Sustainability continues to grow as a driving force in the decision-making of Owners and Specifiers regarding construction materials and placement strategies. Shotcrete offers many significant sustainability advantages. Because shotcrete is simply a method of placing concrete, it offers all of the sustainability benefits of concrete as a building material in addition to a long list of advantages that are unique to the shotcrete method of placement.
This page contains information and links to resources designed to educate readers on the many sustainability advantages that the shotcrete process offers.
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.
Click on a topic below to expand.
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.
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.
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.
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.
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.
In addition to significant sustainability benefits from material resource savings by eliminating formwork, the use of shotcrete can result in a labor savings of up to 50% on a repair project. The shotcrete process offers all the sustainability advantages of concrete as a repair material, plus a significant number of sustainability advantages inherent in the placement process.
In small repair areas, forming is labor-intensive. With shotcrete, the repair material can be gunned in from the open side against a prepared substrate, completely eliminating the need for forming. This not only saves the costs of the forming materials but also the costs of the labor involved in making the forms, securing them in place, the time-consuming procedure of conveying the concrete to the scattered repair areas, and stripping the forms. Additional labor costs are saved by eliminating the need to pour concrete through chutes to adequately fill the small formed repair areas without leaving voids, honeycombs, and air pockets. The ability to transport the shotcrete material through hoses directly to the repair areas eliminates the need for hoists, cranes, buckets, and the additional handwork that is involved in physical concrete placement. Thus, shotcrete placement provides a more efficient and cost-effective method of conveying the repair material.
In overhead areas, shotcrete can be gunned in place from the underside of the repair area and quickly and efficiently placed. On overhead forming and pouring installations, the concrete flows along the bottom side of the form, often leaving a gap or air pockets between the existing overhead concrete substrate and newly placed repair. With shotcrete, the receiving surface is fully visible and the repair material is shot in place from the top down, ensuring an excellent bond to the existing overhead concrete substrate. Additionally, overhead forming requires a great deal of time and labor to secure and often support scaffolding and bracing to hold the formwork in place. This step becomes completely unnecessary with a shotcrete installation.
Shotcrete can easily conform to complex shapes or rounded shapes where forming becomes difficult and expensive. Forming the underside of a dome or arch or building forms for repair areas on a cone or cylinder shape becomes a time-consuming and labor-intensive operation. The shotcrete, when shot in place, will fill in and conform to irregular depths and shapes easily and efficiently, taking the shape of the receiving surface.
In summary, for the rehabilitation of concrete structures, there is no more efficient repair method than using shotcrete. In most repair applications, shotcrete can be gunned directly onto the receiving surface, completely eliminating the need for forming. This yields big savings in time, labor, and material. Additional benefits include: 1) the speed and efficiency of placement; 2) the ease in overhead placement; 3) the elimination of bracing and support scaffolding for the formwork; 4) the reduction or elimination of hoists and handwork; 5) the advantages in material transport and handling; and 6) the unique feature of being able to conform to irregular shapes. In light of all these advantages, it is plainly evident that a 50% reduction in labor costs is easily realized using the shotcrete process for concrete repairs.
Both cars looked the same. When asked why one was more expensive, the salesman had no explanation. “They are the same, one just costs more,” he replied. When we buy automobiles, we expect to pay more for a superior product. We hunt out value by purchasing certain brands that possess attributes such as quality or durability. To the car buyer, two identical cars should cost the same amount.
In the construction industry, costs may vary based on intangibles that may be difficult to initially understand. Obviously, most would be unhappy to realize that they have paid substantially more for an identical product, but this is a common scenario. Concrete is the most common construction material on the planet, but its installed cost will never be the same. The speed and efficiency with which concrete can be placed will determine its installed cost.
