multiscale durability aspects of concrete...1 materials science &technology multiscale...

1

Materials Science & Technology

Multiscale durability aspects of concrete structures exposed to ground waterDr. Michael Romer, Empa

The COE Workshop on Material Science in 21st Century for the Construction Industry -Durability, Repair and Recycling of Concrete Structures

Thursday 11 August, 2005Hokkaido University Conference Hall, Sapporo, Japan Empa, M. Romer, August 2005 Material Science in 21st Century for the Construction Industry

Content

introductionmulti-scale properties of concrete

covercretespatial variabilityporosity

chemical interactionsynthesisconclusions

Multiscale durability aspects of concrete structures exposed to ground water

Empa, M. Romer, August 2005 Material Science in 21st Century for the Construction Industry

Introduction

increasing underground activities: more (road) and longer (rail) tunnels

increasing service life requested (100 years guarantee without major repairs)


Covercrete

concrete layer exposed to the environmentprotection for the reinforcementcritical for the service life of concrete structures

humidity, carbonation, chloride, sulfate, temperature & frost

Re-bars


Covercrete

quality affected bysegregation, bleedingcompactioncuring(microcracking)

how to specify and test covercrete performance ?permeabilityNDT-methods

Re-bars

quality ?


Covercrete characterization (NDT, on-site)

Torrent Permeability Testervacuum pi (leading to controlled air flow)pressure increase in the cell -> permeability coefficient kT in m2

spatial interference with aggregates ?effects of humidity and temperature ?

pi pe

L

Ø 50 mm

Dr. Michael Romer, Empa, Swiss Federal Laboratories for Materials Testing and Research, Duebendorf, Switzerland

www.empa.ch/abt135

2


Size effect for Torrent Permeability Tester

Hypothesis: large aggregates potentially reduce kT because of hindrance of gas flow=> effect reduced with larger cell

Fact: large cell shows slightly higher values and lower standard deviations (but not significant)

0.01

0.1

1

10

0.01 0.1 1 10

conventional cell

larg

e ce

ll

values: kT in 10-16 m2 (one year old concrete)Empa, M. Romer, August 2005 Material Science in 21st Century for the Construction Industry

avaerage of two measurements each

0%

20%

40%

60%

80%

100%

120%

140%

M3(0.40)

M2(0.48)

M5(0.60)

rela

tive

perm

eabi

lity

kT 10°C/35

5°C/35

10°C/70

5°C/70

10°C/90

5°C/90

Effects of humidity and temperature on kT

1 year old samplessix concrete qualitiesstored at 35, 70 and 90% RH

Resultsartificially high kT for 90%RHkT reduced by 50% to 75% if tested at 10°C respectively 5°C

0.1

1

10

30 40 50 60 70

fcm [MPa]

kT [¨

10-1

6m

2 ]

M5

M6

M4

M2

M3

M1

70%RH90%RH

35%RH

Romer M., Materials and Structures 38 (2005) 541-547


Performance of South African durability index teststesting of small (25mm thin) cores is possibleair permeability (OPI) is affected by increased amounts of air voids (chloride conductivity not)adverse curing effects may be detected with the surface samples

1 E-11

1 E-10

1 E-09

30 40 50 60 70fcm [MPa]

k (O

PI)

surface sample

1 E-11

1 E-10

1 E-09

30 40 50 60 70fcm [MPa]

k (O

PI)

M5

M6

M4

M2

M3

M1

interior sample

0.3

0.5

0.7

0.9

1.1

1.3

1.5

30 40 50 60 70fcm [MPa]

Cl-c

ondu

ctiv

ity [m

S/cm

]

surface sample

0.3

0.5

0.7

0.9

1.1

1.3

1.5

30 40 50 60 70fcm [MPa]

Cl-c

ondu

ctiv

ity [m

S/cm

]

M5M6M4M2M3M1

interior sample70%

35%

90%

Romer M. et al., to be published (2005), see www.empa.ch/abt135


Spatial Variability

Hypothesis: concrete quality in larger (vertical) building elements is locally variable due to:

batch production and various transport conditionslocally variable placement and compaction conditions (height of fall, distance to poker, duration, …)effects of segregation / enrichment

Tasks:collect data on real structuresimprove statistical model (service life)improve production process


Spatial Variability

Resultssignificant spatial variation of concrete qualitySCC not generally superior (effects of segregation instead of variable compaction intensity)

60-7050-6040-5030-40

120 60 0 60 120A

B

C

E

D

A

B

C

E

D2400-24502350-24002300-23502250-23002200-22502150-2200

wall 3

2550-26002500-25502450-25002400-24502350-24002300-23502250-23002200-22502150-2200

80-9070-8060-7050-60

B

C

D

E

F

wall 2B

C

D

E

Fself compaction concrete (SCC), 5.0 x 2.0 m

conventionally compacted concrete, 4.0 x 2.5 m

Leemann, A. & Hoffmann C. published (2003), see www.empa.ch/abt135


Pore structure – Permeability

fundamental propertiescomplex relation of porosity and permeability (connectivity, tortuosity)development of a new, quantitative method:FIB-nanotomography

Serial sectioning using dual beam FIB (focused ion beam). The imaging plane (x-y) is parallel to the ion beam (y). SE-images are acquired under an angle of 52° using the SEM-column. Serial sectioning proceeds in z-direction.

