Portal: Pool

Gunite is the original tradename for what we now call dry-mix shotcrete. Shotcrete is a placement method for concrete, so recommendations for curing and protection follow general ACI guidelines for exposed concrete. ASA recommends a minimum of 7 days of continuous (not just a few times a day) wet curing to help control shrinkage issues, increase strength, and reduce permeability 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.

Shotcrete placement with quality materials and proper application techniques generally exceeds the minimum 4000 psi 28-day compressive strength ASA recommends. The statement that 3000 psi is the “industry standard” is not true, as the ACI 350 Code for concrete liquid-containing requires a minimum 28-day compressive strength of 4000 psi for concrete intended to have low permeability when exposed to water. The 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/C1604M provides guidance on taking cores from existing structures. A minimum 3 in. (76 mm) diameter core is recommended. Before coring, it is recommended to use ground-penetrating radar (GPR) or similar equipment to identify the location of reinforcement in the pool section, and then take cores to avoid cutting through the reinforcement 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.

Regarding cracking, the lack of curing will increase the concrete’s shrinkage and correspondingly the potential for cracking. You should verify that there are no significant cracks in the pool shell before the plaster or other interior coating is applied. If there are cracks, the pool contractor should repair those before proceeding with the plaster or coating. Although proper curing would certainly decrease the concrete’s permeability, generally good-quality shotcrete with proper placement and a strength of 4000 psi will be functionally watertight and not allow any significant amount of water to flow through the uncracked concrete thickness. You will find more detailed information on pool compressive strengths and watertightness of pool shells in our ASA Position Statements.

Shotcrete is a placement method for concrete. Both wet-mix and dry-mix produce quality in-place concrete when using quality materials and proper equipment and placement techniques. There are no compatibility problems with shooting wet-mix over dry-mix. Basically, it is just shooting shotcrete on top of already placed concrete. For proper bond, the surface of the existing dry-mix must be roughened, cleaned, and brought to a saturated surface-dry moisture condition before shooting the wet-mix lining.

Shotcrete is a placement method for concrete. Thus, evaluating your pool shell life expectancy will be the same as any concrete structure exposed to water. You may find the technical document ACI 201.1R-08, “Guide for Conducting a Visual Inspection of Concrete in Service,” helpful in evaluating your pool shell. Generally, shotcreted concrete that uses quality materials, proper equipment, and placement techniques will serve for at least 50 to 60 years. ACI 350-06, “Code Requirements for Environmental Engineering Concrete Structures,” states: “When all relevant loading conditions are considered, the design should provide adequate safety and serviceability, with a life expectancy of 50 to 60 years for the structural concrete.” This ACI 350 Code is for liquid-containing concrete structures such as tanks for water and wastewater treatment but the original pool design may or may not meet the Code requirements.

Shooting shotcrete against a one-sided form (what you called a temporary wall) is a common way to build a shotcrete wall. Once the shotcrete sets and builds strength, the plywood form can easily be stripped off the back of the wall. Form release agents (not oil) can be applied to the plywood to make the stripping easier. Once the forms are removed and the concrete has gained adequate strength, the walls can be backfilled with compacted soil or gravel, depending on the drainage needs.

We recommend 7 days of curing. Continuous water curing is best, but if impractical, applying a curing compound on the exposed surface at twice the manufacturer’s recommended rate for a good seal is acceptable. If they remove the forms before 7 days, they should also water cure or apply curing compound to that newly exposed surface. The shotcrete needs to build up enough strength to resist the external force of the backfill, so check with the pool designer to see what they need for the required strength of the concrete before backfilling. With most good-quality shotcrete materials and placement techniques, you can expect about 4000 psi (28 MPa) compressive strength in 7 days.

The engineer designs the wall based on the loads anticipated on it throughout its service life. The minimum spacing for noncontact lap splices as indicated in ACI 506.2, “Specification for Shotcrete,” is:

“Clearance of at least three times the diameter of the largest reinforcing bar; three times the maximum size aggregate; or 2 in., whichever is least”

In your case with No. 5 bars, the 2 in. (50 mm) minimum probably controls and would then provide a 2.6 in. (64 mm) center-to-center spacing. No. 5 bars at 4 in. spacing can be shot properly with proper materials and technique. An ACI-certified shotcrete nozzleman will have had education that explains how this configuration or even closer spacing can be shot properly. Experienced shotcrete contractors doing structural concrete walls do this type of work routinely with excellent results. If you are concerned about encasement of the reinforcing steel in the back curtain of steel you may consider erecting only the back curtain of steel, shooting the wall out to the location of the outer curtain of reinforcement, erecting the outer curtain, and then shooting out to the final surface. Experienced shotcrete contractors have shot in thick sections (36 in. [900 mm] thick or more) with heavy reinforcement (No. 11 at 4 in.) using this technique.

Proper shotcrete placement of concrete depends on high-velocity impact of the concrete materials for full consolidation. Though not clear from your inquiry, it sounds like the additional concrete was just dumped in place without high-velocity impact. Thus, you simply have normal cast concrete that would require some type of external vibration to densify and properly consolidate the concrete. The bond of cast concrete would be inferior to the bond from shotcrete since you do not have the high-velocity impact driving the cement paste into the previously shot material. Also, adding 20 gal. of water at the site would substantially weaken the concrete from the original design strength of the concrete mixture. Further, feathering edging in a joint is not recommended since you end up with a very thin overlay at some point that may tend to spall or delaminate much more easily at the thinner section. A better approach when running short of concrete is to stop and prepare a joint for later shotcreting. For best bond, joints should be cut at a 45-degree angle, roughened, and then cleaned and wetted immediately before shooting.

Thus, answering your final question, in summary the bond will be reduced, and with the higher water content, the concrete will be weaker than properly shotcreted concrete.

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.