Dr. Stelios Antoniou
Head of Structural/ Earthquake Engineering Department at Alfakat
Co-founder, Managing Director and R&D Director at SeismoSoft

Table of Contents

Introduction
Dry Mix VS Wet Mix Shotcrete
Advantages and Disadvantages of Shotcrete
What is it Actually Called, Shotcrete or Gunite?
Materials, Proportioning, and Properties
Mix Proportions for the Dry-Mix Process
Equipment and Crew
Curing and Protection
Testing and Evaluation
References

The term ‘shotcrete’ refers to both the material and the construction method. The material is a concrete or a high-strength mortar, which is literally ‘shot’ into the forms. The method is the application of this material on site.

Strictly speaking shotcrete (or gunite or sprayed concrete, as it is also called) is not a repair or strengthening method for existing buildings. It is a way of placing and compacting concrete and it has numerous applications, other than retrofit. For example [US Army Corps of Engineers, 1993], it can be used (i) to repair spillway surfaces or marine structures that may be damaged by cavitation, abrasion erosion, corrosion of the reinforcement or deterioration of the concrete (ii) in underground engineering, as a temporary measure for slope support and stability, for supplementing or replacing conventional support methods such as steel beams, to seal rock surfaces, or to channel water flows, (iii) in tunnel engineering, in mines, subways, and automobile tunnels (Figure 1a) and (iv) in new structures, e.g. for the construction of pools, tanks or domes. However, due to the restrictions imposed in existing buildings by their structural and non-structural components, cast-in-place concrete in the majority of cases is difficult, expensive or altogether impossible to apply, which makes shotcrete the most usual way of applying concrete in repair and retrofit applications. In fact, the use of shotcrete is so common when constructing RC jackets that the two terms are often used in an interchangeable fashion in strengthening applications.

Figure 1: Application of shotcrete (a) in tunnel engineering , (b) in RC buildings and (c) in masonry buildings*

Shotcrete is the official name of the sprayed high-performance concrete, conveyed through a hose and pneumatically projected at high velocity onto a surface. The application of a fine aggregate cement mixture with the use of pneumatic systems was first introduced in the early 1900’s, and since then many improvements have been made in the equipment and in the specialized techniques required.
Shotcrete usually contains an increased content of cement and aggregates with a small granulometric gradient. It is placed and compacted at the same time, due to the force, with which it is ejected from the nozzle. It can be sprayed onto any type or shape of surface, including vertical or overhead areas (Figure 1). A large proportion of the shotcrete material is expected to bounce off the shooting surface, and falls on the ground as rebound waste. (Figure 2).

Figure 2: Rebound waste after the application of shotcrete*

Shotcrete can be employed in two variations, wet-mix and dry-mix; the distinguishing feature is whether the water, which is required for the mixture hydration, is being injected at the nozzle, immediately before it is discharged onto the receiving surface (dry mix), or beforehand, during the creation of the mix (wet mix). Shotcrete is usually an all-inclusive term for both versions.

The dry mix method involves mixing the ingredients in dry conditions, placing them into a hopper or a bag, and then conveying them pneumatically with a continuous flow through a hose to the nozzle. The cement and aggregate mixture is prepared on site and the water necessary for the cement hydration is injected at the nozzle by a nozzleman, who controls the addition of water. The water and the dry mixture are often not completely mixed at the nozzle, but the hydration is completed as the mixture hits the receiving surface. This process requires a skilled nozzleman, especially in the case of thick or heavily reinforced sections.

Wet-mix shotcrete involves pumping of a previously prepared concrete, typically ready-mixed concrete, to the nozzle. The cementitious material, aggregate, water, and admixtures are thoroughly mixed as would be done for conventional concrete. Compressed air is introduced at the nozzle to impel the mixture onto the receiving surface. The wet-process procedure generally produces less rebound waste and dust compared to the dry-mix process.