The Magic of Shotcrete Efficiency
Since the first use of concrete, it has been cast into forms. Even today, almost all concrete produced is ultimately cast in place. Currently, shotcrete placement methods are capable of creating virtually the same in-place product as traditional cast-in-place concrete construction, but in many applications, shotcrete construction methods are much more efficient than conventional cast-in-place concrete construction. If asked, however, many construction professionals may not be able to clearly explain why.
The “magic” that explains this unique placement method’s efficiency is the nearly nonexistent fluid pressures applied to vertical forming materials during shotcrete placement. Shotcrete is not cast in a fluid state. Therefore, vertical forming materials need only be sufficiently rigid to initially stop the impact of nozzle flow at the receiving surface.
Shotcrete Methods That Save Time
For decades, tunneling, earth retention, and concrete repair contractors have used shotcrete placement methods to speed production. Today, more conventional concrete projects are switching to shotcrete placement methods to save time. A good example is the remodeling of commercial and industrial concrete structures. Window and door infills are regularly formed, placed, and completed on a single overnight shift. Structural improvements, seismic retrofitting, and shear elements commonly use shotcrete to save time. A very common method for seismic upgrading of existing structures is the addition of concrete shear walls. The lack of form pressure from fluid concrete liquid head allows the shotcrete method to often use existing structures as the back form. Many unreinforced masonry buildings can be restored by adding shear walls to the existing structure with very little, if any, added forming. In this same way—that is, using existing surfaces—sea walls, water canals, and erosion control construction are also expedited by the use of shotcrete. On average, shotcrete placement techniques can yield 33 to 50% time savings over traditional cast-in-place methods.
Alternative Shotcrete Form Materials Speed Production
Many shotcrete contractors speed production by using alternative forming materials. Light-grade steel studs, commonly used for commercial and industrial wall framing, provide adequate rigidity as form framing, and standard interior-grade gypsum drywall or expanded metal lath sheets work well as an alternative to plywood or dimensional lumber panels (Fig. 1). Unacceptable to withstand internal casting pressure, as traditional forming materials must, alternative shotcrete forming materials provide unparalleled construction speed. They are tougher than they look! Lightly framed vertical walls higher than 20 ft (6.1 m) can be placed without special precautions. Alternative form materials are proven to dramatically speed production with no compromise in product quality.
There are also times when traditional forming materials cannot be used because of the configuration. The Experienced Music Project (Fig. 2) built in 2000 in downtown Seattle, WA, is an example where forming and cast-in-place methods simply would not work. Using shotcrete placement and a stay-in-place forming system allowed for the construction of an irregular-shaped, 5.5 in. (140 mm) structural concrete shell over 100,000 ft2 (9300 m2) in size.
A Closer Look Reveals More Time Savings
Are low material costs and ultra-fast construction not enough? Alternative shotcrete forming techniques are enormous time savers when compared to conventional retaining wall methods. Traditionally designed as a cast-in-place application, concrete walls constructed against earth embankments require substantial over-excavation to allow safe access to both sides of the wall for construction and removal of forming materials. Although some walls may be shot directly against earth surfaces, common construction details typically require the installation of drainage and waterproofing membranes. Taller walls constructed near excavated earth can become dangerous, creating confined space hazards to workers who can become trapped behind walls in the event of earth movement. Current construction standards mandate that tall concrete walls within close proximity to unsecured earth embankments cannot safely and legally be conventionally constructed without extensively over-excavating the slope or using temporary earth-shoring methods.
Alternative wall-forming techniques can allow shotcrete walls to be safely erected near most earth embankments because the forming system is designed to stay in place. After placement, the wall is simply backfilled with a drainage material. This eliminates the need to place workmen at risk when working between the embankment and the completed wall. All required drainage or waterproofing membranes are incorporated into the face side of the formwork, prior to placement, and will remain in place, undisturbed, and protected by the form materials during backfilling (Fig. 3 through 5).
