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ACI MATERIALS JOURNAL TECHNICAL PAPER Title No. 113-M33 Comparative Evaluation of Transport Properties of Shotcrete Compared to Cast-in-Place Concrete by Lihe Zhang, Dudley Morgan, and Sidney Mindess The question is sometimes asked: “How does the durability of shotcrete compare to that of cast-in-place concrete?” The durability of shotcrete and concrete structures is strongly influenced by their transport properties. While considerable data are available regarding the transport properties of cast-in-place concrete, little has been published concerning shotcrete transport properties. This study is directed at addressing this deficiency so that factual data are made available regarding the comparative transport properties of both wet, and dry-mix shotcretes and comparable cast-in-place concretes. In this study, a comparative evaluation was conducted on cast-in-place concrete; cast wet-mix shotcrete; sprayed wet-mix shotcrete; and sprayed dry-mix shotcrete in mixtures with and without fly ash, silica fume, and accelerators. Plastic concrete and wet-mix shotcrete tests conducted included slump, air content, and setting time. Hardened concrete and shotcrete tests conducted included compressive strength at 7 and 28 days; ASTM C642 boiled absorption and volume of permeable voids; ASTM C1202 rapid chloride permeability (RCP); ASTM C1792 rate of water absorption; and U.S. Navy specification UFGS 03 31 29-3 (chloride permeability test). Calculated transport property values compared included boiled absorption (BA) and volume of permeable voids (VPV), Coulomb values in RCP test, coefficient of diffusion (DiffOH–), effective coefficient of diffusion (DiffOH– x VPV), permeability (k) and tortuosity, in U.S. Navy specification UFGS 03 31 29-3 tests. This study demonstrates that properly applied wet-mix and dry-mix shotcretes can provide equivalent or superior transport properties (for example, ionic diffusion and permeability), and hence durability, to cast-in-place concrete. Keywords: absorption; accelerator; boiled absorption; coefficient of diffusion; dry-mix; durability; ionic diffusion; permeability; rapid chloride penetration; shotcrete; tortuosity; transport properties; volume of permeable voids; wet-mix. INTRODUCTION Shotcreting refers to the process of pneumatically conveying concrete materials at high velocity to a receiving surface to achieve compaction. While shotcrete has been used for over a century, the use of shotcrete instead of conventional cast-in-place concrete has greatly increased in the past several decades, both for new construction and for repair, rehabilitation, and seismic upgrading of existing structures. The range of shotcrete applications is wide, including structural walls and other elements for commercial, industrial, institutional, and residential buildings; repair and rehabilitation for bridges, dams, reservoirs, and marine structures; stabilization of rock faces; and underground support in tunnels and mines. Both wet-mix and dry-mix shotcrete processes are available. Shotcrete can be applied by a nozzleman or by remote control with a robotic sprayer. The advantages of shotcrete are many: • Compared to cast-in-place concrete, shotcrete is able to minimize or eliminate the need for the formwork required for conventional concrete construction; • Shotcrete is compacted by high-velocity impact and can thus achieve increased compaction compared to cast-inplace concrete; • With an aging infrastructure, more and more shotcrete is being used for structural repair and rehabilitation, especially where the use of formwork and access are challenging; and • In underground applications in tunnels and mines, shotcrete has proven to be a cost-effective and safe method of ground support. With the increasing use of shotcrete, however, questions have been raised with regard to its long-term performance and durability. In particular, how does the durability of shotcrete compare to that of cast-in-place concrete? This question is of interest to owners, structural engineers, transportation agencies, architects, and equipment and materials suppliers. Unfortunately, there is a lack of adequate comparative data about the basic durability of shotcrete compared to cast-in-place concrete. Durability factors such as resistance to weathering, corrosion, chemical attack, alkali-aggregate reaction, carbonation, and freezing-and-thawing deterioration are all influenced by the transport properties of the concrete or shotcrete during the service life of the structure. Therefore, the objective of this study is to determine the transport properties of shotcrete compared to conventional cast-in-place concrete. The transport properties evaluated herein include absorption (liquid uptake in a porous medium); diffusion (liquid, gas, or ion movement under a concentration gradient); permeability (resistance to flow of a liquid under a pressure gradient); sorptivity (absorption of a liquid by capillarity); and wicking (capillary transport through a porous medium to a drying surface). The tests that were conducted to quantify the transport properties in concrete and shotcrete were boiled water absorption, water absorption, drying, chloride ponding, chloride diffusion, and rapid chloride penetration for samples from both shotcrete and concrete. RESEARCH SIGNIFICANCE Relatively little has been published about the transport properties of shotcrete. Information on this topic is needed because shotcrete is increasingly being used in various new construction and repair applications. This research program compares the transport properties of cast-in-place concretes with wet-mix shotcretes and dry-mix shotcretes with similar ACI Materials Journal, V. 113, No. 3, May-June 2016. MS No. M-2015-268.R3, doi: 10.14359/51688829, received September 8, 2015, and reviewed under Institute publication policies. Copyright © 2016, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including author’s closure, if any, will be published ten months from this journal’s date if the discussion is received within four months of the paper’s print publication. SAhCotIc Mreatete •r Siaulsm Jmoeur r2n0a1l/6M ay-June 2016 37 335


2016SummerShotcreteEMag
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