Below is a list of the most typical differences between the two types of concrete:

(i) Dry-mix shotcrete is applied at a much slower rate than wet-mix, and the production rates of the latter are considerably higher. Although the production rates depend highly on the in-situ conditions (obstacles, reinforcement, rebound) the maximum productivity of wet-mix can be as high as 4-5 m3/h, whereas that of the dry-mix is less than 1 m3/h.
(ii) With the dry-mix, intermittent use is easily accommodated, as the dry material is easily discharged from the hose; on the contrary wet-mix is better suited to continuous applications.
(iii) The equipment and maintenance cost for the dry-mix is generally lower than those for the wet-mix
(iv) Because the dry-mix that is conveyed through the hose is lighter than the wet-mix, longer hose lengths are possible with the dry-mix
(v) The rebound percentages are generally higher with the dry-mix
(vi) When properly proportioned and applied, the dry-mix has better bond and higher strengths than the wet-mix, allowing more effective placement in overhead and vertical applications without the use of accelerators
(vii) The application of the wet-mix is generally easier; the cementitious materials and the aggregates are mixed with the water and additives prior to the shotcrete application, and the nozzleman does not have to be as skilled as in dry-mix case
(viii) The wet-mix can be used with all ordinary admixtures of common concrete, whilst only accelerators can be added in dry-mixes. The use of air-entraining admixtures (AEA) in shotcrete is practical only in wet-mix, and the resistance of dry-mix to freezing and thawing is poor.

The differences in the equipment cost, maintenance requirements, operational features, placement characteristics, and productivity may make one or the other of the two alternatives more attractive for a particular application. The dry mix process is much more common in repair and retrofit applications, where it is necessary to stop frequently (e.g. to move from one member to the next one), and in general the larger productivity of the wet-mix is not required (small to medium sized projects). On the contrary the wet-mix process is more common in underground works, where there are larger areas without obstacles, and its continuous application is possible.

Shotcrete can be used in lieu of conventional concrete for reasons of convenience or, less frequently, of cost. Shotcrete is advantageous in the situations when formwork is cost prohibitive, impractical or altogether impossible, due to limited access to the work area. Very thin layers of shotcrete can be achieved (up to 3-4 cm when reinforcement is included), considerably thinner than normal casting techniques that require at least 10-12 cm of thickness when reinforcement is present. This makes shotcrete ideal for reinforced concrete jackets.

In retrofit, shotcrete has become a material of vital importance, because of its versatility in shape, which enables the application of concrete in areas with difficult access or totally inaccessible to poured concrete, e.g. columns below floors that are in use and cannot sustain damage. It is placed, consolidated, and compacted at the same time, and the small aggregate size helps improve quality and manageability. It adheres to surfaces, and it has reduced shrinkage and lower permeability. Shotcrete usually provides significantly higher bond strengths to existing materials than does conventional concrete.

However, shotcrete is generally more expensive than traditional cast-in-place concrete, especially in countries with increased labour costs. Furthermore, in the case of the dry-mix method, the concrete is not created in a controlled industrialized environment since there is no way of measuring exactly the amount of water that is added at the nozzle, and correlating it in an accurate fashion to the amount of cement. As a result, increased skill and experience is required by the nozzleman, whilst and continuous attention should be paid by the supervisor, in order not to have a very dry mix, which leads to large rebound waste, or a high water content, which results in the slumping of the concrete.
Because of these difficulties, even though the physical properties of sound shotcrete are comparable or superior to those of conventional cast-in-place concrete, the improper application of shotcrete may lead to unacceptably low strengths. Furthermore, considerably larger variations with respect to the cast-in-place concrete are commonly found in the quality and strength, even within the same project, and a larger mean strength is required in order to achieve the target concrete class (because of the increased standard deviation).

Finally, shotcrete, especially the dry-mix variation, is a relatively ‘dirty’ process, and it suffers from high dust production, and a large proportion of the materials is rebound waste.

Gunite was originally a trademarked name coined by the American taxidermist Carl Akeley in 1909 and patented in North Carolina. Nowadays in strengthening and retrofit shotcrete and gunite mean the same thing, and the two terms are interchangeable. However, this is not the case in other fields of engineering. For example, in pool construction shotcrete refers to the wet mix and gunite to the dry mix. [Jender 2020]. In other areas ‘gunite’ has been used to denote small-aggregate sprayed concrete, and ‘shotcrete’ to denote large-aggregate mixtures.

