A week out: the subgrade you will never see again
A slab fails from the bottom up, and the failure is always older than the crack that reveals it. Before any concrete is ordered, the ground beneath has to be brought to a uniform bearing condition, and uniform matters more than hard. A subgrade that is consistently firm across the whole footprint carries a slab better than one that is rock under three quarters of it and spongy in one corner. Concrete does not object to settling. It objects to settling unevenly.
Compaction happens in lifts or it does not happen at all. Granular fill dumped to full depth in a single drop will not densify through its thickness however many passes the plate makes across the top, so fill goes in at the lift thickness the geotechnical report or the local authority specifies, each lift compacted before the next arrives. Moisture governs how well the material responds; bone-dry sand and saturated clay both shrug off a compactor, for opposite reasons.
Proof-roll before the formwork goes up. Once forms and steel are set, the subgrade becomes invisible and stays invisible for the life of the building. Walk every square metre of it, run a loaded barrow or a truck wheel across, and watch for pumping, rutting, or a patch that simply feels different underfoot.
Softness found now costs an hour with a mattock and a barrow of compacted stone. Softness found later is found by the slab itself: the concrete bridges the void, hogs over the firm ground either side, and cracks, often within the first month, sometimes not until the first hard winter, and always in a position that reads as random until somebody cores it. No repair available at that stage is cheaper than the hour would have been.
- Strip topsoil and organics down to competent material across the full footprint, not just under the walls.
- Place fill in lifts at the thickness the geotechnical report or local authority specifies, compacting each lift before the next.
- Check fill moisture; dry material will not bond and saturated material will not densify.
- Proof-roll with a loaded barrow or truck wheel and mark anything that pumps, ruts, or feels soft.
- Dig out and replace soft spots with compacted stone before a single form board is set.
Divide the depth of fill by the lift thickness the report allows and you have the number of passes the job actually owes — the figure that stops one deep drop being recorded as compaction.
The total height of fill to be placed.
The maximum loose lift thickness for your compaction equipment and material.
Number of lifts needed
16 lifts
They open the calculator with your figures already in it
Backfill Compaction Lift Count Calculator: 16 lifts — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- Lift thickness and fill height have to be measured the same way. This page takes the loose thickness the plant can place in one pass, so where a specification instead states a maximum compacted lift, the finished height gained per lift is less than the figure entered and the true count is higher than the number shown.
- A lift is one layer of material, and it is not brought to density by a single pass of the roller. Reaching the specified density normally takes several passes over each lift at a set overlap and travel speed, so plant hours come from lifts multiplied by passes, and this page asks for neither.
- The count assumes every lift is the same thickness across the whole plan area. It does not cover the thinner first lift often required over a soft or yielding subgrade, the bedding and haunching zone around a pipe where lift thickness is reduced and compaction is done by hand, or the depth of cover that must be built up before heavy plant is allowed to track over a buried service.
- Placing the right number of lifts is not the same as passing compaction testing. Density is checked lift by lift against a laboratory Proctor value, and a lift that fails has to be scarified, moisture-conditioned and recompacted no matter how thin it was placed, with moisture content away from optimum the usual reason a correctly sized lift will not reach density.
- This is a count of layers and nothing else. It produces no volume, tonnage or delivery quantity for the fill, and the loose material ordered exceeds the compacted volume in the void by the material's bulking, which is a separate calculation.
The day before: forms, grade, and the last honest measurement
Forms get set to a datum, never to the existing ground surface. Ground undulates; a laser or a dumpy level does not. Shoot the high point of the subgrade first, because that point sets the minimum top-of-slab elevation unless there is appetite to dig, and everything else falls out of it: form height, screed rail height, the fall to the door threshold or the gully.
Edge form stiffness is worth more attention than it usually gets. Stakes at generous spacing let the form belly outward under the weight of wet concrete, and a bellied form is not a straight edge that can be corrected later; it is a permanent line that every eye entering the building follows. Brace externally, kick the stakes, and check the string line after the last stake is driven rather than before.
Depth checks come last and they cost real money. Over-excavation is a silent invoice: twenty-five millimetres of extra depth across a fifty square metre slab is an extra one and a quarter cubic metres of concrete, paid for at full rate and never seen again. Take gauge stick readings on a grid, not just at the corners, and bring low spots up with compacted stone rather than with the concrete order.
