Concrete
Shotcrete and gunite application field guide for concrete crews
Wet-mix versus dry-mix, the nozzleman who makes or breaks it, managing rebound, encasing the rebar without shadows, and the test panel that proves the work.
Direct answer
Shotcrete is concrete sprayed pneumatically at high velocity onto a surface, where the impact consolidates it without forms on the sprayed side. It builds pools, walls, slopes, tunnels, and repairs. The application is the quality: an ACI-certified nozzleman, the right mix, and managed rebound decide whether it holds. Project specifications and the structural engineer control structural work.
Key takeaways
- Shotcrete is concrete sprayed pneumatically at high velocity; the impact consolidates it without a form on the sprayed side.
- Gunite means the dry-mix process where water is added at the nozzle; wet-mix batches water at the plant and rebounds less.
- Never shoot rebound back into the work or trowel it in; it is aggregate-rich, binder-poor waste that must be removed.
- Gun behind each bar from both sides to encase steel before closing the face, or shadowing leaves voids and sand pockets.
- Prove strength with a cored test panel shot under real conditions, not a hand-cast cylinder; use an ACI-certified nozzleman.
What shotcrete is, and where it fits
Shotcrete is concrete or mortar conveyed through a hose and shot at high velocity onto a surface, where the force of the impact compacts the material in place. There is no vibrator and no closed form pressing it together. The velocity does that work. That single fact is why shotcrete exists: you can build a wall, a pool shell, a tunnel lining, or a slope face with a form on one side or no form at all, because the material consolidates against whatever it lands on.
Gunite is shotcrete. The word predates the modern term and survives in the pool trade, where gunite specifically means the dry-mix process. When a spec says shotcrete it usually means either process; when a pool builder says gunite they mean dry-mix. The next section sorts out the distinction that actually changes how the work runs.
Against cast-in-place concrete, the trade-off is forms versus skill. Cast-in-place needs forms on both faces and a vibrator to consolidate, and the result is only as good as the form pressure and the vibration. Shotcrete needs one face of form or none, but it puts the entire quality of the placement in the hands of the person holding the nozzle. You trade the formwork for the nozzleman. On the right job that is a good trade. On the wrong job, or with the wrong nozzleman, it is how you get a wall full of voids you cannot see until you sound it or core it.
What is the difference between wet-mix and dry-mix (gunite) shotcrete?
The difference is where the water meets the cement. In wet-mix, all the ingredients including water are batched as ready-mixed concrete, pumped through the hose as a wet slurry, and compressed air is added at the nozzle to throw it onto the surface. In dry-mix, which the pool trade calls gunite, the dry materials are conveyed dry through the hose, and water is added at the nozzle through a water ring as the material shoots out. The nozzleman controls the water on dry-mix, in real time, by feel.
That control is the whole character of dry-mix. The nozzleman reads the surface and adjusts the water valve to keep the mix shooting right, drier in a corner, wetter on a flat run. It is a skill that takes years, and the water-cement ratio of the finished work is only as consistent as the hand on the valve. Wet-mix moves the water-cement ratio back to the batch plant, where it is measured and controlled before the material ever reaches the hose, so the mix is more consistent and the result depends less on the nozzleman's water judgment.
Production and waste split the two as well. Wet-mix pumps higher volume and rebounds less, so it dominates large structural work, walls, and most modern pools. Dry-mix shines where you want to start and stop often, shoot long distances, or shut down and clean up fast, because the line carries dry material and is far less prone to setting up and clogging. The choice is driven by volume, run length, the consistency the spec demands, and what the crew is certified and equipped to shoot.
| Factor | Wet-mix shotcrete | Dry-mix (gunite) |
|---|---|---|
| Water added | At the batch plant, before the hose | At the nozzle, by the nozzleman |
| Conveyed as | Wet slurry, pumped | Dry material, air-conveyed |
| Air added | At the nozzle | Throughout, water added at nozzle |
| Water-cement control | Batched and measured | Nozzleman's hand on the valve |
| Typical rebound | Lower | Higher |
| Best at | High volume, walls, structure | Stop-start work, long hose, pools |
The nozzleman is the make-or-break
No part of shotcrete matters more than who holds the nozzle. With cast-in-place you can salvage a marginal pour with good vibration and patience. With shotcrete the placement happens at the nozzle in real time, and a poor nozzleman builds defects into the wall that no later step removes. This is why structural shotcrete is specified around the nozzleman, not just the mix.
