Repair methods for swimming pool gunite

[RichC]

I have a swimming pool under construction that had serious problems with gunite installation. Among other things, the floor and walls were left rough and irregular, and many hairline cracks have developed in the floor. The existing gunite tests at 2500 to 2800 psi. The gunite contractor proposes to shoot a 2-3 inch overlay of new 4000 psi gunite over it all. I understand that with proper preparation new gunite will bond well to old. My questions are: 1) Will the rich 4000 psi mix negatively affect bonding with the original mix (and does the higher compressive strength really provide any advantage in this application)? 2) Is it likely that the existing hairline cracks will transfer through the overlay and finishing plaster over time, and if so, are there any practical measures that can be taken now to minimize such transfer (e.g., reinforcing steel in the overlay, excavation or adhesive filling of the cracks before application of the overlay, etc.)? 3)Is there any sort of resurfacing machinery, other than a jackhammer, that might be used in a swimming pool to roughen the floor and remove an inch or two of the existing gunite in preparation for the overlay?

 

[LPPE]

All I can say is concrete cracks. I'll quote a concrete contractor I know (thick Italian accent) - "Concrete she a'like a woman. You never know whatta she gunna do next."
Doesn't your pool surface need to be smooth so your membrane doesn't puncture? I would grout the cracks smooth along with the rest of the pool. If you can adhere the 4ksi conc to the old, then smooth the 4ksi and I would think you're all set.

 

[ishvaaag]

1) bonding needs not be a special problem after you have roughened and humidified the receiving surface; in fact the overlay will have more binding agents than the reciving part.

Advantage? really I think not necessarily, since normally the higher the strength the bigger the shrinkage, which is itself along with temperature and poor curing the biggest cause of the cracks you may see.

2) Yes, it is likely the cracks will transfer, at least some, them being natural points for the shrinkage action show up. Necessarily? All concrete parts of some size develop hairline cracks, reinforced or not. Then there are several different problems, durability and strength and serviceability. Most pools are surfaced by special products or tiling for best aspect and complete impermeability.

In any case having important percent of binding agents and fines if the concrete has been well made will add to impermeability, since the fine grain particles obstruct pores.

But in any case impermeabilization is better to make rely in special more plastic and impermeable overlays.

Of course adding reinforcement (in proper quantities) reduces cracks, as long the quantity does not serves itself as restraining points and then as lines for cracks appearing. If your overlay is thin, your reinforcement must be weak. Otherwise you risk crack appearing along reinforcement.

Adhesive filling of course would be also good measure...depending what you use (you may create a weak interface), The rationale is the same that the one of using plastic mortar facings for impermeability, but only in the interface.

3) Waterjets or sandblasting can also be used, and any of these will reduce the likelihood of a thin slab being broken when attempting remedial measures.

What is the solution? from my viewpoint ensure by whatever point the mechanical strength and the geometry, then use special surfacing to get the impermeability and aesthetical result. The target of obtaining at ordinary commercial cost uncracked common concrete is not a realistic one.

 

[RichC]

Thanks to Pylko and Ishvaaag for your comments. I'll quote another concrete contractor regarding the guarantees he offers on every job: "I'll guarantee two things-- first, nobody's gonna steal it, and second, it's gonna crack!"

The pool in question doesn't use a membrane (by which I assume you mean a plastic liner?); the gunite forms a structural shell, which as Ishvaaag suggests is then made impermeable by a coating of "pool plaster" (a mixture of white portland cement and crushed marble sand).

The plaster is applied in thin coats that normally are not intended to alter the basic form or finish of the gunite. In our case the gunite finish is so irregular that the plaster won't be able to correct the aesthetic defects. Hence the 2-3 inch layer of new gunite is intended to correct the irregularities prior to application of plaster.

When I asked about machinery to roughen the suface I was referring to making the original gunite rough so the new gunite will have a better chance to adhere. The gunite contractor tried doing this step with a jackhammer, which is crude and uneven at best, and tends to delaminate surface layers. I'd like to find something along the lines of a portable chipper/grinder with a rotating drum that might be a bit more predictable. I've seen roadway resurfacers that operate this way, but they're the size of a dumptruck.

 

[krd]

RichC: You can get a bush-hammer bits for light weight chipping guns. Or your local equipment rental store may be able to locate some "scabblers". These are air tools with a bunch of pistons which will scarify concrete. They come in various sizes from hand held to heavier ones that need to be mounted on a piece of equipment.