- Establish a datum and shoot the high point of the subgrade before setting any form height.
- Set and brace edge forms, then re-check the string line and the diagonals after the last stake goes in.
- Gauge slab depth on a grid and correct low areas with compacted stone.
- Set screed rails or establish wet-screed positions and confirm falls to thresholds and drainage.
Under the steel: the vapour retarder and the argument about it
The vapour retarder goes down after the subgrade is signed off and before any steel is placed. Sheets are lapped generously, laps taped, penetrations sealed around pipes and column boxes, and the membrane turned up at the perimeter so that the slab sits in a tray rather than on a mat. Material classes for these membranes are set out in the ASTM specification covering plastic water vapour retarders used under concrete slabs, and the specified class is not a detail to substitute on site.
Placing concrete directly on the retarder remains the live argument on many jobs. With a membrane immediately beneath, no bleed water can escape downward, so every drop of it has to rise; the slab bleeds longer, the finishing crew waits longer, and the moisture gradient through the slab thickness raises the risk of edge curling. Interposing a granular blotter layer between the membrane and the concrete shortens the bleeding and reduces curl.
Against that, a blotter layer is a reservoir. Rain on it before the pour, or a wet fill placed in autumn, and it holds water directly under a slab that will later be closed off with a floor covering, feeding vapour upward for months and lifting adhesives. Where anything moisture-sensitive is going down over the slab, current guidance in the ACI floor and slab construction document points toward placing directly on the retarder and managing the finishing consequences. Where the slab stays bare and curl control dominates, the blotter argument gets made. The specification decides; the crew does not decide it on the morning of the pour.
The stack a ground-bearing slab sits on
- Slab concrete — taken off the formed dimensions rather than the drawing outline, with an allowance for tolerance and spillage, because running short mid-pour writes a cold joint that stays Concrete Calculator
- Reinforcement — carried at its design height on chairs; lying on the membrane it does nothing, and riding high it has given away the cover that keeps it from rusting out Rebar Calculator
- Granular blotter — gives bleed water somewhere to go downward and shortens the wait before finishing, at the cost of being a reservoir if it is rained on and then covered Gravel Base Layer Tonnage Calculator
- Vapour retarder — the class comes off the specification and is not a site substitution; what the crew controls is the lap, the seal at every penetration and the turn-up at the perimeter
- Compacted fill — placed in lifts and proof-rolled while it can still be corrected, since the day the forms go up is the last day anybody sees it Standard/Modified Proctor Compaction Percentage Calculator
Sheet area is the easy half; whether the membrane ends up continuous is decided at the laps, so take the tape straight off the sheet layout instead of buying one roll and rationing it.
SettingsSettings for this calculation
Waste is set to 5% by hand. Pick a tier above to replace it, or keep your own figure.
The overall width of the slab area being covered by the vapor barrier.
The width of a single vapor barrier roll or sheet.
The overall length of the slab area, running the direction of each seam.
Extra tape to allow for overlaps, patches, and cut waste.
Seam tape needed
154.3 ft
- Number of seams
- 3
They open the calculator with your figures already in it
Under-Slab Vapor Barrier Seam Tape Calculator: 154 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The sheet count divides the slab width by the full roll width, so nothing is deducted for the side lap where adjacent sheets overlap each other. Laps are typically specified somewhere between 150 mm (6 in) and 300 mm (12 in) depending on the barrier manufacturer and the project specification, and once that is taken off the effective coverage of each sheet a wide slab can need one more sheet, and one more seam, than this calculation reports. Enter the lapped coverage width rather than the printed roll width if you want the lap counted.
- Only the longitudinal seams between sheets laid side by side are counted, and each is taken as running the full slab length in one unbroken run. If the roll is shorter than the slab, every run also carries an end lap partway along it, and there is no roll-length input here, so that transverse tape is absent from the total.
- The figure is seam tape between sheets and nothing else. Sealing the barrier to the footing, the foundation wall or the slab edge, and taping around pipes, conduit, column bases and other penetrations are all excluded and have to be estimated separately. On the default 20 m by 15 m slab the perimeter alone is 70 m against the 47.25 m of seam tape returned, so the excluded work can be the larger quantity.
