Shotcrete is a placement method for concrete. Dry-mix shotcrete (the old tradename is gunite) using proper materials, equipment, and application techniques should have easily been able to reach a strength of 4000 psi (28 MPa) in the first month. After 10 years in-place the concrete should be even stronger. The level of concrete strength developed by quality shotcrete should easily accommodate drilling in anchors or concrete screws. In my experience it would have been highly unusual to build an entire 10-story building with shotcrete even in the 1960s. I’d suggest based on the extremely weak material properties and the wall turning to dust what you think is shotcrete may be sprayed plaster or stucco. Those materials don’t have near the same strength as shotcrete and would exhibit many of the problems you have mentioned.
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.
I’m hoping you’d be willing to answer a couple of questions I have about gunite. I am having a swimming pool installed at my house in Florida. I was away when the gunite was shot a few weeks ago and didn’t know at the time that the gunite should be sprayed with water for a few times a day for about a week according to what I have read online. The pool company owner knew I would be away and never mentioned the need for the gunite to be periodically moistened. The owner also made no provision for any of his employees to hose it down or install a sprinkler. When I learned after the fact of the watering requirement and asked him about it, he said it was unnecessary because the gunite was shot at 4000 psi (28 MPa) and not the “industry standard” of 3000 psi (21 MPa). Then he added that with the almost daily rain in Florida at this time of year, all was okay. I’m concerned about the gunite’s integrity—its permeability and the possibility of shrinkage and cracking. Could you tell me if I have a reason to be concerned, and if so, what do you suggest I should do about it?
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.
I have a project where we are designing a shotcrete wall. The contractor plans to mix the shotcrete on site and we need to work with them to come up with a mixture design. Do you have any suggested wet-mix shotcrete designs for on-site mixing applications or could you point me where some may be published?
Shotcrete is simply a placement method for concrete. Most wet-mix shotcrete contractors use a 2 in. (50 mm) diameter delivery hose, so maximum coarse aggregate size should be limited to 3/8 in. (10 mm) nominal. Pumpability usually requires a good paste content. Here’s a link to an article, “Understanding Wet-Mix Shotcrete: Mix Design, Specifications, and Placement,” that should answer many of your questions. It is rare to have wet-mix concrete mixed on site from bulk aggregate and cementitious materials. Most site-batched wet-mix uses dry prepackaged materials that have metered water addition to provide a specific water-cementitious materials ratio (w/cm) for the provided bagged mixture. We also see volumetric mixers used on site that can precisely meter the dry concrete materials and water often with needed water-reducing air-entraining admixtures.
We are applying shotcrete in a slope (8 in. [200 m]) to make it stable. The shotcrete wall has a length of 330 ft (100 m). Do we need to specify construction and an isolation joint? In case it is required as well as an isolation joint, does the separation between joints need to be 30 ft (9 m)? Where can I find information about joints for shotcrete?
Shotcrete is a placement method for concrete. Thus, you should place movement joints (contraction, expansion, isolation) as would be required by your local design codes for concrete. Joint spacing will depend on the amount of reinforcement used in the section to resist temperature and shrinkage volume changes. Construction joint spacing can be determined by the contractor depending on their production rates. Properly prepared construction joints will act as monolithic concrete as long as the joints are properly prepared and proper concrete materials, equipment, and placement techniques are used by the shotcrete contractor. Construction joints should be roughened, cleaned, and then dampened to saturated surface-dry conditions before subsequent shotcrete placement.
The U.S. Bureau of Reclamation has a nice summary document that you may find helpful in your design.
I place shotcrete and I use the wet-mix method. I have been asked by a contractor to repair a pool that was shot with the dry-mix method. He is having trouble convincing his client that with the proper preparation we can shoot the repair with the wet method. Am I missing anything?
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.
When applying 6 in. (152 mm) of shotcrete on an exterior concrete surface, how long should that cure before coating with an acrylic elastomeric roof coating? What is the moisture content of that thickness after 7 days, or after 14 days?
Shotcrete is just a placement method for concrete. Thus, the in-place material will have the same characteristics as concrete. You should consult the coating supplier to establish how long they want the concrete surface cured. They may also specify a certain limit for surface moisture conditions. There are several methods for evaluating the moisture content of the in-place concrete. Here are the ASTM standards that deal with surface moisture:
- ASTM F710, Preparing Concrete Floors to Receive Resilient Flooring; Section 5.2, pH Testing;
- ASTM F1869, Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride;
- ASTM F2170, Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes;
- ASTM F2659, Preliminary Evaluation of Comparative Moisture Condition of Concrete, Gypsum Cement and Other Floor Slabs and Screeds Using a Non-Destructive Electronic Moisture Meter; and
- ASTM F3191, Field Determination of Substrate Water Absorption (Porosity) for Substrates to Receive Resilient Flooring.
In addition, The International Concrete Repair Institute (ICRI) has a certification program, “ICRI Concrete Slab Moisture Testing Technician—Grade 1.”
How thick would you recommend a shotcrete application be to make bedrock reservoir waterproof?
There are many variables that would need to be considered to answer your question. Water flow, depth of the application, and overall geometry can influence the required thickness. This is a question that should be addressed by an experienced engineer who can ascertain the required structural properties to resist the hydraulic loads for the depth and geometry of your application. You may find consultants who are ASA Corporate Members in our Buyers Guide.
I am helping to design radiation shielding for a cyclotron and we do not have much space. The machine produces both neutrons that must be shielded for as well as gamma rays. I have been told that we could get a density of 3.2 g/cc, which helps for gamma rays, but I need to find out more about the water content of the finished product for the neutron shielding part so I can scale between regular 2.4 g/cc concrete and the high-density shotcrete product. Do you have knowledge of this?
Shotcrete is a placement method for concrete. We have shot a wide variety of concrete mixtures from lightweight to heavyweight to the use of alternative cements. Reviewing ACI 304.3R-96, “Heavyweight Concrete: Measuring, Mixing, Transporting, and Placing,” the key to producing heavyweight concrete is the aggregates used. The document also mentions that more cement paste is needed in heavyweight concrete and that helps to increase pumpability, which is important to use with shotcrete placement. Shotcrete will allow you to minimize or eliminate formwork, so it may have benefits of reducing cost and time for construction. Wet-mix shotcrete generally has a low water-cementitious materials ratio (w/cm) of 0.38 to 0.45. Dry-mix shotcrete tends to be even lower with a 0.35 to 0.40 w/cm. Shotcrete materials achieve this using high-range water-reducing admixtures. As concrete hydrates and gains strength, the available free water in the concrete is consumed, and so may be less of a problem for shielding. Reviewing the 304.3R document, it appears reaching the 3.2 g/cc density is fairly easy, and even higher densities may be achievable. You may want to review the ACI 304.3R document, as it provides a good background of heavyweight concrete mixture design and use.
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.