How long will your concrete bridge last?

Norbert MichelManager Infrastructure Disciplines

Acknowledge my colleague at ARRB:

Dr Ahmad ShayanChief Scientist Concrete Technology, Materials Sciences

Acknowledgement

Content of presentation

1. Introduction to durability of concrete structures

2. Durability problems affecting concrete structures

3. Examples of two deterioration mechanisms

4. Investigation of the two durability problems

5. Measures against these durability problems

6. Summary and recommendations

3

Concrete durability: Definition

4

• Resistance against deterioration….

Concrete ingredients

5

Fine aggregate (sand)

Coarse aggregate Water

Cement

Chemical Admixtures (Water reducer, Super-plasticiser)

Supplementary Cementitious Materials(e.g., Fly ash, slag, silica fume)

Features of hardened concrete

6

Note different distribution of aggregate- can influence properties of concrete, e.g., strength and drying shrinkage

Factors affecting durability of structures

Structural design

Quality of individual material components

Mix proportion parameters

Workmanship

Curing

Exposure environment

7

Exposure environment is an important factor in durability

8

Marine conditions:

Chloride-induced corrosion

of reinforcing steel

Wetting & drying cycles

Benign conditions

Major durability problems

Shrinkage and thermal cracking Carbonation-induced corrosion of reinforcement Chloride-induced corrosion of reinforcement Alkali-aggregate reaction (AAR) Sulfate attack Salt attack Frost attack (not serious in Australia) Fire

9

A combination of these problems can be present in some structures

Examples of corrosion

Quality and design….

10

Effects of corrosion

11

Loss of cover concrete due to spalling Loss of steel cross section Weakening of cement-steel bond

Result: Reduction in load capacity

12

Field investigation of steel corrosion

Electrochemical properties of steel Half-cell potential mapping Corrosion rate measurement

Resistivity of concrete (ease of current flow)

High resistivity is desirable

Determination of chloride ingress

13

Criteria for Half-Cell Potentials

Potential Indication of corrosion activity

More positive than –0.20 V

A greater than 90% probability that no reinforcing steel corrosion

is occurring

Between –0.20 V and –0.35 V

Corrosion activity of the reinforcing steel is uncertain

More negative than –0.35 V

A greater than 90% probability that reinforcing steel corrosion is

occurring

Field investigation for reinforcement corrosion

14

X1 X2 X3

Y1

Y2

Y3

Y4

Y5

Y6

Y7

Half Cell Potential MappingColumn 4, Pier 2, Lynchs Bridge

-150-50

-350--150

-550--350

-750--550

mV

0.044 µA/cm²-230 mV

0.185 µA/cm²-422 mV

0.315 µA/cm²-605 mV

Icorr / CSE

Measured electrical resistivity to understand corrosion potential…

No steel corrosion is likely

Steel corrosion probable

15

Measurement of corrosion current density

Corrosion rate categoryIcorr (µA/cm2)

< 0.1 No corrosion expected0.1 to 0.5 Low to moderate rate0.5 to 1.0 Moderate to high rate> 1.0 High rate

X1 X2 X3

Y1

Y2

Y3

Y4

Y5

Y6

Y7

Half Cell Potential MappingColumn 4, Pier 2, Lynchs Bridge

-150-50

-350--150

-550--350

-750--550

mV

0.044 µA/cm²-230 mV

0.185 µA/cm²-422 mV

0.315 µA/cm²-605 mV

Icorr / CSE

No corrosion expected

Low to moderate rate expected

Stage 1 – Service life prediction based on chloride ingress profile

16

ID Location Cl,surf (%) D (m²/s)

Lyn2 Pilecap of Pier 2 1.07 2.85E-12Lyn3 Column 5 of Pier 2, waterline 0.70 1.84E-12Lyn4 Column 5 of Pier 2, 1.5 m above base 0.07 2.13E-12

0

0.2

0.4

0.6

0.8

1

1.2

0 20 40 60 80 100

Depth (mm)

Cl- (

% o

f c

on

cre

te)

0.8 kg Cl-/m3

Lyn2

Lyn3

Lyn4

Cover depth of pier column

Threshold for corrosion

17

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60

Time (years)

Dep

th o

f T

hre

sho

ld C

hlo

rid

e (m

m)

Cap 2006 C5 tidal 2006 Cap 2009 C4 tidal 2009

Present exposure age

Reinforcing bar

Figure 3: Advance of the threshold chloride in concrete. Threshold Cl- =0.4% cement mass. Solid lines: data of 2006; dotted lines: data of 2009.

