LeDeuxions · notes from twenty years on site · 한국어
"We put the waterproofing admixture in the concrete, so it won't leak." Owners believe it. Builders repeat it. I have heard it on site more times than I can count. It is not true, and the reason it is not true is worth understanding before you pour anything.
Concrete is not a sealed stone. It expands in summer heat and contracts in winter cold, and it does this every year for as long as the building stands. That cycle produces hairline cracks. Not always visible ones — but water does not need a visible crack. It travels along the grain of a hairline by capillary action.
Here is the part that decides the argument. An admixture can only be where the concrete was when it was mixed. The crack forms later, somewhere the mix never anticipated. The admixture is not in that crack, because that crack did not exist when the truck poured. You cannot pre-seal a gap that has not happened yet.
This happened on a house I built. The owner did not want any additional waterproofing — just the admixture in the concrete, and done. I told him it would not hold. He did not agree.
Every year after that, a few more damp spots appeared, and every year we did another partial repair. I made the same argument each time. It took five years. In the sixth year he changed to a proper waterproofing system.
Add up what those five years of patch repairs cost and it is well past what doing it correctly at the start would have been — plus five years of living with it.
The membrane belongs on the surface of the concrete, not inside it. Sheet membrane, liquid-applied membrane, modified bitumen — the specific product matters less than the principle. A separate layer sits above the concrete and bridges the cracks the concrete will inevitably develop. The slab can crack and the layer above it still keeps the water out.
That is the whole difference. An admixture is part of something that will crack. A membrane is a separate thing laid over the crack.
"Waterproof concrete" is a sales phrase. It makes the job simpler and the bid look cheaper, and both of those advantages disappear the first winter water shows up inside.
There is no such thing as a house that is both cheap and good. If you believe you have one, it may only mean not enough time has passed for it to show.
In construction there is almost never an absolute — there is usually a better way waiting to be found. On this one the better way is already settled: a membrane, on the outside.
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Written from jobs I worked on, not from a manual.
Small tools I built for everyday work — pdf300.com
And a few things to play with — ledeuxions.com/games