Concrete grout types and equipment baseplate grouting field guide

What grout is, and the job it actually does

Grout is a flowable material that fills a gap and carries load across it. It is mostly cement, water, and fine aggregate, or a resin system, mixed loose enough to flow into a space and harden in place. The job is rarely decorative and never structural in the way a footing is. The job is contact. You set a steel baseplate on a concrete foundation, the bottom of the plate and the top of the concrete never touch across their full area, and the gap between them is where the grout goes so the load has a continuous path from the equipment into the foundation.

Think about what is happening under a pump or a column. The plate sits on shims or leveling screws while it gets aligned, which leaves it floating on a few hard points with air everywhere else. Run the machine or load the column on those few points and you concentrate the load, the plate flexes, and the bearing fails where you cannot see it. Grout fills that space so the whole underside of the plate bears, not just the shims. That is the load-transfer job, and it is the same job a properly installed anchor does in tension. The anchor holds the plate down, the grout spreads what bears underneath. For how the anchor side of that connection works, see the concrete anchor guide.

Grout also patches, beds rail and crane base plates, fills the annular space around a sleeved bolt or pipe, and fills the cells of reinforced masonry. Those are different products and different rules, covered later. The common thread is a material thin enough to fill a space you cannot reach with a trowel and stiff enough, once cured, to carry the load that space has to carry. The strength of the grout is the strength of the gap, and the strength of the foundation under it is the strength of the mix that went in, which is its own subject in the mix-design guide.

What is the difference between grout, mortar, and concrete?

Grout, mortar, and concrete are three cement-based materials with three different jobs, and using one where another belongs is a common and expensive mistake. Grout is flowable and made to fill a space and transfer load through it. Mortar is plastic and made to bond masonry units to each other. Concrete is structural and made to carry load as a mass.

The split comes down to aggregate and consistency. Concrete carries coarse aggregate, the stone, because the stone is what makes it a strong, stiff structural mass for slabs, footings, and walls. Mortar drops the coarse aggregate and stays at a buttery, trowelable consistency because its job is to stick block or brick together at the joint, not to flow. Grout usually has no coarse aggregate either and runs much wetter, because it has to flow into a gap or a cell and reach every corner under hydraulic head. For the structural-concrete side of this, and why the water-cement ratio runs that material, the mix-design guide is the place.

The trap is reaching for a bag of mortar or a sack mix to bed a baseplate because it is on the truck. Mortar shrinks as it cures and pulls away from the gap, so the plate ends up bearing on whatever did not shrink. Concrete will not flow into the gap and will leave the plate bridging on stone. Neither one transfers load under a plate the way a grout formulated for that does. The right material for a baseplate is a grout made for the job, not the nearest cement product.

Non-shrink cementitious grout, the baseplate workhorse

Non-shrink cementitious grout is the standard material for bedding baseplates and filling under columns and bearing plates, because it does not shrink away from the gap as it hardens. That one property is the whole point. An ordinary cement grout shrinks as it cures, lifts off the underside of the plate, and leaves a film of air across the bearing it was supposed to fill. The plate then bears on the shims and the high spots, the load concentrates, and the bearing you paid for is gone. Non-shrink grout is built so the cured grout stays at or slightly above the height you placed it, holding contact under the plate.

It comes pre-bagged as a dry blend of hydraulic cement, fine aggregate, and the additives that control the volume change, and you add only mixing water on site. ASTM C1107 is the specification that covers it. A grout that meets C1107 has been tested to confirm it does not drop below its placed height, which is exactly the failure you are guarding against under a plate. Most general equipment grouting, column base grouting, precast bearing, and machine bases that see static or light dynamic load use a C1107 non-shrink grout.

Non-shrink grout is sold in consistency ranges, from a stiff damp-pack blend up to a high-flow precision grout, and many products are the same powder mixed at different water contents. The strength, the flow, the working time, and the placement temperature limits are all on the bag and the data sheet, and they vary enough between products that you size the pour to the product you have, not to a remembered number. Confirm the strength and the water range on the data sheet for the bag in your hand.

Epoxy grout, the high-strength resin grout

Epoxy grout is a resin grout that trades cost for strength, chemical resistance, and the ability to take vibration and dynamic load without breaking down. It is not cement. It is a three-component system: a liquid resin, a liquid hardener, and a bag of specially graded aggregate. You blend the two liquids, add the aggregate, and place a material that cures by chemical reaction rather than by hydration. The cured grout reaches high compressive strength, has very low shrinkage, bonds hard to clean concrete and steel, and shrugs off oils, solvents, and many process chemicals that would eat a cement grout.