In any case, the preferred and the most commonly used term today for all gunned material is shotcrete, regardless of the aggregate size, in most fields of engineering.

The materials, mixture proportions, and properties of shotcrete are similar in many respects to conventional concrete. The main materials that can be found in a shotcrete mix are the following [US Army Corps of Engineers, 1993]:

Cement

In general, the cement requirements for shotcrete are similar to those for conventional concrete, and the same Standards apply. For instance, Portland cement must meet the requirements of ASTM C 150 [ASTM, 2020] and blended cement must meet the requirements of ASTM C 595 [ASTM, 2020].

Pozzolans

Pozzolans are a broad class of siliceous or siliceous and aluminous materials, which in themselves possess little or no cementitious value but which, in finely divided form and in the presence of water, react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties. The term pozzolan embraces a large number of materials with different origin, composition and properties, and includes both natural and artificial (man-made) materials [Wikipedia 2020, Mehta 1987].

Pozzolans are added to the shotcrete mixes, in order to achieve improved long-term strength performance and lower permeability. When added to a Portland-cement matrix, pozzolan reacts with the calcium hydroxide and water to produce more calcium silicate gel. Pozzolans should conform to the ASTM C 618 Standard [ASTM, 2019].

Some pozzolans (e.g. fly ash) are sometimes added to wet-mix shotcrete to enhance workability, facilitate pumping in longer distances, increase resistance to sulfate attack, and reduce expansion caused by the alkali-silica reaction, however this is not without controversy (for instance the early age strength development is delayed). Natural pozzolans and fly ash are not typically used with dry-mix shotcrete. On the contrary, silica fume is very often used in dry-mix shotcrete and does not delay the strength development.

Silica Fume

Silica fume, also known as microsilica, is an amorphous (non-crystalline) polymorph of silicon dioxide, silica. It is an ultrafine powder, collected as a by-product of the silicon and ferrosilicon alloy production and consists of spherical particles with an average particle diameter of 150 nm. [Wikipedia 2021a]. The material is over 85 percent silica dioxide, it is approximately 100 times finer than portland cement, and it has a specific gravity ranging from 2.1 to 2.6. Because of its chemical and physical properties, it is a very reactive pozzolan, and its main field of application is as an admixture of shotcrete and concrete.

Silica fume is added to a shotcrete mixture as a cement replacement or, more often, as a supplement to cement, in which case its proportion ranges from 7 to 15% by mass of cement. Silica fume significantly increases the concrete strength, decreases permeability, increases the long-term durability in mechanical and chemical attacks, improves the cohesion and bond strength of shotcrete to substrate surfaces, and reduces the shotcrete rebound. Moreover, it enhances the resistance to carbonation and the resistance to highly aggressive chemical environments (e.g. high sulphate concentrations, refineries, or chemical industries) and prevents the ‘washout’, when fresh shotcrete is subject to the action of flowing water. Although its other characteristics are also very important, silica fume is mainly used in shotcrete to achieve high strengths that can reach up to 80 MPa. Because of its extreme fineness, silica fume particles fill the microscopic voids between the cement particles reducing permeability, increasing the density and the strength of the shotcrete.

Aggregates

Aggregate should comply with the quality requirements for ordinary concrete, e.g. ASTM C 33 [ASTM 2018], however, due to the nature of shotcrete and in order to minimize the rebound, aggregates with smaller grain sizes are usually used, and a uniform grading is essential. Similarly to ordinary concrete, the aggregate sizes are provided by Tables or Charts [Table 1]. Note that finer aggregates generally produce shotcrete with greater drying shrinkage, while coarser sands result in more rebound.

Table 1: Acceptable grading limits for the aggregate [US Army Corps of Engineers, 1993]

Water
While there are no special requirements for the curing water, it is highly recommended to use potable water in the shotcrete mix.

