What is Concrete shrinkage?
Concrete shrinkage is the decrease in either length or volume resulting from changes in moisture content that occur as concrete dries and hardens.
When restraint is present to stop the shrinkage process tensile stresses develop than can lead to cracking and deformation, affecting the durability and integrity of concrete structures.
What causes Concrete shrinkage in concrete structures?
There are two primary causes of concrete shrinkage: Plastic shrinkage and Drying shrinkage.
Plastic shrinkage is caused by the loss of water by evaporation from the surface of newly laid concrete or by suction of moisture from underneath. At the surface, plastic shrinkage occurs when the rate of evaporation exceeds the rate of bleeding. High tensile stresses are developed as the surface layer of concrete tries to shrink but is restrained by concrete beneath which is not shrinking and cracks form.
As the concrete has very low strength at this stage, the pattern of plastic shrinkage cracks is usually random but may be influenced by the direction in which finishing operations have been carried out. The cracks tend to be 1-2 mm wide, 300-500 mm long and 20-50 mm deep, though, in some circumstances, they may extend through the full depth of a member.
Drying shrinkage is the decrease in volume or contraction of hardened concrete due to the loss of capillary water and the development of capillary tension. As the concrete is restrained internally by the fact its aggregate will not shrink with the concrete and externally by structural connections, foundations and embedded reinforcements cracking occurs. Unlike plastic shrinkage this happens anywhere from several months to many years after the concrete has been cast and cracks are more uniform and evenly distributed. Drying shrinkage cracks may not be as immediately noticeable as those from plastic shrinkage, but over time, they can significantly affect the durability and aesthetic appearance of the structure.
What are the signs of Concrete shrinkage in concrete structures?
How can I identify Concrete shrinkage in concrete structures?
Visual survey | Non-destructive testing | Destructive testing |
Identification of cracks distribution | - |
How can I prevent Concrete shrinkage in concrete structures?
Preventing concrete shrinkage is a multi-faceted approach that seeks to address the various factors contributing to this phenomenon. It involves a combination of measures designed to reduce water loss, optimize the concrete mix, and mitigate environmental effects.
One of the primary strategies is to control the water to cement ratio, ensuring it's low enough to reduce excess water that could evaporate, yet sufficient for the hydration process. A lower water to cement ratio creates a stronger and more durable concrete, albeit one with a potential increase in shrinkage. The balance here is crucial, as both the quantity of cement and the water to cement ratio influence the volume of hydrated cement paste, which is central to drying shrinkage.
Improving curing conditions is another critical step, which can be achieved through methods such as water curing or the application of curing compounds. These techniques help maintain adequate moisture levels within the concrete, thus limiting shrinkage at early ages. Additionally, the use of shrinkage-reducing and shrinkage-compensating admixtures, along with supplementary cementitious materials like GGBS, Fly Ash or Silica Fume, can further control shrinkage by modifying the concrete mix properties.
Environmental control measures, including the use of windbreaks, shading, or misting, can reduce the rate of moisture evaporation from the concrete surface, especially under extreme weather conditions. These measures are particularly important in the early stages of concrete setting when it is most susceptible to plastic shrinkage. Controlling humidity, wind speed and temperature is vital to reduce surface evaporation.
Concrete shrinkage, both in its plastic and drying stages, poses significant challenges to the durability and structural integrity of concrete. Even small cracks formed when the concrete is fresh can accelerate deterioration, as these can provide an entry point for substances like carbonate or chloride ions to penetrate and degrade the concrete.
How can I repair the damage from Concrete shrinkage on concrete structures?
If cracks are detected, it is important check that they do not progress with time, significantly increase the corrosion risk or penetrate through the structure element with some cases requiring crack monitoring.
In aggressive exposure conditions concrete coatings could be applied to mitigate further deterioration processes, namely, corrosion.