Iaetsd repairing cracks in concrete structures

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Abstract— Studies related to crack formation in concrete is
most interesting because sometimes the same causes produce a
different cracking pattern and sometimes the same cracking
pattern produced by different causes. Sometimes concrete
cracks in allocation where no cause can be found and in other
places it does not crack where there is every reason why it
should. Eighty percent of these cases, however, are straight
forward. In order to determine whether the cracks are active or
dormant, periodic observations are to be done.
Various factors have impeded improvements in the durability
of concrete repairs, including: inadequate condition evaluation
and design; lack of quality construction practices and quality
control; and the choice of repair materials. It is necessary to
reconsider some recent developments in structural repairs from
the view point of extending the service lives of structures under
repairs [1].
Index Terms—Concrete, cracks, reinforcement, repair
I. INTRODUCTION
Cracks can be broadly divided into two categories
namely solitary cracks and pattern cracks. Generally,
solitary crack is due to a positive overstressing of concrete
either due to load or shrinkage, the cause of which will
become apparent when the line of the crack is compared
with the layout of the portion of the concrete, its
reinforcement and the known stresses in it. Overload
cracks are fairly easily identified because they follow the
lines demonstrated in laboratory load test. A crack due to
setting and hardening shrinkage is formed in the first week
of life of the concrete. If length of concrete under
inspection is more than about 9m, it is not likely that there
is a solitary crack, usually there will be another one of a
similar type and the analysis of the second confirms the
finding from the first.
Regular pattern of cracks may occur in the surfacing of
concrete and in thin slabs. The term pattern cracking is
used to indicate that all the cracks visible have occurred
more or less at the same time.
II. TYPES OF CRACKS
Cracks can be divided into two types:-Solitary cracks
and pattern cracks
A. Solitary cracks ( Isolated cracks)
Generally, a solitary crack is due to a positive over
stressing of the concrete either due to load or shrinkage,
the cause of which will become apparent when the line of
the crack is compared with the layout of the portion of the
concrete, is reinforcement and the known stresses in it.
Overload cracks are fairly easily identified because
they follow the lines demonstrated in laboratory load
tests. A crack due to setting and hardening shrinkage is
formed in the first week of life time of concrete. If the
length of concrete under inspection is more than 9m, it is
not likely that there is a solitary crack, usually there will be
another one of a similar type and the analysis of the
second confirms the finding from first. In a long retaining
wall or long channel, the regular formation of cracks
indicates faults in the design rather than the construction,
but an irregular distribution of solitary cracks may indicate
poor construction as well as poor design.
B. Pattern cracking
Regular pattern of cracks may occur in the surfacing
of concrete and in thin slabs. The term pattern cracking is
used to indicate that all the cracks visible have occurred
more or less at the same time.
III. IDENTIFICATION OFCRACKS
Crack movement can be detected by placing a mark at
the end of the crack. Subsequent extension of the crack
beyond the mark indicates probable continuance of the
activity originally producing the defect. The deficiency of
this defect is that it will not show any tendency for the
crack to close or provide any quantitative data on the
movement. In another method, a pair of tooth pick is
lightly wedged into the crack and falls out if there is any
extension of the defect. The deficiencies of this method, as
before, are that there is no indication of closing,
movement or any qualitative measure of the changes
which occur.
A strip of notched tape works similarly. Movement is
indicated by tearing of the tape. An advantage is that same
indication of closure can be realized by observing any
writing on the tape. This device is not reliable; however,
the tape is not dimensionally stable under changing
conditions of humidity, so that one can never be sure
whether the movements are due to shrinkage or swelling
of the marker. A device using a typical vernier caliper is
the most satisfactory of all. Both extension and
compression are indicated and movements of about
(1/100) inch can be measured using a vernier caliper. If
more accurate readings are desired, extensometers can be
used. The reference point must be rigidly constructed and
carefully glued to the surface of concrete, using a
carborundum stone to prepare the bonding surface before
attaching the reference mark.
