The ASA Technical Questions and Answers is a free service offered to all users, but primarily intended for engineers, architects, owners and anyone else who may be specifying the shotcrete process and/or has need for a possible answer to a technical question.
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PoolQ I am building a perimeter overflow pool and need to know the best way to finish the trough. I have one shotcrete company that wants to hand-finish the 6 in. (152 mm) wide trough; another wants to shoot up to foam. Can you tell me which is best and why?
This is a classic performance-versus-prescription situation. The owner or customer should specify the desired end product and the contractor should choose the means and methods. Although they may both provide a great product, what works for one contractor or what he/she is comfortable doing may be completely different than another contractor.
PoolQ I am doing an owner/builder pool. The shotcrete company I hired left an approximately 4 x 4 in. (102 x 102 mm) hole in the deep end of the pool on the side wall. What is the proper way to patch this hole prior to plastering? (The shotcrete was applied a month ago.)
The normal repair is to pressure wash with at least 3000 psi (21 MPa) of water pressure to remove any dirt and laitance on the surface of the concrete. Given the rather small size of the hole, it can be hand-patched with a nonshrink hydraulic cement with at least 4000 psi (28 MPa) 28-day compressive strength to plug the hole. After the patch is completed, roughen the surface that will receive the plaster.
PoolQ I am in the process of hiring a contractor to install a pool at my home. I have a long driveway, about 350 ft (100 m) and the pool will be constructed behind my house. The contractor wants to remove a substantial amount of plantings to allow the shotcrete truck to get closer to the pool area. How far can you reasonably pump shotcrete horizontally and what type/brand pump would be needed?
Shotcrete placement conveys concrete materials through a delivery line that ranges from 11⁄2 in. (38 mm) to 21⁄2 in. (64 mm) in diameter. A 2 in. (50 mm) diameter at the nozzle is most common. Wet-mix shotcrete pumps wet concrete through the line. Dry-mix conveys dry concrete materials through the line and adds water at the nozzle to make concrete. Using proper concrete materials, equipment and placing techniques both wet-mix and dry-mix should provide high strength, durable concrete for your pool. Wet-mix is more difficult to pump as it has more internal friction pushing the low-slump concrete through the line. Shotcrete contractors will try to minimize the delivery line length in wet-mix to make pumping easier and reduce the chance for plugs in the line. With proper planning, concrete mixture selection, and delivery line choices, wet-mix shotcrete can be pumped 500 ft (150 m) or more. The shotcrete contractor can increase the pumping distance by using steel pipe for most of the delivery line with rubber hose the last 50 to 75 ft (15 to 23 m), or using larger diameter hose or pipe and then reducing to the final size closer to the nozzle. If the shotcrete contractor is using dry-mix (often called Gunite) they should easily be able to reach over 500 ft as the delivery hose is mostly full of air conveying the dry concrete materials. This means you don’t have nearly the internal friction that would tend to cause plugging of the line, and thus allows for longer delivery lines.
Thus, if using wet-mix, check with the shotcrete contractor and see if they can make adjustments to their delivery lines to allow a greater pumping distance to accommodate your lot. If using dry-mix, they should have no problems running the hose to your back yard assuming they have enough hose.PoolQ I am looking for some technical assistance concerning temperature guidelines for shooting gunite pools. Is there a suggested range of air temperature and humidity that is recommended? Thanks for your help!
Shotcrete is a method of placing concrete. The basic guidelines for placing concrete or shotcrete can be found in documents ACI 305R-10, “Guide to Hot Weather Concreting,” and ACI 306R-10, “Guide to Cold Weather Concreting.” There are some basic “rules of thumb,” such as 40°F (4°C) and rising for starting concrete/shotcrete operations and 40°F (4°C) and falling for stopping concrete/shotcrete operations. With proper planning and procedures, concrete or shotcrete can be placed at below-freezing temperatures and at very hot temperatures, but only with proper planning, procedures, and likely at some cost.
