Dry-mix shotcrete may have more rebound (coarse aggregate that bounces off the surface) than wet-mix so it may be considered a little less efficient. However, predampening and the use of special nozzles can increase wetting and reduce rebound which makes the dry-mix efficiency approach wet-mix. Rebound may be estimated as 5 to 15%, with an average of 10% of the weight of the concrete materials. The experience and placing technique of the nozzleman can substantially affect the amount of rebound in either dry-mix or wet-mix. Overspray is much less and may depend on wind conditions and placing techniques. When looking at the overall efficiency of shotcrete placement in a given section, dry-mix materials can be tailored much closer to the actual need while wet-mix may have minimum concrete delivery volume and time constraints that would end up not using all the material delivered. Also, dry-mix has approximately ¼ the production rate of wet-mix, so in high-volume placements, wet-mix may have a natural advantage in productivity.
Shotcrete placement provides full consolidation of the concrete by high-velocity impact. Concrete placed into cylinders for testing is consolidated by multiple rodding in three layers. Shotcreting also has some percentage of rebound so the concrete mixture that remains in the panel is more paste-rich than the mixture entering the pump. Thus, shotcrete placement may provide better consolidation, and a more paste-rich in-place mixture resulting in higher compressive strengths. However, shotcrete compressive strength is evaluated by cores extracted from panels. The coring process can create microcracking in the exterior surface of the core and produce slightly lower compressive strength than cylinders that have no damage to the outer surface when removed from the cylinder form and tested. Overall, there doesn’t seem to be a significant difference when evaluating the concrete material’s strength by shotcrete placement or concrete cylinders taken before pumping.
You could establish a correlation on a specific project by taking cylinders before pumping and then shooting material test panels. Then testing the cylinders and cores from the panel at the same age.
We have an ASA document, “Safety Guidelines for Shotcrete,” that addresses shotcrete safety. It is not a job specific safety plan but gives you guidance on the information you may include in your plans. You can find it on our bookstore at https://shotcrete.org/bookstore/?productpage=2. A free copy of the Safety Guidelines is provided to all of our corporate and sustaining members. Membership also provides many other benefits, including discounts on shotcrete nozzleman certification and participation in our committees. We have a committee specifically devoted to Education and Safety for shotcrete that is very active.
Regarding the repair of the rubber hose, there is no procedure to repair any breaches in the hose itself. A damaged or excessively worn hose should never be used as the pressure that builds when a delivery line plugs during pumping is extremely dangerous. Modern pumps can reach 2000 psi (14 MPa) internal concrete pressure when experiencing a plug and the hose must be capable of carrying that high pressure. The wet-mix shotcrete hose is heavily reinforced, and the couplings are designed for high pressure. Your crew must be sure that all clamps are fully engaged on the heavy-duty couplings, and that safety pins are in place.
Shotcrete is a placement method for concrete. Thus, generating the maturity curves would be based on the concrete mixtures. There are several online resources about the maturity method. One that discusses production of the curves is from the Minnesota DOT and available in PDF format at www.dot.state.mn.us/materials/concretedocs/MaturityMethodProcedure.pdf.
Wet-mix shotcrete has been used more and more for structural applications in the past few decades. Recently, wetmix shotcrete was successfully used to construct a mass structural wall with congested reinforcement and minimum dimensions of 1. 0 m in a sewage treatment plant.
Construction has a back problem. After the common cold, back pain is the most common reason why construction workers miss work. Worse, lower back injury is the single leading cause of long-term workplace disability. The back pain crisis plagues the construction industry. Why? It is the physical heavy work that is typical to active construction work.
If asked, could you accurately explain why the choice of slump is so important to a wet-mix shotcrete material’s hardened properties?
Shotcrete is just a placement method for concrete. Shotcrete placement with proper materials and application techniques should provide monolithic concrete with a 28-day compressive strength of at least 4000 lb/in2 (28 MPa). Thus, any systems that work in concrete should be fine. Either mechanically-fixed or epoxy-set anchors are commonly used in concrete. You should consult with the anchor suppliers for the size and type of anchor appropriate for your specific application.
In normal weather conditions test panels should be undisturbed for at least 24 hours. In cold weather you must protect the panels from freezing and preferably keep the concrete surface temperature above 50 to 55 °F. That allowsthe young concrete to gain enough strength to tolerate movement. Also, your test panels are very small in comparison to current ACI 506.2 Specification for Shotcrete requirements that have a minimum of 16 by 16 by 51⁄2 in. (400 by 400 by 140 mm) dimensions. When coring your smaller panels you should be sure to have the nearest edge of the core 3 in. (75 mm) from the sides to preclude the effect that rebound collection in the corners may cause.
