Slab-on-Grade

NOTE This standard covers concrete slabs on ground that serve as building floors: the design basis recorded for them, the subbase and the supporting surface beneath them, the vapor retarder decision and the sequence it fixes, the crack-control strategy and the reinforcement or fibers within the slab, thickened edges, joint layout and load transfer, joint filling, flatness and levelness, the pre-placement verification, and acceptance of the finished floor. (1.1)

NOTE A slab on ground is supported continuously by the soil beneath it, and that single fact separates it from every other concrete element in the building. It does not span, so its thickness is set by wheel and post loads against the stiffness of the subgrade rather than by bending between supports. It shrinks against the friction of the ground it sits on, so it cracks unless the cracks are given somewhere to form. And it is the one element whose top surface is the finished product: the flatness a lift truck feels, the joint edge a pallet jack strikes, and the moisture a flooring adhesive sees are all properties of the slab itself. The decisions this standard records are the ones that determine whether those properties are designed or accidental. (1.2)

NOTE The structural design of the slab, its thickness, its reinforcement, its joint spacing, and the loads it carries, is performed by the Engineer of Record under ACI 360R and recorded on the contract documents. This standard records the design basis the Engineer of Record selected, states how the slab is built to that design, and states how the finished floor is verified; it does not produce the design, and no value in it substitutes for a project-specific analysis. (1.3)

NOTE The work of the following standards applies to slabs on ground and is not restated here. (1.4)

NOTE Moisture testing of the slab before a floor covering is installed, and any topical moisture mitigation applied to the hardened surface, are specified by the standard for the covering: Resilient FlooringResilient FlooringResolves to the current adopted revision.sync/resilient-flooring, Wood FlooringWood FlooringResolves to the current adopted revision.sync/wood-flooring, CarpetCarpetResolves to the current adopted revision.sync/carpet, and Resinous FlooringResinous FlooringResolves to the current adopted revision.sync/resinous-flooring each state the test method, the limit, and the mitigation they require. This standard governs the conditions that let the slab dry from one face only: the vapor retarder beneath it and the sequence of its placement. (1.5)

NOTE Exterior pavements, aprons, sidewalks, and curbs are governed by Concrete PavingConcrete Paving, Curbs, and SidewalksResolves to the current adopted revision.sync/concrete-paving; elevated slabs, structural mats, and transfer slabs that span to a frame or resist column loads are governed by Cast In Place ConcreteCast-in-Place ConcreteResolves to the current adopted revision.sync/cast-in-place-concrete and the structural drawings. (1.6)

NOTE A floor is built in one pass by a crew that cannot stop to consult the documents, and the decisions that govern its quality are the ones every party in the room has to carry out in the same order on the same day. The conference exists so that the finishing foreman, the saw operator, and the testing technician have agreed on timing before the first truck arrives rather than at the first joint. (4.1.4)

NOTE A trial placement is a production-sized panel built with the production crew, mixture, equipment, and method before the floor itself, and measured as the floor will be measured. It costs a day and a panel of concrete, and it buys the F-numbers, the sawing window, and the finishing timing for the actual mixture and the actual weather instead of the values assumed in the plan. It earns its cost where the specified tolerances are tighter than the crew has recently achieved, where the mixture carries fibers or a shrinkage limit the finishers have not worked with, or where the floor will remain exposed as the finished surface. (4.2.1)

NOTE An Owner-engaged agency makes the survey an acceptance test independent of the party whose work it measures, at the Owner's cost; a Contractor-engaged agency keeps the same instrument and procedure on the Contractor's side of the contract, which suits a floor the Contractor is also responsible for correcting; the Contractor's own personnel can survey within the 72 hour window on a remote site where no agency is available, at the cost of an acceptance record produced by the party it judges. (4.3.2)

NOTE ACI 360R sizes a slab on ground for the loads that cross it against the stiffness of the ground that supports it. Three inputs decide the outcome: the modulus of subgrade reaction, the loads, and the crack-control strategy, because the strategy fixes the joint spacing and the joint spacing fixes how much of the slab's capacity is spent on shrinkage restraint before any load is applied. (5.1)

NOTE The modulus is a measure of how much the ground deflects under a loaded plate, and it is the single most influential soil input to slab thickness. It is a property of the prepared subgrade together with the subbase, not of the native soil alone, which is why a granular subbase is specified even where bearing is already adequate: it raises the effective modulus and, more importantly, makes it uniform across the floor. (5.3)

