Wine Country Swimming Pool Shotcrete Shrinkage Cracks

I was recently hired by a homeowner in California's Wine Country, to address some shrinkage cracks in their new shotcrete swimming pool.

The term "shotcrete" can refer to either wet or dry mix pneumatically placed concrete.

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 that naturally occurs when concrete cures.  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) added too much water, the resulting concrete will not have the proper water/cement (W/C) ratio.  Additionally, the structure will be weaker.  Remember, in concrete less water 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.  
Lets 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 the structure remains wet.  But it does not address the horizontal surfaces (tops of beams, cover boxes with drains, etc).  
Only using curing compounds will ensure that EVERY SQUARE INCH of the concrete is protected and ensured of a controlled release of the water.  And after all, it is the practice specified and employed by most highway transportation departments and the DOT for concrete highways, sidewalks, structures and bridges.

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.  On past projects I've had engineers specify that visqueen be placed on the grade (like is required under slab foundations).  This can also provide the structure with some protection from reverse ground water migration.

5.  "Bad concrete" - Really, these is no such thing as bad concrete, only poor mix designs.  Usually they are incorrect for the project or conditions, excessive transportation or waiting times (from initial mix to placement).  Wet concrete experiences "blooms." If it experiences it's second bloom prior to being placed, then you might as well send it back.  There are set retarders and additives that can control these times.  The mix design can be destroyed simply by the pump operator or truck driver adding more water.

6.  Weather conditions - excess heat or high winds can all cause shrinkage cracking by causing rapid evaporation of the water from the concrete surfaces.  Again, there are special additives that can be added to ready-mix concrete to assist in combating some of these conditions.

7.  A hard freeze - water turns to ice, ice expands, concrete cracks, enough said.  ACI 318 has specified parameters for the environmental conditions during and after the placement of concrete.  If it's too hot or going to be too cold, you better learn about and employ some protection.

8.  Shotcrete installed in horizontal lifts (like layers of a cake), instead of in one thickness from one end of the structure to the other, can result in a compromised shell.

9.  The shotcrete company "flashed" the interior of the pool at the end of their job and gave it a pretty broom finish.  
What they are really did, was clean out their hoses and hopper by spraying a watered down soupy mix all over the inside of the 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 & 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.  Trimmed material from the floor of the spa it thrown by hand into benches, seats and stairs.

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 even expansive soils, the pool may be experiencing movement cracks.  Add to this, an 18 inch thick structure with a single curtain of steel, and you have a potential problem.

12.  Excessive foot and shotcrete hose traffic.  This is more commonly seen in dry mix shotcrete (but it can happen in wet mix as well).  Excessive foot traffic or the dragging of the hoses around the floor, reworks the concrete.  This compromises the initial compaction and placement of the concrete and forces water to the surface.  The rapid evaporation of this surface water results in a weakened area of shotcrete.

Any one of these factors can cause shrinkage cracks.  Combining a couple of them will surely result in disaster.

Bottom line, there are many possible causes.  

Without proper structural engineering, construction specifications (mix designs, special inspections, quality assurance testing), builder supervision, documentation and photographs - there is no means to tell exactly what the cause was.

The only real issue is... the pool builder has an expensive mess on his hands. 

Contact the author, Paolo Benedetti of Aquatic Technology Pool and Spa at: info@aquatictechnology.com or 408-776-8220. Visit his website at: www.aquatictechnology.com. All Contents © Aquatic Technology Pool & Spa, 2013. 
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