Crack-chase resin
You cannot fill a crack you have not first opened up
A crack in a slab is either static or it is live, and everything about how it gets treated follows from which one it is. A static crack has finished moving: it was shrinkage, or a settlement that has settled, and it is now simply a line of dirt in the concrete. A live crack is still working, and filling it rigidly only moves the problem a few millimetres sideways.
Assume for a moment it is static. The instinct is to brush resin into it and move on. It does not work, and it fails for a reason worth knowing: a hairline crack is narrower than the resin can enter against its own surface tension, it is full of dust and laitance that no brush reaches, and what you have actually done is bridge the top of it with a film about as strong as a sticker.
The chase is the work, not the resin
Every static crack is cut out along its length with a chase blade to about six millimetres deep, then vacuumed. Widening a crack in order to repair it sounds backwards until you see what it produces: a channel with clean, square, freshly cut sides, which is the first surface in that crack that resin has ever had anything to grip.
The resin itself is almost an afterthought by comparison. It is thin on purpose (around 150 mPa·s, nearer water than honey), so that poured into an open chase it runs to the bottom under its own weight and fills from the base up rather than skinning across the top.
Telling static from live
A crack that is straight, tight and following no obvious line of load is usually static. One that steps, that opens wider at one end, that runs off a re-entrant corner or tracks a wall below is worth watching before anything is poured over it. Where there is doubt, the answer is a tell-tale across it and a fortnight, which is exactly the fortnight most people do not want to give.
A live crack does not get chased and filled. It gets treated as what it actually is (a joint the slab made for itself), and sealed with the polyurea on the previous page, so it can carry on moving under a floor that expects it to.
Where we have got this wrong
We have called a crack static that was not. It was in a workshop extension, it was hairline, it had every appearance of shrinkage, and we chased and filled it. Fourteen months later it reopened along most of its length, and it went back as a warranty job: cut out properly, sealed as a movement joint, recoated.
The lesson was not about the resin, which did exactly what it should. It was that the fortnight of watching is cheap and the callback is not, and we now push for it whenever a crack gives us any reason at all to doubt it.
How to tellA chased crack is a straight cut line, 6 mm wide, usually slightly darker. A crack that was painted over is not straight; it wanders, because cracks do, and in two years it will be a visible line again in the same place.
What to ask
Ask whether cracks will be chased or filled, and what opens them first. If the answer is that they will be filled, ask what happens to a live one. A contractor with no answer to that question has only one repair, and he is about to use it on both kinds.