The use of stay-in-place forming techniques nearly eliminates costly over-excavation, haul-off, and recompaction, while providing identical performance to traditiconcrete’sonally constructed concrete. Innovative shotcrete techniques consistently generate speed savings of 33 to 50% over conventionally cast-in-place concrete. Speed and efficiency directly influence the final cost. Less formwork, labor, and time also significantly enhance the project’s sustainability. Like the two cars, similar products can have vastly different final prices.
When servicing the industrial market, fitting work into short time frames is the nature of the beast. The work must be completed within windows of opportunity—that is, when the production units are temporarily out of service. In the words of Arthur Miller, “It comes with the territory.” During an outage when a boiler, vessel, metal mixer, or cement kiln is “down” for scheduled repairs, all needed repairs are squeezed into the short time duration of the shutdown. The “critical path” dictates the time frame where all routine maintenance, repair, renovation, or upgrade operations must fit into this compressed time frame.
Why the tightly controlled and compressed schedule? As one may suspect, it comes down to money. The lost production time during a shutdown often costs the owner more than the cost of the repairs. Industrial clients consider the lost revenue while the unit isn’t in production an additional cost. In large power plants, the loss of power generation can be as high as $1,000,000 a day, if not higher. The same can be said for blast furnaces, coke plants, vessels, and most other high-output industrial production units. Therefore, getting a unit back into service a few days to a week early can make an enormous difference to the owner’s costs for repairs.
To further complicate this, in many cases, the lion’s share of the allotted downtime is often used for mechanical and structural repairs that must be completed prior to the start of shotcrete work. This further shrinks the open window to complete shotcrete repairs. Thus, the speed of repair in using shotcrete becomes a tremendous advantage. In power plant outages, unforeseen problems are often discovered after the shutdown is started and must be addressed before the shotcrete can be placed. Adding this additional newly discovered work can again compress an already tight schedule for shotcrete placement.
Performing a complete ash hopper refractory reline on the last few days of a power plant outage is not uncommon. With shotcrete, the refractory material can be quickly placed and finished within 24 to 48 hours. For repair work, the refractory material placement can often be completed in one shift or less. With furnaces, coke plant quench towers, and blast-furnace troughs, the downtime can sometimes be as little as a 4- to 8-hour window. Getting in and out quickly and easily and performing the necessary work becomes a requirement for the job. Using shotcrete is often the only way the work can be accomplished within these tight time constraints. No forms are required, the shotcrete conforms to a variety of shapes, material handling issues are eliminated, and the material can be transported directly to the desired location and sprayed in place—readily and efficiently.
Speed of installation is not just an advantage in industrial applications. In infrastructure repair work on tunnels, ramps, and bridges, heavily traveled arteries are often closed at night or on weekends for emergency repairs. One common example is a scheduled weekend shutdown of a tunnel exit ramp for overhead shotcrete repairs. The ramp is shut down on a Friday night, the deteriorated concrete is removed, the reinforcing bars are sandblasted, steel mesh is installed, and shotcrete is placed overhead in the tunnel. All the work is completed and opened to traffic for the Monday morning rush hour.
On projects with tight time frames or a limited amount of time to accomplish the work, the speed and efficiency of installation with shotcrete provides a distinct advantage. Even on projects without severe time constraints, the time, labor, and material savings with shotcrete can produce significant cost savings. Shotcrete provides an invaluable method to efficiently place concrete, refractory, or acid-resistant cements with a speed of installation that would not be possible with any other method. The old adage “time is money” is true for the construction industry, and using shotcrete can help save both on your next project.
Concrete is, by the definition of sustainability, the perfect construction material. It is the most common construction material on the planet. Concrete can be easily produced from widely available, plentiful natural resources, and it is completely recyclable. History has proven that structural elements made from concrete tend to last longer and require less ongoing maintenance than any other common construction material. History has also proven that all concrete elements eventually deteriorate or need to be reconfigured over time.
New concrete construction currently benefits from an array of material advancements and innovative construction techniques that have substantially improved the sustainability of modern elements constructed from concrete. The use of supplementary cementitious materials such as slag, fly ash, and microsilica and the more recent use of other recycled materials in the production of greener concrete reduce the environmental impact of new concrete construction.