The reason it matters under machinery is vibration. A reciprocating compressor, a large pump, or a rail under impact load sends energy into the foundation, and epoxy damps that energy far better than a cement grout, by a wide margin. It also holds tight tolerances, so it keeps a precisely aligned machine where you set it. API 686, the standard for machinery installation, makes epoxy the default for machinery grouting unless the specification says otherwise. That tells you where the industry has landed for rotating and reciprocating equipment.

The cost is real, often several times the price of a cement grout per unit, so epoxy is not the answer for every plate. It also has a temperature ceiling. Many epoxy grouts are rated to roughly 150 degrees F in service and soften above that, while cement grouts tolerate much higher heat, so a hot service can flip the choice back to cementitious. The cure is sensitive to temperature in both directions, and the aggregate has to be fully wetted and placed before it stiffens. Confirm the service temperature limit, the strength, and the working time against the product, because epoxy systems differ more from each other than cement grouts do.

Cementitious or epoxy grout: which do you use?

Use non-shrink cementitious grout for static and general equipment, and use epoxy grout for dynamic, vibrating, chemical, or high-precision service. That is the lean. Cementitious is cheaper, easier to mix and place, tolerates heat, and is plenty for a column base, a static skid, a tank ring, or a pump that does not pound. Epoxy earns its premium where the equipment vibrates, takes impact or dynamic load, sits in chemicals, or has to hold a precise alignment over years.

Where it gets decided is the load and the environment, not the size of the machine. A small reciprocating compressor that hammers can call for epoxy while a large but smooth-running fan base is fine on cementitious. Chemical exposure pushes you to epoxy regardless of load, because cement grout is attacked by acids and many process fluids. High service heat pushes you back to cementitious, because epoxy softens. When API 686 governs the project, epoxy is the default for machinery and you justify cementitious, not the other way around.

The two ways to get this wrong are symmetric. Putting cement grout under a vibrating machine that needed epoxy gets you cracking, loss of bond, and a machine that walks out of alignment. Putting expensive epoxy under a static column base that a cement grout would have held fine just burns money and, in a hot service, can be the worse technical choice. Match the grout to what the equipment does, and let the equipment manufacturer and the spec settle the call when it is close.

What grout consistency do you need?

Grout is placed at four broad consistencies, and you pick the one the gap and the method call for. Fluid grout pours like heavy cream and self-levels, for filling a baseplate gap from one side under head. Flowable grout is thicker but still pours, for shorter or more open spaces. Plastic grout is stiff enough to push or pump but will not pour on its own. Damp-pack grout is a stiff, almost dry blend you hand-pack into place. Same product family, different water and different jobs.

The test for a fluid cementitious grout is the flow cone, ASTM C939. You fill a standard cone, time how long it takes the grout to flow out, and that efflux time tells you the consistency is right and repeatable. The method is built for grouts that flow through the cone in 35 seconds or less, and when a grout is too stiff to run the cone, flowability is judged on a flow table instead. The data sheet gives the target flow or efflux time for each consistency the product supports, and that is the number to hit, not your eye.

The discipline is to flow the grout without overwatering it. A fluid grout has to fill the space and reach every corner, but it cannot be so wet that it bleeds, segregates, and leaves the aggregate at the bottom and a weak laitance on top. The right consistency is the one that flows under the head you can build and still holds the strength and the non-shrink behavior on the data sheet. Reach for more water to make it flow easier and you have started trading away the properties you bought the grout for.

How do you grout an equipment baseplate?

Set and level the plate, prep the surfaces, form a dam, mix the grout, and pour it from one side under head so it pushes the air out ahead of it. That is the sequence, and the order is not negotiable. The plate gets aligned and leveled on shims, wedges, or leveling screws first, with the anchor bolts in place, so the equipment is where it needs to be before any grout goes in. Grout is the last step, after alignment, not a way to fix a plate that is sitting wrong.

Prep both faces. The concrete gets roughened and cleaned to sound material and brought to a saturated-surface-dry condition for a cement grout, and the underside of the plate gets cleaned of oil, paint, and loose rust so the grout makes contact. Build a tight form around the gap, leaving a head box on the pour side and a vent or a low dam on the far side so air has somewhere to go. Mix the grout to the consistency the data sheet calls for, no wetter.