NOTE A slab on ground cracks because the concrete shrinks while the ground holds it still, and every strategy on the list is a different answer to where that crack goes. A plain slab puts it at a closely spaced contraction joint and relies on nothing else, which is economical where the floor is lightly and uniformly loaded and the joint count is acceptable. Welded wire and deformed bars do not stop the crack from forming; they hold it tight between joints so it stays narrow, and they permit the joints to be spaced farther apart at the cost of placing and supporting steel. Steel and synthetic macrofibers do the same job distributed through the section, with no steel to chair and no position to verify, at the cost of a mixture that batches and finishes differently and a surface on which fibers can show. Shrinkage-compensating concrete expands against its reinforcement during the first days and then shrinks back to roughly its original length, so the slab arrives at service with little net shrinkage and few joints, at the cost of a cement the supplier has to source and a curing regime that cannot be shortened. Post-tensioning puts the slab in compression so it does not crack at all and needs no contraction joints, and it is the strategy for large jointless floors and for slabs that must bridge expansive or compressible soil, at the cost of a stressing operation and tendon layout the other strategies do not need. (5.6)

NOTE Thickness is the design output, not a rule of thumb. It is set by the governing wheel, post, or uniform load against the modulus of subgrade reaction, and it is the dimension that a high spot in the subbase silently reduces exactly where a loaded wheel will find it. The thickness tolerance and the subbase surface tolerance are therefore specified together, because the second is what protects the first. (6.3)

NOTE The relation between the slab edge and the foundation decides whether the floor moves with the building or independently of it. A turned-down edge makes the slab its own perimeter foundation, which is the usual construction for a pre-engineered metal building or a lightly loaded single-story structure on competent soil. A free edge separated from the foundation wall by a full-depth isolation joint lets the floor shrink and curl without dragging on a wall that is not moving, which is why it is the usual detail where the building has its own footings. An edge resting on a ledge or haunch cast into the foundation keeps the slab edge from settling with the backfill beside the wall while a bond breaker still lets it move horizontally. A doweled edge ties the slab to the wall for load transfer where the wall and the slab are designed to act together, and it restrains the slab in return. (7.1)

NOTE Uniform support matters more to a slab on ground than strong support. A slab does not span, so when a soft pocket, an uncompacted trench, or a lens of organic material settles under a loaded panel the slab follows it and cracks, and the crack looks exactly like a shrinkage crack. Such cracks are routinely blamed on the concrete. The only defense is a subgrade that was compacted, proof-rolled, and accepted before anything was placed on it, and that was not disturbed afterward. (8.1)

NOTE A subbase buys three things at once: a uniform, higher modulus of subgrade reaction across the whole floor; a capillary break between the soil and the slab; and a working platform that trucks, pump hoses, and finishers can operate on without rutting the subgrade. A dense-graded crushed aggregate base provides all three and trims to a tight surface. An open-graded crushed stone layer provides the strongest capillary break and drains, but its surface cannot be trimmed to tolerance or walked on without displacing stone, so it is finished with a fine-graded choker course on which the vapor retarder lies. A cushion of sand or screenings alone gives the trimmable surface and some protection to the retarder without the stiffness or the drainage of a stone layer. No subbase at all places the slab on the prepared subgrade, which the geotechnical engineer of record permits where the subgrade is granular, uniform, and already drains. (9.1)

NOTE A high spot in the subbase takes its height out of the slab, and a low spot is paid for in concrete; a tolerance of three-quarters of an inch is the common pairing with the ACI 117 thickness tolerance, and a tighter value is set where the slab is thin, where the floor is a defined-traffic floor whose thickness governs its stiffness, or where the quantity of concrete is being controlled closely. (9.8)

NOTE A dry absorptive base pulls water out of the bottom of the slab as it is placed, which stiffens the bottom, starves the hydration there, and sets up the moisture gradient that curls the slab; a saturated base supplies water the slab cannot shed and keeps the bottom wet for months. Damp with no free water is the condition between the two. (9.13)

NOTE Whether a vapor retarder is placed beneath the slab is a design decision about where the slab is allowed to dry, not a detail. With a retarder directly beneath it the slab dries upward only, which takes longer but ends, and the floor covering above it sees no soil moisture. Without one the slab dries from both faces and curls less, but soil moisture moves through it indefinitely, so any adhesive, coating, or covering above it is exposed to that moisture for the life of the floor. ACI 302.1R and ACI 302.2R work through the choice as a sequence of questions about the floor covering, the occupancy, and the soil, and ASTM F710 requires an effective retarder beneath any slab that will receive resilient flooring. (10.1)