PoolQ I am not sure if this is the right place to go… 15 months ago I contracted with a company to remove my pavers, pour a concrete pad, and install spraycrete. It was beautiful! The contractor even has photos on his website. Now, I have hairline cracks all over the place and there are more weekly. My contractor is definitely giving me the runaround regarding this. I explained that I would not have paid more than $16,000 if my pool deck would be cracked a year later. Hurricane Irma took out my pool cage and the insurance money was not enough to replace it, so I used the money for my pool deck. Can someone please tell me what to do? Should these hairline cracks be everywhere? What is the lifespan of the material? Help!
Spray-crete is NOT shotcrete. Shotcrete is high-velocity placement of concrete in thicker structural sections. It appears Spray-crete is a low-velocity sprayed mortar product generally applied in a very thin layer to provide texture to an existing concrete substrate. Since you mentioned the underlaying concrete pad was cast and then the Spray-crete added the cracking could well be originating in the underlaying concrete. Concrete cracks for a variety of reasons, such as drying shrinkage, thermal volume change (summer/winter cycles), inadequate curing, insufficient reinforcing steel, or settlement of the subgrade. You should locate a local professional engineer experienced in concrete slab evaluation who can evaluate your site, materials used, application techniques, and potential causes of the cracking. You may find the Florida Engineering Society and ACEC-FL has a list of firms who offer evaluation services.
PoolQ I am reconfiguring the interior of a spa and am wondering if drains and jets can be relocated without compromising the overall structure and getting cold cracks. Can the entire interior be re-shot to maintain the monolithic form and guarantee against failure? Is there an independent professional who could conduct an on-site inspection and recommend a next step?
We are not able to advise you on the structural integrity of a remodel of a spa or any other structure and would suggest you consult with a local engineer who is familiar with pools and spas. Shotcrete is often used to overlay or patch structures and the success of such overlays and patches is highly dependent upon the quality of the surface preparation prior to the application of the shotcrete. With respect to referrals of independent professionals, we would suggest that you use the directory of Corporate Members in the ASA Buyers Guide.
PoolQ I am working on a water feature formed out of cast-in-place reinforced concrete with a hot-fluid-applied waterproofing system over the concrete. To protect the waterproofing, we plan to install shotcrete over it. What minimum thickness of shotcrete is required? Would welded wire fabric or fiber mesh be required as well?
In general, we would recommend a minimum of 2 in. (50 mm) of shotcrete. Either fibers or wire mesh or both should be used in this application. Please note that there are different types of synthetic fibers (microsynthetic and macrosynthetic). Refer to ACI 506.1R for information on fiber-reinforced shotcrete.
If the surfaces are steep or vertical, wire mesh should be used, but provisions need to be included to stabilize the wire mesh. This would likely require attachment points through the waterproofing system.
PoolQ I am working with a private club in Connecticut and we are are trying to determine what the life expectancy is of a concrete commercial pool shell. There is no evidence of failure or cracking and the pool surface is painted, not plaster. We think it is reinforced shotcrete. Are there any tests or rules we should take into consideration?
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.
PoolQ I had a concrete pool shell installed using gunite (dry shotcrete method) in July 2013. It was never finished due to unfortunate circumstances and has been exposed to the elements of weather over the last 2 years, mostly filled up with water from rain and, in the colder months, frozen like a pond. We would like to finish the pool but were told by the pool company that the concrete looked odd and we should have it strength tested. We had core samples taken from the walls and floor from a certified testing lab. The results from the six samples ranged from 1700 to 2200 psi (12 to15 MPa). When the pool was blown on July 3, 2013, it was to achieve 4000 psi (28 MPa) compressive strength in 28 days. Is it normal for the shotcrete strength to have weakened so much?
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.
PoolQ I have a newly constructed in-ground pool in which shotcrete was used. The pool has been holding dirty water since just after the shotcrete cured. Does the shotcrete have to be cleaned and/or treated before an overlay is applied?
To ensure a good bond between the shotcrete shell and the overlay, the surface should be cleaned and allowed to dry before application of the overlay material.