Transcript Class 27.1 CIVE 2110 Concrete shrinkage creep thermal

Class #27.1
Civil Engineering Materials – CIVE 2110
Concrete Material
Shrinkage
Creep
Thermal Properties
Fall 2010
Dr. Gupta
Dr. Pickett
1
Time-Dependent Volume Changes
(MacGregor, 5th ed., pp. 70-83)
Concrete volume changes over time due to :
 (1)

 (2)

 (3)

Shrinkage;
Negative volume change due to curing, drying,
- shrinkage is less in a structure than in a cylinder,
- rebars restrain shrinkage,
- less exposed surface area per volume,
- structure is built in stages,
shrinkage not simultaneous throughout structure.
- in a parking garage, concrete absorbs CO2 ,
- shrinkage due to carbonation = shrinkage due to drying.
Creep;
Volume change due to load over time,
Thermal Expansion/Contraction;
Volume change due to change in temperature of mass.
2
Time-Dependent Volume Changes

(1) Shrinkage:
(MacGregor, 5th ed., pp. 70-83)
Negative volume change, shortening, under constant temperature due to;
Drying & hardening:
causes water to evaporate from cement paste,
causing shrinkage to occur;
- aggregate does not evaporate, does NOT shrink;
Shrinkage;
 Increases with decreasing humidity,
 Increases with increasing cement-to-aggregate ratio,
 Cement paste shrinks, aggregate does not shrink.
 Increases with increasing water-to-cement ratio,
 More water, less aggregate,




(Fig. 3.22a,
MacGregor, 5th ed.)
water evaporates, paste shrinks, aggregate does not shrink.
Increases with increasing fineness of cement,
 Finer cement absorbs more water, water evaporates, more shrinkage.
Decreases with increasing member size,
3
 More mass per exposed surface area, less shrinkage.
Time-Dependent Volume Changes

(1) Shrinkage:
(MacGregor, 5th ed., pp. 70-83)
Equation for Axial Shrinkage Strain between days ts & t in plain concrete:
 cs t , ts    cso  s t , ts 
 cso = basic shrinkage strain, for a specific concrete & relative humidity,
s t, ts  = coefficient, a function of time and member effective thickness,



t  t s  t1
 s t , t s   

2
 350he h0   t  t s  t1 
0.5
(Fig. 3.24,
MacGregor, 5th ed.)
t = age of the concrete, days, t1 = 1 day
t = age of concrete at end of moist curing, days,
2 Ac As = concrete cross sectional area, in2 .
he 
c
u
u = perimeter of cross section exposed to atmosphere,4 in.
h0 = 4 in.
Time-Dependent Volume Changes
Equation for Axial Shrinkage Strain between days ts & t in plain concrete:
 cs t , ts    cso  s t , ts 
(MacGregor, 5th ed., pp. 70-83)
 cso = basic shrinkage strain, for a specific concrete & relative humidity,
 cso   s  f cm  RH  s  f cm   1.2160  sc  9  f cm  106



f cmo = 1450 psi,
f cm = mean compressive strength, at 28 days, psi,

f cmo 
f cm  f cr' from ACI 318, Sect. 5.3.2.1, f cr'  f c'  1.34s or fcr'  fc'  2.33s  500
'
assuming a standard deviation of s  0.15 f c
f cr'  f c'  1.34 0.15 f c'
f '  f '  2.33 0.15 f '  5000  500

f cr'  1.2 f c'

cr
c
 
f cr'  f c'   1247
'
'
Use f cm = smaller of 1.2 f c or fc 1200psi
 sc = coefficient accounting for type of cement
 sc = 50, for Type I
 sc = 80, for Type III
c

5
Time-Dependent Volume Changes

(1) Shrinkage:
(MacGregor, 5th ed., pp. 70-83)
Equation for Axial Shrinkage Strain between days ts & t in plain concrete:
 cs t , ts    cso  s t , ts 
 cso = basic shrinkage strain, for a specific concrete & relative humidity,
 cso   s  f cm  RH
 RH = coefficient accounting for relative humidity,
 RH = +0.25, for Relative Humidity ≥99%
For Relative Humidity 40% < RH < 99%
 RH
(Fig. 3.23,
MacGregor, 5th ed.)
  RH 3 
 
 1.551  
  RH0  
RH = Relative Humidity of ambient atmosphere, %
RH0 = 100%
6
Time-Dependent Volume Changes

(1) Shrinkage:

Example calculations:
(MacGregor, 5th ed., pp. 74-76)

Underground parking garage,
Floor slab, 6 in. thick, lightly reinforced,
Floor restrained on outside edge by 16 in. thick basement wall,
Walls 26 months old, moist cured, 5 days, cast against ground,
Slab 24 months old, moist cured, 5 days, not on ground,
Relative humidity, roughly constant over period, 50%,
Concrete; Type I cement, f c'  3000psi

Shrinkage for reinforced concrete ≈ 0.75 shrinkage plain concrete

Cracks developed in slab, perpendicular to wall, at roughly every 6 ft.
Assume cracks resulted from restraint by wall of slab shrinkage
parallel to wall.
Calculate crack width.
7








Time-Dependent Volume Changes

(2) Elastic Strain plus Creep Strain:

Example calculations:
(MacGregor, 5th ed., pp. 79-81)

Concrete pedestal, plain (unreinforced), 24”x24”x10’
Moist cured, not on ground,
Applied load 1 month after casting,
Load causes average stress = 1000 psi.,
Temperature, roughly constant over period, 68˚F,
Relative humidity, roughly constant over period, 50%,
Concrete; cement content = 675 Lb/yd3, slump = 3 in.

Compute total shortening in 5 years.






8