By Jay Thomsen, Project Manager
Typically, when someone thinks of curling, they think of the winter sport from the Olympics. (Unless you’re in our industry.) In which case, curling of concrete slabs (or more specifically: how to prevent curling) comes to mind.
Over the past couple of months, the Vogel Bros. team has been preparing for the placement of a 75,000 square foot warehouse slab. To put that in perspective, that’s a little more than 1.5 football fields! In preparation, we reviewed various resources to ensure that our operations would prevent curling. Although most of these factors are fairly well known, it seems that some of them are often forgotten when placing large slabs.
How does concrete curl?
Curling is the by-product of differential shrinkage. This happens when water leaves the top or bottom of a slab more rapidly than it leaves the opposite side. Other factors that affect curling include:
- Vapor retarder in direct contact with the slab – prevents excess mixing water from being drawn into the base
- Concrete with excessive water content
- High concrete compressive strength by increasing Portland cement (and water) content
- Gap graded aggregate portions and/or small maximum aggregate size
- Ambient conditions that cause rapid surface setting
- Excessive concrete temperature at discharge (exceeding 90*F)
How do you reduce curling?
There are several methods that can be used to reduce curling. Not all of the actions can be implemented on every project, but implementing any of them will limit your chances of experiencing curling:
- Design well-graded concrete mixtures
- Include fibers – macro, micro or steel fibers at higher dosage levels
- Incorporate admixtures or fly ash to reduce shrinkage
- Include vapor retarding membranes between the subgrade and the concrete
- Use a frictionless subgrade
- Ensure slab on grade mix is not over designed from a strength perspective. Standard floor strength should be between 3,000 – 3,500psi.
- Use a moisture misting machine to introduce moisture into the air above slab to prevent it from sucking moisture out of the slab.