Shotcrete is simply a placement method for concrete. Thus, fire resistance of any shotcreted concrete section can be evaluated by consulting ACI CODE-216.1- 14(19) Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies. There are many factors that affect the fire resistance so you will need to review the code to establish what’s appropriate for your structure.
Is there a guide to determining pressure applied to a form during shotcrete placement? I have seen references to 50 lb/ft2 (240 kg/m2) but no backup to this. Is the ACI formwork design guide applicable to shotcrete in some way?
Since shotcrete is pneumatically placed against a one-sided form there is no liquid concrete pressure against the form. When bench shooting vertical walls the majority of pressure from the high velocity impact is carried by the previously placed concrete as the concrete is stacked. Plus, this pressure is very localized, only affecting a small zone immediately adjacent to the impact area of the material stream. Here is an article on a research project that quantified the force https://shotcrete.org/wp-content/uploads/2020/05/2007Sum_TechnicalTip.pdf
From the paper conclusion “In normal spraying conditions, wet- and dry-mix shotcretes produced a force on the panel of about 45 and 20 lb (200 and 90 N), respectively. The maximum load recorded is 87 lb (389 N) and it was observed in simulating a water plug.”
If your shotcrete form is exposed to wind forces before placing shotcrete, you may want to consider the wind pressure that may be expected across the entire form during construction to keep the form intact. This should be much less than the 50 lb/ft2 pressure you mention. ACI formwork design is intended for cast concrete where liquid concrete is contained within a two-sided form and is NOT applicable to shotcrete placement. Some sources show a 60 mi/hr (100 km/ hr) wind exerting about 10 lb/ft2 (50 kg/m2) on a vertical wall.
What is the efficiency of dry-mix shotcrete? How much is over spray vs. how much sticks to the surface?
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.
Is it true that shotcrete compression tests are more accurate and are likely to be higher (better) results? Basically, is there a difference when testing wet-mix shotcrete applied pneumatically as compared to simply being taken directly in cylinders from the batch plant or ready-mix delivery?
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.
I was wondering if ASA has any safety meeting/ tailgate presentations on high-pressure shotcrete work. I need to ensure my team knows how to properly disconnect hoses and align proper fitting procedures. Furthermore, do you have any guidance on a repair procedure for the rubber hose?
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.
We have an existing historic 4 in. (100 mm) hollow clay tile wall that is finished with a 3-coat gypsum plaster. We would like to spray shotcrete on the non-finished side of the wall to strengthen it. Will the application of shotcrete on the back side of the wall possibly damage the historic plaster and paint? Will there be too much of a vapor drive from the application for the plaster to hold? Have you experienced plaster deterioration or loose keys from the application of shotcrete on the backside of a wall?
As long as the hollow clay tile wall is rigid and stable, the shotcrete application should not impact the finished side plaster. Shotcrete impacts in a very localized area directly where the material stream is hitting the substrate. Research shows that the localized force is about 90 to 100 lbs (40 to 45 kg) when shooting directly on the substrate. If shooting a thicker wall using a benching method, most of the force is carried by the previously shot material, so it would have less impact. If the clay tile needs to be stiffened, an initial thin layer could be shot to provide additional thickness before the final thickness is placed. Though we don’t have any specific information about vapor transmittal, concrete used in shotcrete placement inherently has a low w/cm and less water in the mixture to bleed or create vapor. We haven’t had any reports of plaster or grout falling off the inside of masonry walls that have been structurally enhanced with shotcrete.
We have a client that has requested a shotcrete application for a dirt crawlspace. The facility is located in the Northeast part of New York. I’m not the designer, but I believe we are looking at a wet-mix, applied at a 4 in. (100 mm) depth throughout the crawlspace. I had a few questions I was hoping could get answered as we move through the technical specifications process. 1) Is there an off-season for the shotcrete product? Does it have to be applied in warm weather? Again, this is upstate NY. 2) What kind of equipment is used to dispense/place the product? (same as concrete?) 3) What is the noise level of the dispensing equipment? Typical of any concrete pour or much louder? The facility is a childcare center, so they are concerned about noise levels.
Here are the answers in the same order as your list.
- Shotcrete is just a placement method for concrete, so there is no “off-season.” However, as with cast concrete, cold weather placement needs more planning and material delivery controls. As wet-mix shotcrete material is predominately delivered by ready-mix trucks, the concrete should be delivered at 50°F (10°C) or higher. Then once shot, if temperatures are less than 50°F, the concrete should be protected by insulated blankets or the area enclosed by vented heaters to keep temperatures above 50°F. We also don’t want to shoot onto frozen surfaces.
- We use small line concrete pumps standard in the concrete industry.
- Concrete pumps and air compressors are generally diesel-powered engines. Pump engines range in horsepower from 75 to over 200 hp, depending on the pump size. Your application is of relatively low volume, so one of the smaller pumps should do fine. We also use relatively small air compressors, and many contractors who work in residential or metropolitan areas use equipment equipped with sound reduction. The sound at the point of placement where the air-accelerated material stream exits the nozzle is generally just the sound of the airflow.
I would like to ask if there are any articles, references, etc, which reference procedures to determine the maturity of the concrete applied via shotcrete? Basically, how to generate the validation curves?
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.
I am working on a project that has existing tunnels made with shotcrete. I am needing to hang 12 in. (300 mm) duct and trying to figure out the best type or suggested anchors to use.
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.
We have shotcrete test panels, 12 by 12 by 6 in. (300 by 300 by 150 mm). My question is it OK to move the panels right after shooting to different location for initial curing or should the panels be left untouched for certain period of time at the spot of shooting?
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.