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2017SummerShotcreteEMag

| SUSTAINABILITY Durability of Shotcrete— Corrosion Protection By Jonathan E. Dongell In a prior Shotcrete magazine article, Spring 2015, “Durability and Exposure Conditions of Cementitious Materials – Deterioration Mechanisms,”1 the author discussed durability as it relates to the placement environment. This article is a continuation of that topic with a basic overview of good shotcrete practices and materials and how each contributes to durability. An example of reinforcement corrosion protection is offered to show the complexities involved in creating durable concrete. PROPER APPLICATION (EQUIPMENT) Shotcrete requires “high” exit velocity (or shot concrete force) to create a lineal stream of material without deviation until contacting a solid form, existing substrate, or previously shot material. High velocity creates the impact force necessary for enhanced surface bonding (adhesion); mixture component bonding (cohesion); and anticipated compressive strength (compaction) to be achieved. Assuming adequate proportioning and constancy of material being shot, there is direct correlation between the material exit velocity (nozzle velocity) and the compressive strength. While a typical concrete placement relies on internal or external vibration for consolidation, shotcrete consolidation is primarily achieved through impact velocity. Of course, the use of proper equipment (pump, hoses, air compressor, nozzle, and so on) to facilitate an adequate flow of material with homogenous and uniform mixing and densification of the material is critical.2-4 QUALIFIED PERSONNEL (APPLICATOR) In the most basic of terms, the nozzleman is responsible to ensure that the necessary in-place shotcrete characteristics for durability are maintained throughout the shotcrete placement. The qualifications of the nozzleman and the shooting practices employed must match the expertise level needed for the project. Thus, the experience and craftsmanship of the nozzleman provide the necessary workmanship for sound, durable shotcrete. Shooting techniques consistent with sound industry practice ensure excellent compaction and consolidation, which lessens the potential for irregular trapped air pockets (or “vugs”) much larger than air-entrainment size within the shotcrete and around reinforcement.2,4,5 DURABILITY BY PRESCRIPTION (QUALIFYING THE MATERIALS) As with any concrete, it is important to use only sound, proven materials (cement, aggregate, sand, pozzolan, polymers, additives, and water) for shotcrete. Each of these materials has at least one ASTM standard that qualifies its use for shotcrete placement of concrete. Proper proportioning of these materials is critical to the shotcrete application. Proper wet- and dry-mix aggregate gradations and mixture designs are addressed in ACI 506R-16, “Guide to Shotcrete.”6 DURABILITY BY PERFORMANCE (QUALIFYING THE APPLICATION) The most commonly used acceptance and performance testing criterion for shotcrete is compressive strength testing (ASTM C1604) in conjunction with testing that quantifies the shotcrete’s overall interconnectivity.7,6 Knowing the shotcrete’s overall density (as correlated by compressive strength testing) and its overall resistance to water/ moisture ingress (as relayed by permeability, absorption, or diffusion testing) directly relates to the shotcreted concrete’s ability to resist most deterioration mechanisms. This in turn also provides a means to forecast shotcrete’s durability.2,7,8 Several test methods have been developed and used to indicate the “apparent” interconnectivity of shotcreted concrete. Such testing includes: rapid chloride permeability (RCP); rapid migration (RM); surface resistivity (SR); boiled water absorption (BWA); and bulk diffusion (BD).7,9 Each of these test methods have been used to quantify or qualify the potential durability of a cementitious material by determining its ability to resist the ingress and movement of ions though the material. The more commonly used test method is the ASTM C1202 RCP Test Method. While the ASTM C1202 RCP Test Method actually measures electrical conductivity and not permeability, it serves as a rapid indicator that many Agencies use to indirectly correlate somewhat closely to long-term bulk diffusion.7 Other performance test methods exist for shotcrete for specific uses, placement conditions, or environments, such as bond strength, shrinkage, freezing and thawing, alkalisilica reaction resistance, and sulfate resistance. 54 Shotcrete | Summer 2017 www.shotcrete.org


2017SummerShotcreteEMag
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