Shotcreting is the method of spraying concrete with a high velocity on vertical surfaces or heightened surfaces. Shotcreting of concrete creates a very strong and durable layer due to the high-velocity impact. The process results in a high-quality bonding of concrete with most substrates and is characterized by the easy application onto curved and complex forms/ surfaces. Compared to the conventional cast-in-place concrete, this method uses significantly lesser formwork and is also more economical. Shotcreting can be done for various types of projects, for example, basement or exterior walls, retaining walls and pool walls.
The application of shotcrete can be in two methods – wet-mix process or dry-mix process. The processes are characterised by the time at which the water is added to the concrete mix.
Dry Mix – The dry mix of cement and aggregates is supplied through the delivery hose. When the mixture reaches the nozzle, water is sprayed to it with high pressure, thereby wetting the dry mix.
The dry mix method creates an increased state of bonding and provides more strength. It is more environment-friendly since there is both controls over water consumption and waste production.
Wet Mix – The wet mix process involves mixing the cement and aggregates with water at a specified ratio. This wet concrete mixture is supplied through the delivery hose and is sprayed onto the surface employing compressed air.
The process is more economical as compared to the dry mix process and also requires lesser labour. It is also much easier since the mix is prepared beforehand. Know More, www.shotcretemelbourne.com.
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Why Shotcreting?
Shotcreting produces a structurally durable bond between the concrete and a surface. This pneumatically propelled process of concreting causes a large force of impact which creates compaction of the shotcrete paste into the fine surface irregularities. This creates an overall smooth surface without pores and provides good adhesion to the surface. The material will not slide or peel due to the forces of gravity.
Other characteristics like low moisture absorption, weather and chemical resistance make it a great alternative in the construction industry. It also costs convenient as additional costs of formwork, scaffolding and equipment can be negated. However, the shotcrete machine can add up to a huge expense.
The most advantageous feature is that it can be used in construction where the generic form of casting concrete would be a difficult option or where different thicknesses of concrete are required to adhere to a single surface. It is generally considered a much better alternative to regular concrete.
Shotcreting can be done only by professionals and provides good results under proper supervision and planning. The skills and attention this process needs makes it high maintenance but provides very good finishes.
Applications
- Repair of damaged surfaces (Concrete, wood, steel etc.)
- Repair of damaged structures like bridges, roads, retaining walls, marine structures and spillway surfaces.
- Underground structures such as tunnels.
- New structures: Pools, compound walls
- Complex Forms: Domes, vaults, curved structures, organic forms
Shotcreting overall provides a great alternative that could transform the process of construction, if adapted more commonly. Its advantages in terms of costs, method and duration make it a very versatile process. When done correctly, it can provide smooth finishes, an overall clean look and enhance the appearance of the structure while giving it structural durability and integrity.
Shotcrete- a brief description
Over the last century, close collaboration between the industry and researchers has led to progress in shotcrete procedures and applications over the previous century.
The history
Equipment technology for shotcrete was developed in the early 1900s. It was Carl Akeley, a reputable hunter and professor who created a procedure to spray plaster onto a wireframe—he had taxidermy purposes, nevertheless. The dry material was pushed with compressed air through a device’s nozzle (similar to a gun) and wetted with water. At the same time, it was blown onto a frame. The result was a thick plaster coating that didn’t set before entirely placed nor fall from the frame.
Later, the professionals have started to use the equipment to patch altered concrete; they would use a mixture made of cement and sand. As the results were excellent, a new cement gun was breveted in 1910 with its “gunite” material.
Professionals used the dry process was widely for 40 years until a new method was born in the 50s. The new technique used pressure tanks to push stiff wet mortar through a nose. The recent wet procedure, along with the dry method, became shotcrete.
How does it work?
Shotcrete is not so much a product or material as it is a process. Before selecting the method for your project, you need to comprehend the various techniques used in shotcrete placement.
Shotcrete impresses with durability, strength, special bond, low permeability, and endless shape possibilities. Both the dry and wet methods give fantastic results, which is why shotcrete surpasses concrete in terms of quality. Development in material and equipment made the interchangeable use of the wet and dry procedures possible—they both have benefits.
The dry process
The method begins with placing pre-blended dry/semi-dampened materials into shotcrete equipment’s hopper; the latter will go into a delivery house. Afterward, compressed air takes material at high speed to the nozzle where water will be added. Then, the material is built upon the surface thanks to the high-impact velocity.
Some of the benefits of the dry process are:
- Effortless setup, shutdown, and clean up
- On-site control of material
- Conveying distances from the gun go as high as 1,000ft horizontally and 500ft vertically.
The wet process
This method starts with mixing all the ingredients (water included) into the shotcrete equipment. Afterward, the wet material is pushed to the nozzle, where compressed air is also forced in. It will create high-velocity installation and set up of the material onto the surface.
