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Home \ Cracking Up

Cracking Up

Pool cracks are no joking matter. They have many causes, but often are reparable.

Source: CUSTOM HOME Magazine
Publication date: 2005-06-02
By Rebecca Robledo

Few people know more about pool cracking that Ron Lacher. This pool builder turned structural engineer provides plans for thousands of new pools and has carved out a niche in pool forensics.

Finding the underlying reason for a pool crack isn't always easy. Problem soils and poor workmanship are often to blame. But the exact nature of the problem, and how to fix it, can be hard to determine.

Here are Lacher's observations about the most prevalent types of pool cracking, how they occur, and how to repair them.

 
Illustrations: Mark Bremmer / www.markbremmer.com
The most common type of cracking takes place across the tile line. To avoid this, follow the model above. Place the expansion material deep enough to completely separate the deck and coping. Otherwise, the bond beam may crack, causing the tile to do the same.


Workmanship issues.
Some of the most common types of cracking result from substandard workmanship. This can include improper gunite or shotcrete application or seemingly minor oversights when installing the concrete pool deck.

The four most common types of workmanship-related cracking are:

Horizontal cracking in the waterline tile
  • What it looks like: This is the most common type of cracking found in pools, and it looks just like the name indicates.
  • What causes it: The concrete deck is not completely separated from the pool's bond beam. The deck moves, putting pressure against the pool wall. As a consequence, the pool's bond beam may crack. If waterline tile is affixed to the problem area, the tile will fracture as well. Similar cracking can result from a common technique used to prepare for the coping. Workers level the top of the bond beam by applying a layer of mortar. If the material doesn't properly affix to the beam, it may delaminate, causing the tile in front of it to crack.
  • How to prevent it: Completely separate the concrete deck from the pool's bond beam. An expansion joint must extend the full depth of the deck. For example, if your deck is 4 inches thick, the expansion joint must be 4 inches deep. This prevents the two bodies from touching. To ensure proper adhesion between the bond beam and that leveling layer of mortar, follow proper workmanship procedures. Moisten and clean the beam before applying the mortar.
  • How to fix it: In either case, the pool builder must remove and replace the cracked materials using proper trade procedures. Often, workers must retile the waterline as well.
Reflective cracking
  • What it looks like: Cracks generally follow the pattern of the reinforcement or piping inside the wall. Older cracks may show rust.
  • What causes it: The reinforcing steel is inadequately covered or improperly encased. This usually stems from one of several possible mistakes: —The shotcrete mix didn't have enough moisture, wasn't shot at a high-enough velocity, or the nozzle man didn't use proper techniques. These errors can result in voids or shadows behind the reinforcing steel. —The rebar wasn't properly secured, so it vibrated during gunite or shotcrete application. This prevents the gunite or shotcrete from adhering to the reinforcement. —The gunite or shotcrete was applied too thinly over the reinforcement. This can especially be a problem on freeform pools, where crews can't set vertical and horizontal string lines to help establish the right thickness.
  • How to prevent it: Use the proper gunite or shotcrete mix, and apply it at the right velocity and thickness.
  • How to fix it: If the cracks aren't too old, they can be injected with an epoxy that is stronger than the concrete itself. Older cracks, however, will have exposed the steel to water, resulting in rust and corrosion. In this case, the area will have to be torn apart and redone.
Delamination
  • What it looks like: Alligator cracking or separation of the outer layer.
  • What causes it: Several errors can cause delamination. Rebound, a byproduct of applying guniteor shotcrete, is a primary culprit. As crews shoot the concrete, some pieces of aggregate bounce off the wall and fall out to the floor. Some builders use it to build steps or benches, or otherwise bury it in the pool shell. However, rebound doesn't have enough cement to form a proper bond, so it may delaminate. Delamination can also be caused by "flashing"—when the applicator conceals rebound or a surface flaw by putting a thin layer of guniteor shotcrete over the area. It may not bond properly.
  • How to prevent it: Keep rebound out of the pool. Haul it off the property. Avoid flashing if possible.
  • How to fix it: Workers must strip away the delaminated area and replace it, making sure it bonds properly to the surface underneath.
Shrinkage cracking
  • What it looks like: There's no set pattern. These cracks are intermittent, random, and angular, generally running throughout the pool. They are typically shallow.
  • What causes it: Improper curing or excessive moisture in the gunite or shotcrete.
  • How to prevent it: Avoid the temptation to add water to the proper shotcrete or gunite mix. After the material is applied, properly hydrate it so it can cure sufficiently.
  • How to fix it: Epoxy injection.
  • It's in the dirt. More serious cracking can occur when soil is unsuitable to support the poolor when more than one type of earth is present under the vessel. The homebuilder should have his soils engineer determine the yard's ability to support a pool as part of the overall soil report.

These are the predominant forms of soils-related cracking:

Differential movement cracking
  • What it looks like: Cracks are usually vertically oriented. They are often wider at the top of the pool wall and get narrower as they move toward the floor.
  • What causes it: Having two different types of soil underneath the pool can cause differential movement cracking. This may occur naturally if, for example, the yard has both bedrock and a softer soil. Or it could result from a cut/fill transition—when the pool straddles backfill and native soil. This can be a problem even if the fill has been properly compacted. Unless the pool rests on uniform earth, the softer material will compress more than the harder, causing unequal movement. The pool may crack from the uneven support.
  • How to prevent it: A soils engineer should examine the site if the home builder does any recompacting or the pool excavation reveals two different kinds of material. A structural engineer may need to design a support system for the pool.
  • How to fix it: Based on Lacher's experience, epoxy injection will work 60 percent to 70 percent of the time after ground movement has run its course. If the soil is expansive or otherwise continues to move, the pool may crack again.
Slope creep cracking
  • What it looks like: Very similar to differential movement cracking.
  • What causes it: On some hillsides, a 10- to 15-foot layer on the face of the slope will slowly move downward. Generally this happens with expansive soil. You can detect slope creep when trees or other vertical elements on the hillside point outward rather than straight up.
  • How to prevent it: Hire both soils and structural engineers to examine any hillside property. The soil can be identified and a proper support system engineered.
  • How to fix it: This very costly repair requires demolishing part of the pool and installing a footing or key to increase the setback from the foundation to the face of the slope.
Expansive soil cracking
  • What it looks like: Cracking located on the pool floor. In the most severe cases, the pool is thrust out of the ground.
  • What causes it: Highly expansive soils aggressively pushing upward on the pool.
  • How to prevent it: Again, hire a soils engineer. If the professional detects expansive soils, a structural engineer can adapt the shell. Solutions will range from thickening the pool floor to building a pier-and-grade-beam foundation.
  • How to fix it: You could try epoxy injection and cross your fingers. If that doesn't work, you may have to remove the floor and add more steel and gunite as specified in an engineer's plans.




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