Strength Development of High Strength High-Performance Concrete at Early Ages

blondglibUrban and Civil

Nov 29, 2013 (3 years and 11 months ago)

93 views


1349

Structural Emgineering, Mechanics and Computation (Vol.2)

A. Zingoni (Editor)

2001 Elsevier Science Ltd. All rights reserved.





Strength Development of High Strength High
-
Performance Concrete


at Early Ages



T. Yen

1
,
K.S.Pann

2
, Y.L.Huang
3


1,3

Departm
ent

of Civil Engineering, Chung
-
Hsiung University,

Taichung, Taiwan

2
Department of Civil Engineering,

Cheng
-
Shiu Institute of Technology

Kaohsiung, Taiwan



ABSTRACT


This study aims to investigate the characteristics of the early strength development of

HPC.
Experimental tests were conducted on concrete with a 28 days’ strength of 56 Mpa starting from 6
hours after mixing. The test methods include ultrasonic pulse velocity measurement and maturity
method in addition to the standard compression test. S
even curing ages (6hrs, 12hrs, 1day, 2days,
3days, 7days and 28days) were selected for the tests.


Test results show that the increase rates of the strength growth of HPC are significantly different
before and after 1 day of curing age. By selecting the
curing age of 1 day as the separating point, a
prediction model is proposed to describe the strength development in the two separated stages and is
further verified by the tests. Moreover, compared to the test results, it is found that the prediction
mode
l releases better agreement than those obtained from the method of maturity.



KEYWORDS


high performance concrete, strength, curing temperature, early age, wave speed, maturity



INTRODUCTION


High
-
performance concrete, HPC, is a new generation of concret
e. Its high strength and high flow

ability are the two basic characteristics that are usually mentioned at the same time

[1].


HPC has
multiple

advantages
, such as

improving environment

and

construction
technique.

I
t will become a
major construction mater
ial in the modern trend for higher construction quality.


Construction work is usually required to speed up and remove shoring and form

work sooner under
the pressure of reducing cost. Yet construction disasters and engineering flaws may occur due to
insu
fficient strength of concrete from these acts. HPC has high flow

ability

and

high strength to last

a
long
period
, but the initial strength after casting is
relatively

small like
conventional

concrete.
It is
therefore quite

important to understand and de
nominate the strength at early age.



1350

This research aims to investigate the strength prediction model for concrete at early age using
ultrasonic wave and maturity method. The speed of ultrasonic wave is quite related to the
constitutions of concrete [2
-
3].

Since the concrete at early age has rapider hydration and the wave
speed of the hydration product C
-
S
-
H is faster than that of water and void, the wave speed of the
early
-
age concrete increases more quickly.


T
emperature and time
are

two major control fa
ctors for the development of concrete strength,
especially, the concrete strength at early age is greatly affected.
The

concrete under higher curing
temperature will reach the maximum temperature earlier.
T
he Maturity Method [
4,5
] was developed
basing on

this concept.

T
ill now, almost all predictions of concrete strength use water
-
cement ratio
as main factor
[
6
].
The others
such as
gradation

of aggregates, property of cement and content

[
7
-
9
]
are often regarded as minor affecting factors. Either water
-
cement theory or maturity prediction has
focused on the long term strength basing on the variations of stress and elastic modulus to form
prediction models

[
10
]. Yet the research reports about the strength at early age have rarely been
seen.


Right after
concrete casting to about 1 day age, the hydration rate of cement is the
most rapid.

After
one
-
day age, the formation of hydration heat slows down and the strength development of paste
becomes slow, too.
R
ecent researcher

[
11
] ha
s

proposed a two
-
period m
odel to predict the strength
of
conventional

concrete at early age. According to the strength developing process, this paper
establishes a strength prediction model of concrete at early age based on the maturity concept,
covering concrete from mixing to 5
6 days age, different curing temperatures

and

ages of paste.