But what about the immense inventory of existing concrete structures? Wear, exposure, deterioration, or obsolescence ultimately mandates the life span of all concrete elements. The ability to repair or reconfigure, rather than replace, existing concrete elements enhances sustainability, as repair methods can significantly extend the structures’ useful service life.
Bonding Becomes the Critical Element
The ability to form a durable bond interface between the existing substrate and the new concrete is critical to nearly every concrete reconstruction or repair. Typical structural designs require both the existing and replacement material to perform as a single cohesive element. Research has proven that durable, permanent bonds can be attained by using properly prepared substrate conditions in conjunction with shotcrete placement methods. Studies focusing on the bond qualities of shotcrete have proven that a sound substrate surface with an adequate roughness profile provides a suitable surface to form a durable bond. Shotcrete applied to a properly prepared substrate offers a significantly stronger and more durable bond than traditional casting placement methods. Other factors, such as surface moisture conditions, impact energy, shrinkage, and the mixture properties of the repair materials, can also affect the long-term bond quality. For many projects, bond quality will have the strongest influence on repair durability.
Why Shotcrete Placement Methods Increase Bond Quality
Pneumatically applied shotcrete is capable of producing a stronger, more durable bond to cementitious, masonry, or stone substrates than any other common application method. Understanding the reason behind this requires insight into the principles of the shotcrete process. Both wet- and dry-mix shotcrete’s superior bond properties are not due to mixture proportion differences between shotcrete and traditional concrete. Bond quality with shotcrete is derived from the very high energy imparted to the surface during shotcrete placement. Traditional casting placement methods rely on new material bonding to an existing substrate, essentially by contact. Conversely, shotcrete placement methods propel material to the substrate at a high velocity. This process significantly modifies the mixture’s proportions at the bond plane. As shotcrete material is initially placed, impact energy causes most of the mixture’s coarser components to bounce, rather than stick, to the substrate surface. Only the mixture’s smallest particles—the fine paste—can accumulate. As the paste layer builds, larger particles become embedded and rebound subsides.
It is shotcrete’s high-velocity nozzle stream, through the tendency of fast-moving larger particles to ricochet off a hard surface, which produces a tight, well-compacted paste layer, driven into the surface irregularities at the bond plane. This perfect material arrangement at the substrate surface facilitates an exceptionally strong crystalline connection—the primary element of a durable bond.
Defining a Durable Bond
Concrete can be designed to possess a very high compressive strength of 4500 to 7500 psi (30 to 50 MPa) or more. Its pull-apart resistance is comparably quite low at 145 psi (1 MPa). Bond strength values between new and existing concrete should be similar to the strength of the existing concrete. Therefore, for a bond to be considered durable, its bond strength should meet or exceed the pull-apart resistance of the underlying material. Typically, pneumatically applied shotcrete, when exposed to pull-apart or tensile loading, does not fail at the bond plane but, rather, within the substrate layer. This proves that the best possible bond has been attained.
Structural designs requiring durable bonding typically specify routine bond pulloff testing. Bond strength is commonly measured by coring through the shotcrete layer and into the substrate. A tensile load is applied to the surface of the core and then increased to the point of failure. The measured load failure divided by the core surface area provides a numerical bond strength. Both wet- and dry-mix shotcrete applications produce very good bond strength, typically 145 psi (1 MPa) or higher.
Bond quality is a primary requirement for repair durability. Shotcrete’s unique material arrangement at the substrate surface enhances durability through improved bond strength.
Note: Bonding agents are not recommended for shotcrete applications. Bonding agents interfere with shotcrete’s natural bond qualities and can create unreliable bonding.
ACI CP 60(09), 2009, Craftsman Workbook, American Concrete Institute, 92 pp.
