KHRI-Webinar(30-04-2020)-Shrinkage induced distress in concrete and remedies- Dr. Piyush Chausali(IITM) - Assistant Director KHRI.pdf

Webinar Series to Promote Quality Construction, Organized by KHRI
Shrinkage-Induced Distress in
Concrete and Remedies
Dr. Piyush Chaunsali
Assistant Professor, Department of Civil Engineering
IIT Madras, Chennai 600036
April 30, 2020

Webinar Series to Promote Quality Construction
Examples of Shrinkage Cracking
2
Concrete Society, UK
Cracks

Examples of Shrinkage Cracking
3
Chaunsali et al., 2013
Cracks
Photo: Illinois DoT

Volume Stability of Concrete
• What triggers the volume change in
concrete?
• Understand the link between stimulus
and response
4
Stimulus
Response
Shrinkage?

Shrinkage of Concrete
• Stimulus: Change in moisture (external
drying, internal drying and chemical
reaction)
• Response: Change in length (volume)
5
Relative
Humidity
Shrinkage

Shrinkage-Induced Cracking: Mechanics
6
Portland Cement Concrete
Fresh Concrete
Hardened
Concrete
T : Tension
Cracking
T
T
Stress due to Shrinkage
Stress Considering
Creep Relaxation
Tensile Strength
Stress
Age
Time of Cracking
No restraint
No cracking
Restraint
Cracking

Influence of Relative Humidity on Shrinkage
• Ultimate shrinkage is greatly
influenced by relative humidity
• Lower relative humidity results
in higher shrinkage
7
Troxell et al., 1958
RH 50%
RH 70%
RH 100%

Variation in Relative Humidity
8
City: Chennai
Duration: March 31 - April 28
City: New Delhi
Duration: March 31 - April 28
Relative
Humidity

Volumetric Deformation
• Plastic shrinkage: Occurs in fresh Concrete while the concrete is still
plastic
• Autogenous shrinkage: Shrinkage without temperature change or
moisture loss (chemical and self-desiccation shrinkage during
hydration)
• Drying shrinkage: Generally considered in hardened concrete
• Thermal shrinkage: Due to temperature change
• Carbonation shrinkage: When hydrated cement paste reacts with CO2
9

Webinar Series to Promote Quality Construction 10
Shrinkage
hours days weeks months years
Time
Plastic
Thermal
(contraction)
Autogenous
Drying
Carbonation
Prof. Gettu

Plastic Shrinkage Cracking
• Where? Random, Diagonal,
Over Reinforcement
• When? 30 mins – 6 hours
• Why? Higher evaporation
than bleeding
11
Concrete Society, UK, Technical Report No. 22

Plastic Shrinkage Cracking
• Plastic shrinkage cracking occurs
when
bleeding rate < evaporation rate
• When the rate of evaporation
exceeds 1 kg/m2/hour, precautionary
measures are required to control
plastic shrinkage cracking
12
Bleeding Evaporation
Courtesy: Prof. Jason Weiss

Estimation of Evaporation Rate
• The chart can be used to estimate
evaporation rate given the climatic conditions
• Based on the air temperature, relative
humidity, concrete temperature and wind
speed, evaporation rate at particular site can
be estimated
13
Hot Weather Concreting,J. ACI, Proc., Vol. 74, No. 8, p. 321, 1977

Effect of Aggregate on Plastic Shrinkage
• Negative capillary pressure generated
due to the loss of moisture causes
plastic shrinkage
• Aggregates reduce the plastic shrinkage
14
Shrinkage
Strain
Drying Period
Concrete
Mortar
Paste

Effect of Fibers on Reducing Plastic Shrinkage
Cracking
15
Sivakumar and Santhanam, 2007
Naaman et al., 2005

Ways to Reduce Plastic Shrinkage Cracking
16
• Moisten aggregates
• Cool aggregates and mixing water
• Dampen subgrade
• Erect temporary windbreaks and sunshades
• Cover concrete
• Fog slab immediately after placing
• Curing compound
• Shrinkage reducing admixtures (SRAs)
• Add fibers

Autogenous Shrinkage
• Autogenous shrinkage is significant in
low w/cm ratio (< 0.4) concrete mixtures
• Chemical shrinkage plus self-desiccation
➢Chemical shrinkage: characteristic of
cement hydration
➢Self-desiccation: internal drying of concrete
due to cement hydration (no moisture loss to
surroundings)
17

Chemical Shrinkage: Measurement
• Observed by Le Chatelier over a century
ago
• Volume reduction associated with the
hydration reactions in a cementitious
material
• ASTM C1608
18

Autogenous Shrinkage: Measurement
19
• Paste/mortar is encapsulated in a
corrugated tube, and the length change is
measured using digital dilatometer
• Measurement can be started immediately
after mixing
• ASTM C1698 Length-to-diameter 400:30 mm

Early-Age Shrinkage in Cement Paste
20
Setting time
Jensen and Hansen, 2001

Drying Shrinkage Cracking
21
Fresh Concrete
Hardened
Concrete
T : Tension
Cracking
T
T
Concrete Society, UK, Technical Report No. 22

Drying Shrinkage Measurement
• Concrete prisms are used
to measure free shrinkage
following ASTM C157
22
Measured
Shrinkage
Drying Period

Restrained Shrinkage Test
ASTM C1581
23
NI DAQ
Concrete Ring
Steel Ring
Computer
0 4 8 12 16 20 24 28
Age (days)
-100
-80
-60
-40
-20
0
20
Strain
in
Steel
Ring
()
Plain Concrete
Appearance of
Crack
According to ASTM C1581, the concrete mix exhibiting cracks beyond 28
days is expected to have low vulnerability to shrinkage cracking in the field.