The technique that separates a pro is boring to describe and hard to do for hours. Hold the nozzle roughly perpendicular to the surface, commonly 3 to 6 ft back, and keep that angle and distance steady while moving in a controlled overlapping pattern. Perpendicular is what drives the material in and consolidates it. The moment the nozzle tilts off square, the stream starts glancing, rebound climbs, and material piles unevenly. Too close and you tear the surface and trap air; too far and the velocity drops and the material lands soft and porous.
The American Concrete Institute runs a nozzleman certification program, carried jointly with the American Shotcrete Association, that certifies in the wet-mix or dry-mix process and in a shooting orientation. Vertical is the base credential; overhead is harder and earned separately, because shooting up against gravity is where skill shows most. Qualification includes a written exam and shooting a test panel that gets cored and evaluated. On structural work, requiring a certified nozzleman in the actual process and orientation of the job is the single most effective specification line there is. The exact certification reference belongs to the project spec and the adopted edition of ACI 506; confirm it rather than guessing the document number.
What is rebound in shotcrete?
Rebound is the material that bounces off the surface instead of sticking. When the stream hits, the coarser, heavier particles ricochet while the paste and finer material embed, so what falls to the floor is aggregate-rich and binder-poor. That is the part that makes the rule absolute: rebound must not be shot back into the work or troweled into the wall. It is not concrete anymore. It is sorted-out coarse aggregate with too little cement, and folding it back in builds a weak, sandy zone exactly where you cannot see it.
Rebound rises with overhead work, with dry-mix, and with a nozzle held off perpendicular. The numbers vary with mix, process, and surface, but the pattern is consistent and worth carrying as a planning figure. Floors and slabs lose the least, vertical walls more, and overhead the most. Dry-mix rebounds more than wet-mix at every orientation. Plan your material yield around it, because the rebound is paid-for material that ends up on the ground and in the dumpster, not in the structure.
Managing it is straight technique and housekeeping. Keep the nozzle square and the velocity up. Shoot the bottom of a wall first and work up so rebound does not collect on fresh material below and get buried. Blow or sweep rebound out of corners, off ledges, and away from the toe before it accumulates. The discipline is to treat every bit of rebound as waste that has to leave, not as material to reclaim.
| Orientation | Rebound, commonly cited range | Note |
|---|---|---|
| Floors and slabs | Lowest | Gravity helps the material stay |
| Vertical walls | Higher | Shoot bottom-up to avoid burying rebound |
| Overhead | Highest | Where skill and process matter most |
| Dry-mix vs wet-mix | Dry-mix higher at every angle | Wet-mix wastes less material |
Encasing the rebar without shadows and voids
Shadowing is the classic structural defect in shotcrete, and it lives behind the reinforcing steel. When the nozzleman shoots straight at a bar, the stream piles material on the front face and the back of the bar sits in a wind shadow where little material reaches it. What forms behind the bar is a void or a sand pocket, a porous binder-starved zone, sometimes packed with trapped rebound. The bar is not fully encased, the bond is compromised, and you have built a path for water to reach the steel.
Beating it is a deliberate technique. The nozzleman cuts the spray angle and guns behind each bar from one side and then the other, encasing the back of the bar before building the cover in front of it. You fill behind the steel first, then close the face. The skill demand climbs as bars get larger and spacing gets tighter, because a curtain of closely spaced big bars is exactly the geometry that wants to shadow. This is also why bar placement matters: lap and tie the steel so the nozzleman can actually get the stream behind it, and avoid bundling that makes encasement impossible.
Sand pockets and voids behind continuous reinforcing steel are not a cosmetic question. They are unacceptable on structural work, and they are the reason the inspector sounds the surface and the engineer may call for cores at congested locations. If the nozzleman cannot reach behind a bar at a given spot, that is a design and detailing conversation before the gun comes out, not a defect to bury and hope nobody sounds.