REPAIRING CRACKS IN CONCRETE STRUCTURES
Anilkumar P M and Dr. J Sudhakumar
B.Tech Student,Professor
Department of Civil Engineering, National Institute of Technology, Calicut
anilkumar_bce11@nitc.ac.in,skumar@nitc.ac.in
INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN ENGINEERING RESEARCH, ICCTER - 2014
INTERNATIONAL ASSOCIATION OF ENGINEERING & TECHNOLOGY FOR SKILL DEVELOPMENT www.iaetsd.in
110
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Where the extreme accuracy is required, resistance-
strain gauges can be glued across the crack. They are
however, expensive, sensitive to changes in humidity, and
easily damaged.
IV. MEHODSOF REPAIR
A. Bonding with epoxies
Cracks as narrow as 0.05mm can be bonded by the
injection of epoxy. The technique generally consist of
establishing entry and venting ports at close intervals
along the crack, sealing the crack on exposed surfaces, and
injecting the epoxy under pressure [2]. Usual practice is to
drill into the crack from the face of the concrete at several
locations; inject water or a solvent to flush out the defect;
allow surface to dry (using hot air jet, if needed);
surface-seal the cracks between the injection points; and
inject the epoxy until it flows out of the adjacent sections
of the crack or begins to bulge out the surface seals, just as
in pressure grouting. Usually the epoxy is injected through
holes of about 0.75 inch diameter and 0.75 inch deep at
about 6 to 12 inch centers (smaller spacing are used for
finer cracks). This method of bonding represents an
application for which there is no real substitute procedure.
However, unless the crack is dormant (or the cause of
cracking is removed, thereby making the crack dormant), it
will probably recur, possibly somewhere else in the
structure. Also, this technique is not applicable if the
defects are actively leaking to the extent that they cannot
be dried out, or where the cracks are numerous.
B. Routing and sealing:
This method involves enlarging the crack along its
exposed face and filling and sealing it with a suitable
material. This is a simplest and most common technique
for sealing cracks and is applicable for sealing both fine
pattern cracks and longer isolated cracks.
The cracks should be dormant. This technique is not
applicable for sealing cracks subjected to a pronounced
hydrostatic pressure. The routing operation consists of
following along the cracks with a concrete saw or with
hand or pneumatic tools, opening the crack sufficiently to
receive the sealant. A minimum surface width of 0.25 inch
is desirable. Smaller openings are difficult to work on. The
surface of the routed joints should be rinsed clan and
permitted to dry before placing the sealant. The method
used for placing the sealant depends on the material to be
used and follows standard technique.
Routing and sealing of leaking cracks should preferably
be done on pressure face so that the water or other
aggressive agents cannot penetrate the interior of the
concrete and cause side effects such as swelling, chemical
attack or corrosion of the bars.
The sealant may be any of several materials, depending
on how tight or permanent a seal is desired. On roadway
pavements it is common to see cracks which have been
sealed by pouring hot tar over them. This is a simple and in
expensive method where thorough water tightness of the
joint is not required and where appearance is not
important.
C. Stitching:
The tensile strength of a cracked concrete section can
be restored by stitching in a manner analogous to sewing
cloth. Concrete can be stitched by iron or steel dogs in the
same way as timber. The best method of stitching is to
bend bars into the shape of a broad flat bottomed letter U
between 1 feet and 3 feet long and with ends about 6
inches long (or less if the thickness of the concrete is less),
and to insert them in holes drilled to match in the
concrete on either side of the crack. The bars are then
grouted up- some grout being placed in the holes in
advance of the bars.
In case of stitching of concrete pavements, after
stitching a longitudinal crack it may be necessary to treat a
nearby longitudinal joint. A primary concern is whether a
crack has formed below the saw cut for longitudinal joints.