Holzer L. et al., J. Microscopy 216 (2004) 84-95


www.empa.ch/abt135

3


Pore structure – Permeability

Resultshigh resolution 3D-reconstruction of the real microstructure

0

20

40

60

80

100

10 100 1000Pore diameter [nm]

sum

[vol

%] FIB-nt / 3D-

analysis

Mercury intrusion porosimetry


numerical calculation of pore connectivity and derivation on transport properties


1 µm

FIB-nanotomography for suspensions and colloids

Shape and positions of particlescement particles in epoxy resin (right)reference particles (SiO2, Ø 680 nm ) with very dense packing (below)

Particle analysis (cement powder) using FIB-nanotomography. Precise 3D-information (Voxel resolution: 30x38x30 nm) can be extracted for each individual particle. In this example the analyzed volume (21x17x4 µm) contains more than 2000 particles.4x4x3 µm

Voxel-resolution: 12 nm



Interfacial Transition Zone (ITZ)

Methodoriented polished sectionsmicrographs around aggregatesquantification of numerical input in shells arounda aggregate (continuous compositional profiles)

0

10

20

30

40

50

60

70

0 50 100 150distance [µm]

poro

us p

aste

[%]

upper ITZ CVC 1lateral ITZ CVC 1lower ITZ CVC 1

0

10

20

30

40

50

60

70


poro

us p

aste

[%]

upper ITZ SCC lateral ITZ SCClower ITZ SCC

SCCconventionally compacted concrete

optical microscopy

numerical input


Interfacial Transition Zone (ITZ)Results

increase of porosity at ITZ less pronounced with SCC (impact of compaction)gravitational effects lead to anisotropic ITZanisotropic permeability properties could result (not jet proofed)

0

10

20

30

40

50

60

70


poro

us p

aste

[%]

upper ITZ CVC 1lateral ITZ CVC 1lower ITZ CVC 1

0

10

20

30

40

50

60

70


poro

us p

aste

[%]

upper ITZ SCC lateral ITZ SCClower ITZ SCC

Leemann A. et al., published (2005)

www.empa.ch/abt135

SCCconventionally compacted concrete


Chemical interaction

existing underground constructions (tunnels) often in contact with ground waterground water often mineralized (carbonate, sulfate, chloride)drainage of ground water restricted -> construction needs to be water tightprotection from ground water difficult (long term risk for water contact and penetration)


Massive concrete deterioration due to Sulfate interaction

[mg/litre]calcium 270

magnesium 270bicarbonate 200

sulfate 1900


www.empa.ch/abt135

4


[mg/litre]sodium 12

potassium 6magnesium 20

calcium 180sulfate 330

bicarbonate 50

Shotcrete tunnel lining


Chemical interactionGeneral finding:

chemical attack is triggered by permeable inhomogeneities (pathways)complex combinations of leaching and (re)precipitationThaumasite not Ettringite (long time interaction)@ high and low sulfate-concentrations (5 – 20 °C)also with sulfate resisting cements (C3A < 3%)

Romer M. et al., published (2002 & 2003) see www.empa.ch/abt135

>>> discrepancy between laboratory and “field” performance>>> ongoing research to clarify the mechanisms

Long time durability: the distribution of concrete quality might be more important then the average concrete performance (practical aspects)


Chemical interaction

Interaction with concrete is affecting the ground water:

increased amount of calciumreduction in pH

Contact with air:precipitation of calcitereduction of cross sectionblocking of drainage system(increased water pressure …)


Synthesis

hours

minutes

days

weeks

months

years

decades

nmnanometer

µmmicrometer

mmmillimeter

mmeter

time

distance

hydration curing

interaction

placement, compaction

mixing, transportITZ segregation

shrinkage cracks

pore

stru

ctur

e

gelpaste

aggregatescover-crete

building

batch

time

physico-chemical processes -> small scalepathways for interaction -> larger scales

processes


Conclusions

Design or prediction of the service life of underground constructions remains a challenge also for the 21th centuryNew reliable (and quantitative) 3D-information on the pore structure availableHigh priority: Linking microstructural features with macroscopic properties (transport, shrinkage)But: Transfer of scientific findings (laboratory) into practice is complicated by many discontinuities Not forget: technical aspects (transport, placement, compaction, curing)

Empa, M. Romer, August 2005 Material Science in 21st Century for the Construction IndustryMaterial Science in 21st Century for the Construction Industry

Thank you for your attention …

Materials Science &Technology


www.empa.ch/abt135

multiscale durability aspects of concrete...1 materials science &technology multiscale...

Documents