TOP SUSTAINABILITY ADVANTAGES OF SHOTCRETE

• Formwork savings of 50 to 100% over conventional cast-in-place construction;
• Formwork does not have to be designed for internal pressures;
• Complex shapes require very little, if any, formwork;
• Crane and other equipment savings or elimination;
• Labor savings of at least 50% in repair applications;
• New construction speed savings of 33 to 50%;
• Speed of repair reduces or eliminates downtime;
• Better bonding to the substrate, which enhances durability;
• Adaptability to repair surfaces that are not cost-effective with other processes; and
• Ability to access restricted space and difficult-to-reach areas, including overhead and underground.

A brief explanation of each of the aforementioned top advantages will be added to this section each quarter as they appear in the “Sustainability” feature of Shotcrete magazine.

Formwork savings of 50 to 100% over conventional cast-in-place construction

One of the most significant sustainability advantages of shotcrete is substantial formwork material savings. With the shotcrete process, the material is shot into its final place, so forming is either completely eliminated or at least reduced by 50% when one-sided forms are necessary. This not only reduces or eliminates the amount of wood or other material used in forming, but also reduces or eliminates the environmental impact of milling the lumber and subsequent transportation to thousands upon thousands of construction sites. Furthermore, the transport and disposal of used formwork is greatly reduced. Even in applications in which one-sided forms are required, the formwork is greatly simplified, as less structural strength is needed and thus, the amount of materials required is significantly reduced.

Projects in which structural walls contained multiple blockouts have experienced a reduction of formwork to one-sixth of what would have been needed for a traditional placement.

In addition to the actual formwork material savings, ancillary formwork materials, such as whalers, bracing, form ties, reinforcing bar standoffs, forming support structures, and release agents are also eliminated or substantially reduced.

Due to the natural consolidation of concrete when placed via shotcrete, consolidation operations are also eliminated.

In overhead work, not only is the formwork eliminated, but the scaffolding and shoring required to support overhead forms are also eliminated. This means less on-site labor and less on-site equipment to unload, move, and load for shipping the forming and shoring materials.

Formwork does not have to be designed for internal pressures

One of the most expensive and labor intensive parts of traditional cast-in-place concrete construction is the design, fabrication, erection, removal and transport of forms used to support fresh concrete until it reaches a strength to be self-supporting. Fresh concrete in its liquid state before set exerts a substantial lateral pressure on the formwork trying to contain it. ACI 347 Guide to Formwork for Concrete provides formulas to estimate the lateral pressure considering the temperature, casting rate and type of concrete with a minimum of 4.16 psi (0.03 MPa). Common casting rates and weather conditions can easily double the pressure. If one considers the outward pressure on a 4’ x 8’ (1.2 x 2.4m)sheet of plywood using just the minimum lateral pressure, nearly 10 tons (9.1 tonnes) of pressure needs to be contained to hold the concrete in place. To put this magnitude of pressure into perspective, think about a parking garage where the design live load is 0.35 psi (0.002 MPa) for car, truck or bus traffic. The minimum concrete pressure inside a cast-in-place form is well over 10 times greater than cars, trucks and busses driving in a garage structure.

For formwork to physically hold together in the desired shape during the casting operation and to withstand these massive lateral pressures requires substantial structural strength and rigidity in the forming system. Thus, you will find properly designed formwork uses a substantial amount of lumber, steel or aluminum depending on the form system. Additionally, the formwork needs to be transported to the site, erected, braced, removed and then trucked off-site either for re-use or disposal. This equates to substantial CO2 emissions from transport, as well as labor and cost for a product that isn’t even incorporated in the final structure.

With the shotcrete process, the concrete is shot in place, so the weight of concrete is carried by the concrete itself. There is little or no lateral pressure because the shotcrete is essentially self-supporting and doesn’t “flow” like fresh concrete does. Forming is reduced by at least 50% of the surface area with the use of one-sided forms. In many cases, no formwork is required at all. In addition to the many benefits of one-sided forms that were discussed in the last issue of this column, when one-sided forms are required, the structural strength requirements of the formwork are substantially reduced because there is no need to design for internal pressure from fluid concrete within the form.

