Application of Shotcrete in the Repair and Rehabilitation of Concrete Structures: A State-of-the-Art Review

Abstract

This state-of-the-art review critically evaluates shotcrete for concrete rehabilitation using an application-based framework covering buildings, bridges, tunnels and underground works, hydraulic and marine structures, and industrial facilities. The review synthesizes deterioration mechanisms, substrate and interface conditions, material and process variables, mechanical and durability performance, quality control, and alternative repair systems. Recent studies are integrated with established guidance and case histories, and quantitative evidence tables are used to facilitate cross-study comparison. Across structural applications, the shotcrete–substrate interface is identified as a critical factor governing rehabilitation performance, with surface condition, moisture state, shrinkage, curing, and environmental exposure affecting bond and durability. The synthesis further demonstrates that performance requirements vary by application, including section restoration, confinement, bond, and durability in buildings and bridges; early-age support, toughness, and residual capacity in tunnels; and low permeability and abrasion/erosion resistance in hydraulic and marine structures. Five research questions are proposed to address key uncertainties in interface behavior, durability, field performance, material development, and quality assurance. The resulting framework provides a systematic basis for evaluating shotcrete rehabilitation strategies and identifying priorities for future research.

1. Introduction

Concrete is one of the most widely used construction materials, yet many reinforced concrete (RC) structures around the world are now showing signs of deterioration. Common problems such as cracking, spalling, reinforcement corrosion, leakage, abrasion, and excessive deflection appear due to aging, poor workmanship, design errors, and harsh environmental exposure [1]. Many studies confirm that RC structures placed in aggressive environments such as coastal areas, marine zones, and regions with freeze–thaw cycles tend to deteriorate faster because of chlorides, moisture, carbonation, temperature variation, and chemical attack [2]. Similar deterioration issues have been observed in underwater and hydraulic structures where flowing water, abrasion, and erosion continuously damage concrete surfaces [3].

In specialized systems like post-tensioned structures, the progression of corrosion inside ducts and tendons can be severe and difficult to detect early, leading to sudden failures if not repaired in time [4]. For conventional RC members such as beams and columns, deterioration often reduces load-carrying capacity, stiffness, and durability, sometimes requiring strengthening or rehabilitation to restore performance [5,6]. Damage in bridge components is also common because many bridges have been exposed to deicing salts, salt spray, and mechanical fatigue over long periods, leading to cracking, corrosion, and loss of serviceability [7].

Because full reconstruction of deteriorated structures is expensive, time-consuming, and environmentally unsustainable, repair and rehabilitation have become essential strategies for maintaining existing infrastructure. Effective repair helps extend service life, improves structural safety, and avoids large financial losses caused by advanced deterioration [8]. Choosing the right repair method requires proper inspection, understanding of the damage mechanism, and selection of materials that are compatible with the existing concrete. Various repair materials such as polymer mortars, cement grouts, fiber-reinforced polymer (FRP) systems, ultra-high-performance fiber-reinforced concrete (UHPFRC) overlays, and concrete jackets have been used to restore strength and durability, each offering specific advantages and limitations [9].

Among the many available repair methods, shotcrete has become one of the most widely used solutions for concrete rehabilitation. Shotcrete is a process where concrete or mortar is sprayed at high velocity onto a surface, allowing it to be placed and compacted at the same time [10]. It is available for both the dry-mix and wet-mix processes, and it can be applied on vertical surfaces, overhead elements, and locations that are difficult to access. Several guideline documents and reviews describe shotcrete as a fast, economical, and durable repair method that requires little or no formwork, provides good bond with existing concrete, and can achieve high strength and low permeability when properly applied [11]. Shotcrete has been used successfully in repairing beams, columns, slabs, retaining walls, bridges, hydraulic structures, tunnels, and mines [11].

Shotcrete is especially useful when rapid construction is required or when working in confined or uneven spaces. Its ability to incorporate fibers, silica fume, admixtures, and high-performance materials allows for the production of tougher, more durable, and more workable mixes suitable for modern repair needs [12]. Shotcrete has also been applied effectively in marine environments, fire-damaged structures, and seismic strengthening projects, where reliable bond, low permeability, and quick application are important for long-term performance [13]. Because deterioration of RC structures continues to increase worldwide and infrastructure systems require reliable and efficient repair solutions, shotcrete has become an important technique in modern rehabilitation practice. This review paper examines the fundamentals of shotcrete technology, material characteristics, construction methods, structural and durability performance, and various repair applications. It also highlights existing standards, challenges, and future research needs to support the continued development of effective shotcrete-based rehabilitation strategies.

In this review, repair denotes localized actions intended to restore damaged concrete or reinforcement protection, while rehabilitation denotes the broader intervention required to recover or upgrade serviceability, durability, or structural capacity. Rehabilitation may therefore include repair alone, strengthening/retrofitting, protective measures, or a combined intervention. This distinction is important because a thin nonstructural patch and a reinforced shotcrete jacket have different design objectives, acceptance criteria, and evidence requirements. The review addresses three central questions: (1) How do deterioration mechanism and structural type change the requirements imposed on shotcrete repair? (2) Which material, substrate, placement, and curing variables most consistently govern interface and mechanical performance? (3) Where is the present evidence sufficiently quantitative to make engineering decisions, and where are the remaining unresolved uncertainties? The contribution is an application-based classification combined with cross-study evidence matrices that connect deterioration mechanism → repair objective → shotcrete function → performance indicator. This structure is used throughout the review to avoid treating buildings, bridges, tunnels, and hydraulic/marine structures as interchangeable applications.

Review Scope and Synthesis Framework

The literature was screened for studies and guidance directly relevant to shotcrete materials, sprayed-concrete construction, concrete repair interfaces, structural rehabilitation, durability, and quality control. Evidence was synthesized by structural application, deterioration mechanism, material/process variables, test method, and reported performance outcome.

2. Fundamentals of Shotcrete Technology

2.1. Definition and Classification

Traditional shotcrete approaches are based on two placement methods: dry-mix and wet-mix. Both use compressed air to propel the material onto the substrate [14]. The method is applied to new construction and repair situations where access is limited or formwork is impractical [10].