Beaupre, D., 1999, “Bond Strength of Shotcrete Repair,” Shotcrete, V. 1, Spring, pp. 12-15.
Duckworth, O., 2011, “Can Nozzleman Skill Affect Bond Quality?” Shotcrete, V. 13, Winter, pp. 30-33.
The Abraham Lincoln Memorial Bridge, located in LaSalle, IL, is the longest bridge in the state, with a total length of 7122 ft (2170 m) and supported by 86 piers, with 43 in each direction. The bridge is elevated approximately 70 ft (21 m) above the Illinois River, numerous local roads, lakes, wetlands, and railroads. The piers range from 50 to 100 ft (15 to 30 m) high, 41 ft (12 m) wide, and 4 ft (1.2 m) thick at the caps while increasing to 6 ft (1.8 m) at the base. The piers span between 135 and 165 ft (41 and 50 m).
Bridge deck access to the piers below was severely limited to a 10 ft (3.048 m) area adjacent to live traffic. Access on the ground was limited to only a handful of piers but they were virtually inaccessible, as they were surrounded by the Illinois River, wetlands, lakes, and the historic Illinois-Michigan Canal. For the piers that were surrounded by land, 70 ft (21 m) tall boom lifts were placed over the side of the bridge with cranes. Platforms set under the deck were constructively engineered as a safe way to be used at all finger-joint piers over the river, lakes, and wetlands that were otherwise inaccessible.
The adaptability of shotcrete provided a superior solution to undertake the repairs. All of the shotcrete work was done from the bridge deck; and the use of shotcrete and its versatility had many advantages compared with other processes, such as form and cast. One example was the ability to remove concrete on the piers segmentally. On several piers that were severely deteriorated, one-third of the pier was prepared and then shotcreted to structurally replace the removed concrete. Once the shotcrete in the repaired section reached 70% of its required strength, the crew remobilized and repeated the procedure on the next one-third of the pier. This eliminated any concern for destabilization of the structure.
Using a packaged, preblended shotcrete material mixed on site as needed allowed a fresh, quality-controlled mixture with a consistent water-cementitious material ratio (w/cm). Ready mixed concrete that would have been used in a “form-and-pour” process would have a short open time to work with after driving to the site from the batch plant. There would have also been risk of a form blowout and polluting the river and wetlands below the working area. Shotcrete was also chosen for safety reasons. The air and water hoses going over the side offer considerably less risk than lowering and roughly handling lumber in the limited spaces underneath the bridge deck. Also, if a problem occurred with concrete placement in the middle of casting the “form-and-cast” repair, it would be hidden inside the form and require removal of the form to correct. Shotcrete placement by an ACI certified nozzleman could immediately follow the surface preparation of the repair area by sandblasting or a high-pressure water blast of the edge of the patch. The shotcrete process allows a visual confirmation of full encapsulation of the reinforcing bars, while casting into a closed form could result with voids that aren’t apparent until the formwork is stripped. The curing of shotcrete by use of wet cotton mats was superior compared to a form left in place because it provided supplemental water to the concrete for curing rather than depending on the original mixture water. Safety, time, quality, adaptability, and cost all significantly contributed to the use of shotcrete, which also resulted in a durable, affordable repair with enhanced sustainability benefits.
Ability to Access Restricted Space and Difficult-to-Reach Areas, Including Overhead and Underground Accessing the site of a concrete pour can often be one of the biggest challenges faced by contractors. Good quality, pumpable concrete mixtures will help minimize the extent of the challenge. But, in the case of form-and-pump placement, forming crews will still be required to install forms and remove them after placement. The extent of the challenges can be magnified when accessing difficult-to-reach locations, such as an elevated bridge structure or a location deep underground in a mine or tunnel. These challenges can often be minimized when shotcrete is the placement method chosen.
