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© 2011 M+P Labs
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© 2011 M+P Labs
www.mandplabs.com 1
Forensic Investigation of
Hardened Concrete:
Julius Bonini, PE - M+P Labs, Schenectady, NY [email protected]
Andrew Smith, PhD - CERAM Research, Stoke-on-Trent, UK [email protected]
Water-Cement Ratio
Outline
• Introduction: Concrete Basics– Constituents and Reactions
– Timeline
• Consequences of Improper Water-Cement Ratio– Performance Impact
– Indicators: Pre-cure and Post-cure
• Methods to Estimate Water-Cement Ratio– Capillary Porosity
– Optical Microscopy
– Paste Volume Analysis
• Other Forensic Methods Applied to Concrete
2
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Water : Cement Ratio in
Concrete
Part 1 – Introduction to Concrete
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Concrete BasicsInert Reactive
Coarse Aggregate + Fine Aggregate + Cement + Water
Admixtures:
• Water
Reducers
• Retarders
• Accelerators
•Air Entrainment
Agents
Tricalcium Silicate (C3S)
Dicalcium Silicate (C2S)
Tricalcium Aluminate (C3A)
Tetracalcium Aluminoferrite (C4AF)
© 2011 M+P Labs 4
Sand AdditivesSilica (Quartz)
Silicates
< 2mm
Dolomite
Sandstone
Etc.
(Regional)
> 2mm
OPC
Flyash
Slag
Microsilica
Other
Components
Pozzolana
Concrete BasicsConcrete: Components and purpose (Theoretical)
Cement – binder
Coarse aggregate – inert bulking
Fine aggregate – inert bulking
Water – activator, catalyst, gives “plasticity/fluidity” to the solids
5
Concrete in the “Modern World”: (Realistic)Cement – binder
Pozzolana – ‘cement replacement’ with binder properties when reacted
with Ca(OH)2 (fly ash (pfa), slag (ggbfs), silica fume (microsilica),
volcanic ash (pumice), glass (recycled), ceramic dust
Coarse aggregate – inert bulking
Fine aggregate – inert bulking
Admixtures – agents to address - air entrainment, water reduction
(plasticisers & organic superplasticisers), waterproofing,
low temperature working
Water – activator, catalyst, gives “plasticity/fluidity” to the solids© 2011 M+P Labs
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Concrete Basics
Definitions:
6
Cementitious Material = cement + flyash + slag + etc.
(Reactive Pozzolana)
w-c =
w-cm =
Weight of water
Weight of cement
Weight of water
Weight of cementitious material
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Cement
Cement Nomenclature (for non-chemists)
CaO – “C” (calcium oxide)
SiO2 – “S” (silicon dioxide)
Al2O3 – “A” (aluminium oxide)
Fe2O3 – “F” (iron oxide)
H2O – “H” (water)
Cement “Clinker Phases” (Shorthand)
C3S Tricalcium Silicate (Alite)
C2S Dicalcium Silicate (Belite)
C3A Tricalcium Aluminate
C4AF Tetracalcium Aluminoferrite
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The Basic ReactionHydration of Silicate & Aluminate Compounds:
2( 2CaO • SiO2 ) + 4H2O → 3CaO • 2SiO2 • 3H2O + Ca(OH)2 + Heat
3CaO • Al2O3 + 6H2O + Ca(OH)2 → 4CaO • Al2O3 • 12H2O (Hexagonal)
converts to 3CaO • Al2O3 • 6H2O (Cubic) over time
8
Water Reaction is Critical
•Fundamentally converts slurry to a solid
•Water-Cement Ratio is critical, and difficult to control and verify
–Optimal theoretical value is 0.25 (0.40 practical)
–Water content can vary during batching, transport, setting
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The Concrete Time Line
Design → Batching → Placement → Hardening
Curing
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Batch
90 Min.
Placement
2 – 8 Hours 28 + Days
Design SetFinal
SetInitial
Set
45 Min.
Setting Curing
Mix Based on Design
Requirements
Aggregates
Cement
AdditivesPozzalana
Water
Mixing & Testing
Batch
Ticket
Transport
To
Site
Tested (Wet)-Slump
-Compaction
-Air Content
-Unit Weight
Poured
Into
Forms
Finishing
Where Problems Occur:
Batching to Placement
• Rinse Water
• Plant Water
• Sand & Aggregate
Moisture
• Transit
– Water
• Slump Adjustment
• Atmospheric Water
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Batch
90 Min.
Placement
2 – 8 Hours 28 + Days
Design SetFinal
SetInitial
Set
45 Min.
Curing
The Results
• Coarse Aggregate
• Fine Aggregate
• Paste Volume
• Fly Ash & Slag Content
• Air Voids: entrained &
entrapped
• Reinforcement Location
11
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SECTIONS EVALUATED IN A
SCANNING ELECTRON MICROSCOPE
SEM
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The Results
• SEM Imaging
• Assess Micro-cracking
• Fine aggregate distribution
• Identify Other Constituents
– Fly Ash
– Slag
– Other Constituents
(Exotics)
13
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IMAGE ANALYSIS SYSTEM
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The Results
Imaging Analysis Results
• % Relative Area of
Constituents
– Coarse & Fine Aggregate
– Paste Volume
– Air Voids
• Depth Profile
15
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Water-Cement Ratio in Concrete
Part 2 – Adding water to cement
16
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Cement Hydration
C = CaO S= SiO2 A= Al2O3 F= Fe2O3 H= H2O
2C3S + 6H → C3S2H3 + 3Ca(OH)2
2C2S + 4H → C3S2H3 + Ca(OH)2
3CaO · Al2O3 + 6H2O + Ca(OH)2 → 4CaO · Al2O3 · 12H2O(Hexagonal)
converts to 3CaO · Al2O3 · 6H2O(Cubic) over time
C4AF + 2Ca(OH)2 + 10H → C3AH6 + C3FH6
17
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Hydration Rates
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Hydration – Strength Development
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Strength Development
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How Much Water is “Enough”?Theoretically, for an OPC cement and a given mix design with
no admixtures:
w-c = 0.25 is required to hydrate all the cement.
however
w-c = 0.15 is physically absorbed by cement paste and thus
not available for hydration.
therefore
w-c = 0.40 represents a minimum water content to achieve
full hydration.
w-c = > 0.40 excess water remains as free water in the
cementitious structure and forms capillary
pores, BLEEDS to the surface, or lost by
evaporation.
w-c = < 0.40 restricted hydration of cement particles.© 2011 M+P Labs
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Water Problems During CuringEarly stage problems with concrete associated with water
Bleeding migration and accumulation of water at the surface of
concrete (horizontal pour)
- loss in volume of concrete (densification)
- segregation of aggregate (settlement)
- slower hydration = retardation of strength gain
Drying Shrinkage
excessive loss of water from the surface of concrete
due to evaporation. Rate of Evaporation > Bleed
- plastic shrinkage crack development
- desiccation of surface concrete
Batch
90 Min.
Placement
2 – 8 Hours 28 + Days
Design SetFinal
SetInitial
Set
45 Min.
Curing
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Consequences of High w-c Ratio
Decrease in Strength
– Concrete is usually mixed
with more water than
required for the hydration
reactions … improves
workability
– Excess water remains in
microstructure pore space
– Pores weaken the concrete
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Consequences of High w-c RatioDecrease in Durability
• Shrinkage occurs as water not consumed by
hydration leaves the system
• The higher the additional water, the higher
the shrinkage potential
• Saw-cut contraction joints absorb normal
shrinkage
• Excess water can result in cracking beyond the
joints, reducing durability:
– Internal cracks: weaken the structure
– Surface cracks: decrease freeze-thaw
resistance, allow ingress of chlorides
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Indicators of High w-c Ratio
Slump
• Slump is a measure of consistency
and workability of wet concrete
• Standardized test .. should yield
consistent results for a given mix
design
• Compare as-mixed to as-placed
slump values:
• Excessive slump could be an
indicator of added water
• However, could also be caused
by other factors (eg, overdose
of air-entrainment)
Pre-cure
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Indicators of High w-c Ratio
