Portal: Pool

You are correct in suspecting quality issues with your pool. These are the specific issues that lead to poor quality, that can affect the serviceability and durability of your pool.

1. Shotcrete placement requires high velocity and impact for compaction of the concrete. Hand-applying “sloughed-off” concrete would not provide proper compaction needed for producing monolithic concrete sections. The resultant fissures and voids in your pool reflect the lack of proper velocity and compaction.
2. Proper preparation of the substrate is essential for good bond and creating a concrete section that acts monolithically. The surface needs to have any materials that would interfere with the bond removed, be roughened, cleaned, and brought to a saturated surface-dry condition before shotcrete placement. This article from Shotcrete magazine gives more details on how and why surface preparation is important (shotcrete.org/wp-content/uploads/2020/01/2014Spr_TechnicalTip.pdf).
3. Concrete cover over the reinforcing bar is critical for maintaining corrosion protection of the embedded steel, and thus providing long-term durability. Low cover will often result in premature corrosion and subsequent spalling of the concrete cover, reducing the serviceability and life of the pool concrete.
4. Curing is important for all concrete, and especially for the relatively cement-rich concrete we use for wet-mix shotcrete. Curing essentially provides additional water to hydrate the cement in the concrete, and produces stronger, less permeable concrete. Not curing concrete yields concrete that is weaker, more permeable, and ultimately less suitable for creating a watertight pool shell.
5. You haven’t indicated the actual time concrete sat for “a while.” Industry standards are that concrete should be placed within 90 minutes of the introduction of water to the mixture unless special precautions are taken. Water is usually added at the ready mix plant. If concrete sits too long it can start to lose workability. At the point of losing workability, some contractors will add additional water on site over and above the concrete mixture design requirements, but this “retempering” produces concrete that is weaker and more permeable than the original design mixture.

Based on your input, you have many good reasons to ask the contractor to provide full remediation of these quality issues.

As you discovered, experience of the shotcrete contractor is key to a successful project. It takes an experienced and knowledgeable shotcrete team (the project manager, supervisor, pump or gun operator, nozzlemen, and finishers) to get a quality job. Answering your specific questions:

1. Can this be done with either a dry or wet mix? Yes, either wet or dry mix will produce good results. You must be sure to properly prepare the substrate including chipping/bush hammering back to sound concrete, fully cleaning the surface and then bringing the surface to a saturated surface-dry condition (SSD).
2. Do you need to bush a recessed area to accept a minimum amount of new material in lieu of a feather edge? Feather edging will create a very thin layer that would have more potential to spall when exposed to shrinkage or seasonal thermal movements. We recommend creating a square shoulder at least 0.75 in. (19.0 mm) deep to create an acceptable thickness of the patching layer.
3. Is a wet mix acceptable to fill these areas given that the aggregate in it is generally up to 0.375 in. or so? With the dry-mix being primarily concrete sand and cement it would seem more practical. Wet-mix with a coarse aggregate can be shot in thin layers, but with a 0.375 in. coarse aggregate may require more finishing due to impact depressions of the aggregate in the shot surface. A dry-mix material without coarse aggregate may be easier to fill in the thin layers. Dry mix is typically less productive in volume placed per hour than wet mix, but in this relatively small-volume repair application, either process should be fine.

Shotcrete is a placement method for concrete. When the pool shotcrete contractor uses quality materials, properly sized and maintained equipment, and experienced crew members, the shotcreted pool shell should be watertight. Proper shotcrete application would also not exhibit voids, honeycomb, or major seepage through the pool shell. Before replastering, the pool shell should be evaluated and all defects (voids, cracks, porous sections, deep holes, and so on) repaired using industry-standard methods and materials. You may consider retaining a professional engineer or experienced pool consultant to evaluate the pool shell and make specific recommendations on the appropriate repair for your specific issues. You can use our Buyers Guide (shotcrete.org/products-services-information/buyers-guide/) to locate consultants in your area. You also may want to review our pool-specific Position Statements on our website at shotcrete.org/products-services-information/resources/. The “Watertight Shotcrete for Swimming Pools” would be particularly informative for you to assist in discussions with your pool contractor.

Because shotcrete is a placement method for concrete, standard concrete repair techniques are applicable. If the repair is being done by shotcreting new concrete on the existing concrete, the repair should include these steps:

1. Chip the poor or weak concrete back to sound concrete.
2. If reinforcing bars are exposed, they should be cleaned of any rust.
3. If more than half of the perimeter of a reinforcing bar is exposed, the concrete should be chipped back to provide at least a 0.75 in. (19 mm) space behind the bar to allow shotcrete to flow around the back of the bar.
4. Before shooting any additional shotcrete, the surface should be cleaned and then wetted to a saturated surface-dry condition (SSD).
5. SSD means the substrate is damp, but there is no flowing water. It should feel damp to the touch, but no water is picked up on your hand when placing it against the surface.
6. Do not use bonding agents, as they can interfere and reduce the bond of the new shotcrete to the old shotcrete surface.
7. When the new shotcrete is in place, the contractor should initiate water curing as soon as possible. In warm, dry, or windy weather, fogging of the fresh surface immediately after finishing would be helpful.
8. Water curing (where the concrete is kept continuously moist) should be supplied for at least 7 days.