Then pour. Place the grout from one side only, continuously, and let it flow across under the plate so the advancing front drives the air out the far side. Keep grout in the head box so the head of material above the gap keeps pushing the front along. Do not pour from both ends and do not chase the grout with a rod or a vibrator, because both trap air, and trapped air is a void, and a void is lost bearing. Strap, link, or chain can drag the grout along under a wide plate, but the head does most of the work. Fill until grout shows at the vents and stands in the head box, then hold the head until it sets so the front cannot pull back.

When it has set, strike the excess, and many specs call for a 45 degree chamfer at the exposed edge under the plate to shed water and reduce edge cracking. Then cure it the way the data sheet says, which for a cement grout means keeping it wet, and leave the equipment alone until the grout has the strength the manufacturer ties to load and final alignment.

Effective bearing area: the no-voids rule

The grout under a baseplate is only doing its job where it actually touches the plate, and the number-one failure in baseplate grouting is voids: air pockets and incomplete fill that leave the plate bearing on less area than the design assumed. The grout can be the right product at the right strength and still fail the equipment if half the underside of the plate is bridging over a void. The load then crowds onto the area that did make contact, the plate flexes, and you get the same concentrated bearing and lost alignment that grout was supposed to prevent.

This is why the placement method is the whole game. Pouring from one side under head, venting the far side, and never rodding or vibrating the grout into place are not preferences. They are how you get the grout to fill the space without folding air into it. A void that forms because someone poured from both ends and trapped a pocket, or because the head dropped and the grout front pulled back, is invisible once the form comes off. The plate looks grouted. It is not bearing.

Effective bearing area is what the engineer designed the plate and anchors around, so losing it is not cosmetic. Confirm full contact by sounding the plate after cure, covered in the QC section, and treat any hollow area as a defect to be pressure-injected or re-grouted, not a cosmetic blemish to ignore. Full contact under the plate is the deliverable. Everything else in the procedure serves it.

Forms, the dam, and the head box

The forms do two jobs: they hold the grout in the gap until it sets, and they create the head that drives a fluid grout across under the plate. A baseplate form is a tight dam around the perimeter of the gap, sealed against the foundation so grout does not run out and leak the head away. Any gap in the seal bleeds pressure and the grout front stalls, so caulk, foam, or a clay seal at the base of the form matters more than it looks.

On the pour side you build the form higher into a head box, a reservoir that sits above the level of the gap. You keep that box full during the pour, and the column of grout standing in it generates the hydraulic head that pushes the grout under the plate and out the far side. The higher the head box and the fuller you keep it, the harder the grout is driven into the space, which is how you reach full contact without rodding. The far side gets a lower dam or a vent so air and the leading grout escape there instead of being trapped.

On a large pour, the same head that helps you can work against you, so forms have to take the pressure of fluid grout without bulging or floating, and big placements get broken into bays or pour lengths the product can fill before it stiffens. Expansion or contraction joints in long grout placements follow the equipment and the foundation, not a habit, so take the joint locations off the design. A form that leaks, bulges, or lets the head fall is the usual reason a textbook pour still ends up with voids.

Surface preparation that lets the grout bond and bear

The grout bonds and bears on what you give it, so both faces of the joint get prepped before anything is mixed. The concrete is roughened to a clean, sound surface with the laitance and any weak top layer removed, so the grout grips real concrete and not a dusty skin. Oil, curing compound, and loose material all have to go, because grout will not bond through them. The underside of the steel plate gets the same treatment: clean steel, free of oil, paint, mill scale where the spec calls for it, and loose rust.

For a cement grout, the concrete is then brought to a saturated-surface-dry condition. You soak the concrete ahead of the pour, often for hours, then remove the standing water so the surface is damp but has no puddles. The reason is simple and the rookies miss it. Dry concrete acts like a sponge and pulls the mixing water out of fresh grout the instant they touch, which starves the grout of the water it needs to hydrate and leaves a shrunk, cracked, debonded layer right at the interface. SSD stops the substrate from stealing that water. Standing puddles do the opposite and dilute the grout, so it is damp, not wet.

Epoxy grout flips one rule. Epoxy bonds to dry, clean concrete and does not want a saturated surface, because water at the interface interferes with the resin bond, so for epoxy you prep clean and dry, not SSD. That single difference catches crews who grout cement one week and epoxy the next out of habit. For surface preparation by substrate and method in more depth, the dedicated prep material is the place; the point here is that the prep is matched to the grout.

