Transcript Cracks in Buildings Made of Fly Ash Bricks and the Remedies

CRACKS IN BUILDINGS :
THE ROLE OF FLYASH
BRICKS AND THE
REMEDIES
P R O F. ( D R . ) A N A N TA K U M A R D A S
D E PA R T M E N T O F C H E M I C A L E N G I N E E R I N G,
D U R G A P U R I N S T I T U T E O F A D VA N C E D
T E C H N O L O G Y A N D M A N A G E M E N T,
DURGAPUR
INTRODUCTION
Cracks in buildings are of common occurrence and are
developed whenever stress in the component exceeds its
strength. This stress caused by external forces such as dead,
live, wind, seismic loads and foundation settlement; or
induced internally due to thermal movements, moisture
changes, chemical actions, weathering actions resulting in
shrinkage or expansion of the bricks, mortar, concrete or due
to corrosion of reinforcement etc. render the structure unsafe.
The other causes may be due to fault of structural design. Fly
ash brick, a building component has a little effect on these
cracks that can be avoided if it is manufactured and used
properly.
CLASSIFICATION OF CRACKS
VERTICAL CRACKS
• Develops
due
to
shrinkage, expansion or
thermal
movement of
brick,
mortar
and
concrete.
• Do not endanger the
safety of the building but
unsightly
• Impression
of
workmanship
faulty
HORIZONTAL CRACKS
• Develops mainly at the
junction of brick masonry
with RCC slab
• Weakens
the
construction and requires
heavy repairing resulting
high cost involvement
STRUCTURAL
• Faulty construction
NON STRUCTURAL
• Internal induced
building materials
stress
in
• Overload
• Penetration of moisture and
weather actions through the
masonry works
• Extensive cracking of RCC
beam
• Resulting in shrinkage
bricks, mortar, concrete
• Incorrect Design
• Resulting
corrosion
of
reinforcement & hence increase
in volume
in
PATTERN OF CRACKS
CLASSIFICATIONS:
• Straight
• Toothed
• Stepped
• Random
• Crazing - Occurrence of closely spaced fine
cracks at surface of a material
STRAIGHT
CRACKS
TOOTHED
CRACKS
STEPPED
CRACKS
RANDOM
CRACKS
CRAZING
CAUSES OF CRACKS
A building component develops cracks whenever
stress developed in the component exceeds its
strength.
External applied force
Internal induced stress
• Due to dead, live, wind or seismic
load
• Due
to
thermal
movements
moisture change and chemical
actions
• Formation of settlement
• In building component leading to
dimensional changes
• Horizontal movements.
• Due to volume change within a
component
resulting
either
expansion or contraction
• Compressive Tensile
Building composition
Shear
in
SUBJECT
TO
CRACKING
•Masonry
•Concrete
•Mortar
WIDTH OF
CRACKS
•Thin – <1mm
•Medium –1 to
2 mm
•Wide – >2mm
PRINCIPAL CAUSES OF CRACKS IN
BUILDING (Non structural)
• Thermal variation
• Chemical reaction
• Moisture movement
• Elastic deformation
• Creep
• Foundation movement & settlement of soil
• Vegetation
• Manufacturing defects
Details in next few slides
THERMAL MOVEMENTS
FACTORS AFFECTING
• Temperature variation
• Dimensions
• Coefficient of expansion
• Physical properties of the materials
COEFFICIENT OF THERMAL
EXPANSION (10-6 / ⁰C)
• Clay Brick & Brick work  5-7
• Cement mortar & concrete  10-14
• Sand lime bricks  11-14
• Fly ash bricks  13-17
SOME FACTORS INFLUENCING IN
THERMAL CRACKING
a.Color & surface characterization (high
reflectivity coefficient) reduces heat load on
the roof
b.Thermal conductivity
c.Provision of an insulating or protective layer
d.Internally generated heat
OTHER FACTORS INFLUENCING IN
THERMAL CRACKING
• Loss of heat by radiation into the atmosphere depends on the proportion of
exposed surface to volume of the component. For instance, if under certain
conditions in a 15 cm thick fly ash brick wall, 95% of heat is lost to the air in 1.5
hours under similar circumstances; same amount of heat will be lost in about one
week when the wall is 1.5 m thick .
• Generally speaking thermal variations in the internal walls are not much and this
does not cause cracking. It is mainly the external walls, especially thin walls
exposed to distinct solar radiation and the roof which are subject to substantial
thermal variations and are this liable to cracking.
• Horizontal crack at the support of an RCC Roof slab due to Thermal movement of
slabs.
CHEMICAL REACTIONS
MOVEMENT DUE TO CHEMICAL
REACTION
Soluble sulphates, which are sometimes present in ground water react with excess lime of fly ash
bricks, form gypsum and calcium aluminate sulphate which occupy much bigger volume than that of
the original constituents. This expansion reaction results in weakening of masonry & plaster and
then formation of cracks.
