What are the advantages of shotcrete over other structural concrete solutions?
Access, strength, prevention of corrosive materials, cost, environmental, low-impact

What is the durability and expected life-span of shotcrete?
The durability of shotcrete is equivalent to high-quality concrete. The life span of a new construction shotcrete structure is more than 50 years. For rehabilitation projects, shotcrete can extend the life of a structure by more than 25 years.

Keys to a long-lasting product are 1) proper surface preparation, 2) appropriate aggregate and ad-mixtures, 3) quality of materials, and 4) skilled placement. Be sure your shotcrete contractor has proven experience for your specific situation.

What is the strength of shotcrete?
Shotcrete is generally stronger than poured concrete because the method of pneumatically applying concrete reduces the water-cement ratio. Shotcrete typically reaches a strength of 3,000 psi in 24 hours or less, and a 28-day strength of 5,000 psi or greater. Ad-mixtures, such as silica fume, can increase the strength to as much as 8,000 psi.

We used shotcrete to repair our concrete structure a few years ago and now it is falling apart. Doesn't this mean that shotcrete doesn't work for my situation?
While this situation can occur, it is rarely due to an inappropriate use of shotcrete. Usually, shotcrete is chosen in situations that demand its superior compressive strength, excellent bond, prevention of corrosive agents, or ability to be applied under severe access and environmental conditions. If a shotcrete project begins to "fall apart" in a few years, it is most likely due to poor workmanship. Potential causes are:

1) surface preparation (i.e. the corroded rebar and/or steel was not sufficiently removed, and/or the deteriorated concrete was not completely removed prior to applying shotcrete)

2) inappropriate aggregate was used which allowed for undesired water penetration (ASTM C-33 concrete sand should be used for the correct gradation of aggregate)

3) poor placement of shotcrete (inappropriate technique used to place shotcrete and/or inexperienced nozzleman)

What is the difference between shotcrete and gunite?

There is really no difference between shotcrete and gunite and the terms can be used interchangebly. Shotcrete has been used as a generic term to describe the process of pneumatically applying concrete. Recently, some have defined shotcrete as utilizing wet-mix application and gunite as utilizing dry-mix application.

What is the difference between wet-mix and dry-mix applications?

Both place concrete by pneumatically projecting the material from a hose. The difference is when water is added to the material. In a wet-mix application, all materials - including cement, aggregate, ad-mixture and water - are mixed together before being pumped through a hose and pneumatically projected. Typically, a concrete truck supplies the wet-mix and it is pumped through a shotcrete rig for placement. Generally, wet-mix is used for large volume placements with easy access for construction vehicles.

In dry-mix applications, all dry materials - including cement, aggregate, and ad-mixture - are mixed together, conveyed pneumatically through a hose and then, at the nozzle via a water ring, water is injected evenly throughout the mix as it is being projected. Generally, dry-mix is used for small to medium volume placements or for conditions with limited vehicular access.

How does shotcrete bond to my existing structure? Are bonding or adhesive agents needed to ensure a long-lasting remedy?

When the existing concrete structure is properly chipped or prepped, the newly applied shotcrete adheres easily to the concrete structure due to the force of the pneumatic application, small particle size used in the aggregate, and lower water-cement ratio. The shotcrete enters the pores of the chipped concrete and "grabs" on well. No bonding or adhesive agents are necessary or recommended.

How much space is needed for setup of equipment?
The only space needed is just enough for a gunite rig and air compressor, which is about the equivalent of 2 pick-up trucks.

Are any additional resources needed on-site for shotcrete operations?
All that is necessary is a nearby water connection that can be accessed, although in certain circumstances a water truck could be utilized if water is not available by any other means.

How soon can water flow in an aqueduct or sewer that has been lined with shotcrete?
24 hours after placement of shotcrete

What is the smallest diameter pipe that can be repaired?
42" diameter pipe, for safety purposes

What are the environmental benefits of shotcrete?
Shotcrete can be used to rehabilitate existing structures as opposed to demolishing a structure, which ends up in a landfill, and rebuilding the structure from new materials.

What are the limitations of shotcrete?
Underwater placement, pipe size

How do I specify shotcrete for my projects? What resources are available for developing performance specificaitons for shotcrete?
ACI 506.2 - Specifications for Materials, Proportioning and Application of Shotcrete

ACI 506R-05 - Guide to Shotcrete

What sort of certifications are there for shotcrete contractors? Is it important to require certification?
Yes, it is important to require certification.

ACI C660 - Certification of Nozzleman

Where do I go to learn about shotcrete?
American Shotcrete Association (ASA) website: www.shotcrete.org

How is shotcrete "green" technology?
1) Utilizes local materials

2) Not much formwork, if any, required so trees are being saved

3) Sustainable