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Design For Crack Control – Expert Opinion

Chart For Crack Control by BCRC durability consultant

Australian Codes include reinforcement stress limits aimed at providing acceptable crack widths, but this simple approach may not achieve the recommended crack widths given in guidance notes (e.g. CIA Z7/06). Required crack widths are not given in the Australian Codes, except the AS 3735 commentary which provides an inaccurate indication of the crack widths that might be achieved using the prescriptive requirements in AS3735, and this can lead to dispute over the acceptability of cracks.

Design for crack width control in Australia has long followed UK methods in BS 8007. In 2007, CIRIA C660 was released as the UK approach to sections in EN 1992 Eurocode 2 for crack control. BCRC introduced C660 to Australia by running a course in Sydney in 2007, with its author and BCRC consultant, Phil Bamforth as the lead speaker.

In 2018, CIA Z7/06 was released as the Australian method of design for crack control and CIRIA updated its approach in C776 in 2019. Conflicts between these documents, and the extensive experience gained by BCRC’s four experts in design for crack control, led BCRC to have a 3 day workshop in August 2019 with all its staff to review and develop the company approach to all aspects related to crack measurement and control, and heat of hydration. A technical guidance note along with spreadsheets to accurately calculate crack widths is under preparation.

BCRC continues its involvement in developing codes for crack control with Frank Papworth participating in an fib international committee developing a position paper on ‘Cracking and Durability’ with the brief of writing relevant paragraphs for Model Code 2020.

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Investigation and Remediation of Construction Defects

faiz khan investigating construction defects with UPV testing device

A common occurrence on active construction sites is the identification and remediation of defects. Regularly, destructive techniques are undertaken which can lead to costly delays from the testing process and subsequent repair of tested areas. To avoid extensive destructive testing, BCRC employs the use of non-destructive testing (NDT) techniques to understand the extent of defects or to identify their presence in areas of concern. A recent project that BCRC was involved with, examined the extent of cracking and presence of voiding within newly cast concrete walls and columns. A column exhibiting cracking is shown in the image below.

Ultrasonic pulse velocity (UPV) testing was undertaken to identify the presence of defects; as well as to quantify their extent. UPV testing is a method where transient waves are used to assess the in-situ condition of a material. Factors that affect the wave velocity include:

  • Density
  • Modulus of elasticity
  • Material homogeneity
  • Defects within the material (i.e. voiding, cracking)

Defects encountered during the wave travel path result in a lower velocity at the test point. By undertaking a comparative analysis between known sound areas and the test area, areas of concern are identified. To allow for the development of a suitable repair methodology, testing needs to identify the areas that required remediation. To ensure that repairs are effective, retesting of the repaired areas is recommended. This approach allows the asset owner to have confidence in knowing the extent of defective concrete has been identified as well as in the soundness of the repaired works that are undertaken.

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Thermal Modelling of Mass Concrete Elements

thermal modeling of mass concrete elements image by BCRC durability consultant

When constructing mass concrete elements, the long term durability of the concrete can be compromised if consideration is not given to limiting peak temperatures and temperature differentials within the concrete. To understand concrete temperature rise, thermal modelling is undertaken considering factors such as the concrete mix design, curing conditions, restraint factors, concrete geometry, tensile strain capacity, coefficient of thermal expansion and reinforcement spacing.

Thermal modelling is used to characterise the behaviour of the concrete during the curing and hardening phases. Limits are placed on concrete temperature rise and distribution to reduce the risk of cracking; as well as to control delayed ettringite formation (DEF). Crack modelling is undertaken to limit crack widths to within allowable code limits.

BCRC recently completed thermal and crack modelling in accordance with standards (such as CIRIA C660, CIRIA 766 and CIA Z7-06) for a transfer wall of a high rise residential complex.

The construction of the transfer wall was proposed to be undertaken in 6 layers with a variable thickness of the wall of 0.6m – 2.3m and a total wall height of 4m. This unique shape and construction sequence created a complex challenge.

FEA analysis was required as the thermal gradient from the lower sections added heat to the newly cast adjacent sections. This analysis could not have been satisfactorily completed using simple spreadsheets due to the complex nature of the project. Field temperatures were measured onsite during a trial pour and were used in the FEA analysis as heat inputs to allow for an accurate determination of the peak temperatures within the concrete.

From our analysis, it was determined that a pour sequence of 6 layers would result in an unacceptable risk to the concrete and therefore the pour sequence was modified to include 8 layers. The modelling process allowed BCRC to provide limits for the concrete placing temperature of the layers to eliminate the risk of DEF formation and to control thermal cracking to within allowable code limits.

The FEA output is provided below.

Output of Thermal Modelling of Mass Concrete Elements