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Jake Hiller, Ph.D. Assistant Professor
Michigan Technological University
Presentation for the 54º Congresso Brasileiro do Concreto
Maceió, Alagoas, Brasil
11 October, 2012
CHARACTERIZATION OF RECYCLED CONCRETE
AGGREGATES FOR REUSE IN R IGID PAVEMENTS
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ACKNOWLEDGEMENTS
Michigan Department of Transportation (MDOT)
Minnesota Department of Transportation (Mn/DOT)
MTU Tomasini Fund
Students
Rita Lederle
Morgan Hansen
Cory Shorkey
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RECYCLED CONCRETE AGGREGATES
(RCA)
RCA is old concrete that has been removed, crushed, and sized for reuse
Old aggregates as well as old mortar and some unhydrated cement
RCA can contain contaminants
RCA differs depending on crushing process
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IMPETUS FOR STUDY
RCA use in concrete goes back to post WWII Europe
MDOT was pioneer of RCA use in rigid pavements in the USA in 1980’s
1050 lane-km constructed
Moratorium for use in rigid pavements in 1991
Costs from landfill, transportation, and quality aggregates has brought issue back into play
Sustainability
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I-94 WEST OF KALAMAZOO,
MICHIGAN, USA
JPCP after 10 years
Shrinkage cracking
Wide joints and cracks
Low load transfer
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SOME RCA ISSUES FOR REUSE IN
CONCRETE SURFACE LAYER
Varied crushing processes and high fines
D-cracking and Material-related distress potential
High alkalinity of water runoff
Leachates (calcium) and high pH for base material
High absorption (mortar / unhydrated cement)
Less volumetric stability
Shrinkage, creep, and carbonation
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LABORATORY STUDY
Aggregate Characterization
Absorption capacity
Specific gravity
Address RCA in comparison with virgin aggregate concrete
Hardened air content
Shrinkage (drying, autogenous, and restrained)
Using three methods
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COARSE AGGREGATE SOURCES
Natural aggregates
Crushed gravel
RCA (with original aggregate type)
RCA limestone
RCA blast furnace slag
RCA crushed gravel
Recycled RCA crushed gravel (3G/Twice recycled)
Fine aggregate natural sands
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DIFFERENCES IN ABSORPTION
CAPACITY AND POROSITY
Standard ASTM C127
24-hour absorption
Visual assessment of SSD High variability
Helium Pycnometer and envelope density analyzer (EDA)
Automated
Assess water absorption?
Image Analysis
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IMAGE ANALYSIS - AGGREGATE
THIN SECTIONS
Surface
porosity
Internal
porosity
Using Impregnated resins with specified
viscosities
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IMAGE ANALYSIS
Imaging software to assess Pore sizes Locations Distribution
Leads into future research in moisture diffusivity and poromechanics
Original Aggregate Surface Porosity Internal Porosity
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Natural aggregates void of larger Feret’s diameter
Number of voids much lower in natural aggregates
PORE SIZE DISTRIBUTION OF
AGGREGATES
0%
10%
20%
30%
40%
50%
60%
70%
80%
0.005005 0.0175 0.0375 0.0625 0.0875
% F
req
ue
ncy o
f P
ore
Dia
me
ter
Diameter (mm)
Gravel RCA Slag RCA Limestone RCA Gravel
0%
1%
2%
3%
4%
5%
6%
7%
0.175 0.375 0.625 0.875 1.75 3.75
% F
req
ue
ncy o
f P
ore
Dia
me
ter
Diameter (mm)
Gravel RCA Slag RCA Limestone RCA Gravel
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SURFACE POROSITY BY MULTIPLE
METHODS
Image Analysis matches He Pyc /EDA
Does ASTM C127 capture porosity/AC for highly porous aggs?
