In water storage tank design, movement joints are strategically placed alongside steel reinforcement to manage concrete movement caused by temperature variations, hydration cooling, and shrinkage. Two primary design methods are employed to control this movement: closely spaced steel reinforcement to limit crack width by shortening crack spacing, or the introduction of movement joints to absorb a portion of the movement.
Consider a 30-meter-long tank wall, subject to a 35°C seasonal temperature change and a 30°C hydration temperature drop. This could result in approximately 8.8mm of movement. This can be either controlled to 0.3mm by using closely spaced reinforcement or absorbed by movement joints. In practice, movement joints typically address the thermal movement caused by seasonal temperature variations.
For critical water-retaining structures like pumping stations, BS8007 dictates even stricter crack width limits (0.2mm for severe and very severe exposure). This poses challenges for designers, who might resort to heavy reinforcement. However, a more efficient alternative is to utilize movement joints, along with adequate reinforcement, to accommodate contraction movement. This approach is exemplified in Water Supplies Department service reservoirs, which incorporate grids of expansion and contraction joints to manage concrete movement effectively.
Ultimately, the choice between closely spaced reinforcement and movement joints depends on factors like structural requirements, cost considerations, and the desired level of crack control. A combination of both methods often yields the most effective solution for ensuring the long-term integrity and performance of water storage tanks and other water-retaining structures.