- Which dimension you enter as the width decides the answer, and the calculator does not compare the two lay directions for you. Running 6.1 m sheets across the 20 m side of a 20 m by 15 m slab gives three seams and 47.25 m of tape, while running them across the 15 m side gives two seams and 42 m. The geometry also assumes one plain rectangle, so an L-shaped or stepped slab has to be broken into rectangles and the results added.
Reinforcement: chairs, laps, and why hooked-up mesh does not stay up
Steel does not stop a slab cracking. It holds the crack tight once it happens, and it can only do that from the correct position through the slab thickness. A bar or a sheet lying on the membrane is doing almost nothing, and a bar riding too high has lost its cover and will rust its way out through the surface within a few winters.
Chairs carry the steel, at spacing close enough that the bar does not sag between them under the weight of a boot. Sheet mesh sits on chairs; rolled mesh fights back and never lies flat enough to trust. The old habit of pulling mesh up with a hook as the concrete goes past is the one to abandon. It lifts the sheet at the point of the hook and nowhere else, the crew walks it straight back down two minutes later, and the finished slab has reinforcement that wanders between the top and the bottom of the section with no record of where it went.
Cover and lap length are code matters. Minimum cover varies with exposure, with the member, and with the edition of the code in force, and ACI 318 governs it for structural work; the drawing takes precedence over any yard rule of thumb. Development and lap lengths depend on bar size, concrete strength, cover, and bar spacing, so a single memorised multiplier is not a substitute for the schedule on the drawing. Mesh sheets are lapped and tied so that the lap acts as a lap and not as two sheets sitting near each other.
Work out the grid quantity, bar count, and total lap allowance before the steel is ordered, so the laps on site match the schedule on the drawing rather than what happens to be left on the stillage.
The length of the slab being reinforced.
The width of the slab being reinforced.
Closer spacing means more reinforcement strength but more material.
Extra bar for the overlap where sticks join across a run longer than one stock length.
Estimated rebar needed
11 x 20 ft sticks
General-purpose slab reinforcement guideline, not a structural engineering design — load-bearing or code-required slabs should follow an engineer's specification.
- Total bar length (incl. 15% lap splice)
- 208.15 ft
- Bars running lengthwise
- 7 bars
- Bars running widthwise
- 9 bars
They open the calculator with your figures already in it
Rebar Calculator: 11 x 20 ft sticks — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Schematic layout — positions are illustrative, quantities are exact. This calculator counts the bars and never says where any of them sits, so they are drawn evenly across the slab. Moving one would change nothing in the numbers above, and cover is not shown because this calculator does not ask for it.
- This calculator does not ask for cover, so the section does not dimension it. Cover is a structural requirement and comes from the engineer's schedule, not from a quantity take-off.
Rebar grid — drawn from your figures
7 bars one way and 9 the other, at 18 in centres, on a 3.96 x 3.05 m slab.
A DXF plan in metres. Opens in AutoCAD, LibreCAD, QCAD and most jobsite viewers.
What this drawing does not show (3)
- Concrete cover. Bars are drawn to the slab edge because that is how the quantity is counted; on site they stop short of it.
- Lap splices, which the quantity applies as a factor rather than placing.
- Bar diameter, chairs, and anything about depth — this is a single plan layer.
It is a setting-out aid drawn from the figures you entered, not a construction drawing, and nobody has checked it against a design.
What this calculation does not cover
- This is a quantity take-off, not a structural design. It does not choose bar size, grade, cover, slab thickness, or the spacing the load actually requires — a slab that carries load, spans between supports, or needs a permit takes an engineer's reinforcement schedule, and this calculator never asks what the slab is for.
- It lays out one flat mat inside one rectangle. A second (top) mat, perimeter and edge bars, corner and opening trim bars, thickened edges, footing or grade-beam steel, and dowels tying the slab to a footing are all outside the count, as is any L-shaped or non-rectangular pour.
- Stick count is total bar length divided by stock length, which assumes every offcut is used somewhere. Bars cut to a slab dimension usually leave a remainder too short for the next full-length bar, so the number of sticks you actually have to buy is often higher than the figure shown.
- The lap allowance is a flat percentage on the whole take-off. Real lap length is set by bar diameter, concrete strength, and whether the splice is in tension or compression — none of which this calculator asks for — so at the 15% default the allowance is short on large bars or high-demand splices and generous on small bars; set it from the bar schedule where there is one.