Shims, leveling, and whether they come out

The plate is leveled and aligned on hard points before grouting, and those points are shims, steel wedges, or leveling screws, sometimes called jackbolts. They carry the plate at the right elevation and attitude while you set the alignment, and they hold it there through the grout pour. Leveling is its own step that has to be finished and checked before grout goes in, because grout does not move a plate that is sitting wrong; it locks in whatever you grouted.

Whether the leveling hardware stays or comes out depends on the design and the product. Some specs leave shims in place permanently, fully embedded in the grout. Others have you remove the shims or back off the leveling screws after the grout has gained enough strength to hold the plate, then fill the resulting pockets, so the grout carries the full bearing and the steel points are not a hard spot under load. Leveling screws are commonly backed off after cure for exactly that reason, so the load goes into grout, not into a handful of screws. Follow the design and the equipment manufacturer on this, because it changes how the load ends up bearing.

The detail crews get wrong is leaving a stack of loose shims as the permanent bearing under a machine that needed full grout contact. The plate then rides on the shims, the grout is along for the ride, and the bearing the design counted on never happens. If the shims are meant to come out, they come out, and the pockets get packed. If they stay, they are placed so they do not create a hard point that defeats the grout.

Anchor bolts and the grout around them

The anchor bolts hold the plate down while the grout spreads what bears underneath, and the two work together. The bolts are set, often in sleeves or pockets that allow some final position adjustment, and the grout fills around them and through their pockets as part of the pour. Grouting the bolt pockets solid is part of getting full contact, and an air pocket around a bolt is the same void problem in a spot that also has to transfer the bolt load into the concrete.

Sequence and torque matter. The bolts are commonly snugged to hold alignment during the pour and then brought to final torque after the grout has the strength to take it, so you are not torquing against fresh grout that has not set. Torque a hold-down before the grout can carry the bearing and you can pull the plate down onto a gap or crush soft grout. The manufacturer's bolt-up procedure and the grout's load strength set the order.

How much load the bolts actually carry, and whether they break the concrete out before the steel yields, is the anchor side of this connection and a subject of its own. The concrete anchor guide covers embedment, edge distance, cracked-concrete ratings, and the cleaning that makes an adhesive anchor hold. The short version here: the grout and the anchors are designed as one connection, so grout the bolts solid and torque them in the order the equipment and the grout strength allow.

How non-shrink grout actually compensates shrinkage

Non-shrink grout does not magically refuse to shrink. It is built so a controlled expansion offsets the normal drying and chemical shrinkage of the cement, so the net change in height stays at or slightly above where you placed it. Different products do that with different mechanisms, and the mechanism is part of why the grades exist. The result that matters in the field is the same: the grout stays in contact with the underside of the plate instead of dropping away.

There are a few common expansion routes. A gas-forming grout uses a fine metallic powder, often aluminum, that reacts with the alkaline cement and releases tiny gas bubbles while the grout is still plastic, expanding it to offset early settlement. A metallic grout uses iron particles that oxidize and expand. Many modern grouts use shrinkage-compensating cement chemistry, where expansive crystals such as ettringite form under confinement and counter the shrinkage as the grout hardens. Each gives a net expansion that is small, commonly a fraction of a percent, just enough to hold contact rather than to push the plate up.

ASTM C1107 sorts non-shrink cementitious grout into three grades, A, B, and C, by how and when the grout makes that volume adjustment, with early-age height change and hardened height change measured against the standard's limits. Which grade a job wants is on the spec and the product data sheet, so confirm it there rather than assuming all non-shrink grouts behave the same. The grade is not a strength rank; it is about the timing and the mechanism of the volume control.

Mixing the grout without ruining it

The single fastest way to wreck a good grout is too much mixing water. The bag gives a water range for each consistency, and you mix to the least water that gets the flow you need. Extra water raises the flow and feels like progress, but it bleeds to the surface, lets the aggregate segregate to the bottom, drops the strength, and brings back the very shrinkage you bought a non-shrink grout to avoid. The slump comes up, the crew is happy, and the grout under the plate is weaker and shrunk a month later. That is the cement-grout version of the same water mistake that haunts a concrete mix, covered in the mix-design guide.

