building it right, from scratch.. (part iii – moving above grade)‏

much of what you’ve endured up until now will go without visual reward. now however, you’re ready to move to the above grade structure.

depending on your structural design, your sloof may be below grade, with another tie-beam structure at grade (colloquially referred to as a “ring” on-site in indonesia), or the sloof themselves may be at or on grade – which means that you would have needed to have all of your drainage pipes, etc mapped out, with sleeves penetrating the sloof, for the later installation of below slab drainage.

however, more often than not, sloof are below grade, and above those sloof a riser wall is installed. this riser wall, takes the sloof axis’ up to grade and will be the footing for the “ring” at slab elevation – which will connect again to your columns though pre-installed rebar anchors coming out of your columns at ring height. column risers from below grade pile caps will have been poured to your slabs elevation and the riser walls will have your drainage and other utility like infrastructure penetrating them, as your pipes cross the building. once your riser walls above your sloof are in place, the voids in between them will be filled with a quality fill – often limestone in bali.

at this stage, one of two things will come next. either your engineer will specify that you now pour your on grade slab, or, more commonly, you’ll continue with the pour of the columns, around the rebar that is already protruding from the depths of the pile caps. these columns should be brought up to the height of the underside of your first storey beams (balok) – this example of course assumes two storey construction.

caution must be exercised in all structural undertakings to ensure that your engineer and architect are on the same page. for example, reflected in architectural drawings may be columns with a dimension of 25cm * 60cm. the engineer must know that these columns will need to take a finishing, and therefore his structural members must be smaller. just how much smaller? that will depend on the finishing – plaster and paint for example need less than stone and marble.

once your columns are up at the elevation of the underside of your slab’s beams, your foreman will quickly set to work erecting all the formwork for your first floor slab and beams. while thin plywood is more economical, it’s also far weaker and it requires more framing. if your project will be employing the use of vibrators (necessary) and / or a concrete pump (potentially), you may very well find that your 6mm plywood formwork provides you with an epic failure – and such failures are not easy to rectify during the course of a pour. thin plywood will also bend more under the weight of the ‘mud’ (wet concrete) – essentially loading your structure with unnecessary weight as slabs and beams bulge to their underside. 9mm plywood (triplek) is the recommended minimum and beware of suppliers trying to sell you compressed particle boards at a lower price – they are cheaper yes, but they lack the tensile strength that laminated plywood provides.

after the formwork is in, the steel rebar starts to get assembled, the drainage and conduit cut-ins are made to the formwork and we find ourselves back again, ready for a pour.

but before we get to the pour stage here are some things to consider while everything’s going in:

  • ribbed steel rebar on suspended slabs is more expensive, but it’s a higher quality grade of steel and also provides better long term traction within the set concrete
  • ensure all rebar at beam joints and column-beam joints is inspected by your engineer thoroughly
  • overdo your minimum rebar overlaps on beam and column joints
  • bathroom walls should have concrete risers poured to assist with waterproofing – 30cm up the wall if possible
  • long spans encounter greater deflection forces. your engineer should calculate these and advise your site staff before they start erecting the formwork if any offsets are needed (this involves rising the formwork and rebar in the center of the span by a number of centimeters to counter the deflection when formwork is removed
  • after all the above is done, we’re ready to pour – and we’re back to the pouring basics. prepare, pour, vibrate, float and cure. don’t rush to pull out your underside formwork – three weeks is the minimum, but this is definitely one area where the old adage is true, “the longer, the better..”.

that’s a very high-level overview of the first above grade structures that tend to go in – in our next piece in this series, we’ll further discuss the components all the way to your roof and if you have any questions regarding the structure of your home, please don’t hesitate to shoot us an email..