Q: We're going to be placing a 6-inch-thick concrete slab in a cold storage warehouse. The concrete has to be placed over 12 inches of expanded polystyrene insulation board. The insulation comes in 4x8 foot sheets that are 6 inches thick, so there will be two layers. A vapor barrier goes on top of the insulation and we place the concrete on top of that. We have several questions:

  • Will the weight of the concrete compress the insulation board?
  • If the specs permit 3/16 inch deviation from flatness in 10 feet, what's the best way to set our side forms so we can meet the speck? We're thinking of using 2x6-inch lumber attached to 2x8-foot sheets of plywood, then screwing plywood side forms to the 2x6s. We'd weigh down the plywood with concrete blocks or sandbags to keep the forms from moving around on the insulation when the vibrating screed is running.
  • Is there anything else we should be worrying about ?

A.: We'll answer the easy question first. A 6-inch thick slab of concrete will put about 75 pounds per square foot of pressure on the insulation board. Regular blue board insulation has a crushing strength of 3,600 pounds per square foot, so crushing isn't a problem. There are several possible ways to control the floor elevation other than the plywood form system you're considering, but we don't know which will work best. Some other options are using leave-in-place concrete screed rails that are set on concrete pads, or using adjustable pipe-screed supports. Can any of our readers suggest a better way to place a floor slab on rigid insulation board and meet the 3/16 inch in 10 feet tolerance? The only other caution is to be prepared for prolonged bleeding. With a vapor barrier beneath the concrete, excess water has to bleed out. If conditions are such that the surface dries quickly, you could have problems with blistering.

Reader Response:

I have a number of comments on both the July 1989 Problem Clinic question and your publication's answer. First, the question specified the insulation would be expanded polystyrene; however, your answer states "blue board insulation." In the cold storage construction industry, blue board most times equates to one of the DOW chemical extruded polystyrene insulation boards. Expanded polystyrene and extruded polystyrene insulations are two different materials, particularly when it comes to compressive strength.Your answer states a compressive strength of 3,600 pounds per square foot (psf) which equals 25 psi. This would be the compressive strength of DOW SM extruded polystyrene. The compressive strength for the most commonly used expanded polystyrene is in the range of 10 to 14 psi which equals 1,440 to 2,016 psf. It should be noted that the compressive strength values listed are at 10% deformation when tested in accordance with ASTM D 1621.

For a cold storage warehouse, expanded polystyrene is not a good choice because of its low compressive strength. A proper slab design analysis must be performed whatever the type of insulation. But it is a fact that, given the same loading, the slab on expanded polystyrene insulation will be thicker than a slab on extruded polystyrene. The modulus of subgrade reaction for the expanded polystyrene is smaller, thus the slab must be thicker. The idea of using a 2x6 attached to plywood is a good method for constructing slab forms and we have had years of experience with this detail. One trick that we have seen frequently is to tie the forms together with metal strap banding so they don't spread when the concrete is placed. The straps are attached to the plywood on the forms and the strap is run across the top of the insulation. It remains under the slab after placement, but it is inexpensive and it works. A normal cold storage floor, in particular a freezer, usually has a base slab below the insulation and most times this slab encases an underfloor heating grid.

This base slab should be placed to a good tolerance to achieve a good top slab tolerance. Also, the manufacturing tolerance for extruded polystyrene insulation is fairly good and consistent.Thickness tolerances, however, are not as good for expanded polystyrene insulation. Thus if the extruded polystyrene insulation is installed properly, the top of the form system should be good. Ballasting the plywood aids in keeping the form set and, if required, shimming could be done under the plywood.There again compressive strength of insulation comes into play. The expanded polystyrene insulation is more prone to damage if the forms are subjected to highly concentrated forces. Using this form detail, we have had cold storage floors placed with exceptional F numbers.

The question referred to the vapor barrier being on top of the insulation. The proper location for the vapor barrier is on the warm side of the insulation, which in the case of a cold room storage building floor is normally below the floor insulation. The questioner may be referring to what is known as a slip sheet. It is normally a thin sheet of polyethylene that is placed, but not sealed, on top of the insulation. This is used to keep any bleed water from migrating into the joints of the insulation and also to act as a bondbreaker. The bleed water must escape through the surface of the concrete and finishers must be careful not to seal or finish trowel the surface too soon.

Also be careful with the support of reinforcement and the control joint dowel bar baskets that are being used. Chairs for reinforcement support should have plates so the legs don't poke down into the insulation. Baskets should be used with continuous runners bearing on the insulation.This question is of considerable interest to me as St. Onge, Ruff and Associates, Inc., is one of the leading designers of refrigerated facilities. Our company has been in the industry for more than 40 years, and we have been involved in the evolution of the design and construction of these types of buildings. I am in charge of the architectural and structural design and also have been involved as an expert witness in numerous cases dealing with refrigerated type buildings. If I can be of further assistance, please write me.

--Barry E. Foreman, P.E. Manager, Architectural & Structural Engineering Depts. St. Onge, Ruff and Associates, Inc. 617 West Market Street P.O. Box M42 York, Pennsylvania 17405