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2016SummerShotcreteEMag

Table 1—Summary of mixtures included in this test program Mixture type Cast concrete Cast wet-mix shotcrete Shot wet-mix shotcrete Portland cement A1 A2 A3 A4 A5 A6 Fly ash modified B1 B2 B3 B4 B5 B6 Silica fume modified C1 C2 C3 C4 C5 C6 Fig. 1—Concrete pan mixer and wet-mix shotcrete pump (left) and after mixing, shotcrete was discharged into shotcrete pump (right). water-cementitious materials ratios (w/cm). These data can be used to provide a comparison of expected durability and predicted service life of structures made with these different materials/systems. The effect of different cementitious materials in the mixtures on transport properties, such as cement, fly ash, and silica fume, was also studied. This research program also provides the opportunity to compare the various testing methods used and assesses their suitability for the purposes of qualification of the mixture, qualification of the nozzleman, and quality control. Current testing methods being used for qualification and quality control of concrete and shotcrete are compared against the findings of the transport properties tests, and recommended testing method(s) are proposed for use in the field. EXPERIMENTAL PROGRAM Mixture designs and materials Determination of the transport properties in this study was based on tests conducted according to: • ASTM C6421: Density, boiled absorption, and volume of permeable voids. This test method is widely used as a qualification and quality control test method in the shotcrete industry. • ASTM C12022: Electrical indication of concrete’s ability to resist chloride ion penetration. This test method is frequently used as a method to qualify the shotcrete or concrete mixture, as well as a quality control test. However, the test itself is controversial in that rather than measuring actual chloride penetration, it measures current flow in Coulombs (1 Coulomb = 1 amp/s), which in turn is related to electrical resistivity (Ω = Volt/Amp). It actually measures the flow of OH– ions as charge carriers, and thus is not just an indirect measure of resistivity through the measurement of the charge passed. • ASTM C17923: Rate of water absorption (drying test) Shot wet-mix shotcrete with 5% accelerator Shot dry-mix shotcrete Shot dry-mix shotcrete with 3% accelerator • Ionic migration test to U.S. Navy Specification UFGS 03 31 29-3.4 This test method is a modified version of the ASTM C1202 test and is described later in this paper. Cast-in-place concrete mixtures, cast shotcrete mixtures, and sprayed shotcrete mixtures (hereafter referred to as shot shotcrete mixtures) were all subjected to these test methods. For each mixture, in addition to cement-only mixtures, mixtures with fly ash or silica fume were included to represent mixtures commonly used in the industry. The different mixtures studied are summarized in Table 1. The as-batched mixture proportions are provided in Tables 2 through 4. All mixtures were dry-batched by the same supplier from Vancouver, BC, Canada and provided as prebagged materials in bulk bin bags. Mixtures of cast concretes (A1, B1, and C1), cast wet-mix shotcretes (A2, B2, and C2), shot wet-mix shotcretes (A3, B3, and C3), and shot wet-mix shotcretes with 5% accelerator (A4, B4, andC4) all have the same w/cm of 0.40. All mixtures used aggregates conforming to the ACI 506R5 Gradation No. 2 requirements. A high-range water-reducing admixture was used for wet-mix shotcretes at the dosages needed to produce the required slump. For the dry-mix shotcretes, the exact w/cm is not available as the water content was controlled by the nozzleman by adjusting the water added at the nozzle. For these specimens, the w/cm of 0.40 is an estimate. Such concretes and shotcretes would satisfy CSA A23.1 Table 26 (equivalent to ACI 318 Table 4.3.17) requirements for a C-1 exposure condition (that is, structurally reinforced concrete exposed to chlorides with a maximum w/cm of 0.40 and minimum 56-day compressive strength of 35 MPa (5076 psi). Additional air entrainment would be required for use of such concrete or shotcrete in freezing-and-thawing exposure environments. Batching, mixing, and production Concrete mixtures A1, B1, and C1 were hand-cast in the laboratory and are described as cast concrete. Test panels with 36 Shotcrete • Summer 2016 374 ACI Materials Journal/May-June 2016


2016SummerShotcreteEMag
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