Self-Consolidating Concrete: Bridging Research and Practice · 2020. 8. 27. · ACI 237 ETS Report....
Transcript of Self-Consolidating Concrete: Bridging Research and Practice · 2020. 8. 27. · ACI 237 ETS Report....
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Self-Consolidating Concrete: Bridging Research and PracticeMATTHEW D’AMBROSIA, PHD, PE
MJ2 CONSULTING, PLLC
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Development of SCC Performance CriteriaDeveloped in Japan in the late 1980’s
Flows into formwork without vibration or mechanical consolidation
Flowable properties achieved with:◦ Ultra high-range water reducer
(polycarboxylate)◦ Viscosity Modifying Admixture (VMA)◦ High cementitious materials or
powder content◦ Small coarse aggregate and
higher sand fraction
ACI 237 ETS Report
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Potential benefits of SCCImproved consolidation
Reduced labor cost
Accelerated construction
Reduced noise
Performance Requirements◦ Flowability into formwork and through reinforcement◦ Stability (resistance to segregation)◦ but what about hardened concrete properties?
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Database of SCC proportions shows a departure from “normal” OPC
0.0
0.5
1.0
1.5
2.0
2.5
50 55 60 65 70 75 80 85 90 95 100
AGGREGATE CONTENT (%)
FA/C
A R
ATI
O
SCC Database
Mixtures studied
SCC4OPC1
SCC3 SCC2 SCC1
Typical non-SCC materials, according to ACI mixture proportioning method
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Workability and RheologyWorkability: “The ease with which [concrete] can be mixed, placed, consolidated, and finished to a homogenous condition.”
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SCC Flow Characteristics
Slump flow test (ASTM C1611)Prestressed Engineering Corp, Blackstone, IL, USA
11
Flowing into double T beam formsACI 237 ETS Report
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Standard tests have been developed
Prestressed Engineering Corp, Blackstone, IL, USA
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J-ring test for passing ability (ASTM C1621)Test is performed using a standard slump cone
Difference in diameter
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Rheology is the study of the flow of matterA concrete rheometer, determines resistance to shear flow at various rates
Bingham model:◦ Yield stress: minimum stress to initiate
or maintain flow (slump)◦ Plastic viscosity: the resistance to flow
once yield stress is exceeded (stickiness)
Slump, slump flow, stability, segregation, are concrete rheology terms
Shear RateShea
r St
ress
The Bingham Model
slope = plastic viscosity (µ)
yield stress (τ0)
Conventional Concrete
SCC
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How to approximate rheological behavior
R2 = 0.90
0
1
2
3
4
5
6
7
8
9
10
0 30 60 90 120
Plastic Viscosity (Pa.s)
T 20 (
s)
Reference: Koehler, E.P., Fowler, D.W. (2008). “Comparison of Workability Test Methods for Self-Consolidating Concrete” Submitted to Journal of ASTM International.
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Stability and Cohesion of SCC
5”
6”"
161
m =18g
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How do we evaluate segregation?
Hardened Visual Stability Index (VSI) Rating Criteria for Concrete Cylinder Specimens
0: StableNo paste or mortar layer visible at top of cylinder, no apparent difference
in the size and area percentage of coarse
aggregate through depth
1: StableNo paste or mortar layer visible at top of cylinder, slight difference in the
size and area percentage of coarse
aggregate through depth
2: UnstableSlight paste or mortar
layer visible (<1”), slight difference in coarse aggregate through
depth
3: UnstableSignificant paste or mortar layer visible
(>1”), obvious difference in the size and area
percentage of coarse aggregate through
depth
0 1 2 3
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Segregation Resistance
( )Rg fluidsphere ρρτ −≥34
0
Buoyancy + Resisting Force-Paste rheology & density-Aggregate shape & texture-Aggregate lattice effect
Gravitational Force-density & size
Saak, A.W., Jennings, H.M., and Shah, S.P. (2001). “New Methodology for Designing Self-Compacting Concrete,” ACI Materials Journal, 98(6), 429-439.