Mix mechanically, not by hand, for anything past a small patch. Use a mortar mixer or a paddle on a slow drill, add most of the water first, then the powder, and mix to a uniform, lump-free consistency without whipping air into it. Mix full bags or consistent partial batches so the water-to-powder ratio stays the same load to load, because eyeballing the water on each batch is how a wall of grout ends up with a soft course in the middle.

Watch the clock. Mixed grout has a working time, and a cement grout that starts to stiffen is past its window. Do not retemper it, meaning do not add water to a load that has begun to set to loosen it back up, because the added water breaks the structure that has started to form and permanently weakens the grout. Mix what you can place in the working time, place it, and throw out what stiffened. The working time shrinks in heat and stretches in cold, so plan batch size around the temperature on the day.

Placement temperature, cure, and early load

Grout cares about temperature at placement and during cure, and the limits are on the data sheet because they vary by product. Cement grout slows down and can be damaged if it freezes before it sets, the same hydration physics as concrete, so cold-weather grouting means warming the materials and the foundation and protecting the placement until it has strength. Hot weather goes the other way: the working time collapses, the grout flashes off, and you have to cool the materials, shade the work, and mix smaller batches to keep up.

Cure follows placement and it is where a cement grout earns or loses its strength. Cement grout needs a wet cure, kept damp under wet burlap, a curing membrane, or whatever the data sheet specifies, because the same dry concrete that steals water before the pour will pull it out of curing grout after. Epoxy cures by chemical reaction and does not want a wet cure, but it is fussy about temperature, gaining strength fast in warmth and slowly, sometimes too slowly, in the cold. Confirm the cure method against the grout type, because curing cement grout dry and trying to wet-cure epoxy are both wrong.

Then the load question. Equipment, alignment, and final bolt torque wait for the strength the manufacturer ties to those steps, not for the calendar. Load a cement grout before it has that strength and you crush it or pull the plate into the soft grout, and the early-age strength climbs slower when it is cold. The early-load and alignment strength is a number on the data sheet at a stated temperature, so figure longer in the cold and verify before you torque or run the machine.

Masonry grout is a different material

Masonry grout is not the grout you use under a baseplate, and confusing the two damages the wall. Masonry grout is a fluid mix of cement, fine aggregate, sometimes pea gravel, and a lot of water, poured into the cells of reinforced concrete block or brick to bond the reinforcing steel into the wall and turn the hollow units into a solid reinforced section. It is covered by ASTM C476 and placed at a high slump, commonly in the 8 to 11 in range, specifically so it flows around the rebar and fills the cells without leaving voids.

It comes as fine grout and coarse grout. Fine grout has only sand and is used in small cells and tight spaces; coarse grout carries pea gravel and is used in larger cells and lifts. The high water content looks alarming next to a baseplate grout, but the block absorbs the excess water after placement, which is part of how masonry grout works and why it can run that wet.

Do not fill CMU cells with non-shrink equipment grout. A non-shrink grout expands early, and inside a confined block cell that expansion can crack the units apart, like water freezing in the cell. It is also too stiff at its working consistency to fill cells the way a fluid masonry grout does. Use masonry grout for masonry and equipment grout for equipment. For the masonry wall side of this in detail, see the dedicated masonry material; the point here is that the word grout covers two different products.

Annular space, void, and abandonment grouting

Grout also fills spaces that have nothing to do with carrying a machine, and the rules change with the job. Annular grouting fills the ring-shaped space between two things, the gap between a pipe and the casing or sleeve it runs through, the space around a bolt sleeve, or the void between a liner and the surrounding wall. The grout there seals and supports, and the consistency is whatever flows into the annulus and stays put.

Void filling and abandonment grouting fill larger empty spaces, an abandoned tank or pipe, a sinkhole under a slab, a hollow between structures, where the goal is volume and stability, not strength. For those, a lean, low-cost grout does the work, and a cellular or foamed grout, lightened with engineered air, fills large volumes with less material and less weight while still flowing into the space. You are not transferring a machine load, so the mix is sized to fill and stay rather than to reach a high strength.

The selection logic is the through-line of this guide applied to a different space: pick the grout for what the gap has to do. A baseplate gap transfers a concentrated load and gets a strong, non-shrink or epoxy grout. An abandoned void just needs to be full and stable and gets a lean or cellular grout. Spend the strength where the load is.

