Concrete is truly a versatile building material. Concretes in use today are formulated with very specific performance characteristics in mind and include lightweight, heavyweight, porous, fiber-reinforced, mass, high-performance and cellular concretes to name just a few. Each provides specific characteristics or properties for their intended use. These properties are achieved by intentional formulation and control of such variables as cement content and type, pozzolan type and content, aggregate type, admixtures used, the addition time and rate of those admixtures, as well as other, often subtle, differences.
By George W. Seegebrecht and Steven H. Gebler
One widely used specialty concrete is known as “shotcrete.” The major difference between shotcrete and its close cousin, concrete, is the placement method. Concrete is discharged from a ready-mix truck, placed on the ground or in forms and then must be vibrated for compaction. By contrast, the shotcrete process, whether using wet or dry material feed, does not require forming or compaction thereby enhancing design creativity and application flexibility, often resulting in a savings of time or money
Shotcrete, was originally called “Gunite” when Carl Akeley designed a doubled chambered cement gun in 1910. His apparatus pneumatically applied a sand-cement mixture at a high velocity to the intended surface. Other trademarks were soon developed known as Guncrete, Pneucrete, Blastcrete, Blocrete, Jetcrete etc. all referring to pneumatically applied concrete. Today Gunite equates to dry-mix process shotcrete while the term “shotcrete” usually describes the wet-mix shotcrete process. At point of application, both are typically referred to as shotcrete.
Dry-mix process shotcrete, introduces and mixes the required water at the application nozzle as the dry cementitious materials (fly ash, slag, silica fume etc.) and aggregates are delivered through the “gun” The nozzleman controls mix consistency, adjusting water addition to suit the changing conditions of the work area. The dry-mix process also is well suited for sporadic application operations since the majority of the water only comes into contact with the cementitious materials as it leaves the nozzle.The wet-mix process utilizes concrete delivered to the job that is thoroughly mixed excluding of any required accelerators. The ingredients are generally delivered in ready-mix trucks as with normal concrete. Accelerators or other admixtures may still be metered into the slurry at the nozzle along with air under pressure to increase the velocity of the material and improve control of the application or “shooting” process.
The impact velocity of properly applied shotcrete instantly compacts the material, yielding an “in-place” mix that is richer in cement and higher in strength than the same mixture prior to placement. Typically, a fine aggregate dry-mix shotcrete mix delivered in a 1:3 cement to aggregate proportion upon entering the application gun results in a 1:2 cement to aggregate ratio when in place. What appears to be a waste of materials and a dust nuisance known in the trade as “rebound” and overspray, actually results in dense, high-strength shotcrete as a portion of the aggregate ricochets off the receiving surface and away from the placement location. The loss through rebound will vary depending upon the dryness of the mix, the shooting distance from the surface, wind conditions, etc. The intended thickness is generally overshot, trimmed back to the design thickness and finished to the desired surface texture and appearance.
While the dry mix process sounds quick and economical, it requires precautions to ensure application quality. The nozzleman’s workmanship and experience are critical, since the nozzleman controls the critical water-to-mix ratio going into application equipment. With the wet-mix process, the nozzleman has no control over the consistency of the mix delivered to the job site, but can control the velocity of the materials and the addition of accelerators as the mix leaves the nozzle.
Just as in concrete mix designs, the water-to-cementitious materials ratio remains the single most important parameter influencing the compressive strength, shrinkage and overall durability of the final product. Application technique is also crucial and less forgiving than ordinary ready-mix. Good “shooting” technique can mean the difference between a dense high-strength material or one that looks good on the finished surface but actually has underlying sand pockets, voids and poorly encased reinforcing steel. Poor application technique increases the probability of cracking and its negative ramifications.The shotcrete process is more versatile than conventional concrete placement. If the shooting surface is sound, clean and accessible, shotcrete can be applied in very difficult or complex shapes or sections where conventional concrete formwork would prove difficult or impossible as well as cost prohibitive. Shotcrete is especially applicable for unique shapes desired in complex shapes, swimming pools and other unique features of aquatic parks. It can also be an excellent overlay and repair material for existing structures because of its potential to achieve good bond strength and low permeability.
The nuances and differences between concrete and shotcrete are too numerous to cover in a short article. Selecting a concrete placement method, whether it be conventional concrete, wet-mix or dry-mix process shotcrete, can be a challenging task, since there are positive aspects of each for almost every application. While it is true that one approach may be more applicable, adaptable or economical than another, the final concrete placement selection for the project should be based on project design, material performance criteria and overall budget.