2.1.1. Dry-Mix Shotcrete

In dry-mix shotcrete, dry materials are conveyed through a hose and water is added at the nozzle. It offers good control of the water content and is suitable for smaller repairs and overhead applications [15]. It also allows for material adjustment on-site and has lower equipment complexity [11], although the nozzleman has greater responsibility for controlling the quality of the finished product than in the wet-mix process. Table 1 summarizes the main differences between dry-mix and wet-mix shotcrete processes.

Table 1. Comparison of Dry-Mix and Wet-Mix Shotcrete Processes [16].

Comparison Criterion	Wet-Mix Process	Dry-Mix Process
Water addition and control	Water is added and controlled during mixing before pumping, allowing more accurate and consistent water measurement.	Water is added at the nozzle, allowing the nozzleman to adjust the water content during placement.
Mixture consistency	Pre-mixing of water with the other constituents generally provides greater uniformity and consistency.	Mixture consistency depends more strongly on water adjustment by the nozzleman during application.
Dust generation	Generally produces less dust during application.	Generally produces more dust because dry materials are pneumatically conveyed before water is added.
Rebound and material loss	Typically produces lower rebound and associated material waste.	Typically produces higher rebound and greater potential material loss.
Production rate	Generally provides higher material output for a given hose diameter and is suitable for larger-volume placement.	Generally provides lower material output and is well suited to smaller-volume placement.
Intermittent operation	Less convenient when placement is frequently stopped because mixed material is already hydrated.	Well suited to intermittent or uncertain placement schedules because water is introduced only at the nozzle.
Hose handling	Pumping the wet mixture generally requires heavier-duty delivery equipment.	Delivery hoses are generally lighter and easier to maneuver.
Specialty materials	Material selection must account for the pumping requirements of the pre-wetted mixture.	Can be advantageous for some lightweight-aggregate or refractory mixtures because water is introduced later in the process.

2.1.2. Wet-Mix Shotcrete

Wet-mix shotcrete uses pre-mixed material pumped through a hose. It provides consistent quality, higher production rates, and lower dust generation, making it suitable for larger repair projects [12]. Figure 1 depicts a schematic of dry and wet shotcrete application processes.

Figure 1. Shotcrete application processes: (1) dry-mix, (2) wet-mix.

2.2. Materials and Mix Design

Shotcrete mixtures generally consist of cementitious materials, fine aggregates, water, admixtures, and sometimes fibers, with performance depending on a cohesive mix that can be pumped and sprayed without segregation. Portland cement or blended cements are commonly used, and silica fume is often added to increase cohesiveness and reduce permeability, especially for durable repair applications [12]. Well-graded fine aggregates are preferred to ensure smooth flow through hoses and to minimize rebound during placement [10]. Admixtures such as accelerators, water reducers, and viscosity-modifying agents are frequently incorporated to improve workability, control setting time, and stabilize the mixture during high-velocity projection [11]. Fiber reinforcement, including steel and synthetic fibers, is also used to enhance toughness, crack resistance, and impact behavior, making fiber-reinforced shotcrete suitable for demanding repair and stabilization projects [10,12].

2.3. Equipment and Application Process

Shotcrete requires pumps or air-delivery systems, hoses, and nozzles. Proper equipment setup ensures continuous flow, minimal blockage, and uniform application [11]. Application technique strongly affects the final quality and must be controlled by an experienced operator [10]. The nozzleman regulates water addition (for dry-mix), spray angle, and layer thickness. Accurate control prevents segregation and ensures strong adhesion [15]. Robotic spraying provides consistent coverage, increased safety, and higher productivity, especially in hazardous or confined environments [11,13].

2.4. Fresh and Hardened Properties

Fresh shotcrete must display cohesive flow and stable spray patterns. Proper mixture design minimizes rebound and ensures uniform deposition [11]. Hardened shotcrete typically develops high compressive strength, strong adhesion, and low permeability. Silica-fume shotcrete demonstrates improved resistance to chloride penetration and enhanced durability [10,11].

3. Deterioration Mechanisms of Concrete Structures and Role of Shotcrete

3.1. Common Deterioration Mechanisms

Concrete structures deteriorate over time due to several interacting mechanisms, including reinforcement corrosion, cracking, spalling, chemical attack, environmental exposure, and mechanical damage. Corrosion of steel reinforcement is one of the most common and severe forms of deterioration, often caused by chloride ingress or moisture penetration, which eventually leads to cracking, delamination, and loss of structural capacity [4]. Many reinforced concrete buildings and bridges also experience deterioration from freeze–thaw cycles, abrasion, erosion, and long-term loading, leading to surface scaling, reduced strength, and serviceability problems [1,9]. Damage in underwater and hydraulic structures can progress faster due to continuous water movement, dissolved chemicals, and physical abrasion from sediments [3]. Structural components such as beams, columns, and slabs may additionally suffer from fire exposure, impact loads, or construction defects, further accelerating cracking, stiffness loss, and material degradation [2,17,18]. Overall, these deterioration mechanisms reduce durability and structural performance, making timely and effective repair essential for extending service life [8].

Application-Specific Deterioration and Repair Objectives

The dominant deterioration mechanism is strongly dependent on structural type. In buildings, corrosion-induced cover delamination, fire damage, impact, construction defects, and seismic deficiencies commonly lead to localized section loss or a need for confinement. Bridges combine chloride/deicing-salt exposure, wet–dry and freeze–thaw cycling, fatigue, impact, and leakage at joints; therefore, bond durability and resistance to transport of water and ions are central. Tunnels and underground works experience ground deformation, cyclic/dynamic loading, groundwater, sulfate exposure, fire, and time-dependent lining loads, making early-age strength, toughness, adhesion, and crack control particularly important. Hydraulic and marine structures are dominated by abrasion–erosion, cavitation, saturation, chloride or sulfate ingress, and difficult access, so low permeability and resistance to surface loss are primary requirements. Industrial facilities may add chemical exposure, thermal cycling, and abrasion. Across all categories, removal of unsound concrete, reinforcement treatment, substrate profiling, and curing remain common requirements; what changes is the governing exposure and structural function of the new shotcrete layer.