Precision and high-strength machinery grout

Precision grout is a high-flow, high-strength grout for machinery that has to sit level and stay aligned to tight tolerances. It is the high end of the consistency range, mixed fluid so it flows into a thin, wide baseplate gap and reaches full contact under hydraulic head with no help from a rod. The high flow and the non-shrink behavior together are what let it fill a precise gap and hold the plate where the millwright set it.

Where precision grout matters most is rotating and aligned equipment whose performance depends on the plate staying flat and level: pumps coupled to motors, turbines, large gear sets. If the grout shrinks, segregates, or leaves voids, the plate distorts, the alignment drifts, the coupling complains, and the bearings pay for it. This is the same family of failure that pushes heavy, vibrating, or chemically exposed machinery to epoxy, and many precision applications use an epoxy precision grout for exactly that reason.

The flatness and level of the finished bearing is the deliverable, so it gets checked, not assumed. The plate is set level within the equipment manufacturer's tolerance, the grout is placed to hold it there, and the alignment is confirmed after the grout cures and the leveling hardware is dealt with. Precision grout is the tool; a flat, level, fully bearing plate is the result you are buying.

Damp-pack and dry-pack grouting

Damp-pack grout, also called dry-pack, is the stiff end of the range, mixed with just enough water to hold together when you squeeze a handful and packed into place by hand and hammer rather than poured. It is used where you cannot build a head box to flow a fluid grout, for narrow or shallow baseplate gaps, for filling under bearing plates from the edge, and for patching, where a stiff mix that stays where you put it beats a fluid one that runs.

The method is what makes or breaks it. You drive the stiff grout into the space in layers with a hardwood stick or a packing tool and a hammer, compacting each layer hard against the last and against the plate, working from the back of the space forward so you do not trap a void behind packed grout. Done right, the compaction gives you contact and density. Done lazily, dry-pack is the easiest way to leave a hollow space under a plate, because nothing about a stiff mix flows in to fill a spot you missed.

Dry-pack is slower and more dependent on the hand doing it than a flowable pour, so it is a choice for the gap that will not take a fluid placement, not a default. When the gap can be flowed under head, a fluid grout reaches full contact more reliably than hand packing does. When it cannot, dry-pack done with discipline is the answer.

QC, testing, and sounding for voids

Grout gets the same quality control concrete does, plus a check concrete does not need: proof that it filled. Strength is verified on molded specimens, 2 in cubes broken per ASTM C109 for cementitious grout, and the resin grouts have their own methods, with compressive strength of chemical-resistant and epoxy grouts run under ASTM C579. Flow or efflux time is confirmed on a fluid cement grout with the flow cone, ASTM C939, so the consistency that went under the plate is documented, not guessed. Record the grout temperature, the ambient temperature, and the water used per batch, because those are the variables that move the result.

The check unique to grouting is sounding for voids. After the grout has cured, you tap across the plate with a hammer or drag a chain and listen. Solid grout under the plate rings; a void sounds hollow and dead. Map the hollow areas, because that is your effective-bearing problem made audible. On critical equipment the sounding is sometimes backed up by drilling and probing or by pressure-injecting the hollow areas with a low-viscosity grout or epoxy to fill what the original pour missed.

What an inspector looks at first on a baseplate grout is the contact, then the strength. A pour with cube breaks that pass and a plate that sounds hollow over a third of its area has not done the job, because the strength means nothing where the grout is not touching. Sound the plate, fix the voids, and keep the cube data and the flow record with the equipment file.

Generators, chillers, and data-center equipment

Critical equipment raises the stakes on every rule above, because the cost of a bad grout job is measured in downtime, not just a callback. A standby generator that vibrates, a large chiller, switchgear lineups, and the rotating equipment in a data center or a process plant all sit on grouted baseplates, and a void or a shrunk grout under one of them shows up as vibration, alignment drift, and premature bearing or coupling wear on a machine the facility cannot afford to lose.

The choices sharpen accordingly. Vibrating and reciprocating critical equipment leans epoxy for the vibration damping and the tight tolerance hold, and the alignment tolerances are tighter, so the level of the plate and the full contact under it get verified rather than assumed. The pour gets planned: enough mixed grout staged to fill the plate in one continuous placement, a head box sized to keep the front moving, and a sounding check after cure before the machine is commissioned. On a generator or a chiller that has to start and carry load on day one, the grout is part of the commissioning, not an afterthought poured by whoever was free.

