1 Scope

NOTE This standard governs the pneumatic placement of hydraulic-cement concrete (shotcrete) by both the wet-mix and dry-mix processes for structural and non-structural applications. (1.1)

NOTE Shotcrete is concrete or mortar conveyed through a hose and projected at high velocity onto a surface, where it consolidates under its own placement energy without formwork on the placed face. The two processes differ in where water is introduced. (1.2)

NOTE In the wet-mix process the concrete is fully batched and mixed — including water — before it enters the delivery equipment; the pump conveys a ready-to-place mix and the nozzle adds only compressed air (and, where used, accelerator). (1.2.1)

NOTE In the dry-mix process the cement and damp aggregate are conveyed dry through the hose and water is added at the nozzle by the nozzleman; the trade name "gunite" refers to this process. (1.2.2)

NOTE Work covered by this standard includes the following applications. (1.3)

1.3.1Shotcrete work governed by this standard includes:

Temporary and permanent shoring walls, including soil-nail walls and tieback walls.
Slope stabilization and rock-face stabilization.
Tunnel primary and secondary linings and underground structures such as vaults, shafts, and chambers.
Structural repair and fill as part of original construction — encasing steel members, filling voids, and restoring profile.
Pool and water-feature shells.
Form-free placement of complex or curved geometries where conventional formwork is impractical.

NOTE This standard addresses only the shotcrete placement and quality-control component of the work; it does not design the structures or ground-support systems that shotcrete forms part of. (1.4)

NOTE Excavation planning, shoring layout, soil-nail and tieback design, global stability, and dewatering are governed by Excavation Support And Dewatering and the geotechnical documents; this standard governs only the shotcrete placed against them. (1.4.1)

NOTE Retaining-wall structural design and global stability are governed by Retaining Walls; reinforcing-bar fabrication, placement tolerances, and splicing are governed by Concrete Reinforcement, although shotcrete encasement of that reinforcement is addressed here. (1.4.2)

NOTE Conventional cast-in-place concrete placed into formwork is governed by Cast In Place Concrete; joints and waterstops between shotcrete placements are governed by Concrete Joints And Waterstops. (1.4.3)

NOTE Where shotcrete is used as a repair material on existing deteriorated concrete, that work is governed by Concrete Repair And Restoration; this standard governs shotcrete only as an original placement method for new elements. (1.4.4)

2 Referenced Standards

2.1Materials, equipment, qualification, placement, and acceptance shall comply with the latest adopted edition of each of the following unless a specific edition is cited in the Contract Documents.

2.2Where referenced standards conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.

NOTE ASTM C1436, Specification for Materials for Shotcrete, was withdrawn in 2022 with no single published replacement; this standard therefore cites the individual material standards (ASTM C150, C33, C1240, C1141) and the materials provisions of ACI 506.2 rather than C1436, and that withdrawn specification shall not be cited as a current requirement. (2.3)

Standard	Title
ACI 506.2	Specification for Shotcrete
ACI PRC-506	Shotcrete — Guide
ACI 506.5R	Guide for Specifying Underground Shotcrete
ACI 506.4R	Guide for the Evaluation of Shotcrete
ACI 318	Building Code Requirements for Structural Concrete (Section 26.10, shotcrete)
ASTM C1604/C1604M	Obtaining and Testing Drilled Cores of Shotcrete
ASTM C1550	Flexural Toughness of Fiber-Reinforced Concrete (Centrally Loaded Round Panel)
ASTM C1141/C1141M	Admixtures for Shotcrete
ASTM C1240	Silica Fume Used in Cementitious Mixtures
ASTM C805/C805M	Rebound Number of Hardened Concrete
ASTM C33/C33M	Concrete Aggregates
ASTM C150/C150M	Portland Cement

3 Submittals

3.1 Action Submittals

3.1.1The Contractor shall submit the following action submittals for review before any shotcrete is placed:

Mix design for each shotcrete mix, identifying process (wet or dry), cement type, aggregate gradation, supplementary cementitious materials, admixtures, fibers, water-cementitious materials ratio, and specified strength.
Nozzleman qualification records, including current ACI Nozzleman (CP-60) certification for each process and orientation proposed, and project qualification panel results where required.
Preconstruction test panel plan, identifying panel size, number of panels, coring locations, and test ages.
Equipment list for delivery, pumping or conveying, and the nozzle assembly, including accelerator dosing equipment where accelerator is used.
Product data for cement, aggregate, silica fume, admixtures, accelerator, fibers, and curing materials.
Placement plan addressing surface preparation, lift sequence, build-up thickness, encasement of reinforcement and embedments, thickness verification, and rebound control.