Table 2 summarizes this application-based classification and the corresponding rehabilitation objectives and governing shotcrete performance requirements.

Table 2. Application-based classification of deterioration mechanisms, rehabilitation objectives, and governing shotcrete performance requirements.

Application	Typical Deterioration	Primary Rehabilitation Objective	Shotcrete Performance Emphasized
Buildings	Corrosion/spalling; fire; seismic deficiency	Section restoration; confinement; stiffness/strength recovery	Bond, shrinkage control, reinforcement encasement, compatibility
Bridges	Chlorides/deicing salts; freeze–thaw; fatigue; leakage	Restore cover/section; protect steel; strengthen members	Bond durability, low permeability, freeze–thaw resistance, constructability
Tunnels/underground	Ground movement; dynamic load; groundwater/sulfate; fire	Rapid support/lining repair; crack control; structural continuity	Early strength, toughness, adhesion, residual capacity, sulfate/fire resistance
Hydraulic/marine	Abrasion/erosion/cavitation; saturation; chlorides/sulfates	Rebuild surfaces and reduce ingress	Low permeability, abrasion resistance, bonding under wet exposure
Industrial	Chemical/thermal attack; abrasion; localized damage	Restore section and protective function	Chemical resistance, thermal compatibility, rapid return to service

3.2. Repair Requirements

Effective repair of deteriorated concrete structures requires understanding the type and extent of damage, ensuring compatibility between the repair material and the existing concrete, and restoring structural performance safely and durably. Proper surface preparation is essential because contaminants, weak layers, or corrosion products can prevent good bonding between new and old materials; therefore, cleaning, roughening, and removing deteriorated concrete are necessary steps before repair application [1,2,8,17,18]. The repair material must be mechanically compatible with the substrate so that differences in stiffness, thermal behavior, and deformation capacity do not create stress concentrations that could lead to future cracking or debonding [9]. When corrosion is present, repair strategies should address both the damaged concrete and the reinforcement by removing rust, restoring section loss, and reducing factors that allow corrosion to continue [4]. Durability requirements must also be considered, especially for structures exposed to freeze–thaw cycles, chlorides, or underwater environments, where low permeability and strong adhesion are critical for long-term performance [3]. Finally, the chosen repair method should be constructible, accessible, and maintainable, particularly for beams, columns, and bridge elements where load demand and geometry influence the suitability of different repair techniques [4,17].

4. Applications of Shotcrete in Repair and Rehabilitation

Shotcrete is widely used for repairing and rehabilitating deteriorated concrete structures because it can restore damaged surfaces, rebuild lost concrete sections, increase load-carrying capacity, and protect structures from further deterioration. Its ability to bond strongly to existing concrete, conform to complex geometries, and be applied vertically or overhead makes it effective in many repair scenarios. Numerous studies and field applications across buildings, bridges, tunnels, marine structures, retaining walls, foundations, and industrial facilities demonstrate its versatility and efficiency in both structural and nonstructural rehabilitation [15,19].

Shotcrete is commonly used to repair spalled and delaminated concrete caused by reinforcement corrosion, freeze–thaw cycles, abrasion, chemical attack, or mechanical damage. In many practical cases, deteriorated cover concrete is removed, reinforcing bars are cleaned or replaced, and a new layer of shotcrete is applied to restore the original section and protect the reinforcement [4,20]. Industrial structures such as retaining walls, storage tanks, and exposed beams often benefit from shotcrete overlays that rebuild the section without requiring formwork, minimize downtime, and provide improved durability through modern admixtures and dense mix designs [21]. Field reports show that wet-mix shotcrete, enhanced with shrinkage-compensating and crystallization admixtures, can effectively rehabilitate old industrial concrete components subjected to marine aerosols, chemical attack, and long-term mechanical stress [22].

Shotcrete is also widely used for strengthening and upgrading structural elements. Many studies show that applying shotcrete jackets around columns, beams, and walls increases confinement, stiffness, and load capacity, making the method suitable for seismic retrofitting and structural upgrades [23,24]. Shotcrete jacketing has been used to improve shear resistance, enhance flexural performance, and restore fire-damaged or impact-damaged components. Experiments demonstrate that properly anchored reinforcement within the shotcrete layer helps mobilize composite action between the new and old concrete, especially when the interface is roughened or provided with dowels [25]. In many bridge applications, shotcrete strengthening has been used to rehabilitate girders, caps, piers, and abutments where geometric constraints prevent formwork installation [26,27].

In hydraulic and marine structures such as dams, spillways, sea walls, culverts, and underwater elements, shotcrete is applied to repair abrasion, erosion, cavitation, and chloride-related deterioration. Its ability to form a dense, low-permeability overlay is documented as an important advantage for protecting structures against water flow and aggressive ions [3]. Historical applications also show that shotcrete can successfully rehabilitate large-scale marine concrete elements, as demonstrated in the restoration of WWII concrete ship hulks, where shotcrete was used to rebuild deteriorated hull panels exposed to harsh seawater conditions [19].

Shotcrete is frequently used in tunnels, mines, slopes, and underground structures for stabilization, surface repair, and protective lining. Figure 2 shows the application of shotcrete in a tunnel. Fiber-reinforced shotcrete provides improved toughness, crack control, and impact resistance, making it suitable for areas subjected to rock movement or dynamic loads [28,29]. Several studies highlight the composite action achieved when fiber-reinforced shotcrete is applied over cracked or damaged concrete, improving both strength and crack distribution while maintaining structural continuity [30,31]. Ultra-high-performance fiber-reinforced shotcrete (UHPFRSC) has also been successfully tested for thin, durable overlays capable of improving mechanical behavior and chloride resistance in heavily deteriorated structures [32].

Figure 2. Application of shotcrete for structural support in Mitchell Point Tunnel. Photograph by Oregon Department of Transportation, licensed under CC BY 2.0.

