“that the shotcrete process be understood and used in . ACI R – Guide to Shotcrete – Gradation No. 1 and 2 Guidelines in ACI R “Guide to. Keywords: dry-mix shotcrete; fiber reinforced shotcrete; nozzleman; shooting . Guide to Certification of Shotcrete Nozzleman. ACI R. Guide to Shotcrete. Guide to Shotcrete ACI R – NEW. This guide provides information on materials and properties of both dry-mix and wet-mix shotcrete. Most facets of the .
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In the shotcrete industry, appropriate aggregate selection and specifications are often the neglected aspects of the shotcrete mixture design. This is certainly a good start, but we believe a little more can be said to support the choice. 506rr-05, the maximal diameter and gradation of the aggregate phase are crucial to obtain optimal properties from a given shotcrete mixture.
This article is written to show and explain why the use of a coarse, well-graded aggregate phase is beneficial to the quality of the 506t-05. Although it is written especially for the dry-mix process, the principles also apply to shotcrets wet-mix shotcrete process.
Although there is no standard regarding the maximum amount of elongated particles, it is safe to say that these particles should be minimized and controlled even more so than in cast-in-place concrete. These aggregates can have many negative effects on shotcrete, such as reducing the shootabilty, forcing reduced hose length plugand causing a more difficult finishability.
Quality certificate reports for the aggregates should always, as with traditional cast-in-place concrete, be reviewed before the beginning of any project.
It is well known in the cast-in-place industry that, for a given mixture design, increasing the size of the larger aggregate increases the shotcretw of the fresh concrete or, conversely, reduces the water demand.
Guide to Shotcrete ACI 506R-05
This effect is directly attributed to the fact that a larger aggregate gradation has a smaller amount of empty space that must be filled in by the cement paste. In dry-mix shotcrete, this either translates into a reduced water-cement ratio or improved placement and reinforcing bar encapsulation properties. In all cases, the use of a coarse aggregate gradation leads to an in-place concrete composition that is closer to that of cast-in-place concrete.
It is often assumed that larger particles tend to rebound more than those found in a finer guise gradation. Whereas this assumption may be correct, the overall rebound values found for shotcrete mixtures with a fine aggregate gradation are comparable with mixtures produced with a coarser aggregate gradation, when both are shot at their wettest stable consistency.
In short, combined with a well-distributed gradation, larger aggregates usually generate better compaction and reduce the required amount of cement paste to an optimized level to build up a given layer of shotcrete.
In dry-mix shotcrete applications, the use of a coarse aggregate gradation reduces the in-place paste content, leaving the installed material with an increased aggregate content. By minimizing the cement paste content of a given shotcrete mixture, total shrinkage is minimized, which greatly reduces the risks of cracking and bond failure at the substrate and shotcrete interface. Additionally, when shotcrete is chosen as a repair material, a higher content of coarse aggregate will more closely reflect that of the parent concrete, resulting in a repair material with more compatible hardened properties for example, modulus of elasticity, coefficient of thermal expansion, and creep.
This compatibility between the repair material and the parent concrete is a crucial factor in achieving a long-term guied repair. It has been proven that the transportation of dry material through a shotcrete hose is more efficient when the mixture contains coarse aggregates.
Shotccrete abrasion of coarse aggregate against the inside lining of the hose reduces the cement buildup and improves material flow. Consequently, in many cases, the use of a coarse aggregate gradation will allow the use of longer transportation hoses.
ACI Committee (Author of ACI )
Application thickness is another aspect of the shotcrete process that should be considered when evaluating the gradation of shotcrete aggregates.
The minimum thickness of any shotcrete application should always be a minimum of shotcrrte times the maximum diameter of the largest aggregate.
In conclusion, the use of coarse aggregate in shotcrete mixtures will provide a wide range of benefits to both plastic and hardened properties.
These benefits can be as simple as improved shootability and fewer disruptions due to plugging or as complex as reduced shrinkage and improved long-term durability. We must always remember, however, that shotcrete is simply a method of placing concrete. We would never design a cast-in-place concrete mixture without coarse aggregate.
The same consideration should be given when designing a concrete mixture that is applied using the shotcrete process. The Effect on Shootability and Paste Content of the In-Place Shotcrete It is well known in the cast-in-place industry that, for a given mixture design, increasing the size of the larger aggregate increases the workability of the fresh concrete or, conversely, reduces the water demand.
Better In-Place Shotcrete Properties In dry-mix shotcrete applications, the use of a coarse aggregate gradation reduces the in-place paste content, leaving the installed material with an increased aggregate content.