Crack In Concrete & Crack width measurement

1. Cracks in concrete

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

3. INTRODUCTION
Cracks in the concrete structures are early signs of distress that have to be diagnosed
properly.
Cracks may affect appearance only, or they may indicate significant structural distress or a
lack of durability.
Efforts should be made to prevent the occurrence 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 them unsafe to inhabit.

4. Cracks in concrete(continue)
Cracks in buildings could be broadly classified as structural and non-structural
cracks.
1. Structural Cracks : These occur due to incorrect design, faulty construction or
overloading and these may endanger the safety of a building. e.g. Extensive
cracking of an RCC beam.
2. Non structural Cracks: These are mostly due to internally induced stresses in
buildings materials and do not endanger safety of a building but may look
unsightly, or may create an impression of faulty work or may give a feeling of
instability.

5. Cracks in concrete(continue)
•In some situations due to penetration of moisture through them non structural
cracks may spoil the internal finishes thus adding to the cost of maintenance, or
corrode the reinforcement, thereby adversely affecting the stability of the
Structure in long run. e.g. Vertical crack in a long compound wall due to
shrinkage or thermal movement.

6. Cracks in concrete(continue)
Cracks may appreciably vary in width from very thin hair crack barely visible to
naked eye to gaping crack. Depending upon the crack width cracks are classified as:
(a) Thin Crack - less than 1 mm in width,
(b) Medium Crack - 1 to 2 mm in width,
(c) Wide Crack - more than 2 mm in width.
(d) Crazing - Occurrence of closely spaced fine cracks at the surface of a material
is called crazing.

7. Cracks in concrete(continue)
The commonly used building material namely masonry, concrete, mortar etc. are weak in
tension and shear. Therefore the stresses of even small magnitude causing tension and shear
stresses can lead to cracking.

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

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

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

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

12. 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.

13. 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 eventually result in the complete deterioration of the structure.

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

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

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

17. 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

18. 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).

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

20. 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.

21. 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

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

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

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

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

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