Unit Weight
• Nominal unit weight (density) can
be calculated from the mix design
• Measured at placement to confirm
proper mix
– A decrease in unit weight
could be an indicator of added
water
© 2011 M+P Labs 26
Pre-cure
Batch
90 Min.
Placement
2 – 8 Hours 28 + Days
Design SetFinal
SetInitial
Set
45 Min.
Curing
Indicators of High w-c Ratio
Effect of added water on unit weight … clues from the batch tickets:
© 2011 M+P Labs 27
Example Batch Calculations: Design vs. Actual
MaterialUnit of
Measure
Specific
Gravity
Mix
Design
Actual
BatchMoisture
Act Water
(gal)
Actual
BatchMoisture
Act Water
(gal)
#2 Stone lbs 2.79 1750 1715 1715
Sand lbs 2.56 1367 1380 3.00% 5.0 1380 3.00% 5.0
Cement lbs 3.12 400 402 402
Fly Ash lbs 2.35 85 80 80
Water lbs 1 32 27 27.0 27 27.0
Air Entrain. oz per 100 wt 2.15 10.4 0.0
Reducer oz per 100 wt 2.00 9.6 0.0
Total Water - Batch: 32.0 Add'l Water (truck/site): 7.0
Total Water - Actual: 39.0
w-cm 0.55 0.55 0.67
Specific Gravity 2.31 2.31 2.23
Unit Weight lbs/ft3 144 144 139
As Mixed As Placed
Pre-cure
Added water reduces unit weight
Indicators of High w-c Ratio
Segregation of Aggregate
• Large amount of excess water
reduces viscosity of cement
paste
• Coarse aggregate segregates,
with larger pieces settling
toward bottom
• Can be exacerbated with
vibratory compaction
Post-cure
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Indicators of High w-c Ratio
Shrinkage Cracks
• Caused when evaporative losses
exceed bleed rate
• Surface cracks or deeper
• Deep cracks evidenced as cracks
in paste only, around the
aggregate
• Correct depth of reinforcement
steel important to keep cracks
tight at surface
Post-cure
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Water : Cement Ratio in
Concrete
Part 3 – Test methods
30
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Measurement of Water in
Hardened Concrete
Where water is or has been in the concrete:
Capillary Porosity Liquid water found within the cement paste as
capillary voids, typically >50µm
Aggregate Porosity Water can be found within the pore structure of the
aggregates used in the concrete
Combined Water For concrete w:c typically between 0.20 and 0.25
(of hydration) often taken as 0.23 in the absence of any additional
information
Lost Water Bleed water that is subsequently evaporated from
the surface of the concrete.
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Measurement of Water in
Hardened Concrete
Complicating factors:
32
Bleed Water Water lost from the surface of the concrete,
by evaporation, during early stage curing.
Damaged Concrete Cracked concrete due to mechanical damage,
frost or chemical attack
Poorly Compacted Concrete Large irregular voids result in inaccurate
measurements of porosity
Carbonated Concrete Carbonation process releases combined water
Cement Content Measurement Inaccurate cement content measurement can
result in incorrect w:c calculations
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Measurement of Water in
Hardened Concrete
Complicating factors (cont’d):
Aggregate Porosity and Combined
Water
The aggregate itself may have a porosity and
or combined (mineral) water in the structure
Special Aggregates Same issues as with aggregate porosity and or
combined water where the values are likely to
be high
Air Entrainment Creates additional voids that are in addition to
the capillary porosity of the concrete
Admixtures Water-proofers and chemical water repellents
can result in poor measurement of the
capillary pore structure
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Measurement of Water in
Hardened Concrete
Methods:
• Capillary Porosity– Chemical analysis to measure free water
– Included in British Standards
• Optical Microscopy
– Impregnate sample with fluorescent epoxy
– Developed in Finland
• Paste Volume by Image Analysis– Estimation based on measured paste volume %
Not covered by ASTM standards
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Capillary Porosity Method
• British Standard: BS 1881-124:1988
• Testing concrete - Part 124 Methods for
analysis of hardened concrete
Clause 7. Original water content
Calculation of :
“Total water : cement ratio” (including
water absorbed into the aggregates)
or
“Free water : cement ratio” (excludes
water absorbed into the aggregates)
35© 2011 M+P Labs
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Capillary Porosity Method
Testing concrete - Part 124 Methods for analysis of hardened concrete
Calculation of :
“Total water : cement ratio” (including water absorbed into the aggregates)
or
“Free water : cement ratio” (excludes water absorbed into the aggregates)
By measuring:
Capillary porosity of the concrete (using 1,1,1-trichlorethane not water)
Combined water of the concrete
