from the casting surface once it has cured. This is a critical step, as improper chemical selection or application will prevent the lifting of the panels, and will entail costly demolition and rework. A rebar grid is constructed inside the forms, after the form release is applied, spaced off the casting surface the desired distance with plastic "chairs". The rebar size and spacing is generally specified by the engineer of record. Concrete is then poured, filling the desired thickness and surrounding all steel inserts, embedded features and rebar. The concrete is then settled through vibration to prevent any voids or honeycomb effects. The forms are removed when the concrete is cured; rigging is attached and a crane tilts the panel or lifts the element into place. In circumstances when space is at a premium, concrete elements can be cast one on top of the other, or stack cast. Quite often a separate casting pad is poured for this purpose and is removed when the panels are erected. Cranes are used to tilt the concrete elements from the casting slab to a vertical position. The slabs are then most often set onto a foundation and secured with braces until the structural steel and the roof diaphragm is in place. Structure Concrete tilt-up walls can be very heavy, as much as or more. Most tilt-up wall panels are engineered to work with the roof structure and/or floor structures to resist all forces; that is, to function as load-bearing walls. The connections to the roof and floors are usually steel plates with headed studs that were secured into the forms prior to concrete placement. These attachment points are bolted or welded. The upper attachment points are made to the roof trusses. Interior walls may be present for additional stiffness in the building structure as necessary, known as shear walls. Insulation can be applied to either side of the panels or cast as an integral part of the panel between two layers of concrete to create sandwich panels. Concrete has the ability to absorb and store energy and is high mass, which regulates interior temperature (thermal mass) and provides soundproofing and durability. Like all concrete construction, tilt-up buildings are fire-resistant. In addition, wall panels can be designed to sag inward when damaged, which minimizes collapse (this can also be done with prefabricated panels). Uses Tilt-up was first used in America circa 1905. In 1908