Shotcrete has also been used effectively in fire-damaged and heat-affected structures. Research shows that areas where concrete has lost strength due to high temperatures can be repaired by removing damaged material, cleaning reinforcement, and applying shotcrete to rebuild the section [33]. In retrofit cases, applying shotcrete layers improves structural performance along with providing a protective barrier against further environmental deterioration.

In heritage and historic structures, shotcrete has been applied in a controlled and minimally invasive manner to stabilize weakened sections and replicate original forms. Repair of large historical concrete shells and architectural elements has been performed using shotcrete overlays to restore structural integrity while preserving appearance [34]. For massive or complex geometries, shotcrete provides a practical solution that avoids large-scale formwork and reduces overall intervention time.

Shotcrete is also used in special-purpose rehabilitation such as strengthening bridge decks, providing protective linings in wastewater structures, and improving durability in chemically aggressive industrial environments. Laboratory studies show improvements in mechanical behavior and reduced transport properties when high-performance or fiber-modified shotcrete mixes are used [33,35]. In addition, optimization studies on shotcrete mix design and spray parameters provide guidance for improving shootability, reducing rebound, and achieving uniform coverage in real-world rehabilitation projects [31,36]. Overall, applications of shotcrete in repair and rehabilitation demonstrate its versatility, constructability, and long-term performance. Across diverse structural types and exposure conditions, shotcrete offers an effective means to restore deteriorated concrete, enhance structural capacity, and improve durability, making it an essential tool for modern infrastructure rehabilitation.

Recent Advances

Recent studies illustrate several recent directions in shotcrete research that are directly relevant to repair and rehabilitation. Kaufmann et al. [37] examined the sulfate resistance of wet-sprayed concrete and showed that cement type, water-to-binder ratio, accelerator chemistry, porosity, and permeability jointly control sulfate-related damage. Menu et al. [38] quantified how mixture parameters and shrinkage-control admixtures affect free shrinkage and restrained cracking in dry-mix repair shotcrete. Yin et al. [39] used 205 field-derived datasets to develop machine-learning models for predicting the compressive strength of sprayed-concrete tunnel linings. Ortiz et al. [40] investigated lightweight polypropylene-fiber shotcrete as a means of reducing the hazard associated with tunnel-lining detachment. Zhu et al. [26] developed high-early-strength steel-fiber-reinforced shotcrete for rapid rehabilitation of disaster-damaged tunnel linings and evaluated its mechanical, durability, and structural repair performance. More recently, Castellón et al. [41] directly compared laboratory-produced and on-site sprayed fiber-reinforced shotcrete, demonstrating the importance of fiber distribution and orientation when laboratory results are transferred to field conditions. Collectively, these studies show that material design, spraying process, exposure condition, and specimen-production method should be considered together when interpreting shotcrete performance. Table 3 summarizes the principal quantitative findings and engineering implications reported in these studies.

Table 3. Quantitative evidence matrix from recent shotcrete studies. Values are reported study outcomes; therefore, materials, curing, test methods, and structural contexts differ.

Study/Year	Application/Material	Quantitative Evidence Reported	Engineering Implication
Kaufmann et al., 2021 [37]	Wet-sprayed concrete for sulfate-exposed tunnel/underground environments; different cements and set accelerators.	Typical w/b = 0.45 (0.48 for selected mixtures); accelerator dosage = 6% of binder (8% for selected mixtures). In the sulfate-resistance test, damage indicated by loss of dynamic modulus occurred only at sulfate expansion well above 1.0‰. Alkali-free accelerators increased porosity/permeability relative to comparable non-accelerated concrete, whereas alkali-based aluminate accelerator produced much higher sulfate expansion.	Sulfate durability cannot be judged from accelerator type alone. Cement type, w/b, pore structure, permeability, and accelerator chemistry must be considered together; sulfate-resistant cement reduces, but does not eliminate, damage potential.
Menu et al., 2022 [38]	Dry-mix shotcrete for repair; restrained-shrinkage cracking and mixture-design effects.	Polymer reduced total free shrinkage by about 13–30% and improved cracking resistance by about 13–35%. Shrinkage-reducing and crack-reducing admixtures reduced total free shrinkage by about 30–60% and improved cracking resistance by 55–78%. Silica fume, alone or with fly ash, increased cracking potential in the tested mixtures.	Mixture changes that improve placement or reduce rebound do not necessarily reduce cracking risk. Shrinkage-control admixtures can substantially improve cracking resistance, so repair-shotcrete qualification should evaluate both constructability and restrained shrinkage.
Yin et al., 2022 [39]	Sprayed-concrete tunnel lining; machine-learning prediction of compressive strength.	205 experimental datasets were used. The RF-QPSO model achieved MAPE = 3.47%, RMSE = 1.30, R2 = 0.93, and a running time of 0.15 s. Water-to-binder ratio was the most influential input variable in the reported importance analysis.	Data-driven prediction can support rapid strength assessment and intelligent quality control of tunnel shotcrete, but application of the model to other projects requires representative field data and validation.
Ortiz et al., 2021 [40]	Lightweight polypropylene-fiber shotcrete for reducing detachment risk in fortified tunnels.	Replacing 70% of conventional aggregate with expanded-glass lightweight aggregate reduced density by about 30% to below 1600 kg/m3 while maintaining 28-day compressive strength above 22.5 MPa. A polypropylene-fiber dosage of 6 kg/m3 produced residual flexural strength values up to approximately 2.93 MPa.	Combining lightweight aggregate with fibers addresses two parts of detachment risk: lower self-weight reduces impact energy, while fibers provide residual load-carrying capacity after debonding.
Zhu et al., 2025 [26]	High-early-strength steel-fiber-reinforced shotcrete for rapid repair of disaster-damaged tunnel linings.	Using steel fibers with an aspect ratio of 50, 28-day compressive and splitting tensile strengths improved by 11% and 70%, respectively. After 300 freeze–thaw cycles, relative dynamic elastic modulus was 83.9% with 1.01% mass loss. The study proposed minimum repair-layer thicknesses of 15 cm and 20 cm for linings with 35% and 50% strength reductions, respectively.	The study links mixture optimization with durability testing and structural-scale repair design, providing performance-based criteria for rapid tunnel rehabilitation rather than relying only on material strength.
Castellón et al., 2025 [41]	Fiber-reinforced shotcrete for tunnels; direct comparison of laboratory-cast and on-site robot-sprayed specimens.	Shotcreted beams exhibited 10–73% lower residual flexural strengths than laboratory specimens. For steel-fiber mixtures, reductions were about 73% for fR1 and 11% for fR3; polyolefin and hybrid mixtures showed reductions of about 20–10%. The 25 kg/m3 steel + 5 kg/m3 polyolefin hybrid mixture provided the most balanced post-cracking response and was easier to pump and spray.	Laboratory specimens can overestimate in-place post-cracking performance. Fiber orientation, effective fiber content, production method, and field spraying conditions should be incorporated into FRS qualification and structural design.

