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Shrinkage Cracks

Pool Design article by Paolo Benedetti

I was recently hired by a homeowner to address some shrinkage cracks in their new pool. Bottom line, there are many possible causes. And without builder documentation, quality control and diligence there is no means to tell exactly what caused it. Remember, the term "shotcrete" means either wet or dry mix.

Possible Causes

  1. Thickened shell without additional steel. A single curtain of steel is inadequate in concrete that is 12 inches (or more) thick. There is not enough steel to control shrinkage. This is why you need a structural engineer to design plans based upon the site's soil conditions.
  2. Excess moisture. If the pump operator, ready-mix driver or nozzleman (depending upon wet or dry mix) adds extra or too much water, the resulting concrete will not have the proper W/C ratio. The water will consume too much of the mix, evaporate & cause shrinkage cracks. Additionally, the structure will be weaker. Remember, with water, less is better.
  3. Improper curing practices. ACI 318 outlines the acceptable methods of curing concrete (shotcrete included). Physical membrane (plastic), fabric that is kept wet (carpets, burlap, etc), wet curing (keeping wet with soaking or flooding), curing compounds. Let’s face it; carpets and plastic are a hassle. Plastic blows around & what do you do with hundreds of yards of wet carpeting? Most wet curing is done poorly - the structure is allowed to dry out between the wetting cycles. Flood curing will ensure that a part of the structure remains wet. It does not address the horizontal surfaces (tops of beams, cover boxes with drains, etc). I have concluded that using curing compounds is the only means to ensure that EVERY SQUARE INCH of the concrete is protected and ensured of a controlled release of the water.
  4. Failure to saturate the sub-grade or rock pack prior to the shotcrete placement. The underlying materials will suck water from the shotcrete from the backside. I've had engineers specify visqueen on the grade. This can also provide the structure with some protection from reverse ground water migration.
  5. Bad concrete - poor mix design for the project or conditions, or
  6. Weather conditions - excess heat or high winds can all cause shrinkage cracking by causing rapid evaporation of the water from the concrete surfaces.
  7. A hard freeze - water turns to ice, ice expands, concrete cracks, enough said.
  8. Shotcrete installed in horizontal lifts (like layers of a cake), instead of in one thickness from one end to the other.
  9. Shotcrete company "flashed" the interior of the pool at the end of their job and gave it a pretty broom finish. What they really did was clean out their hoses and hopper by spraying a watered down soupy mix all over the inside of your newly shot pool. The best laid plans are destroyed by this weak and incompetent crust of waste material. Tell them to bring or build a washout bin for this purpose. When doing wet mix shotcrete, some ready mix companies will allow them to washout back into the truck.
  10. Incompetent shotcrete placement and finishing techniques. For example: trimmings, rebound and reworked material are placed back into the structure; trimmings and loose materials allowed to accumulate beneath steel; trimmings and rebound allowed to accumulate between layers; improper encapsulation of the steel.
  11. Poor soil conditions - the pool may be experiencing movement cracks. If the pool was constructed on fill, soil that could not bear the loads or expansive soils, movement can easily cause a pool to crack. Add to this, an 18 inch thick structure with a single curtain of steel, and you have a potential problem.

Any one of these can cause shrinkage cracks. Getting sloppy and combining a couple of them will surely result in disaster.

So you have shrink cracks, now what?

First, CALL the structural engineer of record. He needs to do a site visit or direct you in a course of action.

Secondly, sound the shell with a crow bar as a "tuning fork." I sound EVERY shell a week after shotcrete placement. A can of paint and an 18" long crow bar are all that you need. Hit the shotcrete with the back of the curved end of the crow bar at about 1 foot intervals. Solid material will resonate with a "PING," while incompetent areas will sound with a "THUD." Place a dot of paint where there are THUDS and you'll soon have the bad areas identified. I have located hollow pockets on pools that did not have shrinkage cracks on the surface.

Thirdly, if there are no shrinkage cracks, but you had a THUD sound, chip out the affected areas. Usually, you'll find an area where the nozzleman (dry mix) had turned up the water or sprayed excess water because the hopper went dry or the gun or hoses clogged. A small chipping hammer usually suffices if the areas are not too large.

Finally, if there are shrinkage cracks, then coring a sample directly over a few of the cracks will let you know if they extend all of the way through the shell.

Paolo Benedetti

Paolo Benedetti

Aquatic Technology Pool & Spa
PO Box 130
Morgan Hill, CA 95023
T (408) 776-8220