( )M1M421
M1-M4 SI+
=
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The Segregation ProbeApplicability:
Rapid surface segregation measurement
Sensitive to small changes in stability of SCC
Suitable for field measurement
Procedure:
Cast fresh concrete into 6 x 12” cylinder
Wait for 15 min, avoid excessive disturbance
Put ring on surface gently
Wait for at least 2 min until ring stops settling
Take reading
5”
6”"
161
m =18g
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Design for Segregation ResistanceSegregation resistance increased with:◦ Higher yield stress (static and dynamic yield stress assumed equal initially)◦ Higher plastic viscosity◦ Higher thixotropy
Reduce Flow
Use smaller aggregate
Use more paste
Reference: Koehler, E.P., and Fowler, D.W. (2008). “Static and Dynamic Yield Stress Measurements of SCC” Proceedings of SCC 2008, Chicago, IL.
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SCC Formwork PressureSCC approaches full hydrostatic pressure during rapid placement
Will SCC change the way we design formwork?
Pressure decrease is a combination of physical (internal friction) and chemi-physical (gelation) phenomena
Internal friction is a function of the aggregate content and the workability of concrete
Set modifying admixtures will affect formwork pressure
Initial pressure drop is independent of cement hydration
Temperature is a significant factor
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Models are needed to describe pressure for design
28
Continuous Pour
0.00.51.01.52.02.53.0
0 50 100 150 200 250 300
Time
Pre
ssure
Model of Pressure Evolution over Time
0
0.2
0.4
0.6
0.8
1
0 50 100 150 200 250Time
C [c
oeff.
of p
ress
. evo
lutio
n ]
The coefficient alpha describesdifferent pressure decay rates over time.
+= −+− ))((
20 2
)1(tbte
BtCC α
)()()(1
tCtRtP i
n
i∑=
∆= γ
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Formwork PressureFormwork pressure is related to concrete rheology◦ Pressure increases with slump◦ SCC exhibits high formwork pressure due to high fluidity
Concrete at rest in forms, static yield stress is relevant◦ Static yield stress is affected by dynamic yield stress and
thixotropy◦ SCC is placed in lifts, which takes advantage of thixotropy
SCC must be designed to flow under its own mass and exert low formwork pressure◦ Low dynamic yield stress (self flow)◦ Fast increase in static yield stress (reduced formwork pressure) –
gelation or structural build up
Place concrete in lifts to allow build-up of thixotropic structure
Limit pour heights and rates based on concrete rheology
Do not vibrate concrete
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Early Age Cracking
0.016” (0.4 mm)
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Stress development in SCC indicates cracking risk
Autogenous shrinkage in low w/c materials generates significant stress at early age
A minimum w/c ratio can reduce early age cracking in restrained concrete
0
50
100
150
200
250
300
350
400
450
0 2 4 6 8 10Age (days)
Shr
inka
ge S
tress
(psi
)
OPC1, w/c = 0.40
SCC1, w/c = 0.39
SCC2, w/c = 0.33
SCC3, w/c = 0.41
SCC4, w/c = 0.34
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Early age shrinkage causes cracking risk
0.39, 37%
0.34, 34%0.41, 33%
0.40, 32%
0.33, 40%
w/b, paste%
-1000
-900
-800
-700
-600
-500
-400
-300
-200
-100
0
0 5 10 15 20 25 30Age (days)
Free
Shr
inka
ge (x
10-6
)
OPC1, w/c = 0.40
SCC1, w/c = 0.39
SCC2, w/c = 0.33
SCC3, w/c = 0.41
SCC5, w/c = 0.34
Typical Concrete –“Safe Zone” ?
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Low w/c drives autogenous shrinkage
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
20
0 5 10 15 20 25 30Age (d)
Aut
ogen
ous
Shr
inka
ge (1
0-6 m
/m)
OPC1, w/c = 0.40
SCC1, w/c = 0.39
SCC2, w/c = 0.33
SCC3, w/c = 0.41
SCC4, w/c = 0.32
Typical Concrete –“Safe Zone” ?