5. Mechanical Performance of Shotcrete Repairs

The mechanical performance of a shotcrete repair is governed by a composite system comprising the substrate, the interface, and the sprayed layer. Interface adhesion and friction, aggregate interlock created by surface profiling, local mechanical interlock around exposed reinforcement, and time-dependent stresses from shrinkage and thermal mismatch collectively determine whether the repaired section behaves monolithically. Consequently, a single bond-strength value cannot fully characterize performance. Pull-off/direct-tension tests primarily quantify tensile adhesion, shear tests emphasize friction and interlock, and fracture tests quantify energy required for crack propagation; the measured value is also sensitive to substrate strength, moisture state, curing, specimen geometry, and failure location [42,43,44,45,46,47,48,49].

Mechanistically, surface preparation increases bonding by removing weak laitance and contaminants and by increasing the real contact area and mechanical interlock. However, excessive roughening is not automatically beneficial if it induces microcracking in the substrate. Moisture conditioning affects suction and the local water-to-binder ratio at the interface, while differential shrinkage and thermal deformation generate self-equilibrating stresses that can initiate debonding before external loading. Freeze–thaw and chemical exposure then exploit connected pores and pre-existing interface defects. This explains why studies using different preparation methods or bond tests can produce different rankings and why the failure mode—interlayer failure, failure in substrate, failure in overlay, or mixed failure—must be reported together with strength [44,45,46,47,48,49].

In buildings and bridge members, the principal mechanical objective is composite action for section restoration, flexural/shear strengthening, or confinement; bond, shrinkage compatibility, and reinforcement anchorage therefore dominate. For tunnels and underground structures, early-age compressive strength must be considered together with residual flexural capacity, toughness, fracture energy, and dynamic response because the sprayed layer may be loaded soon after placement and may experience ground movement. Hydraulic and marine repairs are often less governed by early structural strength than by retention of bond and integrity under saturation, abrasion, and ion ingress. Industrial repairs require case-specific consideration of thermal and chemical exposure. Despite these differences, a common finding is that interface quality and the properties of the as-sprayed, not merely laboratory-cast, material control field performance [42,44,45,46,47,50,51,52,53].

Mechanical Performance by Structural Application

Bond performance between shotcrete and existing concrete is central to repair effectiveness. The measured bond response depends strongly on substrate condition, surface preparation, test method, and environmental exposure. Experimental and long-term investigations of shotcrete–concrete interfaces show that surface preparation and interface condition significantly influence tensile bond performance and failure behavior [44,45,54,55]. Freeze–thaw testing further demonstrates that cyclic environmental exposure can progressively degrade the shotcrete–concrete interface and that the method of substrate preparation affects the retained tensile bond strength [44]. Adhesion testing also shows that shotcrete bond development is associated with the development of the mechanical properties of the sprayed material, while shrinkage and substrate preparation influence long-term interface performance [45].

Fracture mechanics provides additional information that cannot be obtained from bond strength alone. Experimental characterization of the shotcrete–concrete interface demonstrates that fracture energy can be used to quantify resistance to crack initiation and propagation [46]. Mode II fracture is particularly relevant to shotcrete repairs because it represents interface cracking dominated by in-plane shear or sliding along the shotcrete–substrate interface. Such loading can develop when the repaired member transfers shear across the interface or when differential deformation between the substrate and repair layer generates tangential stresses. Numerical analyses based on cohesive-zone modeling under Mode II loading show that interface strength and critical fracture energy influence the initiation and propagation of interface cracks and the resulting failure behavior [47]. Therefore, both strength-based measurements and fracture-based parameters should be considered when assessing the mechanical integrity of shotcrete repairs, particularly when delamination or interface crack propagation is a governing failure mechanism [46,47].

The intrinsic mechanical properties of the sprayed material are also important. Wet-process sprayed concrete can develop satisfactory compressive strength and other hardened properties for structural repair, although its performance may differ from conventionally cast material because of the spraying and placement process [42]. Accelerated shotcrete has also been investigated in terms of mechanical properties, permeability, and durability, demonstrating that rapid strength development must be considered together with longer-term material performance [50]. These characteristics are particularly important in tunnels, transportation structures, and other contexts where early loading or rapid return to service is required.

Tensile and post-cracking behavior become especially important when shotcrete is subjected to cracking, deformation, impact, or dynamic loading. Direct tensile testing provides a method for characterizing the tensile response of shotcrete and evaluating behavior that cannot be fully represented by compressive-strength testing alone [51]. Engineered Cementitious Composites (ECCs) applied through the wet-mix shotcreting process have also demonstrated favorable mechanical behavior for repair applications, including enhanced deformation capacity and crack control [52]. Fiber-reinforced and strain-hardening sprayed materials can therefore be particularly beneficial in applications where residual load-carrying capacity and crack control are important mechanical requirements [51,52].

