WHAT IS SHOTCRETE?

ARE THERE ANY SPECIAL DESIGN CONSIDERATIONS FOR SHOTCRETE?

Shotcrete is simply a method of placing concrete at high velocity – when properly executed the final product is equivalent or superior to formed concrete. No special design considerations are required.

Shotcrete mix designs can be proportioned to meet specified performance requirements including chloride, sulphate, and freeze/thaw resistance.

​

As with any type of concrete placement, qualified installers, material suppliers, and quality control practices are required to ensure the overall quality of the shotcrete.  Shotcrete is covered in ACI 506 as well as 318.

WHAT ABOUT REBAR ENCAPSULATION?

Rebar encapsulation is monitored visually by the project team throughout shotcreting operation. ACI certified and experienced shotcrete nozzlemen apply a variety of techniques to ensure full rebar encapsulation during shotcreting operations. The advantage of shotcrete over a formed wall construction is that the reinforcing steel is visible until it is encapsulated by shotcrete.

HOW DOES SHOTCRETE COMPARE TO CONVENTIONAL FORMED AND POURED CONCRETE WITH RESPECT TO SHRINKAGE CRACKING?

Shrinkage is caused by loss of water from the concrete matrix. As the water demand of a concrete mix increases, shrinkage characteristics generally worsen which increased shrinkage. As the maximum nominal aggregate size decreases, surface area increases, and cement paste demand also increases. Water to cement ratio is related to the required strength, and durability of the mix. As required strength increases, the w:c ratio generally decreases.

Although shotcrete mixes are typically designed with low water to cement ratios, they are generally designed with a maximum nominal aggregate size of 3/8 inch. Compared to a typical wall mix which is designed with a maximum nominal aggregate size of ¾” inch, a 3/8 inch mix has a relatively higher cement paste demand. Due to the increased cement paste demand, at equal water to cement ratio’s a 3/8 inch mix has a higher total water demand than a ¾ inch mix. Additionally, due to the need for shotcrete mixes to be highly cohesive, total cementicious content is further increased. Because of this, in the absence of any other changes, shotcrete mixes would generally be higher shrinkage mixes compared to conventional formed wall mixes. The shrinkage characteristics of shotcrete are moderated by the following:

• lower water to cement ratio than formed and poured mixes

• lower slump requirements;

• use of low water demand supplementary cementing materials (slag/flyash);

• use of supplementary wire mesh reinforcement;

• use of fiber reinforcement to control shrinkage;

• use of shrinkage control admixtures;

• timely and quality curing.

Properly proportioned, and designed, placed, and cured shotcrete mixes will have similar shrinkage characteristics to a conventional formed wall mix.

​

Slump: A 2″ to 3″ slump is typical for a shotcrete mix, compared to a 4″-5″ slump for a cast in place mix. 

Aggregate Size: The large aggregate size used for shotcrete is typically 1/2″ and smaller. Form-and-pour concrete’s large aggregate is regularly 1″ to 1.5″. The size difference is due to the diameter of the concrete pumping equipment; usually consisting of 2″ hose and steel pipes, and a 2″ diameter shotcrete nozzle.

W/CM: A concrete mix used for shotcrete typically has a lower water-to-cementitious material ratio of 0.35 to 0.45.

​

​

WHAT ARE PRECAUTIONS FOR COLD WEATHER AND HOT WEATHER SHOTCRETING?

Same as for concrete. 

Cold Weather: include adjustment of mix to reflect cold weather (warm mix water, accelerated mix) and protection of work area. The fresh shotcrete needs to be insulated or heated to protect against freezing until it reaches 500 psi and to maintain temperatures required for design strength gain. Heated enclosures or draped insulating blankets are often used.

Hot Weather: cold mix water, ice, retarded mix and protection of work area from direct sunlight. In addition to these precautions, curing may need to begin immediately following placement and continued immediately following finishing. Evaporation reducers may be used to minimize surface evaporation prior to finishing. Micro fibers can also be added to reduce plastic shrinkage cracking in severe environments.

With both you must be mindful of any wind or air draft as this could pull the moisture from your finished face and cause cracking as well.  Wind blocks can be used.

HOW IS SHOTCRETE "GREEN" TECHNOLOGY?

1.  Utilizes local materials.
2.  Not much formwork, if any, so trees are being saved.
3.  Sustainable.

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

HOW DO I SPECIFY SHOTCRETE FOR MY PROJECTS?  WHAT RESOURCES ARE AVAILABLE FOR DEVELOPING PERFORMANCE SPECIFICATIONS FOR SHOTCRETE?

ACI 506.2 - Specifications for Materials, Proportioning and Application of Shotcrete.
ACI 506-16 - Guide to Shotcrete

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.

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, surface saturated, and 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.

WHAT IS THE DIFFERENCE BETWEEN SHOTCRETE AND GUNITE?

There is really no difference between shotcrete and gunite and the terms can be used interchangeably.  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.

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 10,000 psi.

WHAT ARE THE ADVANTAGES OF SHOTCRETE OVER OTHER STRUCTURAL CONCRETE SOLUTIONS?

Access, strength, prevention of corrosive materials, cost, environmental, low-impact.  Improved schedule time and flexibility, elimination of time, labor, and materials related to formwork construction.  No erection and stripping of forms as well as re-finishing.  Elimination of crane requirements related to concrete placement.  Patriot utilizes tow behind pumping equipment for all concrete placement requirements – no crane lifts are required for concrete placement.

HOW CAN SHOTCRETE IMPROVE MY PROJECT SCHEDULE?

How much is a day worth in dollars to your overall project? Shotcrete thrives in today’s construction industry for its ability to accelerate a project’s schedule. Unlike cast-in-place, there are no forms to strip. This means no downtime between shotcrete days. A Contractor’s focus typically shifts from shotcrete production goals to making sure trades preparing the work area are able to keep up. Other time-saving shotcrete advantages include:

• No concrete footings required for wall supports.

• Reduced (or the elimination of) crane time for form setting or concrete pouring.

• Faster concrete set-up reduces labor costs.

• Reduction of forming materials by up to 100%

HOW MUCH SHOTCRETE "WASTE" SHOULD I ACCOUNT FOR?

Shotcrete is a method of placing concrete. It has many advantages when compared to the form-and-pour process. The term, “Waste”, and its association with shotcrete should be analyzed carefully. Shotcrete “waste” is generally comprised of rebound and excess cutting/shaping of the plastic concrete. So long as shotcrete “waste” is within industry-accepted parameters, it should be considered a PART of the shotcrete process, not a detriment to it. 

​

Shotcrete “waste” can sometimes be confused with other factors during the course of work. Is the wall true to thickness? A 12″ specified wall that is 13″ thick in the field results in over an 8% increase in concrete consumption. Finally, if a ready mix truck leaves the site with an extra 1-2 yards in the drum, be sure to not consider that waste. That’s simply an over-order of concrete.

​

A project group should be in alignment from the beginning on shotcrete expectations. This topic is no exception.