If a crack has occurred and the joint functions properly,
then no treatment other than joint sealing is warranted.
However, if there is no crack extending below the saw cut
joint, then it is advantageous to fill the saw cut with epoxy
to strengthen the slab at this location. If the joint is not
functioning, but a joint sealant has already installed, then
no further action is recommended [3].
Usually cracks start at one end and run away from the
staring place quicker on the side of the concrete than on
the other. The stitching should be on the side which is
opening up first. The following points should be observed,
in general
i. Any desired degree of strengthening can be
accomplished, but it must be considered that the
strengthening also tends to stiffer the structure locally.
This may accelerate the restraints causing the cracking
and reactivate the condition.
ii. Stitching the cracks will tend to cause the problem to
migrate elsewhere in the structure. If it is decided to
stitch, investigate and if necessary, strengthen the
adjacent areas of the construction to take the
additional stress. In particular, the stitching dogs should
be of variable length and /or orientation and so located
that the tension transmitted across the crack does not
devolve on a single plane of the section, but is spread
out over an area.
iii. Where there is a water problem, the crack should be
sealed as well as stitched so that the stitches are not
corroded, and because the stitching itself will not seal
the crack. Sealing should be completed before stitching
is commenced both to avoid the aforementioned
corrosion and because the presence of dogs tends to
make it difficult to apply the sealant.
iv. Stress concentrations occur at the ends of cracks, and
the spacing of the stitching dogs should be closed up at
such locations.
v. Where possible, stitch both sides of the concrete
section so that further movement of the structure will
not exert any bending action on the dogs. In bending
members it is possible to stitch one side of the crack
only, but this should be the tension side of the section,
where movement is originating. If the members are in
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a state of axial tension, then a symmetrical placement
of the dogs is a must, even if excavation or demolition
is required to gain access to opposing sides of the
section.
vi. As an alternative to stitching cracks at right angles
which method does not resist shear along the crack,
sometimes it is necessary use diagonal stitching. One
set of dogs can be placed on each side of the concrete
if necessary. The length of the dogs is random so that
the anchor points do not form a plane of weakness.
vii. The dogs must be grouted with a non-shrink or
expanding mortar, so that they have a tight fit, and
movement of the cracks will cause the simultaneous
stressing of both old and new sections. If this is not
possible, proportion the stitching to take the entire
load without precipitation.
viii. The dogs are relatively thin and long and so cannot take
much in the way of compressive force. Accordingly, if
there is a tendency for the crack to close as well as to
open, the dogs must be stiffened and strengthened by
encasement in an overlay or by some similar means.
D. External stressing
Development of cracking in concrete is due to tensile
stresses and can be arrested by removing these stresses.
Further, the cracks can be closed by inducing a
compression force sufficient to overcome the tension and
to provide a residual compression. The compressive force
is applied by use of the usual prestressing wires or rods.
The principle is very similar to stitching, except that the
stitches are tensioned; rather than plain bar dogs which
apply no closing force to the crack and which may in fact
have to permit the crack to open up a bit before they
begin to take the load. All the points noted regarding
stitching must be considered. An additional problem is that
of providing an anchorage for the prestressing wires or
rods. Some form of abutment is needed for this purpose.
The effect of the tensioned force on the stress conditions
in the structure should be analyzed.
E. Grouting
Grouting of the cracks can be performed in the same
manner as the injection of an epoxy, and the technique
has the same areas of application and limitations.
However, the use of an epoxy is the better solution except
where considerations of fire resistance or cold weather
prevent such use, in which case grouting is the comparable
alternative. The procedure is similar to other grouting
methods and consist of cleaning the concrete along the
crack; installing built-up seats at intervals along the crack,
sealing the crack between the seats with a cement paint or
grout, flushing the crack to clean it and test the seal; and
then grouting the whole. The grout itself is
high-early-strength Portland cement.
