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2015SpringShotcreteEMag

Testing Air Content of Dry-Mix Shotcrete By Lihe (John) Zhang Iproved by air entrainment. Air-entraining the as-shot air content.air content. Sometimes, the terminology of “aircontent at nozzle” is also used (erroneously) for -t has been well-established that the freezing-and-thawing resistance of concrete is im admixtures are often added to both wet- and dry-mix shotcrete primarily for durability con- Air Content for Steel Fiber- siderations.1-4 If shotcrete is required to have Reinforced Dry-Mix Shotcrete freezing-and-thawing resistance, a typical engi- During a recent project in British Columbia, neering specification will require: Canada, steel fiber­reinforced dry­mix shotcrete • As-shot air content: 2.5 to 5.5%; and was applied for rock stabilization (Fig. 1). • As-batched air content: 4 to 7%, 5 to 8%, or The project specification required: 6 to 9%, depending on exposure condition and • 7-day compressive strength: at least 4400 psi maximum size of the aggregates. (30 MPa); It is a common practice to add air-entraining • 28-day compressive strength: at least 5800 psi admixture into the wet-mix shotcrete because it (40 MPa); is often batched as ready-mix concrete. • Boiled absorption: maximum 8%; However, the question has been raised, from • Volume of permeable voids: maximum 17%; time to time, on whether dry-mix shotcrete should and be air-entrained. About two decades ago, • As-shot air content: 2.5 to 5.5%. researchers and engineers originally developed To meet the project specification and the the concept of air-entrained dry-mix shotcrete,1-3 durability requirement for freezing-and-thawing and its use is well-proven today. resistance, air-entraining admixture was added— It is widely recognized in the shotcrete in a powder format—into the pre-bagged mate- industry that the air content of the as-batched rials of dry­mix steel fiber­reinforced shotcrete. shotcrete will drop during the shooting process. As-shot air content was tested to ASTM C231. This results from high-velocity impact from the The dry­mix steel fiber­reinforced shotcrete was shotcrete process tending to “knock out” air from shot on the ground in a pile. The shotcrete sample the mixture and thus reduce the air content of was taken from the piled shotcrete and tested for the in-place shotcrete. The as-shot air content in as-shot air content. The resultant as-shot air con- the in-place shotcrete is typically below 6% tent is plotted in Fig. 2. Results show that the regardless of how high the as-batched air content as-shot air content ranges from 2.7 to 5.0%. The was before shooting.4 Testing for air content in average as-shot air content is 3.7%, with a coef- the hardened shotcrete can be conducted by air ficient of variation (COV) of 14.5%, and standard voids analysis using the ASTM C457 test pro- deviation of 0.5%. cedure. However, this test is time consuming and expensive and is thus not often used as a quality Plot Air Content versus control (QC) test. This article provides project testing data for the as-shot air content for the Compressive Strength dry-mix shotcrete. A typical air-entrained shotcrete dry-mix An air meter is used to measure the air content design will have an as-shot air content of approx- of the plastic shotcrete or concrete. For the pur- imately 3 to 5%. It is generally known that higher poses of this article, air content measured at the air content will generally reduce the compressive point of discharge from the truck or at the end of strength for cast-in-place (CIP) concrete.5 the pump hose is referred to as-batched air con- The correlation for as-shot air content versus tent. Air content measured by shooting directly 28-day compressive strength is plotted in into the air meter base, or shot against a wall, into Fig. 3 for the dry mix previously introduced in a wheelbarrow, or even into a basket, and then Fig. 2. As expected, there is no clear relationship scraped out to fill the air meter, is called as­shot between the two properties as opposed to CIP 22 Shotcrete • Spring 2015


2015SpringShotcreteEMag
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