Surface prep and the substrate
Shotcrete is only as good as what it lands on, and the prep depends on what the substrate is. On a repair, the bond is everything, and the prep is the same discipline that governs any concrete patch: sound out and remove all deteriorated and delaminated material, square or undercut the edges so you are not feathering, expose and clean the steel, and bring the existing concrete to a clean, roughened, saturated surface dry condition so it neither sucks water out of the fresh mix nor leaves standing water at the bond line. The companion guide on spall repair and restoration walks the substrate and steel work in detail.
On new work the substrate is different in kind. Against earth or rock for a slope, soil nail wall, or tunnel, the face is trimmed and any loose material scaled off, because the shotcrete consolidates against the ground and any soft pocket telegraphs into a thin or unbonded spot. For a pool the gunite shoots against trimmed and compacted excavation over the steel cage. For a wall it shoots against a single form or a sheeted excavation on the blind side.
The bonding-agent question comes up on every repair. The honest field answer is that a properly prepared, clean, saturated surface dry substrate with shotcrete shot at velocity bonds well on its own, and a bonding agent applied wrong, allowed to skin over before the material hits, is worse than none. Follow the spec and the product instructions, and when in doubt, the prep does more for the bond than the bottle.
The shotcrete mix design
A shotcrete mix is concrete proportioned to two jobs at once: it has to pump or convey through the hose, and it has to stick and stack when it lands. Those two are in tension. Pumpability wants the mix wet and slick; shootability wants it stiff enough to hold on a wall or overhead without sliding. The mix design lives in the gap between them, and the companion guide on mix design and water-cement ratio covers the proportioning fundamentals that carry over.
Shotcrete leans on more fines and a smaller top-size aggregate than formed concrete, because the material has to atomize and reconsolidate through a nozzle. The sand-to-total-aggregate fraction runs higher. Silica fume earns its keep here: it raises the viscosity and cohesion so the material resists sliding and sloughing at the receiving surface, and it improves both pumpability and shootability while adding strength and reducing permeability. The cost is a higher risk of plastic shrinkage cracking from its fineness, so curing gets more attention, not less.
Accelerators are a shotcrete signature, especially overhead and in tunnels. They are dosed to grab the material fast so it can build thickness against gravity without sloughing. On wet-mix they are commonly added at the nozzle so the clock does not start in the hose. Dose is a balance: too little and the material sloughs, too much and you can lose long-term strength and durability, so the accelerator type and dosage belong to the qualified mix design and the test panel, not to a field guess. The water-cement ratio is the master variable on shotcrete as on any concrete, and on dry-mix it is set by the nozzleman's hand, which is exactly why the certified hand matters.
The gun, the pump, the air, and the hose
The equipment splits along the wet-dry line. Wet-mix runs a concrete pump that pushes the batched slurry through the line, with compressed air introduced at the nozzle to spray it. Dry-mix runs a gun that meters dry material into the air stream, conveys it dry through the hose, and adds water at the nozzle through a water ring. Either way the air is what gives the material its velocity, and velocity is what consolidates it.
Air volume is the quiet variable that ruins jobs when it is short. Too little air and the material lands soft and rebounds high because it never reached consolidating velocity. The compressor has to deliver the volume the equipment and hose length call for, and a long hose run eats air and pressure, so size the compressor for the actual setup, not the rating on the truck. Hose diameter, length, and condition all matter; a kinked or worn hose changes how the material arrives.
The nozzle is the business end and the nozzleman's instrument. On dry-mix the water ring and valve let the nozzleman tune the water on the fly. On wet-mix the air ring sets the spray. Keeping the nozzle clean, the rings clear, and the water and air balanced is not a side chore; it is part of shooting well. A clogged ring or a worn tip changes the stream and the nozzleman has to feel it and fix it before the wall shows it.
Building thickness, vertical and overhead
Shotcrete builds in passes, not in one shot. You lay a layer the material can support at that orientation, let it stiffen enough to hold the next, and build to the full thickness in lifts. Push a single lift too thick or too wet and it sloughs, the whole mass sags or peels off the wall under its own weight, taking the bond with it. The maximum lift is a function of the mix, the accelerator, the orientation, and the nozzleman's read of how the material is standing.