The severity of sulphate attack depends upon amount of soluble sulphate present, permeability of
fly ash bricks, concrete mortar, proportion of C3A present and duration for which the building
components in quantity remains damp. If pure water free of sulphate is used for manufacturing of fly
ash bricks then sulphate attack can be avoided. Similarly chloride content in water enhances the cell
formation in case of RCC and again propagates the cracks due to increase in volume of Fe3O4.
The chemical reaction proceeds very slowly and it may take about two to more years before the
effect of this reaction becomes apparent.
SEVERITY OF SULPHATE /CHLORIDE
ATTACK
a.Amount of soluble sulphate/Chloride present
b.Permeability of bricks, concrete mortar
c.Proportion of C3A present
d.Duration for which the building components
in quantity remains damp.
MOISTURE MOVEMENT
Initial shrinkage:- Initial shrinkage in fly ash –sand-lime
bricks is about 50% greater than that due to subsequent
wetting and drying from saturation to dry state.
Reversible
In the first instance, moisture
present in the intermolecular
space (absorbed moisture) dries
out, causes some reduction in
volume & shrinkage. This is
reversible in nature.
Irreversible
After capillary water is lost, CaSiO3
gel crystallizes and gives up some
moisture (absorbed moisture) and
individual
molecules
undergo
reduction in size, resulting in
shrinkage which is irreversible
nature.
Most of the cracking in these
materials occurs due to shrinkage at
the time of initial drying.
0.14
Drying Shrinkage
0.12
Fig. Relation between drying
shrinkage and fly ash-sand-lime
compacts of different composition
0.15
0.1
Drying Shrinkage
DRY
SHRINKAGE
VS FLY ASH
– SANDLIME
COMPOSITI
ON
Fig. Relation between drying shrinkage
and fly ash-sand-lime compacts of
different composition
0.08
0.06
0.1
0.05
0
8.0:1.0:1.0 7.5:1.5:1.0 7.0:2.0:1.0 6.0:3.0:1.0 5.5:3.5:1.0 5.0:4.0:1.0
Composition of fly ash - sand - lime
0.04
0.02
0
8.0:1.0:1.0
7.5:1.5:1.0
7.0:2.0:1.0
6.0:3.0:1.0
5.5:3.5:1.0
Composition of fly ash - sand - lime
5.0:4.0:1.0
FLY ASH-SAND-LIME BRICKS VS
CLAY BRICKS
ELASTIC DEFORMATION
Hydration of lime after brick formation causes a
reduction in the volume of the system of silica –
lime – water to an extent of 0.5% of the volume of
the dry compact. This is the plastic strain which
aggravates due to loss of water by evaporation
that causes surface cracks. If the proportions of
each components are properly mixed this problem
will be negligible for making fly ash bricks.
CREEP
The increase of strain of a compact with time
under sustained stress is termed creep, the
shrinkage and creep occurs simultaneously.
The rate of creep decreases with time and the
factors influencing creep are similar to shrinkage
which are described later. In case of fly ash bricks
Creep is negligible if the bricks are matured .
FOUNDATION MOVEMENT &
SETTLEMENT
Fly ash bricks are not responsible for this type of
failure
VEGETATIONThe growth of unwanted plants in the
construction makes the construction weak.
So any growth of plant is to be stopped.
Not related to fly ash bricks
SHRINKAGE
SHRINKAGE OF FLY-ASH BRICKS:
The Factors
a)
b)
c)
d)
e)
f)
g)
h)
i)
LIME / FLY ASH CONTENT
WATER CONTENT
AGGREGATES
ACCELERATORS
CURING
PRESENCE OF EXCESSIVE FINES
HUMIDITY
CEMENT AS A COMPONENT
TEMPERATURE
DETAILS GIVEN IN THE NEXT FEW SLIDES
LIME/ FLY ASH CONTENT
Higher the lime, greater the drying shrinkage.
Conversely larger the volume of aggregate,
lesser the shrinkage for bricks, increasing the
volume of aggregates by 10%, reduction of
shrinkage by 50%.So proper composition to
be maintained.
WATER CONTENT
Greater the quantity of water used in the mix,
greater the shrinkage. Thus a wet mix has
more shrinkage than a dry mix which is
otherwise similar. So better vibration / high
pressure gives less shrinkage. On the other
hand variation in the strength will occure.
AGGREGATES
By using largest possible maximum size of
aggregate in brick and ensuring good grading
– requirement of water is reduced. Aggregates
that are porous and shrink on drying result in
higher shrinkage
ACCELERATORS
Accelerators like CaCl2, MgCl2 is added for
faster reaction towards silicate bonding but
use in high percentage over 0.5 to 2,
shrinkage could be more. In case of steam
curing the addition of accelerators has no
noticeable impact.
CURING
Proper curing should be done started as soon as
initial set has taken place and it is to be continued
for at least for 7 days, then drying shrinkage will be
less, because when hardening takes place under
moist environments, then there is initially some
expansion which offsets a part of subsequent
shrinkage. Steam curing at the time of
manufacturing reduces the liability to shrinkage as
high lime results in pre-carbonation
PRESENCE OF EXCESSIVE FOREIGN
FINES
Like Silt, clay, dust should not be more than 24% in aggregates because it increase surface
area resulting high water requirement and
resistant to bonding in chemical reaction.
