Deformation of Progressively Cracking Reinforced Concrete Beams
CONCRETE CRACKING – WHO IS TO BLAME.pdf
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CONCRETE CRACKING –
WHO IS TO BLAME?
By
Christopher Stanley
TECHNICAL DIRECTOR
UNIBETON READY MIX
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CRACKING FACTS(Concrete Society Technical Report Number 22, non-structural cracks in concrete)
“If a concrete is either cooled or dried, then provided it is free
from restraint, it will reduce in length and no cracks willdevelop”
“Cracks will not form unless there is some form of restraint”
“Because it acts as a form of internal restraint, reinforcement
governs the spacing and width of cracks in hardened concrete
but reinforcement does not have the same effect in plastic
concrete”
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CRACK ASSESSMENTUsually based on:
Critical viewing distance and personal viewpoint
Type of structure, often using an arbitrary or “prestige”
scale (e.g monumental or public buildings, commercial buildings and carparks, public paving, driveways, private housing)
Cracks can be classified thus
Fine cracks - up to 1mm wide
Wide cracks - from 1mm to 6mm wideFractures - over 6mm wide
It is sometimes specified or implied that crack widths of
up to 0.3mm are aesthetically acceptable
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All concrete cracks but some can be prevented
•
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CRACKS OCCURING
BEFORE HARDENING
CONSTRUCTION
MOVEMENT
SUB-GRADE MOVEMENT
FORMWORK MOVEMENT
CRACKS OCCURING AFTER HARDENING
STRUCTURAL
CRACKS
DESIGN LOADS
ACCIDENTALOVERLOAD
CREEP
EXTERNAL SEASONAL
TEMPERATURE VARIATIONS
FREEZE/THAW CYCLES
CHEMICAL
CEMENT
CARBONATION
ALKALI-AGGREGATE
REACTIONS
CORROSION OF
REINFORCEMENT
SHRINKABLE
AGGREGATE
CRACK CLASSIFICATION
PLASTIC CRACKS
PLASTIC SHRINKAGE
PLASTIC SETTLEMENT
CRAZING
DRYING
SHRINKAGE
PHYSICALCRACKS
EXTERNAL
RESTRAINT
INTERNAL
TEMPERATURE
GRADIENTS
EARLY AGE
THERMAL
CONTRACTION
THERMAL
CRACKS
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TYPE OF CRACK -
time of appearance REF *FORM, LOCATION, etc.
PRIMARY/SECONDARY
CAUSESREMEDY * *
PLASTICSETTLEMENT TYPE A Cracks over reinforcementin deep sections Excess bleeding (PC) Reducebleeding
10 minutes to
three hoursTYPE B
“Arching” cracks in
columns Re-vibrate
TYPE CCracks at change of depth
in slab/beam sections
Rapid early drying
conditions
Add Air
entrainment
PLASTIC
SHRINKAGETYPE D
Diagonal cracks in roads
and slabsRapid early drying (PC)
30 minutes to
six hoursTYPE E
Random cracks in
reinforced slabsLow rate of bleeding
Improve early
curing
TYPE FCracks over reinforcement
in slabs
Ditto and steel near
surface
EARLY THERMAL
CONTRACTIONTYPE G
External restraint cracks
in thick walls or columns
Excess heat generation
(PC) rapid cooling
Reduce heat
and/or insulate
One day to three
weeksTYPE H
Internal restraint cracks in
thick slabs
Excess temperature
gradients, rapid cooling
LONG TERM
DRYING SHRINKAGE
weeks - months
TYPE ICracking in thin slabs
and walls
Inefficient joints (PC)
Excess shrinkage and
inefficient curing
Reduce water
content
Improve curing
CRAZINGTYPE J
Cracks “off the form” in
fair-faced concreteImpermeable formwork,
rich mixes, poor curing
Improve curing
and finishing
1 - 7 days
sometimes much later TYPE K
Cracks in power- floated
slabs
over-trowelling.
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SIMPLE CRACK MODEL
Initial state after pouring
r e
s t
r a i n
t
If dried out orcooled with partial
or no restraint
r e s t r a i n tRestraint - shortterm effects
Restraint
Medium/long
term effects
Crack r elieves
tension
Contraction
without stress
n o
r e s t r
a i n
t r
e
s t r
a i n
t
Tension!