Vertical work shoots bottom-up so rebound falls onto the substrate ahead of the gun rather than onto fresh material, where it would get buried as a defect. Corners and the toe of the wall need attention because rebound collects there and because the geometry invites shadows behind congested steel.
Overhead is the hardest shooting there is, and it is why the overhead nozzleman certification is separate. You are building against gravity, every lift wants to fall off, rebound is at its highest, and the mix and accelerator have to be tuned to grab fast. The discipline is thinner lifts, a stiffer shootable mix, and a nozzleman who has shot enough overhead to know when the ceiling is about to let go before it does. If a lift starts to sag or sheet, it comes off and gets reshot; you do not chase a sloughing overhead pass and hope it holds.
Finishing: from gun finish to trowel
The finish is whatever the spec asks for, and the choices run from leaving it as-shot to working it smooth. The natural or gun finish is the surface exactly as it lands, undisturbed, with its sprayed texture intact. It is the most honest surface structurally because nothing has been worked over it, and it is common on tunnel linings, slopes, and anywhere appearance is secondary to integrity. Where a flatter face is needed you cut it to a screed or rod line, then float and trowel.
Timing is the whole game in finishing shotcrete, the same way it is in flatwork. Work the surface too early and you bring paste and water up and weaken it; wait too long and accelerated material is past the window. The cut-and-trowel sequence has to track the set, which an accelerated mix reaches faster than ordinary concrete.
A flash coat is a thin final layer shot over a cut or trowel-cut surface to give a uniform sprayed texture, common on pools and architectural work. It is cosmetic skin, not structure, so it does not substitute for full thickness underneath. One blunt point on finishing: cutting and troweling can hide a multitude of sins in the structural body behind it. A pretty face does not mean the steel is encased. That is what sounding and cores are for, not the finish.
Curing shotcrete
Shotcrete cures the way all concrete does, by keeping moisture in long enough for the cement to hydrate, but it has a problem ordinary concrete does not: a very high ratio of surface area to volume. A thin shotcrete layer on a wall or overhead has far more exposed face per unit of material than a slab or a footing, so it dries out fast, and a dried-out surface stops gaining strength and crazes.
That makes wet curing the gold standard on shotcrete, and it makes neglect expensive quickly. Keep the surface continuously wet with fogging, soaker hoses, or wet burlap for the cure period the spec calls for, commonly seven days and longer in heat or wind. Where wet curing is impractical, a curing compound applied at the right rate and time is the fallback, but the silica-fume mixes that resist sloughing are exactly the ones most prone to plastic shrinkage cracking, so they reward real water curing.
The companion guides cover curing fundamentals by topic. The shotcrete-specific lesson is that the same surface area that lets you build thin walls without forms is the surface area that dries them out, so the cure starts the moment the surface can take water without washing, and it does not get skipped because the wall looks done.
Why use a test panel for shotcrete?
You use a test panel because you cannot make a representative shotcrete cylinder the way you do for ready-mix. Shotcrete gets its properties from being shot, the velocity, the consolidation, the in-place water-cement ratio set at the nozzle. Cast a cylinder from the same material by hand and you have tested something that was never shot. So the trade shoots a panel under the real conditions, lets it cure, then cores it for strength and examines the cores for voids and consolidation. The panel is the only honest sample of the work.
The preconstruction panel does double duty. It qualifies the mix and it qualifies the nozzleman, in the actual process and shooting orientation of the job, before production shooting starts. Practice is to shoot a panel for each proposed mixture, each shooting orientation, and each nozzleman, and to repeat it for any change. The panels and the cores follow the ASTM methods commonly cited for shotcrete: ASTM C1140 for preparing and testing specimens from test panels, and ASTM C1604 for obtaining and testing drilled cores. ACI 506.2 is the specification that ties the test program to acceptance.
Confirm the panel size, core count, age at coring, and acceptance criteria against the project spec and the adopted editions of those standards rather than carrying numbers in your head, because they shift between editions. The principle does not shift: the panel is the proof. A nozzleman who cannot put up a passing panel does not shoot the structure, and a mix that fails a cored panel does not go in the wall.
Where shotcrete gets used
The applications all share the same reason: a form on both sides is impractical, impossible, or not worth it. Pools are the volume application, where gunite or wet-mix shoots a free-form shell over a steel cage against trimmed excavation. Retaining walls, basement walls, and blind-side foundation walls shoot against a single form or a sheeted excavation, building the wall where you could never set a back form.