An alternative and better method, where it can be
performed, is to drill down the length of the crack and
grout it so as to form a key. The grout key functions to
prevent related transverse movements of the sections of
concrete adjacent to the crack. However, this technique is
applicable only where the cracks run in a reasonably
straight line and are accessible at one end. The drilled hole
should preferably be 2 or 3 inches in diameter and flushed
to clean out the crack and permit better penetration of the
grout.
F. Blanketing
Blanketing is similar to routing and sealing, but is used
on a larger scale and is applicable for sealing active as well
as dormant cracks. The following are the principal types of
blanket joints
i. Type 1 – Where an elastic sealant is used: The sealant
material is the one which returns to its original shape
when not under an externally induced stress, i.e., acts
elastically. The recessed configuration is used where
the joint is subjected to traffic or a pressure head. The
strip sealant is applicable where there are no traffic or
pressure problems and is somewhat less costly. The
first consideration in the selection of sealant materials
is the amount of movement anticipated and the
extremes of temperature at which such movement
occur. It should be capable of deforming the required
amount under applicable conditions of temperature.
The material should be able to take traffic, be resistant
to chemical spillage, be capable of being pigmented, if
desired.
ii. Type 2 – Mastic filled joint: The sealant is a mastic
rather than a compound having elastic properties. This
type of joint is for use where the anticipated
movements are small and where trafficabilty or
appearances are not considerations. The advantage is
that the mastic is less costly than the elastic type of
sealant material.
iii. Type 3 – A mortar plugged joint: Proper sealing to the
crack is provide against leakage using a temporary
mortar plug. The mortar plug provides the strength for
the joints. The plug resists the pressure on the joint by
arching the load to the sides of the chase. Where the
pressure acts on the face of the joint, static balance
requires the development of the tensile and shear
stresses between the plug and the sides of the slot.
Compression and shear are developed when the
pressure head is internal. Theoretically, the edges of
the chase should be undercut so that the mortar plug,
which dries more quickly and more thoroughly at the
surface, does not shrink differentially and so pull away
from the sides and bottom of the chase.
iv. Type 4 – A crimped water bar: Mortar is required
where the junctions which bear traffic. A crimped
water stop joint sealant is not applicable for the use
where the crack is subject to a heavy pressure head
from inside the joint, or where movement occurs as a
shear along the length of the crack. In the first case, the
pressure would tend to bulge out the crimp, and in the
second case, the longitudinal movement would tend to
tear the stop. Accordingly, this type of joint is primarily
for use where the anticipated movements are limited
to a simple extension or contraction and where the
pressure head is either small or acts from the face of
the joint. However, while not specifically intended for
such applications, a rubber type or similar crimped joint
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material will take a little longitudinal movement, and
the crimped stop will function under limited amount of
relative transverse displacement.
v. Dealing with the reinforcement: What ever be the
detail used, when cutting the chase, it is possible that
some of the reinforcement will be exposed. If this is the
case, cut deep is enough so that the sealant will be
behind the reinforcing, clean the bars, and paint them
with bitumen as a protection against moisture
penetration and corrosion. If the crack in an active one
with substantial movements, cut the bars so that they
do not impede the movement of the joint.
G. Overlays:
Overlays may be used to seal cracks and are very
useful and desirable where there are large numbers of
cracks and treatment of each individual defect would be
too expensive.
i. Active cracks: Sealing of active cracks by use of an
overlay requires that the overlay be extensible. The
occurrence or prolongation of crack automatically
means that there has been an elongation of the surface
fibers of the concrete. Accordingly, an overlay which is
flexible but not extensible, i.e., can be bent but cannot
be stretched, will not seal a crack that is active. A two
or three-ply membrane of roofing felt laid in a mop
coat of tar, with tar between the plies; the whole
covered with a protective course of gravel, concrete, or
brick, functions very well for this purpose. The type of
protective course depends on the use to which it will
be subject. Gravel is typically used for applications such
as roofs, and concrete or brick is applicable where fills
is to be placed against the overlay. An asphalt block
pavement also works well and may be used where the
area is subjected to heavy traffic. If the cracks are
subjected to longitudinal movements parallel to their
axis, the overlay will wrinkle or tear. Be very careful in
repairing such joints. Blanketing may be a better
solution.