Fibre Reinforcement

Fibres are used to increase the ultimate strength, particularly the tensile strength of shotcrete, as well as its ductility and energy absorption capacity. Furthermore, fibres allow for better crack control and may decrease the width of shrinkage cracks in the material. Fibres are discontinuous and unlike conventional reinforcement, they are distributed randomly throughout the concrete matrix. In shotcrete they are available in the following general forms [US Army Corps of Engineers, 1993]:

(i) steel fibres
(ii) glass fibres that consist of chopped glass fibres with a resin binder
(iii) synthetic macro or micro-fibres (e.g. polypropylene, polyethylene, polyester, or rayon), with polypropylene fibres being the most widely used material. Synthetic fibres are derived from organic polymers. Synthetic fibre types used in concrete
(iv) natural fibres, but these are not commonly used in shotcrete
a. Steel fibres. Steel fibres have been used since the late 1950’s in shotcrete to increase its mechanical properties. While steel fibres for reinforced concrete are commercially available in various sizes, the typical fibre lengths for shotcrete range from 2 to 4 cm. The typical proportion is between 1 and 2% by volume.

Steel fibres are found in different shapes, round, flat, or irregular, while additional anchorage is provided by deformations along the fibre length or at the ends, e.g. fibres with hooked or flat ends, with crimps, corrugations, or undulated fibres (Figure 3). Steel fibres provide a modest increase in the tensile and flexural strength, but limited increase in the compressive strength of shotcrete, however they contribute to improved load carrying capacity after the cracking of the member.

b. Synthetic and polypropylene-fibres. The technology of using synthetic fibres in shotcrete is relatively new, compared to steel fibres, but their use is growing rapidly, and nowadays they are preferred to steel fibres (Figure 4). A wide range of types is available: aramid, polypropylene, polyethylene, polyester, or rayon, but polypropylene is mostly used in retrofit applications. The most common specified lengths for polypropylene are between 2 and 4 cm, however longer lengths can also be accommodated. The typical amount to be added to the shotcrete mix is between 1.0 and 3.0 kg/m³, and it does not require any change in the mixing ratios of the other concrete materials.

The most important use of synthetic fibres is to control plastic shrinkage, but synthetic fibres also increase the toughness and the tensile, and to a limited extent the compressive, strength of the shotcrete.

c. Glass fibres. The fibres consist of alkali-resistant glass (designated AR glass), which is protected from the alkalinity of cement. Alkali resistance is achieved by adding zirconia to the glass, the higher the zirconia content the better the resistance. The application of glass-fibre shotcrete cannot be done with the conventional equipment for shotcrete, but requires a special gun and delivery system, hence glass reinforced shotcrete is not widely used in strengthening and retrofit operations.

d. Natural fibres. These are natural fibres of different origin, such as bamboo, sisal and coconut, but they are rarely used in shotcrete nowadays.

Chemical Admixtures & Accelerators

As mentioned above, accelerators are the only admixtures that can be added in dry-mix shotcrete. All the other chemical admixtures, including air-entraining (AEA), water-reducing and retarding admixtures, can only be used with the wet-mixes.

Nowadays, there is a large variety of accelerators, both powdered and liquid, that may have different effects depending on their chemistry, their dosage rate, and the chemistry of the cement and the aggregates [US Army Corps of Engineers, 1993]. Powdered accelerators are mostly used for dry-mix shotcrete, whereas liquid accelerators can be used with both dry and wet mixes, and they are added at the nozzle prior to the application. In the case of the dry-mix process the admixtures are usually premixed with the water.

Because of the limitations related to the required equipment, the use of admixtures in shotcrete is not the same as in conventional concrete. Furthermore, some admixtures may adversely affect the shotcrete properties, for instance, some accelerators may reduce the compressive shotcrete strength as high as 40%, or reduce its frost resistance. Therefore, the shotcrete mix that contains the admixtures should be tested in the field prior to application, in order to ensure that the desired properties are achieved.

Reinforcing Steel

In general, the same specifications as for conventional concrete should be met by the reinforcing rebars in shotcrete. However, because of the sprayed placement method, the use of bars larger than Ø20mm should be avoided. Similarly, large rebar concentrations interfere with the placement, prevent the correct build-up of good quality shotcrete, and can leave large voids behind the reinforcement. Rebar spacing of at least 15 cm is recommended in at least one of the directions in the plane of the shotcrete application.