Slope and soil work is shotcrete's natural ground. Soil nail walls, slope faces, channel linings, and rock stabilization get a shotcrete skin shot directly against the earth or rock, often over a welded-wire or bar mat, to hold the face and shed water. Tunnels and underground works use shotcrete as initial and final lining, frequently wet-mix with accelerator so the crew can build lining behind the advance without forms.
Repair is the other big bucket. Spalled and deteriorated concrete on bridges, parking structures, dams, water tanks, and marine structures gets restored with shotcrete because it can rebuild section overhead and on vertical faces without forming the whole element. The repair guide covers the assessment and substrate side. The common thread across every one of these is that the structural performance rides on the nozzleman and the mix, so the same rules about encasement, rebound, and test panels apply whether you are building a pool or lining a tunnel.
Structural shotcrete and the engineer
Structural shotcrete is shotcrete the engineer is counting on to carry load, and it lives under a tighter regime than a decorative or protective coat. When the shotcrete is part of the structure, the design strength, the reinforcing, the cover, the encasement, and the acceptance testing all come from the structural engineer of record and the project specification, and ACI 506 is the family of documents that frames how it gets specified and evaluated.
The line that matters on the ground: structural shotcrete is only structural if it was placed by a qualified nozzleman, with a qualified mix, proven on a test panel, and verified in place. The engineer is relying on full encasement of the steel and the cored strength, both of which depend entirely on placement quality. That is why the spec on a structural job names the certification, the panel program, and the acceptance criteria, and why the engineer or the testing agency is on site for the shooting.
Do not let anyone treat structural shotcrete as a finish trade. The decisions about mix, accelerator, lift thickness, and where the steel can actually be encased are engineering decisions with field consequences, and the right time to raise a congested-steel or thin-cover problem is before the gun comes out, with the engineer in the loop.
Cold joints and construction joints
Shotcrete stops and starts, and where it does you get a joint that has to be made on purpose or it becomes a defect. A construction joint in shotcrete is not a sawn line; it is a shot detail. The standard practice is to taper the edge of the day's work, sloping it down rather than leaving a square cold face, so the next placement can be shot against it and consolidated into it without trapping rebound or air at the seam.
The square cold edge is the trap. Leave a vertical face where you quit, come back the next day, and the stream piles against it and shadows just like it does behind a bar, leaving a weak unbonded line right at the joint. So the rule is to taper to a thin feathered edge while shooting, clean and dampen that edge before resuming, and shoot the new work into it so the joint consolidates rather than abuts.
Plan joints where the structure can take them and where the engineer puts them, not where the crew happens to run out of day. On a water-retaining structure or a structural wall the joint location and treatment are part of the design, because a joint is the most likely place for a leak or a plane of weakness. Treat every stop as a joint to be made well, even an unplanned one.
Dust, rebound, and safety
Shotcrete throws hazards that ordinary placement does not. Dry-mix in particular generates respirable crystalline silica dust at the nozzle and at the gun, and that dust is the long-term killer of the trade. Respiratory protection rated for silica, dust controls at the gun, and the engineering and work-practice controls the standards require are not optional gear; silicosis is the bill that comes due years after the job. The applicable occupational safety rules for respirable crystalline silica govern, and they have teeth.
Rebound is the immediate hazard. Material is bouncing off the surface at velocity, so eye protection and full skin coverage are baseline, and everyone stays out of the rebound cone and out from under overhead work. The nozzle stream itself will injure anyone who walks into it. Keep the shooting zone clear and controlled.
The rest is concrete safety plus pressure. You are running compressed air and pumped material at pressure through hoses and couplings, so a blown coupling or a whipping hose is a real injury, and the lines get bled and the system depressurized before anyone breaks a connection. The fresh shotcrete is caustic like any cement, so the skin and eye protection that applies to concrete applies here too, with the added energy of the spray.