EXPERIMENTS


Materials


Type I Portland Cement is used
,

the maximum size of coarse aggregate is 1/2", the fineness modulus
of fine aggregate is 2.9, and the
superplasticizer

of HICON HPC100 whi
ch meets the requirements of
ASTM G
-
Type is used.


Variables


The variables considered in serial tests are shown in the following:

Curing temperature: T
cured
=10
0
C, 23
0
C(ambient temperature), 40
0
C

Water/binder ratio: w/(c+p)=0.33, 0.36, 0.40

Age: t=6, 12 hr
s, 1 day, 2, 3, 7, 14, 28 and 56 days

Overfill ratio of paste: n=1.5, 1.6, 1.8

ratio of cement replaced by fly ash: 0%, 10%, 20%, 30%


Where w s
tands for

the weight of
water
, c is the weight of cement, p is the weight of
p
o
zzolanic
material, and n is the r
atio of
the volume of paste to the voids between aggregates
.
The mix
proportions of HPC are shown in Table 1.


Specimens and Test Methods


The size of cylinder specimen for compression test is
ψ
10
×
20cm. They are tested according to
ASTM C109 specifications.

The variation of internal temperature was measured with a thermometer
within 48 hours after concrete casting. Readings were taken eve
ry two hours to calculate the
maturity and related compressive strength.


The heights of the specimens were measured, the both
ends were smeared with Vaseline and the wave speeds were detected before every compressive

1351

0.34
0.36
0.38
0.4
W
a
t
e
r

/

B
i
n
d
e
r
2000
3000
4000
5000
P

-

W
a
v
e

v
e
l
o
c
i
t
y


m
/
S
e
c

n
=
1
.
6
28Day,Y = -3728.306732 * X + 5850,R=0.993
14Day,Y = -3273.823091 * X + 5608,R=0.9778
7Day,Y = -3970.590742 * X + 5814,R=0.978
5Day, Y = -4184.429219 * X + 5795,R=0.985
3Day,Y = -6075.000693 * X + 6338,R=0.9965
2Day,Y = -6105.307195 * X + 6188,R=0.999
1Day,Y = -11400.50017 * X + 7713,R=0.976
0.5Day, Y = -12693.43878 * X + 7945,R=1
experiment was conducted.


TABLE
1

T
HE MIX PROPORTIONS O
F HPC
(
kg/m
3
)


w/(c+p)

0.33

0.36

0.36

0.36

0.40

Overfill ratio
of paste
(

n )

1.6

1.8

1.6

1.5

1.6

Cement

395

435

369

333

334

Fly ash

158

147

157

163

158

S
lag

17.2

22.8

19.4

17.5

17.6

Sand

633

589

629

652

633

Aggregate

967

901

962

996

967

Water

167

200

180

167

189

S.P.

20.8

18.3

16.4

17.9

15.3



RESULTS AND DISCUSSI
ONS


Wave Speed of HPC


During early age, the hydration product, C
-
S
-
H, forms quickly to fill the voids in paste. Therefore,
the increase rate of the wave speed at
the early age is larger than in any later ages. As can be seen in
Table 2, the largest increase rate of wave speed occurs at the age of one day. The relationship
between curing age and wave speed of HPC with n equal to 1.6 and cured at 23

C is shown in F
ig. 1.
According to the slope of the regressive line in Fig. 1, an earlier
-
age concrete possesses a larger
increase rate of wave speed whatever the water
-
paste ratio is and the maximum increase rate occurs at
the age of one day or 0.5 day. Basically, the

age of one day is the separating point at which the
increase rate of wave speed changes.


