Expansive Additive and

frizzflowerUrban and Civil

Nov 29, 2013 (3 years and 4 months ago)

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Mechanical Properties of HPC with

Expansive Additive and

Shrinkage Reducing Admixture
under Simulated Completely
-
Restrained Condition at Early Age

Takafumi Noguchi

The University of Tokyo, Japan

Park Sun
-
Gyu


Yonsei University, Korea

Ippei Maruyama


Hiroshima University, Japan

Background


High
-
performance Concrete (Low W/C)


Self
-
desiccation


Autogenous Shrinkage


Under Restraint Condition


Cracking


Influence on Durability & Aesthetic


Use of


Expansive Addition


Shrinkage Reducing Admixture

Objective of Study


Behaviour of Early Age HPC with and without


Expansive Addition


Shrinkage Reducing Admixture


Restraint Free Condition


Shrinkage Strain


Under Restraint Condition


Strain & Stress


Creep Behaviour


Control of Autogenous Shrinkage Cracking

Variable Restraint Testing Machine

Fresh concrete is cast into the framework of the testing machine.

Specimen size is 1500 mm in length and
100 mm x 100 mm in cross sectional area

The ends of specimen are fixed to the cross
-
head,
which is fixed to the frame, by claws which hold
the concrete specimen and are able to exert
tensile or compressive force.

The load through the specimen is monitored
by a load cell with accuracy of 1 N.

The longitudinal deformation of concrete specimen is
monitored by four LVDTs with accuracy of 0.125 μm.

Experiment is commenced
after the concrete setting.

Program Flow of Simulated
Completely
-
Restrained Test

Completely restrained condition is simulated by
maintaining the total deformation of the
specimen within a threshold, which is defined as
the permissible change in the length of the
specimen
.

There are two controlling triggers.

One is
stress

trigger.

Another is
strain

trigger.

While repeating this process in VRTM,

a completely
-
restrained condition is achieved and

the stress generated by shrinkage is measured.

Mix Proportions of Concrete

Composition

NHC

EHC

SHC

Cement (kg/m
3
)

550

530

550

Expansive additive (kg/m
3
)

0

20

0

Shrinkage reducing admixture (kg/m
3
)

0

0

6

Water (kg/m
3
)

165

165

165

Fine aggregate (kg/m
3
)

781

781

781

Coarse

aggregate (kg/m
3
)

869

869

869

High
-
range water
-
reducing admixture
(cement weight %)

0.7

0.7

0.7

Experiments


Compressive Strength


Tensile Strength


Modulus of Elasticity


Free Autogenous Shrinkage


Sealed with a polyester film at 20 ºC


Stress Development under Simulated
Completely
-
Restraint


Sealed with a polyester film at 20 ºC


Trigger of stress and strain : 0.01 MPa and 2 x 10
-
6

Mechanical Properties of Concrete

Compressive Strength
(MPa)

Tensile Strength
(MPa)

Modulus of Elasticity


(GPa)

1

day

3

days

5

days

1

day

3

days

5

days

1

day

3

days

5

days

NHC

25.6

56.5

65.4

2.2

4.4

4.9

22.5

30.6

32.3

EHC

25.1

51.4

57.7

2.4

3.6

4.1

22.6

29.7

33.1

SHC

24.9

53.8

64.1

2.3

3.1

3.8

23.1

29.9

33.0

Autogenous Shrinkage

-500
-400
-300
-200
-100
0
0
1
2
3
4
5
6
Age (days)
Autogenous Shrinkage (x10
-6
)
NHC
EHC
SHC
Autogenous shrinkages of NHC and SHC occurres at a rapid rate
in the first few hours and the rate decreased afterward.

In EHC, after a few hours expansion is observed.

Expansive addition and shrinkage reducing admixture can
obviously reduce the autogenous shrinkage of HPC.

Temperature Histories

0
5
10
15
20
25
30
0
1
2
3
4
5
6
Age(days)
Temperature(ºC)
NHC
EHC
SHC
Almost constant temperature never causes significant expansion.

Strain under Simulated Completely
-
Restraint

-4
-3
-2
-1
0
1
2
3
4
0
1
2
3
4
5
6
Age (days)
Strain (x10
-6
)
NHC
EHC
SHC
Deformation is well controlled

within the range of the threshold value, 1



Tension

Compression

Stress under Simulated Completely
-
Restraint

-0.5
0.0
0.5
1.0
1.5
2.0
0
1
2
3
4
5
6
Age (days)
Tensile Stress (MPa)
NHC
EHC
SHC
Invisible Crack

Tensile Strength of NHC at 1 day = 2.2 MPa (x 0.7 = 1.54 MPa)

In EHC and SHC,

lower tensile stress
and no cracking.

Schematic Diagram for Creep Estimation

Age

Strain

Strain Trigger

Accumulation of

Elastic Strain

Free Autogenous

Shrinkage

Creep Strain

ε
i,creep

= ε
i,free

-

ε
i,elastic

Elastic Strain
measured from
the recovery cycles of VRTM

Creep Strain

-400
-350
-300
-250
-200
-150
-100
-50
0
0
1
2
3
4
5
Age (days)
Creep Strain (x10
-6
)
NHC
EHC
SHC
Creep is quite significant in the deformation of HPC at early
age,
correspond
ing

to
90 % of the free shrinkage strain.

Creep strain shows the tendency to increase rapidly
immediately after the setting up to 10 hours.

A considerable tensile stress in HPC can be
relaxed under restraint at early age.

Creep Coefficient in Each Step

0
5
10
15
20
25
30
0
1
2
3
4
5
Age (days)
Creep Coefficient
NHC
EHC
SHC
ε
i,co
-
creep

= ε
i,creep

/ ε
i,elastic

Creep coefficient of NHC is lower than
those of EHC and SHC in the beginning.

0
2
4
6
8
10
0
0.2
0.4
0.6
0.8
1
Age (days)
Creep Coefficient
NHC
EHC
SHC
Tensile stress in restrained EHC and SHC is lower
than that in NHC at early age.

Concluding Remarks (1st)


The variable restraint testing machine can
show how tensile stress and strain develop
under restrained condition in HPC with and
without expansive addition and shrinkage
reducing admixture.


The tensile stress in HPC with expansive
addition or shrinkage reducing admixture
under completely restrained condition at early
age was lower than that of normal HPC.

Concluding Remarks (2nd)


Normal HPC shows larger creep strain but
smaller creep coefficient than concrete with
expansive addition or shrinkage reducing
admixture. Normal HPC is sensitive to
autogenous shrinkage cracking.


Expansive addition and shrinkage reducing
admixture make a crack prevention effect on
HPC at early age.