0
4
8
12
16
20
Gravel Slag RCA Limestone RCA Gravel RCA
Surf
ace
Po
rosi
ty (
%)
Aggregate Type
ASTM Helium Pycnometer/EDA Image Analysis
An
GB
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RCA CONCRETE MIX
PROPORTIONING
0.42 water-to-cement ratio
No SCMs
362 kg cement per m3 of concrete
72% bulk volume of aggregate to vol. of concrete
Paving mixes high in coarse aggregate content
Air entrained (target= 6.5%, range of 5.5 – 8.5%)
Target slump 50mm
Monitor fresh properties over time
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FRESH PROPERTIES
Measured slumps consistently higher than expected for RCA concretes
Met specs for natural aggregate concretes
Slump loss was more dramatic in RCA concrete
Shorter window of workability
Harsh mixes with poorer consolidation
Superplasticizers helped to some degree
Fresh air content specification difficult to meet
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HARDENED AIR CONTENT
RCA occupies same bulk volume
Contains more void space
Moisture / chloride movement
0
2
4
6
8
10
12
14
16
18
Crushed Gravel Slag RCA Limestone RCA Crushed Gravel RCA
To
tal A
ir C
on
ten
t (%
)
Air Content in RCA Mortar Fraction Air Content in New Mortar
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AIR VOID SYSTEM QUALITY
RCA concretes showed better air void system connectivity in general
Dependent on old concrete’s quality
Counterintuitive
Aggregate Void Frequency Within
Criteria
Specific Surface
Area (mm-1)
Within
Criteria
Spacing Factor
(mm)
Within
Criteria
Crushed Gravel 0.187 N 16.3 N 0.303 N
Slag RCA 0.339 Y 18.4 N 0.237 N
Limestone RCA 0.522 Y 23.8 N 0.140 Y
Crushed Gravel RCA 0.509 Y 29.8 Y 0.153 Y
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OPTICAL AND SCANNING
ELECTRON MICROSCOPY
Porous interfacial transition zones (ITZ)
Ettringite filled pores of 3G concrete samples
Good performance of 3G concrete
Microcracking in old mortar from crushing process
Reduced fracture resistance
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DISTINCT MORTAR PHASES
Some evidence of multiple ITZs
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SHRINKAGE SPECIMENS
1-D shrinkage
ASTM C 157
Sealed (autogenous)
Unsealed
Difference is drying shrinkage
Stored at constant relative humidity, then switched after 1 year, then every 30-45 days
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MECHANISM OF SHRINKAGE
Shrinkage in concrete is dominated by capillary surface tension mechanisms
As water leaves pore system, curved menisci develop, creating reduction in RH and “vacuum” within the pore fluid
Internal relative humidity (RH)
Hydration
product Hydration
product
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VOIDS IN CONCRETE
RCA concrete contains higher percentage of entrained air through gel pores
Affects concrete durability and structural properties
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Capillary stresses present in pores with radii between 2-50 nm
C-S-H makes up ~70% of hydration product
Majority of capillary stresses present in C-S-H network
VISUALIZING THE SCALE OF SHRINKAGE
MECHANISMS
Note the
dimensions
Scanning electron
micrograph from Taylor
“Cement Chemistry”
(originally taken by S.
Diamond 1976)
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DRYING SHRINKAGE IN RCA
CONCRETE
Thought to be a driving force in deterioration of JPCPs in many cases
Higher amount of capillary pores from attached mortar
Unhydrated cement particles
Previous pavement design methods have not taken this into account
Pavement mechanics are now being utilized to capture effects
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DRYING SHRINKAGE AT 50% RH
Significantly higher for RCA concrete
Capillary porosity exists in RCA and new mortar
Crushing -600
-500
-400
-300
-200
-100
0
0 10 20 30 40 50 60
Dry
ing
Shri
nka
ge (µ
ε)
Age (days)
Crushed Gravel Slag RCA Limestone RCA Crushed Gravel RCA 3G RCA
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RESTRAINED SHRINKAGE STRAINS
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RCA CONCRETE SLABS UNDER
RESTRAINT
While drying shrinkage is higher in RCA concretes …
Increased creep characteristics of RCA concrete can relax strains at early ages
At later ages, creep is less effective in controlling strain magnitudes and deflections
Rigid pavement is restrained from dowels, tie bars, slab-base friction/bonding, and self-weight
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MOISTURE IN CONCRETE
PAVEMENT SLABS
Due to this semi-porous nature of concrete
Moisture can get in and out of pores
Due to simplicity of pavement geometry, typically through surface
Below depth of 50-100mm in concrete slab
Moisture content is nearly saturated and consistent
Top 50-100mm varies with rain events, ambient relative humidity, wind, etc.
Leads to highly non-linear moisture gradients in slab
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MOISTURE LOSS EFFECTS
Occurs through both self-desiccation and drying
Both cause volumetric changes
Autogenous (self-desiccation) happens throughout concrete
Drying shrinkage is a gradient within concrete
Drying shrinkage and warping are linked
Tied to gel pores and smaller capillary pores
Differential drying shrinkage – permanent
Warping – reversible portion of shrinkage
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WHY CURL AND WARP MATTER
Stresses induced by environmental loading can be enough to crack a slab
Slab corners can be unsupported
Changing boundary conditions
Change in primary failure mechanism
Premature fatigue failure
Top-down cracking from combined
environmental and traffic loading
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DRYING SHRINKAGE LEADING TO
SLAB DEFORMATION
Warping and built-in curling issues can lead to alternative fatigue cracking development
Gaps under slab corners
Interaction with external loads
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VOLUME CHANGE COMPONENTS IN THE MECHANISTIC-
EMPIRICAL PAVEMENT DESIGN GUIDE (MEPDG)
Total Equivalent Temperature
Difference
Equivalent Temperature
Gradient (ETG)
Temperature Gradient
Moisture Gradient
Built-in Curl (BIC)
Construction Curl
Differential Drying Shrinkage
- Creep
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TEMPERATURE SHIFT DUE TO
BUILT-IN CURL
Shift can be large enough to never “curl down”
No Built-in Curl
-18oC Built-in
Curl
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REVERSIBLE SHRINKAGE -
INTRODUCTION
Portion of shrinkage is reversible
Assumed to be 50% in MEPDG
Mindness and Young,
(1981.) Concrete.
Prentice Hall, Englewood
Cliffs, NJ.