Action submittals required before placementcheckbox

☑ Shotcrete mix designs (each mix)

☑ Nozzleman qualification and certification records

☑ Preconstruction test panel plan

☐ Equipment list (delivery, pump/conveyance, nozzle, accelerator dosing)

☑ Material product data

☑ Placement plan (prep, lifts, thickness, rebound, encasement)

3.2 Informational Submittals

3.2.1The Contractor shall submit the following informational submittals:

Preconstruction test panel core test reports, including individual and average compressive strengths.
Fiber-reinforced shotcrete toughness test reports per ASTM C1550 where fiber-reinforced shotcrete is specified.
Accelerator compatibility data demonstrating the 28-day strength of the accelerated mix relative to the companion unaccelerated mix.
Curing records, identifying method, start time, and duration for each placement.

Informational submittals requiredcheckbox

☑ Test panel core test reports

☐ ASTM C1550 toughness reports (fiber mixes)

☐ Accelerator compatibility / strength-retention data

☑ Curing records

3.3 Closeout Submittals

3.3.1The Contractor shall submit the following closeout submittals:

In-place core test reports for any cores taken from the completed work.
Thickness verification records correlating depth-pin or probe readings to specified minimum thickness.
Record of nonconforming areas and the approved remediation for each.

Closeout submittals requiredcheckbox

☑ In-place core test reports

☑ Thickness verification records

☐ Nonconformance and remediation record

4 Process Selection

NOTE The wet-mix and dry-mix processes each suit different conditions; the Contract Documents either specify the process or allow the Contractor to select it, subject to qualification and test-panel acceptance. (4.1)

NOTE Process suitability depends on placement volume, water control, orientation, hose travel distance, and the availability of qualified operators. (4.2)

NOTE Wet-mix is generally preferred for high, continuous placement volume because the mix is pre-batched and the placement rate is limited by the pump rather than by the nozzleman; water content is controlled at the batch plant, giving more consistent water-cementitious ratio. (4.2.1)

NOTE Dry-mix is generally preferred for intermittent placement, remote or access-constrained sites, and applications needing long hose runs or instant start-stop, because the dry stream can be held in the hose between placements; water control depends on the nozzleman's skill. (4.2.2)

NOTE The specifier should select or allow a process rather than mandate one without justification; forcing a single process against the site conditions invites requests for information and value-engineering proposals. (4.2.3)

4.2.4Where the Contractor selects the process, the selected process shall be qualified by preconstruction test panels for each orientation in which shotcrete will be placed.

Shotcrete placement processradio

○ Wet-mix process

○ Dry-mix process

● Contractor's option (qualified by test panel)

Primary application categoryselect

Temporary shoring / soil-nail / tieback wall

Permanent structural wall or lining

Slope or rock-face stabilization

Tunnel or underground structure

Void fill / member encasement

Pool or water feature

5 Materials

5.1 Cementitious Materials

5.1.1Portland cement shall conform to ASTM C150, Type I or Type II, except that Type III may be used where rapid early-strength gain is required and is permitted by the mix design submittal.

5.1.2Silica fume, where specified, shall conform to ASTM C1240.

NOTE Silica fume reduces permeability and improves cohesion and overhead build-up; it is commonly specified for underground, corrosion-sensitive, and water-retaining applications at 8% to 12% of cementitious weight, with 8% to 15% used in slurry-blended underground mixes. (5.1.3)

Portland cement typeradio

● Type I/II (general)

○ Type III (rapid early strength)

Silica fume content (by cementitious weight)range

%

015

Default: 8 %

5.2 Aggregates

5.2.1Fine and coarse aggregate shall conform to ASTM C33, with gradation within the limits established by the approved mix design.

NOTE Shotcrete aggregate is finer and more closely graded than aggregate for cast-in-place concrete, because the mix must pump or convey through a hose and consolidate on impact without segregation; oversize or gap-graded aggregate increases rebound and creates voids behind reinforcement. (5.2.2)

5.3 Admixtures and Accelerators

5.3.1Admixtures for shotcrete shall conform to ASTM C1141.

5.3.2Accelerators, where used, shall be alkali-free unless an alkali-based accelerator is expressly approved in writing.