Quality control and acceptance
Acceptance on shotcrete rests on three checks, because the defects you care about are mostly hidden. First, the cores from the test panels and, where specified, cores from the in-place work give you compressive strength and a direct look at consolidation and voids, tested under ASTM C1604. Second, sounding the hardened surface with a hammer or chain finds the voids and delaminations behind the face that you cannot see, especially the shadows behind congested steel. A hollow or drummy sound is a void until proven otherwise.
Third is thickness and cover. Shotcrete shot without a back form can come out thin, and thin is where it fails, so depth gauges, probes, or shot-in pins and depth checks against the design thickness are part of the work, along with verifying the cover over the steel. On a slope or tunnel the thickness check is also a quantity and a structural check at once.
Visual acceptance catches the rest: sloughs, sand streaks, dry patches, overspray and rebound trapped in the surface, and cracking. The inspector who knows shotcrete sounds the wall and looks behind the finish, because a troweled face can sit over a void. Acceptance criteria, core frequency, and tolerances come from the project spec and the adopted edition of ACI 506.2; confirm them rather than assuming, and tie every result back to the nozzleman and the panel that qualified the work.
What to document
The voids in shotcrete are the part you cannot see once the wall is up, so the cores and the panel results are what carry the weight later when a section gets questioned. The record ties each area of placed work to the mix it was shot with, the certified nozzleman who shot it, the qualifying panel and its cores, and the verified thickness, so when a question comes up the trail exists.
Capture the area or element, the process and mix used, the nozzleman by name and certification, the orientation shot, the test panel and core results, the verified thickness and cover, the curing method and duration, and any joints or repairs. If a section was reshot for sloughing or a panel was rerun, write down why. The next person reading it needs to know which nozzleman and which mix stand behind the wall they are looking at.
| Field to record | Why it matters |
|---|---|
| Area or element placed | Ties the record to a location |
| Process and mix design | Wet vs dry, accelerator, the proportions shot |
| Nozzleman name and certification | The certified hand stands behind the work |
| Shooting orientation | Vertical or overhead, matches the qualification |
| Test panel and core results | The only honest strength sample |
| Verified thickness and cover | Thin and short cover are where it fails |
| Curing method and duration | High surface area dries fast |
Common mistakes
- Shooting rebound back into the work or troweling it into the wall instead of removing it.
- Leaving voids and sand pockets behind the rebar by shooting straight at congested steel instead of gunning behind each bar.
- Putting an unqualified nozzleman on structural work, or one not certified in the actual process and orientation.
- Building a lift too thick or too wet so it sloughs, then chasing the sag instead of reshooting it.
- Holding the nozzle off perpendicular or at the wrong distance, which spikes rebound and leaves soft, porous material.
- Skipping the preconstruction test panel, so the first proof of strength is a failing core from the structure.
- Leaving a square cold joint instead of tapering the edge and shooting the next placement into it.
- Letting the high surface area dry out by skipping or shortening the wet cure.
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
Standards and references
ACI 506 is the family of documents for shotcrete. ACI 506R is the guide to shotcrete, the how and why; ACI 506.2 is the specification used to require and accept it; and ACI maintains the related evaluation and materials guidance. Cite the document and the adopted edition the project actually references rather than a section number from memory, because the numbering and the content shift between cycles.
Nozzleman qualification runs through the ACI certification program, carried with the American Shotcrete Association, certifying in the wet-mix or dry-mix process and in a shooting orientation, with overhead earned separately from vertical. The test methods for the panel program are commonly ASTM C1140 for preparing and testing specimens from shotcrete test panels and ASTM C1604 for obtaining and testing drilled cores of shotcrete. Verify the exact certification and edition references against the project spec.
Above all of these sits the structural engineer of record and the project specification, which set the design strength, the reinforcing and cover, the certification required, the panel program, and the acceptance criteria. The applicable occupational safety regulations for respirable crystalline silica govern the dust controls and respiratory protection. When a specification or the engineer is stricter than a general guide, the stricter requirement governs, and the adopted code edition and any local amendments control.
Units, terms, and conversions
Shotcrete carries its own vocabulary, and the same idea reads differently across a pool spec, a tunnel spec, and a structural drawing set.
Shotcrete is the general term for pneumatically applied concrete or mortar; gunite is the older name that survives for the dry-mix process, especially in pools. Thickness and cover are given in inches or millimeters, strength in psi or MPa, and the water-cement ratio as a unitless decimal. Rebound is expressed as a percentage of material shot. The two processes are wet-mix, water batched before the hose, and dry-mix, water added at the nozzle.
