Lecture #13 Properties of Hardening Concrete Curing.
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Transcript of Lecture #13 Properties of Hardening Concrete Curing.
Lecture #13
Properties of Hardening Concrete
Curing
Cracking Factors
Temperature and Evaporation
Thermal Stress
TE T Temperature change
Coefficient of thermal expansion
Concrete stiffness
Cracking stress
Concrete Thermal Contraction
= = Coefficient of thermal expansion ~ 5*10-6 /oF
= Difference in concrete temperature (T) and the concrete setting temperature (T set)
= T set - T
T = Variation of the average concrete temperature after placement. Assume this variation tracks closely to the 24-hour ambient air temperature cycle (after a 72 hour period).
T set = 0.95(T conc + TH)
th CTEΔTα
CTEΔT
Concrete Thermal Contraction (con’t)
T conc = Concrete placement temperature at construction (oF).
Assume this value (approx. 80 oF) = Change in concrete temperature due to heat of hydration = Hu = Total heat of hydration per gram (kJ/g)
= 0.007 (Tconc) – 3x10-5 (Tconc)2 –0.0787
C = amount of cement (grams) per m3
= Degree of hydration (estimate to be approximately 0.15-0.2) cp = Specific heat of cement = 1.044 kJ/g
= Density of concrete ~ 2400 kg/m3
HΔT ρcCαH pdu
d
Strength(ft) vs. Time
0
50
100
150
200
250
0 50 100 150 200
Time (hours)
Ten
sile
Str
eng
th (
psi
)
ft = a log (t) + b
TE
MP
ER
AT
UR
E (
C)
40
30
20
10
0
-10
-200 12 24 36 48 60 72 84 96
TIME (hours)
FIGURE 1. The Nurse-Saul Maturity Function
S
M, te
Maturity
Maturity Concepts
Nurse - Saul Equation (units: Temp – Time)
Maturity: Product of time & temperature
To = Datum TemperatureT = Average Concrete Temperature over Time “t”M = Maturity
tTTM o
t
o
ARRHENIUS EQUATIONS
tet rTTR
Et
e
273
1
273
1
0
E = Activation Energy
R = Gas Constant
te = Equivalent Age or Time
LABORATORY TESTING FIELD MEASUREMENT
Procedures for using maturity method involve laboratorytesting and field measurements.
Elastic Modulus
Concrete E-Modulus vs. Hydration
0
5
10
15
20
25
30
35
40
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Degree of hydratiom
E-M
odulu
s
(G
Pa)
D\4 Sawn JointD\4 Sawn Joint
Sawcut Timing and Depth
Curing
StrengthFactors
MPa
cw
Pcap
3
3
32.0
68.01001100
Relative Humidity at ¾ inch
50
60
70
80
90
100
0 5 10 15 20 25
Time (hours)
RH
of C
oncr
ete
(%)
EffectiveCuringThickness
Effective Curing Thickness
Curing Quality
0
50
100
150
200
0 1 2 3 4 5 6
Age of Concrete (hours)
Acc
umul
ativ
e E
vapo
ratio
n (g
ram
s)
0.00
0.20
0.40
0.60
0.80
1.00
Eva
pora
tion
Rat
e (k
g/m
2/hr
)
Accumulative Evaporation Evaporation Rate
0.0
1.0
2.0
0 1 2 3 4 5 6
Age of Concrete (hours)
Effe
ctiv
e C
urin
g Th
ickn
ess
(inch
es)
0
20
40
60
80
100
120
140
0 2 4 6 8 10 12
Age of Concrete (hours)
Acc
umul
ativ
e E
vapo
ratio
n (g
ram
s)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Eva
pora
tion
Rat
e (k
g/m
2/hr
)
Accumulative Evaporation Evaporation Rate
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0 2 4 6 8 10 12
Age of Concrete (hours)
Effe
ctiv
e Cu
ring
Thic
knes
s (in
ches
)
WindWind
No WindNo Wind
Model of CSHStructure of CSH
Nature of Concrete Creep and Shrinkage
Typical creep curve for cement paste.
