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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
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