Windshield

The outer RCC shell which encloses the flue ducts is called windshield. This is the main supporting structure for flue ducts and it resists air pressure. Slip form construction method is used for the construction of large height windshields.

Slipform Chimney Construction "Slipform" assembly : it is an extremely complicated form. A lot of controlled functions are especially designed to adjust this form's diameter, taper and the poured shell thickness, Below is a summary of some key construction features:

1. The form rises non-stop (continuously) at a rate of approx 1 foot/hour

2. Each Jack is mounted to a yoke, to which is secured the inside and outside form sheets and the working platforms.

3. Each Jackrod is removed upon completion of the shell. To protect from water entrainment after the chimney is completed, the jack rod voids are capped top and bottom with grout. 

4. Radiused walers extend between the yoke frames on both the outside and the inside surfaces of the concrete wall, and additional form sheets (which lap the sheets fixed to the yokes) will be attached to these walers. .

5. The diameter of the slipform assembly is controlled by a radial adjustment fitted to each yoke frame, and the taper of the column or slope of the wall is set by an adjustment at the top of each yoke frame. 

6. The thickness of the concrete wall will be controlled by adjustments that are secured to the inside form sheets and the yoke frames.

7. The construction of the shaft, that is the slipping of the form, is usually carried out continuously on a 24-hour day, 5-day week basis, with a temporary shutdown each weekend.

8. Cast-in inserts are not installed due to great difficulty experienced in the industry when a slipform is used. The practice is to anchor all interior and exterior securements (For balconies, fixtures, and accessory items) using Bulldog (or equal) self-drilling expansion shields.

9. It is the usual practice of the slipform construction team to require the customer to provide centerline markings for starting the chimney, such as lines to demarcate the orientation of chimney centerpoint, flue, access,or other openings, as well as foundation slab elevation data.

10. Hoisting the work personnel to the top platform of the slipform is based on the utilization of a "man-cage" rigged with cable grabbers and two guide cables, so that in the event of a hoist line cable breakage, the grabbers would actuate and suspend the cage from the guide cables. This man-cage is typically operated by a safety hoist, equipped with limit switches and operated under power in both directions.  The customer supplies the hoist, if there is no acceptance of this lifting system.