TABLE 2

WAVE SPEED AND INCREASED RATE OF CONCRETE AT VARIOUS AGES


ages

w
/
(c+p)

0.5 day

1 day

2 day

3 day

5 day

7 day

14 day

28 day

Wave speed (m/sec)

0.33

3763

3
962

4170

4326

4448

4499

4550

4622

0
.36

3365

3591

3996

4165

4280

4373

4450

4505

0
.4
0

2873

3161

3
683

3903

4147

4222

4315

4361


Increased wave speed (m/day)

0.33

---

398

207

156

61.1

25.3

7.30

5.15

0.36

---

452

405

169

57.5

46.5

11.0

3.92

0.4
0

---

577

5
2
1

21
9

122

37.6

13.3

3.24


Development of
the
Compressive Strength of HPC


T
he
developments

of compressive strength of HPC are
summarized

as shown
in Fig.
2
. By
observ
ing,

it
i
s found
from the figures
that before
one
-
day age, except for the one with cur
ing
temperature of 10
0
C, the strength of HPC is far greater than the
increase

rate after
one
-
day age.
In
addition
, the lower the water
-
cement
is, the greater the strength gain becomes. At 0.5 day age and
Fig.
1
Relationship between w/(c+p) and
wave speed


1352

curing temperature of 23
0
C and 10
0

C, most strengt
h
increase

rate
s

d
o

not exceed 1% of that at 56
days age.
While for

the curing temperature of 40
0
C, the strength
increase

rate at 0.5 day age has
reached about 2%
.


It

means
enhance the
curing temperature can stimulate the development of
strength at early

age.














(a) cured at 40

C





(b)cured at 23

C





(c)cured at 10

C


Fig.2

S
trength growth
of HPC versus curing age


Effects of Curing Temperature on Strength of HPC


Fig.
3

indicates the strength development of HPC. It can be observed that
all the
strengths of HPC
increase
with

age
in

a parabol
a.

High and low curing temperatures have certain effect
s

on the
strength of HPC. Under low curing temperature of 10
0
C, the strength of HPC obviously decrease
s

as
shown in Fig.
4
. Under high curing t
emperature of 40
0
C, the strength of HPC increased within early
age but the strength after 7 days age
does

not increase. It is concluded that high curing temperature
is in favor of strength of HPC for early age but not for the long age.


0
20
40
60
Age(day)
0
20
40
60
80
C
o
m
p
r
e
s
s
i
v
e

s
t
r
e
n
g
t
h

(
M
p
a
)
T(cured)=23C
w/(c+p)=0.33
w/(c+p)=0.36
w/(c+p)=0.4
predicte

Fig.3 Developm
ent of compressive


strength of HPC



Temperature Change inside Concrete


The temperature change
s

inside concrete from casting to 48 hours age under three curing temperatures
for

HPC with water/binder ratio=0.33 are shown in Fig.
5
.


In Fig.
5(a)
,
the

curing temperature is
23
0
C
, similar to

the trend of hydration heat releasing curve of cement,
the concrete

has two peaks on
the temperature curve. The second peak occurs within 20 to 24 hours after casting. Fig.
5(b)

0
20
40
60
Curing temperature (C)
0
10
20
30
40
50
60
70
C
o
m
p
r
e
s
s
i
v
e

s
t
r
e
n
g
t
h

(
M
p
a
)
n=1.6
w/(c+p)=0.36
1 day
7 days
14 days
28 days
56 days
0.25
0.50
1.00
2.00
3.00
7.00
14.00
28.00
56.00
Age (days)
0.00
40.00
80.00
120.00
C
o
m
p
r
e
s
s
i
v
e

s
t
r
e
n
g
t
h

(

f
/
f
(
5
6
)

)

w/(c+p)=0.33
n=1.6
T(cured)=40 C
0.37
1.85
30.96
48.46
51.59
69.76
82.66
91.08
100
0.25
0.50
1.00
2.00
3.00
7.00
14.00
28.00
56.00
Age (days)
0.00
40.00
80.00
120.00
C
o
m
p
r
e
s
s
i
v
e

s
t
r
e
n
g
t
h

(

f
/
f
(
5
6
)

