Deck Slabs - Normal and lightweight concretes.

One of the principal advantages of steel decking is that it acts as a working  platform. The loads that it supports arise mainly during the concreting operation.

Lightweight concrete (LWC), therefore, has advantages in terms of its reduced self-weight in comparison to normal-weight concrete (NWC). In the UK the main form of LWC comprises Lytag and sand with a dry density range of 1800–1900kg/m3.The compressive strength of Lytag LWC is similar to NWC, although the elastic modulus is lower.

Concrete is usually placed by pump,mainly for reasons of speed, but also because it is difficult to ‘skip’ the concrete with the decking in place above. Indeed, it is usual practice to deck-out a number of floors ahead of the concreting  operation. The decking is attached to its supports by shot-fired pins or self-tapping screws and later by welded shear-connectors, welding or bolting.

Unlike traditional reinforced concrete, there is no need to restrict bay sizes during construction because the decking serves to distribute early age and shrinkage strains, thereby eliminating the formation of wide cracks.Typical pumped pours are 500–1000m2. LWC also has a higher tensile strain capacity than NWC, reducing the tendency for cracking.The mesh reinforcement is of a nominal size to control cracking at supports, and to act as ‘fire reinforcement’. A142 or A193 are the common sizes specified.

The concrete grade (cube strength in N/mm2) is normally specified as C30 to C40. Pumped concrete also contains additives to aid lubrication. For  this reason the slump test is not a good measure of workability and so the flow-meter method is often used. The concrete is usually tamped level by fixing the tamping rails to the support beams. As these beams deflect, the slab level adopts that  of the support beams. In propped construction, the slab level deflects further on removal of the props.

0 comentarios:

Post a Comment