Whilst Australian codes provide some guidance on durability design, they are often limited, conflicting, and the advice in some cases may not always deliver the desired outcome. This is where construction materials and durability consultants BCRC enters the picture.
For structures like bridges, reservoirs and tanks, or those exposed to harsh environments such as wharves, coastal buildings and sewage works, asset owners may have developed specification clauses to deal with Australian code inadequacies where the requirements are reviewed by independent durability consultants.
Major infrastructure projects can incorporate different durability mechanisms, but many require the experience of multidisciplinary durability experts. With over 30 years’ experience, the BCRC team is in tune with the role durability design plays in extending the structural life of an asset.
Understanding and managing the heat of hydration in mass concrete elements is paramount, given its potential to trigger serious issues such as cracking, delayed ettringite formation (DEF), and long-term durability problems. In the construction industry where the strength and durability of structures are non-negotiable, knowledge of concrete’s complex properties, composition and behaviour under varying conditions, is essential.
BCRC technical director Dr Inam Khan brings a profound level of experience and a next-level skillset to address key challenges typically encountered in large mass concrete pours – from heat of hydration, thermal cracking and DEF, to heat mitigation, crack prevention and material selection, to name a few.
As an expert in durability and modelling, Dr Khan is well aware of the complex processes, their profound implications for the industry, and the requirements for strengthening and extending the life of critical infrastructure.
Heat of hydration
“At the core of the concrete curing process is the heat of hydration, a chemical reaction that occurs when water and cement interact,” said Dr Khan. “As the cement hydrates, it generates heat, contributing to concrete hardening – a process crucial to the overall strength and durability of concrete structures.”
“Excessive heat of hydration can lead to issues like DEF, thermal cracking, premature drying and structural weakness, particularly in large-scale construction projects.”
When the heat generated by hydration is improperly managed, it can lead to thermal cracking. This cracking generally arises when the temperature differential between the interior and exterior of a concrete structure is too significant. As the interior heats up and expands while the exterior cools and contracts, it creates tensile stresses within the structure that can exceed the tensile strength of the concrete, therefore causing thermal cracking.
Dr Khan highlights that thermal cracking is a considerable concern in the construction industry as it can severely compromise the structural integrity of concrete structures.
“This can create significant safety hazards and potential operational shutdowns,” he says. “Additionally, cracks provide a pathway for aggressive substances that can potentially cause reinforcement corrosion and a reduction in the structure’s longevity.”
Delayed ettringite formation
While heat of hydration and thermal cracking are well recognised challenges, another less obvious yet critical issue is DEF, a harmful chemical reaction that occurs in hardened concrete.
Ettringite is a mineral commonly formed during the hydration process and is generally harmless in properly cured concrete. However, when the concrete’s internal temperature exceeds a critical limit – typically considered to be 70°C – 80°C during curing depending on the concrete mix composition – this mineral can dissolve and reprecipitate later as the concrete cools, causing internal pressure and eventual cracking.
DEF is a significant concern in construction due to the risk of long-term structural damage. This form of degradation can lead to a substantial reduction in the load-bearing capacity of concrete structures, with symptoms often not surfacing until several years after construction.
Mitigating heat and preventing cracks
According to Dr Khan, several strategies can be employed to manage the heat of hydration and prevent thermal cracking in concrete structures.
Selection of materials – Low-heat cement, which releases heat at a slower rate during hydration, can be utilised for large concrete structures. Additionally, incorporating pozzolanic materials like fly ash or slag in the mix can reduce the heat of hydration.
Pour sequencing – In large concrete pours, strategic pour sequencing plays a vital role in mitigating restrained cracking issues. Carefully planning the sequence and timing of pours minimises the restraint imposed by previously poured concrete. This reduced restraint can significantly reduce the risk of cracking due to differential thermal contraction between adjacent pours.
Pre-cooling methods – Techniques such as use of ice in the concrete mix, utilising thermal blankets, and night-time concreting when temperatures are lower can help manage thermal differentials during the curing process.
Post-cooling methods – Techniques such as embedded pipe cooling, where cold water is circulated through pipes embedded in the concrete, can be used to dissipate heat from the interior of large concrete structures.
Testing and monitoring – Regular testing for signs of DEF and consistent monitoring of concrete temperatures during curing can assist in early detection and timely interventions.
Addressing these challenges from the design phase is integral to achieving successful project outcomes. Selecting the right concrete materials, intelligent pour sequencing and appropriate construction techniques can pre-emptively curb potential thermal issues.
Advanced thermal modelling techniques, such as finite element modelling, can predict temperature rises, assess the risk of thermal cracking, and support the design of an effective thermal control plan. This proactive approach during the design stage greatly minimises the risk of thermal issues during construction and throughout the structure’s lifetime.
As emphasised by Dr Khan, these design considerations form the bedrock of strategies to tackle the heat of hydration and thermal cracking. They demonstrate the importance of design in preserving the durability of infrastructure within construction and reinforce the notion that prevention is, indeed, better than cure.
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