Production of precast elements is done in casting yard. Moulds of adequate stiffness have been used and installed as per issued GFC drawings. The sequence of activities involved in the production of precast elements is as follows:
1. Mould cleaning and preparation
2. Shuttering/assembling the mould components
3. Fixing of rebars / cast-in-fittings
4. Pre-concrete check
8. Final inspection
Stacking of Precast Elements. Lifting and handling of precast elements have been done using gantry cranes and other approved lifting devices. Number and location of lifting points have been decided such that the ‘handling stresses’ were always within allowable limits. For members having unsymmetrical geometry or projecting sections required supplementary lifting point. Precast elements such as precast slabs, beams, spandrels and staircase landing elements were stacked horizontally and supported with strips of wood or batten across the full width of the bearing points. Precast walls, column-beam-wall panels shall be stacked in vertical position supporting their self-weight using racks. Precast elements were transported to the site on a flatbed or low bed trailers. The delivery of precast elements was planned according to the erection sequence to avoid or minimize unnecessary handling or storage at the site. The precast elements are being loaded using gantry crane of 10 to 15 ton of varying capacity and delivered to the site with proper supports, frames and cushioning to prevent transit damages. The elements were delivered in a manner in which they can be lifted directly and erected without much change in the orientation or Sequence. Average 136 numbers of elements were to be transported per day to the site with the help of around 10 numbers of trailers.
Before the erection of precast elements, the following preparatory works shall be carried out to achieve efficient and quality installation.
1. Site accessibility shall be checked for the delivery of precast elements.
2. Check whether the right element is dispatched or not.
3. Visual inspection shall be done to check the concrete finishes and damages if any.
4. Crane shall be checked for its working clearance for hoisting the precast element.
5. The elements shall be stored using “First In First Out” principle according to the erection sequence
During the erection of vertical members, the reference line and offset line was set out to determine the position of the element to be installed. Then shim plates or level pads were provided for setting the level of the elements. For external wall or column elements, a compressible form of the backer rod was fixed on the outer perimeter of the wall. The element was placed in the designated location and secured with diagonal props (push-pull jacks) and then check for the alignment. After installing the element, grouting work was carried out. Approved grouting material was prepared as per specifications and applied to seal the gaps along the bottom edge of the inner side of the element. This is a technique of post-tensioning; where elements are placed first and then by inserting the grout material, tension is imparted in the element to achieve the highest strength. Erection of horizontal element is similar to that of the vertical element; but the difference is, props were kept ready before bringing the horizontal element to the location, and levelling is done. Screed concrete of 60mm thickness was placed over the slab as specified in the drawings.