I have noted that curtain wall panels precast on the site bulge outward only when erected and never inward. Why is this and is there anything that can be done about bulging?
Bulging (or warping) of curtain wall panels, and particularly of the sandwich type, is largely due to differential shrinkage and is difficult to control. To precast a panel in the horizontal position is virtually the same as casting any slab on the ground. The rate of curing and evaporation of moisture from the two faces can never be uniform and a tendency to warp is the natural result. Also since curing and measurable shrinkage of concrete can continue for as long as 3 years, the movement of a concrete panel due to non-uniform exposure of the two faces can be expected long after erection. Because the internal face of panels in a building will be exposed to reasonably constant conditions, simulating uniform curing, and because of the physical restraint imposed by the method of connection, the result is a tendency to bulge outward. Accelerated curing, demolding as soon as possible and storage with equal exposure on both faces until erection can do much to prevent excessive warping. Use of low-slump concrete will also help.