Service life prediction based on Chloride ingress profile

More realistic corrosion model

18

Corrosion Damage Curve

0

20

40

60

80

100

Time (Year)

Dam

ag

e

1st crackCorrosion initiation

Major repair is needed at the end of service life

Mitigation of corrosion damage

19

Existing structures: How to address these in-situ? What is practical? What is efficient and suitable?

New structures in aggressive environment: What are the design considerations that are needed? What would be effective? How much does it cost?

The above needs testing and verification.

Alkali-Aggregate Reaction (AAR)

20

AAR gel is highly hydrated in the presence of water and reacted aggregate develops expansion

Result: Expansion and cracking of concrete

Alkali hydroxide

Silica in aggregate

Water AAR gel+ +

Visual features of concrete interior

21

View of reacted aggregate particles in concrete

Example of AAR in bridge pylon

22

Manifestation of combined AAR and corrosion

23

Appearance of a seriously deteriorated bridge pile affected by AAR and steel corrosion

24

Effects of AAR on concrete

Strength properties

Concrete cracking

Overall effect: Reduced service life

Diagnosis of AAR

Visual observation

Microstructural examinations, including petrographic examination and Scanning Electron Microscopy (SEM) / Energy Dispersive X-ray (EDX)

Residual alkali content

Residual expansion

Residual strength

25

Petrographic thin section

26

SEM/EDX of AAR Products

27

0 2 4 6 8 1 0E n e rg y (k e V )

0

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0C o u n ts

C

O

N a

M gA l

S i

SC l K

C a

C a

0 2 4 6 8 1 0E n e rg y (k e V )

0

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0C o u n ts

C

O

N aM g

A l

S i

SC l

KC a

C a

Repair of AAR-affected concrete

28

38°C, 100%RH

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 50 100 150 200 250 300 350 400

Exposure time (day)

Exp

ansi

on

(%

)

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0 50 100 150 200 250 300 350 400

Exposure (day)

Exp

ansi

on

(%

)

Pier5 Pile18 Pier3 Pile28 Pier6 Pile18

Residual expansion of concrete must be determined

Expansion ongoing = more cracking expected

Minimal expansion

Repair technique would depend on condition of affected concrete, residual expansion and exposure

conditions

Prevention of AAR damage

29

Methodology for preventing AAR damage in new structuresSelect aggregates that are not susceptible to AAR by: testing aggregates by the Accelerated Mortar Bar Test testing aggregates by Concrete Prism Test

0

0.05

0.1

0.15

0.2

0.25

0.3

0 5 10 15 20 25

Time (day)

Exp

ansi

on (%

)

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0 100 200 300 400 500

Exposure time (day)

Exp

ansi

on (%

)

If non-reactive aggregate is not available, modify concrete mix by using SCMs (slag, fly ash, silica fume)

30

So how long will your bridge last?

Corrosion of reinforcement ?

Alkali-aggregate reaction ?

End of life ?

Investigations such as those described would determine the end of service life...

31

Recommendations

To avoid these deterioration problems…. Select non-reactive aggregate Check cement composition (C3A, SO3, alkali) Use supplementary cementitious materials in correct quantity Use appropriate admixtures to reduce water content Verify low permeability of cover concrete

The Reward: You won’t have to face premature deterioration!

Norbert MichelManager Infrastructure DisciplinesARRB Group - Research and Consulting

P: +61 3 9881 1580 M: +61 (0) 412 357 249norbert.michel@arrb.com.au

Thank you