NOTE Alkali-based accelerators reduce long-term strength and present a significant skin and eye hazard to the nozzle crew; alkali-free accelerators are current practice and should be specified for overhead, rapid-demold, and rapid-early-strength work. (5.3.3)

5.3.4Accelerator dosage shall not reduce the 28-day compressive strength of the accelerated mix to less than 80% of the companion unaccelerated mix.

Accelerator typeradio

● None

○ Alkali-free liquid accelerator

○ Alkali-based accelerator (written approval required)

Alkali-free accelerator dosage (by cement weight)range

%

08

5.4 Reinforcement and Fibers

5.4.1Fiber type and dosage shall be as specified in the Contract Documents where fiber-reinforced shotcrete is indicated, and shall not be substituted without approval.

NOTE Where fiber-reinforced shotcrete is shown on the drawings, the fiber type and dosage rate shall be stated; without them the Contractor cannot price the work or submit a mix design. (5.4.2)

NOTE Steel fibers are typically dosed at 30 to 50 lb/yd³ for structural shoring ductility and 50 to 75 lb/yd³ for tunnel ground support; synthetic macro-fibers are typically dosed at 3 to 7.5 lb/yd³. The choice between steel and synthetic depends on required toughness, durability exposure, and finish. (5.4.3)

5.4.4Conventional reinforcing bar encased in shotcrete shall be positioned and supported so the shotcrete stream fully encases each bar without shadow voids; bar spacing and cover shall be as shown and as governed by Concrete Reinforcement.

Reinforcement typeradio

● Conventional reinforcing bar

○ Steel fiber reinforced

○ Synthetic macro-fiber reinforced

○ Plain (unreinforced)

Steel fiber dosagerange

lb/yd³

075

Default: 40 lb/yd³

Synthetic macro-fiber dosagerange

lb/yd³

07.5

6 Mix Design and Strength

NOTE Each shotcrete mix shall develop the specified compressive strength when sampled, cured, and tested in accordance with this standard and ACI 506.2. (6.1)

NOTE Specified compressive strength (f'c) at 28 days is the primary measure of structural adequacy and the usual basis of acceptance. (6.2)

6.2.1Each shotcrete mix shall achieve the specified 28-day compressive strength established for its application.

NOTE Structural shoring walls and similar elements commonly specify 4,000 psi at 28 days; permanent underground linings commonly specify a 5,000 psi minimum; the practical range for shotcrete work is 3,000 psi to 6,000 psi. (6.2.2)

6.2.3The water-cementitious materials ratio shall not exceed the value established by the approved mix design.

NOTE Wet-mix water-cementitious ratio is typically 0.40 to 0.50, reduced to 0.38 to 0.45 where silica fume is used; dry-mix water content is controlled by the nozzleman and is verified through the test-panel program rather than by batch records. (6.2.4)

6.2.5Structural shotcrete elements designed as structural members shall meet the minimum strength and cover requirements of ACI 318 Section 26.10 in addition to the procurement requirements of ACI 506.2.

Specified compressive strength (f'c) at 28 daysrange

psi

30006000

Default: 4000 psi

Maximum water-cementitious materials ratio (w/cm)range

ratio

0.380.5

Default: 0.45 ratio

7 Nozzleman Qualification

NOTE The nozzleman is the single most influential factor in shotcrete quality; placement quality, consolidation, and rebound control depend directly on nozzleman technique. (7.1)

7.1.1Each nozzleman shall hold a current ACI Nozzleman (CP-60) certification for the process — wet-mix or dry-mix — and the orientation — vertical or overhead — in which they will place shotcrete.

NOTE ACI Nozzleman certification requires both a written examination and a practical examination in which the candidate shoots a qualification panel; certification is valid for five years and is issued by ACI, while the American Shotcrete Association provides training. (7.1.2)

7.1.3Where the Contract Documents require a project-specific qualification program in addition to certification, each nozzleman shall successfully shoot project qualification panels before placing shotcrete in the work.

NOTE Failure to require ACI certification explicitly allows uncertified operators to be proposed and is a frequent source of quality disputes; certification shall be verified before any placement. (7.1.4)

Nozzleman qualification requirementradio

○ ACI CP-60 certification only

● ACI CP-60 certification plus project qualification panel

Certified orientations requiredcheckbox

☑ Vertical

☐ Overhead

8 Preconstruction Test Panels

NOTE Preconstruction test panels qualify the mix, the equipment, and each nozzleman together as a placement system before production work begins; they are the basis of acceptance and shall not be omitted. (8.1)

8.1.1Each nozzleman shall shoot preconstruction test panels for each mix, process, and orientation they will use in the work, using the production equipment.