NOTE A retarder is omitted where the slab will remain bare or will receive only a breathable covering, where the space is unconditioned, and where the geotechnical engineer of record confirms that the subbase drains and the soil carries no moisture source the slab needs protection from; the floors of an open warehouse, an unconditioned industrial bay, or a covered loading area are the usual cases. Omitting it where any adhered or coated finish is later installed converts a saved roll of polyethylene into a flooring failure. (10.3)

NOTE A retarder directly beneath the slab removes the base that would otherwise absorb bleed water, so a mixture placed on it bleeds longer, finishes later, and curls more than the same mixture on a granular base. The adjustments that compensate, a lower water content, a larger aggregate, and a cementitious content that does not depend on the base to take water away, are mixture decisions made under Cast In Place ConcreteCast-in-Place ConcreteResolves to the current adopted revision.sync/cast-in-place-concrete with the retarder in view. (10.9)

NOTE Strength is the property a cylinder measures and shrinkage is the property a floor exhibits, and the two are set by different parts of the mixture. A floor cracks between its joints, curls at them, and spalls along them in proportion to how much water and paste it carried, not to how many pounds per square inch it reached, so a slab mixture is controlled through its water content, its aggregate volume, and a measured shrinkage limit where joint spacing or an exposed finish makes shrinkage the governing property. (11.3)

NOTE Reinforcement in a slab on ground controls the width of the cracks that form; it does not prevent them. A light mat of welded wire or bars adds nothing to the slab's capacity to carry load and nothing to its resistance to cracking; its entire value is in holding a crack closed after it opens, which it does only if it is in the upper part of the slab where the crack opens widest and only if it is still there, in that position, after the concrete is placed. (12.1)

NOTE Reinforcement from a roll holds the curvature it was wound to, and in a slab the spring-back lifts the mat off its supports and up through the surface in one place while it lies on the subbase in another; flat sheets lie where they are chaired, which is why ACI 301 requires them and rolls are selected only for the lightest mats, where the Contractor will flatten and secure them in a way the Engineer of Record accepts. (12.2.2)

NOTE A shrinkage crack opens widest at the top of the slab and narrows toward the bottom, so the steel that holds it closed belongs in the upper part of the section, 2 in. below the surface on a slab of ordinary thickness and at about one-third of the depth on a thick one. Placed higher it loses its cover and rusts at the first crack; placed at mid-depth or below it is simply not where the crack is. (12.4.2)

NOTE Reinforcement lifted into position by hand from beneath the fresh concrete ends up wherever the hook left it, which the record of every cored slab shows is the bottom. Supports close enough that a finisher's weight between them does not push the mat down are the only method that puts the steel where the design assumes it. (12.4.6)

NOTE Steel that crosses a contraction joint restrains it, and a joint that cannot open does not relieve the shrinkage, which then cracks the panel somewhere else. Discontinuing the reinforcement at every joint lets the joint work and relies on dowels or aggregate interlock for load transfer across it. Cutting alternate wires or bars halves the restraint while keeping some steel across the joint to hold it from opening wide, a compromise used where no dowels are provided and the joint will be filled. Continuing the reinforcement without interruption is the detail for a joint that is intended not to open, which makes sense only where the design accepts cracking between joints or where the steel area is large enough to hold those cracks tight. (12.5.2)

NOTE Cold-drawn wire fibers, usually with hooked or crimped ends, give the highest residual strength per pound and are the type the floor market is built around; cut sheet and mill-cut fibers are shorter and stiffer, pump and finish more easily, and deliver less post-crack toughness per pound; melt-extracted fibers are the corrosion-resistant stainless product for an exposed or aggressive environment; modified cold-drawn wire covers the proprietary geometries that trade anchorage against fiber count at a fixed dosage. (12.6.2)

NOTE Steel fibers and synthetic macrofibers occupy different parts of the dosage scale for the same crack-control effect because they differ by roughly an order of magnitude in density, so a dosage is meaningful only beside the fiber it was set for; the design value that compares them is the residual strength the fiber-reinforced concrete develops in the ASTM C1609/C1609M beam test at the proposed dosage. (12.6.6)

NOTE Fibrillated or monofilament microsynthetic fibers at a fraction of a pound per cubic yard hold the surface together during the first hours, while the concrete is plastic and evaporation is pulling it apart; they do nothing for drying shrinkage cracking afterward and are not a crack-control strategy, which is why they are specified as a separate decision and never in place of one. (12.6.11)