The discipline is the same discipline as any baseplate, applied where the consequence is highest: the right grout for the load and the environment, full contact with no voids, and a record that proves both. The difference is only that on critical equipment, nobody gets to find out later.

What to document

Baseplate grout disappears under steel the moment the plate is set, so when alignment or vibration trouble shows up later there is nothing to inspect except what you wrote down at placement. The record is what answers, months out, whether the right grout went in at the right consistency and whether it actually filled. Capture the grout product and type, the batch water and consistency or flow, the temperatures, the surface prep and SSD or dry condition, the placement method, the cure, the strength results, and the void check.

Keep it with the equipment file, not loose in a daily report, because the person who needs it is the one troubleshooting vibration or alignment two years on. If you sounded the plate and fixed voids, record where they were and how they were filled, because a repaired void is information the next person wants.

Common mistakes

• Using mortar, sack mix, or a shrink grout under a baseplate, which pulls away and loses the bearing.
• Leaving voids and incomplete fill under the plate, so it bears on a fraction of the design area.
• Adding too much mixing water to ease flow, which bleeds, segregates, weakens, and brings back shrinkage.
• Pouring from both sides or from the middle, trapping air that becomes a void.
• Rodding or vibrating fluid grout into place, which whips in air instead of driving it out.
• Letting the head box drop during the pour, so the grout front pulls back and leaves a gap.
• Putting epoxy under a static base where cement grout was fine, or cement grout under a vibrating machine that needed epoxy.
• Skipping surface prep, grouting dry concrete that steals the water, or grouting an oily plate that will not bond.
• Filling CMU cells with non-shrink equipment grout, which can crack the block apart.
• Loading the equipment or final-torquing the anchors before the grout has the strength the manufacturer requires.

Field checklist

Standards and references

ASTM C1107 is the specification for packaged dry, non-shrink hydraulic-cement grout, the workhorse for baseplates and load transfer, and it sorts the material into grades A, B, and C by how the grout controls its volume change. The height change behind that classification is measured at early age by ASTM C827 and in the hardened state by ASTM C1090. Compressive strength of cementitious grout is run on 2 in cubes per ASTM C109, and the flow of a fluid grout is measured with the flow cone in ASTM C939.

For epoxy and chemical-resistant grouts, the test methods differ. Compressive strength is commonly determined under ASTM C579, and the linear shrinkage of chemical-resistant mortars and grouts under ASTM C531. Masonry grout, a separate material, is specified by ASTM C476 and placed fluid to fill reinforced block cells, not to bed equipment. The exact designations and editions move over time, so confirm the current standard and edition the spec calls out.

On the equipment side, API 686 covers machinery installation and grouting and makes epoxy the default for machinery grouting unless the specification says otherwise, which is the reference to know for rotating and reciprocating equipment. ACI Committee 351 addresses foundations and grouting for the support of equipment and machinery and is the concrete-industry reference for the topic. Above all of these sits the grout manufacturer's data sheet and the project specification, which control the strength, the water, the consistency, the cure, and the load timing for the actual product in your hands. Hedge any strength or water number to that data sheet, because it varies by product.

Units, terms, and conversions

Grouting carries a few terms that cross between the cement and the resin worlds and a few units that show up on the data sheets and the specs.

Compressive strength is given in psi in the United States and in MPa in metric sources, where roughly 1000 psi is about 6.9 MPa. Flow for a fluid grout is reported as an efflux time in seconds from the flow cone, or as a percent spread on a flow table. Volume change is reported as a percent of height, and a non-shrink grout holds a small net expansion rather than a loss. Consistency runs from damp-pack at the stiff end through plastic and flowable to fluid at the wet end, and the same product often spans that range at different water contents.

Non-shrink grout

Grout built so controlled expansion offsets shrinkage, holding contact under a plate; ASTM C1107

Epoxy grout

Three-component resin, hardener, and aggregate grout for dynamic, chemical, and high-precision service

Effective bearing area

The plate area actually in contact with grout, where load transfers; voids subtract from it

Head box

A reservoir built above the gap on the pour side that creates hydraulic head to drive grout across

SSD

Saturated surface dry, concrete damp with no standing water, the prep condition for cement grout

Damp-pack / dry-pack

A stiff grout mixed near-dry and hand-packed where a fluid pour will not work

Flow cone

ASTM C939 test timing how long fluid grout flows through a standard cone, an efflux time

Related tools

Calculators and readiness checks for this work

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