Drying Shrinkage: Influence of Specimen
Size
• Higher surface/volume ratio results in
higher rate of shrinkage
• Ultimate shrinkage values are similar
24
Shrinkage
Strain
Drying Period
4 inch cube
6 inch cube

Influence of Aggregate Stiffness and Volume on
Shrinkage
25
n: 1.2 to 1.7
ACI Monograph, 1971 Troxell et al., 1958
𝜀𝑐𝑜𝑛𝑐𝑟𝑒𝑡𝑒 = 𝜀𝑝𝑎𝑠𝑡𝑒 1 − 𝑉𝑓𝑎𝑔𝑔
𝑛
• Increase in aggregate
volume or reduction in
paste volume results in
lower shrinkage
• Stiffer aggregates
exhibit lower shrinkage
Low modulus
High modulus

Influence of Water Content
26
ACI 224R- 2008
For the same w/cm ratio, increase in water
content also results in higher drying shrinkage
due to increase in paste volume.
Concrete shrinkage increases with w/cm ratio for
given aggregate content. The effect is more
pronounced with lower aggregate contents.

Influence of Drying and Wetting on Shrinkage
27
Mindess and Young, 1981

Mechanism of Shrinkage
• Capillary stress (RH > 45 %)
• Disjoining pressure (RH > 45%)
➢Van der Waals forces attract adjacent particles
• Surface energy (RH < 45%)
➢Surface free energy increases as the most strongly adsorbed water is
removed
28
ln 𝑅𝐻 = −
2𝛾𝑉
𝑚
𝑟𝑅𝑇
𝜎𝐶𝑎𝑝 =
2𝛾
𝑟

’s s
29
Relative vapor
pressure
H. Roper, 1966
I: Water lost from big pores
II: Water from smaller pores
III: Menisci break
IV: Interlayer

Factors Affecting Shrinkage
• Mix Proportions
➢w/c (w/c  εsh)
➢Paste/Aggregate (agg. vol.  ε sh )
• Material Parameters
➢Aggregate stiffness
➢Mix composition (silica fume)
• Environmental Conditions
➢Curing conditions (R.H., temp, wind)
➢Age of drying (age  ε sh )
➢Volume-to-surface area ratio (v/s  ε sh )
30

Other Shrinkage Mitigation Strategies
• Shrinkage reducing admixtures (SRAs)
• Expansive cements
• Light-weight aggregates (LWAs)
31

Shrinkage Mitigation Using SRAs
SRAs reduce the surface tension of pore water, and hence the capillary stress is also reduced.
32
Restrained Shrinkage Test
Free Shrinkage Test
Control With SRA

Shrinkage Mitigation Using Expansive
Cements
• Examples: Type K, Type M, Type S
and Type G cements (ACI 223)
33
0 20 40 60 80 100 120
Age (days)
-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
0.08
0.1
Deformation
(

)
-1000
-800
-600
-400
-200
0
200
400
600
800
1000
Deformation
(

)
Control
Type K
Type G
Expansion
Shrinkage
Start of Drying (7d)
Fresh Concrete
Hardened
Concrete
Tensile stresses due to shrinkage
C C
C C
T T

Application of Type K Cement for Preventing
Shrinkage Cracking
34
Transverse
Crack
No cracks
CSA Concrete Deck
OPC Concrete Deck
Photo: Illinois DoT

Shrinkage Mitigation Using LWAs
35
Henkensiefken et al., 2009
Castro et al., 2010
Delay in cracking

Thermal Shrinkage
• Thermal shrinkage is of great importance in massive concrete
structures
• Factors affecting thermal shrinkage
➢Degree of restraint
➢Temperature change
➢Coefficient of thermal expansion of aggregates
➢Cement type and fineness
36

Carbonation Shrinkage
• Surface zone of concrete undergoes shrinkage
due to the carbonation of the hydrated cement
paste.
• Carbonation shrinkage is probably caused by
the dissolving of the Ca(OH)2 crystals and the
depositing of CaCO3 in spaces free from stress.
• Carbonation increases the shrinkage at
intermediate relative humidity.
➢At low humidity, there is insufficient water in the pores
to form carbonic acid with the CO2.
➢On the other hand, when the pores are saturated, the
diffusion of CO2 is slow.
37
G. Verbeck, 1958

Contraction Joints
38
D
D
Portland Cement Association

Main Takeaways
• Plastic shrinkage cracking: Efforts must be directed to reduce evaporation rate
• Importance of aggregate volume, aggregate stiffness, w/cm ratio and surface-
to-volume ratio of concrete element in influencing shrinkage of concrete
• SRAs, fibers, expansive cements and LWAs can be used to mitigate shrinkage-
induced cracking
• Autogenous shrinkage is significant in low w/cm ratio mixtures
• Thermal shrinkage is greatly influenced by adiabatic temperature rise and is
prominent in thick concrete elements
39

KHRI-Webinar(30-04-2020)-Shrinkage induced distress in concrete and remedies- Dr. Piyush Chausali(IITM) - Assistant Director KHRI.pdf

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