Base materials are not "self-compacting" merely by dumping them on the ground. To ensure longevity of the concrete pool decks proper compaction procedures must be followed.
Soils Report
A thorough soils report will make minimum recommendations for the preparation of the pool deck sub-base. This may include over excavation, removal and recompaction, or the importation of materials.
The most common method is the over excavation of the pool decks when the pool is being dug. After back filling the plumbing and utility trenches, a base material is imported to build the site back up to the proper elevation.
The soils engineer will specify the materials to be used. It may be sand, road base or crushed rock. The thickness of this material is specified by the soils engineer.
Proper Compaction
The thickness of this material will determine the compaction equipment that is to be used. The confinement of the site will also dictate is larger, more efficient equipment can be used.
Jumping jack tampers are reserved for compacting trenches and open pits. Vibratory plates are used for thin layer of materials or final compacting. Walk behind or ride on compactors are sued when the material is to be compacted in 4-6" lifts.
Rolling compactors are the most efficient, as the size of the roller quickly compacts a lot of terra firma.
Using the improper equipment is not only a waste of time and money, it will not result in a stable base. Spreading a foot or more of material, and expecting to compact it with a vibratory plate is a fool hardy endeavor.
While the top may become compacted, the bottom will remain loose. Over time that material will settle, resulting in deck movement and cracking.
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.
All rights reserved.
Plan for the Worst Case Scenario
Whenever designing or building a swimming pool in expansive soils conditions, you must plan for the worst case scenario, says swimming pool designer and expert witness Paolo Benedetti of Aquatic Technology Pool and Spa at www.aquatictechnology.com.
This means that you must adhere to to the following procedures:
- Obtain a Geotechnical report (soils report)
- Have the structure engineered to withstand saturated soil conditions when the pool is empty.
- Ensure that water does not collect around the outside of the structure.
- Isolate the structure from the soils.
Soils Report
The structural engineer cannot possibly know how strong to design the structure if he does not know that characteristics of the soils.
The soils report will define the load bearing capacity of the soils. It will also quantify the expansiveness of the soils. From these criteria, the structural engineer can determine the strength of the structure.
Structural Engineering
Whether one uses generic or custom structural engineering, it must exceed the forces the soils will exert upon it (these are known as soils surcharges).
The structural engineering will define the thickness of the walls and floor; the size, placement and spacing of the reinforcement steel, PSI strength of the concrete and any special mix design requirements.
Custom engineering is always recommended, as the structural engineer will also review other potential surcharges (from buildings, slopes, snow, surf, wind, etc.).
Empty and Saturated Condition
The structural engineer will design the structure to resist surcharges when the pool is empty. Why empty? Because some day the pool will need to be drained to be remodeled or refinished.
And because the engineer cannot predict the condition of the soils when the pool is drained, he must assume that the soils will be fully saturated and swollen.
Keeping the soils dry
Installing concrete decks around the pool will help keep the soils adjacent to the structure dry.
Drainage should be installed to prevent water from sheeting off of the decks and into the surrounding landscaping. This will only force the water to seep back under the decks into the sand/rock base, defeating the intent of keeping the soils around the pool dry.
Placing drainage rock or drainage panels under or around the pool is fruitless, unless there is a means to discharge any water that collects under the pool. This could be a drain to daylight (if there were a slope nearby) or a dewatering well with a sump pump in it.
Isolate the pool from the expansive soils
Another method is to reduce the surcharges around the pool from expansive soils. This can be achieved by over excavating the soils and later back filling with non-expansive materials. Of course this requires that the pool be built differently, since the concrete cannot be shot against the soils.
Sand bags, void forms or geo-voids can also be installed behind the reinforcement steel. Properly braced, the concrete can be shot against these surfaces.
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.
All rights reserved.
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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.
All rights reserved.
