Cracks in concrete structures can be classified as thin (less than 1mm), medium (1-2mm), or wide (more than 2mm). Cracks are caused by factors like plastic shrinkage during curing, drying shrinkage due to moisture loss, thermal stresses from temperature changes, chemical reactions between aggregates and alkalis, and weathering from freezing and thawing. Cracks are evaluated based on their location, extent, and width. Repair methods include epoxy injection into cracks, routing and sealing cracks, reinforcing around cracks, and cement grouting of cracks. Left unrepaired, cracks can compromise the structural integrity and durability of concrete.

2. Simple answer is… … …
“Cracks can be treated as Cancer in R.C. Structures”

3. About CRACKS
Cracks in buildings are common occurrence. It develops when the stress in a
component exceeds the strength.
Classification of Cracks
Structural crack Non Structural crack
1. Incorrect design 1. Internal induced stress in building material
2. Faulty construction
3. Overloading

4. Classification of Cracks

5. OUTLINE
Introduction
Cracks Classification
Cause of cracking
Evaluation of cracking
Selection of repair procedures
Methods of crack repair
Referemces

6. INTRODUCTION
Cracks in the concrete structures are early signs of distress which
have to be diagnosed properly.
Cracks may affect appearance only, or they may indicate
significance structural distress or a lack of durability.
Efforts should be made to prevent the occurance of cracks.
Cracks are analogous to ‘CANCER’for R.C.C structures & their
presence may not only deteriorate the R.C.C structures but also
makes it unsafe to inhabit.

7. Cracks Classification
THIN
- less than 1 mm
in width.
MEDIUM
- 1 to 2 mm in
width.
WIDE
- more than 2
mm in width.

8. CAUSE OF CRACKING
1. Cracking of plastic concrete:
A. Plastic shrinkage cracking:
Fig. 1.1- Typical Plastic shrinkage cracking

9. When moisture evaporates from the surface of freshly
placed concrete faster than it is replaced by bleed water, the
surface concrete shrinks. Due to the restraint provided by
the concrete below the drying surface layer, tensile stresses
develop in the weak, stiffening plastic concrete. This results
in shallow cracks of varying depths that may form a random,
polygonal pattern, or be essentially parallel to one another
(Fig. 1.1).

10. B. Settlement cracking:
Fig. 1.2- Crack formed due to
obstructed settlement

11. Concrete has a tendency to continue to consolidate
after initial placement, vibration, and finishing.
During this period, the plastic concrete may be
locally restrained by reinforcing steel, a previous
concrete placement, or formwork. This local
restraint may result in voids, cracks, or both,
adjacent to the restraining element(Fig. 1.2).

12. 2. Cracking of hardened concrete:
A. Drying shrinkage cracking:
Fig. 1.3- Shrinkage crack in restrained
and unrestrained drying

13. A common cause of cracking in concrete is
restrained drying shrinkage. Drying shrinkage is
caused by the loss of moisture from the cement
paste constituent. If the shrinkage of concrete could
take place without restraint, the concrete would not
crack. It is the combination of shrinkage and
restraint that causes tensile stresses to develop.
When the tensile strength of the material is
exceeded, concrete will crack.

14. B. Thermal stresses:
Temperature differences within a concrete structure
may be caused by portions of the structure losing
heat of hydration at different rates or by the
weather conditions cooling or heating one portion of
the structure to a different degree or at a different
rate than another portion of the structure. These
temperature differences result in differential volume
changes. When the tensile stresses due to the
differential volume changes exceed the tensile
strength, concrete will crack.

15. C. Chemical Reaction:
Concrete may crack with time as the result of slowly
developing expansive reactions between aggregate
containing active silica and alkalis derived from
cement hydration, admixtures. When the alkalis in
cement react with susceptible aggregate particles, a
reaction rim of alkali-silica gel is formed around the
aggregate. If this gel is exposed to moisture, it
expands, causing an increase in volume of the
concrete mass that will result in cracking and may

16. D. Weathering:
Fig. 1.4- Crack in concrete due to
weathering

17. The weathering processes that can cause cracking
include freezing and thawing, wetting and drying,
and heating and cooling. Damage from freezing and
thawing is the most common weather-related
physical deterioration. Concrete may be damaged by
freezing of water in the paste, in the aggregate, or in
both . Damage in hardened cement paste from
freezing is caused by the movement of water to
freezing sites and, for water in larger voids, by
hydraulic pressure generated by the growth of ice
crystals .

18. F. Poor construction practice:
Fig. 1.5- Crack in basement wall due
to poor construction practice

19. Foremost among these is the common practice of
adding water to concrete to increase workability.
Added water has the effect of reducing strength,
increasing settlement, and increasing drying
shrinkage. An increase in water content will also
mean an increase in the temperature differential
between the interior and exterior portions of the
structure, resulting in increased thermal stresses
and possible cracking.

20. G. Error in design and detailing:
Fig. 1.6- Typical crack patterns at re-
entrant corners

21. Errors in design and detailing that may result in
unacceptable cracking include use of poorly detailed
re-entrant corners in walls, precast members, and
slabs; improper selection or detailing of
reinforcement, or both; restraint of members
subjected to volume changes caused by variations in
temperature and moisture; lack of adequate
contraction joints; and improper design of
foundations, resulting in differential movement
within the structure .

22. 1. Determination of location and extent of
concrete cracking:
A. Direct observation method:
EVALUATION OF CRACKING
Fig. 1.7- Comparator for
measuring crack widths
Fig. 1.8- Card
used to
measure
crack width

23. The locations and widths of cracks should be noted
on a sketch of the structure. A grid marked on the
surface of the structure can be useful to accurately
locate cracks on the sketch. Crack widths can be
measured to an accuracy of approximately 0.001 in.
(0.025 mm) using a crack comparator, which is a
small, hand-held microscope with a scale on the lens
closest to the surface being viewed (Fig.1.7). It is
generally more convenient, however, to estimate
crack widths using a clear card having lines of
specified width marked on the card, as shown in
(Fig. 1.8).

24. B. Nondestructive testing method:
Fig. 1.9- Rebound
Hammer
Fig. 1.10- Ultrasonic testing through
transmission technique

25. Nondestructive tests can be made to determine the
presence of internal cracks and voids and the depth
of penetration of cracks visible at the surface.
• In Rebound Hammer tapping of the surface is
used to identify laminar cracking near the surface .
A hollow sound indicates one or more crack below
and parallel to the surface .
• In ultrasonic testing machine a mechanical pulse is
transmitted to one face of the concrete member
and received at the opposite face, as shown in Fig.
1.1o. The time taken for the pulse to pass through
the member is measured electronically.

26. SELECTION OF REPAIR
PROCEDURES :1. Restore or increase strength
2. Restore or increase stiffness.
3. Improve functional performance.
4. Provide water tightness.
5. Improve appearance of the concrete surface.
6. Improve durability

27. METHOD OF CRACK REPAIR :
1. Epoxy injection:
Fig. 1.11- Epoxy Injection in progress

28. 2. Routing and sealing:
Fig. 1.11- Repair of crack by routing
and sealing

29. 3. Near-surface reinforcing and pinning:
Fig. 1.12- Repair of crack by near
surface mounted reinforcement

30. 4. Cement Grouting:
Fig. 1.13- Cement grouting machine

31. References:
Sites:
www.Colorado.edu
www.pdfgeni.com
Books:
Concrete technology:
M.S.Shetty