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REVERSIBLE SHRINKAGE – PAST
RESEARCH
Some research in Europe from 1940-1975
40-70% of shrinkage is reversible
Mortars stored in water for an extended cure had complete shrinkage reversibility L’Hermite, R.L. (1947) “Le Retrait Des Ciments,
Mortiets et Betons.” Laboratioires du Batiment et des
Travaux Publics.
Time in Days
Sh
rinka
ge
Str
ain
(μ
ε)
Air Air Water
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MECHANISM OF REVERSIBLE
SHRINKAGE
The mechanism is not well understood
Neville hypothesized that C-S-H gels form bonds when they are in close proximity during drying phase
When the concrete is again exposed to moisture, these bonds swell, but hold the matrix together
Preventing shrinkage from being fully reversible
Neville, A.M., Properties
of Concrete. 4th ed. 1997,
New York: Wiley & Sons,
Inc.
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MECHANISM OF REVERSIBLE
SHRINKAGE
Others hypothesize that a portion of irreversible shrinkage is related to microcracking
During the shrinkage phase, microcracks are formed and open
During swelling, cracks close either partially or fully
Because these cracks cannot be reversed, some portion of drying shrinkage is believed to be irreversible
Granger L., Torrenti J.M.,
and Acker P., Thoughts
About Drying Shrinkage:
Scale Effect and
Modelling. Materials and
Structures, 1997. 30(3):
pp. 96-105.
Sellier A., and Buffo-
Lacarrière L., "Vers une
Modélisation Simple et
Unifiée du Fluage Propre,
du Retrait et du Fluage en
Dessiccation du Béton.
Revue Européenne de
Génie Civil, 2009. 13(10).
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% REVERSIBLE BY AGG TYPE
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
% R1 dry cure % R2 dry cure % R3 dry cure % R1 wet cure % R2 wet cure
Pe
rce
nt
reve
rsib
le s
hri
nka
ge
virgin
slag RCA
limestone RCA
gravel RCA
twice recycled
lightweight
Initially low humidity (50% RH) Initially high humidity (100% RH)
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WARPING & DDS - FINAL
MODELS
Non-linear model for equivalent temp. difference due to drying shrinkage in concrete pavements
Warping model
Differential Drying Shrinkage model
Lederle, R.E., and Hiller,
J.E., Development of New
Warping and Differential
Drying Shrinkage Models
for Jointed Plain Concrete
Pavements Derived with a
Nonlinear Shrinkage
Distribution, Accepted to
the Transportation
Research Record, 2012.
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WARPING & DDS – DESIGN AIDS
h 25cm
hs 7.5cm
w/c 0.4
ϕ 0.5
εsu 1200µε
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HOW THIS AFFECTS CONCRETE
PAVEMENTS IN AUSTRALIA
Non-linear model for equivalent temp. difference due to drying shrinkage
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HOW THIS AFFECTS CONCRETE
PAVEMENTS IN AUSTRALIA
For RCA concretes
Higher drying shrinkage
More change in coastal regions
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SIGNIFICANCE OF HIGH LEVELS OF PERMANENT
DIFFERENTIAL DRYING SHRINKAGE?
Poor curing High irreversible drying shrinkage
Leads to high levels of built-in curling in JPCP
Essentially causes equivalent temperature shift for specific climatic location
MEPDG captures this effect to some degree
Only predicts transverse fatigue cracking
To account for fatigue as transverse, longitudinal, or corner cracking
Rigid Pavement Analysis for Design in California
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RADICAL
Mechanistic rigid pavement analysis tool
Predict fatigue cracking at multiple locations
Transverse, longitudinal, corner cracking
Top-down or bottom-up
Site-specific conditions, traffic, materials
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LOW LOAD TRANSFER AND 4.6M
JOINT SPACING
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EBITD = Equivalent Built-
in Temperature Difference
For temperature
differences, °C = 5/9 °F
For San Francisco
climate, traffic, soils, etc.
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LOW LOAD TRANSFER AND 3.7M
JOINT SPACING
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RCA CONCLUSIONS
More unpredictable fresh and hardened properties using RCA in concrete
Need to account for existing air content in RCA
RCA concrete shows higher drying shrinkage, but better creep characteristics
Account for some factors in mechanistic design methods
Creep may be advantageous in restrained pavements
Must understand differences and plan for multi-generational use Promote Sustainable Practices
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RCA IN PVTS CONCLUSIONS
Past pavement design/analysis has not taken moisture warping into consideration directly
Not site-specific design
MEPDG attempts to account for this … unsuccessfully
Amount of reversible warping can be controlled
Macro-level curing conditions, w/c ratio, SCMs
Affects nano/micro-level C-S-H gel spacing, micro-cracking
RCA concretes show no difference with virgin agg concretes
Now have ability to capture these effects in pavement design, analysis, and construction practices
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RCA OPPORTUNITIES IN RIGID
PAVEMENTS
Cost savings for right projects
Typically urban with crushing on-site
Michigan environmental policy to use RCA if up to 10% greater cost than virgin aggregates
Knowledge and control of original project aggs
Lower cement content can yield good performance
Design adjustments to accommodate differences
Joint spacing, thickness, limited restraint, materials?
Two-lift construction?
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THANK YOU …
ANY QUESTIONS?