)

w/(c+p)=0.33
n=1.6
T(cured)=23 C
0.28
0.70
14.59
33.85
45.78
61.42
69.20
87.88
100
0.25
0.50
1.00
2.00
3.00
7.00
14.00
28.00
56.00
Age (days)
0.00
40.00
80.00
120.00
C
o
m
p
r
e
s
s
i
v
e

s
t
r
e
n
g
t
h

(

f
/
f
(
5
6
)

)
w/(c+p)=0.33
n=1.6
T(cured)=10 C
0.36
0.63
1.68
15.28
37.43
62.63
72.23
85.09
100

Fig. 4 Effects of curing temperatures on the compressive
strength of HPC


1353

shows the temperature change under c
uring temperature of 40
0
C. It indicates the temperature inside
concrete increases rapidly
right
after mixing due to high curing temperature. The second peak
occurs within 12 to 20 hours after casting
,

which is earlier than that under curing temperature o
f 23
0
C.
Fig.
5(c)

shows the result of 10
0
C curing temperature. It differs from the former in the
occurring

of
the second peak
,

which delays to within 35 to 45 hours after casting.
It can be thus

drawn that the
higher the curing temperature is, the soone
r the second peak reaches.


0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
50.00
Age (hours)
21.00
22.00
23.00
24.00
25.00
26.00
27.00
28.00
29.00
30.00
31.00
T
e
m
p
a
t
u
r
e

(
C
)
T=23 C
W/(C+P)=0.33
n=1.6
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
50.00
Age (hours)
22.00
23.00
24.00
25.00
26.00
27.00
28.00
29.00
30.00
31.00
32.00
33.00
34.00
35.00
36.00
37.00
38.00
39.00
40.00
41.00
42.00
T
e
m
p
a
t
u
r
e

(
C
)
T=40 C
W/(C+P)=0.33
n=1.6
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
50.00
Age (hours)
10.00
11.00
12.00
13.00
14.00
15.00
16.00
17.00
18.00
19.00
20.00
21.00
22.00
23.00
24.00
T
e
m
p
a
t
u
r
e

(
C
)
T=10 C
W/(C+P)=0.33
n=1.6


(a) cured at 23

C





(b)cured at 40

C





(c)cured at 10

C

Fig.5 Temperature change in the interior of concrete


Prediction by Maturity Method


According to Nurse
-
Saul Function method suggested by ASTM C 1074, the maturit
y of concrete can
be determined by temperature and time factors as following equation:


M =





T
T
t
t



0
0









(1)


where

M = maturity when age is t



T = average temperature of concrete in the period of
Δ
t

T
0

= basic temperature (when curing

temperature is within 5
~
43
0
C,
for Type I cement
without any admixture, T
0

=
-
10
0
C;
if curing temperature is not in this range,
T
0

=
4.4
0
C)


According to Plowman[
12
], the relation between maturity and strength can be represented as follows:


S
S
A
B
M
e
28
1000



log








(
2
)


where

S
e

= compressive strength when maturity
is M



S
28

= compressive strength of 28 days



A, B = constants



M = maturity (F
-
hr)


Prediction Model for Strength of HPC at Early Age


From the tests, it is found that except for t
he curing temperature of
10
0
C all other cases
have

relatively
large

slope change of curve at all ages before 1 day. The slope change between 1 day and 7 days
reduces gradually. After 7 days, the slope change is even smaller. Most strength
increase

rates

of
HPC at
before and after

1 day age have significant difference which
is similar to

the heat
evolution

of

1354

Portland Cement

[1
3
].
S
imilar result is
also observed in the wave speed detection
.
Accordingly,
taking 1 day as the
separating

point is acceptable
.


