Mechanical Properties of Mechanical Properties of HPC with HPC with Expansive Additive and Expansive Additive and Shrinkage Reducing Shrinkage Reducing Admixture under Simulated Admixture under Simulated Completely-Restrained Completely-Restrained Condition at Early Age Condition at Early Age Takafumi Noguchi Takafumi Noguchi The University of The University of Tokyo, Japan Tokyo, Japan Park Sun-Gyu Park Sun-Gyu Yonsei University, Yonsei University, Korea Korea Ippei Maruyama Ippei Maruyama Hiroshima Hiroshima University, Japan University, Japan
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Mechanical Properties of HPC with Expansive Additive and Shrinkage Reducing Admixture under Simulated Completely- Restrained Condition at Early Age Takafumi.
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Mechanical Properties of Mechanical Properties of HPC withHPC with
Admixture under Simulated Admixture under Simulated Completely-Restrained Completely-Restrained Condition at Early AgeCondition at Early Age
Takafumi NoguchiTakafumi Noguchi The University of The University of Tokyo, JapanTokyo, JapanPark Sun-GyuPark Sun-Gyu Yonsei University, KoreaYonsei University, KoreaIppei MaruyamaIppei Maruyama Hiroshima Hiroshima University, JapanUniversity, Japan
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 Program Flow of Simulated Completely-Restrained TestCompletely-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 stressstress trigger. Another is strainstrain
trigger.
While repeating this process in VRTM,a completely-restrained condition is achieved andthe stress generated by shrinkage is measured.
Mix Proportions of Mix Proportions of ConcreteConcrete
Compressive Strength Compressive Strength Tensile StrengthTensile Strength Modulus of ElasticityModulus of Elasticity Free Autogenous ShrinkageFree Autogenous Shrinkage
Sealed with a polyester film at 20 ºCSealed with a polyester film at 20 ºC Stress Development under Simulated Stress Development under Simulated
Completely-RestraintCompletely-Restraint Sealed with a polyester film at 20 ºCSealed with a polyester film at 20 ºC Trigger of stress and strain : 0.01 MPa and Trigger of stress and strain : 0.01 MPa and
2 x 102 x 10-6-6
Mechanical Properties of Mechanical Properties of ConcreteConcrete
Autogenous shrinkages of NHC and SHC occurres Autogenous shrinkages of NHC and SHC occurres at a rapid rate in the first few hours and the rate at a rapid rate in the first few hours and the rate decreased afterward.decreased afterward.
In EHC, after a few hours expansion In EHC, after a few hours expansion is observed.is observed.
Expansive addition and shrinkage reducing Expansive addition and shrinkage reducing admixture can obviously reduce the admixture can obviously reduce the autogenous shrinkage of HPC.autogenous shrinkage of HPC.
Temperature Histories Temperature Histories
0
5
10
15
20
25
30
0 1 2 3 4 5 6
Age(days)
Tem
pera
ture
(ºC)
NHC
EHC
SHC
Almost constant temperature never causes Almost constant temperature never causes significant expansion.significant expansion.
Strain under Simulated Strain under Simulated Completely-RestraintCompletely-Restraint
-4-3-2-101234
0 1 2 3 4 5 6Age (days)
Stra
in (x
10-6)
NHC
EHC
SHC
Deformation is well controlledDeformation is well controlledwithin the range of the threshold within the range of the threshold value, 1value, 1m.m.
Ten
sion
Com
pre
ssio
n
Stress under Simulated Stress under Simulated Completely-RestraintCompletely-Restraint
-0.5
0.0
0.5
1.0
1.5
2.0
0 1 2 3 4 5 6Age (days)
Ten
sile
Stre
ss (M
Pa) NHC
EHC
SHC
Invisible Invisible CrackCrack
Tensile Strength of NHC at 1 day = 2.2 MPa (x 0.7 = 1.54 MTensile Strength of NHC at 1 day = 2.2 MPa (x 0.7 = 1.54 MPa) Pa)
In EHC and In EHC and SHC,SHC,lower tensile lower tensile stress and no stress and no cracking.cracking.
Schematic Diagram for Creep Schematic Diagram for Creep EstimationEstimation
Elastic Strain Elastic Strain measured measured from the recovery from the recovery cycles of VRTMcycles of VRTM
Creep StrainCreep Strain
-400
-350
-300
-250
-200
-150
-100
-50
0
0 1 2 3 4 5
Age (days)
Cre
ep S
train
(x10
-6)
NHC
EHC
SHC
Creep is quite significant in the deformation of Creep is quite significant in the deformation of HPC at early age, corresponding to 90 % of the HPC at early age, corresponding to 90 % of the free shrinkage strain.free shrinkage strain.
Creep strain shows the tendency to Creep strain shows the tendency to increase rapidly immediately after the increase rapidly immediately after the setting up to 10 hours.setting up to 10 hours.
A considerable tensile stress in A considerable tensile stress in HPC can be relaxed under HPC can be relaxed under restraint at early age.restraint at early age.
Creep Coefficient in Each Creep Coefficient in Each StepStep
0
5
10
15
20
25
30
0 1 2 3 4 5
Age (days)
Cre
ep C
oefficient NHC
EHC
SHC
εi,co-creep = εi,creep / εi,elastic
Creep coefficient of NHC is Creep coefficient of NHC is lower than those of EHC and lower than those of EHC and SHC in the beginning.SHC in the beginning.
0
2
4
6
8
10
0 0.2 0.4 0.6 0.8 1
Age (days)
Cre
ep C
oefficient NHC
EHC
SHC
Tensile stress in restrained EHC and Tensile stress in restrained EHC and SHC is lower than that in NHC at SHC is lower than that in NHC at early age.early age.
Concluding Remarks Concluding Remarks (1st)(1st)
The variable restraint testing machine The variable restraint testing machine can show how tensile stress and strain can show how tensile stress and strain develop under restrained condition in develop under restrained condition in HPC with and without expansive HPC with and without expansive addition and shrinkage reducing addition and shrinkage reducing admixture.admixture.
The tensile stress in HPC with The tensile stress in HPC with expansive addition or shrinkage expansive addition or shrinkage reducing admixture under completely reducing admixture under completely restrained condition at early age was restrained condition at early age was lower than that of normal HPC.lower than that of normal HPC.
Concluding Remarks Concluding Remarks (2nd)(2nd)
Normal HPC shows larger creep strain Normal HPC shows larger creep strain but smaller creep coefficient than but smaller creep coefficient than concrete with expansive addition or concrete with expansive addition or shrinkage reducing admixture. shrinkage reducing admixture. Normal HPC is sensitive to autogenous Normal HPC is sensitive to autogenous shrinkage cracking.shrinkage cracking.
Expansive addition and shrinkage Expansive addition and shrinkage reducing admixture make a crack reducing admixture make a crack prevention effect on HPC at early age.prevention effect on HPC at early age.