The reduction or elimination of formwork minimizes movement of materials, a key benefit provided by the shotcrete process. In difficult-to-access locations, a shotcrete nozzleman can place concrete thousands of feet (1000 ft = 300 m) away from the material source, and is limited only by the amount of available air required to move the material for dry-mix applications and the pump selection in wet-mix applications.
When faced with the challenge of difficult access, it is important for specifiers to have a strong understanding of the capabilities and limitations of the shotcrete placement process. Understanding these capabilities and limitations will allow them to specify shotcrete for applications where conventional form-and-pump placement methods are difficult and costly to undertake.
Dry-Mix Shotcrete Process
When placing dry-mix shotcrete, the concrete mixture is pneumatically conveyed through a shotcrete hose or other conveying pipe until it reaches the nozzle, where the water required for hydration is added by the nozzleman. The benefits in terms of access are obvious. The distance that a dry concrete mixture can be conveyed is dependent on a number of factors—the most important being volume of air. A minimum of 185 ft3/m (5.25 m3/m) is usually required to convey a dry shotcrete mixture a distance of 100 ft (30 m), allowing the shotcrete nozzleman to access elevated locations such as a bridge deck or confined spaces such as a sewer pipe, while the remainder of the crew’s material and equipment are staged at an easier-to-access location. If higher volumes of air (and longer hose lengths) are available, the distance over which the shotcrete can be conveyed can be much greater.
Wet-Mix Shotcrete Process
When placing wet-mix shotcrete, the concrete mixture (with water already added) is pumped through a hose and air is added at the nozzle to increase the speed of the mixture to achieve a high velocity. Benefits in terms of access are the same as those experienced when using dry-mix shotcrete. The distance that wet-mix shotcrete can be pumped is dependent on the capabilities of the concrete pump and, of course, the available length of the hose. One important difference between the two placement methods is the weight of the hose. In the dry-mix shotcrete process, much of the material composition (as it travels through the hose) consists of air. In the wet-mix shotcrete process, the material composition also contains admixtures and the water required for hydration. The result is a much heavier material hose that is more difficult to maneuver.
Access in Underground Environments
Difficult access is a common challenge when placing concrete in an underground environment. In mining environments, concrete placement is often required in locations that are extremely difficult to reach. One example would be ore bins that often require concrete lining; or shafts and raises that require stabilization. A nozzleman can usually access a raise (often from a working platform) hundreds of feet (100 ft = 30 m) above the access point where concrete can be placed using either the dry or wet shotcrete process. Both processes have challenges that are common to many underground environments—the most common being communication between the nozzleman and the gun/pump operator.
It is imperative from a safety standpoint that communication between the nozzleman and the gun/pump operator always be maintained. A constant line of communication is critical to ensure that the material flow is cut off in the event of a plug or an injury. It should be noted, however, that in the case of underground environments, visible contact is not always possible and must be substituted with verbal communication, usually through the use of voice-activated headsets.
For an application in which dry-mix shotcrete material is conveyed vertically over significant distances, consideration should also be given to the available water pressure. Although the volume of air may be sufficient to convey the material to a waiting nozzleman, the water pressure must also be sufficient to ensure the mixing water also reaches the nozzle. This can be of particular concern in a confined space where lack of water at the nozzle would result in dry, cementitious material filling the space. This provides another example of the importance of communication between the nozzleman and the gun operator.
When shotcrete equipment is located above the placement site and the shotcrete material is conveyed down hundreds of feet (100 ft = 30 m) into a raise or ore bin, the increased velocity of the material can often lead to inconsistent material flow and hose blockages. Depending on the process (wet or dry), there are a number of steps that can be taken to minimize the effect of gravity on the material conveyed down to the point of placement. First, the hose can be “looped” to decrease the velocity of the material prior to it exiting the nozzle and to help balance the speed of the material. This is especially true in the dry-mix process when combined with a decrease in conveyance air, as required.
