The Steel reinforced concrete dome in five phases
The construction of a steel reinforced concrete dome begins by building a circular foundation of reinforced concrete called the ringbeam. The ringbeam is designed to support not only the weight of the completed dome, but also the forces of uplift from inflation of the airform during construction and pressures created by wind passing over the form.
The ringbeam is a steel reinforced continuous circular concrete footing which anchors the dome even through hurricane and tornado force winds, seismic activity, impact from debris, and soil settlement.
Next comes the process of placing and inflating the airform. The airform is both the form for construction of the dome and the outer roof membrane of the shell when it‘s finished. The airform is fabricated from a durable multi-ply material similar to that of commercial roofing membranes. The shape and size of the airform matches that of the completed structure. It’s attached to the ringbeam foundation and inflated using large blower fans. The fans run throughout the construction of the dome and are closely monitored to ensure that the airform remains properly inflated.
Prior to inflation and final attachment of the airform to the ringbeam, the reinforcing steel, construction equipment, and materials are all stockpiled within the airform. This allows construction of the dome to continue regardless of weather conditions, which can result in significant savings in time and construction costs.
Once the airform is inflated, the third step in the process is applying polyurethane foam to the inside of the form. The foam initially provides the required rigidity to support the reinforcing steel of the concrete dome. After construction is completed, the insulating foam works with the thermal mass of the concrete dome to provide exceptional energy efficiency. A dome is capable of sustaining comfortable temperatures year round as air penetration is eliminated and the insulating foam layer reduces extreme outdoor temperature fluctuations on the concrete.
Because temperature ranges inside a dome are less drastic than in a conventional building, the time required to raise or lower temperatures throughout the structure is reduced. This decreases demand on the HVAC systems and provides substantial energy savings.
The fourth phase in the construction process is the setting of the steel reinforcing bars. To expedite the construction process, the reinforcing steel and all the equipment required to place the reinforcing steel is staged inside the perimeter of the ringbeam and the airform prior to inflation. Once the form is inflated and the insulating polyurethane foam is applied, special fasteners are imbedded in the foam and the reinforcing bars are attached. The quantity and diameter of reinforcing bars are engineered to maximize the performance of the dome based on the requirements of the application.
With the reinforcing steel in place, the fifth phase of the process can begin – applying the shotcrete. Shotcrete is concrete that is sprayed on using a machine.
The most crucial part of dome construction is the proper placement of the shotcrete. Because the shotcrete is being applied to a flexible air supported form, it must be applied in progressive layers to prevent movement of the airform. Working in circles, from bottom to top, shotcrete is applied uniformly until the dome acquires its pre-engineered thickness. Once the shotcrete is fully applied and cured, it becomes entirely self-supporting and able to resist incredible loads applied to its exterior or suspended from its interior.