Summing up the above analysis and discussions with reference to
the
reports of the hydration of
cement on strength development by Young[1
3
] and Oluokun[1
4
], it can be estimated roughly that the
strength of HPC develops rapidly
before

1day age. After 1
day, it reduces gradually so no any single
curve can be used to reasonably predict the strength development of concrete. This paper divides the
curing age from casting to 56 days into two periods. First period starts from 0.25 day to 1 day (2
days when c
uring temperature is 10
0
C). The second period starts from 1 day (or 2 days) to 56 days.
The first period is depicted by

the

equation (3) and the second period by
the
equation (4):


y

= a

x
b











(
3
)


y = a

(x
-
c)
b










(
4
)


Substitute in a
ll test data and run linear regression to get coefficients in the above equations as shown
in Table
3
.


TABLE 3

COEFFICIENTS OF A, B

AND C IN E
qs
. (3) AND (4)


Curing Temp.

T(

)







眯⡣+瀩

〮㌳

〮㌳

〮㌶

〮㌶

〮㌶

〮㐰

〮㌳

佶O牦楬氠牡瑩漠潦o
灡獴s

⡮(

ㄮ1

ㄮ1

ㄮ1

ㄮ1

ㄮ1

ㄮ1

ㄮ1

〮㈵

A

ㄴ⸱㜳

㘮㐳6

㐮㤵4

㔮㈶5

㌮㠷3

㌮㌹3

〮㈵

ㄮ㈹1



B

㌮ㄹ㤹

㈮㠳㤸

㈮㜴㤳

㈮㐹㤶

㈮㌳㌶

㈮㔵㜸



ㄮ㜶㔱

1

C

0

0

0

0

0

0

2

0

摡y

R
2

0.
9762

0.9095

0.9519

0.9983

0.8949

0.9369

day

0.9132

1

a

26.997

23.433

17.919

14.971

17.29

14.239

2

16.375



b

〮ㄸ㠷

〮㈵ㄲ

〮㌳㜱

〮㌶0

〮㈹㌳

〮㌳㌶



〮㈷ㄴ



c

〮㤱㔴

〮㤷0

〮㤵㐶

〮㤳㔶

〮㤸㌴

〮㤷㔴



ㄮ㤵

摡ys

R
2

0.9
878

0.
9931

0.
9991

0.
9981

0.
9
965

0.
9911

day

0.
9916












-2
0
2
4
6
Log(X) (X:day)
-4
-2
0
2
4
6
L
o
g
(
y
)
:

(
y
:
M
p
a
)
y2
y1
y3
y4
y5
y6
T(cured)=23C, n=1.6
w/(c+p)=0.33
w/(c+p)=0.36
w/(c+p)=0.40
predicted
y1=6.43*(x-0)**2.8398,Rsq=0.9095
y2=23.4325*(x-0.978)**0.2512,Rsq=0.9931
y3=5.268*(x-0)**2.4996,Rsq=0.9983
y4=14.971*(x-0.9356)**0.364,Rsq=0.9981
y5=3.39*(x-0)**2.5578,Rsq=0.9369
y6=14.2385*(x-0.9754)**0.3336,Rsq=0.9911
-2
0
2
4
6
Log(X) (X:day)
-2
0
2
4
6
L
o
g
(
y
)

(
y
:
M
p
a
)
T(cured)=40C,n=1.6
w/(c+p)=0.33
predicted
y1=14.173*(x-0)**3.1999,Rsq=0.9762
y2=26.99736*(x-0.9154)**0.18873,Rsq=0.98779
y1
y2
-2
0
2
4
6
Log(X) (X:day)
-2
0
2
4
L
o
g
(
y
)

(
y
:
M
p
a
)
T(cured)=10,n=1.6
w/(c+p)=0.33
predicted
y1=1.295*(x-0)**1.7651,Rsq=0.9132
y2=10.801*(x-0.999)**0.3903,Rsq=0.9376
y1
y2




(a)







(b)








(c)


Fig.6 Strength Development under (a)23


(b)40


(c)10


curing temperature


1355


Fig.6 shows the prediction curve.
In average, for the case under curing temperature of 10
0
C, the
predicted strength of concrete within 1 day age is much off. The other two curing temperature cases
of 23
0
C and 40
0
C have
more acceptable predicted
results
. For 1 day and over to 56 days age cases,
all three have fairly reliable results.


