cause of cracking, the easiest approach to investigate for corrosion entails the
removal of a portion of the concrete to directly observe the steel. Corrosion potential can be detected by electrical potential
measurements using a suitable reference half-cell.
Fig. 6 Pachometer reinforcing bar indicator
C. Tests on concrete cores:
Necessary information can be obtained from cores taken from selected locations and selected positions within the structure.
Cores and core holes is used to measure the width and depth of cracks. In addition, the strength of concrete can be determined
by using cores an indication of concrete quality can be obtained from compressive strength tests; however, cores that contain
cracks should not be used to determine concrete strength. Chemical tests for the presence of excessive chlorides indicate the
potential for corrosion of embedded reinforcement.
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8. Fig.7 Collection of core samples
VI.CONCLUSIONS AND RECOMMENDATIONS.
Control of cracking in fresh concrete and hardened concrete is of great importance to ensure a desired service life and
function of structures. The paper is divided into three parts. First Part contains different types of cracks, Second part contains
different causes of cracks and preventive action. Final part contains evaluation of cracks. This paper on a whole focuses on
possible causes and evaluation of cracks in R.C.C structures. Pachomerer is used in determining concrete cover, size and
location of reinforcement. The following preventive measures to be taken to prevent cracking in the concrete.
 Reduce heat of hydration by optimum utilization of cementicious material
 Cure properly as soon as finishing has been completed. Ensure concrete is properly placed,
compacted
 Use cooler concrete in hot weather and avoid excessively high concrete temperature in cold weather condition.
 Dry sub grade shall be properly wetted before placing concrete or a separate polythene sheet
 cover the concrete surface by plastic sheet or hessian cloth after about 1 to 1.5 hours of finishing the surface when
impression of finger on pressing does not appear. If minor cracks develop during this period steel trowel finish
should be done to remove them.
 Plastic settlement cracking can be avoided by use of a dry mix, good compaction, pacing in layers and by not
allowing too fast a rate of build up of concrete. Also top cover over reinforcement bars is to be maintained properly
to avoid such cracks.
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9. VII. ACKNOWLEDGMENT
The authors would like to thank the officials and management of SRM University for their support in doing this work.
VIII. REFERENCES
[1] ACI 224.3R-95: Joints in Concrete Construction (Reapproved 2013)
[2] ACI 224.2R-92: Cracking of Concrete Members in Direct Tension (Reapproved 2004)
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[4] ASTM C881 “Standard Specification for Epoxy-Resin-Base Bonding Systems for Concrete”
[5] Concrete Technology by M. S. Shety, Publication of S. Chand & Company Ltd, Delhi, 2005
[6] IS 456:2000, “Indian Standard of Plain and Reinforced Concrete Code of Practice.
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[10] S. Zhang, C. Zhu, J. K. O. Sin, and P. K. T. Mok, “A novel ultrathin elevated channel low-temperature poly-Si TFT,” IEEE Electron Device Lett., vol. 20,
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[11] ACI Committee 308 Standard Practice for Curing Concrete ACI Manual of Concrete Practice, Part 2, 2001.
[12] Ajdukiewicz A., Kliszczewicz A., Głuszak B.; Destrukcja termiczna zbiorników żelbetowych we wczesnym okresie dojrzewania (Thermal
destructionof RC tanks in early age of concrete). XXXIX Konferencja Naukowa KILiW PAN i KN PZITB Krynica 1993, Vol.5, p.5-12 (in Polish)
[13] Guide to concrete repair U.S Department of the Interior Bureau of Reclamation Technical service center.
[14] Appendix E Avoiding Coating Failures Due to Cracking of Concrete Coating Manual
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