Gunite, the dry mix process, is used most often for swimming pools. It can also be used for structural applications, but the wet mix process is more common for structural uses. As with all concrete construction, success depends on good design and implementation. Shotcrete is generally higher quality concrete than CIP. Most structures that can be built with CIP can be built with shotcrete.

In general concrete construction design for jointing, concrete handling, hot and cold weather and curing are identical. Shotcrete is only different in how it is placed.

The concrete used in shotcrete and CIP is virtually identical.

Shotcrete is used often. But, it depends on where you work or practice and what industries you serve. In addition to that, shotcrete does have barriers to entry for the installer. The successful use of shotcrete requires significant capital investment, extensive safety and use training, knowledge of mix design, commitment to R&D and resources for education of clients and designers who may have a need for shotcrete.

Shotcrete is well suited for horizontal, vertical and overhead work. We use a practical limit of 90 CY per day per team. Applications requiring higher volumes can be appropriate with low finish requirements.

Slabs on grade and slabs on deck are not good fits for shotcrete. Concrete walls and footings with greater than 36” thickness can be difficult to shoot in one pass. Footings placed separately from the walls do not make sense for shotcrete.

Most of our work involving shotcrete consists of one sided wall construction, soil stabilization, underpinning, two sided wall construction, tunnels, and structural stabilization. The list continues to grow. While two of our nozzlemen have extensive experience with pools, skate parks and free form art, as a company we do not currently do this type of work.

Shotcrete is tested using a test panel. The test panel has standard dimensions. It can include a preset rebar grid or a grid that represents the actual work being shot on site. After the test panel has hardened, cores are taken and shipped to the lab to be broken in similar fashion to molded cylinders used for CIP testing.

We often test shotcrete using a traditional cylinder mold taken at the truck or at the end of the nozzle as well. We have not seen substantial differences in results among the different methods.

Yes, and maybe no. Building with shotcrete requires a high pressure pump, a large air compressor, pipes and hoses, different nozzles and clamps. Admixtures may be used to accelerate the set of the concrete, slow the set of the concrete, raise the slump, lower the slump, and add silica fume. Most times our standard proprietary shotcrete mix reaches 5,000 PSI in 7 days. Prepackaged mixes are available that attain 3000 psi in under 4 hours.

ACI 301 2020, Specifications for Structural Concrete, ACI 318 2019 Building Code Requirements for Structural Concrete, ACI 305R, Guide to Hot Weather Concreting, ACI 305.1, Specification for Hot Weather Concreting, ACI 306R, Guide to Cold Weather Concreting, ACI 306.1, Standard Specification for Cold Weather Concreting, ACI 506R, Guide to Shotcrete, ACI 506.1R, Guide to Fiber Reinforced Shotcrete, ACI 506.2, Specification for Shotcrete, ACI 506.4R, Guide for the Evaluation of Shotcrete, ACI 506.6T, Visual Shotcrete Core Quality Evaluation, ACI CCS-4, Shotcrete for the Craftsman, ASTM C1140, Standard Practice for Preparing and Testing Specimens from Shotcrete Test Panels, ASTM C1604, Standard Test Method for Obtaining and Testing Drilled Cores of Shotcrete, ASA, Saftey Guidelines for Shotcrete.

Done well, shotcrete usually has fewer problems that traditional CIP work. The majority of concrete problems come from the use of excess water in the concrete mix. This leads to low strength, high shrinkage, excessive cracking and high permeability.

By its nature, shotcrete needs to have a very low W/C ratio. This eliminates most potential problems from over watering. That being said, there are some challenges unique to shotcrete that must be managed for a successful outcome.

One potential problem is ghosting. This is a phenomenon where the rebar in the piece to be shot is not properly encapsulated. This can be caused by excessive rebar congestion, poor nozzle technique, an excessively low slump or vibration of the reinforcing grid. As long as the piece is shot correctly and the reinforcing cage is rigid ghosting is not a problem. Correct rebar encapsulation will be verified in the test panel. In pieces containing large or tightly spaced rebar grids non contact lap splices should be used to minimize the chance of ghosting. Another common challenge with shotcrete is sand pockets caused by rebound. Rebound is the amount of material that bounces off of the piece to be shot. Once shotcrete is rebounded it should not be used in the final work. It will lack sufficient cement paste to form a cohesive concrete matrix. The percentage of placed shotcrete for the wet mix process that becomes rebound is around 12%. This amount is even lower with mixes containing silica fume. In larger scale projects such as foundation walls the rebound must removed from the area immediately ahead of the nozzleman with an air lance or blowpipe.

In smaller pieces such as infills, underpinning or similar the volume of rebound is so small that good nozzleing technique is all that is required to prevent rebound build up. A typical infill or underpin piece of 3’ width x 4’ height is generally shot in about 100 seconds. This fairly swift time window does not allow a significant amount of rebound buildup. As with rebar ghosting, rebound build up will be evident in the test panel.

Shotcrete is high strength concrete. The lowest PSI shotcrete mix we use 4000 PSI with a W/CM ratio of .38. This typically breaks are well over 6000 PSI at 28 days. Assuming, however, it does break at 4000 at 28 days the strength gain is 6 PSI per hour. This equates to 864 PSF per hour or harder than most ground soils in four hours at almost 3,500 PSF. However, this logic is seriously flawed and conservative to the point of misleading. Concrete strength gain is decidedly nonlinear. Typically strength is gained rapidly in the first 7 days and slows considerably thereafter. Most of our 4000 PSI shotcrete tests break at over 5000 PSI in 7 days.

Using these numbers, the concrete gains almost 30 PSI per hour or 4,320 PSF per hour. In other other words, harder than most ground soils in 60 minutes after placement. Even this estimate is low because it still ignores the fact the concrete strength gain is far from linear. It is this unique property of shotcrete that makes it an ideal material for overhead use, structural support in mines, tunnels and for underpinning.

Hire the right company with the equipment, experience and staff to do the job. In addition to an ACI certified nozzleman on site, the team itself needs to have the experience to deliver the required quality level. A certified nozzleman is required to have at least 500 hours before completing the examination for certification. We currently have one certified nozzleman and two nozzlemen in training. In addition we hire a nozzleman with over 44,000 hours on the nozzle for challenging or unique projects. He also consults for us on creating execution plans and handles our continuing education. 

If it looks like a mess, it probably is. Keep in mind, however, shotcrete is not a back the truck in and start dumping in a wheelbarrow proposition. Many times the mix will need to adjusted. It may be too stiff or it may be too loose. Sometimes impurities in the concrete mix cause the pump line to jam.

Although shotcrete uses a 3/8” (#8) maximum course aggregate size, this does not mean that a truck will not have larger stone left over from a prior batch or perhaps pieces of concrete that break off from the mixing fins. But, once the process us up and running, it should be smooth and rapid. In addition, inspect the work after it is complete.

Does the concrete look dense? After the forms are removed does it look consolidated? If the work is segmented such is as the case for infills or underpinning, what does the interface between the shotcrete and the supported building look like? Is it tight? Done correctly, there will be little to no space between the shotcrete and the area supported. This is typical for the full depth of the infill or underpin. This is in direct contrast to the traditional use of a nonshrink grout joint between the old work and the new CIP concrete support. In many cases this grout joint looks super at the surface, but 2’ back into the pin? No contest is typically the reaction once most professionals see the results of shotcrete vs CIP for infills or segmental supports such as underpins. Finally, evaluate the test panel. How does it look and what do the breaks tell you?