HUMIDITY
Shrinkage is much less in coastal areas where
relative humidity remains high. Low relative
humidity causes plastic shrinkage.
CEMENT AS A COMPONENT
Rapid hardening cement to be avoided because it has
greater shrinkage than ordinary Portland cement of
higher proportion of CaSiO3 & lower proportion of
alkalis like sodium oxide and potassium oxide to be
used. PPC cement is preferable.
Otherwise difference in strength development
between fly ash bricks and cement mortar will cause
cracks.
TEMPERATURE
If the temperature of the mix is lowered from
38⁰C to 10⁰C, it would results reduction of
water requirements and hence lower
shrinkage. It is thus follows that in a tropical
countries like India, brick work done by fly ash
bricks in mild winter would have much less
tendency for cracking than that done in hot
summer. So the aggregates and mixing water
should be shaded from direct sun.
MEASURES FOR CONTROLLING
CRACKS DUE TO SHRINKAGE
On account of drying out of moisture content in building materials/
components.
Shrinkage in a material induces tensile stress when there is some
restraint to movement where the stress exceeds the strength,
cracking occurs, this relating the stress. Cracks get localized at weak
sections such as door and window opening or staircase walls.
Avoiding use of rich cement mortar in masonry made of fly ash
bricks. Delaying plaster work till masonry has dried after proper
curing. Shrinkage is made to take place without any restraint.
MEASURES FOR CONTROLLING
CRACKS DUE TO SHRINKAGE(Cont.)
Coat of plastering on masonry is restrained from shrinkage to some extent
by its adhesive bond to non shrinking background, the later having already
undergone shrinkage.
Shrinkage of a rich and strong mortar is known to extent sufficient force to
tear off the surface layer of weak bricks.
In summer 1 cement: 6 sand
In winter 1 cement: 5 sand
If a wall exceeds 5-7m length, provide control joints at weak sections.
Curing of masonry should be done sparingly to avoid body of the blocks
getting wet.
Avoid excessive welting of masonry at the time of plastering so that
moisture doesn’t reach the body of the blocks.
DRYING SHRINING OF FLY ASHSAND-LIME BRICKS(Comparison)
FRESH MIXTURE BRICK
STRENGTH
75-85 kg/cm2
3 days
95-105 kg/cm2
15 days
DEAD MIXTURE BRICK
STRENGTH
107-112 kg/cm2 30 days
DRYING
0.084%
SHRINKAGE
SORPTION
21%
40-50 kg/cm2
3 days
52-55 kg/cm2
15 days
55-60 kg/cm2
30 days
56-62 kg/cm2
45 days
DRYING
0.128%
SHRINKAGE
SORPTION
34%
MORTOR SHRINKAGE
MORTOR SHRINKAGE
(sand: cement=5:1)
Rate of
SHRINAGE
MORTOR SHRINKAGE
(sand: cement=6:1)
190.275
10 days
16 days
190.155
15 days
190.500
18 days
190.160
18 days
190.465
21 days
190.130
25 days
190.455
23 days
190.090
30 days
190.435
30 days
190.585
10 days
190.535
0.079%
SRENGTH
SHRINAGE
0.097%
MANUFACTURING DEFECTS
SOURCES OF SUCH DEFECTS – Role of fly ash bricks
• To use proper and standard raw materials
• Heterogeneity in mixing causes unequal stress development and subsequently
cracks in bricks and in structure
• If moisture is high in the mixture the coarse aggregate will settle down and lime
will come up on the surface during compaction resulting less strength
development
• If the filling of the materials in the mould box is not proper, unequal pressure
will develop on the brick mixture and improper compact will be formed. So crack
will develop in the brick itself and in the construction
• If numbers of bricks are more in one cycle then deviations in compaction will be
much more resulting different quality bricks
• Old mixture in manufacturing bricks to be avoided
SOME CONSTRUCTIONS
CONSTRUCTION
ON PROCESS
WITH FLY ASH
BRICKS
COMPLETE
CONTRUCTION
WITH FLY ASH
BRICKS
A 3-Storied
building
constructed
by Fly ash
bricks
A BEAUTIFUL
CONSTRUCTION
WITH FLY ASH
BRICKS
CONCLUSION
It is found that fly ash-sand-lime bricks are the
only alternative to burnt clay bricks to avoid
pollution in the atmosphere, to save huge energy
consumed in brick kiln by burning coal and to save
huge fertile land. If the product is manufactured
with proper technological method and construction
is done with proper way it will be the only
alternative to conventional clay brick. This is the
prime importance of the manufacturers, users and
engineers to find out the solution how the
ecofriendly bricks can be consumed for all
construction purposes for the sustainability of the
Society and our Earth.
THANK YOU