We are casting sandwich wall panels and experiencing cracking over the sheets of insulation. The panels are eight inches thick and approximately 11 feet wide and 35 feet long. Insulation sheets are one inch thick. Some sheets are two by four feet and some two by eight feet. Sixteen of these are laid on a grid pattern with eight to eighteen inches of space from the edge of one sheet to the edge of another. There seems to be no consistency to the cracking. Some panels crack and others do not. Of those that do crack the cracks sometimes appear over all the sheets of insulation and sometimes over only a few. The panels are cast on tilt tables, with curing provided by radiant heat directly below the surface of the table. Curing temperatures vary from 90 degrees F to 120 degrees F. The insulation is closed cell urethane.
The best analysis we have been able to obtain about your problem of cracking of insulated sandwich panels is that the cracking is caused by a panel design that is not responsive to shrinkage. The panel has severe edge restraint. The large panel that you are producing can be thought of as a full-thickness concrete grid into which have been laid two by four-foot and two by eight-foot panels of concrete and insulation. The concrete is subjected to heat and moisture during the curing process. Then when it has become strong and rigid it is subjected to cooling and drying. The rectangular surface areas enclosing the insulation dry slightly more rapidly than the portions in which the concrete goes all the way through the panel because there is less concrete thickness and hence less moisture there. This difference in shrinkage sets up stresses. The thin areas over the insulation can withstand less stress than the rest of the panel, so the drying stresses eventually exceed the concrete strength in this area first and cause cracking over the insulation. The fact that not all rectangular areas crack and that the pattern of the cracking varies from area to area can probably be charged to the fact that concrete is a nonhomogeneous material whose strength is not completely uniform. We believe that the distribution of insulation needs redesigning. Is it not possible to use a single sheet of insulation covering the whole area and to tie the concrete layers together with ties? This should reduce or eliminate your shrinkage cracking troubles and have the additional advantage of providing continuous uniform thermal insulation over the whole area. The present design has the additional disadvantage of providing opportunity for heat leakage between sheets. Some good background on the performance of sandwich wall panels designed with ties is given in three reports available from the Portland Cement Association, Old Orchard Road, Skokie, Illinois 60076: "Precast Concrete Wall Panels: Flexural Stiffness of Sandwich Panels," by D. W. Pfeifer and J. A. Hanson, Bulletin D99. "Cast-in-Place Concrete Residences with Insulated Walls," by Harry L. Scoggin, Bulletin D104. "Cast-in-Place Concrete Residences with Insulated Walls Influence of Shear Connectors on Flexural Resistance," by Harry L. Scoggin and Donald W. Pfeifer, Bulletin D118.