Why slabs curl

But curling can also occur at later stages of the slab. Join Join TheConstructor to ask questions, answer questions, write articles, and connect with other people. Have an account? Log in Now. Join for free or log in to continue reading Remember Me!

Don't have account, Join Here. Forgot Password Lost your password? Ask A Question. How to Prevent Concrete Curling? Prevent Drying Shrinkage 2. Prevent Bleeding 3. Reduce cement content 4. Quality Curing of Concrete Slab 5. Thickness of the concrete slab 6. Proper Steel Reinforcement 7. Upward and Downward Curling of the Concrete Slab.

What is Concrete Slab Curling? Email optional. Log in or register to add a comment. Keep in touch. All rights reserved. This website uses cookies to improve your browsing experience. This means the moisture in drying concrete is going to leave essentially only from the top, causing the slab to shrink faster concrete shrinks as it dries on the top than the bottom, and curl upward in the process.

Differential drying can be exacerbated by adverse ambient conditions. In these conditions, when no vapor barrier is present, the slab at least has a chance to compensate for this by permitting some moisture to move downward, which is likely why the researchers saw more cracking in the test slabs using polyethylene sheeting.

For nearly a quarter-century, this was the beginning and end of the conversation. Vapor barriers were a nuisance and a contributor to cracking and slab curl. How can you blame the contractor for avoiding their use? The subsequent tips in this article continue our story of how this myth was dispelled and standards were improved. Just as the committee recommended to contractors in to avoid the use of a vapor barrier, they exacerbated the problem with their recommendation of using a blotter layer.

Initial guidance in the revision called for a three-inch layer of dampened sand or cement-treated sand mixture between the vapor barrier and the slab.

The thinking was the same with each revision: give the slab some room to release moisture downward upon placement to avoid cracking and short-term curling. The unintended consequences of this guidance only led to vapor barriers being viewed as the culprit again: while the blotter layer may have temporarily provided room for the slab to dry downward, where was all that moisture ultimately to go?

On a long enough timeline — with a vapor barrier taking the ground out of play — the only direction was back up through the slab. Blotter layers resulted in widespread floor failures, the very thing the vapor barrier was supposed to protect against:. In other cases, it was determined that tears, punctures or improperly sealed penetrations had provided an avenue for moisture to enter the fill course and travel freely beneath the slab which over time increased the moisture level within the slab.

In , ACI Committee updated its standard, removing blotter layers from the guidance and recommending concrete placement directly on top of the vapor barrier. Nearly two decades later, I still get questions about blotter layers. Most industry experts will tell you not to use them. After removing the blotter layer issue and its negative impacts on flooring failures and long-term curling, in ACI Committee set out to understand the relationship between the many other variables in slab design, placement and warping.

Tarr and James A. Twenty-eight years after the study on concrete cracking, the edition of ACI had undone, corrected or improved nearly every finding in the original study. On the issue of curling and warping, the standard practices for slab design, vapor barrier installation and concrete placement have remained remarkably consistent ever since. One primary characteristic of concrete that affects curling is drying shrinkage.

Anything that increases drying shrinkage of concrete will tend to increase curling. The most common occurrence of curling is when the top surface of the slab dries and shrinks with respect to the bottom. This causes an upward curling of the edges of the slab see figure on right. Curling of a slab soon after placement is most likely related to poor curing and rapid surface drying. In slabs, excessive bleeding due to high water content in the concrete or water sprayed on the surface, or a lack of surface moisture due to poor or inadequate curing, can create increased surface drying shrinkage relative to the bottom of the slab.

Bleeding is accentuated in slabs placed directly on a vapor retarder polyethylene sheeting , or when topping mixtures are placed on concrete slabs. Shrinkage differences from top to bottom in these cases are larger than for slabs on absorptive subgrades. Thin slabs and long joint spacing tend to increase curling. For this reason, thin unbonded toppings need to have a fairly close joint spacing. In industrial floors, close joint spacing may be undesirable because of the increased number of joints and increased joint maintenance problems.

However, this must be balanced against the probability of intermediate random cracks and increased curling at the joints. The other factor that can cause curling is temperature differences between the top and bottom of the slab.