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Effective crack control in water-retaining structures

Effective crack control in water-retaining structures

Water-retaining structures, like wastewater treatment plants, require stringent crack control measures to maintain structural integrity and functionality. Concrete experiences volume changes due to early-age thermal contractions, autogenous shrinkage and drying shrinkage. When these volume changes are restrained, shrinkage-induced stresses develop, leading to tensile stress. If these stresses exceed the concrete’s tensile strength, cracking can occur. Excessive concrete cracking is one of the most common forms of damage in reinforced concrete structures.

Controlling crack widths in concrete is often achieved by adding steel reinforcement, which distributes strains along the member length, producing many narrow and closely spaced cracks. Inadequate reinforcement, however, leads to fewer but wider cracks. Cracking can cause significant issues, including leakage, reduced structural performance and accelerated deterioration due to the ingress of harmful substances.

Mitigating these risks requires a comprehensive approach to crack control, one that involves careful planning of the pouring sequence, appropriate reinforcement design and selecting the right concrete mix. These measures help ensure the durability and functionality of structures over their intended lifespan.

Pouring sequence

The construction of wastewater treatment tanks typically involves multiple pours. The sequence in which these pours are executed plays a critical role in controlling cracks. Improper sequencing can result in excessive restrained stresses between different sections of the tank, leading to cracking.

Dr Inam Khan, technical director at BCRC, emphasised the significance of planning the pouring sequence in minimising the risk of end restraint cracking.

“The illustration below is an example of pour sequencing in a wastewater treatment plant foundation where two different approaches are shown,” said Dr Khan. “The approach on the left is less susceptible to cracking compared to the approach on the right due to the lesser degree of restraint from existing pours.”

Different pouring sequence arrangement. (Image: BCRC)
Different pouring sequence arrangement. (Image: BCRC)

Design reinforcement

In water-retaining structures, reinforcement is essential in controlling cracks as it helps distribute tensile stresses from shrinkage and thermal contractions, thereby preventing wide cracks.

“Proper reinforcement distributes strains along the member length, resulting in multiple narrow, closely spaced cracks rather than a few wide ones,” said Dr Khan.

“Determining the amount of reinforcement needed to control crack widths in concrete remains challenging, despite the many prediction formulas developed.”

Concrete mix selection

The right concrete mix also plays a critical role in crack control within water-retaining structures. A well-designed concrete mix can significantly reduce the risk of cracking by minimising shrinkage and thermal strains, while enhancing overall performance. High-performance concrete mixes with low water-cement ratios, and the inclusion of shrinkage-reducing admixtures, can help achieve the desired durability.

“Supplementary cementitious materials like fly ash or slag help reduce the heat of hydration and, in turn, the thermal strains,” said Dr Khan. “This only further highlights the importance of material selection in achieving long-term durability and crack control.”

BCRC’s approach to crack control

BCRC employs a comprehensive approach to crack control by leveraging its advanced modelling and analysis expertise to address the unique challenges of water-retaining structures.

Risk assessment

BCRC conducts thorough risk assessments to identify potential crack formation areas based on the restraints and evaluates the susceptibility of these elements to thermal and shrinkage-induced cracking.

Material selection

Choosing appropriate cementitious materials is critical. BCRC promotes the use of supplementary materials like fly ash and slag to reduce heat generation during hydration, thereby reducing restrained thermal strains and the risk of cracking.

Advanced crack modelling techniques

BCRC uses state-of-the-art modelling techniques to accurately predict and control crack formation. Utilising advanced finite element analysis (FEA), BCRC can simulate the complex interactions within concrete structures, allowing for precise predictions of thermal strains, stress distributions and potential crack locations.

Effective crack control in water-retaining structures
Example of an FEA model for a water tank foundation. (Image: BCRC)
Reinforcement optimisation

BCRC calculates the required reinforcement in each element to ensure effective crack control. Its extensive experience in designing reinforcement that effectively controls crack widths ensures they remain within acceptable limits for water tightness and durability. Carefully selecting the type, placement and amount of reinforcement is essential to maintain the structural integrity of these structures.

Controlling cracks in water-retaining structures like wastewater treatment plants involves a comprehensive understanding of the types of cracking, their causes and appropriate preventative measures. By considering factors such as the pouring sequence, degree of restraint, design reinforcement and concrete mix selection, it is possible to minimise the risk of cracking and ensure the structural integrity of these critical assets.

BCRC’s expertise in this specialised field has enabled the company to support clients in not only achieving their desired outcomes, but also ensuring their critical structures remain safe, durable and functional over their intended operational life.

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