8.1.2Test panels shall have a minimum face dimension of 16 in by 16 in and a minimum depth of 5 in, shot at the same nozzle angle and reinforcement configuration as the work they represent.

8.1.3At least one test panel shall be shot per nozzleman per day, or per 50 yd³ placed, whichever results in more panels.

NOTE Omitting or under-specifying the panel program leaves the acceptance basis ambiguous and creates delay and rejection disputes; the number of panels and the test ages shall be stated in the Contract Documents. (8.1.4)

Test panel frequency basisradio

○ One panel per nozzleman per day

○ One panel per 50 yd³ placed

● Whichever governs (more panels)

Test panel minimum face dimensionrange

in

1624

Default: 16 in

Test panel minimum depthrange

in

58

Default: 5 in

9 Testing and Acceptance

NOTE Compressive-strength acceptance is based on cores drilled from test panels and from the in-place work, not on cast cylinders. (9.1)

9.1.1Compressive-strength specimens shall be cores obtained and tested in accordance with ASTM C1604; cylinders cast from the shotcrete stream shall not be used as the basis of acceptance.

NOTE Shotcrete is placed by impact, not poured; a cast cylinder does not represent the consolidation of the placed material, which is why ACI 506.2 requires cores. Confusing the two is a common acceptance error. (9.1.2)

9.1.3At least three cores shall be taken from each test panel for compressive-strength testing.

9.1.4No individual core shall test below 0.85 f'c, and the average of three cores from a panel shall not be less than f'c.

9.1.5Where fiber-reinforced shotcrete is specified, flexural toughness shall be determined by ASTM C1550 on round panels, with the energy-absorption class stated in the Contract Documents.

NOTE For underground and ground-support fiber-reinforced shotcrete an energy absorption of at least 270 J at 40 mm deflection (a D500-equivalent class) is commonly specified; the required class shall match the ground-support design. (9.1.6)

9.1.7Rebound number testing per ASTM C805 may be used as a supplemental in-place uniformity check but shall not be used as the basis of strength acceptance.

Cores per test panelrange

cores

35

Default: 3 cores

Core acceptance criteriaradio

● Individual ≥ 0.85 f'c and average of 3 ≥ f'c (ACI 506.2)

Fiber toughness acceptance (ASTM C1550 energy at 40 mm)range

J

0500

10 Surface Preparation

NOTE Sound bond to the substrate is essential; inadequate surface preparation is a leading cause of delamination and rejection. (10.1)

10.1.1The receiving surface shall be cleaned of loose material, laitance, dust, oil, and debris before shotcrete placement.

10.1.2Earth and rock surfaces shall be trimmed to line and brought to a stable, damp condition; flowing water shall be controlled or diverted so it does not wash the freshly placed shotcrete.

10.1.3Existing concrete and masonry substrates shall be scarified to expose sound material and brought to a saturated surface-dry condition before placement.

NOTE Where shotcrete is placed over a waterproofing membrane, existing concrete, or other bond-critical surface, the surface preparation required to achieve bond shall be specified; omitting it leads to delamination. (10.1.4)

Substrate typeselect

Soil / rock face

Existing concrete or masonry

Over waterproofing membrane

Lagging / sheeting

Required substrate condition at placementradio

● Damp, no free water

○ Saturated surface-dry

11 Placement

NOTE Shotcrete shall be placed by qualified nozzlemen using technique that consolidates the material on impact and fully encases reinforcement and embedments without shadow voids. (11.1)

11.1.1The nozzle shall be held approximately perpendicular to the receiving surface and at the distance that produces dense, well-consolidated material with minimum rebound.

11.1.2Shotcrete shall be built up in layers that will not sag or slough; the build-up thickness in a single pass shall not exceed the thickness the material can support in the orientation being placed.

NOTE Vertical single-pass build-up is commonly limited to about 2 in, with structural walls reaching 4 in to 6 in total in multiple passes; overhead single-pass build-up is limited to roughly 2 in to 3 in without accelerator. (11.1.3)

11.1.4Reinforcement and embedments shall be encased from the back forward so the shotcrete stream does not create voids or sand pockets behind bars, plates, or anchors.

