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Technical Tip Specified shotcrete bond strength for shotcrete to properly prepared concrete substrates generally ranges from 100 to 150 psi (0.69 to 1.00 MPa). If a curing compound is used on a layer, it should be completely removed before shooting subse- quent layers of shotcrete. 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: “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 Sawed side of shotcrete test panel shot with multiple layers 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 Charles S. Hanskat is Man- material, laitance, overspray, or other mate- aging Principal at Hanskat rial that may compromise the bond of subse- Consulting Group, LLC, a firm quent layers. Bring the surface to a saturated in Northbrook, IL, which he surface-dry (SSD) condition at the time of founded in 2012. He received application of the next layer of shotcrete.” his BS and MS in civil engi- The shotcrete specification is actually more neering from the University of stringent than ACI 318-11, Section 6.4, on con- Florida. Hanskat is a licensed struction joints, because it requires removal of professional engineer in 23 states. He has been all potential bond-breaking materials immedi- involved in the design, construction, and evalua- ately after initial set, as well as the cleaning and tion of environmental concrete and shotcrete SSD conditions provided for in 3.4.2.2. structures for over 35 years. Hanskat is the ASA Thus, shotcrete placed in layers does not President, a Board member, and Chair of the ASA produce a “cold joint” as defined by ACI, Sustainability Committee. He is also a member of because it produces excellent bond between the ACI Committees 301, Specifications for Concrete; layers. This has been confirmed by visual 350, Environmental Engineering Concrete Struc- inspection of numerous cores taken through tures; 371, Elevated Tanks with Concrete Pedes- multiple layers of shotcrete, where it is often tals; 372, Tanks Wrapped with Wire or Strand; impossible to identify where one layer stops and 376, Concrete Structures for Refrigerated Lique- the other starts, unlike cold joints in cast-in- fied Gas Containment; and 506, Shotcreting; and place work where the difference between lifts Joint ACI-ASCE Committee 334, Concrete Shell is readily apparent. Design and Construction. Hanskat’s service to the American Society of Civil Engineers (ASCE), References the National Society of Professional Engineers ACI Committee 309, 2005, “Guide for Consolidation of (NSPE), and the Florida Engineering Society Concrete (ACI 309R-05),” American Concrete Institute, (FES) in over 50 committee and officer positions Farmington Hills, MI, 36 pp. ACI Committee 506, 2013, “Specification for Shotcrete at the national, state, and local levels was high- (ACI 506.2-13),” American Concrete Institute, Farmington lighted when he served as State President of FES Hills, MI, 40 pp. and then as National Director of NSPE. He served ACI Committee 318, 2011, “Building Code Requirements as a District Director of Tau Beta Pi from 1977 for Structural Concrete and Commentary (ACI 318-11),” to 2002. He is a Fellow of ACI, ASCE, and FES American Concrete Institute, Farmington Hills, MI, 503 pp. Beaupré, D., 1999, “Bond Strength of Shotcrete Repair,” and a member of ASA, NSPE, ASTM International, Shotcrete, V. 1, No. 2, Spring, pp. 12-15. AWWA, NFPA, and ASCC. Shotcrete • Spring 2014 41


2014SprShotcreteEMag
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