Restrained shrinkage crack control in locks
Crack control in concrete elements is important in most civil structures, for reasons relating to water tightness and durability. This is especially important for sea locks with all its technical elements below (ground) water level and their exposure to the sea or brackish water. This Blog item discusses the difficulties of shrinkage relatied cracks in concrete.
Shrinkage related cracks in concrete are induced by restrained contraction or a temperature differential within the concrete. It’s a complex phenomenon influenced most importantly by the following factors:
- temperature rise during the setting of the concrete affected by the cement content and type, the element thickness, the casting conditions and the after-treatment of the concrete. During setting, concrete tends to shrink. When this shrinkage is restrained, cracks occur;
- autogenous and drying shrinkage of the concrete due to the cement content and type and by the climate conditions during the lifetime of the element;
- the restraint of movement offered by adjacent elements caused by the casting sequence and the stiffness ratio of the different elements; and
- the ability of the concrete to resist tensile strain.
There are different methods available to calculate the crack-inducing strain and to design the necessary reinforcement to control the crack width, but a lot of discussions are still ongoing on how these methods have to be applied. The most used methods in Belgium are:
- Eurocodes: NBN EN 1992-1-1 and NBN EN 1992-3;
- Betonconstructies onder temperatuur- en krimpvervormingen: theorie en praktijk – K. van Breugel; and
- CIRIA C660.
In most cases, a combination of two or all three of these is used because none of them is complete. To address the entire problem a combination of the different methods has to be used and even these combinations use simplifications which aren’t generally acknowledged in all cases. Enough cause for discussion.
For example, no method clearly describes how the calculated strain due to restrained thermal contraction has to be combined with external forces. More precisely, it’s not described what has to be done when it’s uncertain which of these two effects will induce the first cracks, a situation that is very common in civil structures. It is very conservative to use the largest crack width of these two effects. The Dutch Rijkswaterstaat addresses this partly by capping the crack width but this is not very scientific and not an international acknowledged rule.
Another point of discussion is the exact level of restraint that has to be taken into account. To test this, field tests have to be performed.
There’s a need for a uniform European method that is able to combine shrinkage strains and external forces for all types of restraint. To reach this goal, further research has to be conducted to address these issues. This research will have to include large field testing campaigns, for example, on a new lock project with massive concrete elements, like the Royers lock in Antwerp.
Renovation works at the Van Cauwelaert lock (before and after):
Click here for more information about this project.