11.1.5Rebound — material that ricochets off the surface — shall be discarded and shall not be incorporated into the completed work.

NOTE Rebound is the largest single waste stream in shotcrete; trowelling rebound into toes, corners, and re-entrant angles is a frequent quality defect, and the Contract Documents shall state that rebound is excluded from the work. (11.1.6)

NOTE Typical rebound is 5% to 15% for wet-mix, 15% to 30% for dry-mix walls, and up to 35% for overhead dry-mix; rebound shall be removed from the work area before it can be entrapped by subsequent passes. (11.1.7)

Total specified shotcrete thicknessrange

in

28

Default: 4 in

Maximum single-pass build-up (vertical)range

in

14

Default: 2 in

12 Thickness Verification

NOTE Because shotcrete has no formed back face, placed thickness cannot be read from formwork and shall be verified by an independent method. (12.1)

12.1.1Minimum placed thickness shall be verified using depth gauge pins, thickness probes, or drilled-and-measured checks at a stated frequency.

12.1.2Depth pins or gauge wires shall be installed before placement at a spacing that lets the nozzleman judge build-up, and the verified minimum thickness shall meet or exceed the specified thickness.

NOTE Thickness enforcement is commonly overlooked because there is no formed surface to measure against; the verification method and frequency shall be specified rather than left to the Contractor. (12.1.3)

Thickness verification methodradio

● Depth gauge pins / wires

○ Thickness probe or pull-out check

○ Drilled and measured cores

Thickness verification frequencyrange

ft²/check

25200

Default: 100 ft²/check

13 Curing

NOTE Shotcrete has a high surface-area-to-volume ratio and a thin cross-section, so it is especially vulnerable to early-age moisture loss; effective curing is essential to develop strength and limit shrinkage cracking. (13.1)

13.1.1Shotcrete shall be moist-cured for a minimum of 7 days, or cured by an approved curing compound or membrane providing equivalent moisture retention.

13.1.2Curing shall begin as soon as the surface has hardened enough that the curing method will not mar or erode it.

NOTE Where a curing compound is used on a surface that will receive a bonded subsequent layer, the compound shall be compatible with that bond or removed before the next placement, because most curing compounds act as bond breakers. (13.1.3)

Curing methodradio

● Continuous moist curing (water/fog/wet covering)

○ Curing compound / membrane

○ Combination

Minimum curing durationrange

days

714

Default: 7 days

14 Quality Assurance

NOTE Quality assurance combines qualified personnel, qualified mixes, and ongoing field verification; the nozzleman qualification and test-panel programs above are its foundation. (14.1)

14.1.1The Contractor shall maintain ACI-certified nozzlemen on the work at all times during shotcrete placement.

14.1.2The Owner's testing agency shall observe placement, obtain and test cores, and verify that placement, encasement, rebound control, and thickness conform to this standard.

14.1.3Defective shotcrete — including delaminated, voided, sand-pocketed, rebound-contaminated, or under-strength material — shall be removed and replaced with conforming shotcrete.

NOTE Coordination between the shotcrete contractor and the geotechnical and structural engineers is required on shoring and ground-support work; this standard governs placement and quality, while load and stability design is governed by Excavation Support And Dewatering and the structural documents. (14.1.4)

Field quality-control inspectioncheckbox

☑ Continuous inspection during placement

☐ Periodic inspection

☑ Core sampling by Owner's testing agency

☑ Thickness verification witnessing

15 Delivery, Storage, and Handling

NOTE Material conditioning before placement affects mix consistency and the nozzleman's ability to control water and consolidation. (15.1)

15.1.1Bagged dry-mix and cementitious materials shall be stored off the ground, under cover, and protected from moisture until use; material showing signs of hydration or lumping shall be discarded.

15.1.2Pre-batched wet-mix shotcrete shall be placed within the time limit established by the approved mix design and shall not be retempered with water at the pump beyond the approved water-cementitious ratio.

15.1.3Fibers shall be stored to prevent corrosion (steel) or degradation (synthetic) and shall be batched at the rate established by the approved mix design.

16 Warranty

NOTE The warranty assures that placed shotcrete remains sound and bonded for the warranty period under the service conditions for which it was designed. (16.1)

16.1.1The Contractor shall warrant the shotcrete work against delamination, spalling, and strength deficiency for the period stated in the Contract Documents, and shall remove and replace nonconforming work discovered within that period at no cost to the Owner.

Workmanship warranty periodrange

years

15

Default: 1 years