Comparison of Measured
Results with

the
Predicted Strength from Maturity Method and Prediction
Model


The predicted strength
from

maturity

method
and the prediction model
and the measured strengths are
listed in Table
4
. The strength percentage by maturity method shows
a
negative
value

before 6 hours
age
,

which is obviously not an applicable result. After 12 hours age, under curing tempera
ture of
10
0
C, the predicted strength at 1 day age by maturity method still has large deviation. For curing
temperature of 23
0
C and 40
0
C, the predicted strength at 1 day over to 2 days has smaller deviation.
Comparing the predicted strengths
of

the predic
tion model

and the maturity method

with
the
measured strengths,
we can

found that the prediction model is obviously superior to maturity method
,
because the former has smaller deviation
.


TABLE 4

COMPARISON OF
THE STRENGTH OF
MEASURED
,

MATURITY METHOD

AND

PREDICTION MODEL


w/(c+p)

n

T(

)

䅧A

⡨()

S
瑲t湧瑨⁲a瑩漠
⠠(


䵥a獵牥d

P牥摩捴楯d

䵡瑵物ty⁍ 瑨潤





F
-
桲)

Se⽓㈸

〮㌳

ㄮ1



6


〮 㐲


〮 ㈹


㈷ 〮 㠱


-
㈮ 㜸




〮 㜴


〮 㤹


㐹 㜮 㘱


㈮ 㐱




ㄮ 㤸


㌮ ㌶


㤴 㔮 㤹


㜮 㠸




ㄷ ⸹
6


1 ㄮ 㐳

ㄸ 㐶 ⸷ 1


ㄳ ⸵ 1



6


〮 ㌲


〮 ㈳


㌷ 㤮 㜱


-
㔮 㐹




〮 㜹


ㄮ 㘲


㜵 㤮 ㈴


㜮 㐹




ㄶ ⸶ 1


1 ㄮ 㘰

ㄵ ㌲ ⸹ 1


㈰ ⸶ 2




㌸ ⸵ 3


㐰 ⸰ 4

㌰ 㔹 ⸲ 3


㌳ ⸵ 3



6


〮 㐰


〮 ㌳


㐹 㤮 ㄴ


-
㘮 ㈰




㈮ 〳


㌮ 〳

㄰ ㄴ ⸶ 1


ㄲ ⸸ 1




㌳ ⸹ 3


㈷ ⸸ 2

㈰ 㜰 ⸹ 2


㌲ ⸰ 3




㔳 ⸲ 5


㔳 ⸳ 5

㐱 㔹 ⸷ 4


㔰 ⸸ 5



C ON C L U S I ON S


A c c o r d i n g t o t h e t e s t s, a n a l y s i s a n d d i s c u s s i o n s, t h e f o l l o w i n g c o n c l u s i o n s a r e d r a w n:


1.

E a r l i e r
-
a g e H P C h a s l a r g e r i n c r e a s e r a t e o f w a v e s p e e d, e s p e c i a l l y b e f o r e t h e a g e o f 1 d a y. A
f t e r
3 d a y, t h e i n c r e a s e r a t e b e c o me s s mo o t h a p p a r e n t l y.

2.

F o r H P C w i t h d i f f e r e n t w a t e r/b i n d e r r a t i o s, u n d e r a mb i e n t c u r i n g t e mp e r a t u r e ( 2 3
0
C ), t h e s t r e n g t h
d e v e l o p s t h e f a s t e s t w i t h i n 1 d a y f r o m
c a s t i n g.

T h e d e v e l o p me n t o f s t r e n g t h t h e n r e d u c e s
o b v i o u s l y a
f t e r 1 d a y
.

3.

T h e
s t r e
n g t h d e v e l o p me n t

o f H P C

i s r e l a t e d t o t h e w a t e r/b i n d e r r a t i o a n d i s
a
f f e c t e d b y t h e c u r i n g
t e mp e r a t u r e.