NOTE A slab on ground is going to crack, and the joint layout is the choice of where. Contraction joints create planes of weakness so that the shrinkage cracks form in straight lines beneath a saw cut instead of wandering across the floor. Isolation joints separate the slab from the columns, walls, footings, and pads that would otherwise hold it still while it shrinks, so that the restraint of those elements does not crack the field. Construction joints close out each placement and are the one joint type located by the crew's production rate rather than by the shrinkage of the slab, which is why they are made to coincide with the contraction joint lines. ACI 224.3R and ACI 302.1R set out the mechanisms; the layout that applies them is drawn for each floor. (13.1)

NOTE At 24 to 36 times the thickness, which is 2 to 3 ft of spacing for each inch of slab, a plain slab of ordinary shrinkage cracks at its joints and nowhere else; a mixture with a measured shrinkage limit, a fiber-reinforced slab, or a slab with a shrinkage-reducing admixture is carried to the upper end of that band or beyond it. Every foot added to the spacing is a joint removed from the floor and a foot added to the opening of the joints that remain, so a wider spacing is paid for in dowels, in filler, and in the width the filler has to bridge. (13.3.2)

NOTE A long narrow panel cracks across its middle because the shrinkage along its long dimension accumulates faster than the joints at its ends can relieve it; panels kept close to square crack at their joints. (13.3.4)

NOTE A conventional saw cuts a quarter of the thickness and has to wait until the concrete is hard enough not to ravel, which on a hot day is after the slab has already cracked somewhere else; an early-entry saw cuts a shallower joint within an hour or two of finishing, before the shrinkage stress develops, and relies on the still-weak concrete to crack beneath its shallower cut; a preformed insert pressed into the plastic concrete needs no sawing at all and leaves a joint edge that is harder to fill and more easily damaged, which keeps it to floors without hard-wheeled traffic. (13.3.9)

NOTE Cutting too late is the single most common joint failure on a floor. Shrinkage stress builds from the moment the slab stops bleeding, and in hot or windy weather a conventional saw cannot get on the slab before the first random crack forms; the window is set against the sawing method and the weather, and the trial placement is where it is confirmed. (13.3.11)

NOTE Anything that holds the slab while it shrinks becomes the origin of a crack. Columns are the classic case: without an isolation joint a crack radiates diagonally from each column corner across the panel, and no reinforcement in the slab stops it. The joint is full depth so that nothing at the bottom of the section restrains what the top is free to do. (13.4.2)

NOTE A diamond turned to the joint lines puts a corner of the isolation joint on each contraction joint, so the joints meet the column and the panel has no re-entrant corner; a circular isolation joint has no corners to crack from and is the detail for a floor that stays exposed, at the cost of a formed circle; a square boxout aligned with the contraction joints is the simplest to form and works only where the joints are run to its corners, because a square with a joint arriving at the middle of a side leaves a re-entrant corner that cracks. (13.4.6)

NOTE Where a loaded wheel crosses a joint, the two panels either deflect together or the loaded edge drops below the unloaded one, and every pass of the wheel then strikes the far edge. Hard wheels on a pallet jack or a lift truck spall an undoweled joint within months. A load-transfer device makes the two panels deflect together while still letting the joint open as the slab shrinks, which is the combination that aggregate interlock alone loses as soon as the joint opens more than a few hundredths of an inch. (13.6.1)

NOTE A construction joint is a formed, full-depth separation with no aggregate interlock at all, so it is doweled on nearly every floor that carries anything on wheels; contraction joints keep their interlock until they open, which on a floor with light traffic is enough, and on a floor with hard wheels is not, so they are doweled where the traffic runs and left to interlock elsewhere; doweling every joint is the choice for a floor whose traffic pattern is not known or will change; no devices at all suits a slab that carries foot traffic and pneumatic tires only. (13.6.3)

NOTE A smooth round dowel is the least expensive device and the one most sensitive to alignment, because a bar skewed in the joint locks it as the slab shrinks; a square or diamond plate dowel in a sleeve lets the joint move both along and across the bar, so it tolerates the misalignment that locks a round bar and can be used at a joint where panels shrink in two directions; a tapered plate keeps its bearing tight as the joint opens, which holds the deflection down at a joint that will open wide; a keyed joint transfers load through the concrete key itself and loses that transfer as soon as the joint opens, so it serves only a lightly loaded floor; aggregate interlock without a device serves where no hard wheel will cross the joint. (13.6.5)