Although the effect of gravity must be monitored closely in the dry-mix process, the wet-mix process is more susceptible to hose blockages and nozzle pulsation due to the force of gravity on the material within the pipeline. As the conveying hose or pipe is filled, segregation can occur as the material enters free-fall, and a plug can arise before shooting begins. The use of a sponge ball or “Go Devil” can eliminate plugs caused by segregation, as this ball restricts material flow and forces the concrete pump to naturally convey material through the line. As with the dry-mix process, a “loop” in the hose or the addition of an elbow (configured similarly to a “P Trap” under a sink) can help to regulate material flow and allow the pump to convey material, instead of gravity conveying the material. For this reason, it is often easier to convey concrete up, rather than down, especially when the conveying distance is in excess of 100 ft (30 m).
Access in Elevated Environments
The rehabilitation of elevated bridge structures is a common example of an application where the shotcrete process can minimize the problems related to access. Once deteriorated concrete is removed, it must be replaced to ensure structural integrity. On elevated structures, especially over water, railways, or heavy traffic areas, it can be extremely challenging to access the areas where concrete placement is required. In a form-and-pump application, forms and form hardware must be lifted into place and removed several days later, after the concrete reaches sufficient strength. This can be an arduous task, especially when several patches throughout the structure require repair. Through the use of the shotcrete process, a nozzleman and finisher can shoot, cut, finish, and cure several patches in succession, without having to return to the same location on the structure. This results in lower labor costs and a reduced construction schedule. The shotcrete process also allows the contractor to stage materials and equipment from one central location, accessing several repair areas from that location, thereby minimizing movement, planning, and time necessary to shift operations.
Access in Remote or Isolated Environments
Concrete placement challenges can also come from projects where access to the project itself is the biggest challenge. Repairs to an isolated lighthouse and construction of a remote dam are two examples. For these types of projects, a lack of proximity to a batch plant, cement supply, and aggregate source generally makes pre-blended, pre-bagged materials the best choice for material supply. As with other projects where access is a challenge, the less formwork required, the more efficient the concrete placement process will be. When the Haut-fond Prince Lighthouse rehabilitation was tendered in 1996, shotcrete was specified as the placement method and a highly accelerated, durable, steel-fiber shotcrete mixture, supplied in lined marine tote bags, was specified for the material supply. The site was located in the middle of the St. Lawrence River, 5 miles (8 km) from the coast of Tadoussac, QC, Canada, and was only accessible by barge and only when the weather was cooperative. Shotcrete nozzlemen worked from an inflatable zodiac, with all material, equipment, potable water, and other personnel located on the main barge. The shotcrete was placed ahead of the rising tides through a 490 ft (149 m) long hose that ran from the barge to the zodiac. The shotcrete process allowed easier access to the site and eliminated any need for forms. It played a significant role in the successful completion of this project.
Access in Challenging Terrains
Whether selecting a wet or dry shotcrete process, it is much easier for a nozzleman to reach a difficult-to-access placement site while the crew, material, and equipment are stationed in an accessible location. Difficult access, however, can mean different things to different people. The location of a backyard swimming pool, for example, can be considered a difficult-to-access location for the pumps and other materials supply equipment. Other examples of difficult access can be much more extreme and include projects that require skilled nozzlemen who are also skilled rock climbers.
An example of the latter occurred in 2003, when New Jersey Transit was faced with the challenge of stabilizing the rock face of the King’s Bluff Slope and Weehawken Tunnel East Portal along the Hudson-Bergen Light Rail Transit Line. Shotcrete crews from Atlantic Underground Services Ltd. (AUSL) of Riverview, NB, Canada, were contracted to place up to 4 in. (101 mm) of steel fiber-reinforced, dry-mix shotcrete over a rock face situated up to 200 ft (61 m) above the rail line. In addition to being skilled in the art of dry-mix shotcrete placement, the AUSL nozzlemen had also mastered the skill of rappelling down rock faces. This skill allowed the AUSL nozzlemen to place over 236 yd3 (181 m3) of material over a period of several weeks, while the remainder of the crew handled the material delivery and equipment operation at the base of the bluff.
