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Implementing an effective thermal control plan in mass concrete projects

Implementing an effective thermal control plan in mass concrete projects

Mass concrete elements, characterised by their large volume and considerable thickness, present unique challenges, particularly in managing peak temperature and temperature differentials throughout the curing process.

Uncontrolled heat generation during hydration can lead to delayed ettringite formation (DEF), thermal cracking, reduced durability and compromised structural integrity. In order to mitigate these risks, the implementation of a comprehensive thermal control plan is critical.

With a highly experienced team spread across Australia, BCRC utilises advanced modelling expertise to tackle the key challenges commonly faced in developing an effective thermal control plan aimed at minimising the risk of thermal cracking and DEF.

Peak temperature and temperature differentials in mass concrete elements are primarily caused by the heat of hydration. The internal heat generated cannot dissipate quickly enough, resulting in a rise in temperature. If left unchecked, this heat can induce the risk of delayed ettringite formation and thermal stresses, leading to cracking and other structural issues.

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Thermal cracking in a 1.5-metre-deep foundation slab. (Image: BCRC)
Thermal cracking in a 1.5-metre-deep foundation slab. (Image: BCRC)

BCRC principal engineer Dr Herman Jong explained the seriousness of thermal cracking in the construction industry.

“It can severely compromise the structural integrity of concrete structures,” said Dr Jong. “Government agencies such as state road authorities specify strict controls over concrete temperature while curing.”

“Cracks provide a pathway for aggressive substances that can potentially cause reinforcement corrosion and a reduction in the structure’s longevity. It can also lead to low in-situ strengths.”

When it comes to thermal control planning, this process should commence during the durability design phase and continue throughout construction. This is where the involvement of durability experts during the planning phase proves vital, specifically in the assessment of potential risks and development of tailored thermal control strategies to mitigate them.

By adopting a collaborative approach involving design engineers, contractors and material suppliers, durability experts can provide valuable insights into material selection, curing methods and structural design considerations that focus on minimising the risk of thermally induced cracking and DEF.

BCRC’s approach to thermal control planning involves the following core components:

Risk assessment – Conducting a thorough risk assessment to identify potential elements and evaluate the susceptibility of these elements to thermal cracking.

Material selection – Selecting an appropriate combination of cementitious materials. This includes incorporating supplementary cementitious materials like fly ash and slag to minimise heat generation. The actual heat generated by concrete is generally measured in a hot box test and assessed as heat flux for input into the thermal modelling.

Heat of hydration analysis – BCRC utilises modern methods, such as finite element analysis, to simulate the heat of hydration within the concrete structure. This analysis helps to accurately predict peak temperature and temperature differentials in elements with complex shapes including Tee-Roff end blocks. Traditional 1D analysis often fails to provide accurate predictions in such cases, leading to potential non-conformance.

BCRC technical director Dr Inam Khan, who has collaborated with leading designers on this innovative approach to thermal analysis, highlighted the significant advancements made possible by more sophisticated software in this field.

“In relation to the design of cooling systems, there are situations where it’s not possible to control heat of hydration by the appropriate binder type and content,” said Dr Khan. “If this occurs, BCRC provides design of internal cooling systems, such as embedded pipes, that facilitate the circulation of chilled water through the concrete mass, dissipating heat more effectively.”

“We have successfully designed cooling pipes for a number of large-scale infrastructure projects across the country.”

Dr Khan added that the use of appropriate thermal insulation over a certain period helps minimise the temperature differentials and hence the risk of cracking.

“We are also capable of developing a construction sequence that optimises the pour sequencing of concrete to minimise the risk of cracking,” he said.

An example of heat of hydration analysis using finite element analysis. (Image: BCRC)
An example of heat of hydration analysis using finite element analysis. (Image: BCRC)

The specialist field of durability is one where BCRC truly delivers. The company brings a niche level of technical knowledge of local cementitious materials and their behaviour in heat of hydration to every project. This ensures that thermal control plans are not only effective in managing temperature differentials, but also address long-term durability concerns.

That’s why various factors such as aggregate selection, cementitious materials and environmental conditions, must be carefully considered by a durability expert to optimise the performance of mass concrete structures over their service life.

Frank Papworth, founding director of BCRC, emphasised the critical importance of implementing a robust thermal control plan for the successful construction of most structures.

“By carefully managing temperature differentials and controlling heat generation during hydration, engineers can minimise the risk of cracking, enhance durability and ensure the long-term performance of concrete elements,” said Papworth.

“From early planning and broad stakeholder collaboration to construction and adherence to best practices, the challenges associated with thermal and crack control can be effectively addressed, paving the way for safer, more durable and sustainable infrastructure.”

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