In the wet-mix process, batching and mixing are practically identical to conventional concrete, thus allowing for very good control and more versatile mix designs. On the contrary, with the dry-mix process the preparation of the cement and aggregates mix and the addition of water is carried out on site with conditions that cannot be strictly controlled. A very good preparation is required so as to minimize the deviations from the correct material proportions (which are frequently inevitable in the on-site conditions).

Initially, the mix proportions are calculated, based on the water to cement ratio (usually between 0.30 and 0.50 by weight), the aggregates to cement ratio (usually between 2.50 and 4.50 by weight), and the actual specific weights of the materials used. During the shotcrete application on site, it is preferable to measure the cement and sand by weight rather than by volume. The most efficient and practical way to do so on site, is with the use of buckets; buckets filled with cement and the aggregates are weighed and the final dry mix is formed by mixing, for instance 3 buckets of cement, 4 buckets of sand and 2 buckets of coarse aggregates. Because the sand particles should be thoroughly coated with cement, mixing of at least one minute in a drum – type mixer is required. It is noted that the in-place cement proportion will be higher, and the in-place aggregate grading will be finer than the batched grading due to rebound, especially if larger aggregate sizes are used.

The application of shotcrete should be closely supervised by an experienced engineer, in order to guard against large rebound (which is an indication of an overly dry mix or aggregate problems) or slumping of the concrete (which is an indication of high water content). Furthermore, field testing of the dry-mix proportions is highly recommended, especially if no field data exist for a given dry-mix. Because the shotcrete strength can vary even for the same mix proportions, depending on the aggregates strength, and the abilities and the proficiency of the nozzleman, prior to the final application of the shotcrete it is advisable to test 2-3 different mixtures, in order to check that an appropriate mix will be applied.

Dry-mix Process

The cementitious materials and the damp aggregates are thoroughly mixed and bagged, possibly well in advance of the shotcrete application (provided that they are kept in dry conditions). Prior to the shotcrete gunning, it is often advantageous to premoisturize the mix to 3-6% by dry mass using a premoisturizer, an apparatus that distributes and mixes water to the dry materials.

The cement-aggregate mixture is then fed to the gunning machine and introduced into the delivery hose through a metering device such as a feed wheel to ensure a constant feed is passed. Compressed air generated by an air compressor is added at the gun and the mixture is carried through the delivery hose to the nozzle. A perforated water ring is fitted at the nozzle, through which the water and the admixtures are introduced. The materials are all mixed to concrete, as they go through the nozzle. The concrete is propelled from the nozzle at high velocity onto the receiving surface.

Dry-mix guns are classified in two categories:

(v) The double chamber gun shown in Figure 5, which was first introduced in the early 1900’s, but its use limited nowadays. The material enters the upper chamber in batches, but the valve arrangement is such that the discharge from the lower chamber is continuous.
(vi) The continuous feed gun, which is more common today and is shown in Figure 6 and in Figure 7. It was first introduced in the early 1960’s. A rotary gun is employed and it is continuously fed using an open hopper.

Figure 5: Sketch of the double chamber gun for shotcrete [US Army Corps of Engineers, 1993]

Figure 6: Sketch of the continuous feed gun for shotcrete [US Army Corps of Engineers, 1993]

Figure 7: The continuous feed gun for shotcrete on site*

Dry-mix nozzles come in a wide variety of nozzle tips, nozzle sizes, and configurations. A typical nozzle consists of a tip, water ring, control valve, and nozzle body arranged, as depicted in Figure 8.

An air compressor of ample capacity should be employed. The compressor should maintain a supply of clean, dry, oil-free air adequate in order to provide the pressure that drives the material from the delivery equipment into and through the hose, and to maintain sufficient nozzle velocity at all parts of the work. The air pressure should be steady (non-pulsating). Typical air compressors are characterised by their capacity in terms of the guaranteed air delivery (e.g. 275, 400, 600 ft3/min in the Imperial system; m3/min or lt/sec in the metric system) at the norm effective working pressure (typically 100 psi in the Imperial system or 7bar in SI units or higher). For larger hose diameters and lengths, larger nozzles and larger productivities, a larger capacity of the compressor is required.