ii. Dormant cracks: If the cracks are dormant, almost any
type of overlay may be used, provided that it will take
the traffic to which it is subject and that it is either
adequately bonded or thick enough so that curling due
to differential deformations is not a problem. Epoxy
compounds are coming into increasingly frequent use
for this purpose.
H. Autogenous healing
The inherent ability of concrete to heal, cracks within
itself is termed as autogenous healing, and is a
phenomenon which has been known for some time. It has
practical application for sealing dormant cracks, such as in
the repair of precast units cracked in handling, reknitting
of cracks developed driving the driving of precast piling,
sealing cracks in water tanks, and sealing of cracks which
are result of temporary conditions or inadvertencies. The
effect also provides some increase in strength of concrete
damaged by vibration during set and of concrete disrupted
by the effects of freezing and thawing.
The mechanism whereby healing occurs is the
carbonation of calcium oxide and the calcium hydroxide in
the cement paste by carbon dioxide in the surrounding air
and water. The resulting calcium carbonate and calcium
hydroxide crystals precipitate, accumulate and grow out
from cracks. The crystals interlace and twine producing a
mechanical bonding effect, which is supplemented by a
chemical bonding between adjacent crystals and the
surfaces of the paste and aggregate. As a result, some of
the tensile strength of the concrete is restored across the
cracked section, and the crack may become sealed.
Saturation of the cracks and the adjacent concrete with
water during the healing process is essential for the
development of any substantial strength. Submergence of
the cracked section is desirable. Alternatively, water may
be ponded so that the crack is saturated. Upto 25% of the
normal tensile strength may be restored by healing under
conditions of submergence in water. The saturation must
be continuous for the entire period of healing. A single
cycle of drying and re-immersion will produce a drastic
reduction in the amount of healing strength. Healing
should be commenced as soon as after the crack appears
as possible. Delayed healing results in less restoration of
strength. Healing will not occur if the crack is active.
Healing also will not occur if there is a positive flow of
water through the crack, which dissolves and washes away
the lime deposit, unless the flow is slow so that complete
evaporation occurs at the exposed face causing
re-deposition of the dissolved salts.
Concrete cracks both dormant and active subjected to
water pressure are able to themselves with time. The
greatest autogenous healing effect occurs between the
first 3 to 5 days of water exposure. In additional skin
reinforcement proves to be highly effective in supporting
the autogenous healing effect. Growth rate of calcium
carbonate crystals depends on crack width and water
pressure, whereas concrete composition and type of water
has no influence on the autogenous healing rate [4].
V. CONCLUSION
General precautions to be followed for the repair of
cracks in concrete as listed below.
i. Do not fill the cracks with new concrete or mortar
ii. Try to avoid the use of brittle overlay to seal an active
crack.
iii. Do not fail to remove the restraints causing the crack.
iv. Do not surface –seal cracks over corroded
reinforcement without encasing the bar.
v. Do not bury or hide a joint so that it is in accessible.
REFERENCES
[1] C.S. Suryawanshi, “Structural concrete repair – A durability
based revised approach is needed”, The Indian Concrete
Journal, 2012.
[2] “Causes, Evaluation and Repair of Cracks in Concrete
Structures”, Reported by ACI Committee 224.
[3] “Stitching concrete Pavement”, International Grooving and
Grinding Association, June 2010.
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[4] Carola Edvardsen, “Water permeability and Autogenous
healing of cracks in concrete”, ACI Material journal, Title no
96-M 56, July/August 1999.
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Iaetsd repairing cracks in concrete structures

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Iaetsd repairing cracks in concrete structures