curling nightmares

[2300]

Subject: curling problems in our buildings



Hi,we are having curling problems in two of our buildings.One of the building is a refrigarator,the other one is a printing facility.
We have novocon steel fiber in both floors 30 pounds per cubic meter ,six inches slab,lazer screed,water cured for ten days,60 pounds hardener, ashford formula for sealer,water reducer"super plastifier",plastic vapor barrier.Thanks for taking the time .
In the printing facility we maintain a 45 % humidity level.
The curling started around six months after the finishing.

 

[Ron]

2300...looks like you've covered a lot of bases in your process. Have you looked at mix issues with your concrete mix? Several things can be done in the mix to reduce shrinkage, and thus, curling. Use the largest practicable aggregate and keep the cement content as low as practicable. This will help tremendously.

I suspect you have a reasonably high strength concrete for durability (4000 psi or better) and it was likely placed by pumping. Since you maintain the environment at 45%RH, you are constantly depleting the moisture of the concrete at the surface, while allowing it to remain at depth. This causes curling. The relative humidity inside the concrete is more likely in the 80 percent range. Since you obviously cannot maintain 80 percent RH in your operation, you have to mitigate by reducing the potential for shrinkage.

One way to do this is to provide a barrier coating, but these rarely work well for this purpose. One other way to do this is to cut evenly spaced grooves in the concrete and backfill these grooves with a semi-rigid epoxy. Here's the picture...

Let's say you have a slab that is jointed with resulting sections that are 15 feet long, 12 feet wide. Starting at the transverse joint, sawcut a groove at one foot in and parallel to the joint, then another at 2 feet in and parallel to the joint. The groove should be about 1/2 inch deep. Fill the groove with a semi-rigid epoxy.

Now when your shrinkage occurs, it will pull on the epoxy rather than on the concrete, thus less curling is likely.

 

[emmgjld]

To add to Ron's comments:

ACI 360R-92(97) (Guide for Design of Slabs on Grade), Note Chapter 9, Reducing the effects of Slab Shrinkage and Curling, provides an excellant discussion.

All small particles in the mix need to be minimized. Decreasing sand %, increasing aggregate size, decreasing cement and decreasing the mix water are very important. However, recognizing that many of the chemical additives (such as water reducer, super plasticizer) may actually increase the shrinkage (and curling), even while decreasing the amount of mix water.

I usually recommend that the actual mix be designed for the lowest, reasonable compressive strength and I actually recommend a maximum design strength. The actual field samples may go over the maximum but, the mix is designed (proportioned) at a 'not to exceed' value in order to hold down the cement content. Stronger is not always better.

I recommend that any granular subgrade be only slightly moist prior to placing concrete. Do not overwet, as subgrade saturation must not be allowed to occur. The subgrade must be able to absorb Slab Bottom Bleed Water.

If a vapor barrier is required beneath slabs, I recommend that it be overlain by at least 2 inches of sand to decrease the likelihood of curing problems. In my area, I recommend an alternate method of reducing finishing problems would be to place the vapor barrier beneath approximately 6 inches of a minus 3/4 inch, well graded, gravel fill. This method must be carefully accomplished to minimize excessive puncturing and tearing of the vapor barrier.

I recommend that critical slabs be reinforced with a bar matte or with significant wire fabric. Slab Reinforcing to be placed 2" below finished slab top. The purpose of this reinforcement in the top is to resist the slab deflection (curling).

To avoid surface delamination during and after finishing, No Entrained Air for indoor slabs (very Low Freeze-Thaw Potential).

 

[dik]

Also the use of perforated vapour barrier where possible can sometimes even the moisture content across the slab...

 

[Ron]

Emmgjld's comment about specifying a MAXIMUM strength is an excellent one, often overlooked in concrete specifications. More strength at an early age is rarely the proper solution to good quality concrete (there are exceptions!), but generally we should be more concerned with proper mix proportions and LIMITING the cement content, not indiscriminately adding cement just to moderate the potential for lower strengths at 7 or 28 days.

A design-build client of ours routinely does this and their results are much better. It drives the small ready-mix suppliers nuts (would that be a short trip anyway?), but it is an effective way to control the important properties of concrete that must be controlled for long-term performance, not just short term strength.

 

[JAE]

Great comments!
I remember an older engineer once saying "there's nothing an extra bag of cement in the mix won't fix"...seems not to be totally true.

In slabs that will be left exposed (i.e. in warehouse, industrial settings) we do not use a vapor barrier due to fears of curling and no significant human occupancy where moisture would impact the function of the space.

In finished areas, such as offices, corridors, bathrooms, etc., we use a vapor barrier and usually allow it to be placed directly under the slab. This does affect the slab cure, and allow potential curing, but the slab will eventually be covered and usually loaded very lightly so future cracking doesn't really matter.

Placing vapor barrier under gravel - I always feared would allow numerous punctures of the barrier and thus defeat its purpose. Also would not want to do this in areas where the slab will be placed prior to the roof as rain can drain into the slab and be perched atop the plastic.

Have any of you had luck with VB under gravel?

 

[Ron]

JAE...I have specified the VB to be placed under both gravel and compacted fill, just so it wouldn't be right under the slab. In our corner of the world, this was met with great resistance by both the contractor and the local building officials. It took a bit of "education" to get them to come around. It still isn't done very often.

There are lots of advantages to putting the VB in with a protective layer on top. The most promininent is the control of slab thickness. It it must easier to get a consistent slab thickness on compacted soil than it is with a VB on top. On many occasions I have extracted cores through cracks in concrete to find a fold in the VB had created a "control joint" thus contributing to "random cracking".

One detriment to consider in the placement of the VB below a protective layer is the "bathtub effect". If there is a lateral breach of the VB and particularly if you have poor drainage to begin, the section above the VB and below the slab becomes a reservoir for water. This creates a whole host of problems, particularly with vertical vapor migration, slab sweating, barrier coating adhesion, tile adhesion, etc.