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2014SprShotcreteEMag

Technical Tip Shotcrete Placed in Multiple Layers does NOT Create Cold Joints By Charles S. Hanskat Dbuilding out a section with cold joints vides thorough consolidation and densification by-Shotcrete does not use internal vibration forconsolidation of concrete. Instead, shotcrete pro-esigners and inspectors often confuse placement of multiple layers of shotcrete in experienced in cast-in-place concrete construc- high-velocity impact of fresh concrete material tion. The American Concrete Institute’s (ACI’s) on the receiving surface. It is well proven in Concrete Terminology defines cold joint as: laboratory testing that properly placed shotcrete “Cold joint—a joint or discontinuity resulting is very well consolidated, and provides excellent from a delay in placement of sufficient dura- strength and durability. The high-velocity impact tion to preclude intermingling and bonding of of shotcrete on a hardened, previously shot layer the material, or where mortar or plaster rejoin (or existing concrete surface) provides a strong, or meet.” abrasive blast to open up the surface, and then In cast-in-place concrete construction, internal provides an immediate exposure of that hardened vibration is the most common method for pro- surface to fresh cement paste. As a result, shot- viding adequate consolidation of the placed crete exhibits excellent bond to concrete and concrete. In cast-in-place work, a cold joint is previously shot surfaces. formed when an initial lift of concrete becomes A study on shotcrete bond to concrete repair too stiff for penetration by the vibrator used to surfaces that included work on multi-layer shot- consolidate a subsequent lift. This thus precludes crete bond was conducted at Laval University the “intermingling” of material in the definition. (Beaupré 1999). The study looked at bond with However, ACI 309R-05, “Guide for Consolidation multiple layers of shotcrete shot 4 hours, 1 day, of Concrete,” indicates that if bond is obtained and 28 days apart with four levels of surface between cast sections, a cold joint is avoided. ACI finishing (no surface finishing, scratched with 309R-05, Section 7.2, states: steel trowel, scratched and finished with wood “When the placement consists of several layers, trowel, rough broom finish). Table 1 shows the concrete delivery should be scheduled so that results from Beaupre’s (1999) report. The report each layer is placed while the preceding one is concluded that “it can be seen that, for the still plastic to avoid cold joints. If the under- waiting period and the types of finish studied, lying layer has stiffened just beyond the point there is no significant influence of these param- where it can be penetrated by the vibrator, bond eters on bond strength” and “With respect to the can still be obtained by thoroughly and system- multi-layer bond strength of shotcrete, the pres- atically vibrating the new concrete into contact ence of shotcrete/shotcrete interfaces does not with the previously placed concrete; however, seem to create a large reduction in shotcrete an unavoidable layer line will show on the quality in terms of mechanical bond if no curing surface when the form is removed.” compound is used.” Table 1: Multi-layer bond strength in MPa (psi) (Beaupré 1999) Type of finish between layers (results with no curing compound) Roughen with Time None Scratch Scratch + wood broom 4 hours 2.1 (300) 1.8 (260) 2.1 (300) 1.9 (275) 1 day NA 2.1 (300) 2.1 (300) NA 28 days NA 1.8 (260) NA 2.0 (290) Note: NA is not available 40 Shotcrete • Spring 2014


2014SprShotcreteEMag
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