Shotcrete is a placement method for concrete. All normal concrete experiences drying shrinkage that creates a volume change in the hardened concrete. Pools will also experience volume change in the concrete due to thermal changes, especially summer to winter seasonal swings. Contraction and expansion joints are common in all kinds of concrete liquid-containing structures especially with walls of this length. Though we can’t provide a firm design recommendation you should consider these factors:

• What are the weather conditions when the pool is anticipated to be built? If during hot summer months, could there be enough seasonal temperature swing to require expansion joints?
• If expansion joints wouldn’t be needed would contraction joints be needed to handle anticipated temperature swings, and drying shrinkage?
• Will the pool be empty for extended times (this could lead to more shrinkage or direct exposure to solar gain or cold conditions?
• Is the pool to be kept full or empty during the winter months? (if the pool is in a geographic region where extending freezing conditions are prevalent)

Overall, the design for a shotcrete pool should be the same as one for a cast concrete pool.

ASA recommends a minimum of 7 days curing to help control shrinkage issues in young concrete sections. Lack of curing, and exposure to windy, hot or dry conditions will certainly increase the potential for shrinkage and cracking of the concrete. Lack of curing will prevent the concrete from achieving its maximum potential strength. However, shotcrete generally exceeds the minimum 4000 psi (28 MPa) 28-day compressive strength ASA recommends, and required strength depends on the pool design. If you want to confirm the compressive strength of your in-place concrete, cores taken from the pool should be tested for compressive strength by a qualified testing lab. ASTM C1604 Standard Test Method for Obtaining and Testing Drilled Cores of Shotcrete gives guidance on taking cores from existing structures. A minimum 3” diameter core is recommended. Before coring it is recommended to use ground penetrating radar (GPR) or similar equipment to identify the location of reinforcing in the pool section, and then take cores to avoid cutting through the reinforcing wherever possible. The core holes would then need to be filled with a high strength, non-shrink cementitious grout. Once you learn the actual strength, you would need to check with the pool design engineer to verify the strength is adequate for the design. If the strengths are not adequate, you should consult with the pool designer or a licensed professional engineer experienced in pool design for potential solutions.

There are many variables that can cause cracking. Vertical cracking is often the result of drying shrinkage of the concrete. You said you cured (watered) the wall properly, but don’t give any specifics. ASA recommends a minimum of 7 days of curing, with a wet cure preferred over a spray-applied membrane. You should have a licensed engineer evaluate the structural sections, and determine if there were any problems with the amount or placement of reinforcing in your wall section.

Air entrainment will generally slightly reduce the compressive strength of concrete, but significantly increase the resistance to freezing-and-thawing exposure. Dry-mix shotcrete (gunite) is generally a very paste-rich mixture. With modern cements, the normal 28-day compressive strengths easily exceed commonly specified compressive strengths. ASA recommends a minimum of 4000 psi (28 MPa) for shotcrete, and 4000 psi to 5000 psi (28 to 34 MPa) strengths are routinely specified.

1. Air entrainment increases the workability. The small air bubbles act as a form of lubricant to ease internal friction between the concrete mixture components.
2. You should contact one of our material supplier members to see what they offer. You can use our Buyers Guide at shotcrete.org/products-services-information/buyers-guide/, and limit your search to “Admixture Sales” with the “Air-Entraining” subcategory.
3. Quality shotcrete shot against a properly prepared concrete substrate should produce a watertight interface. The hydrophilic waterstop at the joint could be considered a secondary method of making the joint watertight. Though not necessary, it is kind of a “belt and suspenders” approach with a relatively low cost to place.
4. In shotcrete construction, surface preparation between layers to provide full bond is important. ACI 506.2-13, “Specification for Shotcrete,” specifically addresses this in the requirements of Sections 3.4.2.1 and 3.4.2.2 that state:

“3.4.2.1 When applying more than one layer of shotcrete, use a cutting rod, brush with a stiff bristle, or other suitable equipment to remove all loose material, overspray, laitance, or other material that may compromise the bond of the subsequent layer of shotcrete. Conduct removal immediately after shotcrete reaches initial set.
“3.4.2.2 Allow shotcrete to stiffen sufficiently before applying subsequent layers. If shotcrete has hardened, clean the surface of all loose material, laitance, overspray, or other material that may compromise the bond of subsequent layers. Bring the surface to a saturated surface-dry condition at the time of application of the next layer of shotcrete.”

For more details on bond between shotcrete layers, you may want to refer to an article in the Spring 2014 issue of Shotcrete magazine, “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints.” A PDF of the article can be found at shotcrete.org/wp-content/uploads/2020/01/2014Spr_TechnicalTip.pdf.

Properly produced concrete material shotcreted in place should gain strength over time, not lose strength. ASA recommends that concrete placed by the shotcrete method have a minimum compressive strength at 28 days of 4000 psi (28 MPa). Coring does damage the sample somewhat, so it is common to require core strength to meet 85% of the specified compressive strength. Cores should be no less than a nominal 3 in. (76 mm) in diameter for representative results because smaller cores (less than 3 in. [76 mm] diameter) are more subject to damage from the core extraction, affecting the reported strength. Thus, at 85% of 4000 psi (28 MPa) the minimum should be 3400 psi (23 MPa). Based on the reported values, and assuming a 3 in. (76 mm) diameter or greater core, the concrete strength is well below ASA’s recommended strength, and the strength you originally specified in 2013.

ASA recommends a minimum of 7 days curing for all exposed shotcrete surfaces. Wet curing is preferred to supply additional water to the concrete surface. If a spray-on curing membrane is used instead of water curing, the material should be applied at twice the manufacturer’s recommended rate for formed surfaces. Curing is important to allow the concrete to develop as much strength as possible and to help resist cracking from internal shrinkage of the concrete. Low humidity, wind, and exposure to sun will increase the need for proper curing. If the site is dewatered, the groundwater is below the concrete work, and not effective in curing the exposed shotcrete surface. The American Concrete Institute (ACI) has an excellent reference, ACI 308R-01, “Guide to Curing Concrete.” It appears your contractor is not following the industry standards as documented by ACI.