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.

We are working on a repair/renovation project in Boston, MA. A long, concealed wall next to an adjacent property is now visible, as the adjacent property is being renovated. We have been told that our wall must now have a 2-hour fire rating. Our wall is comprised of concrete masonry units (CMUs) and exposed structural steel members. Applying shotcrete to the CMUs and steel is a good solution for several reasons. Can you provide or point me to a shotcrete specification that will have a 2-hour fire rating on CMUs and structural steel?

Shotcrete is a placement method for concrete. Thus, the fire resistance for shotcrete placement is the same as concrete. The primary reference for fire resistance of concrete is ACI 216.1-14(19), “Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies.”

I have a 24 in. (610 mm) thick shotcrete wall that needs to be scanned for voids. The project has been struggling to locate a local expert who has the capability to scan this thick of a wall. In addition, this new 24 in. thick shotcrete wall was dowelled and made an “as-one-unit” together with an existing 24 in. form-and-pour wall. Any thoughts and ideas would be greatly appreciated.

Shotcrete is a placement method for concrete. Thus, all nondestructive testing (NDT) applicable to concrete walls would be usable on your wall. However, it is difficult to get good results with a scanning system for heavily reinforced concrete walls of your thickness. Impact echo and impulse response are two one-sided techniques that can provide good results for a portion of the 24 in. thickness, although they would likely not be able to scan the entire depth. Ultrasonic pulse velocity is a potential if you can access both sides of the wall. For one-sided investigation at greater depth, you may be able to use a MIRA system. It is a sophisticated tomographic system that says it can test from 50 to 800 mm (32 in.) thickness. There are national consulting firms that provide these investigation systems. Each requires a highly trained, experienced operator—so be sure to verify the firm can document successful experience using the method.

A contractor has proposed using shotcrete to repair the concrete in the elbows of a draft tube. I have not heard of shotcrete being used in a draft tube. Velocities would range from 10.5 to 3.8 ft/s (3.2 to 1.2 m/s). I am concerned about whether the shotcrete would delaminate after time or be abraded away, as there is considerable abrasion present where the concrete cover has been abraded away on the floor of the elbow. Any guidance would be helpful.

Shotcrete has been used in many dam repairs, including large-diameter draft tubes. Shotcrete is high-velocity (60 to 80 mph [100 to 130 km/h]) placement of concrete. When shotcreting with proper concrete materials, equipment, placement, and curing techniques, along with complete surface preparation, you can expect a tensile bond strength of at least 150 psi (1 MPa) between the existing concrete and the newly shotcreted material. Original Portland Cement Association research by Felt from 1956 showed that 200 psi (1.4 MPa) bond shear strength is required for bonded overlays to act monolithically in flexure. Research by Silfwerbrand in 2003 showed that the ratio of bond shear strength to direct tensile bond strength ranges from 1.9 to 3.1. Thus, using the low value of the range with a 150 psi tensile bond strength yields a shear strength of at least 285 psi (2 MPa), well above the 200 psi needed. You may find more information on the bond between concrete and shotcrete layers in the article “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints” that can be found in our article archive.

Regarding the abrasion, shotcrete displays good toughness in a wide variety of demanding applications. Quality shotcrete should have at least a 4000 psi 28-day compressive strength and, with attention to mixture design using silica fume and a low water-cementitious materials ratio (w/cm), can comfortably reach 6000 to 8000 psi (40 to 55 MPa) or more. Shotcrete also can easily use steel or synthetic fiber to significantly increase the toughness of the in-place concrete.

Finally, because shotcrete requires no formwork or bonding agent for a high-quality repair, you will find the shotcrete process provides an economical solution.

An article about a draft tube modification project can be found in our article archive.

We are constructing a new custom roundabout with water running through the bridges on the Coast of Zintan, Tripoli, Libya, using a three-dimensional (3-D) panel system. It’s basically a system with an expanded polystyrene (EPS) panel with a wire mesh and shotcrete on both sides. Because of the heat, sea salt, and high humidity of the region, we are looking for a mixture formula for a waterproof shotcrete for the exterior coating with the right aggregate size to help prevent moisture migration to the interior and prevent cracks. Do you have any recommendations for the shotcrete?

If looking for a low-permeability concrete mixture for shotcrete placement, you should consider using supplementary cementitious materials (SCMs) such as silica fume, fly ash, or slag to reduce the permeability. You should also be sure to require a minimum 4000 psi 28-day compressive strength to assure good paste content and the ability to fully encase your reinforcement. The addition of microfibers and early wetting of the finished shotcrete surface will help to reduce the potential for early-age plastic shrinkage cracking. Proper curing for at least 7 days is also important to help increase strength gain and reduce the potential for drying shrinkage cracking. You can consult our Buyer’s Guide to locate our corporate members, who may consult with you on the mixture design. However, please be aware that many of the panel systems with an EPS core don’t use high-velocity (60 to 80 mph) shotcrete for consolidation and compaction of the sprayed concrete, but use a low-velocity sprayed mortar (LVSM). Because LVSM doesn’t have the compaction of shotcrete impact, it depends on a more sophisticated and expensive cementitious mixture, often with a latex or other admixture to improve adhesion and reduce permeability. You’ll need to consult with the material supplier of the LVSM product to verify the permeability of their in-place product.

We’re building a pool 25 x 45 ft (7.6 x 14 m) and had a massive cave-in on our deep end. The builder wants to build temporary walls to shoot the shotcrete against, then remove the plywood walls and backfill with gravel. I’m wondering if this will work and if they will be able to remove the plywood without damaging the shotcrete walls. I know with typical forms you would prep the form with oil so the concrete doesn’t stick. Would that be necessary for shotcrete, too? Also, is there a recommended wait time for curing before we backfill?

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.