aM
icro
stra
in
Time after loading
Creepstrain
Elasticrecovery
Creeprecovery
Irreversible creep
Concreteunloaded
Elastic strain
Burger Model
Constant Stress(Creep)
time
Strain
Creep of cement under simultaneous loading & drying.62sh=free shrinkage; bc=basic creep (specimen loaded but not drying); dc=drying creep; cr=total creep strain;tot=total strain (simultaneous loading & drying)
b Free shrinkage (no load)
Basic creep (no drying)
Loading and drying
Mic
rost
rain
Time
sh
sh
bc
bc
dccr
tot
Spring-Loaded Creep Frame
C
C
C
C
C
6 X 3 IN. PLUG (CONCRETE)
C = 6 X 12 IN. TEST CYLINDERS
6 X 3 IN. PLUG (CONCRETE)
UPPER JACK PLATE
LOAD BARSLOWER JACK PLATE
UPPER LOAD PLATE
LOWER LOAD PLATEUPPER BASE PLATE
SPRINGSLOWER BASE PLATE
Horizontal Mold for Creep Specimens
Cracking Frame
The Cracking Frame Test
specimen strain gauge
T = 1.010-6K-1
T = 1210-6K-1
Crack in Specimen
Preparation of Fracture Specimens
Determination of Creep
vs
crp ec c
F
E A
where crp = Creep strain
v = Shrinkage strain (ASTM C 157)
e = Frame strain
Fs = Force in concrete (F)
Ec = Modulus of elasticity of concrete (F/L-2)
Ac = Specimen cross sectional area (L2)
Accumulative vs. Time
0.0E+00
5.0E-05
1.0E-04
1.5E-04
2.0E-04
2.5E-04
0 20 40 60 80
Age of Concrete (hours)
Acc
umul
ativ
e C
reep
Str
ain
Equation 1 Net Difference
Time of Cracking
Burger Model
Constant Stress(Creep)
time
Strain
Aggregate Effects
napcon V 1
Effects of Paste Properties
Effect of age of loadingon the creep strain.
Effect of w/c ratio on the shrinkage strain.
Mechanisms of Creep and Shrinkage
•CreepIt is a complex process involving slipping of surfaces
past one another within the structure of C-S-H. It is a function of pore structure and ease of slippage of C-S-H particles. Asthe space between particles becomes less and less the degree of creep becomes less and less.
•Drying ShrinkageMoisture loss is driven by the ambient relative humidity.
As moisture escapes from the capillaries, menisci are created and capillary stresses are developed. As more moisture is evaporated, smaller and smaller menisci are created. This action creates stress
and causes slippage between C-S-H particles.
This method is based upon a method proposed by Bransonand Christiason (2.3) and was developed by ACI Committee 209(2.4) In 1982, ACI Special Publication 76 (2.5) gives an updatedbut not significantly changed version of this method.
This method uses the
as the creep coefficient.
ACI Committee 209 Method
creep strain
elastic strain at the time of loading t
Shrinkage – ACI
The shrinkage strain at t days after the end of initial curing is
where
= ultimate shrinkage strain= 415 to 1070 micro-strain
= 0.9 to 1.10and f = 20 to 130 days
In the absence of specific data for local aggregate and conditions Committee 209 suggests thatWith
= product of applicable correction factors
The equations for the correction factors are given in Table A
ushtsh t
t
35
sh t
sh t
780 (micro-strains)sh shu
sh
usha
a
tshtf
t
at h s c
The creep coefficient at t days after loading is given by
where= ultimate creep coefficient
= 1.30 to 4.15 = 0.40 to 0.80 d = 6 to 30 days
In the absence of specific data for local aggregates and conditionsCommittee 209 suggests thatwhere
= product of applicable correction factors
The equations for the correction factors are given in Table A
uttd
t
utt
t 60.0
60.0
10
t
u
2.35u C C
Creep – ACI 209
at h s c
The concrete strength at t days is given by
with suggested values of a = 4.0 days = 0.85
for most cured ordinary Portland cement concrete. The modulusof elasticity Ec at t days is given by
which is often taken as
when E and f’c are in MPa
28'' cttc fa
tf
30.043 ( ' )ct c tE w f
tcct fE )'(4730
Strength and Modulus of Elasticity
Notes Correction FactorsCreep Shrinkage
Loading Age
0.1181.25 at
Relative Humidity
Average h=average 1.14 - 0.00092 h 1.23 - 0.0015 h during 1st year Thickness thickness in mm for 1st yr. of
150 < h < 300 loading 1.10 - 0.00067 h 1.17 - 0.0014 h ultimate value ultimate value
Concrete s= slump in mm 0.82 + 0.00264s 0.89 + .00161s Composition
c= cement content - kg/m3 0.75 + .00061c
Table A ACI Creep and Shrinkage Correction Factors
loading age
in days
at
% relative
humidity
1.27 0.0067
40%for
1.40 0.010 for 40%< <80%
3.00-0.030 for 80% <100%
% fine aggregate 0.88 0.00240.30 0.014 for <50%
0.90+0.002 for >50%
0.95 0.0080.46 0.09 1 % air content