Forensic Investigation of Hardened Concrete: Water · PDF fileForensic Investigation of...

© 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

© 2011 M+P Labs 10

Batch

90 Min.

Placement


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


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


Design SetFinal

SetInitial

Set

45 Min.

Curing

© 2011 M+P Labs


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

© 2011 M+P Labs


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

© 2011 M+P Labs


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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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


Design SetFinal

SetInitial

Set

45 Min.

Curing


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


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

© 2011 M+P Labs



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

© 2011 M+P Labs



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.

© 2011 M+P Labs



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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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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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)

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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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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


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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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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



(Concrete Society 2010)February 2011 41


4mm 4mm


(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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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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Element Mapping of ASR Gel


(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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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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