Complex shapes require very little if any formwork

One of major benefits of the shotcrete process is that it can be sprayed in place on vertical and overhead surfaces without the need for forming. Most conventional formed and placed concrete uses flat surfaced shapes, as these are by far the easiest to form. Curved or even just tapered sections may be used in form and cast work, but the formwork is much more expensive to construct both in time, labor and materials.

Using shotcrete allows total flexibility in the finished shape and surface treatment. Variable thicknesses, curves, or virtually any combination of shapes are readily available to the designer to produce the most efficient structure possible with the least amount of materials. Shotcrete construction of structurally efficient, yet complex shapes often provides added benefits of reduced formwork, quicker completion and reduced project costs. This is a primary reason shotcrete is routinely used for free-form pools, faux-rock surfaces for fountains and zoo enclosures, and continuously tapered walls of liquid storage concrete tanks. Also, as the finished shotcrete surface is evident immediately when placed, there is no question what the final finish will look like. The finish is limited only by the creativity of the architect or engineer and the talents of the shotcrete contractor.

In dry process shotcreting, the material is conveyed through the hose in a dry or damp state and water is added at the nozzle. The resulting water content is much lower than you would find in normal conventionally-placed concrete. The dry process shotcrete is gunned in place in what is essentially, a zero slump pneumatically placed concrete. The shotcrete adheres to both vertical and overhead surfaces, and because of this, it can be easily gunned in place to conform to complex shapes. The same is true for the wet-process where the shotcrete is pumped and air added at the nozzle, to accelerate the concrete mix with a high velocity into place. With the wet-process, you will have somewhat higher water content, because the material must be of a flowable consistency to be pumped, so accelerators are commonly used. None the less, it conforms to the shapes where it is sprayed applied. With both methods you are placing material without the need to form or hold the material in place. This in itself is a tremendous benefit, because it eliminates the additional time, labor, lumber, and other forming materials necessary in casting, containing and supporting conventional placed concrete.

Shotcrete also allows you to place large quantities of material quickly and efficiently. In past years in the steel industry, we gunned Treadwell ladles commonly referred to as “ torpedo” or “submarine” ladles a few times a week. They are long tapered football shaped ladles, mounted on rail cars that are used to convey molten iron from blast furnaces to BOF shops. The Treadwell ladles required approximately 20 tons of refractory to line and were typically gunned in less than 5 hours. No forming or forming materials were necessary for the shotcrete installation and the refractory material was gunned in place to conform to the unusual internal shape.

Because shotcrete is sprayed in place, it can easily be placed over irregular shapes and surfaces, to simulate natural rock formations. In pool and spa construction, shotcrete is often placed in rounded or kidney shapes. As long as there is a solid surface to shoot against, shotcrete can easily conform to any shape. Building forms for irregular shapes is inordinately labor intensive, time consuming, and requires a great deal of custom highly skilled hand work. Lumber, bracing, and support systems also become necessary in forming and pouring operations with conventionally-placed concrete. Shotcrete is routinely used in concrete repair applications to gun round or oval shapes bridge piers, for overhead arches, in tunnels and sewers, in domes, and on irregular rock surfaces in slope stabilization or in mining operations.

In industrial applications, shotcrete is used for gunning refractory linings in vessels, steel ladles, round electric furnaces, smoke stacks, ductwork, and in cylindrically shaped cyclones. There are areas in power plants at tube penetrations areas, where it isn’t feasible to form and pour, and where shotcrete provides the only viable method of installing refractory materials. Imagine the additional labor and material costs involved in trying to form and pour 36 cone shaped burners inside of a power plant boiler. With shotcrete, there is no need to need to custom cut, fit and form each burner, there is no need for purchasing the lumber and building the forms. The shotcrete can be sprayed and contoured to fit the receiving surface and an additional benefit, is that any variance in the burners and burner tube spacing can be adjusted to, with a little more or a little less material.

