building it right, from scratch.. (part ii – below grade structures)

pools, basements, water tanks, fish ponds, balancing tanks – they all have one thing in common; an overwhelming desire to leak, and there probably isn’t a place on earth with more leaking below grade structures than right here in bali. why? almost as compelling as the nature of water to escape or intrude is the nature of some folks to cut corners and save money – not realising that these savings will amount to massive future outlays.

all of these structures should be waterproofed, reinforced concrete – period! there’s no getting around that – and given that waterproofed, reinforced concrete is many times the cost of bricks and cement plaster, it’s no surprise that the poor old red brick is often summoned to perform the task – but it can’t, and it won’t. it’s like trying to use a fork to transfer broth. so not just must these structures be from reinforced concrete, they also will require significant foundations (refer to part i) and often basements will warrant specialised drainage provisions to minimise water sitting on the face of the wall – all of these factors add up, making below grade structures a considerable chunk in the total cost of your home.

so what are some means of ensuring a good outcome?

firstly, think excess – that’s right, concrete is your structure, and together with reinforcement (aka “rebar”, typically steel) it forms a monolithic material that literally changed the face of the earth. while its monolithic nature makes reinforced concrete a modern day engineer’s best friend, the monolithic nature of it dictates that failure in either element (the concrete or the reinforcement) renders it failed. with any concrete work, particularly below grade applications, our first priority is protecting the rebar – and one of the simplest, most practical ways to ensure such is to make those walls just an inch or two wider, offsetting the rebar to the inside – this gives us an extra margin of safety, assuming that we pour it right. so instead of forming up our wall at 15cm width (12cm isn’t even an option), we form it up at 18cm. sure, that’s an extra 20% added to the volume of mud you’ll use, but it will not go unrewarded. we keep the rebar in the same position we would have it in if we were doing a pour of 15cm, and gain our extra 3cm on the outer face. your internal rebar (particularly in pools, ponds, balancing and water tanks) must still have at least the minimum coverage, typically taken as twice the width of the rebar – but again, throwing an extra 10mm of coverage on the inner face won’t be a regrettable decision. wider form-work will also reduce the risk of honeycombing by allowing installers to better vibrate within the form-work throughout the pour.

secondly, we need to look at the mud itself. concrete comes in various qualities – some tragic, others decent, and a few superior. in indonesia, the compressive strength of concrete is represented as ‘k###”, where ### represents the compressive strength of the concrete in kilograms per square centimeter. in most countries, mpa (metric) or psi (imperial) is used, but we’ll stick with the indonesian standard for the purpose of this topic. you want to avoid any mix design where the first # is a ‘2’. the minimum you should be using for below grade structures is k300 (actually, a decent builder won’t go below this grade for any part of the structure, and will often use an even higher grade of concrete for below grade structures, e.g. k400). why is this important? well there’s a generally accepted correlation between compressive strength and density. and a higher density grade of concrete generally means that your end result, installed correctly, will be less porous – in essence providing a greater water barrier between the surface and the inner rebar. always take test samples on site and send them to a lab (udayana has one) for analysis – its cheap, easy and it’s the only way to ensure your ready-mix supplier is being held to account.

thirdly, you must ensure the concrete is properly cured after the pour. all the best preparation and materials will fail miserably if you don’t cure your concrete well. curing essentially means not allowing the concrete to set too fast. the curing of concrete is a field of its own, and there are many resources online that will give you all the detailed information needed – but at a high level, curing involves some basic common sense. wet your form-work thoroughly before the pour, pour at night when possible, keep your form-work (and slabs) wet and the pour protected from direct sunlight in the weeks following the pour. all of this effort, just to slow down the hydration reaction – because when this reaction runs wild, a lot heat is generated, and when this reaction isn’t initiated in a controlled environment it races ahead of itself and your concrete shrinks too quickly – your concrete weakens and hairline cracks develop – the rebar can become exposed, no longer protected by it’s dry mud companion. a kind note to remind you that while concrete will in fact cure completely submerged in water, you do not want to wash away the cement while the concrete is still wet.

with some extra padding of decent quality concrete and the proper installation and curing, you’re 90% there.

finally we come to waterproofing your concrete. in many countries, this is done as an initial ad-mixture to your mud – and sure, this is preferable. problem? in indonesia, this is not common and as such, most ready-mix companies don’t have a waterproof mix design that they stand by (or at least one that they are justified to stand by). ad-mixtures are tricky and qc at the ready-mix depot is imperative and since we can’t control this, we believe it’s best to assume it doesn’t exist. there are a few products we recommend for waterproofing concrete; bondall and sika have fantastic waterproofing products, but the application is imperative. surfaces must be clean, free of loose and flaked debris, and application instructions from the manufacturer must be followed precisely. most cementitious waterproofing compounds should be covered for protection, by either plaster, tiles or other rigid products.

and with that you’re done – you’ll have a below grade structure that will stand the test of time. in our next piece in this series, we promise we’ll get to walls, columns, beams, slabs, etc.