Cement content of the concrete
Aggregate porosity (water absorption value or 1,1,1-trichlorethane method)
Aggregate combined water
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Capillary Porosity Method
Aggregate Control Sample
(direct measurements on representative sample of aggregate used)
Measure the capillary porosity of the aggregate (q) as %
Measure the combined water content of the aggregate (Y) as %
Measure the capillary porosity of the concrete (Q) as %
Measure the combined water content of the concrete (X) as %
Determine cement content of concrete (C) as %
Determine the aggregate content of the concrete (F) as a %
Such that:
Wtotal= Q + X – (YF/100) therefore Total w-c ratio = Wtotal/C
Wfree= Q + X – F/100 (q+Y) therefore Free w-c ratio = Wfree/C
37
Calculation of Original w:c Ratio
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Calculation of Original w:c RatioWithout Aggregate Control Sample
(direct measurements on representative sample of aggregate not available)
Measure the capillary porosity of the aggregate (q) as % or assume equal to
water absorption value. (If not available only Total w:c ration can be
calculated)
Measure the capillary porosity of the concrete (Q) as %
Determine cement content of concrete (C) as %
Determine the aggregate content of the concrete (F) as a %
Assuming combined water of hydration in concrete = 0.23 x Cement content
of the concrete
Such that:
Total w-c ratio = Q/C + 0.23
Free w-c ratio = (Q/C – qF/100) + 0.23
38
Capillary Porosity Method
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Optical Microscopy Method
NORDTEST Method NT Build 361
November 1999
Impregnation of concrete sample
using fluorescent epoxy under
vacuum
Or
Fluorescent Liquid Replacement
(FLR) - not applicable above w-c 0.50
39
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Optical Microscopy Method
From Concrete Petrography
(Concrete Society 2010)
w:c = 0.35 w:c = 0.50
w:c = 0.60 w:c = 0.70
February 2011 40
Optical Microscopy Method
From Concrete Petrography
(Concrete Society 2010)February 2011 41
Optical Microscopy Method
4mm 4mm
From Concrete Petrography
(Concrete Society 2010)
Estimated w-c = > 0.70 Known w-c = 0.55
February 2011 42
From: Concrete Petrography, St.
John D.A. et al. Elsevier, 2005February 2011 43
Paste Volume by Image Analysis
• Measured paste volume
% in hardened concrete
sample
– Relative area by image
analysis techniques
– Point-count method
• Weight of cement per
unit volume
– Based on batch ticket
Inputs:By Estimating Nomograph:
DC
CVD
C
W
⋅
−
=
W/C Estimated water-cement ratio
C Weight of cement (in kg) per cubic meter of concrete
CM Weight of cementitious material (in kg) per cubic meter of
concrete
V Volume of paste (%)
- excluding the air voids > 0.15 mm
- including aggregate < 0.15 mm
D Density of cement (kg/m3)
DM Weighted average density of cementitious material (kg/m3)
44
Paste Volume by Image Analysis
From: French, WJ “Concrete
Petrography, Quarterly Journal of
Engineering Geology, 24, 17-48, 1991
By calculation:
DMCM
CMVD
CM
W
⋅
−
=
Pure OPC Binder Blended w/ Cementitious Material
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Water : Cement Ratio in
Concrete
Part 4 – Other issues
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46
Other Forensic Methods for
Hardened Concrete
• Unit Weight and Absorption
• Aggregate Volume and Distribution
• Air Void Content and Distribution
• Cement Content
• Presence of “Cement Replacement” Materials: flyash, slag, microsilica
• Placement/Depth of Reinforcement
• Mineralogy Assessment of Aggregate
• ASR Assessment (Alkali-Silica Reaction)
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ASR
47
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Element Mapping of ASR Gel
From Concrete Petrography
(Concrete Society 2010)February 2011 48
HIGH FLY ASH CONTENT
SEM Image & EDS Spectrum
Powder Sample of Poor Concrete
Very High Fly Ash Content
SEM Image & EDS Spectrum
Powder Sample of Good Concrete
Normal Paste © 2011 M+P Labs
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Reactive Recycled Aggregate
50© 2011 M+P Labs
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51
Questions
1) What are the four main components of Portland cement?
C2S, C3S, C3A & C4AF
2) What is the maximum time generally allowed between batching
and placement of concrete?
90 Minutes
3) What is the ideal w-c ratio?
0.25 theorectical 0.40 full hydration
4) As w-c ratio increases, what is the expected impact on strength and
durability of the concrete?
Strength and durability both decrease
5) What are three methods to determine w-c ratio of hardened concrete?
Capillary Porosity, Optical Microscopy and Paste Volume by Image Analysis© 2011 M+P Labs
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