- The grid runs edge to edge across the slab dimensions you enter, so concrete cover at the perimeter is not deducted. Chairs, bar supports, and tie wire are not counted at all.
Booking the truck: volume, slump, and the weather window
Volume gets calculated from the formed dimensions and then adjusted upward. Subgrade tolerance, form deflection, and spillage all eat concrete, and running short mid-pour is far more expensive than a small over-order, because a cold joint through the middle of a slab is permanent. A waste allowance in the region of five to ten per cent is common practice, tightened when the subgrade has been laser-screeded and loosened when it has not.
Slump belongs on the ticket and gets verified on site. The test method is ASTM C143, and the reason for running it is not paperwork. Water added at the truck to make placing easier raises the water-cement ratio, and that single act reduces compressive strength, increases drying shrinkage, weakens the wearing surface, and widens every crack the slab was going to get anyway. Any water addition on site is governed by the ready-mixed concrete specification and the mix design, and it is not a decision for whoever happens to be holding the chute.
Weather is the other half of the booking. Concrete temperature, ambient temperature, relative humidity, and wind speed together set the rate at which the surface loses water, and it is wind that catches crews out; a warm still day is far more forgiving than a cool day with a stiff breeze across an open slab. High evaporation rates produce plastic shrinkage cracking in the first hours, before the concrete has any strength to resist it. Hot conditions push the start time earlier; cold conditions demand that the ground is not frozen and that the slab is protected before nightfall.
Crew size decides the pour rate, not the pump. Concrete arriving faster than the finishing crew can close it is the most common scheduling error on a slab, and it turns the back half of the pour into a scramble against a surface that is already stiffening.
Take the formed dimensions off the setting-out and get the slab volume plus a waste allowance before you ring the plant, so the order is a number rather than an estimate made at the gate.
SettingsSettings for this calculation
The standard allowance most suppliers and estimating guides assume for ordinary work.
The length of the slab or footing.
The width of the slab or footing.
How deep the concrete pour is.
Extra concrete for spillage, uneven subgrade, and forming imprecision.
Estimated concrete needed
1.358 yd³
- Volume (no waste)
- 1.23 yd³
- Volume with waste factor
- 1.36 yd³
- Cubic feet
- 36.67 ft³
- 80 lb bags needed
- 62 bags
They open the calculator with your figures already in it
Concrete Calculator: 1.36 yd³ — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Geometry is one rectangular prism: length x width x a single uniform thickness. Thickened edges, integral footings, haunches, steps, curbs and any non-rectangular outline are not in the figure, and nothing is subtracted for block-outs or openings. Take those off as separate volumes and add them.
- It assumes a flat, compacted subgrade sitting at exactly the depth you entered. Ruts, soft spots, over-excavation and a base that dishes in the middle all take concrete the geometry never sees, and a flat waste percentage is not a measurement of that. On a rough base, check depth across the whole pour rather than trusting the allowance.
- This is a volume take-off, not a structural decision. It accepts whatever thickness you type without sizing it, and says nothing about mix strength, aggregate size, air entrainment, fibre, or rebar and mesh. Slabs carrying vehicles, footings, and anything supporting a structure are a code and engineering question.
- The bag count assumes an 80 lb (36 kg) bag yields about 0.6 cubic feet (17 litres) of mixed concrete, and rounds up to whole bags. Real yield shifts with the product and with how much water goes in, and no other bag size is converted for you.
- The volume is not an order quantity. Ready-mix is sold in fixed increments with a minimum load and its own short-load charges, and concrete left in the drum, the chute or the pump line is not counted. The waste factor covers spillage and forming slop, not the plant's ordering rules.
Code thresholds this tool can check
Code thresholds this tool can check
Checked for United States. Each check below names the body that published the limit it uses. Switching market re-runs them. This is not a code review and has no official standing.
These checks cover only the specific numeric limits listed below. They are not a complete code review: fire separation, egress, structural capacity and accessibility provisions are outside their scope, and only the handful of local amendments offered in the selector are modelled — your municipality may have others. Passing every check here does not make a design compliant. Final approval rests with your local building authority.
WITHIN LIMIT — Concrete floor slabs on ground: minimum 3.5 in (89 mm) thick.
Slab thickness 4.00 in meets the 3.5 in IRC floor-slab minimum. Expansive soils are handled separately under IRC R403.1.8, and any slab carrying vehicles or point loads should be designed rather than taken from the code minimum.
