Arcadis, formerly Hyder Consulting

• Design and Consultancy firm for natural and built assets

• 27,000 people

• Over 70 countries

David Talbot – Senior Bridge Engineer at Arcadis, based in Sydney

David.Talbot@arcadis.com

Summary

• Overview on early age thermal and shrinkage restraint cracking

• Current provisions in Australian Standards

• CIRIA C660 guide

• Parametric study of CIRIA C660

• Case Study 1 – Abutment curtain wall

• Case Study 2 – Pier blade wall

• Recommended application of CIRIA C660

• Conclusion and take aways

Cracking in general

• Reinforced concrete is designed to crack

• Concerns arise with durability, unpredictable cracking or

serviceability (appearance, function)

• Durability issues due to cracking is still contentious

Overview on early age thermal and shrinkage restraint cracking

• Typically an issue for thicker elements or large changes in section

• Internal restraint

• Differential temperature due to both heating and cooling,

due to reinforcement

• External restraint

• Edge restraint, end restraint, partial restraint

• Delayed Ettringite Formation (DEF)

External surface cooling / contracting

Internal surface heating / expanding

Typical Internal Restraint

Typical External Restraint

Current provisions in Australian Standards

AS5100.5

• Clause 2.8 - Deemed to comply minimum reinforcement 500 mm2/m

• 100 year design life

• Supplementary recommends 0.3mm crack width for most structures

AS3600

• Clause 2.3.3 and relevant sections - Deemed to comply minimum reinforcement depending on element type

• 40-60 year design life

• No guidance but 0.3mm crack width for most structures is considered appropriate

CIRIA C660 Early-age thermal crack control in concrete, P B Bamforth 2007

1. Define the design crack width

2. Estimate the magnitude of restraint

3. Estimate the crack inducing strain

4. Design the reinforcement to control crack spacing and width

CIRIA C660 Early-age thermal crack control in concrete, P B Bamforth 2007

1. Define the allowable design crack width

2. Estimate the magnitude of restraint

3. Estimate the crack inducing strain

4. Design the reinforcement to control crack spacing and width

0.3 mm crack width is recommended for general exposure classification and

reinforced concrete elements

CIRIA C660 Early-age thermal crack control in concrete, P B Bamforth 2007

1. Define the allowable design crack width

2. Estimate the magnitude of restraint

3. Estimate the crack inducing strain

4. Design the reinforcement to control crack spacing and width

CIRIA C660 Early-age thermal crack control in concrete, P B Bamforth 2007

1. Define the allowable design crack width

2. Estimate the magnitude of restraint

3. Estimate the crack inducing strain

4. Design the reinforcement to control crack spacing and width

Need to consider the probability of cracking ratio to determinethe appropriate age at which cracking occurs

CIRIA C660 Early-age thermal crack control in concrete, P B Bamforth 2007

1. Define the allowable design crack width

2. Estimate the magnitude of restraint

3. Estimate the crack inducing strain

4. Design the reinforcement to control crack spacing and width

Based on CIRIA C660

wk, crack width

= Sr,max εcr

Sr,max = 3.4 c + 0.425 k1φ

ρp,eff

εcr = crack inducing

strain

ε cr = K1 { [αc T1 + ε ca]R1 + αc T2 R2 + ε

EXTERNAL RESTRAINT

ε cr = K1 ΔT.αc R - 0.5 ε

INTERNAL RESTRAINT

Parametric study of CIRIA C660

• Peak core temperature

• Differential temperature between the core and the surface

• Peak core temperature and reinforcement quantity on long term crack width

• Minimum reinforcement requirements

Peak Core Temperature

• Higher percentage of fly ash reduces the peak temperature

• Placing temperature has a direct correlation to the peak temperature

• Delayed Ettringite Formation

Differential Temperature

• The required stripping time to control differential temperature is highly variable with member thickness

• Stripping times have a limited benefit

• Higher percentage of fly ash reduces the differential temperature

Peak Temperature and Reinforcement Ratio on long term crack width (taken at 28 days)

• Reinforcement spacing has a greater influence on reducing the crack width than increased bar size

For example:• N16 @ 100 = 2010 mm2 / m• N20 @ 150 = 2093 mm2 / m

Similar area of steel but closer spacing with smaller bars results in a smaller crack width

Minimum Reinforcement Requirement

• The age at which cracking occurs is an important parameter as the minimum reinforcement required is directly related to the tensile strength of the concrete

• The minimum reinforcement requirements increase with relation to concrete thickness for large members, this is different to the Australian Standards where the requirement remains constant for thicknesses above 500mm

