Q.: We have a number of tilt-up panels, 8 inches thick, lying in place on a 6-inch floor slab. We can't get them free of the base slab so that we can start tilting. It seems that someone forgot to put bond breaker on the foundation slab. We were unsuccessful in wedging the slabs free. What else can be done?
A.: Wedging, which you tried, is the usual remedy, and it works well on captive panels that are not too strongly bonded. Steel wedges are driven in at the top edge of the panel and at insert lines while attempting to lift the panel and slowly peel it off. The initial pull should be more than the lift weight but not enough to jerk the panel up. It is good practice to pause briefly after the initial pull on the panel. A minute or so is sometimes needed before the panel can overcome bond with the casting surface.
Small panels that stick have been successfully lifted by jacking parallel to the floor slab. This method rarely works with large panels because it is difficult to find an object large enough to remain stable at the base of the jack.
A third possibility that has worked on occasion with both large and small panels is freezing with dry ice. Fist-size pieces of dry ice are spread uniformly over the panel at the rate of about 1/3 pound per square foot of panel. A 6-mil layer of polyethylene film is then placed over the dry ice, forming a tent. Contraction of the concrete and the reinforcing steel sets up a shear plane between the casting slab and the panel. The time required varies, depending on the ambient temperature, the amount of dry ice, and the severity of the sticking problem.
If there is a crane on the job overnight, the panel may be stacked with dry ice, with crane lines hooked to the panel and a slight amount of tension applied. Reportedly, the panel is usually free by the following day.
We queried Peter Courtois of Dayton Superior Corp., a past-president of the Tilt-Up Concrete Association. He confirmed wedging, jacking, and freezing as the available alternatives for releasing problem panels. However, he expressed the view that on some large jobs the economical alternative might be simply to fill in any openings in the tilt-up panels and raise the grade of the floor slab.
Considering the horror of a tilt-up panel not separating from the floor slab, it is rare that the bond breaker is actually forgotten. However, in hot, windy weather some added precautions may be required. If the base slab surface begins to dry before the bond breaker-curing agent is put down, the material can be drawn into the concrete pores instead of remaining on top. Under such conditions it is a good idea to use a fog-mist spray on the floor slab to fill the concrete pores before the curing and bond breaking agent is applied. It also could help to apply a second coat of bond breaker at right angles to the first, before any reinforcing steel for the tilt-up panel is in place.