I t i s f o u n d t h a t o n l y

c u r i n g t e mp e r a t u r e a b o v e t h e a mb i e n t t e mp e r a t u r e c a n
a d e q u a t e l y
s t i mu l a t e t h e d e v e l o p me n t o f s t r e n g t h a t e a r l
y a g e b u t n o t
f o r
t h e l o w c u r i n g
t e mp e r a t u r e. H o w e v e r, t h i s e f f e c t b e c o me s l e s s s i g n i f i c a n t a f t e r 7 d a y s.


1356

4.

For HPC under ambient curing temperature (23
0
C) and 40
0
C, the two
-
period prediction model
using
one
-
day age as the
separating

point is proven to hav
e fairly good reliability. As for curing
temperature of 10
0
C case, 2 days are taken as the
separating

point for the prediction model.

5.

For the strength of HPC within
one
-
day age, although the suggested two
-
period prediction model
has

relatively

large devia
tion, it has
rather
better accuracy than the presently used maturity method.



REFERENCES


1.

Stphan, D.E. (1994). High Performance Concrete,
Proceedings of International Workshop on HPC
,
Bangkok. Thailand.

2.

Lai C. P. (1998).
Effects of the Composition of Conc
rete on it’s Flow Properties and the Ultrasonic
Pulse Velocity in Concrete
, Ph.D.
dissertation
, National Chung
-
Hsiung University, Taiwan.

3.

Stephen P.Pessiki and Matthew R. Johnson. (1996). Nondestructive Evaluation of Early
-
Age
Concrete Strength in Plate St
ructures by the Impact
-
Echo Method,
ACI Materials Journal,

Vol.93,
No.3
, 260
-
271.

4.

ASTM C1074
-
87. (1992). Standard Practice for Estimating Concrete Strength by the Maturity
Method,
Annual Book of ASTM Standard,
Vol.04.02
.

5.

Nicholas J. Carino,
The Maturity Me
thod
, CRC Handbook on Nondestructive Testing of Concrete,
101
-
143.

6.

Gilkey, H.J. (1961). Water
-
Cement ratio versus strength another look,
J. of the ACI,

V.57,
No.10
, .1287
-
1312.

7.

Walker.Snton and Bloom. (1960). Effect of Aggregate size on properties of concr
ete,
ACI Journal
,
V.57, No.9
, 283
-
298.

8.

Aitcin,P.C. and Metha,P.K. (1990). Effect of coarse Aggregate characteristics on mechanical
properties of high strength concrete,
ACI Materials Journal
, Mar.
-
Apr., 103
-
107.

9.

Williams,R.I.T. (1962). Effect of cement con
tent on the strength and elastic properties of dry lean
concrete, Technical Report No.TRA/323,
Cement and Concrete Association
, London, Nov., 28.

10.

Hirsch,T.J. (1962). Modulus of Elasticity of Concrete Affected by Elastic Modulus of Cement
Paste Matrix and A
ggregate,
Journal of American Concrete Institute
,
Vol.59, No.3
, 427
-
452.

11.

Yen, T. (1993). The Development of Concrete in Early Age,
Proceedings of 17
th

National
Conference on Theoretical and Applied Mechanics
, 817
-
821.

12.

Plowman, J.M
. (
1956
)
. Maturity and Str
ength of Concrete,
Magazine of Concrete Research

(London),
V. 8,

No. 22
,

Mar., 13
-
22.

13.

Young, J.F. (1981)
.
Hydration of Portland Cement,
EMMSE
, Material Research Lab. PA.

14.

F.A. Oluokun, E.G. Burdette, J.H. Deatherage
.

(
1990
).

Early Age Concrete Strength Pred
iction by
Maturity
-
Another Look,
ACI Materials Journal
, Nov.
-
Dec.,

565
-
572.