Free to shrink
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Common crack types – Plastic shrinkage cracking (Type A)
Caused by BLEEDING (“A special case of sedimentation” - T.C.Powers,1939)
The phenomenon of water rising to the surface of plasticconcrete, caused by gravity pulling heavier particles
downward, the latter being known as sedimentation
Bleeding is not a result of poor compaction, and it cannot be
eliminated by improved compaction
Sedimentation
bleed water evaporates - volume change - shrinkage - tension - restraint - crack?
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PLASTIC SETTLEMENT CRACKSType A cracks
Water void formed under
steel – desiccation - water
evaporates leaving dry void
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Settlement cracks occurring at
changes in slab depth
Type C cracks
PLASTIC SETTLEMENT CRACKS
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Tension
Item cast into slab
acts as crackinducement - crack
follows line of least
resistance
Uneven sub base
acts as crack
inducement - crack
follows line of least
resistance
PLASTIC SHRINKAGE CRACKS
Type E - random
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Type F – over reinforcement
PLASTIC SHRINKAGE CRACKS
Insufficient reinforcement cover ?
(cracks induced by steel proximity
to surface which is in tension
due to rapid drying)
Differentiated from plastic settlement
cracks because plastic shrinkage cracks
tend to pass through slab depth
Tension
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Plastic Shrinkage Cracking
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Plastic Shrinkage Cracking
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Plastic Shrinkage Cracking
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Plastic Shrinkage Cracking
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Plastic Shrinkage Cracking
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Thermal cracking - specification
Question - Why 70°C max. temperature?
Past experience with mass concrete and accelerated curing e.g. in precasting
suggests that the quality of the cement hydrate at elevated temperatures >70°C is
inferior to that in a normally cured concrete so mechanical strength tends to be
lower - in addition a phenomenon known as Delayed Ettringite Formation (DEF)may affect durability
Question - why require a maximum temperature differential of 20°C?
Raw materials for concrete expand at different rates when heated up - thismay lead to “micro-cracking” when the respective coefficients of expansion
of cement paste and aggregates are significantly different in some cases
Most aggregates can absorb a degree of strain from temperature
movements (“tensile strain capacity”) therefore it does not automaticallyfollow that aggregates with significantly different E. coef.are going to cause
or influence cracking.
20°C taken as a conservative limit on differentials due to lack of knowledge
of local aggregate expansion coefficients
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20°C
EARLY THERMAL CONTRACTION CRACKS
Core
temperature
Surface
temperature
Higher
Temperature
differential -
Possibility ofcracks?
Time after casting - hours
T e m p e
r a t u r e ° C
>20°C?≤20°C?
Temperature
differential
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EARLY THERMAL CHANGES - 24 HOURS
Heat and
Expansion
75°C?
Sections > 0.5m
thick considered
“Self-Insulating”
I n s
u l a t i o n p r o v i d
e d b y f o r m w o r k ?
Ambient
temperature
say 35°C?
Formwork temperature say 55°CCover to Insulate?
I n
s u l a t i o n p r o v i d e d b y f o r m w o
r k ?
Maybe base
restraint from
mature concrete or
sub base material
No edge
restraintFree to shrink?
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EARLY REMOVAL OF FORMWORK - THERMAL SHOCK
Core Heat
say 75°C
BASE RESTRAINT
Remove
forms early?Remove
forms early?Expansion
R a p i d c o o l i n g t o a m b i e n t
t e m p e r a t u r
e – s a y 3 5 ° C ?
R a p i d c o o l i n g t o a m b i e n t
t e m p e r a t u r
e – s a y 3 5 ° C ?
Tension - Plastic shrinkage
T en si
on!
T e n
s i o n !
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METHOD CONTRIBUTION DISADVANTAGE
Pour concrete
continuously
Improves uniformity of
pouring temperatures
Places additional demands
on production and
handling
Pour concrete at night
Negligible.Depends on
speed and volume placed
and nature of hydration of
cement
Extra logistical
considerations for night
working
Delay removal of formworkSignificant contributionPrevents thermal shock
Allows uniform controlled
cooling to take place
Formwork re-use delayed
Use insulated curing
methods
Significant reduction in
temperature differentials.