Environmental conditions may alter these mechanical properties and should be considered according to structural application. High-temperature exposure can modify the microstructure and mechanical properties of shotcrete, which is particularly relevant to tunnels, underground facilities, industrial structures, and fire- or heat-exposed repairs [53]. Shrinkage can also generate restrained stresses at the shotcrete–substrate interface and contribute to cracking or loss of adhesion, particularly when deformation of the repair layer is restrained by the existing substrate [45]. Recycled concrete aggregate can influence the porosity, mechanical behavior, and durability of dry-mix shotcrete and therefore requires appropriate mixture proportioning when used in repair applications [43].

The governing mechanical requirements also vary with structural application. In buildings and bridge members, reliable composite action between the substrate and the shotcrete layer is required for section restoration, confinement, and structural strengthening; consequently, interface bond, shrinkage compatibility, and load transfer are particularly important [44,45,46,47]. In tunnels and underground structures, early-age strength must be considered together with toughness, residual capacity, fracture behavior, and resistance to environmental deterioration [46,47,50,52]. Hydraulic and marine repairs require the interface and sprayed material to retain mechanical integrity under persistent moisture, abrasion, erosion, and aggressive-ion exposure. Industrial repairs may additionally require resistance to thermal and chemical actions [53]. Despite these application-specific differences, successful mechanical performance depends on the combined behavior of the substrate, interface, and as-sprayed material rather than on the compressive strength of the shotcrete alone [44,45,46,47,50]. Figure 3 illustrates the principal failure modes that may occur at or near the shotcrete–concrete interface, while Table 4 summarizes the key variables governing interface performance and their engineering implications.

Figure 3. Interface bond failure modes between shotcrete overlay and concrete substrate: (a) interlayer failure, (b) failure within the shotcrete, (c) failure within the concrete, and (d) mixed failure.

Table 4. Mechanistic framework for shotcrete–substrate interface performance.

Interface Variable	Mechanism	Decision Relevance	Supporting References
Surface roughness/profile	Increases effective contact area, mechanical interlock, and friction at the shotcrete–substrate interface.	Bond-strength results should be interpreted with consideration of substrate preparation and surface profile.	[42,44,45,48,51]
Cleanliness	Removal of dust, laitance, loose particles, and contaminants reduces the likelihood of a weak boundary layer at the interface.	Inadequate cleaning may contribute to adhesive/interface failure and complicate comparison among studies.	[48,49]
Moisture state	Substrate moisture condition influences water exchange between the existing concrete and fresh shotcrete and can consequently affect interface development.	Differences in substrate moisture condition may contribute to variability in measured bond performance.	[42,48,51]
Shrinkage/thermal mismatch	Differential deformation between the repair layer and substrate can generate restrained stresses at or near the interface and promote cracking or debonding.	Compatibility between the repair material and substrate should be considered when assessing long-term interface integrity.	[42,45,48,51]
Environmental cycling	Repeated environmental exposure can progressively damage the interface through microcracking and changes in moisture/pore condition, reducing retained bond performance.	Environmental exposure should be considered when interface durability governs repair service life.	[44,48,49]
Test method	Test configuration and loading mode alter the stress state imposed on the interface and therefore influence the measured bond strength or fracture response.	Results obtained using different interface test methods should not be directly pooled or compared without accounting for differences in test configuration and failure mechanism.	[42,46,47,48,49]

6. Construction Practices, Quality Control, and Standards

Effective shotcrete repair depends on proper construction practices, consistent quality control, and adherence to established standards. High-quality results begin with correct surface preparation, since debris, laitance, weak concrete, or corrosion products can prevent proper adhesion. Studies show that techniques such as chipping, hydrodemolition, sandblasting, and pressure washing significantly improve surface cleanliness and interface performance, reducing the risk of delamination and premature failure [4,42,44]. Field projects also demonstrate that roughened or water-jet-scaled surfaces provide more reliable bonding when subjected to environmental exposure or cyclic loading [54,56].

During placement, control of nozzle angle, spray distance, and layer thickness is critical to achieving uniform consolidation and minimizing rebound. Practical guidance from construction reports and case studies shows that incorrect nozzle motion or excessive spraying distance can create voids, segregation, or weak layers, especially in overhead or confined areas [10,15,55]. Wet-mix shotcrete applications in structural repair projects, such as pier rehabilitation and dam restoration, demonstrate the importance of controlled pump pressure and consistent flow to avoid variability in strength and density [57,58]. When repairing marine or industrial structures, quality control also involves monitoring ambient conditions such as moisture, temperature, chemical exposure, and wind, which can affect setting, bonding, and long-term durability [19,22].

Material quality control is equally important. Laboratory and field evaluations highlight that proper control of water content, admixture dosage, and fiber distribution improves shootability and mechanical performance [50,59]. Tests on chloride-exposed and high-temperature environments show that mix adjustments, such as silica fume addition, optimized aggregate grading, and steel or synthetic fiber reinforcement, contribute to improved resistance against cracking, shrinkage, and chemical attack [53].

Standards and guidelines provide the framework for proper construction and evaluation. The USACE EM 1110-2-2005 standard [11] outlines material requirements, curing procedures, equipment selection, surface preparation, and acceptance criteria for structural repairs. Industry guidelines place strong emphasis on structured quality control throughout the shotcrete process. The ACI 506R-16 Guide to Shotcrete outlines detailed requirements for preconstruction testing, mock-up panels, nozzleman qualification, material submittals, and continuous QA/QC monitoring during placement to ensure proper reinforcement encasement and uniform consolidation [60]. For sewer-tunnel rehabilitation, laboratory testing has shown that the acid resistance of pozzolan-modified shotcrete depends on pozzolan type, replacement level, and exposure acidity [61]. These practices are complemented by project specifications from dam, tunnel, and structural repair case studies, which highlight the importance of documenting field conditions, verifying mixture properties, and performing in-place testing to confirm that shotcrete achieves the expected performance [58].

Overall, successful shotcrete repair relies on a combination of proper substrate preparation, controlled application techniques, thorough material quality checks, and compliance with established standards. The combined evidence across numerous laboratory studies, field investigations, and rehabilitation case histories confirms that disciplined construction and quality control practices are essential for achieving long-lasting and structurally reliable shotcrete repairs. To summarize the main construction quality-control requirements used in shotcrete repair projects, Table 5 compiles the most widely used QC parameters found in ACI guides, military standards, and project specifications from the collected literature. These requirements emphasize proper substrate preparation, nozzleman qualification, controlled placement procedures, environmental monitoring, and in-place performance verification.