The layout of a typical plant for dry-mix shotcreting is shown in Figure 9.

Figure 8: A typical dry-mix nozzle [US Army Corps of Engineers, 1993]

Figure 9: A typical plant layout for dry-mix shotcreting [US Army Corps of Engineers, 1993]

Wet-mix Process

The cement, the aggregates, and the admixtures (except accelerators) are mixed, and the mixture is fed into the wet-mix gun and propelled through the delivery hose to the nozzle by compressed air or pneumatic or mechanical pumping. Air is injected at the nozzle to disperse the stream of concrete and generate the velocity for shotcrete placement.

A typical wet-mix nozzle consists of a rubber nozzle tip, an air injection ring, a control valve, and nozzle body, as shown in Figure 10.

Figure 10: A typical wet-mix nozzle [US Army Corps of Engineers, 1993]

The curing of shotcrete is extremely important, in order to ensure the proper hydration and bond strength development, and to prevent cracking due to shrinkage. It is noted that the relatively thin sections commonly used in retrofit applications of shotcrete are particularly susceptible to drying shrinkage, and that the development of bond strength is significantly slower than that of the compressive or tensile strength.

The shotcrete surfaces should be kept continuously moist for at least 7 days; after this time interval the shotcrete has gained sufficient tensile strength to resist shrinkage strains.

The strength of shotcrete should be verified at established intervals. The testing is usually carried out with cylindrical specimens extracted from rectangular or square test panels mounted in a framework (Figure 11 and Figure 12). The size of the testing panels should be large enough, so as to obtain all the test specimens needed with the same level or uniformity and quality that can be expected in the structure. Hence, square panel of dimensions of at least 70×70 cm (preferably even more) should be employed. The thickness of the shotcrete should be no less than 12 cm, in order to allow the extraction of specimens at least 10 cm high. The distance between the cores and from the panel edges should be at least 10 cm, and the cores should be extracted after at least 7 days of standard curing, so that to attain sufficient strength and allow movement to the testing laboratory.

Since the quality of shotcrete is highly dependent on the abilities of the nozzleman, a separate panel should be employed for each nozzleman, as well as for each shooting position in the structure.

Figure 11: Test panel support system [Mahar et al. 1975]

Figure 12: Test panel on site*

• ASTM International (2018). ASTM C33 / C33M-18, Standard Specification for Concrete Aggregates, ASTM International, West Conshohocken, PA, 2018
• ASTM International (2020a). ASTM C150 / C150M-20, Standard Specification for Portland Cement. https://doi.org/10.1520/C0150_C0150M-20.
• ASTM International (2020b). ASTM C595 / C595M-20, Standard Specification for Blended Hydraulic Cements, ASTM International, West Conshohocken, PA.
• ASTM International (2019). ASTM C618-19, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete, ASTM International, West Conshohocken, PA, 2019.
• Jender H (2020). Gunite vs. Shotcrete: What’s the Difference? Available at: https://www.riverpoolsandspas.com/blog/gunite-vs-shotcrete.
• Mahar, J.W., Parker, H.W. and Wuellner, W.W. (1975). Shotcrete practice in underground construction. US Dept. Transportation Report FRA-OR&D 75-90. Springfield, VA: Nat. Tech. Info. Service.
• Mehta, P.K. (1987). Natural pozzolans: Supplementary cementing materials in concrete. CANMET Special Publication. 86: 1–33.
• US Army Corps of Engineers (1993). Standard Practice for Shotcrete. Engineering Manual EM 1110-2-2005, 31 January 1993.
• Wikipedia (2020). Pozzolan. Last modified date July 9, 2020. Available at: https://en.wikipedia.org/wiki/Pozzolan
• Wikipedia (2021a). Silica fume. Last modified date February 5, 2021. Available at: https://en.wikipedia.org/wiki/Silica_fume
• *All the pictures, which are in the article and are not referenced, are courtesy of Alfakat (www.alfakat.gr), an official partner of Seismosoft.
• Structural Assessment, Strengthening & Retrofitting carried out using SeismoSoft Earthquake Engineering Software.