Sustainability is about conserving resources and eliminating waste in materials, transportation, and in labor. When you can remove the forming of complex shapes from the project, many additional benefits go with it. If complex shapes require triple or quadruple the labor to form and cast, then being able to shotcrete the material in place without having to build forms, becomes even more important. Reduction of transportation is another sustainability benefit. Eliminating forming materials removes the costs and carbon footprint in the production of the lumber; from tree cutting and transporting to the lumber mill and distributors and ultimately to the job site. Not to mention the waste and disposal of the forming materials after the project is completed. There are greater demands for sustainability than ever before and we need to make the most of our available resources. The phrase “Time is money” is as relevant today as it ever was, and innovative designs and construction methods that improve efficiency are becoming more and more important to explore.

Crane and Other Equipment Savings or Elimination

An often overlooked sustainability benefit of the shotcrete process is one of the most basic of construction problems: the conveying and handling of materials. Because shotcrete transports material over distances efficiently, it can often eliminate the need for concrete buckets, cranes, hoists, freight elevators, motorized concrete buggies, and other costly methods of transporting cementitious materials to the desired work areas.

Shotcrete applications in both the dry- and wet-mix processes convey concrete materials over distances and propel the material directly onto the receiving surface. Shotcrete is placed under high velocity in what is essentially a sprayed installation of concrete material. These methods of transport provide us with several advantages.

On elevated structures or scaffolding, shotcrete hoses or pipe can run directly to the level where the material is needed and completely eliminate the need for cranes, hoists, concrete buckets, or other lifting equipment. The material runs through the shotcrete hose or piping in a continuous manner straight to the work area.

In industrial applications, shotcrete provides an efficient method of transporting material through man-doors or observation ports and up into boilers, vessels, and units, where double handling of materials is often necessary. The same can be said for smokestacks and ductwork, where concrete materials or refractory can be moved with shotcrete equipment, eli­minating the need to use hoists or climbers, which often results in double handling of the material. One industrial project in particular, a power plant scrubber, required the contractor to load refractory brick onto a bucket, lift the materials up to a work platform, lower them down to the access door, and then hoist them up into the vessel. The handling of material multiple times could have easily been eliminated by using shotcrete.

In sewers, tunnel inverts, and underground culverts, shotcrete can be placed by feeding hoses through manhole openings and other access points and transported long distances underground, eliminating the need for hoists and concrete buggies. On a past tunnel project some years back, the contractor set up operations on the roadway, where the distance between manholes was 600 ft (183 m). Shotcrete hoses were run horizontally in each direction to transport repair materials to each repair area. Shotcrete eliminated the need for hoists and motorized buggies and provided a more effective method of material placement.

On concrete bridges, the shotcrete material hose can be strung directly to the arch, beam, pier, or abutment where the repair material is needed without using any cranes or hoists. By using shotcrete, the placement process is simplified and, in many cases, eliminates the extra work involved in the double handling of materials.

The need for lifting equipment can be eliminated or substantially reduced anywhere a shotcrete hose can be run or shotcrete can be used for the placement of cement-based materials. Sustainability is about reducing costs in material, energy, and transportation. Moving material on site exactly to where it’s needed is just one area where shotcrete always provides an advantage. Remember, shotcrete is not a product—it is a process for placing concrete. It’s a method that saves time, money, and in many cases, unnecessary labor.

Labor Savings of at Least 50% in Repair Applications

Shotcrete has substantial benefits for enhanced sustainability in the repair industry. Shotcrete is an efficient repair method that offers—in addition to significant material savings—exceptional labor and speed advantages in many repair applications, all of which are critical sustainability advantages.

Using shotcrete allows the repair contractor to economically and efficiently address a wide range of concrete repairs with these labor-saving benefits:

* The use of minimal, if any, formwork;
* The allowance of unique overhead placement quality and efficiency;
* Increased speed of placement;
* The ability to provide the precise shape and thickness required for the structural or aesthetic functionality of the repaired concrete members in a significantly more efficient manner than form-and-pour;
* The possible elimination—or at least reduction—of shoring and scaffolding that would be needed for form-and-pour repair methods; and
* The possible elimination of the need for labor to operate heavy lifting equipment or forklifts on the site that would be needed for form-and-pour methods to build, set, and strip formwork.