NOTE Whether a joint is filled depends on what crosses it. A hard wheel striking an unfilled saw cut chips the joint edge on every pass; a semi-rigid filler supports those edges from inside the cut and is the only treatment that protects them. A joint that carries no hard wheels can be sealed to keep water and debris out of it or simply left open. The filler that supports an edge is also the one that cannot stretch, so it has to go in after the slab has done most of its shrinking, or the joint opens and splits it. (13.7.1)

NOTE A semi-rigid filler supports the joint edge against wheel impact and is installed full depth without a backer rod, at the cost of a product that cannot follow a joint that keeps opening; an elastomeric sealant follows joint movement and keeps the joint clean and watertight but is soft and offers the edge no support; an open joint costs nothing and collects whatever falls into it, which suits a floor that is covered, bare under foot traffic only, or scheduled for a later finish that fills the joints itself. (13.7.3)

NOTE Most of a slab's drying shrinkage occurs in its first months, and a joint filled early is pulled apart as the joint continues to open; delaying the filling as long as the construction schedule permits leaves less movement for the filler to absorb, at the cost of a floor whose joints stay unprotected until the filler goes in. (13.7.8)

NOTE Floor flatness and levelness are specified by F-numbers because a straightedge cannot measure a floor. FF describes bumpiness over short distances, the waviness a lift truck mast amplifies into sway; FL describes tilt over longer distances, the slope that leaves a rack leg short of the floor. Both are measured by the procedure of ASTM E1155 as an overall value for the whole floor and a minimum local value for each test section, because a floor can average well and still carry a hump that stops a pallet jack. A defined-traffic floor, where very narrow aisle trucks run on fixed wheel paths between high racks, is measured differently: along the wheel paths themselves, by F-min numbers that bound the change in elevation across and along each path. (14.1)

NOTE The F-number ladder tracks what runs on the floor. A slab that will be covered by a thick finish or that carries foot traffic only is built to the low end; an office or retail floor under a thin covering needs the flatness that keeps a resilient tile from telegraphing every bump; a warehouse with lift trucks in open aisles needs the flatness that keeps the mast from swaying and the levelness that keeps the racks plumb; and a floor that stays exposed as the finished surface is built to the number its owner can see. Local values are set at roughly two-thirds of the overall values, so that an individual section can be rougher than the floor's average without being bad enough to notice. Each step up the ladder is paid for in the strike-off method, in narrower placement strips, in the crew's skill, and in the number of sections that fail and are ground. (14.7)

NOTE Measured later than 72 hours, the survey records curling at the joints rather than the surface the finishers produced, and a slab that was within tolerance when it was finished can then fail a test that no finishing could have passed; measuring early fixes responsibility for the as-finished surface on the finishing crew and for later curling on the mixture and the curing. (14.11)

NOTE The strike-off method is what the F-numbers are bought with. A laser-guided screed or a vibrating truss screed riding on set forms produces a flat strip; hand strike-off from wet screeds produces whatever the screed-setter's eye produces. Long narrow placement strips with the screed riding on hard edge forms are how a tight tolerance is reached, and a wide placement finished from the middle is how a loose one is. (15.2.4)

NOTE Troweling bleed water back into the surface traps it under a dense skin, and the blisters and delaminations that result appear weeks later as the trapped water tries to leave. The finishers' waiting for the sheen to go is the step most often skipped on a cold day and most often regretted. (15.3.4)

NOTE Early loading drives a wheel across joints that have not yet been cut or filled and over concrete that has not reached the strength the dowels bear on, and the joint edges it breaks are the ones the finished floor will show for its life. (15.4.2)

NOTE A hairline crack in a bare warehouse floor is a cosmetic record of where the shrinkage went; the same crack beneath a resilient tile or an epoxy coating telegraphs through the finish and keeps moving under it, which is why the width at which a crack is reported is set by what the floor will carry rather than by the structural significance of the crack, which for a slab on ground is usually nil. (16.4)

NOTE Joint filler installed at an age of a few months is installed into a joint that is still opening, and some separation from the joint face is the expected result rather than a defect; a scheduled return to refill the separated joints after the first year costs a day of work and leaves the floor with filler that is bonded to the joint it will serve, where leaving it to the warranty claim process costs the same work plus the dispute over whether the separation is a defect. (18.5)