We have a pool designed with the cast-in-place concrete construction method in mind. The project has been awarded to a dry-mix gunite contractor. To accommodate the contractor’s placement method, we have been working with him on the details. They are planning on casting the floor and shooting the walls. There are two main areas of concern/questions that we have. First is in regard to the air entrainment and the admixtures that are appropriate for gunite. They have not used air-entrainment admixtures prior to this project. What is the effect to durability without using air? What are the workability effects of adding air entrainment? Which product is recommended? Second, we have specified a hydrophilic waterstop between the cast-in-place floor and the wall. With the walls using a gunite application, what is the best method for preventing water infiltration in the construction joint? Does it hurt the integrity of the joint by installing a hydrophilic waterstop? If the water stop is omitted, what does the surface roughness need to be to provide a monolithic-type connection?

Air entrainment will generally slightly reduce the compressive strength of concrete, but significantly increase the resistance to freezing-and-thawing exposure. Dry-mix shotcrete (gunite) is generally a very paste-rich mixture. With modern cements, the normal 28-day compressive strengths easily exceed commonly specified compressive strengths. ASA recommends a minimum of 4000 psi (28 MPa) for shotcrete, and 4000 psi to 5000 psi (28 to 34 MPa) strengths are routinely specified.

1. Air entrainment increases the workability. The small air bubbles act as a form of lubricant to ease internal friction between the concrete mixture components.
2. You should contact one of our material supplier members to see what they offer. You can use our Buyers Guide at www.shotcrete.org/products-services-information/buyers-guide/, and limit your search to “Admixture Sales” with the “Air-Entraining” subcategory.
3. Quality shotcrete shot against a properly prepared concrete substrate should produce a watertight interface. The hydrophilic waterstop at the joint could be considered a secondary method of making the joint watertight. Though not necessary, it is kind of a “belt and suspenders” approach with a relatively low cost to place.
4. In shotcrete construction, surface preparation between layers to provide full bond is important. ACI 506.2-13, “Specification for Shotcrete,” specifically addresses this in the requirements of Sections 3.4.2.1 and 3.4.2.2 that state:

“3.4.2.1 When applying more than one layer of shotcrete, use a cutting rod, brush with a stiff bristle, or other suitable equipment to remove all loose material, overspray, laitance, or other material that may compromise the bond of the subsequent layer of shotcrete. Conduct removal immediately after shotcrete reaches initial set.
“3.4.2.2 Allow shotcrete to stiffen sufficiently before applying subsequent layers. If shotcrete has hardened, clean the surface of all loose material, laitance, overspray, or other material that may compromise the bond of subsequent layers. Bring the surface to a saturated surface-dry condition at the time of application of the next layer of shotcrete.”

For more details on bond between shotcrete layers, you may want to refer to an article in the Spring 2014 issue of Shotcrete magazine, “Shotcrete Placed in Multiple Layers does NOT Create Cold Joints.” A PDF of the article can be found at www.shotcrete.org/wp-content/uploads/2020/01/2014Spr_TechnicalTip.pdf.