Enables more uniform
temperature rise and fall
Large scale use of
insulation materials may be
expensive
METHODS EMPLOYED TO REDUCE EARLY AGE THERMAL CRACKING
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METHOD CONTRIBUTION DISADVANTAGE
Install sacrificial cooling
system in the concrete in
the concrete massDepends on efficiency of
cooling system
Very wasteful.
Durability problems from
embedded pipes
Reduce design strength
margin or observe 60 or 90
day compliance
Significance based on
amount of cement reduced
28 day requirements?
Durability may be
compromised
Use superplastisizing
admixtures
Can significantly reducecement content.
Significance dependant on
amount of reduction
achieved
Extra cost of
superplasticizer over
normal admixture cost
Use admixtures formulatedfor hydration control
and/or significantly
increase dosage
Can significantly reducecement content ,delay
hydration and reduce peak
temperatures
Extra cost of admixtureover normal admixture
Setting times extended
Formwork removal may be
delayed
METHODS EMPLOYED TO REDUCE EARLY AGE THERMAL CRACKING
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METHODS EMPLOYED TO REDUCE EARLY AGE THERMAL CRACKING
METHOD CONTRIBUTION DISADVANTAGE
Use chilled water to partly
or wholly replace mixingwater
Significant
- 4oC water temperature =
Approx - 1oC concretetemperature
Plant equipment expensive.
Consumption usually
exceeds supply capacity.Storage tanks usually
required
Use ice to partly or wholly
replace mixing water
Significant
50% ice = approximately
- 10oC concrete temperature
100% ice =
approximately - 17oC
Usually expensive.
Handling difficult, must be
weighed.
May not be available in
remote areas.
Nitrogen gas injection Depends on quantity of
Nitrogen gas injected
Expensive
Handling difficult in remoteareas. Difficult to control.
Efficiency lost due to gas
leakage to atmosphere
during injection
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METHODS EMPLOYED TO REDUCE EARLY AGE THERMAL CRACKING
METHOD CONTRIBUTION DISADVANTAGE
Shade stockpiles
Significant
Exposed stockpile
temperatures can reach>50°C in mid day direct
sunlight
Extensive shading required
- combination of shading
and fresh deliveries ofaggregates to manage
temperatures effectively
Water spraying of
stockpiles
Depends on requirements
of pour and aggregatedemand
Some difficulty in
maintaining uniform,
effective, large scalespraying for mass pours -
moisture uniformity can be
compromised
Control temperature of
fresh cement
Influence of cementtemperature not significant
as volume of cement is only
about 12% of concrete
mass
Logistical difficulties aslarge pour cement demand
maximizes available silage
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STOCKPILE TEMPERATURE MEASUREMENT
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Base restraint
from mature
concrete
No edge restraintFree to shrink?
Less “Self-insulation”
from lower section
thickness
Sections > 0.5m
thick considered
“Self-Insulating”Thermal
contraction of
concrete after
hardening
EARLY THERMAL CONTRACTION CRACKS
Internal
restraintType H cracks
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Type I cracks
LONG TERM DRYING SHRINKAGE CRACKS
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Primary causes - Impermeable form-face materials
- Over-trowelling
Secondary causes
- Rich, pasty mixes
- Poor curing- Thermal shock (application of cool water on hot surfaces)
Time of appearance - 1 - 7 days, sometimes much later
Remedial measures
Improve curing
Avoid over-trowelling
CRAZING
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Type J cracks (crazing)
CRAZING
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Heat of hydration
Heat of hydration
SIMPLE INSULATION OF FORMWORK for CONCRETE
BREAKWATERS – GUAM 1998
Plastic cling
film wrap
creates cells
Plastic
cling film
wrap
Cell heats up
to higher
temperature
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Site Location
Date concrete placed
Grade/type of concrete/slump
Weather/site conditions at time of pouring e.g dry, wet, sunny,changeable,cloudy, sunlight,shaded, exposed
Temperature range ºC, (check met.reports?)
Wind conditions/speed (check met.reports?)
Relative humidity (check met.reports?)
Curing system used
Type of structure
Approximate dimensions
Immediate sub-base
Details of reinforcement especially top steel
When was cracking first noted – hours/days
Cement type Coarse aggregate Fine aggregate Admixture
kg/m³ 20mm kg/m³ (1) kg/m³ (1) mls/100kg
Fly Ash kg/m³ 10mm kg/m³ (2) kg/m³ (2) mls/100kg
CRACK FIELD REPORT