ICC · IRC R506.1
Pour day, first hour: placing, consolidating, screeding
Discharge as close to final position as the access allows. Concrete dragged any distance with a rake segregates: the mortar travels, the coarse aggregate stays, and the slab ends up with a weak, sandy zone that will craze and dust later. Move the truck or move the pump line rather than moving the concrete.
Placing runs against a live edge. Each load goes against the face of the last, in a continuous advancing front, so nothing sits long enough to start setting before its neighbour arrives. A slab placed in scattered heaps and joined afterwards has cold joints running through it that nobody will find until they show as cracks.
Consolidation gets the air out and the concrete around the steel and into the form corners. Vibrate deliberately rather than dragging a poker sideways to move material, which is another route to segregation. Screed immediately behind placing, off rails or wet screeds, and screed once; repeated passes bring fines and water to the surface and build the weak layer that finishing problems come from.
Bull float straight after the screed, while the surface is still plastic and before bleed water has appeared. One or two passes, and then the crew stops and waits. Stopping is the hard part.
- Discharge close to final position; never rake concrete across the slab.
- Keep a live edge and place continuously against the previous load.
- Consolidate around steel, edges, and penetrations without using the poker to move material.
- Screed once, immediately behind placing.
- Bull float straight away, before any bleed water shows, then stop.
Bleed water and the finishing window that moves
Troweling too early is the single most common finishing error on a concrete slab, and it is the one with the most expensive consequences. While bleed water is on the surface or a wet sheen still shows, any tool that touches it works that water back down into the top few millimetres. The result is a surface layer with a far higher water-cement ratio than the slab beneath, which then dusts under traffic, scales in frost, crazes into a mosaic of fine cracks, or blisters where a sealed skin has trapped rising air and water. Sprinkling dry cement to dry the surface, or a hose to lubricate the trowel, does the same damage by another route.
Bleed water goes away by evaporating or by being dragged off with a hose or a squeegee, never by being troweled in. The classic readiness check still works: when the sheen has gone and a boot leaves an impression of a few millimetres rather than sinking, the surface will take a float. Beyond that point the operations follow the concrete rather than the clock.
Temperature and wind move the whole window, and they can move it in both directions. On a hot, breezy, low-humidity day the surface may show no bleed water at all and crust within minutes while the body of the slab is still soft, which produces blisters and a delaminated skin if the crew reads the dry surface as readiness. On a cool damp day the same mix will bleed for hours, the crew stands around, and finishing runs past dark under lights. Neither situation is unusual, and neither is solved by working to the time the last slab took.
Broom finishes close earlier than hard-troweled ones and are the safer choice for external slabs, where a burnished surface is a slip hazard and holds less air entrainment near the top. Hard troweling is a sequence of passes with progressively tighter blade angles, each one waiting on the concrete, not on the shift ending.
| Operation | Cue on the slab | What going early produces |
|---|---|---|
| Bull float | Immediately behind the screed, surface still plastic, no bleed water yet | Nothing, provided it stops before bleed water rises; extra passes bring fines up |
| Waiting | Bleed water present or a wet sheen showing | Any tool used here works water into the surface layer |
| Float | Sheen gone, boot leaves a shallow impression rather than sinking | Dusting, crazing, and a soft wearing surface |
| Broom | Immediately after floating, while the texture will still take | A texture too shallow to give slip resistance, or a torn surface if too late |
| Hard trowel | Successive passes as the slab stiffens, blade angle tightened each time | Blistering and delamination where a sealed skin traps rising air and water |
Four to twelve hours: cutting the joints before the slab cuts its own
The saw is the last operation of the pour, and its timing has almost no slack in it. Cutting is possible once the surface has hardened enough that the blade does not ravel the edges of the cut, and it must happen before drying shrinkage stress exceeds the tensile strength of the young concrete. With a conventional wet saw that window commonly falls somewhere in the first four to twelve hours after finishing; early-entry equipment is designed to go in far sooner, sometimes within an hour or two. Set time, mix, and ambient temperature move it, so somebody has to be on site watching rather than booking a slot.