Table 5. Practical construction quality-control (QC) requirements for shotcrete repair, synthesized from major standards, guidelines, and project specifications.

Reference(s)	Requirements/Best Practices Found in Standards	QC Parameter
[11,16,54,60]	Substrate must be clean; saturated-surface-dry (SSD); free of laitance, oil, dust, scaling; roughened to promote mechanical bond.	Substrate condition
[15,54,60]	Profiling should be checked (ICRI CSP 6–9 scale), weak concrete removed, reinforcing steel exposed and cleaned.	Surface preparation verification
[11,15,58]	Substrate must not be bone-dry or water-logged; moisture inspected before shooting.	Moisture condition
[11,15,16]	Ensure proper bar exposure, corrosion removal, correct lap, adequate clearance for shooting; confirm bar cleanliness before shotcrete placement.	Reinforcement inspection
[11,16]	Verify mix proportions, admixture compatibility, fiber dosage, aggregate grading, and batch records.	Shotcrete material documentation
[16]	Mandatory for nozzleman qualification and performance verification; panels tested for strength, absorption, density, and core quality.	Preconstruction test panels (mock-ups)
[11,15,16]	Nozzleman should be appropriately qualified/certified for the applicable wet-mix or dry-mix process.	Nozzleman certification
[11,15]	Check air pressure, pump output, water metering, hose cleanliness, nozzle alignment before operation.	Equipment calibration
[16]	Maintain proper nozzle distance (0.5–1.0 m), perpendicular shooting for first layer; angle adjustments monitored during placement.	Shooting distance & angle control
[16]	Inspect layer build-up to avoid sagging, hollow pockets, and insufficient consolidation.	Layer thickness & buildup control
[11]	Excessive rebound indicates poor technique or mix inconsistency; rebound waste should be removed and not reintroduced.	Rebound monitoring
[60]	Record ambient temperature, humidity, wind speed, substrate temperature; restrict placement under extreme temperatures.	Environmental condition logging
[16]	Consistency, pumpability, air content, setting time, slump/slump-flow monitored depending on wet/dry mix.	Fresh-property testing
[11]	Moist curing or curing compounds applied immediately after final set; verify minimum curing time.	Curing requirements
[60]	Cores extracted for testing of compressive strength and bond quality, void detection, and petrographic review.	In-place quality verification (core tests)
[15]	Daily logs must include weather, batch records, equipment checks, nozzleman ID, rebound %, curing actions, and field test results.	Documentation & QC reporting

7. Comparison of Shotcrete with Alternative Repair Methods

Shotcrete is frequently compared with other concrete repair and strengthening methods such as cast-in-place (CIP) concrete, fiber-reinforced polymer (FRP) systems, textile-reinforced mortar (TRM), ferrocement, grout/epoxy injection, and seismic retrofitting techniques to determine the most suitable option for different structural conditions. Across many studies, shotcrete provides strong advantages in constructability, access, and performance, particularly for deteriorated concrete, masonry, tunnels, and buried pipelines.

Compared with cast-in-place repair, shotcrete eliminates most formwork and achieves superior compaction due to high-velocity placement, producing lower permeability and better transport properties, which translates to improved durability [62,63]. Several comparative durability studies demonstrate that well-executed shotcrete can match or exceed CIP concrete in chloride resistance, absorption, and diffusion, making it well suited for marine structures, dams, and bridge components [62]. In repair of large areas or overhead surfaces, shotcrete also offers significant labor and cost efficiencies by reducing installation time [63].

In contrast, FRP and TRM systems, while widely used for seismic strengthening of masonry and concrete walls, provide high tensile capacity and improved ductility but often require smooth, well-prepared substrates and controlled environmental conditions during installation. Reviews of seismic retrofitting approaches emphasize that FRP/TRM overlays significantly enhance the in-plane and out-of-plane shear resistance of masonry but may pose compatibility and reversibility concerns in heritage structures [64,65,66]. Shotcrete, especially steel-fiber or mesh-reinforced types, remains a preferred option when both section enlargement and structural confinement are needed, such as in bridges, piers, and load-bearing walls [32,67].

For masonry retrofitting, several studies show that FRP/TRM systems are lighter and less intrusive, but shotcrete provides greater stiffness and enhances integrity by forming a continuous shell. This makes it particularly effective in seismic rehabilitation of tunnels and rubble-masonry systems, where structural mass and ductility must be increased to resist ground deformation [67]. In heritage-sensitive contexts, however, shotcrete may be considered too invasive compared with reversible strengthening methods such as grout injection or textile overlays [68].

When compared with traditional reinforced concrete linings for buried pipes or culverts, shotcrete offers faster placement, reduced material consumption, and improved adaptability to irregular geometries. Research on steel pipeline rehabilitation shows that modern fiber-reinforced shotcrete and repair mortars deliver comparable or superior structural performance with better economic efficiency relative to thick concrete linings [32].

Taken together, studies consistently indicate that shotcrete excels when rapid construction, difficult access, geometry adaptability, or the combination of structural strengthening and surface restoration are primary objectives. Alternative methods such as FRP, TRM, ferrocement, or grout injection remain valuable for lightweight applications, heritage preservation, or where minimal intervention is needed, but shotcrete stands out as a robust, versatile, and cost-effective solution for many concrete and masonry rehabilitation scenarios. To provide a clear comparison between shotcrete and the most commonly used repair and strengthening techniques, Table 6 summarizes the key differences among shotcrete and alternative repair and strengthening methods in terms of formwork requirements, accessibility, structural performance, durability, cost, and suitability for different repair conditions. The comparison is an evidence-based qualitative synthesis of the literature, in which terms such as “high,” “moderate,” and “low” are used as relative screening descriptors. Cost and application speed are particularly project-dependent because they vary depending on access, project scale, labor, formwork requirements, and mobilization. Therefore, the comparison provides a basis for preliminary method selection while recognizing the need for project-specific structural design and life-cycle cost assessment.