Accessibility is a key factor when choosing which concrete placement method is best suited for a specific application. On projects where access is difficult, the traditional form-and-pour school of thought will often leave contractors with challenges that are difficult and costly to overcome. The benefits offered by shotcrete will provide specifiers with an effective alternative that is less labor-intensive, more sustainable, and less costly.
The Summer 2013 issue of Shotcrete magazine’s Sustainability column marked the completion of a 10-article series detailing the “Top 10 Sustainability Benefits of Shotcrete.” We’ve had great contributions to the series by various authors, including Cathy Burkert, Michael Cotter, Oscar Duckworth, Charles Hanskat, Joe Hutter, Ray Schallom III, Ted Sofis, and Marcus von der Hofen.
As a reminder, our “Top 10” series covered these topics:
1. Formwork savings of 50 to 100% over conventional cast-in-place construction.
2. Formwork does not have to be designed for internal pressures.
3. Complex shapes require very little—if any—formwork.
4. Crane and other equipment savings or elimination.
5. Labor savings of at least 50% in repair applications.
6. New construction speed savings of 33 to 50%.
7. Speed of repair reduces or eliminates downtime.
8. Better bonding to the substrate enhances durability.
9. Adaptability to repair surfaces that are not cost-effective with other processes.
10. Ability to access restricted space and difficult-to-reach areas, including overhead and underground.
Additionally, over the course of the 2-1/2 years our “Sustainability Top 10” series has run, we’ve had many other articles in Shotcrete magazine that have addressed sustainability topics, either directly or indirectly. In many ways, sustainability is becoming a key aspect of all types of shotcrete work we report on in Shotcrete magazine. In fact, three articles in last Fall’s issue directly relate to sustainability!
The Technical Tip, “Material Velocity at the Nozzle,” by Nicolas Ginouse and Marc Jolin, detailed practical research on nozzle material velocity with the intent to develop future guidance on optimizing—and hopefully reducing—rebound. This enhances sustainability because we can use less material and labor to produce the same structural element.
“Limestone Cement in Shotcrete” looked at the growing use of ground limestone “filler” as partial portland cement replacement in shotcrete mixtures. Although the amount of limestone “filler” that can produce equal durability in combination with lesser amounts of portland cement is being widely debated, the substitution of any amount of cement has sustainability benefits due to the reduced production of greenhouse gases associated with cement production.
“The Use of Recycled Glass in Shotcrete,” by Isabelle Fily-Paré and Marc Jolin, looked at replacement of a portion of portland cement in our mixtures with recycled glass. Cutting cement and the greenhouse gases associated with cement production and recycling glass that may otherwise go into a landfill are both factors that lead to enhanced sustainability.
Take a moment to review our Top 10 list on the previous page… When you look closely, it is clearly evident that nearly everything we do with the shotcrete process is more sustainable than normal cast-in-place concrete. Shotcrete inherently creates enhanced sustainability because we have:
Fewer forms, and when we use forms they create much lighter construction;
Easy construction of curved or variable-thickness shapes, which allows maximum structural efficiency with the least amount of material;
Less heavy equipment on the job site;
Substantially less labor with the reduction in formwork activities;
Less material and labor, which equates to faster completion of a given structural section; and
Adaptability to nearly any repair, renovation, or repurposing of a structure, which means we can substantially prolong the life of existing concrete structures with more durable shotcrete rather than require demolition and rebuilding of the structure that uses more resources.
If you find yourself promoting the use of shotcrete in lieu of cast concrete on a project, pull out this Top 10 List, our ASA Sustainability brochure, or go to the shotcrete sustainability web link, shotcrete.org/why-shotcrete/sustainability, to prove that not only will the project have cost and time benefits but by enhancing the sustainability, you will also ultimately help preserve our world for future generations.
It’s clear from everything we do as an industry: Shotcrete = Sustainability.