CREEP.ppt

Concrete Creep

1. Creep of Concrete
1-Introduction:














2. Creep of Concrete
1-Introduction:
•Figure(1) shows the components of time-dependent strain of
drying concrete, which applies to general structural elements.
The total creep consists of basic-plus-drying creep.
Fig.(1)
Components of
strain at age of
drying concrete
subjected to
load at age ,t0

3. Creep of Concrete
1-Introduction:
•Creep is a partly recoverable phenomenon. When a sustained
load is removed after some time, there is an immediate
recovery of elastic strain (generally smaller than the initial
elastic strain because the modulus of elasticity has increased),
followed by a gradual decrease in strain, called creep recovery
(Figure-2).

4. Creep of Concrete
1-Introduction:
Ty x
y

 
bB
yb25

yy
z

5. Creep of Concrete
Nature of creep:
Type of deformationInstantaneous Time Dependent
Reversible Elastic Delayed-elastic
Irreversible Plastic Set Viscous
Various types of deformation can
be summarized as shown in table:

6. Creep of Concrete
1-Measurment of Creep:
•The only standard test available for the general measurement of
creep of any type of concrete under uniaxial compression is that
of ASTM C512 (1987). However, BS EN 1355: 1997 exists for the
determination of creep of test specimens taken from
prefabricated components of autoclaved aerated concrete or
lightweight concrete with an open structure.

7. Creep of Concrete
2-Measurment of Creep:
F(3)
A



8. Creep of Concrete
3-Mechanism of creep:
by

bb
xb
M

yM
byB(2001)
b

by

 b
T
yb

y

y

9. 4-Factors influencing creep:
4-1 Influence of stress and strength:
I
yb



b
Creep
Stress/Strength ratio

10. 4-Factors influencing creep:
4-2 Influence of cement and aggregates:
T

qy 

x

T 



T

y



Fx



11. 4-Factors influencing creep:
4-3 Influence of size of member:
Figure (5) illustrates the
influence of size of member.
The influence of shape of
member on creep is a secondary
factor and is normally neglected
in prediction methods.
Basic creep of sealed concrete
or large-volume concrete is
unaffected by size of member.
F5 Influence of member size on 30-year creep
coefficient of concrete stored at a relative humidityof 60 per
cent (based on Neville and Brooks, 2001).

12. 4-Factors influencing creep:
4-4 Influence of ambient relative humidity:
For drying concrete, creep is greater the lower the relative
humidity of the surrounding environment as shown in fig.(6)
F6 Influence of relative humidity on creep of
concrete .

13. 4-Factors influencing creep:
4-5 Influence of age at application of load:
The age at which the load is applied greatly affects the magnitude
of creep and its influence can be also be perceived by the change in
strength.
Fig.(7) Influence of age at
application of load on
creep of concrete relative
to creep of concrete
loaded at 7 days

14. 4-Factors influencing creep:
4-6 Influence of Temperature:
The rate of creep increases with temperature up to 70⁰C when, for a
1:7 mix with a W/C ratio of 0.6, is approximately 3.5 times higher than
at 21⁰C. Between 70⁰C and 96⁰C the rate drops off to 1.7 times the rate
at 21⁰C. These differences in rate persist at least for 15 months under
load.
This behavior is believed to be due to desorption of water from the
surface of the gel so that gradually the gel itself becomes the sole
phase subject to molecular diffusion and shear flow :subsequently the
rate of creep decreases.
It is also possible that a part of the increase in the creep of
concrete loaded at elevated temperatures may be due to the lower
strength of concrete at high temperatures.
As far as low temperatures ar concerned, freezing produces a higher
initial rate of creep but it quickly drops to zero. At temperatures
between -10 ⁰C and -30⁰C creep is about one-half of the creep at 20 ⁰C.

15. 5-prediction of creep:
TEB -FIP(1990)
q
 

yz
bT

x
OBS
1881:P2(1985)

xb


 ф
∞
.

16. 6-Effects of creep:
In general the effects of creep are disadvantageous in concrete
structures, although there are some benefits.
Creep increases the deflection of reinforced concrete beams and
causes a loss of pre-stress in pre-stressed concrete beams.
In reinforced concrete columns, creep causes a gradual transfer of
load from the concrete to the reinforcement. Once the steel yields,
the concrete takes any increase in load, so that the full strength of
concrete and steel is developed before failure occurs.
Creep increases the deflection of eccentrically slender columns and
can lead to buckling.

17. 6-Effects of creep:
On the beneficial side, in statically indeterminate structures, creep
can relieve stress concentrations induced by shrinkage, temperature
changes or movement of supports.
Moreover, in all concrete structures creep reduces internal stress due
to non-uniform or restrained shrinkage so that there is a reduction in
cracking.
Conversely, in mass concrete, creep may be a cause of cracking when
restrained concrete undergoes a cycle of temperature due to heat of
hydration and subsequent cooling.
Creep may also cause excessive deflection in tall buildings and long
bridges.
Cracking of partitions and failure of rigidly fixed external cladding can
occur due to differential movement.