Table 6. Comparison of shotcrete with major alternative repair and strengthening methods.

Criteria	Shotcrete	CIP Concrete	FRP	TRM/FRCM	Ferrocement/Thin RC Overlay
Formwork	Minimal/none [62,63]	Required [62,63]	None [64,65,66,68]	None [64,65,66,68]	None [65,66]
Access/geometry	Excellent; irregular/confined areas [62,63]	Limited by formwork [62,63]	Good; surface access needed [64,65]	Good; surface access needed [65,66]	Good [65,66]
Structural strengthening	High; section enlargement/confinement [65,67]	High; section enlargement [67]	High; flexural/shear [64,65,66]	High; shear/ductility [65,66]	Moderate–high [65,66]
Bond/interface	Good; preparation critical [63,65]	Preparation/anchorage dependent [66]	Adhesive/interface sensitive [64,65,66]	Surface/matrix dependent [65,66]	Mesh/anchorage dependent [65,66]
Durability	High when properly applied [62,63]	High; placement dependent [62,66]	Exposure/resin dependent [64,65,66]	Generally good; system dependent [65,66]	Good; mesh corrosion possible [65,66]
Thickness/added mass	Thin–moderate; multilayer possible [63]	High [66,67]	Very low [65,66]	Low [65,66]	Low–moderate; ~10–50 mm [65,66]
Application speed	Fast [63,65]	Slow–moderate [63,66]	Fast [65,68]	Moderate–fast [65,66]	Moderate [65,66]
Skill/QC	High [63,65]	Moderate–high [66]	High [65]	Moderate [65,66]	Low–moderate [65]
Relative cost	Project-dependent; often economical in repair [63,65]	Project-dependent; formwork/labor intensive [63,66]	Generally high [65]	Moderate–high [65]	Low–moderate [65]
Best suited for	Structural repair, tunnels, bridges, marine works [62,63,67]	Heavy reconstruction/jacketing [66,67]	Lightweight flexural/shear retrofit [64,65,66]	Masonry/seismic strengthening [65,66]	Low-cost masonry strengthening [65,66]

8. Research Gaps and Future Directions

The synthesis identifies five research questions (RQs) that are specific enough to be tested experimentally or numerically:

• RQ1: How can mixed-mode fracture parameters measured with different interface test geometries be converted into transferable design properties? Current studies use non-equivalent stress states and specimen geometries, which limits comparison; coordinated round-robin testing and inverse identification of cohesive-zone parameters are needed.
• RQ2: How does the shotcrete–substrate interface deteriorate under coupled chloride, freeze–thaw, moisture, thermal, and fatigue exposure rather than isolated laboratory actions? The main limitation is the duration and complexity of multi-factor testing; accelerated protocols should be calibrated against instrumented field repairs.
• RQ3: What correction factors are required to translate laboratory-cast or laboratory-sprayed material properties to in-place shotcrete? Recent evidence shows substantial differences in fiber orientation and residual strength between laboratory and field specimens; paired field/laboratory datasets should quantify process-induced variability.
• RQ4: Can low-clinker, geopolymer, recycled-aggregate, and other low-carbon shotcretes achieve equivalent service life without sacrificing shootability, bonding, or early-age performance? Future studies should report embodied carbon together with rebound, pumpability, early strength, shrinkage, transport properties, and interface durability.
• RQ5: Which real-time measurements can reliably support automated quality assurance during robotic or conventional spraying? Nozzle distance/angle, flow rate, accelerator dosage, layer thickness, rebound, temperature, and surface condition should be linked to in-place density, bond, and mechanical properties through validated sensing and data-driven models.

9. Conclusions

This review reframes shotcrete rehabilitation according to structural application, deterioration mechanisms, and interface performance. Repair is defined as the localized restoration of material integrity and serviceability, whereas rehabilitation represents a broader intervention that may encompass repair, strengthening, protection, and retrofitting. For buildings and bridges, the primary performance requirements generally include section restoration, confinement, reliable interface bond, and durability. In tunnels and underground structures, greater emphasis is placed on early-age support, toughness, residual load-carrying capacity, and resistance to environmental deterioration. For hydraulic and marine structures, low permeability and resistance to abrasion and erosion are particularly important. Despite these application-specific requirements, the shotcrete–substrate interface remains a critical factor governing rehabilitation performance across structural applications. Its behavior should therefore be evaluated considering substrate surface condition, moisture state, curing, shrinkage and thermal compatibility, environmental exposure, test method, and failure mode. Recent studies also highlight differences between laboratory and as-sprayed properties and advances in low-carbon materials, fibers, and automated construction. Future research should focus on establishing transferable relationships among mixture design, spraying process, interface performance, durability, and structural response.

• The mechanical behavior of shotcrete repairs depends heavily on the quality of the interface, which governs load transfer and long-term integrity. Research confirms that roughened and cleaned substrates, optimized mixtures, and fiber reinforcement significantly enhance bond strength, fracture resistance, and toughness. Modern materials such as silica fume shotcrete, fiber-reinforced shotcrete, and engineered cementitious composites further improve shrinkage control, environmental durability, and crack behavior.
• Construction practices and quality control play a major role in achieving reliable repairs. Standards such as ACI 506R-16 and USACE guidelines USACE EM 1110-2-2005 emphasize proper surface preparation, preconstruction testing, nozzleman qualification, and monitoring of mixture properties and environmental conditions. Field studies demonstrate that strict adherence to these procedures reduces variability and enhances in-place performance.
• Despite extensive research and successful field applications, several knowledge gaps remain, especially regarding interface fracture mechanics, long-term field monitoring, sustainability of modern materials, performance in harsh environments, and opportunities for automation. Addressing these gaps will enable the development of more durable, efficient, and environmentally responsible shotcrete repair solutions. Overall, the evidence confirms that shotcrete is a robust and versatile technique that will continue to play a central role in the future of concrete infrastructure rehabilitation.