Summary of the Parametric Study

Reducing core temperature• Lower quantity of cementitious binder• Increased use of supplementary cementitious material, e.g. fly ash• Lower temperature of concrete placement

These parameters are all related to the design and supply of the concrete mix and are relatively easy to control at the source

Reducing differential temperature• Lower quantity of cementitious binder• Increased use of supplementary cementitious material, e.g. fly ash• Leave formwork in place for longer times

It has been demonstrated that the benefit of leaving the formwork in place is finite and should be specifically considered rather than imposing onerous restrictions on the contractor

Case Studies – Project: Tintenbar to Ewingsdale, Pacific Highway Upgrade

BRISBANE

SYDNEY

Case Study 1 – Abutment curtain wall

Demonstrates potential inadequacies of AS5100 crack control requirements

• B1 Exposure Classification• 45mm cover• 40 MPa concrete

300 mm thick wall cast onto a 1.7 m x 1.5 m concrete headstock

The base of the wall is considered as continuous edge restraint

Case Study 1 – Abutment curtain wall

AS5100 requirements are the greater of:

1. N12 at 200 mm spacing or similar (565mm2) on each

face to satisfy Clause 2.8

2. N16 at 225 mm spacing (900 mm2) on each face to

satisfy Clause 11.6.2

CIRIA C660 was adopted for a crack limit of 0.3 mm with

a determined steel requirement for the horizontal

reinforcement of N16 at 150 mm spacing, approximately

45% higher than AS5100 steel requirements

Case Study 1 – Abutment curtain wall

• Crack mapping was carried out at 28 days

• Typical cracks of 0.2 mm with peaks of 0.3 mm

• AS5100 reinforcement would likely have

resulted cracks exceeding the allowable

0.3 mm limit

• Shows that CIRIA C660 should also be

considered for thinner sections that have high

restraint

Case Study 2 – Pier blade wall

Demonstrates the accuracy of CIRIA C660 predictions

compared to actual data

• 1.5 m thick blade wall

• Element with higher visual considerations

• Horizontal reinforcement concentrated towards the

base where higher restraint exists

Case Study 2 – Pier blade wall

• Thermocouples located in the core of the concrete to continually log the peak temperature

• AS5100 minimum steel requirement 2000 mm2

per m• CIRIA C660 minimum steel requirement

2362 mm2 per m• Actual steel at the base N20 at 90 mm spacing

= 3491 mm2 with a predicted 28 day crack width of 0.12

Case Study 2 – Pier blade wall

• Theoretical peak temperatures and theoretical age to peak temperatures are in good correlation between the predicted and actual data

• The key parameter of the predicted crack width of 0.11 mm to 0.13 mm is in good agreement with 39 out of 40 samples measured crack widths of 0.05 mm to 0.1 mm (one outlier of 0.15 mm)

Pier Location Theoretical peak

temperature

Measured peak

temperature

Pier 1 Northbound 77 degrees 81 degrees

Pier 1 Southbound 77 degrees 81 degrees

Pier 2 Southbound 78 degrees 83 degrees

Pier Location Theoretical age at

peak temperature

Measured age at

peak temperature

Pier 1 Northbound 28 hours 36.23 hours*

Pier 1 Southbound 28 hours 29 hours

Pier 2 Southbound 28 hours 29.15 hours

Pier Location Theoretical 28 day

crack width

Measured 28 day

crack width

Pier 1 Northbound 0.11mm 7 samples at 0.05mm

5 samples at 0.1mm

Pier 1 Southbound 0.12mm 6 samples at 0.05mm

6 samples at 0.1mm

Pier 2 Southbound 0.13mm 8 samples at 0.05mm

7 samples at 0.1mm

1 samples at 0.15mm

Table 1 - Peak core temperature

Table 2 – Time until peak core temperature

Table 3 – 28 day crack width

Conclusion and questions

• AS5100 includes provision for minimum reinforcement to control cracking for restrained elements

• CIRIA C660 provides an alternative, more refined method. Better design control of early cracking to reduce

potential for rectification works.

• Case studies from the Tintenbar to Ewingsdale project have shown good correlation between predicted and actual

temperatures and crack widths

• Parametric study of CIRIA C660 identified the major influencing factors to be total cementitious content,

percentage of supplementary cementitious materials such as fly ash and reducing the placing temperature

• Option to design for cracking criteria to allow departure from deemed to comply requirements, e.g. stripping times

David.Talbot@arcadis.com