Ready-mixed concrete is mainly used for the wet process. The mix design and consistency of supply will ensure the proper mix that is workable to being pumped through small-diameter hoses.
The wet process is beneficial on many levels:
- It’s best for irregular surfaces
- It doesn’t require much formwork
- It’s affordable
- The rate of application is higher than with dry methods
- It eases the material handling in areas with tricky access
Budget, the material you can use, equipment, site access, and experience of the contractor and nozzlemen are factors that weigh in when selecting the application method.
Wet or dry shotcrete? How should you choose?
When you pick the placement method, the finished product isn’t the primary factor to consider. Even if some differences exist, materials are similar once they hit the substrate. Other factors weigh in when selecting the procedure.
Scope of work
Every job is unique, and quantity is often a factor. Concrete pumps have been developed a lot in recent years, and faster placement of wet process material is possible. Therefore, the wet process is the primary choice for large projects.
Projects like repairs of parking garage often include small areas that require finish with a steel trowel. The contractor has to stop and start repairs, and the wet process is tricky to use. The machine is easily turned off with dry shotcrete, so the method is excellent for more minor repairs.
Material availability
Pre-blended bulk bags of material are popular on remote sites for a wet process where the ready mix isn’t available.
Contractor’s personal preference
Some contractors have more experience with one method over the other. Each method requires specific equipment, so it’s understandable why the contractor will turn to the wet/dry process.
Temperature
Temperatures affect the curing times of the materials. In cold weather, shotcrete doesn’t hang very well. It may not adhere to the surface and rebound. In warm weather, the material may need more curing to avoid cracking.
Location
Some overhead locations at specific depths can recommend one method over another. Some areas won’t allow effective nozzeling and distance. For instance, in mining, nozzlmen need to stand on their knees, stomachs and backs away/closer to the substrate than usual. The particularities of the location will make one method more suited than the other.
What kind of equipment do you need?
Many professionals in civil engineering consider shotcrete as the most advanced method to place concrete. Civil, mortar, refractory, and underground industries are some of the most common applications. Here are some examples where shotcrete is the main procedure:
- Tunneling
- Soil nailing
- Repairing and restoring parking garages, bridges, seismic retrofits, dams and reservoirs, sewers, and marine structures
- Ground stabilization
- Specialty application
- Mining
- New foundations, tanks, and omes
- Architectural landscaping
Professionals use a wide range of equipment to place shotcrete:
- Pumps
- Pre-dampeners
- Gunning machines
- Dosing pumps
- Nozzle carriers
- Combination mixer-pumps
Some of the equipment is appropriate for the dry process, whereas other is best used for the dry method.
Over the last years, manufacturers discovered that shotcrete equipment that handles many jobs is a better investment than buying several pieces of equipment. As a result, more and more multifaceted pieces of equipment make for the no.1 choice for people in the industry.
The shotcrete pumps
Shotcrete pumps with built-in accelerator pumps are all the rage right now in the industry. For years, admixtures were used in shotcrete, mainly on the East Coast, whereas conventional concrete was used on the West Coast. Nowadays, though, many contractors on the West Coast switch to admixtures as well.
Since more and more professionals use admixtures, it’s understandable why shotcrete pumps with built0 accelerator pumps are widely used. They manage abrasive fluids and don’t wear out quickly. It’s because only the hose and hose inserts touch the material being pumped. Additionally, the hoses inside the casing ensure impressive volumetric precision needed for transfer, sampling, metering jobs. They also let solids in the material pass.
Today, professionals can choose from a great variety of pieces of equipment for various shotcrete projects. The purpose of the job and how the equipment can help place the shotcrete are the main factors when picking the equipment.
Frequently Asked Questions
Is it challenging to do shotcrete?
The shotcrete application method means that you make a durable pool shell and become even more robust in time. Without the knowledge, materials, and equipment, it will be impossible to set up a good shell made of concrete that you apply with the shotcrete process.
Is shotcrete stronger than concrete?
Shotcrete is mainly used in underground projects. Shotcrete is pneumatically spread at high speed through a hose and nozzle. Since the spray application procedures decrease the water/cement ratio, shotcrete is more robust than concrete.
How long does it take for shotcrete to cure?
When you apply concrete through the shotcrete procedure, it will need seven days to cure. The most effective way to curing is water.
Shotcrete is expensive. Why is so?
When you use too much water in the premixed concrete, the risk of cracking is significant. Some builders add water to the cement truck so that it doesn’t harden while working. Since the time window is narrower than with regular procedures, shotcrete is pricier.
Is it possible to do shotcrete in the rain?
You have to protect shotcrete from rain until you get the final set, which takes around 5 hours. Once the last set is done, it will be wet cured for four days, and seven days is ideal. Exposure to rain is beneficial because it provides moisture for continuous curing.