0.39, 37%
0.34, 34%
0.41, 33%
0.40, 32%
0.33, 40%
w/b, paste%
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NEED: Measurement of cracking tendencyASTM C1581
•Concrete shrinks around the steel ring causing tensile stress in concrete•Stress relaxes due to tensile creep•Strain measurements in steel are proportional to stress in concrete•When tensile stress exceeds strength, cracking occurs
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Cracking time is improved with higher w/cm, and/or lower paste content
w/cm = 0.42, 33% paste
-100
-80
-60
-40
-20
0
20
0 5 10 15 20 25 30Concrete Age, days
Stra
in, i
n/in
x10-
6
PP-2
P-100
RG-C2
TR-C2w/cm = 0.40, 35% paste
w/cm = 0.37, 37% paste
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NEED: Concrete temperature rise
25% fly ash
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NEED: Shrinkage and cracking mitigationSRA – Shrinkage reduction through chemistry
SLA – Shrinkage reduction through internal curing
Optimization – Reduce cement paste %
SCMs – Reduce autogenous, temperature, modulus
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Preventing SCC crackingSome SCC mixtures have high risk for cracking, specs should utilize performance tests
Mechanical properties controlled by paste content and w/cm
Autogenous shrinkage is early age issueRecommendations:◦ Use the lowest possible cement paste content that still achieves desired
flow characteristics◦ Avoid low w/b ratios that lead to high autogenous shrinkage
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Back to Specifications…What is the “real performance” we need to ensure?◦ More that strength
Spec writers need to assert more control ◦ Example: SCC needs limits on segregation, shrinkage cracking, min. aggregate
content, min. w/c◦ Limit temperature rise for mass concrete applications◦ Modulus and Creep◦ Ring test for cracking
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Suggestions for Practice
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42
Acceptance Criteria: w/cm ratio
0.42 eliminates autogenous shrinkage
Application specific limits◦ High Restraint: 0.42◦ Med Restraint: 0.38◦ Low Restraint: 0.32
0
100
200
300
400
500
600
700
800
900
0.30 0.32 0.34 0.36 0.38 0.40 0.42
w/cm
Aut
ogen
ous
Shr
inka
ge S
train
(x10
-6)
Autogenous Shrinkage (28d)
Total Shrinkage (28d)
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43
Water-cementitious ratio “safe zone”
0.25 0.450.35 0.55 0.65
Below 0.32 autogenous shrinkage can cause severe cracking when concrete is restrained
Above 0.50, drying shrinkage is a concern
Strength and durability also affected
Good range for minimum “total” shrinkage is 0.38 to 0.44
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44
0
100
200
300
400
500
600
700
800
900
30% 32% 34% 36% 38% 40% 42%
Paste Content by Volume
Aut
ogen
ous
Shr
inka
ge S
train
(x10
-6)
Autogenous Shrinkage (28d)
Total Shrinkage (28d)
Acceptance Criteria: Paste Content
IDOT max cement factor: 7.05 cwt/yd3
At 705 lb/yd3, 0.44 w/cm = 34% paste
Below 32%, SCC has questionable fresh properties
Application specific limits ◦ High Restraint: 30%◦ Med Restraint: 34%◦ Low Restraint: 36%
TABLE 4.3 – From ACI 237 ETS
Summary of Self-Consolidating Concrete Proportioning Trial Mix Parameters
Coarse aggregate by volume 28% - 32%
Paste Content by volume 34% - 40%
Mortar Fraction by volume 68%-72%
Typical w/cm 0.32 – 0.45
Typical powder content 650* – 800 pounds
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45
Aggregate gradation analysis
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
1 3/4 5/8 1/2 3/8 1/4 1/8 1/16 1/30 1/50 1/100 1/200 PanSieve Size (inches)
Per
cent
Ret
aine
d
Min
OPC1
SCC1
SCC2
SCC3
SCC5
Max
15%
5%
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46
Shrinkage reduced through aggregate optimization
-600
-500
-400
-300
-200
-100
0
0 50 100 150 200Age (d)
Free
Shr
inka
ge (x
10-6
)
OPC 42
DOC 42
w/cm = 0.4222% paste
w/cm = 0.4232% paste
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47
RecommendationsPlace limits on maximum paste content
Place lower limit on w/cm
Use well graded aggregates to reduce the need for high paste%Recommend supplementary cementitious materials to replace cement
Recommend autogenous shrinkage test for w/cm below 0.38
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SCC: Bridging Research and PracticeSCC has many benefits◦ Improved consolidation for tight forms or bar spacing◦ Labor cost savings◦ Aesthetic finish◦ Rapid placement
Research is available to help solve problems◦ Rheology testing is becoming more common◦ Avoid segregation problems with proper testing in the lab and field◦ Pressure models assist formwork design◦ Cracking can be avoided through careful mixture development or mitigation
Specifications must ◦ Limit w/b and paste content or utilize mitigation to avoid cracking◦ Use performance testing for rheological characterization, avoid segregation, form
pressure, and pumpability problems