Depth matters as much as timing. A joint cut too shallow does not create a plane of weakness strong enough to attract the crack, and the slab cracks somewhere else instead, usually a few hundred millimetres away and running at an angle. The conventional working rule for a wet-sawn joint is a depth of about one quarter of the slab thickness; early-entry systems are cut shallower by design, and the project specification governs which applies. Spacing follows the rule of thumb of roughly twenty-four to thirty-six times the slab thickness in consistent units, adjusted for mix shrinkage and reinforcement, with panels kept close to square. Long thin panels and re-entrant corners crack regardless of how well the rest is laid out, so those get jointed deliberately.
Cutting late is not a partial success; it is a total failure of the operation. Concrete that has already relieved its shrinkage stress through a random crack has nothing left to give the saw cut, and the cut becomes decoration around a crack that now runs across the floor and through the joint line. Curing continues around the sawing: the cure is applied as soon as the finish closes, interrupted only for the length of the cut, and restored immediately after.
| Variable | Working rule on site | What governs it |
|---|---|---|
| Cut timing | Conventional wet saw commonly four to twelve hours after finishing; early-entry sooner | Set time of the mix and ambient conditions; the point at which the cut stops ravelling |
| Cut depth | About one quarter of slab thickness for a conventional wet-sawn joint | The project specification; early-entry systems are cut shallower by design |
| Joint spacing | Roughly twenty-four to thirty-six times the slab thickness, in the same units | Mix shrinkage, reinforcement, ACI 302.1R guidance, and the specification |
| Panel layout | Keep panels close to square | Re-entrant corners, columns, and long thin panels crack regardless of spacing |
Panels get laid out on paper long before the saw is fuelled, so run the slab thickness through the spacing rule now and mark the lines while keeping them square is still free.
The thickness of the concrete slab.
Maximum joint spacing
10 ft (maximum spacing)
This is a general rule of thumb — actual joint layout should also follow the slab's panel shape (aim for roughly square panels, not long narrow strips), re-entrant corners at any cutouts, and any project-specific structural engineering requirements.
- Slab thickness
- 4 in
They open the calculator with your figures already in it
Concrete Control Joint Spacing Calculator: 10 ft (maximum spacing) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- Thickness is the only input. What comes back is a spacing ceiling, not a joint layout: the calculator never sees the slab's plan dimensions, so it cannot count the cuts, place them, keep the panels near square, or flag that an inside corner or a block-out for a bollard or column base needs a joint run out of it.
- The multiplier is fixed at the middle of the cited range and does not move for the conditions that decide where in that range a slab belongs. A high-shrinkage mix, small maximum aggregate, a hot windy pour, or restraint from a rough sub-base, thickened edges or footings all argue for tighter spacing than this returns.
- This is a rule of thumb for a plain slab on ground, not a design. It does not apply to a structurally reinforced slab, a post-tensioned slab, a suspended or elevated floor, or a steel-fibre jointless floor, and where a project specification, a standard drawing or a local authority sets a spacing, that figure governs and this one has no standing.
- Spacing is all it returns. Cut depth — the joint has to remove enough section to force the crack at that plane, traditionally about a quarter of the slab thickness — and the sawing window after finishing are both outside the calculation, and a joint at perfect spacing cut too shallow or too late controls nothing.
- Control joints are one of three joint types and the only one modelled here. Isolation joints where the slab meets columns, walls and footings, and construction joints at pour breaks, are separate decisions with their own positions; a construction joint that lands off the control-joint grid gives the slab two competing instructions about where to crack.
Days one to seven: curing, the step most often skipped
Curing is the step most often skipped, and it gets skipped because nothing visible happens on the day it is skipped. Cement hydration needs water. A slab that dries out at the surface stops gaining strength exactly in the top layer that carries abrasion, impact, and freeze-thaw, so an uncured slab can test acceptably on cylinders taken from the truck and still dust, scale, and wear badly within a year.
Methods vary and any of them beat nothing: ponding or continuous wetting, wet hessian kept genuinely wet rather than allowed to dry and act as a wick, plastic sheeting lapped and weighted down at the edges, or a membrane-forming curing compound applied at the specified coverage rate. Sheeting can mottle the surface where it touches unevenly, which matters on exposed floors. Curing compounds have to be compatible with whatever coating or adhesive comes later, and a compound applied under a floor covering that needs a clean substrate is a problem bought for the tiling contractor.
