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.
- Air entrainment increases the workability. The small air bubbles act as a form of lubricant to ease internal friction between the concrete mixture components.
- 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.
- 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.
- 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 184.108.40.206 and 220.127.116.11 that state:
“18.104.22.168 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.
“22.214.171.124 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.