Frequently Asked Questions

1. What is shotcrete?

Shotcrete is a term that is used to describe concrete that is pneumatically projected at high velocity onto a surface. Shotcrete is typically classified into two categories:

  • wet mix process: concrete is batched, delivered to site, and pumped to the shotcrete nozzle where compressed air is added to the flowing material just prior to shooting.
  • dry mix process: concrete ingredients excluding water are mixed and delivered to the shotcrete nozzle using compressed air. Water is added to the dry materials at the nozzle, just prior to shooting.

Structural shotcrete is typically completed using a wet mix process however, for remote areas or limited quantities, dry mix may be considered due to its versatility and lack of reliance on the availability of ready mix concrete.

2. Are there any special design considerations for shotcrete?

Shotcreting is a method of placing concrete – when properly executed the final product is equivalent or superior to formed concrete. No special design considerations are required – CSA 23.3 and 23.4 apply to shotcrete works.

Shotcrete mix designs can be proportioned to meet specified performance requirements including chloride, sulfate, 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.

 

3. What are the similarities/differences between a structural shotcrete mix design and a typical formed wall mix design?

Compressive Strength: structural shotcrete is designed to meet or exceed specified compressive strength requirements.

Cement Content: shotcrete mixes are typically high cementicious mixes (420kg/m3 to 450kg/m3) compared to conventional formed wall mixes (310kg/m3 to 330kg/m3).

Water to Cement Ratio: due to the nature of placing shotcrete, mixes are typically designed with a low water to cement ratio (maximum 0.5) to maintain a high degree of plastic material cohesion. As with any properly proportioned concrete mix, lower water to cement ratios will typically yield improved strength, permeability, and durability results.

Aggregate: Shotcrete differs from conventional mixes in aggregate makeup.

Typical concrete mixes contain high volumes of coarse aggregate (40 to 70% volume/volume of concrete depending on nominal maximum aggregate size) and only moderate volumes of sand fill (5% to 10% volume/volume of concrete). Wall mixes will generally utilize a 20mm nominal maximum size of coarse aggregate.

Shotcrete mixes contain relatively high volumes of sand (40% to 60% volume/volume shotcrete) and moderate volumes of coarse aggregate (10% to 20% volume/volume shotcrete). Further, coarse aggregate size is generally limited to nominal maximum size of 14mm.

4. How does shotcrete compare to conventional formed wall construction 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 (increased shrinkage). The primary factors that affect water demand are water to cement ratio and cement paste demand. Cement paste demand is related to the surface area of aggregate being used. 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 of the mix. As required strength increases, the w:c ratio generally decreases.

Shotcrete mixes are generally designed with a maximum nominal aggregate size of 14mm. Compared to a typical wall mix which is designed with a maximum nominal aggregate size of 20mm, a 14mm mix has a relatively higher cement paste demand. Due to the increased cement paste demand, at equal water to cement ratio’s a 14mm mix has a higher total water demand than a 20mm mix. 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 wall mixes;
  • lower slump requirements;
  • use of low water demand supplementary cementing materials (slag);
  • use of supplementary wire mesh reinforcement;
  • use of fiber reinforcement to control shrinkage; and
  • use of shrinkage control admixtures.

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

5. Are control joints required?

Control joints cannot be easily added to formed walls – without control joints, shrinkage cracking will not be contained within a set pattern. In shotcrete applications, because the face of the wall is accessible, it is possible to introduce regular control joints (tooled or cut) to contain shrinkage cracking.

6. How are cold joints treated?

Cold joints in shotcrete walls are similar to cold joints in formed walls. Depending on the designer’s requirements, a key may be required at cold joint locations. Similar to formed walls, exposed reinforcing steel that is not being cast in concrete must be kept clean or cleaned following concrete placement.

7. What are curing requirements for shotcrete?

Shotcrete should be cured in the same way as any type of exposed concrete. Formed walls are typically not wet cured as they have formwork in place to guard against excessive water loss. Shotcrete can be cured in any or a combination of the following ways:

  • continuous misting – best practice
  • wet medium (i.e burlap) – good practice
  • curing compound – acceptable method

We recommend wet curing (misting and/or wet medium) for the first 4 to 7 days where possible.

8. What precautions need to be taken for cold weather and hot weather shotcreting?

Cold Weather: typical precautions including adjustment of mix to reflect cold weather (warm mix water, accelerated mix) and protection of work area. Similar to formed wall construction, the fresh shotcrete needs to be insulated or heated to protect against freezing and to maintain temperatures required for design strength gain. In the absence of formwork to secure insulation to, heated enclosures or draped insulating blankets are used.

Hot Weather: typical precautions including adjustment of mix to reflect 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 (if finishing is not immediately possible) 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.

 

References:

  • “ACI 209.1R-05 – Report on Factors Affecting Shrinkage and Creep of Hardened Concrete” ACI Manual of Concrete Practice. Comp. American Concrete Institute. 2012 ed. Vol. 6. Farmington Hills, MI: American Concrete Institute, 2012.
  • “ACI 306R-10 – Guide to Cold Weather Concreting” ACI Manual of Concrete Practice. Comp. American Concrete Institute. 2012 ed. Vol. 6. Farmington Hills, MI: American Concrete Institute, 2012.
  • “ACI 308R-01 – Guide to Curing Concrete” ACI Manual of Concrete Practice. Comp. American Concrete Institute. 2012 ed. Vol. 6. Farmington Hills, MI: American Concrete Institute, 2012.
  • “ACI 506R-05 – Guide to Shotcrete” ACI Manual of Concrete Practice. Comp. American Concrete Institute. 2012 ed. Vol. 6. Farmington Hills, MI: American Concrete Institute, 2012.
  • Morgan, D. R. “Understanding and Controlling Shrinkage and Cracking in Shotcrete.”Shotcrete: A Compilation of Papers. Farmington Hills, MI: American Shotcrete Association, 2008.