Duration is set by code and by the mix rather than by convenience. Minimum curing periods are given in ACI 318 and set out in detail in the ACI specification for curing concrete, and they are commonly on the order of a week for ordinary mixes and shorter for high-early-strength concrete, with the applicable edition and the local building control having the final say. Protection from freezing during that period is separate from and additional to curing. Leaving side forms in place for the first days protects the arrises, which are the first thing to chip and the last thing anyone wants to patch.
- Start curing as soon as the finish is closed, not at the end of the shift.
- Choose a method compatible with the finish and with any covering that follows.
- Keep wet coverings genuinely wet; a dry hessian sheet draws water out of the slab.
- Maintain the cure for the period the specification and the code in force require, and protect against freezing separately.
- Leave side forms on through the early days to protect the edges.
A membrane cure performs at the coverage rate printed on the drum and nowhere near it if the drum runs out, so turn the slab area into litres before the finish closes rather than after.
The total finished concrete surface area to be cured.
The specific product's coverage rate from its technical data sheet.
Curing compound needed
10.8 gal
Actual coverage varies by application method (spray vs. roller), surface texture, and porosity — always confirm the specific product's rated coverage against your own trial application.
- Equivalent in gallons
- 10.75 gal
They open the calculator with your figures already in it
Concrete Curing Compound Coverage Calculator: 10.75 gal — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The figure is the theoretical volume for the area you enter. It carries no allowance for overspray past the slab edge, wind drift, or material left in the sprayer and hose, and it does not round up to whole pails or drums.
- Plan area is not wetted area. A broomed, tined or exposed-aggregate finish presents more surface than its footprint and takes more compound than a smooth trowelled slab at the same rated coverage. Formed edges, curbs and slab returns are excluded entirely.
- This sizes a single application at a single rate. Where the specification calls for two coats, a second pass at right angles, or a heavier rate on textured pavement, you have to multiply the figure yourself — there is no coat count in the model.
- It does not check the rate you enter against anything. Specifications and product listings often cap the application rate below the coverage a product can physically achieve, and the calculator will size an order at a rate your specification does not permit.
- Nothing here decides whether a membrane-forming cure is the right choice. VOC rules restrict solvent-based compounds in many regions, and many compounds interfere with adhesives and coatings applied later, sometimes needing mechanical removal — confirm the product against the specification and the future floor finish before ordering.
The month after: strength, movement, and what the slab tells you
Twenty-eight days is the conventional age for the specified compressive strength, and cylinders or cubes made and cured in accordance with the relevant ASTM field practice are what settle any argument about the concrete supplied. Delivery tickets, slump and temperature results, and a note of when curing started and stopped are worth keeping in one folder; they are the only evidence that exists if a floor is questioned two years later.
Loading matters through this period. A slab reaching design strength at twenty-eight days is not at design strength when the scaffold tower goes up on day four, and racking, wheeled plant, and stacked material placed early leave permanent marks or cracks in a slab that would have carried them comfortably a month on.
Movement shows up in the first weeks and then slows. Joints open as the slab shrinks, edges curl slightly upward at panel corners where the top dries faster than the bottom, and fine surface crazing may appear on floated finishes. Filling joints with a semi-rigid material is usually deferred until most drying shrinkage has occurred, which is a matter of months rather than weeks, because a joint filled while it is still opening will simply split or debond. Cracks that appear in this window are worth recording with a date and a width; the pattern tells you whether you are looking at plastic shrinkage from the first hours, drying shrinkage that the joints failed to control, or a subgrade problem that started the week before the pour.
Slab takeoff: from setting-out to the order
Run these in the order the pour happens. Dimensions come off the formed slab, not the drawing outline, and the steel quantity is settled before the concrete is booked so the chairs and laps are on site the day before the truck is.
- Slab volume — Formed length, width, and thickness, plus a waste allowance in the region of five to ten per cent for subgrade tolerance, form deflection, and spillage.
- Reinforcement grid — Bar or mesh spacing, sheet layout, and the total lap allowance; check the result against the schedule on the drawing rather than a memorised multiplier.
- Chairs and accessories — Chair count follows the grid spacing closely enough that the steel does not sag underfoot; count them at the same time as the bar.
- Bag equivalent — For small pours, hand-mixed sections, or topping up after a short load, convert the volume to bag counts before deciding between bags and a truck.
- Order and window — Confirm slump on the ticket, the start time against the day's temperature and wind, and the pour rate against the size of the finishing crew.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.