- Shotcrete / gunite
- Pneumatically applied concrete or mortar; gunite is the dry-mix process, mainly in the pool trade
- Wet-mix / dry-mix
- Water batched before the hose (wet) versus water added at the nozzle by the nozzleman (dry)
- Rebound
- Aggregate-rich material that bounces off and must not be reincorporated; rises overhead and on dry-mix
- Shadowing
- Voids or sand pockets behind reinforcing steel from shooting straight at the bar instead of behind it
- Test panel
- Shot sample cored for strength and consolidation, since a cast cylinder does not represent shotcrete
- Shootability vs pumpability
- The stiffness to hold on a wall or overhead versus the flow to move through the hose
- Flash coat
- A thin final sprayed layer for uniform texture over a cut or troweled surface; cosmetic, not structural
FAQ
What is shotcrete?
Shotcrete is concrete or mortar sprayed through a hose at high velocity onto a surface, where the impact compacts it in place without a form on the sprayed side. It builds pools, retaining walls, slopes, tunnel linings, and structural repairs, trading formwork for the skill of the nozzleman holding the nozzle.
What is the difference between shotcrete and gunite?
Gunite is shotcrete applied by the dry-mix process, a name that survives mainly in the pool trade. In dry-mix gunite, dry materials are conveyed dry and water is added at the nozzle by the nozzleman. In wet-mix shotcrete, the concrete is batched with water and pumped, with air added at the nozzle.
What is rebound in shotcrete?
Rebound is the material that bounces off the surface instead of sticking, the coarser aggregate that ricochets while the paste embeds. It is aggregate-rich and binder-poor, so it must never be shot back or troweled into the work. Rebound is highest overhead and on dry-mix, and it is paid-for material lost as waste.
Why use a test panel for shotcrete?
Because you cannot cast a representative shotcrete cylinder by hand; shotcrete gets its strength from being shot. The trade shoots a panel under real conditions, cures it, then cores it for strength and consolidation. The preconstruction panel qualifies the mix and the nozzleman in the actual process and orientation before production shooting starts.
Do you need a certified nozzleman for structural shotcrete?
On structural work, yes, and it is the single most effective specification line. ACI, with the American Shotcrete Association, certifies nozzlemen in the wet-mix or dry-mix process and shooting orientation, overhead earned separately from vertical. The placement is the quality, so structural specs name the certification in the actual process and orientation of the job.
What causes voids behind rebar in shotcrete?
Shooting straight at a bar so the stream piles material on the front and leaves the back in a wind shadow, a void or sand pocket the trade calls shadowing. The fix is to cut the angle and gun behind each bar from both sides, encasing the steel before closing the face. Congested big bars make it worst.
How thick can shotcrete be applied in one pass?
It varies with the mix, accelerator, orientation, and nozzleman, so there is no universal number. Shotcrete builds to full thickness in supportable lifts, not one shot. A lift too thick or too wet sloughs, sagging or peeling off the wall. Overhead takes the thinnest lifts. Reshoot a sloughing lift rather than chasing the sag.
Can rebound be reused in shotcrete?
No. Rebound is sorted-out coarse aggregate with too little cement, so it is not standard-compliant concrete anymore. Shooting it back or troweling it in builds a weak, sandy zone right where you cannot see it. Remove rebound from corners, ledges, and the toe before it accumulates, and treat all of it as waste.
How is shotcrete cured?
By keeping moisture in while the cement hydrates, and shotcrete needs it more than most concrete because its high surface-area-to-volume ratio dries it out fast. Wet curing with fogging, soaker hoses, or wet burlap for the specified period, commonly seven days and longer in heat, is the standard. A curing compound is the fallback where wet curing is impractical.
Is shotcrete as strong as cast-in-place concrete?
Properly placed shotcrete meets or exceeds the same strengths because it is consolidated by velocity at low water content. The catch is the placement: an unqualified nozzleman builds hidden voids and shadows that cast-in-place vibration would not leave. Strength is proven by cores from a shot test panel under ASTM C1604, not by a hand-cast cylinder.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.